Evidence of an abrupt environmental disruption during the Mid Paleocene Biotic Event (Zumaia...

For permission to copy, contact [email protected]© 2007 Geological Society of America

785

ABSTRACT

An abrupt environmental disruption occurred in the photic zone and at the seafl oor during the mid-Paleocene biotic event (MPBE). Calcareous nannoplankton, planktic foraminifer, and benthic foramini-fer assemblages at Zumaia section (western Pyrenees) underwent a rapid and remark-able transformation. The major calcareous plankton assemblage changes suggest a shift from relatively cooler mesotrophic to warmer, more oligotrophic conditions, indi-cating a disturbed environment due to the warming of the ocean. Benthic foraminifer assemblages were also signifi cantly affected by the MPBE; diversity of the assemblages and buliminids show net decline and the low food and opportunistic taxa increase in abundance. The reorganization of the plank-tic ecosystem possibly involved changes in the food fl ux (type and quantity) to the

seafl oor, thus triggering changes in the ben-thic communities.

A 1‰ negative δ13C shift and a 30% car-bonate content decrease are recorded in con-nection with the biotic event. This suggests that during the MPBE, as in the Paleocene-Eocene Thermal Maximum (PETM), an input of a large mass of isotopically depleted carbon into the ocean and atmosphere could have lowered the deep-sea pH, triggering a rapid shoaling of the lysocline and contribut-ing to greenhouse warming.

The MPBE was short lived: according to the counting of limestone-marl couplets, the stratigraphic expression of precession cycles throughout the Zumaia section, the MPBE lasted for ~52–53 k.y., with the core of the event representing ~10–11 k.y.

The Zumaia section is the fi rst land-based locality in which the MPBE is recognized and described in detail. Due to its expanded character and excellent paleontological record, this section may prove to be a global reference section for the study of this short-lived event.

Keywords: mid-Paleocene biotic event, calcar-eous nannofossils, planktic foraminifera, ben-thic foraminifera, Zumaia, Pyrenees.

INTRODUCTION

Recent studies of deep-sea sections at Shatsky Rise (Ocean Drilling Program [ODP] Leg 198, central Pacifi c) and Walvis Ridge (ODP Leg 208, South Atlantic) have confi rmed the existence of several dissolution levels inter-spersed in the carbonate sediments of the late Paleocene–early Eocene, i.e., the Elmo horizon, X event layer, and mid-Paleocene biotic event layer (Bralower et al., 2002; Zachos et al., 2004; Lourens et al., 2005). Similar to the Paleocene-Eocene Thermal Maximum (PETM) interval, these levels are characterized by the presence of a distinct reddish clay layer associated with an abrupt drop in carbonate content and a pro-nounced peak in magnetic susceptibility (MS) that refl ects an increase in clay content. These dissolution intervals have been interpreted as the sedimentary response to abrupt climatic changes; the few micropaleontological analyses performed in these intervals indicate short-lived events of considerable evolutionary signifi -cance probably related to hyperthermal events (Thomas and Zachos, 2000; Petrizzo, 2005). This idea is supported by a decrease in oxy-gen isotope (δ18O) values measured in planktic foraminifera from some of the aforementioned intervals (Lourens et al., 2005).

Evidence of an abrupt environmental disruption during the mid-Paleocene biotic event (Zumaia section, western Pyrenees)

Gilen Bernaola†

Juan Ignacio Baceta§

Xabier Orue-Etxebarria#

Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain

Laia Alegret*Área de Estratigrafía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain

Maite Martín-Rubio‡

Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain

Javier Arostegui##

Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain

Jaume Dinarès-Turell**Istituto Nazionale di Geofi sica e Vulcanologia, Via de Vigna Murata 605, 00143 Rome, Italy

†corresponding author: [email protected] §[email protected]#[email protected]*[email protected]‡[email protected]##[email protected]**[email protected]

GSA Bulletin; July/August 2007; v. 119; no. 7/8; p. 785–795; doi: 10.1130/B26132.1; 5 fi gures; Data Repository item 2007147.

Bernaola et al.

786 Geological Society of America Bulletin, July/August 2007

The foremost hypothesis to explain the most relevant global warming event during the early Paleogene, the PETM, is the input of signifi cant CO

2 into the ocean by dissociation of submarine

methane hydrates (Dickens et al., 1995, 1997). However, there is not yet enough evidence to know whether the other late Paleocene–early Eocene hyperthermal events were related to the input of massive amounts of greenhouse gas.

Recent orbital tunning analyses suggest that these late Paleocene–early Eocene hyperthermal events were controlled by maxima in the short-term eccentricity cycles (Cramer et al., 2003; Lourens et al., 2005); this implies that they may be astronomically forced.

The mid-Paleocene biotic event (MPBE) has not been described in onshore sections before. The Zumaia section is one of the most complete and expanded land-based, deep-marine Paleo-cene sections reported (Schmitz et al., 1997; Pujalte et al., 1998; Baceta et al., 2000). It com-prises a rhythmic alternation of (hemi)pelagic limestones and marlstones with intercalations of thin-bedded (<5 cm) siliciclastic, calcareous, or mixed turbidites (Baceta, 1996; Baceta et al., 2000). The excellent paleontological record of this section makes it ideal to document the evolutionary trends of different fossil groups after the Cretaceous-Paleogene (K-Pg) bound-ary, including transient biotic events such as the PETM (Schmitz et al., 1997; Orue-Etxebarria et al., 2004). Moreover, its strong paleomag-netic signal and the preservation of orbitally paced bedding cycles allow the defi nition of the approximate timing of each of these events (Ten Kate and Sprenger, 1993; Dinarès-Turell et al., 2002, 2003).

Ongoing studies in the mid-Paleocene suc-cession of Zumaia have shown the presence of a distinct clay-rich interval characterized by important calcareous nannofossil and foramini-fer assemblage changes (Bernaola et al., 2006). This interval, which is also characterized by a signifi cant drop in carbonate content and a pro-nounced peak in MS, is located ~4.5 m above the fi rst occurrence of Heliolithus kleinpellii, the Zone NP6 marker, ~2 m below the C26r-C26n reversal and within planktic foraminifera Zone P4. According to this position, it seems to be unmistakably equivalent to the red clay layer of the MPBE found at Shatsky Rise and Walvis Ridge (Bralower et al., 2002; Zachos et al., 2004) (Fig. 1). In Shatsky Rise, the clay-rich layer of the MPBE is enriched in phillipsite crystals and contains fi sh teeth and phosphatic micronodules. Planktic foraminifers are charac-terized by a low diversity and largely dissolved assemblage dominated by the genus Igorina, whereas the calcareous nannofossil assemblage has a sharp increase in Ericsonia, a genus that is

most abundant at low latitude, warm-water sites (Wei and Wise, 1990; Kelly et al., 1996). How-ever, only the planktic foraminiferal assemblage changes of the MPBE clay-rich layer of Leg 198 at Shatsky Rise have been studied in detail (Petrizzo, 2005), and there is still little evidence regarding the response of other marine micro-organisms, particularly benthic foraminifers and calcareous nannoplankton, to the MPBE.

The aim of this study is to show the most relevant changes that took place in the calcareous plankton and benthos at the time of the MPBE, based on the micropaleontologic analysis carried out in the mid-Paleocene of the Zumaia section.

MATERIAL AND METHODS

The Zumaia section (lat 43°18′02″N, long 2°15′34″W) is the most representative and complete outcrop of the (hemi)pelagic succes-sions that were deposited in the Pyrenean Basin during the Late Cretaceous–early Paleogene (Schmitz et al., 1997; Pujalte et al., 1998).

In this section, the Paleocene is compara-tively expanded (150 m thick); the estimated sedimentation rate is ~1.5 cm/k.y. (Pujalte et al., 1998). According to Pujalte et al. (1998) and our data, the Paleocene succession was deposited at water depths of ~1000 m and does not show any signifi cant stratigraphic gaps. Detailed calcare-ous nannofossil and planktic foraminiferal bio-stratigraphic studies confi rm the continuity and expanded character of this section (e.g., Are ni-llas and Molina, 1995; Bernaola, 2002).

We took 60 samples at moderate resolution through 4.3 m of the mid-Paleocene interval containing the MPBE for paleontological, litho-logical, and geochemical studies (Fig. 2; see GSA Data Repository Tables DR1–DR41).

The paleontological studies have focused on the analysis of calcareous nannofossils, planktic foraminifera, and benthic foraminifera.

