Dept. of Earth Sciences, University of Ottawa, P.O. Box 450, Stn. A, Ottawa, ON, Canada
The new database for strontium, carbon and oxygen isotopic composition of Phanerozoic seawater, based on some 4500 LMC shells, provides a record of environmental change with a resolution of about a biozone. In particular, the detrended 18O pattern shows a clear correlation with the reconstructed greenhouse/icehouse modes, including the advocated cold mode during the late Jurassic - early Cretaceous (Frakes et al., 1992). The highly significant correlation with 87Sr/86Sr of paleo-seawater shows that these greenhouse/icehouse modes were driven by tectonic forces. The high resolution of the isotope record enables also partial recognition of glacial/interglacial episodes, similar to those of the Quaternary.
The wavelet, discontinuity and sliding window correlation dimension analysis for cyclicities and non-stationarities in the isotope signals indicate discontinuities in the oxygen and carbon isotope records at ~ 500, 385, 290 Ma, and particularly at 210 Ma (Norian/Rhaetian) and 65 (K/T) transitions. The data also indicate a long term quasi-periodicity at about 110 ± 15 Ma, with superimposed intermittent 53 ± 5 and 32 ± 3 Ma oscillations, the latter particularly prominent during the late Paleozoic - Mesozoic interval. In the Mesozoic, from about the Late Norian to K/T boundary, five 32 Ma intervals generate similar signals (low 18O and 87Sr/86Sr, and high 13C) that often coincide with the major anoxic events. A possible scenario that may explain all these observations could be invoked along the oceanic - plateau model (Kerr, 1998), where the emplacement of large oceanic plateaus (mantle-like 87Sr/86Sr) leads to transgression, increased atmospheric CO2 and sea surface temperatures (lighter 18O), and high organic productivity and burial rates causing large scale deposition of black shales (hence 13C enriched ocean). Yet, the comparable cycles during the Late Paleozoic were developed mostly during the icehouse interval, at times of high but falling sea levels. The actual climatic forcing scenario is therefore more complex than the above model.
Frakes LA, Francis, JE & Syktus JI, Climate Modes of the Phanerozoic. Cambridge University Press, (1992).
Kerr AC, J. Geol. Soc. London, 155, 619-626, (1988).
B09 : 4A/03 : G5
Mid-Cretaceous North Polar Vegetation and Climate: A High Resolution Study
2 Department of Geology, Lund University, Solvegatan 13, SE 223 62 Lund, Sweden
3 Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky Pereulok, 109017 Moscow, Russia
The characterisation of polar climates is critical for understanding the global climate system. Land based climate proxies give a direct reading of atmospheric conditions but often the precise age of non-marine sediments is difficult to pinpoint. Moreover sediment accumulations used as proxies may take many thousands to millions of years to form and thus represent climate averaged over that time.
The latest Albian /earliest Cenomanian (96 Ma) Grebenka Flora (Krivorechenskaya Formation) in Northeastern Russia represents a range of plant communities in several seral stages of development growing very close to the mid Cretaceous north pole. The age of the flora is constrained by 40Ar/39Ar analyses of biotites in air-fall tephras and minimally water transported volcaniclastics, coupled with biostratigraphic correlation of the overlying marine beds of the Dugovskaya Formation.
Our detailed sedimentological studies have shown that rapidly aggrading alluvial volcaniclastics 'captured' the different communities. From palaeosol development and sedimentary characteristics we estimate that a thickness of 100 m of sediments was deposited in no more than a few thousand years. This high rate of deposition led to excellent preservation and also accounts for the diversity of this flora (85 megafossil foliage taxa, plus woods) because both pioneer species and mature forest communities are represented.
The geologically short time span involved means that palaeoclimatic analyses derived from the flora represent a 'snapshot' of mid Cretaceous climate. The flora represents vegetational communities that grew under climatic conditions associated with one small part of a precession cycle, i.e., the shortest Milankovitch cycle of orbital climate forcing. No longer term averaging of climatic change caused by tectonic factors is involved.
The high temporal resolution and floristic diversity of this site provides an extremely reliable quantitative climatic signal of conditions near the mid Cretaceous North Pole. Using leaf physiognomic multivariate analysis (CLAMP) on the whole composite flora the Mean Annual Temperature experienced by the Grebenka vegetation was 13.0 ± 1.8oC with a cold month mean temperature of 5.5 ± 3.3oC. However analyses of individual florules yield slightly different results and help constrain the uncertainties inherent in such an approach. These and other foliar physiognomic data are compared across the Arctic and comparisons are made with climate model simulations for the region.
B09 : 4A/04 : G5
Geochemistry of Late Early Cretaceous Deposits in Northern Germany: Black Shale Formation, Cyclicity and Palaeoenvironment
Ralf Littke (littke@lek.rwth-aachen.de)1,
Hans Brumsack
(H.Brumsack@geo.icbm.uni-oldenburg.de)2,
Jörg Mutterlose
(Joerg.Mutterlose@rz.ruhr-uni-bochum.de)3 &
Jürgen Rullkötter
(J.Rullkoetter@ogc.icbm.uni-oldenburg.de)2
1 Lehrstuhl für Geologie, Geochemie und Lagerstätten des Erdöls und der Kohle, Lochnerstr. 4-20, 52056 Aachen, Germany
2 Institut für Chemie und Biologie des Meeres (ICBM), Universität Oldenburg, 26111 Oldenburg, Germany
In the framework of a multidisciplinary research programme, a combined organic and inorganic geochemical study was carried out on several cores covering the Barremian and Lower Aptian deposits that were deposited in the central part of the Lower Saxony Basin (northern Germany). In addition a core which comprises a 250 m thick sequence of light-coloured, Upper Albian marlstones was studied geochemically.
The Barremian to Aptian sequence is characterized by highly variable, often high, organic carbon concentrations which reach maximum values in the laminated paper shale ("Blätterton") units and in the Fish Shale ("Fischschiefer"; up to 7% Corg). Petrographically, the sediments contain mainly alginite and bituminite; only above the "Fischschiefer", terrigenous organic matter becomes dominant. In most of the section, the organic matter is moderately well preserved as expressed by Hydrogen Index values from Rock-Eval pyrolysis between 150 and 450 mg HC/g Corg for most of the samples. For part of the Upper Albian sequence, high-resolution measurements of carbonate content reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g m-2 a-1. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks.
To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on depositional patterns than global events.
