Earth and Planetary Science Letters 301 (2011) 365–372

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Earth and Planetary Science Letters

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Stepwise atmospheric carbon-isotope excursion during the Toarcian Oceanic AnoxicEvent (Early Jurassic, Polish Basin)

Stephen P. Hesselbo a,⁎, Grzegorz Pieńkowski b

a Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UKb Polish Geological Institute - National Research Institute, Rakowiecka 4, PL-00-975 Warszawa, Poland

⁎ Corresponding author.E-mail addresses: stephen.hesselbo@earth.ox.ac.uk (

grzegorz.pienkowski@pgi.gov.pl (G. Pieńkowski).

0012-821X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.epsl.2010.11.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 July 2010Received in revised form 9 November 2010Accepted 9 November 2010Available online 8 December 2010

Editor: M.L. Delaney

Keywords:Pliensbachian–Toarcianpalaeoclimatecarbon cycleOceanic Anoxic Eventsea-level changeterrestrial organic matter

During the Mesozoic (250–64 Ma) intervals of about 0.5 Myr were subject to severe environmental changes,including high sea-surface temperature and very low oxygen content of marine water. These Oceanic AnoxicEvents, or OAEs, occurred simultaneously with profound disturbance to the carbon cycle. The carbon-isotopeanomaly in the Early Jurassic that marks the Toarcian Oceanic Anoxic Event (T-OAE) at ~182 Ma ischaracterized in marine sections by a series of dramatic steps towards lighter values. Herein we present newcarbon-isotope data from terrestrial organic matter (phytoclast separates), collected through a LatePliensbachian–Middle Toarcian coastal and marginal marine succession in the Polish Basin, a setting wherehinterland climate and sea-level change are well recorded. The results show that the shift to light carbon-isotope values in the woody organic matter, and therefore also in atmospheric carbon dioxide, similarlyoccurred in major steps. The steps are here correlated with those identified from marine organic matter,where they have previously been attributed to 100 kyr eccentricity forcing of climate. The results providestrong support for orbitally and climatically controlled release of isotopically light carbon from gas hydratesinto the ocean–atmosphere system in a series of rapid bursts. Additionally, a link between the carbon-isotopesteps and shoreline movements can be demonstrated. Individual peaks of the negative excursion are mostlyassociated with facies indicative of sea-level rise (flooding surfaces). However, at the same time inferredhigher atmospheric carbon-dioxide content may be expected to have resulted in increased rainfall andtemperature, leading to accelerated weathering and erosion, and consequently increased sediment supply,progradation and regression, causing some mismatches between isotope shifts and inferred sea-levelchanges. Enhanced abundance of megaspores derived from hydrophilic plant groups, and marked increase inkaolinite, are coincident with the overall development of the negative isotope excursion. The combined datasuggest that each 100-kyr cycle in carbon-isotope values was characterized by increasingly severepalaeoclimatic change, culminating in extremely hot and humid conditions co-incident with the peak ofthe final most negative carbon-isotope excursion. The chemostratigraphic correlation allows very precisedating of the Late Pliensbachian–Middle Toarcian coastal and marginal marine sedimentary succession in thePolish Basin.

S.P. Hesselbo),

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The most profound environmental change in the Jurassic Periodtook place during the Early Toarcian Oceanic Anoxic Event (T-OAE)(Jenkyns, 1988). Marine depositional settings across Europe showevidence for widespread anoxia in the form of coeval black shale, andhigh seawater palaeotemperatures are inferred from isotopic andelemental anomalies (Cohen et al., 2007; Jenkyns, 2010; Jenkyns et al.,2002; McArthur et al., 2008). A negative carbon-isotope excursion,with an average δ13C amplitude of approximately −7‰ (VPDB),possibly the largest such anomaly in whole Phanerozoic, has been

described from marine and terrestrial materials. High-resolutiondatasets in marine sections have shown that the shifts to light carbon-isotope values occur as a series of stratigraphically abrupt steps(Hermoso et al., 2009; Jenkyns et al., 2001; Kemp et al., 2005) — forexample as determined from Yorkshire, UK (Fig. 1).