Smear slides for the study of calcareous nan-nofossils were prepared following the standard methods of sample preparations for quantitative analysis (Bown, 1998). The slides were ana-lyzed under a Leica DMLP petrographic micro-scope at 1500× magnifi cation (2000× for details and/or small taxa). We identifi ed 43 Paleocene taxa; however, only 8 groups have been used for detailed paleoecological interpretations (Fig. 3). These eight groups were formed on the basis of taxonomic and paleoecological affi nities in order to enhance trends. For the quantitative analysis, at least 500 nannofossil specimens per

sample were counted along a random traverse on the slide. Toweius pertusus and Coccolithus pelagicus exceeded 50% of the assemblage in all samples, which obscured the abundance changes of other important but less prevalent species. In order to increase the number of spec-imens of these taxa to a statistically signifi cant level, counts were continued until the number of specimens other than C. pelagicus and T. pertu-sus reached 200.

Samples for the study of planktic and ben-thic foraminifera were processed following standard micropaleontological procedures: 250 g of sediment per sample were washed and screened to obtain residues of a 63–1000 µm size fraction that were studied under binocu-lar microscope. After separation with an Otto microsplitter, relative abundances of the differ-ent species of both planktic and benthic fora-minifera were estimated based on counts of at least 300 individuals of each group. The resi-due from all samples, except for sample Ev-34, which is almost void of planktic foraminifera, contained planktic and benthic foraminifera in suffi cient quantity and degree of preservation to permit a quantitative analysis.

To determine the planktic foraminifera shell fragmentation index, routinely used to delin-eate stratigraphic changes in planktic fora-miniferal preservation (e.g., Thunell, 1976; Howard and Prell, 1994; Kelly et al., 2005), a split (>63 μm) of each sample was examined. The fragmentation index is expressed as shell fragments/(whole shells + shell fragments). To characterize the planktic foraminiferal test size changes across the MPBE we determined the mean value of the fi ve largest specimens of each planktic foraminiferal group (morozovellids, acarininids, igorinids, and subbotinids) outside and within the MPBE (Fig. 4). The reason for focusing on the test size of the largest specimens in each sample rather than the mean of the entire population is that the latter would be strongly infl uenced by the number and size of juvenile specimens counted. The total number of plank-tic specimens screened in each sample ranged from 258 to 376.

The planktic/benthic ratio [P/B ratio, expressed as 100 × P/(P + B)] and the number of planktic and benthic foraminifera per gram of dry sediment were also calculated. As prox-ies for benthic foraminiferal diversity, we calcu-lated the Fisher-α index and the H(S) Shannon-Weaver information function (Murray, 1991).

The calcium carbonate (CaCO3) and low-

fi eld MS analyses were performed using standard methods at the Universidad del País Vasco and the Istituto Nazionale di Geofi sica e Vulcanologia, respectively. The MS was mea-sured in a Kappabridge KLY-3 apparatus and

1GSA Data Repository item 2007147, Tables DR1–DR4, is available at http://www.geosociety.org/pubs/ft2007.htm or by request to [email protected].

Mid-Paleocene Biotic Event at Zumaia

Geological Society of America Bulletin, July/August 2007 787

C

B

MPBE

A

Figure 1. (A) Global map showing early Paleogene paleogeography and locations of Zumaia section, Walvis Ridge, and Shatsky Rise, and early Paleogene paleogeography of western Europe. (B) Synthetic sketch of the mid-Paleocene outcrops in the Zumaia section, showing the fault system (F1–F3) that disrupts the succession and the location of the mid-Paleocene biotic event (MPBE) and x-y correlation levels. (C) Detailed view of the MPBE interval showing main lithological beds; a–d reference beds in Figure 2. Scale bar represents 2 m.

Bernaola et al.


Figure 2. Integrated lithostratigraphy, biostratigraphy, and cyclostratigraphy of the mid-Paleocene of the Zumaia section. Arrows show main calcareous nannofossil and magnetostratigraphic events. FO is fi rst occurrence. Calcareous nannofossil biostratigraphy is from Ber-naola (2002) according to Martini (1971). Planktic foraminifer biostratigraphy is from Orue-Etxebarria et al. (2006). Details of CaCO3 wt% content, magnetic susceptibility (MS), and bulk carbonate δ13C chemostratigraphy across the mid-Paleocene biotic event (shaded) are shown. All δ13C values are expressed as per mil differences with respect to Peedee belemnite (PDB) standard (a–d are reference beds in Figure 1). D is Discoaster; H. is Heliolithus.

Figure 3. Relative abundance of selected calcareous nannofossil taxa in the study interval. Shaded area corresponds to the mid-Paleocene biotic event. Star indicates occurrence of abundant radiolaria.



weight normalized. The percentage of carbon-ate was determined volumetrically in a gauge calcimeter. Stable isotope (δ13C) analyses were performed on bulk samples with a VG-Isotech SIRA-II mass spectrometer attached to an Iso-carb automated carbonate preparation system at the Isotope Laboratory of the Universidad de Salamanca.

Mineralogical analyses of clay minerals were performed by X-ray diffraction (XRD) for the <2 µm fraction (Mg and K homoionic) of each sample on the oriented aggregates; samples were air dried, treated with glycerol and dymetil-sulfoxide, and heated at 300 and 500 °C. Clay minerals were identifi ed by characteristic basal refl ections (Moore and Reynolds, 1997). Semi-quantitative determination was carried out using the intensity (area) of the major XRD refl ections with correcting factors.

RESULTS

Lithostratigraphy, Geochemistry, and Mineralogy

The Paleocene of the Zumaia section is made up of a rhythmic alternation of (hemi)pelagic limestones and marlstones. The MPBE repre-sents a marked break in the general increasing trend of carbonate content recorded throughout

the Selandian–middle Thanetian (NP5–NP8), and defi nes a prominent entrant within the cliff face of the outcrop (Fig. 1).

The sedimentological analyses confi rm the continuous nature of the MPBE succession at Zumaia. Throughout the studied interval lime-stone marlstone bed transitions are always grad-ual, and no abrupt lithological changes, distinct mineral concentration levels, or evidence for hardgrounds have been recorded (Figs. 1 and 2). The presence of turbidites does not involve ero-sion, because they occur as thin beds (<5 cm) with T

c-e Bouma sequences that do not show any

evidence of channeling. Moreover, trace fos-sils throughout the interval, mainly Zoophycus and Chondrites, show no evidence of trunca-tion. The precise bed-by-bed correlation of the studied interval between the Zumaia and Ibaeta sections (Dinarès-Turell et al., 2006), separated from each other by ~15 km, supports the lack of any distinct hiatuses.

According to the carbonate content and MS values, the 4.3-m-thick succession containing the MPBE can be subdivided into 4 intervals (Figs. 1 and 2). The average carbonate content in interval 1 is ~50%, in interval 2 is ~40%, and in interval 3 (also called the clay-rich layer) decreases to <30%. From the top of interval 3 and across interval 4 the average carbonate con-tent is >40%, increasing gradually upsection

(Fig. 2). Magnetic susceptibility is negatively correlated with carbonate content and reaches the highest values (6.58) in interval 3 (Fig. 2).

The δ13C record shows a relatively stable pattern in intervals 1, 2, and 4; most values are in the range 2.2‰–2.5‰ (Fig. 2). These val-ues are very similar to those obtained from the same stratigraphic interval by Schmitz et al. (1997). At the base of the clay-rich layer (inter-val 3), δ13C shows a negative shift of ~1‰. Val-ues of ~2.2‰ δ13C drop to ~1.2‰ in sample Ev-34 (Fig. 2). Throughout interval 3 and the lowermost part of interval 4, we recorded the progressive recovery of the δ13C to more posi-tive values, exceeding 2‰ in the lowermost part of interval 4.

The MPBE succession consists of gray indu-rated marlstones and marly limestones contain-ing 25%–40% clay minerals, as well as traces of quartz, dolomite, plagioclase, and Fe oxides and sulfi des. In interval 3, however, clays become dominant, reaching values of 55% in sample Ev-34. The clay mineral associations in the <2 µm fraction consist of smectite (46%–60%), illite (35%–51%), and minor chlorite (2%–10%): these values are almost constant in all samples (Table DR1). The complete absence of kaolin-ite is remarkable, and in contrast to what it is observed in Walvis Ridge and Shatsky Rise, no trace of phillipsite was found.

Percentile 95

Figure 4. Relative abundance of selected planktic foramini-fer taxa, planktic and benthic foraminifera total abundance, and planktic/benthic (P/B) ratio across the study interval. Shaded area corresponds to the mid-Paleocene biotic even (MPBE). Circles correspond to samples cited in the text. The table shows test size in microns of the main planktic foraminiferal groups outside and within the MPBE. The 95-percentile represents the diam-eter separating the 5% largest specimens of each planktic foraminiferal group from the rest of the specimens of the group. Asterisk represents the percentage reduction of each planktic foraminiferal group within the MPBE.