B09 : 4A/05 : G5
Early Lower Cretaceous (Berriasian - Valanginian) Wave - Current System of the Northern Hemisphere (Sea - way Regulation of Upvellings: Modelling)
Evgenij J. Baraboshkin (Barabosh@geol.msu.ru) &
Yuri V. Volkov (Barabosh@geol.msu.ru)
Dept. Historical and Regional Geology Geological Faculty, Moscow State University, 119899, Vorobjovy Gory, Moscow, RUSSIA
The analysis of ammonite distribution (Berriasian - Early Hauterivian, the latest compilation) in the Northern Hemisphere provides a possibility for reconstruction of sea-ways and fauna migration (Baraboshkin, 1997). Those reconstructions gave a possibility to applicate Volkov's (1997) methodics of wave-current system modelling of the Northern Hemisphere. An ancient upvelling system was reconstructed in the result. The plate - kinematik reconstructions by Rowley and Lottes (1988) were used for the work.The high pressure zone was considerably reduced during the Early Berriasian (=Late Volgian) up to Early Hauterivian. An opening of the Polish Furrow sea-way determined an intensive northward fauna migration in the late Early Berriasian. The Boreal / Tethyan connection sea-connection via the Russian Platform did not exist yet, but Trans-Urals sea-way was opened (it was determined firstly). It provided the development of strong circle-directed currents and upvelling systems in the Russian Platform and West Siberia, where phosphorites and oil-shales ("Bazhenov Horizon") were deposited.The situation has changed considerably when North-Caspian sea-way was opened in the Late Berriasian ("Ryasanian"). The strong Tethyan water input through that sea-way lead to the reduction of phosphate sedimentation in the Russian Platform. The narrowing of Trans-Urals sea-way resulted in expansion of the upvelling area in the Bazhenov sea. An expansion of that strait in the Late Valanginian effected significantly on the organic matter productivity of the West Siberia Basin. The Tethyan sea-connection with the Russian Platform was strongly reduced or even missing during that time. It determined the development of endemic fauna in the Russian Platform and very strong decreasing of the sea productivity.The work was done with the financial support of RBSF (Grants 97-05-65567 and 98-05-64195).
Sedimentary, Biological and Isotopical Record of a Late Early Cretaceous Paleoclimatic Event in the Pieniny Klippen Belt, Slovakian Western Carpathians
Jozef Michalik (geolmich@savba.savba.sk)1,
Daniela Rehakova (geolreha@savba.savba.sk)1,
Otilia Lintnerova (lintnerova@fns.uniba.sk)2,
Daniela Boorova ((boorova@gssr.sk))3,
Eva Halasova (halasova@fns.uniba.sk)4 &
Jana Hladikova6
1 Geological Institute SAS, Dubravska 9, 842 26 Bratislava, Slovakia
2 Dept.Mineral Resources, Comenius University, 842 15 Bratislava, Slovakia
3 Geological Survey of Slovak Republic, Mlynska dolina 1, 817 04 Bratislava, Slovakia
4 Dept.of Geology, Comenius University, 842 15 Bratislava, Slovakia
Sedimentological, biological and chemical proxies of Barremian / Aptian climate perturbations have been studied in Mt Rochovica (Pieniny Klippen Belt). Its pelagic carbonate sequence influenced by a clastic input from elevated Czorsztyn Ridge (Tithonian / Berriasian limestone microbreccia) and by fluxoturbidites derived from an unknown carbonate buildup has been deposited on a distal edge of the Paleoeuropean shelf. Interruption of carbonate deposition by terrigeneous Koòhora Formation has been interpreted as the consequence of a humid event of starting Mid Cretaceous greenhouse. Rhythmic pattern comparable with precipitation / runoff cycles is punctuated 1) by limestone / marl alternation in Valanginian- and also in highstands of upper part of the Pieniny Lst Formation, 2) by contourite intercalations of Hauterivian transgressive tracts of the Pieniny Fm, as well as 3) by fine alternating terrigenous and biogene laminae in the Koòhora Fm. 4) This pattern was disturbed by calciturbidite regime of the Barremian Vranie Member reflecting carbonate platform growth in shallowing sea. Similar tendences appeared in the overlying Upper Aptian/Lower Albian Brodno Formation. During late Early Aptian humide climatic event, a huge amount of terrigeneous material was repeatedly transported into Kysuca Basin. Deposition of the material brought by rivers has been interrupted by temporal drier seasons with high radiolarian- and (later) nannoconid - foraminiferal primary production. Three (detrital, productive and stagnant) models of anoxia have been recognized in the sedimentary regime of the Koòhora Formation. Two major partial 13C excursions have been recognized in the Upper Barremian - lowermost Albian record of this isotope ratio. The lower one coincides with the increased Corg content in the uppermost part of the Vranie Member. The upper one starts in the uppermost parts of the Koòhora Fm and continues in the Brodno Formation. These two peaks are separated by less positive part of the 13C curve belonging to the Koòhora Fm, representing runoff / temperature perturbation of the marine productivity regime.
B09 : 4A/09 : G5
The Distribution of Planktonic Foraminifera and Their Contribution to our Understanding of Cretaceous Palaeoclimates
Malcolm Barrie Hart (mhart@plymouth.ac.uk)
Dept. of Geological Sciences, University of Plymouth, Plymouth PL4 8AA, United Kingdom
During the Cretaceous the planktonic Foraminifera underwent a major diversification, colonising a wider range of water depths and geographical regions. Bandy (1961) was the first to recognise the potential of using this regional distribution to reconstruct different palaeoceanographic regimes. Using new data from the South Atlantic Ocean, Antarctic Ocean and Indian Ocean it is possible to show the poleward migration of warm-water taxa during the Cretaceous and compare it with data already available for the North Atlantic Ocean. Instead of the present-day 9 latitudinal zones based on planktonic Foraminifera it is only possible to identify 5 with any degree of reliability. These are, from north-to-south, the 'boreal', 'transitional', tropical, transitional and 'austral'. In some of the developing oceans during the Cretaceous (e.g., eastern Indian Ocean) there are local biogeoprovinces with quite distinctive local assemblages or morphotypes. While these do not reflect on the climatic zonation they provide an interesting insight into the development of the plankton. Migration into higher latitudes can be demonstrated against currently published time scales. The foraminiferal distributions are compared with climatic maps produced by the assessment of other data sources.
Bandy OL, Micropaleontology, 13, 1-31, (1967).
B09 : 4A/10 : G5
High Resolution Stable Isotope Records from the Upper Cretaceous of Italy and Spain: Glacial Episodes in a Greenhouse Planet?
