Stepwi atmospheric carbon-isotope excursion during the Toarcian Oceanic Anoxic Event (Early Jurassic,Polish Basin)
Stephen P.Heslbo a ,⁎,Grzegorz Pie ńkowski b
a Department of Earth Sciences,University of Oxford,South Parks Road,Oxford OX13AN,UK
b
Polish Geological Institute -National Rearch Institute,Rakowiecka 4,PL-00-975Warszawa,Poland什么是三权分立
a b s t r a c t
a r t i c l e i n f o Article history:
Received 8July 2010
Received in revid form 9November 2010Accepted 9November 2010Available online xxxx Editor:M.L.Delaney Keywords:
Pliensbachian –Toarcian palaeoclimate carbon cycle
Oceanic Anoxic Event a-level change
terrestrial organic matter
During the Mesozoic (250–64Ma)intervals of about 0.5Myr were subject to vere environmental changes,including high a-surface temperature and very low oxygen content of marine water.The Oceanic Anoxic Events,or OAEs,occurred simultaneously with profound disturbance to the carbon cycle.The carbon-isotope anomaly in the Early Jurassic that marks the Toarcian Oceanic Anoxic Event (T-OAE)at ~182Ma is characterized in marine ctions by a ries of dramatic steps towards lighter values.Herein we prent new carbon-isotope data from terrestrial organic matter (phytoclast parates),collected through a Late Pliensbachian –Middle Toarcian coastal and marginal marine succession in the Polish Basin,a tting where hinterland climate and a-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,similarly occurred in major steps.The steps are here correlated with tho identi fied from marine organic matter,where they have previously been attributed to 100kyr eccentricity forcing of climate.The results provide strong support for orbitally and climatically controlled relea of isotopically light carbon from gas hydrates into the ocean –atmosphere system in a ries of rapid bursts.Additionally,a link between the carbon-isotope
steps and shoreline movements can be demonstrated.Individual peaks of the negative excursion are mostly associated with facies indicative of a-level ri (flooding surfaces).However,at the same time inferred higher atmospheric carbon-dioxide content may be expected to have resulted in incread rainfall and temperature,leading to accelerated weathering and erosion,and conquently incread diment supply,progradation and regression,causing some mismatches between isotope shifts and inferred a-level changes.Enhanced abundance of megaspores derived from hydrophilic plant groups,and marked increa in kaolinite,are coincident with the overall development of the negative isotope excursion.The combined data suggest that each 100-kyr cycle in carbon-isotope values was characterized by increasingly vere palaeoclimatic change,culminating in extremely hot and humid conditions co-incident with the peak of the final most negative carbon-isotope excursion.The chemostratigraphic correlation allows very preci dating of the Late Pliensbachian –Middle Toarcian coastal and marginal marine dimentary succession in the Polish Basin.
©2010Elvier B.V.All rights rerved.
1.Introduction
The most profound environmental change in the Jurassic Period took place during the Early Toarcian
Oceanic Anoxic Event (T-OAE)(Jenkyns,1988).Marine depositional ttings across Europe show evidence for widespread anoxia in the form of coeval black shale,and high awater palaeotemperatures are inferred from isotopic and elemental 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-resolution datats in marine ctions have shown that the shifts to light carbon-isotope values occur as a ries of stratigraphically abrupt steps (Hermoso et al.,2009;Jenkyns et al.,2001;Kemp et al.,2005)—for example as determined from Yorkshire,UK (Fig.1).
However,what is still poorly known about this event is its manifestation in non-marine and marginal marine environments.Although the prominent negative carbon-isotope anomaly has been described,at a relatively low resolution,from terrestrial organic matter in fully marine deposits (Heslbo et al.,2000,2007),the same excursion towards light isotopic values has been documented from only a single marginal marine site,on the island of Bornholm,Denmark (Heslbo et al.,2000,2007;McElwain et al.,2005).Furthermore,the stepwi character of the excursion has not hit
herto been identi fied from non-marine materials.
Earth and Planetary Science Letters xxx (2010)xxx –xxx
⁎Corresponding author.
E-mail address:stephen.ac.uk (S.P.Heslbo),grzegorz.v.pl (G.Pie ńkowski).
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EPSL-10668;No of Pages 8
0012-821X/$–e front matter ©2010Elvier B.V.All rights rerved.doi:10.1016/j.epsl.2010.11.021
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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 Heslbo 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 Littler et al.(2010).Numbering of steps in the carbon-isotope curve is from this study.
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The prent paper describes an extensive new carbon-isotope datat generated from terrestrial organic matter in thick Toarcian ctions fromfive boreholes and one clay pit in the Polish Basin, northern-central Europe(Fig.2).The new data allow us to address important unresolved questions concerning the origin of the negative carbon-isotope excursion,the detailed structure that it exhibits,and accompanying environmental changes.Additionally,the results demonstrate the power of terrestrial carbon-isotope stratigraphy as a new tool for a high-resolution correlation of Late Pliensbachian to Middle Toarcian strata,important in this ca for proving stratigraphic integrity of one of most important potential als for future CO2 questration systems in Poland.
