Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 128–137

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Productivity modes in the Mediterranean Sea duringDansgaard–Oeschger (20,000–70,000 yr ago) oscillations

Alessandro Incarbona a,⁎, Mario Sprovieri b, Agata Di Stefano c, Enrico Di Stefano a, Daniela Salvagio Manta d,Nicola Pelosi d, Maurizio Ribera d'Alcalà e, Rodolfo Sprovieri a, Patrizia Ziveri f,g

a Università degli Studi di Palermo, Dipartimento di Scienze della Terra e del Mare, Via Archirafi 22, 90134 Palermo, Italyb Consiglio Nazionale delle Ricerche, Istituto per l'Ambiente Marino Costiero U.O. Capo Granitola, Via del Mare 3, 91021 Torretta Granitola (Campobello di Mazara, Trapani), Italyc Università di Catania, Dipartimento di Scienze Geologiche, Corso Italia 55, 95129 Catania, Italyd Consiglio Nazionale delle Ricerche, Istituto per l'Ambiente Marino Costiero, Calata Porta di Massa, Interno Porto di Napoli, 80133 Naples, Italye Laboratorio di Oceanografia Biologica, Stazione Zoologica ‘A. Dohrn’, Villa Comunale, Naples, Italyf Universitat Autònoma de Barcelona (UAB), Institute of Environmental Science and Technology (ICTA), Edifici Cn Campus de la UAB, 08193 Bellaterra (Cerdanyola del Vallès), Barcelona, Spaing Vrjie Universiteit Amsterdam, Department of Earth Sciences, Faculty of Earth and Life Sciences, de Boelelaan 1085, 1081HV Amsterdam, The Netherlands

⁎ Corresponding author. Tel.: +39 09123864648; fax:E-mail address: alessandro.incarbona@unipa.it (A. Inc

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.palaeo.2013.09.023

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 May 2013Received in revised form 16 September 2013Accepted 19 September 2013Available online 27 September 2013

Keywords:PaleoproductivityDansgaard–OeschgerMediterraneanPlanktonic ForaminiferaCoccolithophores

The study of planktonic organisms during abrupt climatic variations of the last glacial period (Dansgaard–Oeschger oscillations, D–O)may reveal important insights on climatic, oceanographic and biological interactions.Here we present planktic foraminifera and coccolithophore data collected at the Ocean Drilling Program Site 963(Sicily Channel), with a mean sampling resolution of respectively 43.5 and 98.9 yr, over the interval between70,000 and 20,000 yr ago. The paleoenvironmental reconstruction suggests that three different scenarios canbe seen across each D–O cycle: 1. oligotrophic surface water and a deep thermocline for the early Interstadials;2. a Deep ChlorophyllMaximumand coccolithophorewinter/spring blooming in the late Interstadials; 3. reducedproductivity togetherwith the shallowing of the nutricline depth during Stadials andHeinrich events. The uniquemode of productivity dynamics is corroborated by comparing our paleoecological results with those publishedfrom high-resolution cores in the Alboran Sea clearly indicating reduced trophic levels during Stadials andHeinrich events. Finally, we argue that the density contrast between the Atlantic water inflow and subsurfacewater may have affected productivity dynamics in such a large area. The strong vertical density gradient mayhave hampered the vertical convection of thewater column, producing a negative effect on biological productivity,especially during Stadial phases.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

High-frequency climatic variations in the Northern Hemisphere, notdue to the orbital motion of the Earth, can retrospectively be seen in theGreenland ice cores of Camp Century (Dansgaard et al., 1969), even ifthe first evidence of this phenomenon came from the sedimentary re-cord of the northern North Atlantic, with the deposition of ice rafteddetritus levels (Heinrich events—He), happened every about 8 kiloyears(kyr) (Heinrich, 1988). A correlation among 23 interstadial (warm) and24 stadial (cold) fluctuations were found over the last glacial period(between about 20 and 70 kyr ago) byGISP andGRIP Greenland drillingprojects (Grootes et al., 1993). These suborbital climatic fluctuations,the so-called Dansgaard–Oeschger (D–O) oscillations, occurred everyabout 1.5 kyr and can be traced across the whole Northern Hemisphere(Bond et al., 1993; Voelker and SCOR-IMAGES workshop participants,2002; Rohling et al., 2003; Petersen et al., 2013). Recent studies

+39 0916166908.arbona).

ghts reserved.

demonstrated that suborbital-scale climatic fluctuations could even beidentified in the Mediterranean Sea. Stadial and interstadial eventsseem to be simultaneous to those in the Greenland ice cores and NorthAtlantic sediments, possibly by teleconnection phenomena. Stadials intheMediterranean Sea are characterized by low SST, saline water massesand enhanced thermohaline circulation (Cacho et al., 1999;Martrat et al.,2004; Frigola et al., 2008).