1 Dept. de Geologia, Univ. de Oviedo, c/Arias de Velasco, 33005 Oviedo, Asturias, SPAIN
2 Dept. of Earth and Planetary Sciences, Harvard University, 20 Oxford St, Cambridge, MA 02138, USA
High resolution 13C and 18O records from Upper Albian to Lower Santonian pelagic carbonates of the Contessa Quarry section in Italy exhibit large positive oxygen isotopic excursions of ~ 1 permil in the Lower Cenomanian and Upper Turonian - Coniacian. Within the uncertainties of biostratigraphic correlation, these positive excursions appear to correspond to times of large sea level regressions in global sequence stratigraphic sea level curves. Several lines of evidence suggest that the major 18O excursions in Contessa reflect episodes of global cooling and not differential diagenesis. Numerical models of oxygen isotope exchange during diagenesis show that a high contrast in the degree of alteration would be required to produce these signals as artefacts of diagenesis, and sediment preservation data provide no evidence for such large contrasts in the degree of alteration. Furthermore, the general sequence of major positive excursions is reproduced in the Santa Ines 18O record from the south of Spain; it is unlikely that differential diagenesis would produce similar artefacts in multiple sites. One explanation for the link between episodes of global cooling and sea level falls is that global cooling events led to polar ice sheet accumulation, lowering sea level. While not independently verifiable, the ice sheet explanation for the link between rapid sea level regressions and climatic cooling is appealing because it provides a mechanism for the origin of the rapid global sea level changes observed in the sequence stratigraphic record, as well as their correlation with climate events.
B09 : 4A/11 : G5
Cretaceous Laminated Sediments as Climate Archive
Jürgen Thurow (j.thurow@ucl.ac.uk) &
Alexandra J. Nederbragt
Department geological Sciences, University College London, Gower Street, London, UK
In the modern icehouse climate, ice volume fluctuations control and enhance climatic trends, which are recorded in sediments. However, for long periods of the Phanerozoic, significant iceshields did not exist, yet apparently climate was controlled by processes similar to modern ones, e.g. Milankovitch cyclicity, solar activities. It is crucial for climate modelling to understand the natural background variability. The extent of decadal to millennial scale climatic variability in a greenhouse world can be documented from Cenomanian/ Turonian laminated sediments of the Tarfaya Basin (S. Morocco). In this high-productivity, open marine, but anoxic basin, sedimentation rates were high to the extent that the upper Cenomanian to middle Turonian is expanded to 200 m. Individual lamina couples consist of a simple alternation between well preserved coccoliths (light) and organic carbon (dark), allowing for an ultra-high resolution study of carbonate flux. We established a Milankovitch stratigraphy on the presence of pronounced 40 ka obliquity cycles, attributed to variations in the trade winds pattern. Milankovitch-cycles are on the meter-scale, and are overprinted by light dark/alternations at scales ranging from decimeters to micrometers. Decimeter-to-centimeter-scale variation is best expressed in high-resolution (2 mm) carbonate and iron measurements. Laminae-scale variation was analysed from a continuous colour record spanning the whole event, using digital images with a resolution of 0.05 mm. The Milankovitch cyclicity provides a first order estimate of a ca. 11 year duration for pronounced light/dark couples, with bundling into ca. 100 year and longer cycles. Detailed investigations show higher frequency variations, which may correspond, to annual productivity changes. Potential annual couples, however, must be on a micrometer scale reflecting the almost negligible terrigenous input from the Saharan hinterland. SEM techniques reveal very thin flasers (clayey? organic-rich?) intercalated between much thicker layers, often rich in faecal pellets consisting of monospecific coccoliths accumulations. The wide range of frequencies of variation in the Tarfaya record provide the opportunity to unravel the natural variability of background climate signals in a Greenhouse-setting.
B09 : 4A/12 : G5
Milankovitch-Scale Palaeoclimate Changes in Pale-Dark Bedding Rhythms from the Cretaceous of NW Europe: Results of a Multidisciplinary Approach
Alastair Ruffell (a.ruffell@qub.ac.uk)1 &
Joerg Mutterlose
(Joerg.Mutterlose@ruhr-uni.bocum.de)2
1 School of Geosciences, Queen's University, Belfast, N. Ireland
Abstract Marine early Cretaceous sediments of the North Sea borderlands (north Germany, eastern England) are dominated by fine-grained siliciclastic deposition with minor sands and carbonate components. Occasionally this depositional system is characterised by visually striking bedding rhythms (Nebe & Mutterlose, 1998). These comprise calcareous and less calcareous clays or pale - dark alternations (P - D rhythms) on a scale of 0.5 - 1 m for each of the rhythms. Such rhythmic sedimentation becomes most dominant in the early to late Hauterivian, parts of the early Barremian and in the Aptian. This study concentrates on late Hauterivian sections in England and Germany. Although these P - D rhythms lack the contrasting lithologies of the limestone - shale alternations found in Tethyan successions to the south (Cottilon, 1987), they have remained relatively unaffected by deep burial, tectonism or diagenesis. For this reason, changes in geochemistry can be related to floral/faunal alternations and thus directly to palaeoenvironment. The range of palaeontological and geochemical analyses we have conducted show conclusively how bedding rhythms, developed in the Milankovitch band, are the result of palaeoclimate change. Pale layers were "Tethyan influenced" in their microflora/fauna. They indicate warm surface waters, poor in nutrients developed during warm, seasonally arid hinterland climates. Dark layers were "Boreal influenced", displaying features consistent with cooler waters, rich in nutrients. Such dark layers reflect cooler, more humid hinterland conditions. The coincidence of Tethyan taxa with groups ("bundles") of pale layers displaying high gamma-ray emission and elevated radioactive element contents suggests transgressive or condensed beds. Thus the bundling of P - D rhythms coincides with both the long-term Milankovitch Eccentricity Cycle and the duration of the parasequence; they are most likely one and the same. This presentation will document a direct link between the reaction of marine floras and floras to sequence stratigraphic and Milankovitch-scale changes in bed lithology and thickness.
Cotillon, P Bed-scale cyclicity of pelagic Cretaceous successions as aresult of world-wide control, Marine geology, 78, 109-123, (1987).
Nebe, DW and Mutterlose, J Lower Cretaceous pale-dark bedding rhythms, a result of Milankovitch cycles, Zbl. Geol. Palaont, Teil I 98, in press, (1998).