Bernaola et al.


Calcareous Nannofossil Assemblages

According to the preservation criteria pro-posed by Roth and Thierstein (1972), all the studied samples yield moderately to well-pre-served calcareous nannofossil assemblages and no marked preservation changes were observed throughout the succession. The high species richness (an average of 27 species per sample) and total abundance of calcareous nannofos-sils are remarkably regular across the studied interval. Reworked Cretaceous nannofossils occur in all samples but never reach 3% of the total assemblage. The calcareous nannofos-sil assemblages are dominated by T. pertusus and C. pelagicus, with common to abundant Prinsius bisulcus, Sphenolithus primus, and Toweius eminens, and with less common Prin-sius martinii and Biscutum spp. Relative abun-dances of selected taxa are shown in Figure 3.

Assemblage counts indicate signifi cant changes in nannoplankton populations during the MPBE, particularly across interval 2 and the clay-rich layer (Fig. 3). At interval 2 there is a gradual change in the relatively stable cal-careous nannofossil assemblages of interval 1. There is a decrease in the relative abundance of T. pertusus and Prinsius, and a slight increase in that of C. pelagicus, Ericsonia, Sphenoli-thus, and T. tovae (Fig. 3). The relative abun-dance of Fasciculithus increased in the lower-most part of interval 2, but shortly afterward began to decrease.

T. pertusus, Prinsius, and Fasciculithus reach their minimum relative abundance values (<20%, 10%, and 1%, respectively) in the clay-rich layer, in connection with the δ13C nega-tive excursion. In this interval, C. pelagicus, Ericsonia, Heliolithus, and T. tovae increase in relative abundance, doubling the values in the underlying interval. The relative abundance of Sphenolithus is high, and it does not show any signifi cant changes.

In the overlaying interval 4, where δ13C returns to pre-excursion values, the calcareous nannofossil assemblages again show a distinct change. The relative abundance levels of T. pertusus, Prinsius, and Fasciculithus increase, whereas those of C. pelagicus, Ericsonia, Heliolithus, Sphenolithus, and T. tovae abruptly decrease, recording relative abundance values similar to those of interval 1.

Planktic Foraminifera

The assemblage preservation of planktic for-aminifera is moderate to good, and no signifi -cant changes have been observed throughout the succession. Fragmentation and the preser-vation of delicate structural features have been

used to determine the presence of dissolution in the samples. The fragmentation index is low throughout the section and does not vary mark-edly. These data, together with the preservation of fragile structures of delicate taxa in all sam-ples except for Ev-34, suggest that the assem-blages do not have signifi cant postdepositional or taphonomic bias.

The quantitative analysis indicates that planktic foraminiferal assemblages underwent important changes during the MPBE (Fig. 4). Assemblages from interval 1 are characterized by relatively high total abundance and diversity values, with ~600–1000 specimens/g and 16–19 species/sample (Fig. 4). The assemblages con-sist of abundant Subbotina (35%), especially S. linaperta and S. triangularis, and common Acarinina (20%) and Igorina (20%), particularly A. subsphaerica and I. pusilla. In coincidence with the slight carbonate decrease recorded from interval 1 to interval 2, planktic forami-niferal assemblages show a signifi cant change. Species richness decreases from 16–19 to 10–11 species/sample and the absolute abundance declines from ~1000 to ~250 specimens/g. As benthic foraminifer absolute abundance remains rather stable across intervals 1 and 2, the P/B ratio drops from ~87 to 50. In interval 2, Acarin-ina, Morozovella, and Globanomalina decrease in species richness and relative abundances, whereas Igorina shows a distinct increase in its relative abundance, reaching a maximum of 46% in sample Ev-30 (Fig. 4).

We observed a second distinct fall in absolute abundance and diversity of planktic foraminif-eral assemblages at the level with the clay-rich layer and the δ13C negative shift. In this inter-val, the species richness is 5–6 species/sample, the absolute abundance does not exceed 25 specimens/g, and the P/B ratios remain very low (<10). Globanomalina is absent and only one species of each of the genera Morozovella, Acarinina, and Igorina is present, the latter showing a decrease in relative abundance from ~30% to <10%. Subbotina is the less affected planktic foraminiferal genus at the clay-rich layer, and as a result, it is the only one that records an increase in relative abundance (Fig. 4). The test size of planktic foraminif-eral taxa decreases signifi cantly in intervals 2 and 3, affecting all the planktic foraminiferal groups, thus suggesting that it cannot be due to the assemblage change registered in these intervals (Fig. 4).

Above the clay-rich layer, at the beginning of interval 4, planktic foraminiferal species richness, total abundance, and the P/B ratio increase abruptly, reaching pre-event values (Fig. 4). The relative abundance of Subbotina abruptly decreased, whereas the percentages of

Acarinina, Morozovella, Globanomalina, and Igorina increased signifi cantly.

Benthic Foraminifera

Benthic foraminiferal assemblages from the studied section contain 50%–60% of calcareous foraminifera (e.g., laevidentalinids, Stensioeina beccariiformis, Anomalinoides spp., Cibicidoi-des pseudoperlucidus, Gyroidinoides beisseli, Pullenia spp.). Several groups of trochamminids dominate among agglutinated taxa throughout the studied section. The high diversity (Fisher-α index ~22) and heterogeneity [H(S) index ~3.6–4]) of the assemblages throughout the section, together with the presence of abundant species typical from bathyal settings (e.g., Van Mork-hoven et al., 1986; Alegret et al., 2003), such as Bulimina trinitatensis, Cibicidoides hyphalus, Gyroidinoides globosus, Nuttallides truempyi, Nuttallinella fl orealis, Osangularia velascoen-sis, and Stensioeina beccariiformis, indicate a middle bathyal (~1000 m) depth of deposition.

Calcareous assemblages from interval 1 are dominated by laevidentalinids and buliminids, whereas the percentages of Recurvoides fl uc-tuate among agglutinated groups, which are dominated by trochamminids throughout the section. The percentages of laevidentalinids, trochamminids, and Recurvoides strongly fl uctuate in interval 2, where the relative abun-dance of some agglutinated taxa (e.g., Haplo-phragmoides, astrorhizids) increases and the percentages of buliminids gradually decrease. An increase in the absolute abundance of ben-thic foraminifera has been observed toward the top of this interval.

Major changes in composition of the benthic foraminiferal assemblages have been recorded in coincidence with the clay-rich layer and the δ13C isotopic shift (Fig. 5). Heterogeneity of the assemblages, although still high, reached a minimum (3.59), whereas the percentage of the agglutinated taxa, especially Haplophrag-moides spp., Karrerulina spp., and Recurvoides spp., reached a maximum, making up 55.6% of the assemblages. Among the calcareous taxa, Nuttallides truempyi and Stensioeina beccari-iformis increased in relative abundance. In con-trast, the percentage of agglutinated suspension feeders (e.g., Bathysiphon, Hyperammina, Rhi-zammina) and calcareous infaunal groups such as laevidentalinids and buliminids decreased at the clay-rich layer (Fig. 5). Absolute abundance of benthic foraminifera is at a maximum in the lower part of interval 3, and it drops signifi cantly toward the top of this interval.

Assemblages from interval 4 are similar to those from interval 1, being dominated again by trochamminids and laevidentalinids.



Siliceous Microfossils

The occurrence of radiolaria in the Upper Cretaceous–lower Paleogene succession of Zumaia is very rare, and until now has not been documented in this section. However, during the study of the fi nest fractions (63 μm) of samples from the uppermost part of the clay-rich layer (close to the recovery of δ13C to pre-event values), relatively abundant radiolarian assemblages were identifi ed (Fig. 3).

DISCUSSION

Important changes in calcareous plank-ton and benthic foraminiferal assemblages have been recorded across the distinct clay-rich interval studied at the Zumaia section. According to its stratigraphic position, ~4 m above the base of the Zone NP6 and ~2 m below the C26r-C26n reversal, the 30-cm-thick clay rich interval defi ning the core of the MPBE at Zumaia is undoubtedly equivalent to the red 5–30-cm-thick clay-rich layer of the MPBE found in ODP sites at Shasky Rise and Walvis Ridge.

At Zumaia, however, the lithological and biological changes that characterize the MPBE started earlier, in interval 2. The presence of this ~52-cm-thick interval is remarkable because it records some gradual biological and lithological changes preceding the more abrupt biotic turnover and the negative 1‰ isotopic excursion at the clay-rich layer (interval 3). No equivalent intervals have been described below

the MPBE clay-rich layer at Walvis Ridge and Shatsky Rise.