2.Depositional ttingfin
A comprehensive depositional model,quence stratigraphy,and inferred relative a-level history,has previously been developed for the Early Jurassic succession in the Polish Basin,bad on dimentological and palaeontological study of boreholes and outcrops(Pieńkowski, 2004).Latest Pliensbachian to Early Toarcian strata are assigned to a single unconformity-bounded depositional quence,numbered VIII in the succession,which has itlf been objectively subdivided into paraquences(VIII a,b,c,d,e:Fig.72in Pieńkowski,2004)).The oldest deposits of the quence,partly of latest Pliensbachian age,rest on a regional erosional surface(quence boundary)and compri al
luvial and deltaic diments,assigned to a number of lithostratigraphic units of local significance.The quence boundary reprents the uppermost of a t of three Late Pliensbachian quence boundaries(VI–VIII),all marking significant a-level falls,attributed by veral authors to recurrent Late Pliensbachian glaciation,followed by rapid global warming at the beginning of Toarcian(Morard et al.,2003;Pieńkowski, 2004;Price,1999;Rogov and Zakharov,2010;Suan et al.,2008a,2010).
The bulk of the quence is assigned to the Ciechocinek Formation, which is prent across the whole Polish Basin,and is compod of poorly consolidated grey-green mudstone and heterolithic silty mud-stone(‘verdine’facies),and siltstone with intercalation offine sand and sandstone((Leonowicz,2005;Pieńkowski,2004);Figs.3–5).The ba of the Ciechocinek Formation is a transgressive surface of regional extent,recognized in a number of boreholes across the Polish Basin, marking one of the most prominent quence stratigraphic horizons in the Early Jurassic strata in Poland(Pieńkowski,2004).A Toarcian age for the Ciechocinek Formation is confirmed by the continuous and abundant occurrence of the Paxillitriletes phyllicus megaspore asm-blage(Marcinkiewicz,1971).Overlying an erosion surface at the top of the quence are strata of mostly Late Toarcian age that are assigned to the Borucice Formation.The deposits comprifine to medium sandstone laid down in alluvial or subordinately deltaic channels,and the unconformity at the ba of the Borucice Formation is a major quence boundary(Figs.4and5).
The Ciechocinek Formation has the maximum geographic extent of Early Jurassic dimentary strata in the Polish Basin,consistent with the Early Toarcian a-level ri also recorded from other European basins(Dadlez,1969;Hallam,2001;Pieńkowski,2004).Depositional environments in the Polish Basin at this time did not host organisms normally associated with well-oxygenated marine conditions.The marine fossil fauna is uncommon and shows low diversity,made up mostly of ostracods,foraminifers,gastropods,a bivalve(Meleagrinella substriata(Münster)),andfish teeth(Kopik and Marcinkiewicz, 1997).Rare marine phytoplankton(dinoflagellate cysts,acritarchs) are also reported(Barski and Leonowicz,2002).Trace fossil asmblages are also of low diversity and dominated by simple deposit feeding forms such as Planolites,whilst the traces of suspension feeders,such as the more stenohaline Diplocraterion,are rare(Leonowicz,2009;Pieńkowski,2004).Plant remains are abundant,particularly phytoclasts and sporomorphs.Organic matter
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Fig.2.Palaeographic map and summary profile for the Early Jurassic epicontinental basin in Poland.The main parts of the basin are indicated,as well as locations of profiles sampled (modified from Pieńkowski,2004).
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Fig.4.Carbon-isotope pro files for studied locations in the Polish Basin,shown with a compilation of dimentological,quence stratigraphic and palaeoclimatic indices (graphic logs after Pie ńkowski,2004,simpli fied and modi fied).See Figure 2for pro file locations.Int:previously published data from Korsodde,Bornholm,Denmark (Heslbo et al.,2000)and Yorkshire,UK (modi fied from co
mpilation shown in Fig.1).Megaspore data from Marcinkiewicz (1962)and Marcinkiewicz (1971).Kaolinite/illite ratios from Bra ński (2010).Correlation lines are bad upon interpreted steps in carbon-isotope excursions (labelled Pl/To and 1–4).The unconformities at the bas of depositional quences VIII and IX (Borucice Formation)are also shown (u/c).For key e Figure 5.
Fig.3.The Koz łowice outcrop,showing embayment/lagoonal deposits of the Ciechocinek Formation (Early Toarcian age),and the interpreted position of δ13C steps 1,2and 3of the T-OAE.Int photographs show dimentary structures of sandy delta lobes incid into embayment deposits with storm intercalations.Lower sandstone layer shows progradation from NW.Upper sandstone layer reprents sandy shoal,possibly wave-reworked delta front diment,forming a submerged barrier.Note progradational paraquences (b,c,d)with flooding surfaces at the tops (b,c).The whole succession shows an overall shallowing-upward trend,from open embayment,through mi-clod lagoon-embayment,to more restricted lagoon.Inferred equivalence to marine ammonite biozones is indicated on right.