B09 : 4A/13 : G5
The Impact of Sea Level Changes on Mid-Cretaceous Climate
Frédéric Fluteau (fluteau@ipgp.jussieu.fr)1,
Jean Besse1 &
Gilles Ramstein (ramstin@lsce.saclay.cea.fr)2
1 Labo. de Paléomagnétisme, IPGP - 4 place Jussieu, PARIS cedex 05, FRANCE
2 LSCE/DSM, Bat 701- CEA Saclay, 991191 Gif sur Yvette, France
According paleodata, the Mid-Cretaceous climate is relatively warm, equable, with little evidence of glaciation. Numerical simulations performed with Atmospheric General Circulation Model (AGCM) (Barron, 93) showed that such a climate may be reproduced using both an elevated atmospheric carbon dioxide rate and a stronger oceanic heat transport from equator to pole, the paleogeography playing a minor role. However, the Mid-Cretaceous is punctuated by important sea level changes, reaching +150 m between the Aptian (120 Ma) and the Cenomanian (94 Ma). Until now, the impact of marine transgressive event on the Mid-Cretaceous climate has been poorly investigated. Accurate paleogeographic global reconstructions accounting for sea level changes and based on paleomagnetic and geological data have been realised for the Cenomanian and the Aptian and used to constrain the AGCM. A set of numerical simulations highlights the impact of sea level fluctuation and other paleogeographical changes on the climate and the atmospheric circulation. Paleoclimatic indicators are then used to confirm these numerical results.
Barron et al., Paleoceanography, 8, 785-798, (1993).
B09 : 4A/14 : G5
Mid-Turonian Climatic Change and Atmospheric CO2 Level Evidence from Paleosols in Israel
Amir Sandler (sandler@mail.gsi.gov.il)1 &
Eytan Sass (esassa@vms.huji.ac.il)2
1 Geological Survey of Israel, 30 Malkhei Yisrael St., Jerusalem 95501, Israel
2 Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
During the Mid-Turonian, the central area of Israel, a part of the Tethyan carbonate platform, emerged and was governed by continental conditions for a few hundred thousands of years. The exposed flat limestone terrain was affected by vadose karst activity which created vertical shafts down to a depth of ~20 m. Braided and low-angle meandering rivers deposited sand in channels and sandy clay in over-bank environments on top of the karstic relief. Soils that developed on these siliciclastic sediments are characterized by prismatic and blocky structures, clay cutans, slickensides, masepic and associated microfabrics, rootlet structures, and calcite glaebules in the Bk horizon, which in places coalesce into a rigid calcrete. Accordingly, these paleosols are regarded as an equivalent to modern calcic vertisols, which develop characteristically in clayey smectitic substrate in seasonal tropical to warm temperate climates. The described paleosols are distinguished, however, by the predominance of palygorskite, which often exceeds 90% of the clay fraction. Several macro and micro features indicate that the palygorskite is authigenic and an essential part of the soil forming processes. All these point to a progressive aridization from a Caribbean-type climate right after exposure, through warm climate with pronounced seasonality under which original vertisols were formed, to semi-arid climate when calcretization and palygorskitization of the vertisols took place. Stable isotope analysis of pedogenic calcite, underlying and overlying marine limestone beds resulted in mean values (permil, relative to PDB) of -6.6±0.7, -3.8±1.6, -1.7±2.0 respectively for 13C, and of -5.3±1.0, -6.4±1.2, -6.1±0.80 respectively for 18O. Since the studied calcitic glaebules fulfill the criteria put forward by Cerling (a pre-C4 plants stage, formation of pedogenic calcite at >30 cm depth, no modification by late diagenesis), their 13C results were used to calculate CO2 partial pressure of the Mid-Turonian atmosphere by his diffusion-reaction model. The range of the 13C data were calculated to have CO2 concentrations similar to the 2500 to 3000 ppmV calculated by Cerling for the Lower Cretaceous and are consistent with other model calculations. These data and calculations seem to be the first ones made for the Turonian, and reduce the existing gap of calculated pCO2 record between the Albian and the Senonian.
Session B09:4B
B09 : 4B/25 : G5
The Isotopic Evolution of Triassic Seawater: Paleoenvironmental Implications
Christoph Korte
(Christoph.Korte@ruhr-uni-bochum.de)1 &
JánVeizer (veizer@geol.uottawa.ca)2
1 Institut für Geologie, Ruhr-Universität Bochum, 44801 Bochum, Germany
2 Ottawa-Carleton Geoscience Center, University of Ottawa, Ontario K1N 6N5, Canada
The new high resolution 87Sr/86Sr, 18O and 13C isotope curves for Triassic seawater were established by measurements on brachiopods and conodonts. Their degree of preservation was monitored by optical microscopy, SEM, ICP-AES and PIXE. For the brachiopod samples, only samples with less than 250 ppm Mn and more than 400 ppm Sr were utilized. The 18O of brachiopod shells varied between -0.3 and -4.9‰ (PDB), indicating that the paleotemperatures of the Tethyan sea were about 10 - 30°C. In contrast, the values for the Muschelkalk brachiopods were -1.9 to -6.2‰, suggesting paleotemperatures for the mid-Triassic German basin of about 15 - 36°C, about 4 - 6°C warmer than the Tethyan counterpart. The 13C of Triassic brachiopods oscillates around 0.5 to 0.8‰ (PDB) up to the Ladinian, followed by a 2‰ positive jump at the Ladinian/Carnian boundary, values around 3‰ in the Late Carnian and a decline to 2‰ in the Rhaetian. The Ladinian/Carnian jump coincides with the termination of the so called "coal gap". In the Late Triassic, the 13C appears to correlate positively with sea level stands. Overall, the 13C oscillations likely reflect the degree of 12C withdrawal by deposition of organic matter on land and sea and its return by weathering processes into the ocean/atmosphere system. The 87Sr/86Sr trend shows a steep rise, from 0.7074 to 0.7082, during the early Triassic (Brahman to Olenek). This coincides with the uplift of the Appalachians and shedding of large quantities of clastics into the Tethys. The subsequent waning of this clastic input is accompanied by a decline in 87Sr/86Sr to 0.7075. A renewed clastic phase, commencing with the Raibl-event (Jul) and reflecting the uplift of Kimmerides, Gondwanides and Indonesiades, coincides again with an 87Sr/86Sr enhancement to 0.7082 in the Upper Norian.