However, what is still poorly known about this event is itsmanifestation in non-marine and marginal marine environments.Although the prominent negative carbon-isotope anomaly has beendescribed, at a relatively low resolution, from terrestrial organicmatter in fully marine deposits (Hesselbo et al., 2000, 2007), the sameexcursion towards light isotopic values has been documented fromonly a single marginal marine site, on the island of Bornholm,Denmark (Hesselbo et al., 2000, 2007; McElwain et al., 2005).Furthermore, the stepwise character of the excursion has not hithertobeen identified from non-marine materials.

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Fig. 1. Summary graphic log for the Pliensbachian–Toarcian boundary and T-OAE at Hawsker Bottoms, Yorkshire, UK. Graphic log from Hesselbo and Jenkyns (1995) and Littler et al.(2010). Ammonite range data from Howarth (1992) and Page (2004). Carbon-isotope data from Hawsker Bottoms and other Yorkshire localities from Kemp et al. (2005) and Littleret al. (2010). Numbering of steps in the carbon-isotope curve is from this study.

366 S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

367S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

The present paper describes an extensive new carbon-isotopedataset generated from terrestrial organic matter in thick Toarciansections from five boreholes and one clay pit in the Polish Basin,northern-central Europe (Fig. 2). These new data allow us to addressimportant unresolved questions concerning the origin of the negativecarbon-isotope excursion, the detailed structure that it exhibits, andaccompanying environmental changes. Additionally, the resultsdemonstrate the power of terrestrial carbon-isotope stratigraphy asa new tool for a high-resolution correlation of Late Pliensbachian toMiddle Toarcian strata, important in this case for proving stratigraphicintegrity of one of most important potential seals for future CO2

sequestration systems in Poland.

2. Depositional setting

A comprehensive depositional model, sequence stratigraphy, andinferred relative sea-level history, has previously beendeveloped for theEarly Jurassic succession in the Polish Basin, based on sedimentologicaland palaeontological study of boreholes and outcrops (Pieńkowski,2004). Latest Pliensbachian to Early Toarcian strata are assigned to asingle unconformity-bounded depositional sequence, numbered VIII inthe succession, which has itself been objectively subdivided intoparasequences (VIII a, b, c, d, e: Fig. 72 in Pieńkowski, 2004)). Theoldest deposits of the sequence, partly of latest Pliensbachian age, reston a regional erosional surface (sequence boundary) and comprisealluvial anddeltaic sediments, assigned to anumber of lithostratigraphicunits of local significance. The sequence boundary represents theuppermost of a set of three LatePliensbachian sequenceboundaries (VI–VIII), all marking significant sea-level falls, attributed by several authorsto recurrent Late Pliensbachian glaciation, followed by rapid globalwarming at the beginning of Toarcian (Morard et al., 2003; Pieńkowski,2004; Price, 1999; Rogov and Zakharov, 2010; Suan et al., 2008a, 2010).

Mecho

Gorzinferred maximum rangeof the Lower Jurassicdeposits

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Fig. 2. Palaeographic map and summary profile for the Early Jurassic epicontinental basin in P(modified from Pieńkowski, 2004).

The bulk of the sequence is assigned to the Ciechocinek Formation,which is present across the whole Polish Basin, and is composed ofpoorly consolidated grey-green mudstone and heterolithic silty mud-stone (‘verdine’ facies), and siltstonewith intercalation of fine sand andsandstone ((Leonowicz, 2005; Pieńkowski, 2004); Figs. 3–5). The baseof the Ciechocinek Formation is a transgressive surface of regionalextent, recognized in a number of boreholes across the Polish Basin,marking one of the most prominent sequence stratigraphic horizons inthe Early Jurassic strata in Poland (Pieńkowski, 2004). A Toarcian age forthe Ciechocinek Formation is confirmed by the continuous andabundant occurrence of the Paxillitriletes phyllicus megaspore assem-blage (Marcinkiewicz, 1971). Overlying an erosion surface at the top ofthe sequence are strata of mostly Late Toarcian age that are assigned tothe Borucice Formation. These deposits comprise fine to mediumsandstone laid down in alluvial or subordinately deltaic channels, andthe unconformity at the base of the Borucice Formation is a majorsequence boundary (Figs. 4 and 5).