The MPBE at Zumaia is characterized by increased MS and a 30% decrease in the carbon-ate content that creates a marked break in the gen-eral carbonate-increasing trend identifi ed across the Selandian–Thanetian interval (Fig. 2) (Baceta et al., 2006). This drop in carbonate content does not seem to be associated with an increase in the siliciclastic supply, as no signifi cant changes in the number of turbidite intercalations have been observed (Fig. 2). The carbonate decrease is not related to any signifi cant changes in the clay mineral associations, which are dominated by smectite and illite and remain almost constant throughout the event (Table DR1). The absence of volcanic activity in the Pyrenean domain dur-ing the Paleocene precludes the volcanogenic ori-gin of smectite. This clay mineral suite is mainly interpreted as the result of inheritance from a source area; the smectite from soils developed under a warm seasonal climate, and the illite and chlorite as a result of moderate mechanical ero-sion of topographic highs (Chamley, 1998).

In Zumaia, however, the drop in carbonate content coincides with a negative δ13C shift. Until now, no high-resolution isotope studies had been carried out at the MPBE at Walvis Ridge and Shasky Rise, and therefore this iso-topic anomaly has not been detected. However, similar or even greater negative carbon isoto-pic anomalies have been observed in connec-tion with a carbonate content decrease in other hyperthermal events such as the Elmo (Cramer et al., 2003; Koch et al., 2003; Lourens et al.,

2005) and especially the PETM (e.g., Bralower et al., 1995; Schmitz et al., 1997). The δ13C shift at the PETM at Zumaia and elsewhere has been interpreted as an input of large mass of isotopically depleted carbon into the ocean and atmosphere (Schmitz et al.; 1997, Zachos et al., 2005). Oceanic dissolution of this CO

2 would

lower the pH and carbonate ion content [CO32–]

of seawater, and this would cause the rise in the level of the lysocline and calcite compensation depth (CCD), resulting in enhanced dissolution of the carbonates at the seafl oor.

The simultaneous decrease in carbonate content and δ13C at the MPBE in the Zumaia section may be related to a succession of pro-cesses similar to those recorded at the PETM, but of a lower magnitude. Therefore, the marked decrease in carbonate associated to the MPBE at Zumaia could be attributed to partial dissolution taking place above the CCD, but below a raised lysocline.

Although no isotopic anomalies were reported from Shasky Rise and Walvis Ridge, the carbon-ate decreases recognized in the MPBE have been attributed to dissolution due to the shoaling of the CCD, as in the PETM (Zachos et al., 2004; Petrizzo, 2005). In the MPBE of Zumaia, dis-solution is much less intense than that recorded at Shatsky Rise and Walvis Ridge, where peak dissolution resulted in poorly preserved plank-tic foraminifera (Petrizzo, 2005). This is prob-ably due to a shallower depositional depth of the Zumaia section (~1000 m) compared to ODP Legs 198 and 208 sediments, which were deposited in water depths of 2500–3900 m and

Figure 5. Relative abundance of selected benthic foraminifer taxa, and agglutinated/calcareous ratio in the study interval. Shaded area cor-responds to the mid-Paleocene biotic event.

Bernaola et al.


1500–3600 m, respectively (Bralower et al., 2002; Zachos et al., 2005). Dissolution in the MPBE at Zumaia is also less intense than that recorded across the PETM in the same section, where the core of the dissolution results in a 1.7-m-thick carbonate-depleted interval (Schmitz et al., 1997).

Photic Zone

The calcareous nannofossil assemblage data can be used to provide information on tempera-ture, productivity, and the thermal structure of the upper water column during the MPBE. Most of the Paleocene and Eocene taxa have no recent representatives, so their paleoecologic affi nities are deduced from paleobiogeographic studies of Paleogene nannofossils (Haq and Lohmann, 1976; Haq et al., 1977; Wei and Wise, 1990; Aubry, 1992, 1998; Erba et al., 1992; Bralower, 2002). However, ecological preferences of indi-vidual taxa can change with time; therefore the paleoecologic interpretations based on analo-gies with recent counterparts should be used with caution. Coccolithus pelagicus, which is a major constituent of middle- and low-latitude assemblages during the Paleocene (Haq and Lohmann, 1976; Wei and Wise, 1990), is today a common component only in high-latitude assemblages (McIntyre and Bé, 1967; Okada and McIntyre, 1977).

In this study we have adopted the paleoecologic affi nities of the most important taxa of the MPBE as summarized by Bralower (2002), sub dividing the assemblages into taxa adapted to warmer waters and increased oligotrophy (Ericsonia, Fasciculithus, Heliolithus, Sphenolitus) and taxa adapted to mesotrophic to eutrophic conditions and cooler waters (Prinsius bisulcus, Prinsius martini, Toweius). The paleoecologic affi nities of T. tovae are not well known. In the studied inter-val this taxon shares the same variations in rela-tive abundance as the K-selected taxa Ericsonia, C. pelagicus, and H. kleinpellii, and therefore it has been interpreted as a taxon adapted to warm waters and increased oligotrophy.

During the MPBE at Zumaia, the calcareous nannofossil assemblages did not undergo any signifi cant changes in the species richness or total abundance. The only response of the calcar-eous nannoplankton to the event was a change in populations, indicating that the MPBE was not catastrophic for this group.

During the deposition of interval 2 and the clay-rich layer, R-mode specialists such as T. pertusus and P. bisulcus decreased in abundance and were replaced by warmer, more oligotrophic taxa such as Ericsonia, Sphenolithus, H. klein-pellii, and C. pelagicus, which are thought to be K-mode specialists (Haq and Lohmann, 1976;

Bralower, 2002). This change suggests a shift from a relatively cooler, more productive water column to a warmer, more stable, and stratifi ed water column characterized by increased oli-gotrophy during the MPBE. The faunal change is not major and probably refl ects a shift from warm mesotrophic to warmer oligotrophic con-ditions. Surface-water warming and probable increased oligotrophy in surface waters have also been inferred from calcareous nannofos-sils across the MPBE at Shatsky Rise (Bralower et al., 2003), as well as in other hyperthermal events, such as the PETM, in several sections (Kelly et al., 1996; Bralower, 2002).

The relative abundance of Fasciculithus increased in the lowermost part of interval 2, although it shows variations in relative abun-dance similar to T. pertusus and Prinsius higher in the section. The genus Fasciculithus has dis-tinctly different nutrient preferences from the later two R-selected taxa, and consequently its abundance distribution was probably infl uenced by other taphonomical or ecological factors. A similar decrease in the relative abundance of Fasciculithus has been recorded across the PETM of several low-latitude sections (Mone-chi et al., 2000; Agnini et al., 2005), where few species of this genus survived into the Eocene and became extinct shortly thereafter.

A two-stepped pattern of change of calcare-ous nannofossil assemblages has been observed across the MPBE. In the fi rst stage, during the deposition of interval 2, the replacement of R-selected taxa by K-selected taxa is gradual and not very distinct. The calcareous nannofossil turnover became more abrupt in the clay-rich layer (interval 3), where R-selected species record minimum values and the K-selected spe-cies peak in abundance. This suggests that sur-face water conditions gradually became slightly more oligotrophic and warmer during interval 2, and only became markedly more oligotrophic and warmer in interval 3.

Planktic foraminiferal assemblages were more severely affected by the MPBE than the calcare-ous nannofossils. In contrast to what we have observed in calcareous nannofossils, the total abundance of planktic foraminifers decreased signifi cantly in interval 2, and dropped abruptly in the clay-rich layer, where they were almost absent. This drop in abundance of planktic fora-minifera fi rst affected the surface-dweller acari-ninids and morozovellids (interval 2), and later the intermediate and deeper dwelling igorinids and subbotinids (clay-rich interval). Subboti-nids were the less affected planktic foraminif-eral group during the MPBE, and although their total abundance decreased in the clay-rich layer, their relative abundance increased, exceed-ing 80% of the total assemblage. In terms of

ecological strategies, K-mode specialists (aca-rininids and morozovellids) disappeared fi rst, whereas R-mode strategists were less affected by the MPBE, the expected norm for planktic foraminifers under stress conditions.

Changes in the planktic foraminiferal assem-blages are consistent with a top-down disrup-tion of planktic marine biota during the MPBE, possibly triggered by a warming of the ocean. Abundance and size maxima of many planktic foraminiferal taxa tend to occur at specifi c tem-peratures (Kennett, 1976; Hecht, 1976; Schmidt et al., 2004). The low total abundance and size minima observed during the MPBE (intervals 2 and 3) suggest that these assemblages consist of species living outside their ecological optima (e.g., higher temperatures). Today, some spe-cies of planktic foraminifers that are known to be deep-dwelling species, such as Globorotalia infl ata and G. truncatulinoides, show several abundance optima with respect to global tem-perature variability (Prell et al., 1999). However, their increase in relative abundance seems to be related to the absence of other species (Schmidt et al., 2004). We suggest that the Paleocene subbotinids were deep-dwelling, persistent spe-cies dominant in zones of major environmental perturbation, as are the recent G. infl ata and G. truncatulinoides.