B09 : 4B/26 : G5
Jurassic Oceans: Sea-Surface Temperatures and Thermal Structure at Tropical Latitudes (the Paris Basin)
S. Picard (spicard@ens-lyon.fr)1,
Christophe Lécuyer
(clecuyer@geologie.ens-lyon.fr)1,
Jean-Pierre Garcia (jpgarcia@u-bourgogne.fr)2 &
Simon Sheppard
(ssheppar@geologie.ens-lyon.fr)1
1 Laboratoire de Sciences de la Terre, CNRS UMR 5570, Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon cedex 07, France
2 Biogéosciences-Dijon, Centre des Sciences de la Terre, CNRS UMR 5561,
To determine the thermal structure of Jurassic seas at tropical latitudes, we propose to evaluate horizontal and vertical temperature variations in the water column over the Paris basin (500 km x 500 km). 18O versus SMOW differences in the water column were obtained from analyses on teeth (apatite) from surface water dwellers, marine reptiles and fish (selacians and pycnodonts) and on co-existing benthic brachiopods (calcite). 12 selacians of the same genera show a restricted range of 18O values (18.7‰ to 20.5‰), close to those of pycnodonts (18.7‰ to 20.6‰) and reptiles (19.4‰ and 19.7‰) with an isolated value for one reptile (21.7‰). Brachiopods have 18O values increasing from 26‰ to 30.9‰ from the shallowest to the deepest environments defined on sedimentologic criteria. Assuming 18O of sea water ( W) is constant, measured 18O-calcite - 18O-apatite differences of 8 to 11.2‰ imply vertical temperature differences from 0 to 12°C going from the shallowest (shoreline) to the deepest parts (offshore below photic zone) of the basin. With water depths < 200 m based on sedimentologic criteria, a surface to bottom water temperature difference of 12°C implies a vertical thermal gradient close to modern tropical shelf sea values 1/14°C.m-1.
Sea surface temperatures (SST) estimated from fish apatite range from 19°C to 27°C for an ice-free world and about 4°C higher for oceans with an ice-cap similar to present ( W = 0‰). Such SST are consistent with SST measured on modern analogs such as Bahamas or West-Australian shelves (~18-30°C) but are lower than proposed Cretaceous values (around 30°C). Based on 20 samples, maximum SST tend to increase from the Bajocian (23°C) to the Upper Bathonian-Lower Callovian (26-27°C) but are lower in the Upper Oxfordian (22°C), supporting palynological data.
The maximum latitudinal SST thermal gradient of 2°C per degree of latitude is recorded in the Upper Callovian from 25 to 30°N on apatite data. Thus, we propose to shift to 30°N the limit between a warm tropical seasonless belt and a variable cool zone, previously defined at 45°N. Temperature variations with latitude, compiled from previous work, show a marked asymmetry between both hemispheres with temperature in the South decreasing more progressively. Previous climatic models for the Upper Jurassic, based on a CO2 level of 1120 ppm, predict a similar form for the thermal gradient but at higher global temperatures.
B09 : 4B/27 : G5
Palaeoproductivity During the Toarcian Oceanic Anoxic Event as Recorded by Nitrogen-Isotope Ratios from the Jet Rock (Mulgrave Shale, Yorkshire, UK)
Darren Gröcke (darreng@earth.ox.ac.uk) &
Hugh Jenkyns (hughj@earth.ox.ac.uk)
Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, UK
Organic matter in the black shales of the Jet Rock (Mulgrave Shale Member of the Whitby Mudstone, Lower Toarcian, north-east Yorkshire) has been analysed for nitrogen-isotope, carbon-isotope and and total organic carbon (TOC) in order to investigate palaeoproductivity changes during the early Toarcian Oceanic Anoxic Event. The sequence is characterized by a negative carbon-isotope excursion in 13Corg centred in the lower exaratum Subzone, followed by a positive shift centred in the upper exaratum Subzone. This positive shift is also registered as 13Ccarb in belemnites from the Jet Rock, where values exceed 6‰. Maximum values of TOC correlate with relatively low values of 13Corg in the lower exaratum Subzone; and nitrogen-isotope ratios show a positive excursion at this same level. In recent oceans, relatively high values of 15N are characteristic of water masses where nitrate utilization is high due to enhanced biological uptake and/or where denitrification has taken place. Hence the correlation between more positive 15N values and higher TOC values can be simply related to relatively elevated rates of productivity of organic-walled plankton, high fluxes of organic matter to the sea floor and concomitant high rates of burial of this material. It is possible that intense upwelling of poorly oxygenated waters, where much organic carbon had been oxidized and denitrification had taken place, returned 13C-poor and 15N-rich waters to the photic zone where they could be directly utilized by phytoplankton: this would explain the isotopic records from the lower exaratum Subzone of the Jet Rock. The main positive 13Corg and 13Ccarb excursion is registered, wherever it is accurately dated in the UK, in the upper exaratum Subzone; in southern Europe it is dated simply as falciferum Zone. In all investigated localities, however, this excursion occurs higher in the succession than the most carbon-rich sediments. This presents something of a paradox, as positive 13C excursions are conventionally interpreted as reflecting an increase in the size of the global reduced carbon reservoir, most typically by an increase in productivity and rate of burial of organic matter in the world ocean. If this model is correct, the global depositional rate of organic carbon must have been at its maximum during late exaratum-Zone time, and the major sinks for this material may lie outside Europe.
B09 : 4B/28 : G5
Long Term Sedimentological and Palaeontological Variations in the Early Cretaceous: Climatic Versus Sea-Level Control
Jörg Mutterlose
(joerg.mutterlose@ruhr-uni-bochum.de) &
Max G. E. Wippich
(max.wippich@ruhr-uni-bochum.de)
Institut für Geologie, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
Sedimentological and palaeontological variations recorded in the stratigraphy of a certain area reflect the interaction of local tectonics, global eustasy and - most difficult to decipher - climate. Large scale changes of sedimentation pattern of the early Cretaceous were not necessarily related to major tectonic events or dramatic changes in global sea-level. It is here suggested that climate was superimposed on both tectonics and eustasy. Humid climate led to enhanced weathering and abundant clastic supply, resulting in expanded successions of coarse clastics without major hiatuses. Arid climate diminished detritic supply and favoured carbonate sedimentation as well as frequent stratigraphic condensation. Variations of biota are illustrated by the changing biogeographic distribution of early Cretaceous cephalopods (ammonites, belemnites). Most of them were restricted to climatically controlled faunal realms. Eustasy and tectonics provided marine sea-ways allowing, or prohibiting, faunal migration between different basins. Climatic changes, on the other hand, shifted the boundaries of biogeographic realms. The resulting pattern is that of an alternation of endemic with cosmopolitan faunas through time.The impact of earliest Creatceous (Berriasian-Valanginian) climatic change is discussed in a case study based on data from two palaeoceanographically different settings. The shallow epicontinental NW German Basin occupies a position intermediate between the Artic-Boreal and Tethyan realms. Regressive conditions during the Berriasian are represented by non-marine Wealden-type sediments. Narrow sea-ways opened in earliest Valanginian times towards both faunal realms. An episode of enhanced clastic sedimentation is evident in late early to late Valanginian times.The Agadir-Essaouira Basin (Morocco) allows for onshore field studies of the Central Atlantic passive continental margin. Carbonate sedimentation dominated in Berriasian to earliest Valanginian times with extensive carbonate platforms being developed on the shelf. These became drowned due to an abrupt shift towards siliciclastic sedimentation in late early Valanginian times. The sudden increase of clastic supply is synchronous in both study-areas and coincide with an overall rise in sea-level. The shift towards clastic sedimentation is here interpreted to be caused by a major change from arid to humid climate in late early Valanginian times.
