Volcanism, CO2 and Palaeoclimate: A Late Jurassic-Early Cretaceous Carbon and Oxygen Isotope Record
Weissert, Helmut, Erba, Elisabetta, Journal of the Geological Society
A composite Tethyan Late Jurassic-Harly Cretaceous carbon and oxygen isotope curve is presented. C-isotope data provide information on the evolution and perturbation of the global carbon cycle. O-isotope data are used as a palaeotemperature proxy in combination with palaeontological information. The resulting trends in climate and in palaeoecanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification. Positive C-isotope anomalies in the Valanginian and Aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide. A major warming pulse accompanies the Aptian but not the Valanginian C-isotope event. The observed change in Early Aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion. Both C-isotope anomalies are accompanied by biocalcification crises that may have been triggered by pCO^sub 2^-induced changes in climate and in surface water chemistry. Elevated nutrient levels in river-influenced coastal waters and in upwelling regions further weakened marine calcification. These conditions contrast with 'normal' trophic conditions prevailing in the latest Jurassic and favouring biocalcification. The C-and O-isotope curves record a stable mode of carbon cycling and stable temperatures. We conclude that biocalcification is mostly triggered (and inhibited) by C02 conditions in the atmosphere-ocean system.
Keywords: Mesozoic, ^sup 13^C, ^sup 18^O, carbon cycle, palaeoclimate.
The Late Jurassic low-latitude oceans favoured reef growth and the expansion of calcareous plankton into pelagic realms whereas the Early Cretaceous was punctuated by repeated 'biocalcification crises'. These times of contrast provide an excellent opportunity to investigate boundary conditions favouring or inhibiting biocalcification in marine ecosystems. We first trace carbon cycling from the Late Jurassic into the Early Cretaceous using a pelagic bulk carbonate carbon isotope record established on Tethyan limestone successions. Correlation of the C-isotope record with major trends in marine biocalcification provides insights into environmental controls on biomineralization in shallow- and deep-water carbonate systems. 'Biocalcification crises' are defined here as times of reduced calcification rates of a variety of neritic and planktic organisms, causing widespread carbonate platform collapse and changing calcareous nanno-plankton assemblages with time scales of around 100 ka. We next present a bulk oxygen isotope record through the Late Jurassic and Early Cretaceous and discuss its potential as a palaeotemperature proxy. Comparison of the oxygen isotope data with palaeontological palaeotemperature indices allows the discrimination between changes in palaeosalinity and palaeotemperature and, consequently, the identification of warming and cooling episodes and trends. The combined palaeotemperature-carbon cycling information is the baseline for the evaluation of environmental conditions favouring or inhibiting biocalcification.
Integrated stratigraphy and radiometric dating provide the framework to link volcanic activity and elevated CO2 to climate change, nutrient recycling and biocalcification in Late Jurassic-Early Cretaceous oceans.
A composite carbon isotope curve from Tethys sediments
Weissert et al. (1998) have published a composite carbon isotope curve through the Late Jurassic and Early Cretaceous. Here we present a revised and extended composite C-isotope curve based entirely on the analysis of bulk-rock samples (Figs 1 and 2). The analytical methods were presented by Weissert et al. (1985). Sediments outcropping along the Southern Alps and deposited at a palacodepth of about 1-2 km along the southern …
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Publication information: Article title: Volcanism, CO2 and Palaeoclimate: A Late Jurassic-Early Cretaceous Carbon and Oxygen Isotope Record. Contributors: Weissert, Helmut - Author, Erba, Elisabetta - Author. Journal title: Journal of the Geological Society. Volume: 161. Publication date: July 2004. Page number: 695. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.