The Ciechocinek Formation has themaximumgeographic extent ofEarly Jurassic sedimentary strata in the Polish Basin, consistent withthe Early Toarcian sea-level rise also recorded from other Europeanbasins (Dadlez, 1969; Hallam, 2001; Pieńkowski, 2004). Depositionalenvironments in the Polish Basin at this time did not host organismsnormally associated with well-oxygenated marine conditions. Themarine fossil fauna is uncommon and shows low diversity, made upmostly of ostracods, foraminifers, gastropods, a bivalve (Meleagrinellasubstriata (Münster)), and fish teeth (Kopik and Marcinkiewicz,1997). Rare marine phytoplankton (dinoflagellate cysts, acritarchs)are also reported (Barski and Leonowicz, 2002). Trace fossilassemblages are also of low diversity and dominated by simpledeposit feeding forms such as Planolites, whilst the traces ofsuspension feeders, such as the more stenohaline Diplocraterion, arerare (Leonowicz, 2009; Pieńkowski, 2004). Plant remains areabundant, particularly phytoclasts and sporomorphs. Organic matter

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Fig. 4. Carbon-isotope profiles for studied locations in the Polish Basin, shown with a compilation of sedimentological, sequence stratigraphic and palaeoclimatic indices (graphiclogs after Pieńkowski, 2004, simplified and modified). See Figure 2 for profile locations. Inset: previously published data from Korsodde, Bornholm, Denmark (Hesselbo et al., 2000)and Yorkshire, UK (modified from compilation shown in Fig. 1). Megaspore data from Marcinkiewicz (1962) and Marcinkiewicz (1971). Kaolinite/illite ratios from Brański (2010).Correlation lines are based upon interpreted steps in carbon-isotope excursions (labelled Pl/To and 1–4). The unconformities at the bases of depositional sequences VIII and IX(Borucice Formation) are also shown (u/c). For key see Figure 5.

Fig. 3. The Kozłowice outcrop, showing embayment/lagoonal deposits of the Ciechocinek Formation (Early Toarcian age), and the interpreted position of δ13C steps 1, 2 and 3 of theT-OAE. Inset photographs show sedimentary structures of sandy delta lobes incised into embayment deposits with storm intercalations. Lower sandstone layer shows progradationfrom NW. Upper sandstone layer represents sandy shoal, possibly wave-reworked delta front sediment, forming a submerged barrier. Note progradational parasequences (b, c, d)with flooding surfaces at the tops (b, c). The whole succession shows an overall shallowing-upward trend, from open embayment, through semi-closed lagoon-embayment, to morerestricted lagoon. Inferred equivalence to marine ammonite biozones is indicated on right.

368 S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

Pl/To

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Fig. 5. Carbon-isotope profiles for studied locations in the Polish Basin, shown with a compilation of sedimentological, sequence stratigraphical and palaeoclimatic indices (graphiclogs after Pieńkowski, 2004, simplified and modified). See Figure 2 for profile locations. Kaolinite/illite ratios from Brański (2010). Correlation lines are based upon interpreted stepsin carbon-isotope excursions (labelled Pl/To and 1–5). The unconformities at the bases of depositional sequences VIII and IX (Borucice Formation) are also shown (u/c). Key alsoapplies to Figure 4.

369S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

present in mudstone and clay in the Polish Basin is immature, asindicated by the low Thermal Alteration Index of palynomorphs (thebackground ‘thermal’ colours of Early Jurassic palynomorphs are darkyellow to pale orange, i.e. thermal index=2, rarely 2.5–3 (Pieńkowskiand Waksmundzka, 2009)), and biomarkers have been recoveredwith biological configurations (Marynowski and Simoneit, 2009).

Overall the formation is considered to have been deposited in avariety of shallow proximal environments including restrictedoffshore marine through to barrier and barrier-associated lagoons,particularly in the lower part of the formation, as well as small deltasystems which, when present, predominate in the upper part andmarginal areas of the basin (Pieńkowski, 2004).

3. Carbon-isotope excursion in terrestrial materials

3.1. Background and method

The organic content of the deposits studied consists mainly ofphytoclasts (wood fragments, cuticle) and spores, and is thusmade upof material that originates almost entirely in the terrestrial environ-ment. The content of marine palynomorphs is negligible and, ifpresent, does not exceed 1–3% — see also Barski and Leonowicz(2002). The principal new dataset reported here comprises N400carbon-isotope analyses of woody phytoclasts separated manuallyfrom palynological preparations. The carbon-isotope composition ofthe phytoclasts is an integrated representation of the standingvegetation, and so consequently the values are strongly affected bythe carbon isotopic composition of contemporaneous atmosphericcarbon dioxide (cf. Hasegawa, 1997).

