CORRELATION OF EARLY CRETACEOUS CARBON-ISOTOPE STRATIGRAPHY AND PLATFORM DROWNING EVENTS - A POSSIBLE LINK

The Early Cretaceous carbonate carbon isotope record is marked by thre e positive high-amplitude (> 1.5%) excursions each covering time spans of more than 10(6) years. They are of late Valanginian-Hauterivian an d early and late Aptian age. In a case study along a transect across t he western Tethys Ocean we identified a coincidence between delta(13)C excursions, black shale formation, and widespread carbonate platform drowning events. We conclude that the delta(13)C excursions reflect a change in partitioning of carbon between the organic and carbonate car bon sinks which was triggered by climate induced ecological changes in Cretaceous pelagic and neritic environments. Episodes of intensified greenhouse climate conditions led to an increase in weathering, erosio n and runoff rates and to elevated nutrient transfer rates from contin ents into oceans. The resulting increase in oceanic nutrient levels fa voured marine phytoplankton production and black shale deposition whil e conditions for carbonate producing biotas became unfavourable. Parti al choking of carbonate production along river influenced coasts resul ted in widespread carbonate platform drowning during times of sea-leve l rise in the Valanginian and Aptian. Widespread contemporaneous black shale deposits and drowned carbonate platforms therefore reflect the contrasting response of the marine organic and carbonate carbon pumps to nutrient-enhanced phytoplankton productivity. The change in marine carbon partitioning is mirrored in a shift of the delta(13)C record to wards more positive values. The transition to the delta(13)C peak valu es lasted up to several hundred thousand years. The peaks of the excur sions, also covering a time span of up to several hundred thousand yea rs, reflect a new stabilisation of the carbon partitioning between car bonate and organic carbon sinks. A renewed intensification of carbonat e sedimentation under mesotrophic conditions was facilitated by stabil isation of the sea-level rise at a high level. Decreasing delta(13)C v alues record increasing carbonate carbon burial rates at constant or d ecreasing organic carbon accumulation rates. These changes contributed to the stabilisation of the marine carbon budget and the global carbo n cycle up to millions of years after its initial perturbation. (C) 19 98 Elsevier Science B.V.