Seafl oor

The highly diverse and heterogeneous ben-thic foraminiferal assemblages from the studied section indicate well-oxygenated bottom waters and mesotrophic conditions at the seafl oor. Never theless, the composition of the assem-blages suggests different environmental condi-tions during the deposition of the clay-rich layer. The signifi cant decrease in calcium carbonate content can be interpreted in terms of a shal-lowing trend in the lysocline. We suggest that, under such conditions, agglutinated (e.g., Hap-lophragmoides, Karrerulina) rather than calcar-eous (e.g., laevidentalinids, buliminids) species dominated the infaunal niche.

The drop in relative abundance of laeviden-talinids and buliminids, and the increase in the percentage of deep-water species such as Sten-sioeina beccariiformis and Nuttallides truempyi, suggest oligotrophic conditions at the seafl oor (Nomura, 1995; Jorissen et al., 1995; Fontanier et al., 2002).

Haplophragmoides, Karrerulina, and Recur-voides are agglutinated benthic foraminifera that show quantitative peaks in the clay-rich layer. These taxa have been reported to be opportun-ists that tolerate low oxygen conditions as well as changes in the food supply (see Alegret et al., 2005). We found no evidence for oxygen



defi ciency at the seafl oor, and the calcareous spe-cies that are common during interval 3 belong to the “oxic” group identifi ed by Kaiho (1994) (see species assignments to Kaiho’s groups in Alegret et al., 2003). Because no evidence for low oxy-gen conditions has been found, we suggest that Haplophragmoides, Karrerulina, and Recur-voides peaks may be related to changes in the trophic conditions at the seafl oor. Opportunistic species of Haplophragmoides and Karrerulina also peak in relative abundance at the PETM in other bathyal sections from the Basque Basin (Ortiz, 1995; Orue-Etxebarria et al., 1996), and abundant Karrerulina have been observed in recent abyssal plains with a very low organic carbon fl ux (Kaminski and Gradstein, 2005). Increased percentages of Recurvoides have also been observed directly above the K-Pg boundary clay layer at the nearby Sopelana section, and related to the oligotrophic Strangelove Ocean (Kuhnt and Kaminski, 1993). We also observed a decrease in the percentage of epifaunal sus-pension feeders, as expected under oligotrophic conditions (Jorissen et al., 1995).

The benthic foraminiferal turnover observed in Zumaia at the MPBE is similar to that reported from other early Eocene hyperthermal events from bathyal and abyssal settings from the southeastern Atlantic ODP Leg 208 in Walvis Ridge, such as the PETM, Elmo, and X events (Lourens et al., 2005; Thomas, 2005; Röhl et al., 2005), where diversity of the assemblages and buliminids show a net decline, and the low-food species Nuttallides truempyi and opportunistic taxa such as abyssaminids increase in abun-dance. These faunal changes indicate disturbed environments during the hyperthermal episodes due to the warming of the ocean, which possi-bly affected metabolic rates of deep-sea faunas (Thomas, 2005).

In addition, the benthic foraminiferal turn-over may be linked to the planktic turnover. Both surface productivity and the transfer of organic matter from surface water to the seafl oor may have been affected by the reorganization of the planktic ecosystem, which involved changes in primary producers (calcareous nannofos-sils; Fig. 3) and a decrease in the abundance of planktic foraminifera (zooplankton; Fig. 4). In the present-day oceans, the food supply is the major limiting factor for deep-sea benthic foraminifera (e.g., Gooday, 2003). The reor-ganization of the planktic ecosystem possibly involved changes in the food fl ux (type, quan-tity) to the seafl oor, thus triggering changes in the benthic communities. This hypothesis is also supported by the abundance of opportu-nistic benthic foraminiferal taxa, which may have taken advantage of a high-stress, probably strongly fl uctuating, environment.

Recovery

The MPBE was short lived. Planktic and benthic assemblages returned to pre-event con-ditions in connection with the recovery of δ13C to more positive values. The faunal recovery was relatively fast, and at the beginning it was marked by the common occurrence of radiolar-ians, which are absent or nearly absent through-out the Paleocene succession at Zumaia. The presence of radiolaria following the MPBE could refl ect upwelling conditions. Increased surface ocean productivity with a strengthened biological pump as well as increased weathering and runoff of silicate rocks have been proposed as a means of lowering atmospheric CO

2 levels

during the PETM (Bains et al., 2000; Kelly et al., 2005). According to Zachos et al. (2005), increased weathering of silicate rocks, besides taking up one unit of CO

2 for each unit of sili-

cate weathered, would also contribute with one unit of SiO

2, thus facilitating the origination

and/or preservation of radiolarian skeletons and playing an important role in carbon sequestra-tion and CO

2 drawdown.

Timing

The uppermost Cretaceous–lower Eocene (hemi)pelagic deposits at Zumaia are defi ned by a distinct cyclic (or rhythmic) bedding pat-tern, which in some intervals is persistent and identifi able bed by bed, even between sections 100 km apart (Pujalte et al., 1998). The basic noticeable cycles are the bedding couplets, from 15 to 66 cm thick, which are expressed as alter-nating pairs of marl and limestone beds. These limestone-marl couplets can be grouped into bundles containing 4–6 couplets, which usu-ally range from 0.5 to 1.5 m in thickness. The precession and short eccentricity origin of the couplets and bundles, respectively, was roughly confi rmed when the duration of magnetozones and biostratigraphic intervals retrieved from the upper Maastrichtian, Danian–lower Selandian, and Paleocene-Eocene transition in the Zumaia section were compared to standard time scales (Ten Kate and Sprenger, 1993; Dinarès-Turell et al., 2002, 2003).

In the absence of accurate radiometric ages, the measurement of orbital-climatic cycles within the Milankovitch band is a practical method to determine the approximate duration of discrete intervals such as some individual early Paleogene magnetochrons and the distinct PETM event (Norris and Röhl, 1999; Cramer, 2001; Röhl et al., 2000, 2001, 2003; Dinarès-Turell et al., 2002, 2003). The “fl oating” astronomical time scales constructed for such discrete intervals are based on the number of

cycles with short periodicities recorded within the target interval (mainly precession and obliq-uity cycles), multiplied by the estimated period of the corresponding astronomical cycle at that time. According to Herbert et al. (1995), it is reasonable to assume the modern mean duration of 21 and 41 k.y., respectively, for the preces-sion and obliquity cycles identifi ed in series not older than Late Cretaceous, but always bear-ing in mind the cumulative errors derived from the secular variation of the orbital parameters (Berger et al., 1992).

The biostratigraphic and magnetostrati-graphic data from Shatsky Rise and Walvis Ridge revealed that the MPBE is located above the NP5-NP6 zone boundary and below the C26r-C26n magnetic reversal (Bralower et al., 2002; Zachos et al., 2004; Petrizzo, 2005). This intermediate position is clearly confi rmed in the Zumaia section, where the stratigraphic inter-val between the NP5-NP6 boundary and the C26r-C26n reversal is 4–4.5 times thicker than in Shatsky Rise and Walvis Ridge. In addition, precession-related bedding couplets are particu-larly well recorded across the studied interval, allowing a detailed cycle counting and, fol-lowing the above-mentioned procedure, some approximate estimation of its position rela-tive to reference datum and its duration. In the 7.4-m-thick stratigraphic interval between the NP5-NP6 zone boundary and the C26r-C26n magnetic reversal, 21 bedding couplets (pre-cession cycles) can be recognized. Assuming a mean period of 21 k.y., these cycles would rep-resent an interval of ~440 k.y., consistent with the 489 k.y. established in Berggren et al. (1995) for the same interval. The clay-rich layer (inter-val 3), which marks the core of the MPBE, is located 8 bedding couplets (precession cycles) below of the base of chron C26n and 13 bed-ding couplets (precession cycles) above the fi rst occurrence of H. kleinpellii (Zone NP6 marker). Cycle counting reveals that interval 2 comprises 2 bedding couplets, whereas the core only rep-resents half a cycle (Fig. 2). Thus, it is possible to tentatively conclude that the duration of the MPBE in the Zumaia section was ~52–53 k.y., with the core of the event (clay-rich layer, inter-val 3) representing ~10–11 k.y.