For the present study, organic matter was concentrated usingstandard palynological techniques: about 30–50 g sediment wascrushed and treated twice with cold HCl (30%) and cold HF (38%);the first residues were washed in water with hydrogen iodide acid,then heavy minerals (such as pyrite) were separated using CdJ and KJsalt solution, and finally the residue was washed in distilled water toneutral pH. During the preparation of the organic residues nochemical oxidation, organic reagents, or deliberate physical separa-tion was used. Subsequently, phytoclasts weremanually separated forisotope analyses using a needle. In this case, only woody material wasincluded in the samples, not cuticle. Analyses were carried out at theNERC Isotope Geoscience Laboratory (NIGL) at Keyworth, UK, using a

Carlo Erba 1500 mass spectrometer on-line to a TripleTrap device,with a secondary cryogenic trap just before the mass spectrometerinlet. The 2σ on analyses of 82 standards (δ13CV-PDB) was 0.11‰.Twenty-two duplicate analyses of samples from Mechowo have a 2σvalue of 0.56‰ (see Supplementary data Table 1). This variabilityreflects sample heterogeneity rather than analytical uncertainty.

3.2. Results and discussion

All profiles analyzed show the same well-developed excursiontowards exceptionally light isotopic values through the Early Toarcian(Figs. 4 and 5). The magnitude of the excursion, as expressed in thestandard δ13CV-PDB notation, ranges from about ~−4‰ to ~−7‰. Thispattern is perfectly compatible with previous reports of the negativecarbon-isotope excursion in organic matter from the T-OAE, includingthe example from Bornholm shown as an inset in Figure 4. Oneparticularly important feature of all the isotopic profiles of the EarlyToarcian presented here is the development of a number of abruptsteps leading up to the climax of the negative excursion.

A number of general inferences can be made on the basis of thisnew dataset. 1) We can deduce that at several sites in the Polish Basinthere is a very expanded record of the T-OAE. In particular, theMechowo borehole has a record of the OAE that is N60 m thick (Fig. 4).2) Based on the abrupt upward return to relatively heavy carbon-isotope values, the record of the OAE is commonly truncated byerosion prior to the deposition of the later Toarcian sediments of theBorucice Formation; Mechowo again provides a good example of this,and a similar conclusion was also previously reached for theBornholm section (Hesselbo et al., 2007). 3) In two other profiles,Brody-Lubienia and Parkoszowice (Fig. 5), the sedimentary record ofthe upper part of the T-OAE is more complete, probably as a result oflocally reduced erosion at the overlying sequence boundary. 4) It isclear that there is a subordinate negative carbon-isotope excursion atabout the Pliensbachian–Toarcian boundary (Pl/To — Figs. 4 and 5),likewise recently reported from two other European locations andfrom Morocco (Bodin et al., 2010; Hesselbo et al., 2007; Littler et al.,2010). 5) Perhaps most importantly, we can conclude that thestepped nature of the excursion, previously only known from marinematerials, is also expressed in terrestrial organic matter.

The stepped nature of the excursion is now very well documentedfrom a number of wholly marine successions in Europe, in both

370 S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

carbonate and organic matter records (Hermoso et al., 2009; Jenkynset al., 2001; Kemp et al., 2005; Sabatino et al., 2009). In the case of theSancerre Borehole, in the Paris Basin, the steps are manifested in datafrom separated size fractions that include diagenetically pristinenannofossils (Hermoso et al., 2009). In all these marine profiles thesteps occur as inflections on the falling limbs of quasi-sinusoidalisotope profiles and thus cannot be explained simply as an artefact ofsedimentary hiatus interrupting a linear shift towards lighter isotopevalues; the same observation and conclusion applies to the results ofthe present study, especially well illustrated by the Brody–Lubieniaprofile in Figure 5. Each step has been interpreted, controversially, as aphase of rapid and voluminous release of isotopically light carbonfrom an external source such as marine gas hydrate, with pacingmodulated by climatic forcing at an astronomical frequency, thoughtinitially to be ~20 kyr precession (Kemp et al., 2005, 2006; Wignallet al., 2006). On the basis of analysis of high-resolution lithologicaldata from Portugal and Italy, these regular cyclic steps in the negativecarbon-isotope excursion were subsequently reassigned to the100 kyr astronomical eccentricity cycle (Sabatino et al., 2009; Suanet al., 2008b). Although Kemp et al. (2005) settled for an interpre-tation of the Yorkshire cycles as precession driven, these authorscould not demonstrate the occurrence of longer wavelengths in theiranalyses; on the basis of a steep gradient in 87Sr/86Sr ratios inbelemnite fossils, McArthur et al. (2000) argued for a longer durationfor the T-OAE and implied that Kemp et al. had underestimated thedegree of condensation in the Yorkshire sections.