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prent in mudstone and clay in the Polish Basin is immature,as indicated by the low Thermal Alterat
ion Index of palynomorphs (the background ‘thermal ’colours of Early Jurassic palynomorphs are dark yellow to pale hermal index=2,rarely 2.5–3(Pie ńkowski and Waksmundzka,2009)),and biomarkers have been recovered with biological con figurations (Marynowski and Simoneit,2009).
Overall the formation is considered to have been deposited in a variety of shallow proximal environments including restricted offshore marine through to barrier and barrier-associated lagoons,particularly in the lower part of the formation,as well as small delta systems which,when prent,predominate in the upper part and marginal areas of the basin (Pie ńkowski,2004).3.Carbon-isotope excursion in terrestrial materials 3.1.Background and method
The organic content of the deposits studied consists mainly of phytoclasts (wood fragments,cuticle)and spores,and is thus made up of material that originates almost entirely in the terrestrial environ-ment.The content of marine palynomorphs is negligible and,if prent,does not exceed 1–3%—e also Barski and Leonowicz (2002).The principal new datat reported here compris N 400carbon-isotope analys of woody phytoclasts parated manually from palynological preparations.The carbon-isotope composition of the phytoclasts is an integrated reprentation of the standing vegetation,and so conquently the values are strongly affected by th
e carbon isotopic composition of contemporaneous atmospheric carbon dioxide (cf.Hagawa,1997).
For the prent study,organic matter was concentrated using standard palynological techniques:about 30–50g diment was crushed 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 parated using CdJ and KJ salt solution,and finally the residue was washed in distilled water to neutral pH.During the preparation of the organic residues no chemical oxidation,organic reagents,or deliberate physical para-tion was ud.Subquently,phytoclasts were manually parated for isotope analys using a needle.In this ca,only woody material was included in the samples,not cuticle.Analys were carried out at the NERC Isotope Geoscience Laboratory (NIGL)at Keyworth,UK,using a
Carlo Erba 1500mass spectrometer on-line to a TripleTrap device,with a condary cryogenic trap just before the mass spectrometer inlet.The 2σon analys of 82standards (δ13C V-PDB )was 0.11‰.Twenty-two duplicate analys of samples from Mechowo have a 2σvalue of 0.56‰(e Supplementary data Table 1).This variability re flects sample heterogeneity rather than analytical uncertainty.3.2.Results and discussion
All pro files analyzed show the same well-developed excursion towards exceptionally light isotopic values through the Early Toarcian (Figs.4and 5).The magnitude of the excursion,as expresd in the standard δ13C V-PDB notation,ranges from about ~−4‰to ~−7‰.This pattern is perfectly compatible with previous reports of the negative carbon-isotope excursion in organic matter from the T-OAE,including the example from Bornholm shown as an int in Figure 4.One particularly important feature of all the isotopic pro files of the Early Toarcian prented here is the development of a number of abrupt steps leading up to the climax of the negative excursion.
A number of general inferences can be made on the basis of this new datat.1)We can deduce that at veral sites in the Polish Basin there is a very expanded record of the T-OAE.In particular,the Mechowo borehole has a record of the OAE that is N 60m thick (Fig.4).2)Bad on the abrupt upward return to relatively heavy carbon-isotope values,the record of the OAE is commonly truncated by erosion prior to the deposition of the later Toarcian diments of the Borucice Formation;Mechowo again provides a good example of this,and a similar conclusion was also previously reached for the Bornholm ction (Heslbo et al.,2007).3)In two other pro files,Brody-Lubienia and Parkoszowice (Fig.5),the dimentary record of the upper part of the T-OAE is more complete,probably as a result of locally reduced erosion at the overlying quence boundary.4)It is cl
ear that there is a subordinate negative carbon-isotope excursion at about the Pliensbachian –Toarcian boundary (Pl/To —Figs.4and 5),likewi recently reported from two other European locations and from Morocco (Bodin et al.,2010;Heslbo et al.,2007;Littler et al.,2010).5)Perhaps most importantly,we can conclude that the stepped nature of the excursion,previously only known from marine materials,is also expresd in terrestrial organic matter.
The stepped nature of the excursion is now very well documented from a number of wholly marine successions in Europe,in both
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S e q u e Fig.5.Carbon-isotope pro files for studied locations in the Polish Basin,shown with a compilation of dimentological,quence stratigraphical and palaeoclimatic indices (graphic logs after Pie ńkowski,2004,simpli fied and modi fied).See Figure 2for pro file locations.Kaolinite/illite ratio
s from Bra ński (2010).Correlation lines are bad upon interpreted steps in carbon-isotope excursions (labelled Pl/To and 1–5).The unconformities at the bas of depositional quences VIII and IX (Borucice Formation)are also shown (u/c).Key also applies to Figure 4.
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