CONCLUSIONS

The quantitative analyses of calcareous plankton and benthic foraminifera across the MPBE at the Zumaia section have evidenced abrupt environmental disruptions in both the photic zone and the seafl oor. Changes in cal-careous nannoplankton assemblages mainly consist of a replacement of R-mode specialists such as T. pertusus and P. bisulcus by warmer,

Bernaola et al.


more oligotrophic taxa such as Ericsonia, Sphenolithus, H. kleinpellii, and C. pelagicus (which are thought to be K-mode special-ists), suggesting a shift from relatively cooler mesotrophic to warmer oligotrophic conditions during the event. Planktic foraminiferal assem-blages were more severely affected during the MPBE, as is evidenced by a sharp drop in their total abundance and species richness. In addi-tion, test-size minima would indicate a clear disruption of the photic zone, with most plank-tic foraminiferal species living outside their ecological optima. Benthic foraminifers were also signifi cantly affected during the MPBE, as recorded by the drop in relative abundance of laevidentalinids and buliminids, and the increase of deep-water (e.g., S. beccariiformis and N. truempyi) and opportunistic species (Haplophragmoides, Karreru lina, and Recur-voides). These changes suggest oligotrophic and unstable conditions at the seafl oor during the MPBE, similar to those reported in early Eocene hyperthermal events from Walvis Ridge (Thomas, 2005). The reorganization of plank-tic ecosystems may have affected both surface productivity and the transfer of organic matter from the surface to the seafl oor, thus triggering changes in the benthic communities.

All these biotic changes at the photic zone and the seafl oor started at the base of interval 2 and reached a maximum within the clay-rich level defi ning interval 3 (core of the MPBE), indicating a gradual change in paleoenviron-mental conditions rather than a sudden event. In contrast to its initiation, the return to pre-event conditions was relatively fast.

The core of the biotic event coincides with a negative excursion of δ13C that, as in the PETM of this section and elsewhere, may be inter-preted as an input of a large mass of isotopically depleted carbon into the ocean and atmosphere. The CO

2 input lowered deep-sea pH, triggering

a rapid shoaling of the lysocline and contribut-ing to a greenhouse warming.

The MPBE was short lived: according to lime-stone-marl couplets (the stratigraphic expression of precession cycles throughout the Zumaia sec-tion), the event lasted for ~52–53 k.y., with the core of the event, characterized by the negative δ13C excursion, representing ~10–11 k.y. This cyclostratigraphic approach also allows pin-pointing the approximate position of the MPBE relative to the NP5-NP6 zone boundary (~13 precession cycles above) and the base of C26n (~8 precession cycles below).

The Zumaia section is the fi rst land-based locality with a well-preserved record of the MPBE and thus may prove to be a reference locality for the study of this and other short-lived events during the early Paleogene.

ACKNOWLEDGMENTS

We thank Brian Pratt and two anonymous reviewers for helpful suggestions that improved the manuscript. Bernaola acknowledges support from a post-doctoral grant from the Basque Country Government, and Baceta and Alegret acknowledge a “Ramon y Cajal” research grant from the Spanish Ministry of Science and Technology. This research is a contribution to Projects CGL2005–01721/BTE (Ministry of Science and Tech-nology, Spanish Government) and 9/UPV00121.310–1455/2002 (University of the Basque Country).

REFERENCES CITED

Agnini, C., Fornaciari, E., Giusberti, L., Backman, J., Capraro, L., Grandesso, P., Luciani, V., Muttoni, G., Rio, D., and Tateo, F., 2005, The early Paleogene of the Valbelluna. Early Cenozoic extreme climates (ODP Leg 208 Post Cruise Meeting): Field trip guidebook: Padova, Libraria University, 32 p.

Alegret, L., Molina, E., and Thomas, E., 2003, Benthic fora-miniferal turnover across the Cretaceous/Paleogene boundary at Agost (southeastern Spain): Paleoenviron-mental inferences: Marine Micropaleontology, v. 48, p. 251–279.

Alegret, L., Ortiz, S., Arenillas, I., and Molina, E., 2005, Paleoenvironmental turnover across the Palaeocene/Eocene boundary at the stratotype section in Dababiya (Egypt) based on benthic foraminifera: Terra Nova, v. 17, p. 526–536, doi: 10.1111/j.1365-3121.2005.00645.x.

Arenillas, I., and Molina, E., 1995, Análisis cuantitativo de los foraminíferos planctónicos del Paleoceno en Zumaya: implicaciones paleoambientales y eventos paleoceanográfi cos: Geogaceta, v. 17, p. 23–26.

Aubry, M.-P., 1992, Late Paleogene calcareous nanno-plankton evolution: A tale of climatic deterioration, in Prothero, D.R., and Berggren, W.A., eds., Eocene-Oli-gocene climatic and biotic evolution: Princeton, New Jersey, Princeton University Press, p. 272–309.

Aubry, M.-P., 1998, Early Paleogene calcareous nannoplank-ton evolution: A tale of climatic deterioration, in Aubry, M.-P., et al., eds., Late Paleocene–early Eocene climatic and biotic events in the marine and terrestrial record: New York, Columbia University Press, p. 158–203.

Baceta, J.I., 1996, El Maastrichtiense superior, Paleoceno e Ilerdiense inferior de la Región Vasco-Cantábrica: Secuencias Deposicionales, Facies y Evolución Paleo-geográfi ca [Ph.D. thesis]: Leioa, University of the Basque Country, 372 p.

Baceta, J.I., Pujalte, V., Dinarès-Turell, J., Payros, A., Orue-Etxebarria, X., and Bernaola, G., 2000, The Paleo-cene/Eocene boundary interval in the Zumaia Section (Gipuzkoa, Basque Basin): Magnetostratigraphy, and high-resolution lithostratigraphy: Revista de la Socie-dad Geológica de España, v. 13, no. 2, p. 375–391.

Baceta, J.I., Bernaola, G., and Arostegi, J., 2006, Lithostratig-raphy of the mid-Paleocene interval at Zumaia section, in Bernaola, G., et al., eds., The Paleocene and lower Eocene of the Zumaia section (Basque Basin): Post conference fi eld trip guidebook, Climate & Bioata of the Early Paleogene: Bilbao, Spain, p. 38–42.

Bains, S., Norris, R.D., Corfi eld, R.M., and Faul, K., 2000, Termination of global warmth at the Paleoecene/Eocene boundary through productivity feedback: Nature, v. 407, p. 171–174, doi: 10.1038/35025035.

Berger, A., Loutre, M.F., and Laskar, J., 1992, Stability of the astronomical frequencies over the earth’s history for paleoclimate studies: Science, v. 255, p. 560–566, doi: 10.1126/science.255.5044.560.

Berggren, W.A., Kent, D.V., Swisher, C.C., and Aubry, M.-P., 1995, A revised Cenozoic geochronology and chronostratigraphy, in Berggren, W.A., et al., eds., Geochronology, time scales and global stratigraphic correlation: SEPM (Society for Sedimentary Geology) Special Publication 54, p. 129–212.

Bernaola, G., 2002, Los nannofósiles calcáreos del Paleo-ceno en el dominio Pirenaico. Bioestratigrafía, crono-estratigrafía y paleoecología [Ph.D. thesis]: Leioa, University of the Basque Country, 445 p.

Bernaola, G., Baceta, J.I., Orue-Etxebarria, X., Alegret, L., Martín-Rubio, M., Arostegui, J., and Dinarès-Turell, J., 2006, The mid Paleocene biotic event, in Bernaola, G., et al., eds., The Paleocene and lower Eocene of the Zumaia section (Basque Basin): Post conference fi eld trip guidebook, Climate & Bioata of the Early Paleo-gene: Bilbao, Spain, p. 49–51.

Bown, P.R., 1998, Calcareous nannofossil biostratigraphy: British Micropalaeontological Society Publication Series: London, Chapman and Hall Ltd., 315 p.

Bralower, T.J., Zachos, J.C., Thomas, E., Parrow, M., Paull, C.K., Kelly, D.C., Premoli Silva, I., Sliter, W.V., and Lohmann, K.C., 1995, Late Paleocene to Eocene pale-oceanography of the equatorial Pacifi c Ocean: Stable isotopes recorded at Ocean Drilling Program Site 865: Allison Guyot: Paleoceanography, v. 10, p. 841–865, doi: 10.1029/95PA01143.

Bralower, T.J., 2002, Evidence of surface water oligotrophy during the Paleocene-Eocene thermal maximum: Nan-nofossil assemblage data from Ocean Drilling Program Site 690, Maud Rise: Weddell Sea: Paleoceanography, v. 17, p. 1023, doi: 10.1029/2001PA000662.