The late Quaternary sedimentary record of the Sicily Channel is char-acterized by very high sedimentation rates. Geochemical andmicropale-ontological studies led to the detection of suborbital climatic fluctuationsduring the last (Marine Isotopic Stage 5) and present (Holocene) inter-glacials, suggesting an evident interplay between oceanographic and at-mospheric processes (Sprovieri et al., 2003, 2006; Incarbona et al., 2008,2010a,b). Recently, thewhole sequence of D–O oscillations has been rec-ognized by variations of oxygen isotopic values in both planktic and ben-thic foraminifera shells, pointing to significant changes in the physical/chemical character (e.g. temperature and/or salinity) of both surfaceand bottomwatermasses (Sprovieri et al., 2012). The study also showedthe distribution pattern of some key species,Globigerinoides ruber amongplanktic foraminifera and Florisphaera profunda among coccolithophores,

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highlighting the rapid transformation of the marine ecosystem in re-sponse to climatic/oceanographic changes.

The work presented here focuses on the modification that occurredat a suborbital time-scale in the surface marine environment, with aspecial emphasis on productivity variations, during the last glacial,between 70 and 20 ka. We show data of planktic foraminifera andcoccolithophore assemblages (sampling resolution respectively of43.5 yr and 98.9 yr) from the same site (Ocean Drilling Program –

ODP – Site 963) and the same sedimentary material of Sprovieri et al.(2012). Finally,we compare Sicily Channel results to data fromdifferentMediterranean areas, in order to reveal whether there is a unified basinresponse of ecosystems to abrupt climate changes or a strong imprint oflocal oceanographic processes at the sub-basin scale.

2. Material and methods

The sedimentary sequence between 9.48 and 31.24 m below seafloor of the ODP Site 963 Hole A (37°02.148′N, 13°10.686′E, 480 mdepth), recovered in the Sicily Channel between the Adventure Bankand theGela basin (Fig. 1), has been selected. Thematerial is constituted

Fig. 1. A. Bathymetric map of the Mediterranean Sea, surface water (MAW) circulation in windiscussed in text are illustrated.White circles show: 1: Alboran Sea; 2: Balearic Sea; 3: Algero–BSea; 9: Cretan Sea; 10: Aegean Sea; 11: Levantine Sea; 12: Black Sea. Arrows show: AC:Mid-Mediterranean Jet. White dashed surfaces show the Western Mediterranean Deep Wate(D) formation areas. Modified from POEM group (1992), Millot (1999) and Pinardi and Mase1999, 2000; Pérez-Folgado et al., 2003; Colmenero-Hidalgo et al., 2004;Martrat et al., 2004); 2:Met al., 2012; presentwork). B. Bathymetricmapof the Sicily Channel, surfacewater (MAW) circulocation of the cyclone gyre discussed in the text (ABV— adventure Bank Vortex). Modified fro

of homogeneous olive to gray nannofossil clays with disseminated py-rite. No lithological evidence of sapropels, turbidites and resedimentedlevels occurs, but amillimetric quartz sand horizon exists in the interval56–60 cm of Section 5, core 3H (Emeis et al., 1996). It is a tephra layerwith a bimodal (pantellerite and trachyte) composition. The chemicalcomposition suggests a correlation of this layer with the Green Tuffexplosive eruption from the close Pantelleria Island (Tamburrinoet al., 2012).

Planktonic foraminifera analysis was carried out on 961 samples(every 2 cm on average). Samples were dried and washed on a 63 μmsieve. The relative abundance of each species was obtained from splitsof about 300 specimens, in the residue greater than 125 μm. Fifteen spe-cies or species groups were distinguished, following Bolli et al. (1985)and Hemleben et al. (1989) taxonomic concepts. G. ruber includesGlobigerinoides elongatus and Globigerina bulloides includes Globigerinafalconensis. The ecological preferences of selected planktonic foraminif-era species are shortly commented in Table 1.

Coccolithophore analysis was carried out on 440 samples (every5 cm on average), by a light polarized microscope with about 1250×magnification. Smear slides were prepared following the standard

ter and Mediterranean cores' location. Mediterranean seas and basins and major currentsalearic Basin; 4: Ligurian Sea; 5: Tyrrhenian Sea; 6: Sicily Channel; 7: Ionian Sea; 8: AdriaticAlgerian Current; AIS: Atlantic Ionian Stream; ATC: Atlantic Tunisian Current; MMJ:r (A), Eastern Mediterranean Deep Water (B and C) and Levantine Intermediate Watertti (2000). White squares represent: 1: MD95-2043 and ODP Site 977 cores (Cacho et al.,D01-2434 andMD01-2472 cores (Toucanne et al., 2012); 3: ODP Site 963 cores (Sprovieri

lation (black arrows) andODP Site 963 location (blue circle). The bluedashed line show them Robinson et al. (1999) and Lermusiaux and Robinson (2001).

Table 1Ecological preference of selected planktonic foraminifera species, from Hemleben et al.(1989), Pujol and Vergnaud-Grazzini (1995), Žarić et al. (2005) and Rigual-Hernándezet al. (2012).

Taxa Ecological preference

G. ruber Warm and oligotrophic surface waterN. pachyderma dx Deep Chlorophyll MaximumT. quinqueloba High fertility surface waterG. bulloides High fertility surface water

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procedure. The semi-quantitative analysis was performed on 300–500coccoliths of about 20 taxonomic units generally following the taxo-nomic concepts on living coccolithophores of Young et al. (2003).All the specimens of Gephyrocapsa spp. smaller than 3 μm weregrouped into small gephyrocapsids, whereas the taxonomic unit‘small placoliths’ includes Reticulofenestra spp. and probable verysmall specimens of Emiliania huxleyi and Gephyrocapsa spp. with slightdiagenetic problems. Finally, F. profunda includes rare specimens ofGladiolithus flabellatus.