B09 : 4B/29 : G5
Evidence of Climatic Change During the Middle Jurassic, UK
Catherine Elizabeth Ross (lizr@earth.ox.ac.uk)
Department of Earth Sciences, Oxford University, Parks Road, Oxford, Britain
The Middle Jurassic climate in Britain has traditionally been regarded as warm and humid. Recent palynological (Boulter & Windle, 1993) and isotopic evidence (Hendry, 1997) yields tentative indications that the late Bajocian and Bathonian may have contained one or more periods of relative aridity in comparison to the Aalenian. A detailed quantitative palynofacies study was conducted on the predominantly fluvio-deltaic Aalenian to Bathonian, Ravenscar Group of the Cleveland Basin, UK. Palynofacies analysis, which is the study of microscopic organic matter, can, in association with sedimentological and palaeoecological data, provide temporal and spatial resolution in the characterisation of sedimentary environments and potentially, vegetation and climate. Variations in the palynofacies assemblages from the Aalenian to Bathonian, reflect temporal and spatial changes, including variations in the redox status, hydrodynamics, proximity to source, and salinity, and permit the identification of palynofacies sequences. An increase in the abundance of black fragments (charcoal) and classopollis pollen, an indicator of increased intensity of warmth or seasonally dry periods, was detected in the early Bajocian, in comparison to the Aalenian. This suggests that the early Bajocian probably included warmer periods and/or was more seasonal which is earlier than has been previously thought. The increased abundance of charcoal is based upon the detection of an increase in the ratio of black to brown fragments in the early Bajocian, in comparison to the Aalenian. This increase was detected in three sections which were probably sourced from different parts of the basin. The black fragments occur; in both oxic and anoxic sediments, in conjunction with brown fragments, and in a range of lithologies. These varied associations suggest that the black fragments are probably charcoal rather than post-depositionally altered organic matter. The lower ratio of black to brown fragments in the Aalenian, which contains similar depositional environments to the Bajocian, implies that the change is due to an increase in production of carbonised material, probably in response to increased conflagration, and is not a result of preferential deposition.
Boulter MC & Windle T, Special Papers in Palaeontology, 49, 125-154, (1993).
Hendry J, British Sedimentological Research Group, Abstract Volume, 40, (1997).
B09 : 4B/30 : G5
The Posidonia Shales (Toarcian) of SW-Germany: A new Depositional Model Based on High Resolution Geochemical and Palaeoecological Investigations
Wolfgang Oschmann
(wolfgang.oschmann@uni-tuebingen.de)1,
Annette Schmid-Röhl
(annette.schmid@uni-tuebingen.de)2,
Jochen Röhl (jochen.roehl@uni-tuebingen.de)2,
Andreas Frimmel
(andreas.frimmel@uni-tuebingen.de)2 &
Lorenz Schwark
(schwarkl@geozip.geo.uni-koeln.de)3
1 Geologisch-Paläontologisches Institut, Senckenberganlage 32-34, Postfach 111932 Johann Wolfgang Goethe Universität, D-60054 Frankfurt
2 Institut für Geologie und Paläontologie, Universität Tübingen, Sigwartstr. 10,
3 Geologisches Institut, Universität zu Köln, Zülpicher Straße 49a, D-50674 Köln
The Posidonia Shales (Toarcian, Lower Jurassic) in SW-Germany are famous for their unique fossils. Skeletons of vertebrates and invertebrates are excellently preserved. The rocks of the Posidonia Shales in general consist of fine grained mixed clastic and carbonate sediments with a high content of organic matter. There is an ongoing discussion since decades weather or not there existed a permanent redox boundary within the water column of the Posidonia Shale Sea and resulting anoxic to euxinic bottom water conditions. On various sections within the Toarcian of SW-Germany detailed investigations have been carried out with a vertical sample spacing of approximately 2 cm combining geochemical, palaeoecological and sedimentological methods in order to contribute to the controversial discussion.
The Posidonia Shale rocks can be described as a four component system which consists of organic mater, carbonate, pyrite and siliciclastics. The lateral consistence reveals a striking similarity in geochemical parameters (e.g. total organic carbon, sulphur). Also, faunal distribution and sedimentological data correlate well laterally. The faunal investigations and c/s-plots (based on organic carbon and sulphur) were used to determine the benthic oxygen levels for narrow intervals and were combined to a high resolution oxygen curve throughout the Posidonia Shale. In general the environment was strongly oxygen depleted to anoxic, however, punctuated by short oxygenated intervals which may last from several month to several years.
Analysis of stable isotopes ( 13C-TOC, 13C-carb. and d18O-arb.) favour sea-level changes within the Torcian epicontinental sea as the primary factor controlling the depositional environment. The 18O-arb. analysis indicate a surface water body with slightly reduced salinity values and a monsoonal typ climate. The combination of all data permits description of a new seasonal accentuated and sea-level controlled depositional model.