Bralower, T.J., Premoli Silva, I., Malone, M.J., and 24 oth-ers, 2002, Proceedings of the Ocean Drilling Program, Initial reports, Leg 198, http://www-odp.tamu.edu/publications/198_IR/198ir.htm.

Bralower, T.J., Roehl, U., Petrizzo, M.R., Zachos, J., Pre-moli-Silva, I., and Thomas, D., 2003, Constraining the controls on carbonate accumulation in deep sea sequences: A global dissolution event in the early late Paleocene: Geological Society of America Abstracts with Programs, v. 35, no. 6, p. 205.

Chamley, H., 1998, Clay mineral sedimentation in the Ocean, in Paquet, H., and Clauer, N., eds., Soil and sediments (mineralogy and geochemistry): Berlin, Springer- Verlag, p. 269–302.

Cramer, B.S., 2001, Latest Palaeocene–earliest Eocene cyclostratigraphy: Using core photographs for recon-naissance geophysical logging: Earth and Planetary Science Letters, v. 186, p. 231–244, doi: 10.1016/S0012-821X(01)00249-7.

Cramer, B.S., Wright, J.D., Kent, D.V., and Aubry, M.-P., 2003, Orbital climate forcing of δ13C excursions in the late Paleocene–early Eocene (chrons C24n–C25n): Paleoceanography, v. 18, doi: 10.1029/2003PA000909.

Dickens, G.R., O’Neil, J.R., Rea, D.K., and Owen, R.M., 1995, Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene: Paleoceanography, v. 10, p. 965–971, doi: 10.1029/95PA02087.

Dickens, G.R., Castillo, M.M., and Walker, J.G.C., 1997, A blast of gas in the latest Paleocene: Simulating fi rst-order effects of massive dissociation of oceanic methane hydrate: Geology, v. 25, p. 259–262, doi: 10.1130/0091-7613(1997)025<0259:ABOGIT>2.3.CO;2.

Dinarès-Turell, J., Baceta, J.I., Pujalte, V., Orue-Etxebarria, X., and Bernaola, G., 2002, Magnetostratigraphic and cyclostratigraphic calibration of a prospective Paleo-cene/Eocene stratotype at Zumaia (Basque Basin, northern Spain): Terra Nova, v. 14, p. 371–378, doi: 10.1046/j.1365-3121.2002.00431.x.

Dinarès-Turell, J., Baceta, J.I., Pujalte, V., Orue-Etxebarria, X., Bernaola, G., and Lorito, S., 2003, Untangling the Palaeocene climatic rhythm: An astronomically cali-brated early Palaeocene magnetostratigraphy and bio-stratigraphy at Zumaia (Basque basin, northern Spain): Earth and Planetary Science Letters, v. 216, p. 483–500, doi: 10.1016/S0012-821X(03)00557-0.

Dinarès-Turell, J., Baceta, J.I., and Pujalte, V., 2006, Mag-neto- and cyclostratigraphy of the mid-Paleocene inter-val, in Bernaola, G., et al., eds., The Paleocene and lower Eocene of the Zumaia section (Basque Basin): Post conference fi eld trip guidebook, ,Climate & Bio-ata of the Early Paleogene: Bilbao, Spain, p. 42–44.

Erba, E., Castradori, O., Guasti, G., and Ripepe, M., 1992, Cal-careous nannofossils and Milankovitch cycles: The exam-ple of Albian Gault Clay Formation (southern England): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 93, p. 47–69, doi: 10.1016/0031-0182(92)90183-6.

Fontanier, C., Jorissen, F.J., Licari, L., Alexandre, A., Anschutz, P., and Carbonel, P., 2002, Live benthic foraminiferal faunas from the Bay of Biscay: Faunal density, composition and microhabitats: Deep-Sea



Research. Part I, Oceanographic Research Papers, v. 49, p. 751–785, doi: 10.1016/S0967-0637(01)00078-4.

Gooday, A.J., 2003, Benthic foraminifera (Protista) as tools in deep-water palaeoceanography: Environmen-tal infl uences on faunal characteristics: Advances in Marine Biology, v. 46, p. 1–90.

Haq, B.U., and Lohmann, G.P., 1976, Early Cenozoic cal-careous nannoplankton biogeography of the Atlantic Ocean: Marine Micropaleontology, v. 1, p. 119–194, doi: 10.1016/0377-8398(76)90008-6.

Haq, B.U., Premoli-Silva, I., and Lohmann, G.P., 1977, Calcar-eous plankton paleobiogeographic evidence for major cli-matic fl uctuations in the early Cenozoic Atlantic Ocean: Journal of Geophysical Research, v. 82, p. 3861–3867.

Hecht, A.D., 1976, An ecologic model for test size varia-tion recent planktonic foraminifera: Application to the fossil record: Journal of Foraminifer Research, v. 6, p. 295–311.

Herbert, T.D., Premoli Silva, I., Erba, E., and Fischer, A.G., 1995, Orbital chronology of Cretaceous- Paleogene marine strata, in Berggren, W.A., et al., eds., Geochro-nology, time scales and global stratigraphic correla-tion: SEPM (Society for Sedimentary Geology) Spe-cial Publication 54, p. 81–93.

Howard, W.R., and Prell, W.L., 1994, Late Quaternary CaCO

3 production and preservation in the Southern

Ocean: Implications for oceanic and atmospheric car-bon cycling: Paleoceanography, v. 9, p. 453–482, doi: 10.1029/93PA03524.

Jorissen, F.J., De Stigter, H.C., and Widmark, J.G.V., 1995, A conceptual model explaining benthic foraminiferal microhabitats: Marine Micropaleontology, v. 26, p. 3–15, doi: 10.1016/0377-8398(95)00047-X.

Kaiho, K., 1994, Benthic foraminiferal dissolved oxygen index and dissolved oxygen levels in the modern ocean: Geology, v. 22, p. 719–722, doi: 10.1130/0091-7613(1994)022<0719:BFDOIA>2.3.CO; 2.

Kaminski, M.A., and Gradstein, F., 2005, Atlas of Paleogene cosmopolitan deep-water agglutinated foraminifera: Grzybowski Foundation Special Publication 10, 548 p.

Kelly, D.C., Bralower, T.J., Zachos, J.C., Premoli Silva, I., and Thomas, E., 1996, Rapid diversifi cation of plank-tonic foraminifera in the tropical Pacifi c (ODP Site 865) during the late Paleocene thermal maximum: Geology, v. 24, p. 423–426, doi: 10.1130/0091-7613(1996)024<0423:RDOPFI>2.3.CO;2.

Kelly, D.C., Zachos, J.C., Bralower, T.J., and Schellen-berg, S.A., 2005, Enhanced terrestrial weathering/runoff and surface ocean carbonate production dur-ing recovery stages of the Paleocene-Eocene thermal maximum: Paleoceanography, v. 20, p. PA4023, doi: 10.1029/2005PA001163.

Kennett, J.P., 1976, Phenotypic variation in some Recent and late Cenozoic planktonic foraminifera, in Hedley, R.H., and Adams, C.G., eds., Foraminifera: New York, Academic Press, p. 111–170.

Koch, P.L., Clyde, W.C., Hepple, R.P., Fogel, M.L., Wing, S.L., and Zachos, J.C., 2003, Carbon and oxygen iso-tope records from paleosols spanning the Paleocene-Eocene boundary, Bighorn Basin, Wyoming, in Wing, S.L., et al., eds., Causes and consequences of globally warm climates in the early Paleogene: Geological Society of America Special Paper 369, p. 49–64.

Kuhnt, W., and Kaminski, M.A., 1993, Changes in the community structure of deep water agglutinated fora-minifers across the K/T boundary in the Basque Basin (northern Spain): Revista Española de Micropaleon-tología, v. 25, p. 57–92.

Lourens, J.L., Sluijs, A., Kroon, D., Zachos, J.C., Thomas, E., Röhl, U., Bowles, J., and Raffi , I., 2005, Astronomi-cal pacing of late Paleocene to early Eocene global warming events: Nature, v. 435, p. 1083–1087, doi: 10.1038/nature03814.

Martini, E., 1971, Standard Tertiary and Quaternary calcare-ous nannoplankton zonation, in Farinacci, A., ed., Pro-ceedings of the Second Planktonic Conference, Roma, 1970: Rome, Edizioni Tecnoscienza, v. 2, p. 739–785.

McIntyre, A., and Bé, A.W.H., 1967, Modern coccolitho-phoridae of the Atlantic ocean—1. Placoliths and cyr-toliths: Deep-Sea Research. v. 14, p. 561–597.