In order to obtain a synthetic and immediate vision of coccolithophoreenvironmental indications, taxa were grouped into ‘placoliths’, ‘miscella-neous group’, ‘upper photic zone (UPZ) group’ and ‘lower photic zone(LPZ) group’ (Incarbona et al., 2010c). Placoliths include E. huxleyi,small placoliths, small Gephyrocapsa, Gephyrocapsa caribbeanica,Gephyrocapsa muellerae and Gephyrocapsa oceanica. Miscellaneousgroup includes Helicosphaera spp., Coccolithus pelagicus, Syracosphaerahistrica, Pontosphaera spp., Calcidiscus leptoporus, Pleurochrysis spp.,Oolithotus fragilis, Braarudosphaera spp. and specimens of all the otherspecies. UPZ group includes Syracosphaera pulchra, Umbellosphaeraspp., Discosphaera tubifera, Rhabdosphaera spp., Umbilicosphaera spp.,Calciosolenia spp., holococcoliths, Ceratolithus spp. and the dinoflagel-late Thoracosphaera heimii (Tangen et al., 1982). Finally, given the rarityof G. flabellatus and given that Algirosphaera robusta was not found inany sample, F. profunda is themain species of the LPZ group. The ecolog-ical preferences of coccolithophore groups and some selected taxa aresummarized in Table 2.

3. The Mediterranean Sea: Oceanography and climatology

3.1. Main water masses

The Mediterranean is an elongated and semi-enclosed sea, with ananti-estuarine circulation pattern forced by the negative hydrologicalbalance and the density gradient with the Atlantic Ocean (Robinsonand Golnaraghi, 1994). Surface waters, called Modified Atlantic Water(MAW), enter from the Atlantic Ocean (Fig. 1) and occupy the first100–200 m of the water column. They are characterized by a salinity

Table 2Ecological preference of selected coccolithophore taxa. The compilation is not exhaustiveespecially for those taxa with contrasting ecological reports (e.g. H. carteri). Numberrefer to bibliographic references: 1) Young, 1994; 2) Broerse et al., 2000; 3) Flores et al.,2000; 4) De Bernardi et al., 2005; 5) Molfino and McIntyre, 1990; 6) Beaufort et al.,1997; 7) Incarbona et al., 2008; 8) Grelaud et al., 2012; 9) Andruleit et al., 2003; 10)Boeckel and Baumann, 2004; 11) Okada and McIntyre, 1979; 12) Roth and Coulbourn,1982; 13) Ziveri et al., 1995; 14) Colmenero-Hidalgo et al., 2004; 15) Álvarez et al.,2005; 16) Jordan et al., 1996.

Taxa Ecological preference

Placoliths R-strategist taxa (1), high surface productivity (2–4)F. profunda Deep nutricline (5), low prodictivity levels (6–8)UPZ taxa K-strategist taxa (1), surface oligotrophy (9–10)H. carteri Abundant in tropical and subtropical waters (11–12), high

productivity levels (13), abundant in turbid and fresh waters(14) or in areas influenced by continental runoff (15)

Syracosphaera spp. Abundant inwarm, nutrient-poor and stratifiedwaters (12, 16),abundant in turbid and fresh waters (14) or in areas influencedby continental runoff (15)

of about 36.5‰, which due to evaporation and mixing reach the valueof over 39‰ in the easternmost part of the basin (POEM group, 1992;Millot, 1999). In the western Alboran Sea, MAW describes a quasi per-manent anticyclonic gyre and thus flows along the North African coastas theAlgerian Current (Millot, 1999). At the entrance of the Sicily Strait,MAW splits into two branches (Millot, 1987): two-thirds of these watermasses enter the Sicily Channel, the remainder flows into the TyrrhenianSea and follows the northern coast of Sicily (Bethoux, 1980). Into the Sic-ily Channel, MAW is again split into two streams, southeast of Pantelleria(Robinson et al., 1999; Béranger et al., 2004): the Atlantic Tunisian Cur-rent follows the 200 m isobath, reaching the African coast and flowingeastwards as a coastal current (Onken et al., 2003; Béranger et al.,2004). The northern branch, called the Atlantic Ionian Stream (AIS), con-tributes to the MAW transport into the eastern Mediterranean off thesouthern coast of Sicily. Three semipermanent mesoscale summer fea-tures are associatedwithAISmeanders,mainly in response to topograph-ical effects (Lermusiaux and Robinson, 2001; Béranger et al., 2004). Inparticular, ODP Site 963 is located within the cyclonic gyre of the Adven-ture Bank Vortex. Finally, AIS meanders into the Ionian Sea and feeds theMid-Mediterranean Jet that flows in the central Levantine basin up toCyprus.