There is no continuous carbon-isotope record from bulk, organiccarbon in the section at Peniche in Portugal with which to compare tothe Polish data (Hesselbo et al., 2007). However, a very detailedcarbonate carbon-isotope record does exist for Peniche that exhibitsunambiguously one step and, if the carbon-isotope spikes areinterpreted as primary rather than reflecting introduction of diage-netic calcite, possibly two higher steps (at 12.5 and 19.5 m in Fig. 3 ofHesselbo et al., 2007). If these spikes are correlated with the steps inYorkshire, Poland, and Italy, then the cyclostratigraphy based oncarbonate content at Peniche (Suan et al., 2008b) would have to havebeen be at a higher frequency than those authors suppose, andpossibly also irregular. On the other hand, it is known that carbonatecarbon-isotope records commonly show lower amplitude changesthan the same records from organic matter (e.g. Hesselbo et al., 2007),and this may also partly explain the poor development of steps in thePeniche bulk carbonate isotope profile.

As argued in Kemp et al. (2005), evidence for astronomical control,of whatever frequency, is important because an isotopic signal ofglobal significance under astronomical control would rule out a keyalternative mechanism for rapid (i.e. b20 kyr) input of isotopicallylight carbon to the atmosphere: methane vented from the metamor-phic aureoles of giant sills within older marine and terrestrial organic-rich sediments during the formation of the Karoo–Ferrar large igneousprovince (McElwain et al., 2005; Svensen et al., 2007). The new dataare particularly significant in this regard because they demonstratethat the stepped pattern was characteristic not just of surfaceseawater, in which it might be argued the isotope signal onlyrepresents localized oceanographic processes, but also the contem-poraneous atmosphere and terrestrial biosphere, where localizedsystematic anomalies matching those in the ocean are inconceivable.Methane from metamorphic aureoles still remains an importantpotential source of isotopically light carbon that could explain theoverall carbon-isotope anomaly and also have served as a trigger forthe whole event (Cohen et al., 2007; Suan et al., 2008b).

Originally only a single step was recognized in the carbon-isotopeprofile from the Yorkshire coastal sections (Hesselbo et al., 2000).Subsequently, on the basis of a high-resolution dataset, the single stepwas resolved into three separate steps labelled A–C by Kemp et al.(2005), with a stratigraphically higher fourth step highlighted byCohen et al. (2007) and labelled step D. The dataset of Kemp et al.

(2005) also clearly shows at least two strongly expressed cycles incarbon-isotope values that occur below their step A (Fig. 1). Althoughthere is a scope for miscorrelation between the steps shown in theYorkshire dataset and those established from the Polish Basin in thepresent work, we can achieve more confident correlation of thesuccessions between these basins on the basis that each shift incarbon-isotope values becomes progressively lighter, culminatingabove step C of Kemp et al. (2005). Thus we correlate our step 3 withstep A of Kemp et al. (2005).

Biostratigraphy also supports the chemostratigraphic correlation.The stratigraphically important dinoflagellate cyst Luehndea spinosa(Morgenroth) found in the Kozłowice outcrop (Barski and Leonowicz,2002) is known to span the age of three successive amimonitebiozones: margaritatus, spinatum (Late Pliensbachian) and tenuicos-tatum (earliest Toarcian) (Riding and Thomas, 1992). At KozłowiceLuehndea spinosa (Morgenroth) occurs within strata above thegreatest negative excursion following step 3, which, based on ourchemostratigraphic correlation with the Yorkshire profile, is equiva-lent to the tenuicostatum Zone and not the overlying falciferum Zone(Fig. 1).

