Isotopic evidence for Early to mid-Cretaceous ocean temperature variability

Stable isotopic measurements have been made on planktonic and benthic foraminifera of Early to midCretaceous (Early Albian-Cenomanian) age from DSDP site 463 located in the Pacific Ocean. The degree of alteration of the foraminifera has been assessed through the application of scanning electron microscopy. The oxygen isotope data support the view that during the Cenomanian 'globigerine' foraminifera, such as the genus Hedbergella, inhabited near-surface waters, whilst the flattened, keeled, morphotypes lived deeper within the water column. The carbon isotope data reveal a more complex pattern as the delta(13)C values from planktonic species essentially show a large overlap, which is interpreted in terms of a weak delta(13)C gradient existing in the upper part of the ocean. In contrast, the oxygen isotope data from planktonic foraminifera from the Late Albian display no discernible depth stratification pattern and the delta(13)C data also show a large overlap of values indicative of a complete disruption of the upper part of the thermocline. A relatively low surface to bottom water temperature gradient within the Early-Middle Albian part of the succession may reflect a well-mixed ocean. Inferred cool ocean temperatures (and/or high salinities) for the Early Albian may reflect increased oceanic poleward heat transport. An increase in temperature (and/or decrease in salinity) in the Cenomanian suggests a reduction of the poleward heat flux, promoting the build-up of limited polar ice during this period of tropical warmth. (C) 2002 Elsevier Science B.V. All rights reserved.