Levantine Intermediate Water (LIW) forms in the eastern basin inFebruary–March as a process of surface cooling on water masseswhichundergo a severe salt enrichment, in a large area between Rhodesand Cyprus (POEM group, 1992) (Fig. 1). LIW occupies a depth between150–200 and 600 m and constitutes the main source of water that out-flows across the Gibraltar Strait (POEM group, 1992; Millot, 1999).Dense water forms in the northern part of the basin, in the Gulf ofLions (WesternMediterranean DeepWater) and in the Adriatic and Ae-gean Sea (EasternMediterranean DeepWater), and is preconditionatedby the presence of permanent cyclonic gyres (Fig. 1).

3.2. Nutrient dynamics

The trophic levels of the Mediterranean Sea are among the poorestin theworld's oceans. The anti-estuarine circulation pattern contributesto its maintenance, since, at the Strait of Gibraltar, surface waterscoming from the Atlantic Ocean are nutrient depleted, with respectto outflowing waters, mainly constituted by LIW (Bethoux, 1979;Sarmiento et al., 1988).

The main factor which controls the seasonal changes in pelagicprimary production is the dynamics of the water column. The winterconvection, and less frequent frontal zones or upwelling areas, bringnutrients into the photic zone (mesotrophic regime) (Klein and Coste,1984). An oligotrophic regime, characterized by a much lower level ofproduction, occurs in summer, when a stable stratification, due to thedeepening of the summer thermocline up to about 90 m, takes place(Klein and Coste, 1984; Krom et al., 1992).

The Mediterranean Sea shows a significant west-east trophic gra-dient, with a reduction in primary productivity in the eastern basinby a factor of 3, which follows the same increasing nutrient depletiontrend, particularly exacerbated for phosphorous (Kromet al., 1991, 2010).

3.3. Atmospheric pattern

Most of theMediterranean region is indirectly under the influence ofNorth Atlantic SST, one of the most important factors that drives thenorthern hemisphere atmospheric circulation pattern (Luterbacherand Xoplaki, 2003). Rainy westerlies provide moisture and lower thetemperature in winter, whereas the penetration of the Azorean high-pressure cell in summer causes a general drought. Even on a longertime-scale, the Mediterranean region is linked to the North Atlanticthrough the North Atlantic Oscillation (NAO) variability, defined asthe normalized winter difference in the sea-level pressure betweenthe Azorean high and the Icelandic low cells (Hurrell, 1995). Duringperiods of positive NAO-index, westerlies blow over the western parts

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of northern Europe, while dry conditions are experienced in southernEurope and northern Africa. The situation is reversed during negativeNAO-index periods.

The easternmost part of the Mediterranean basin is influenced bythe Indian Monsoon, through the seasonal shift in the position of theIntertropical Convergence Zone (ITCZ) and the Hadley cell (Bolle,2003). In winter, the ITCZ is located at 10°N and precipitation occursin the central-eastern sector of Africa. In summer, the ITCZ penetratesnorthward, up to 20°N, influencing the north-eastern sector of Africaand part of the Middle East.

4. Chronological framework

The chronological framework here presented follows Sprovieri et al.(2012). It has been assessed through a peak to peak correlation betweenclimatological/oceanographic signals (peaks in abundance of G. ruberand depleted oxygen isotope values) at ODP Site 963 and interstadialsidentified in the δ18ONGRIP curve (Fig. 2) (NGRIP members, 2004;Andersen et al., 2006; Svensson et al., 2006, 2008). In addition, twocalibrated ages from radiocarbon measurements, were available inthe upper part of the interval. The agemodel benefited from an interpo-lating cubic spline function that ensured limited loss of amplitudes athigher frequencies and reduced bias effect.

Fig. 2.Distribution patterns of selected planktonic foraminifera species at ODP Site 963 and com(kyr). A: oxygen isotopic record of Greenland ice cores (NGRIPmembers, 2004); B–C: planktonibetween benthic (C) and planktonic (B) oxygen isotopic values, presented as a 5-point runni(G) T. quinqueloba and (H) N. pachyderma dx at ODP Site 963 (present study), expressed as pODP Site 977 (Pérez-Folgado et al., 2003), expressed as percentage values; Light gray bandsHeinrich events. Dashed black lines correlate Heinrich events (and corresponding Stadials) bSea (identified by N. pachyderma sx peaks) records. Note the missing correlation between HStadial–Interstadial transitions in the ODP 977 Site, from the visual inspection of Fig. 4 and

As introduced in Section 2, the only tephra layer present in the stud-ied sedimentary sequence may be ascribed to the Green Tuff explosiveeruption and its Y-6 tephra equivalent recorded throughout the centraland eastern Mediterranean (Tamburrino et al., 2012). It is locatedwithin the Stadial S12 and the age estimate that can be extrapolatedat Site 963 is 45.0 kyr. This value is very close to the new 40Ar/39Arage of 45.7 ± 1.0 kyr proposed by Scaillet et al. (2013).

5. Results

5.1. Planktic foraminifera assemblages

Planktic foraminifera specimens are abundant and well preserved.Four species, G. bulloides, G. ruber, Neogloboquadrina pachyderma rightcoiling (dx) and Turborotalita quinqueloba are dominant throughoutthe sedimentary interval (Fig. 2E–H). The remaining taxa occur withvery low percentage values (b5%).