Monechi, S., Angori, E., and von Salis, K., 2000, Calcare-ous nannofossil turnover around the Paleocene/Eocene transition at Alamedilla (southern Spain): Bulletin de la Société Géologique de France, v. 171, p. 477–489, doi: 10.2113/171.4.477.

Moore, M., and Reynolds, R.C., 1997, X-ray diffraction and the identifi cation and analysis of Clay Minerals: Oxford, Oxford University Press, 378 p.

Murray, J.W., 1991, Ecology and palaeoecology of benthic foraminifera: Amsterdam, Elsevier, 397 p.

Nomura, R., 1995, Paleogene to Neogene deep-sea pale-oceanography in the eastern Indian Ocean: Benthic for-aminifera from ODP Sites 747, 757 and 758: Micropa-leontology, v. 41, p. 251–290, doi: 10.2307/1485862.

Norris, R.D., and Röhl, U., 1999, Carbon cycling and chro-nology of climate warming during the Palaeocene/Eocene transition: Nature, v. 401, p. 775–778, doi: 10.1038/44545.

Okada, H., and McIntyre, A., 1977, Modern coccolithophores of the Pacifi c and North Atlantic Oceans: Micropaleon-tology, v. 23, p. 1–55, doi: 10.2307/1485309.

Ortiz, N., 1995, Differential patterns of benthic foraminiferal extinctions near the Paleocene/Eocene boundary in the North Atlantic and the western Tethys: Marine Micro-paleontology, v. 26, p. 341–359, doi: 10.1016/0377-8398(95)00039-9.

Orue-Etxebarria, X., Apellaniz, E., Baceta, J.I., Coccioni, R., di Leo, R., Dinarès-Turrell, J., Galeotti, S., Mone-chi, S., Núñez-Betelu, K., Pares, J.M., Payros, A., Pujalte, V., Samso, J.M., Serra-Kiel, J., Schmitz, B., and Tosquella, J., 1996, Physical and biostratigraphic analysis of two prospective Paleocene-Eocene bound-ary stratotypes in the intermediate-deep water Basque Basin, western Pyrenees: The Trabakua Pass and Ermua sections: Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, v. 201, p. 179–242.

Orue-Etxebarria, X., Bernaola, G., Baceta, J.I., Angori, E., Caballero, F., Monechi, S., Pujalte, V., Dinarès-Turell, J., Apellaniz, E., and Payros, A., 2004, New constraints on the evolution of planktic foraminifera and calcare-ous nannofossils across the Paleocene-Eocene bound-ary interval: The Zumaia section revisited: Neues Jah-rbuch für Geologie und Paläontologie Abhandlungen, v. 234, p. 223–259.

Orue-Etxebarria, X., Caballero, F., and Apellaniz, E., 2006, New planktic foraminifera biozonation from the mid-Paleocene, in Bernaola, G., et al., eds., The Paleocene and lower Eocene of the Zumaia section (Basque Basin): Post conference fi eld trip guidebook, Cli-mate & Bioata of the Early Paleogene: Bilbao, Spain, p. 46–47.

Petrizzo, M.R., 2005, An early late Paleocene event on Shatsky Rise, northwest Pacifi c Ocean (ODP Leg 198): Evidence from planktonic foraminiferal assemblages, in Bralower, T.J., et al., Proceedings of the Ocean Drilling Program, Scientifi c results, Volume 198: http://www-odp.tamu.edu/publications/198_SR/102/102.htm.

Prell, W., Martin, A., Cullen, J., and Trend, M., 1999, The Brown University Foraminiferal Data Base, GBP PAGES/World Data Center-A for Paleoclimatology Data Contribution Series 1999–027: Boulder, Colo-rado, National Oceanic and Atmospheric Administra-tion/National Geophysical Data Center Paleoclimatol-ogy Program [software].

Pujalte, V., Baceta, J.I., Orue-Etxebarria, X., and Payros, A., 1998, Paleocene strata of the Basque Country, western Pyrenees, northern Spain: Facies and sequence devel-opment in a deep-water starved basin, in Graciansky, P.C., et al., eds., Mesozoic and Cenozoic sequence stra-tigraphy of European basins: SEPM (Society for Sedi-mentary Geology) Special Publication 60, p. 311–325.

Röhl, U., Bralower, T.J., Norris, R.D., and Wefer, G., 2000, New chronology for the late Paleocene ther-mal maximum and its environmental implications:

Geology, v. 28, p. 927–930, doi: 10.1130/0091-7613(2000)28<927:NCFTLP>2.0.CO;2.

Röhl, U., Ogg, J.G., Geib, T.L., and Wefer, G., 2001, Astro-nomical calibration of the Danian time scale, in Nor-ris, R.D., et al., eds., Western North Atlantic Paleogene and Cretaceous paleoceanography: Geological Society [London] Special Publication 183, p. 163–183.

Röhl, U., Norris, R.D., and Ogg, J.G., 2003, Cyclostratigraphy of upper Paleocene and late Eocene sediments at Blake Nose Site 1051 (western North Atlantic), in Wing, S.L., et al., eds., Causes and consequences of globally warm climates in the early Paleogene: Geological Society of America Special Paper 369, p. 567–588.

Röhl, U., Westerhold, T., Monechi, S., Thomas, E., Zachos, J.C., and Donner, B., 2005, The third and fi nal Early Eocene thermal maximum: Characteristics, timing, and mechanisms of the “X” event: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 264.

Roth, P.H., and Thierstein, H., 1972, Calcareous nanno-plankton: Leg 14 of the Deep Sea Drilling Project, in Hayes, D.E., et al., Initial reports of the Deep Sea Drilling Project, Volume 14: Washington, D.C., U.S. Government Printing Offi ce, p. 421–485.

Schmidt, D.N., Renaud, S., Bollmann, J., Schiebel, R., and Thierstein, H.R., 2004, Size distribution of Holocene planktic foraminifer assemblages: Biogeography, ecol-ogy and adaptation: Marine Micropaleontology, v. 50, p. 319–338, doi: 10.1016/S0377-8398(03)00098-7.

Schmitz, B., Asaro, F., Molina, E., Monechi, S., Von Salis, K., and Speijer, R., 1997, High-resolution iridium, δ13C, δ18C, foraminifera and nannofossil profi les across the latest Paleocene benthic extinction event at Zumaya: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 133, p. 49–68, doi: 10.1016/S0031-0182(97)00024-2.

Ten Kate, W.G., and Sprenger, A., 1993, Orbital cyclicities above and below the Cretaceous/Paleogene boundary at Zumaya (N Spain), Agost and Relleu (SE Spain): Sedi-mentary Geology, v. 87, p. 69–101, doi: 10.1016/0037-0738(93)90037-6.

Thomas, E., 2005, Benthic foraminifera and early Eocene hyperthermal events (SE Atlantic ocean): Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 413.

Thomas, E., and Zachos, J.C., 2000, Was the late Paleo-cene thermal maximum a unique event?: GFF, v. 122, p. 169–170.

Thunell, R.C., 1976, Optimal indices of calcium carbon-ate dissolution in deep-sea sediments: Geology, v. 4, p. 525–528, doi: 10.1130/0091-7613(1976)4<525:OIOCCD>2.0.CO;2.

Van Morkhoven, F.P.C.M., Berggren, W.A., and Edwards, A.S., 1986, Cenozoic cosmopolitan deep-water benthic foraminifera: Bulletin de Centre de Recherches Explo-ration et Production Elf-Aquitaine, v. 11, 421 p.

Wei, W., and Wise, S.W., 1990, Biogeographic gradients of middle Eocene–Oligocene calcareous nannoplank-ton in the South Atlantic Ocean: Palaeogeography, Pa laeoclimatology, Palaeoecology, v. 79, p. 29–61, doi: 10.1016/0031-0182(90)90104-F.

Zachos, J.C., Kroon, D., and 25 others , 2004, Early Cenozoic extreme climates: The Walvis Ridge transect: Proceed-ings of the Ocean Drilling Program, Leg 208: http://www-odp.tamu.edu/publications/208-IR/208ir.htm.

Zachos, J.C., Röhl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi , I., Lourens, L.J., McCarren, H., and Kroon, D., 2005, Rapid acidifi cation of the ocean during the Paleocene-Eocene Thermal Maximum: Science, v. 308, p. 1611–1615, doi: 10.1126/science.1109004.

MANUSCRIPT RECEIVED 23 OCTOBER 2006REVISED MANUSCRIPT RECEIVED 5 MARCH 2007MANUSCRIPT ACCEPTED 23 MARCH 2007

Printed in the USA

Evidence of an abrupt environmental disruption during the Mid Paleocene Biotic Event (Zumaia...

Documents

Transcript of Evidence of an abrupt environmental disruption during the Mid Paleocene Biotic Event (Zumaia...