Session B09:4P
B09 : 4P/01 : PO
Late Jurassic Isotopic and Sedimentary Response to Greenhouse Climate and Sea Level Change
Maureen Padden (padden@erdw.ethz.ch) &
Helmut Weissert (helmi@erdw.ethz.ch)
Geolgical Institute, ETH-Zentrum, 8092 Zurich, Switzerland
Late Jurassic paleoenvironmental reconstructions provide an opportunity to observe the response of marine carbon pumps to high (at least 400 ppm according to previous estimates) atmospheric carbon dioxide concentrations. Climatic reconstructions suggest that the breakup of the supercontinent of Pangaea forced the climatic evolution from a monsoonal rainfall pattern to increasingly zonal circulation in the Cretaceous. Continuous Late Jurassic pelagic and platform limestone sequences deposited along the Tethyan margin offer the opportunity to trace the response of marine carbon system to changing oceanic circulation and climate. These sequences were sampled in the Northern Alps, Southern Alps and the Jura mountains. Sedimentary and isotope analysis of a paleotransect from near-platform to basinal deposits reveal both local variations of the Tethyan depositional environment as well as aspects of the carbon cycle with global implications. A composite carbon isotope record displays a highly variable (1 to 2 per mil) signal with distinct cyclicity. The amplitude of these variations decreases from the Oxfordian to the Kimmeridgian. In the late Kimmeridgian, the carbon isotopes become more depleted, a trend which continues through the Tithonian and is consistent with previous nannofossil data which suggest increasingly oligotrophic conditions in the Tethys towards the early Cretaceous. Oxygen isotope interpretations are complicated to some degree by alpine diagenesis but the general trend of the composite record displays an intriguing divergence during the Kimmeridgian-Tithonian. Enrichment of oxygen isotope from the Kimmeridgian through the Tithonian in the Southern Alps may indicate local cooling of surface water. Previous isotopic analysis of bulk carbonate in the Helvetic Alps and at Atlantic DSDP sites, and of belemnite rostra from the Swabian Alb all display oxygen isotope values which become more depleted in the Tithonian suggesting surface water warming.
B09 : 4P/02 : PO
Late Jurassic to Early Cretaceous Climate Change: New Data from the Volga Basin, Southeast Russia
Gregory D. Price (g.price@plymouth.ac.uk)1,
Alastair H. Ruffell2,
Joerg Mutterlose3,
Evgenij J. Baraboshkin4 &
Robert M. Kalin5
1 Dept. Geological Sciences, University of Plymouth, Plymouth, PL4 8AA, UK
2 Department of Geology, Queen's University of Belfast, Belfast, BT7 1NN, UK
4 Department of Historical & Regional Geology, Moscow State University, Vorobjovy Gory, 199899, Moscow, Russia
5 Department of Civil Engineering, The Queens University of Belfast, Belfast, BT7 1NN, UK
A broad pattern of climate change from the late Jurassic to early Cretaceous is now well established. Cool, arid climates are thought to typify the late Kimmeridgian to late Ryazanian and Barremian to early Aptian, separated by generally warm and wet periods. Although minor reversals of such trends are common, the broad pattern may be applied to many of the successions of northern and western Europe. In order test the validity of these European-based models of palaeoclimatic and environmental change, the boreal succession of the Volga Basin (near Ulyanovsk) southeast Russia was investigated.
The Volga Basin is an area of intra-cratonic subsidence bounded to the south and west by the foreland of the Caucasus, to the east by the Urals and to the north by a gentle tectonic slope onto the Russian Platform. The Jurassic - Cretaceous succession is dominated by mudstones with subordinate sandstone, black shale and phosphatic nodule beds. This research concentrates on clay mineralogy, oxygen and carbon isotopes and fauna analysis, conducted using a bed-by-bed sampling programme. Clay mineral analysis shows a correlatable progressive reduction in kaolinite concentrations and increases in smectite and illite clays through the Volgian and dominance of smectite in the Aptian black shales. The smectite within these sequences may represent to "last to settle" clay mineral, deposited during periods of anoxia in the water column, aridity in the hinterland and the early stages of a rise in regional sea-level. Preliminary analysis of oxygen and carbon isotopes determined from well preserved belemnites (including Acroteuthids and Praeacroteuthids) from the succession display a pattern compatible with coeval boreal isotopic profiles. Such observations suggest that the overall climatic signature in the Volga Basin is similar to the western European successions but with important differences and emphasises the need for such studies.
B09 : 4P/03 : PO
Monsoon-Like Climate During Bajocian: Microfacies and Geochemistry of Rhythmic Deep-Water Marl/Limestone Deposits from the Mecsek Mountains, Southern Hungary
Béla Raucsik (raucsik@almos.vein.hu)1,
Attila Demény (demeny@sparc.core.hu)2,
Gyula Szabó (szgy@cseles.atomki.hu)3 &
Ildikó Borbély-Kiss (ibkiss@cseles.atomki.hu)3
1 University of Veszprém, Department of Earth and Environmental Sciences, H-8201 Veszprém, P.O.B. 158., Hungary
2 Laboratory for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaõrsi út 45., Hungary
3 Institute of Nuclear Research, Hungarian Academy of Sciences, H-4001 Debrecen, P.O.B. 51., Hungary
In the Mecsek Mountains (Tisza Terrain, Southern Hungary) an Aalenian-Bajocian rhythmic limestone-marlstone alternation crops out. To explain the origin of characteristic bedding phenomenon of Komló Calcareous Marl Formation microfacies, clay mineralogical composition, major, trace element and stable isotope geochemistry were examined. Samples are collected from a profile representing the major part of the Bajocian stage.According to results of thin section analysis, hemipelagic processes without any erosive redeposition seem to be the main control on sedimentation. The clay fraction is dominated by illite and illite/smectite mixed-layer phases. Kaolinite is rarely found. Mixed-layer illite/smectite is characterized by 40-70 percent illite proportion and mostly R=0 or R=1 type ordered interstratification (Reynolds and Hower, 1970). This group of clay fraction documents erosion of smectite-rich soils developing under warm and seasonally humid climate. Illite proportion in mixed-layer structure indicates 100-110°C maximum heating temperature during burial (Hoffman and Hower, 1979). Discrete illite is abundant, it seems to be not altered by burial diagenesis. The sparse occurrence of kaolinite, chlorite and abundant mixed-layer phases besides of illite suggest a relatively distant source area during deposition. The abundance of the clay mineral types not correlate with the lithologies suggesting, that processes forming the alternation of carbonate-rich/carbonate-poor semicouplets could not affect directly the formation of clay minerals. Stable isotope values of samples show higher 18OPDB and 13CPDB values in carbonate-rich beds relative to carbonate-poor beds. Diagenetic carbonate dissolution and reprecipitation seem to be not efficient to produce rhythmic layering. Some diagenetic overprint on the stable isotope signal can not be excluded. Carbonate-rich semicouplets were forming during periods characterized by enhanced bioproductivity and relatively dry and/or cool climate. Carbonate-poor semicouplets, in the contrary, seem to be formed during periods characterized by enhanced dilution from continental terrigenous sources and relatively wet and/or hot climatic conditions.Carbonate-rich semicouplets are characterized by good oxigenation according to Fe/Mn ratio, and concentrations of redox sensitive trace elements. High P/Ti, Sr/Ti ratios, low Ti values indicate enhanced bioproductivity and moderate terrigenous supply during their deposition. The carbonate-poor semicouplets are characterized by opposite trend of geochemical data. Dysoxic conditions and moderate bioproductivity prevailed during their deposition. The sedimentation of Komló Formation was controlled by changing conditions. These alternating conditions are 1) efficient mixing, highly fertile surface waters, and well-oxygenated seafloor to 2) enhanced runoff or decreased evaporation, or both, and weakly oxygenated bottom waters. The changes could be results of alternating estuarine/anti-estuarine circulation of the basin controlled by seasonally alternating evaporation and freshwater runoff, i. e. monsoon-like climatic conditions. This observation is in accordance with climatic models for western Tethyan realm during Jurassic (Weissert and Mohr, 1996).