G. ruber abundances range between 0 and 50%. Remarkable relativeabundance peaks coincide with each Interstadial, while this species isabsent in Stadials. However, a closer examination reveals that G. ruberoccurrence is sometimes discontinuos (Is14 and Is12) and the highestvalues cover only the lower part of Interstadial horizons (see for in-stance Is16, Is13, Is11, Is8 and Is7). N. pachyderma dx replaces G. ruber

parison with geochemical and micropaleontological records, plotted versus calibrated agec and benthic oxygen isotopic records at ODP Site 963 (Sprovieri et al., 2012); D: differenceng average; downcore variations of (E) G. ruber (Sprovieri et al., 2012), (F) G. bulloides,ercentage values; downcore variations of (I) T. quinqueloba and (J) N. pachyderma dx atmark Interstadials, white bands mark Stadials and dark gray bands mark North Atlanticetween the Sicily Channel (identified by light oxygen isotopic values) and the Alborane5 and He4 and between Is17 and Is14, because it was not possible to identify certainTable 2 in Pérez-Folgado et al. (2003).

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in both the upper part of Interstadials and where a discontinuos occur-rence exists. In general, this species exhibits wide abundance fluctua-tions (0–55%) with the highest values during the Interstadials and thelowest ones during Stadials. Abundance peaks of G.bulloides are associ-ated to Stadials, except for increasing values during Is14 and little orno influence between Is16 and Is15 and above Is8. Finally, T. quinquelobashows a quite regular pattern with positive abundance fluctuations as-sociated to Stadials and decreasing values associated to Interstadials.

5.2. Coccolithophore assemblages

Distribution patterns of selected coccolithophore taxa are showed inFig. 3C–I.G.muellerae in the lower part and E. huxleyi in theupper part ofthe record are the dominant taxa. The dominance reversal is placed atabout 48.8 kyr, just below He5. G. muellerae increases in abundanceduringmost of Interstadials and decreases during Stadials. The distribu-tion pattern of E. huxleyi shows that, despite the occurrence of widefluctuations, the response to D–O variability is less evident than inG. muellerae. The most relevant peaks of small Gephyrocapsa are as-sociated to Is14, Is12 and Is8. More regular seems to be the responseof Helicosphaera carteri and S. histrica, with increasing abundanceduring Stadials and vice versa. Umbellosphaera spp. shows a fewslight increases in abundance during Is17, Is16, Is11 and Is10 andUmbilicosphaera spp. is significantly present up to the base of Is14.The response of coccolithophores to D–O climatic variability becomesmore evident once taxa are grouped (Fig. 4C–F). Placoliths increase in

Fig. 3. Distribution patterns of selected coccolithophore taxa at ODP Site 963 and comparison woxygen isotopic records at ODP Site 963 (Sprovieri et al., 2012); downcore variations of (C) EGephyrocapsa, (F) H. carteri, (G) S. histrica, (H) Umbellosphaera spp. and (I) Umbilicosphaeramark Stadials and dark gray bands mark North Atlantic Heinrich events.

abundance during Interstadials and decrease during Stadials, while“Miscellaneous” taxa show an opposite behavior, especially aboveIs13. Abundance fluctuations of these groups are significant, evenover the interval He5–He3 where they range between about 7% and15%, given that the error associated to countings, for a 95% confi-dence level, is on average of 3.4% for placoliths and 2.7% for the“Miscellaneous” group. Finally, F. profunda (LPZ taxa), as alreadycommented in Sprovieri et al. (2012), exhibits increasing abundancevalues in coincidence of most of Interstadials. However, likewise tothe distribution of G. ruber, the abundance peak is often limited tothe lower part of the interval.

6. Discussion

6.1. Productivity variations during D–O oscillations in the Sicily Channel

Both calcareous plankton groups suggest three productivity scenar-ios for each D–O cycle. Planktic foraminifera assemblages indicate oligo-trophic conditions in the basal part of Interstadials (G. ruber abundancepeaks) and a subsequent increase in productivity (N. pachyderma dxabundance peaks). High productivity levels can also be ascribed toStadials, because of increasing values of T. quinqueloba and in manycases of G. bulloides. Thus, the passage between Interstadials andStadials can be seen as a shallowing in the nutricline depth, from adistinct Deep Chlorophyll Maximum (DCM) to highly productive sur-face waters (Rohling and Gieskes, 1989; Pujol and Vergnaud-Grazzini,

ith geochemical records, plotted versus calibrated age (kyr). A–B: planktonic and benthic. huxleyi (black line) and G. muellerae (dashed black line), (D) small placoliths, (E) smallspp., expressed as percentage values. Light gray bands mark Interstadials, white bands