4. Relation to parallel environmental changes

4.1. Relative sea-level change and sediment supply

It has long been observed that worldwide changes in sedimentaryfacies provide substantial evidence for global rise in sea-level from thelatest Pliensbachian to the Early Toarcian (Hallam, 2001), and thegeographical extent of the Ciechocinek Formation is part of the veryextensive evidencebase for this eustatic event. At a smaller stratigraphicscale it has been more difficult to disentangle the effects of sea-levelchange from change in sediment supply (e.g. see Suan et al. (2008a,b)for discussion regarding the Early Toarcian of the Lusitanian Basin,Portugal). In the marginal-marine succession of the Polish Basin, theEarly Toarcian strata have been subdivided into 5–20 m thick shallow-ing-upwards sedimentary cycles, or parasequences (Pieńkowski, 2004)(Figs. 4 and 5). These parasequences are defined by lithological cycles,and four phases of progradation fall within the T-OAE (Pieńkowski,2004) (Figs. 4 and 5). In Brody–Lubienia, additional parasequencesabove T-OAE are also preserved as a result of locally reduced middleToarcian erosion (Fig. 5).

Progradational phases culminate in the deposition of shoreface/foreshore, barrier island, deltaic or lagoonal/marsh sediments, withthe most regressive deposits evincing root traces or palaeosols, inplaces (usually at the proximal locations) associated also with non-deposition or erosion (Pieńkowski, 2004). In Figures 4 and 5 changesin depositional environment are indicated by the dashed black lines,which represent shifts between more proximal and more distalsettings. In theory, parasequences may be ascribed purely toautocyclic sedimentary processes without the involvement of relativesea-level change (sensu Posamentier et al., 1988) but in view of thesize of the carbon-isotope perturbations represented by each cyclicstep in the isotope profile, it would be remarkable were there to havebeen no element of sea-level change also playing a strong role in thegeneration of the parasequences. Also, the basinwide extent of theparasequences indicates regional (Pieńkowski, 2004) rather than localscale, an argument against autocyclic processes such as delta lobeswitching. The parasequences are therefore here related to sea-levelchanges: sea-level rise created accommodation space and causedretrogradation (flooding), and highstand of sea-level or even slowsea-level rise allowed passive progradation into available space drivenby excess sediment supply.

In some cases, but not all (e.g. Fig. 3), the steps in the carbon-isotope curves occur at flooding surfaces at the top of eachparasequence. The individual peaks that make up the negativeexcursion are mostly associated with facies indicative of flooding

371S.P. Hesselbo, G. Pieńkowski / Earth and Planetary Science Letters 301 (2011) 365–372

and deepening. At the same time, inferred higher atmosphericcarbon-dioxide content associated with the negative peaks at boththe smaller and larger scale, may be expected to have resulted inincreased rainfall and temperature (cf. Sluijs et al. (2009) for aCenozoic example), leading to accelerated weathering and erosion,and consequently to increased sediment supply, progradation, andregression, particularly at points of sediment delivery such as rivermouths (e.g. Hesselbo et al., 2007).

Interestingly, the match between the carbon-isotope shifts andrelative sea-level rise in the Polish Basin seems to be almost exactlycoincident for the Pl/To excursion and step 1 of the T-OAE, while morediscrepancies appear in steps 2, 3 and 4. In the case of the higher steps,the negative isotope shifts occurmostly below the lithologically definedflooding surface. This observation can be explained by intensification ofchemical weathering in the source areas; higher steps show progres-sively lighter carbon-isotope values, corresponding to a likely progres-sively more humid and warmer climate. We infer progressive delay ininitiation of flooding in each carbon-isotope cycle through the T-OAE assediment supply and progradation rate increased. Moreover, the higherparasequences (‘c’ and particularly ‘d’, ‘e’ and ‘f’) showmuch shallowerfacies, which we interpret to result from the significant increase inintensity of weathering and rates of sediment supply concomitant withthe peak of the T-OAE. If the sea-level changes were glacioeustatic anyremnant ice would have melted early in the progress of the T-OAE, solater in the event theweathering signal would be expected to dominateover sea level. In places, particularly in the marginal parts of the basin,non-deposition or erosion also reduced the sedimentary record ofhigher parasequences.