Reynolds Jr RC & Hower J, Clays and Clay Mins, 18, 25-36, (1970).
New Stable-Isotopic Evidence for Long-Term Cretaceous Oceanographic and Climatic Change
Leon Clarke (l.clarke@ic.ac.uk)1 &
Hugh Jenkyns (hughj@earth.ox.ac.uk)2
1 NERC ICP-MS Facility,TH Huxley School, Imperial College at Silwood Park, Ascot, Berkshire SL5 7TE, UK
2 Department of Earth Sciences, University of Oxford, Parks Road, Oxford, Oxfordshire OX1 3PR, UK
Fine-fraction isotopic records from low-latitude Pacific DSDP Sites 167 and 463 illustrate long-term Cretaceous oceanographic and climatic change. Composite fine-fraction 13C and 18O records for the southern-hemisphere mid-latitudes are compiled from Indian Ocean Exmouth Plateau ODP Holes 762C, 763B and 766A. Assessment of low-latitude, composite mid-latitude and previously published 18O data confirms that a Cretaceous global climatic optimum was achieved sometime between Cenomanian-Turonian boundary time and the Middle Turonian. During this interval, surface-ocean palaeotemperatures were the highest of the last 115 million years. Superimposed upon Cretaceous long-term temporal climatic trends were periods of cooling and warming that reversed the general patterns. Pacific low-latitude palaeotemperatures can be extrapolated using mid-latitude temporal climatic evolution and palaeotemperature data from well-preserved Maastrichtian rudistid aragonite and high-magnesium calcite from Pacific guyots. Such an extrapolation suggests that Pacific mid-Cretaceous low-latitudes were at times as warm as ~33-38°C.
Foraminiferal 13C and 18O records, from low-latitude Pacific DSDP Site 463 and mid-latitude Indian Ocean ODP Hole 762C, broadly parallel the long-term temporal oceanographic and climatic trends observed from fine-fraction isotopic data described above. Planktonic and benthic foraminiferal isotopic data indicate intensification of mid- to Late Cretaceous surface- to deep-ocean isotopic gradients. Assessment of two palaeoproductivity proxies, surface- to deep-ocean carbon-isotope gradients ( 13C) and Babiogenic, however, indicates no evidence for an increase in surface-ocean palaeoproductivity through the mid- to Late Cretaceous. Climatic cooling during this interval could not, therefore, have been the result of a continual inverse greenhouse effect driven by sequestration of atmospheric CO2 due to expanding marine biomass. Intensification of mid- to Late Cretaceous 13C could alternatively have been due to enhanced oxidative potential in deep-ocean waters, since intensification of 13C was controlled by a greater temporal change in benthic 13C than that observed in planktonic 13C. Long-term mid- to Late Cretaceous cooling would have increased the solubility of O2 in the oceans and hence enhanced oxidation of organic matter in deep waters.
New fine-fraction and foraminiferal 18O data presented herein, in combination with previously published data from Europe, are consistent with the hypothesis that the Cenomanian-Turonian boundary Oceanic Anoxic Event was broadly coincident with a palaeotemperature maximum. Mid- to Late Cretaceous climatic cooling was thereby initiated by an inverse greenhouse effect due to sequestration of atmospheric CO2 as sedimentary organic carbon.
B09 : 4P/05 : PO
Short-Term Climate Changes Around the Middle-Upper Turonian Boundary Stable Isotopic Evidence from Bulk Sediments and Brachiopod Shells from S-England and N-Germany
Silke Voigt (svoigt@geo.tu-freiberg.de)1 &
Frank Wiese (frwiese@berlin.snafu.de)2
1 Geological Institute, Bergakademie Freiberg, Freiberg, Germany
2 Palaeontological Institute, Freie Universität Berlin, Berlin, Germany
The comparison of four Middle-Late Turonian bulk rock 18O curves in Western and Central Europe shows that they, albeit diagenetically altered, still contain primary paleoenvironmental signals. The primary signature is mainly preserved as a 18O value of the averaged curve. Within a high resolution stratigraphy, three positive 18O peaks at the 105 years time scale occur synchronous in the investigated areas around the Middle-Upper Turonian boundary. Two of them can be shown to be associated with a southward migration of boreal nectonic and benthic macrofauna. They are interpreted to be an expression of a stepwise climatic cooling. The 18O increases covary with a corresponding increase of the 13C values that indicates changes in the global carbon cycling as cause for the observed climate pertubations. Enhanced carbon uptake in the ocean and export to the sediments could have caused a drawdown of atmospheric CO2 and a decrease of temperature.
About 190 well preserved brachiopods could be collected from the Middle-Upper Turonian boundary interval from two localities in the Lower Saxony basin (N Germany), and from seven sections in southern England. Most specimens are filled with sediment, and only small amounts of sparitic cements could be observed. Some individuals are completely unfilled. Cathodoluminescence studies show that the majority of shells are non-luminescent, whereas the matrix sediments have a light orange luminescence, showing an early submarine cementation. 13C and 18O measurements on shells will be done to recognize the two short-term 18O variations obtained from the bulk-rock signal, and to calculate the corresponding decrease of paleotemperature. First results of non-luminescent brachiopod shells from Lower Saxony fall within a narrow range with mean values of 3.7‰ for carbon and -1.1‰ for oxygen (std. dev. = 0.1), and provide an early late Turonian bottom water paleotemperature of about 17°C ( 18OW = -1‰ vs. SMOW for ice free water).
The short-term Middle-Late Turonian climate cooling occurred at the end of the mid-Cretaceous greenhouse when the atmospheric pCO2 value was high. The atmospheric pCO2 value decreased during the Late Cretaceous, and could probably have reached a threshold value in the Middle-Late Turonian time interval. We suggest that the stepwise cooling was a positive feedback on the long-term decrease of atmospheric pCO2. This transient climate may have caused a reorganization of the thermohaline circulation resulting in the southward expansion of colder water-masses in the mid-latitudes.
EUG 10 28th March - 1st April, 1999 Strasbourg, France