Fig. 4.Distribution patterns of coccolithophore groups at ODP Site 963 and comparisonwith geochemical andmicropaleontological records, plotted versus calibrated age (kyr). A–B: plank-tonic and benthic oxygen isotopic records at ODP Site 963 (Sprovieri et al., 2012); downcore variations of (C) placoliths, (D) Miscellaneous taxa, (E) UPZ taxa (present study) and(F) F. profunda (LPZ taxa) (Sprovieri et al., 2012), at ODP Site 963, expressed as percentage values; downcore variations of (G) C37 alkenones from Site MD95-2043, Alboran Sea (Morenoet al., 2004); downcore variations of (H) C37:4 alkenones from Sites MD01-2444 and MD01-2443, Iberian Margin (Martrat et al., 2007; Incarbona et al., 2010d). Light gray bands markInterstadials, white bands mark Stadials and dark gray bands mark North Atlantic Heinrich events. Dashed black lines correlate Heinrich events (and corresponding Stadials) amongthe Sicily Channel (identified by light oxygen isotopic values in Sprovieri et al., (2012), the Alboran Sea (identified byN. pachyderma sx peaks in Cacho et al., (1999) and the IberianMargin(identified by the lowest SST in Martrat et al., (2007) records.

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1995). Based on today's species ecological preference (Table 1), it is re-ally hard to establish the entity of productivity variations across theInterstadial/Stadial switch. In otherwords,we are unable to inferwhetherproductivity was higher duringN. pachyderma dx or T. quinqueloba peaks.

The same three-scenario framework for productivity variationsduring D–O is showed by coccolithophores. High abundance values ofF. profunda in the early Interstadials indicate the occurrence of a deepnutricline, likely due to a strong summer thermocline, and lowerproductivity (e.g. Molfino and McIntyre, 1990; Beaufort et al., 1997;Incarbona et al., 2008). A clear evidence of reduced productivity duringStadials, with respect to the late Interstadials, is given by the decreasingdistribution pattern of placoliths, a group formed by r-stategist taxablooming in winter/spring (Table 2). Reduced productivity in corre-spondence of Stadials is further supported by increasing abundance of“Miscellaneous” taxa. Independently from a well-established ecologicalpreference of the most abundant taxa of this group, H. carteri andS. histrica (e.g. low salinity and high turbidity following Colmenero-Hidalgo et al., 2004; Grelaud et al., 2012), they are certainly suggestiveof a lower trophic level than E. huxleyi and gephyrocapsids (e.g. Young,1994; Ziveri et al., 1995; Incarbona et al., 2010c).

During the early Interstadials, G. ruber and F. profunda are often asso-ciatedwithN. pachyderma dx and placoliths (E. huxleyi andG. muellerae).This apparent contrasting ecological indication can easily be explainedby a seasonal offset. Both G. ruber and F. profunda are typical of the sum-mer/early autumn period in the Mediterranean Sea (Knappertsbusch,

1993; Pujol and Vergnaud-Grazzini, 1995; Rigual-Hernández et al.,2012). Their coexistence with high productivity winter/spring taxa isthe result of a marked seasonal contrast or more likely of short/weak summer conditions in the water column (development of surfaceoligotrophy and thermocline deepening), that are instead missing dur-ing the upper part of Interstadials and Stadials.

The three scenarios that describe the D–O variability in the SicilyChannel environment are illustrated and summarized in Fig. 5.

6.2. Comparison with Mediterranean records

The cold spell/high productivity and warm water/low productivitybinomial reported for suborbital fluctuations of MIS 5, of the last degla-ciation and of the Holocene in the Sicily Channel (Sprovieri et al., 2003,2006; Incarbona et al., 2008, 2010a,b) was in contrast with evidencefromnumerous geochemical andmicropaleontological records showingD–O productivity variability in the western Mediterranean Sea (Cachoet al., 2000, 2002; Colmenero-Hidalgo et al., 2004; Moreno et al., 2004,2005; Toucanne et al., 2012). The new faunal and floral record presentedhere reconciles evidences of paleoproductivity at millennium-scale in alarge portion of the Mediterranean Sea. Unfortunately, no further evi-dence exists from the eastern Mediterranean record, even if climaticsuborbital-scale variations affected the isotopic character of LIW duringthe last interglacial and the last glacial (Incarbona et al., 2011; Sprovieriet al., 2012; Toucanne et al., 2012).

Fig. 5. Cartoon showing productivity variations in the photic zone of the Sicily Channel during each D–O cycle. A, lower part of Interstadials characterized by surface oligotrophy (G. ruberpeaks) and a deep thermocline (F. profundapeaks). Note that the coexistence ofG. ruber and F. profundawith high abundance values ofN. pachyderma dx and placoliths (like in the scenarioB) implies a marked seasonal contrast. B, upper part of Interstadials characterized by a distinct Deep Chlorophyll Maximum (N. pachyderma dx peaks) and winter/spring coccolithophoreblooming (placolith peaks). C, Stadials characterized by surface fertility (T. quinqueloba and G. bulloides). Coccolithophore assemblages suggest decreased productivity with respect to sce-nario B, because independently from a well-established ecological preference of H. carteri and S. histrica (Miscellaneous taxa), they indicate a lower trophic level than placoliths. Possibly,cold and low-saline surface waters from the Atlantic Ocean perturbed nutrient and productivity dynamics.