4.2. Other evidence for palaeoclimatic change

The Polish Basin succession contains further independent evidencefor palaeoclimatic conditions prior to, during, and after the T-OAE.Palynofacies fromthe LatePliensbachianshowstrongpollen-over-sporedominance, in contrast to palynofacies from the Early Toarcian stratawhich show strong dominance of spores over pollen; this has beeninterpreted in Pieńkowski and Waksmundzka (2009) as a shift from arelatively cold and dry climate in the Pliensbachian to a very warm andhumid climate in the Early Toarcian. Data from the expandedMechowoand Gorzow Wielkopolski boreholes also show enhanced megasporeabundance coincidentwith development of the overall negative isotopeexcursion, and the highest frequencies of megaspores are observedin the four highest parasequences (‘c’, ‘d’, ‘e’ and ‘f’ in Fig. 4). Themegaspores, such as the most common Paxillitriletes phyllicus (Murray)Hall et Nicolson 1973, and Minerisporites institus Marcinkiewicz, 1971(Marcinkiewicz, 1962, 1971, 1989), are derived from the hydrophilicplant groups Lycopsida (club mosses) and Isoetaceae (quill worts).These data strongly support the inferences made by other authors thatthe T-OAE negative excursion corresponds to a time of extremely warmand humid climate and very high atmospheric CO2 content (Jenkyns,2003).

Additionally, clay mineral distributions have been documentedfrom the Polish Basin (Brański, 2010). Most of the mineral grains inthe Ciechocinek Formation are of detrital origin. Early diageneticphases include Fe-rich chlorite, as well as siderite and pyrite, thesegenetically linked with the brackish-marine environment. Detritalsources were strongly weathered metamorphic and sedimentaryrocks (Leonowicz, 2005, 2007). Amongst other factors, detrital claymineral assemblages may be expected to have responded to changesin palaeoclimate, with an increase in the formation of kaolinite overillite during hot humidweathering of source terrains. A relatively low-resolution study of Early Jurassic strata in Europe noted theprevalence of kaolinite for the Early Toarcian (Dera et al., 2008).These authors demonstrated a distinct palaeolatitudinal distributionin detrital clay mineralogy, which they interpreted to reflect thedevelopment of a humid belt north of the Tethys Ocean. Within the

Polish Basin similar conditions occurred with a marked increase inkaolinite within the interval of the negative carbon isotope excursion.The new and old data combined suggest that each 100-kyr cycle incarbon-isotope values was characterized by increasingly severepalaeoclimatic change, culminating in extremely hot and humidconditions co-incident with the peak of the most negative carbon-isotope excursion.

5. Conclusions

A pattern of cyclic and abruptly stepwise negative shifts in carbon-isotope values during the initial phase of the Toarcian Oceanic AnoxicEvent are demonstrated from terrestrial organic materials for the firsttime. Correlation between marine and terrestrial carbon-isotopeprofiles implies that stepwise shifts occurred with 100 kyr frequencyand thus an orbital control on atmospheric as well as marine carbon-isotope values can be inferred. This confirming evidence forastronomical control of what must be a global isotope signatureprovides strong support to the hypothesis that each step representsan episode of enhanced gas hydrate dissociation and release ofmethane. A link between the carbon-isotope steps and shorelinetransgression and regression can be demonstrated, but is complicatedbecause of increased sediment supply through the OAE. If the high-frequency sea-level change observed were glacioeustatic in origin,then the data imply that ice melted in the first one or two steps, andsubsequently there was little high-frequency sea-level rise to begained when more greenhouse gas was added, only greaterweathering and sediment supply in the hot and humid conditions.The close coincidence between mineralogical and palynologicalindicators of hot, humid climates, and the peak of the overall negativecarbon-isotope excursion, provides compelling evidence of extremeclimate change at the high point of the OAE. Carbon-isotopechemostratigraphy provides a means of very precise stratigraphiccorrelation for strata deposited in diverse marine and non-marinedepositional environments during the Late Pliensbachian to MiddleToarcian interval, both within the Polish Basin, and at a global scale.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.epsl.2010.11.021.

Acknowledgements

We are very grateful to Melanie Leng for the carbon-isotopeanalyses carried out at NIGL and Marta Hodbod for palynologicalpreparation. The comments of three anonymous reviewers are greatlyappreciated. This study was financed by the Polish Scientific Funds2006/2007 project N30701131/094, the scientific funds of the PolishGeological Institute, project 6.10.0003.00.0, and the UK NaturalEnvironment Research Council, project NE/F011067/1. This is acontribution to IGCP project 506.

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