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The Alboran Sea core MD95-2043 has been investigated by severalgeochemical and micropaleontological studies. Results indicate a clearStadial/low productivity and Interstadial/high productivity pattern inthe last glacial period (Cacho et al., 2000, 2002; Colmenero-Hidalgoet al., 2004; Moreno et al., 2004, 2005). There is little doubt that anin-depth evaluation of MD95-2043 records points to the binomial in In-terstadial productivity, as already described for ODP Site 963. In fact,mass accumulation rates of organic and inorganic geochemical proxiesshow that the highest productivity conditions are reached just at thetop of Interstadials, for instance during Is12, Is11, Is8 and Is7 (Fig. 4)(Moreno et al., 2004).

Sicily Channel planktic foraminifera and coccolithophore assem-blages show a rather high degree of similarity with their AlboranSea counterparts, collected at ODP Site 977 and MD95-2043 (Pérez-Folgado et al., 2003; Colmenero-Hidalgo et al., 2004), although dif-ferent methodologies were applied (e.g. the use of different size inthe residue of planktic foraminifera). Minor discrepancies, such asthe rarity of G. ruber and F. profunda or the significant occurrence ofN. pachyderma sx in the Alboran Sea, are possibly due to the west-easttrophic and thermal gradient across the Mediterranean Sea.

N. pachyderma dx and T. quinqueloba seem to be constantlyanticorrelated in both ODP Site 977 and MD95-2043 cores (butonly data from Site 977 are plotted in Fig. 2I–J) like in the Sicily Channeland this fact is more evident during Heinrich events, where the lowestabundances of the former species can be seen. The correlation of thesetwo species between the Sicily Channel and the Alboran Sea records(Fig. 2G–J) is problematic for a series of reasons such as the adoption

of different chronologies, the apparent different duration of Heinrichevents, respectively based on light isotopic values and peaks inN. pachyderma sx, and the detectment of D–O oscillations in some in-tervals (by the visual inspection of Fig. 4 in Pérez-Folgado et al., 2003).However, the same pattern across D–O cycles emerges from the compar-ison and point to the shallowing of the nutricline depth at the passagebetween Interstadials and Stadials (Fig. 5B–C) in a wide Mediterraneandomain.

Alboran Sea coccolithophore assemblages show the increase in rela-tive abundance of small placoliths (especially small Gephyrocapsa andE. huxleyi b4 μm) during Interstadials and ofH. carteri and Syracosphaeraspp. during Stadials and Heinrich events (Colmenero-Hidalgo et al.,2004), like in the Sicily Channel. The comparison with diunsaturatedand triunsaturated alkenones of core MD95-2043 (Cacho et al., 2000,2002; Moreno et al., 2005), synthesized by coccolithophore bloomingspecies (E. huxleyi and gephyrocapsids), is also interesting. Alboran Seaalkenones and Sicily Channel placoliths display a general correspon-dence, the same alternation during D–O cycles and the highest peakswithin the upper part of Interstadials (Fig. 4), once again suggesting thesame productivity dynamics for the two areas.

6.3. Climatic forcing of productivity variations

Enhanced productivity during D–O Interstadials in the Alboran Seahas been explained by: 1) the establishment of a wetter climate in theneighboring land and the subsequent continental nutrient supply pro-vided by increased river runoff; 2) Nutrient fertilization induced by

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enhanced surface water mixing associated with an energetic inflow jetof Atlantic water, presumably resulting from stable low pressure overthe western Mediterranean Sea (Moreno et al., 2004).

Lags among SSTs, productivity, and fluvial input proxies deny thatnutrient enrichment by river discharge was a critical factor limiting pri-mary productivity in the Alboran Sea during the glacial period (Morenoet al., 2005).

High-speed surface currents would produce contrasting productivi-ty dynamics among Site MD95-2043 located in the area between twoanticyclonic gyres, ODP Site 977 located within an anticyclone gyreand ODP Site 963 located within a cyclonic gyre. However, as discussedin the Section 6.2, the three sites show the same behavior, in terms ofproductivity variations, during D–O oscillations. We argue that thedensity, rather than the energy, of the Atlantic water inflow may haveaffected productivity dynamics in such a large area and specificallymay have altered the equilibrium of the vertical mixing during Stadialphases.

During the last glacial period, the lower global sea level led to astrong decrease of the Atlantic/Mediterranean exchange and thus to asignificant density gradient between surface and subsurface water(e.g. Mikolajewicz, 2011; Rogerson et al., 2012). The higher buoyancygradient may have suppressed vertical mixing and may have impactedon themarine ecosystem. However, independently from contrasting re-constructions (Siddall et al., 2003; Frigola et al., 2012), sea-level changesof up to 35 macrossD–Ooscillations (Siddall et al., 2003) seem to be toomodest to drive the three different productivity scenarios drawn inChapter 6.1.

The Atlantic Meridional Overturning Circulation slowdown/shutdown during Heinrich events led to the shift of the Polar Front andto the deposition of IRD as far south as the Iberian Margin (e.g. Zahnet al., 1997; Bard et al., 2000; de Abreu et al., 2003; Eynaud et al., 2009).However, the C37:4 alkenone record in cores MD01-2443 and MD01-2444 testifies to the occurrence of cold and less saline polar water inthis region during each Stadial (Fig. 4H) (Martrat et al., 2007), differentlyfrom IRDdeposition thatmay be influenced by icebergs' route and prov-enance (Hemming, 2004).

No IRD levels exist in the Mediterranean Basin, but the incursion oflow-salinity waters is documented in the Alboran Sea and in the SicilyChannel by light oxygen isotopic values in planktic foraminifera shells,during most of North Atlantic Heinrich events (Sierro et al., 2005;Rogerson et al., 2010; Sprovieri et al., 2012).We assume that the inflowof fresher water of polar originmay also have happened during Stadials,although the entity of the phenomenon was certainly smaller andwithout the presence of the Polar Front at the Iberian Margin latitude.The reduced magnitude of the phenomenonwould explain the absenceof N. pachyderma sx in the Alboran sedimentary record, while the isoto-pic signature of the freshening may have been masked by concomitantlow SST.

In order to gather the signal of possible freshening inputs duringStadials, we plot the curve that deals with the difference betweenbenthic and planktonic oxygen isotopic values (Fig. 2D). The offset inthe isotopic values should represent a homogenization (low offset) ora stratification (high offset) between the lower part of the photiczone, where N. pachyderma dx thrives, and the core of LIW that flowson the Sicily Channel sea floor. Results demonstrate that intervals ofstratification were common, consistently with the high glacial densitygradient in the water column, and possibly reinforced by differentcauses. The most remarkable offsets fall in coincidence of He5, He4and He3 (dark gray bands in Fig. 2) and show the impact of water com-ing from themelting of icebergs in the IberianMargin. Note that there isevidence of prolonged stratification, or at least different fresher waterpulses for instance during He5 and He2, with respect to the short dura-tion assumed by Sierro et al.(2005) and Sprovieri et al. (2012). Othersignificant offset peaks mark the lower part of interstadials, such asIs16, Is15, Is12, Is11, Is8, Is7, Is6 and Is4. They can be associated to theoccurrence of surface oligotrophic waters and of a deep thermocline

(Scenario A in Fig. 5A). Finally, we point out that minor offset intervalsmatch with Stadials, for instance S14, S10, S8 and S4, but also at thetop of S15, S12, S11, S7 and S6. These episodes of stratification happeneddespite the strengthening of north-westerly winds (Cacho et al., 2000;Moreno et al., 2004, 2005), that would have produced vertical mixingand water column homogenization, and support the hypothesis thatAtlantic fresher water penetrated into theMediterranean Sea even dur-ing Stadials.

The potential impact of freshening due to iceberg armadasmelting issignificant for regional circulation, impeding vertical and promotinglateral flow of water masses (Eynaud et al., 2009; Rogerson et al.,2010; Petersen et al., 2013). The negative effect of this process on bi-ological productivity has been reported in numerous works andmodeling (e.g. Pailler and Bard, 2002; Schmittner, 2005; Nave et al.,2007; Incarbona et al., 2010d) and may be suitable to explain reducedtrophic levels in the Mediterranean Sea Stadials.

7. Conclusion

Contrasting evidence on productivity dynamics for Alboran Searecords, MD95-2043 core and ODP Site 977, during D–O oscillations(Cacho et al., 2000; Moreno et al., 2005) and for Sicily Channel ODPSite 963 during the last interglacial, the last deglaciation and HoloceneBond cycles (Sprovieri et al., 2003, 2006; Incarbona et al., 2008, 2010b)have previously been reported. New faunal and floral data, during D–Ooscillations at ODP Site 963, reconciles productivity modes at suborbitaltime-scale between the two regions.

We suggest that the base of Interstadials were characterized bysurface oligotrophy and a deep thermocline (G. ruber and F. profundapeaks) and by a subsequent stage with a distinct DCM (N. pachydermadx peaks) and winter/spring coccolithophore blooming (placolithspeaks). The transition to Stadials/Heinrich events led to the shallowingof the nutricline depth (T. quinqueloba and G. bulloides peaks), while theincrease of coccolithophore species like H. carteri and S. histrica, at ex-penses of placoliths, point to reduced trophic levels.

The same response of marine planktonic ecosystems to D–O oscilla-tions in thewestern-centralMediterranean Sea need to be explained bymechanisms other than an energetic inflow jet of Atlantic water orincreased river runoff. The strong vertical gradient density in theupper part of the water column, due to the global lower sea-level andthe inflow through the Strait of Gibraltar of fresher water of polar originduring Stadials and Heinrich events, may disturb vertical convectionand may be suitable to explain reduced productivity. The offset in theoxygen isotopic values betweenplanktic and benthic foraminifera shellsat Site ODP 963 supports the occurrence of water column stratificationduring Heinrich events and, even if with less intensity, during most ofStadials.

Acknowledgments

We are grateful to two anonymous reviewers for their valuablecomments and suggestions. We also would like to thank Belen Martratfor the friendly discussions. The research has benefit from the fundingfrom: European Community's Seventh Framework programme, undergrant agreement 265103 (Project MedSeA) and from the MarinERA-MedEcos project (ERAC-CT-2004-51587) supported by the Ministeriode Ciencia e Innovación; the MIUR Italian Flagship project RITMARE;MIUR ex 60% grants to E. Di Stefano; Ateneo (University of Catania)grants to A. Di Stefano.

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