| Abstract: | Within data supporting warm mid-Cretaceous climates, paleotemperature estimates from the late Turonian at DSDP Site 511 (Falkland Plateau) stand out as problematic. Huber et al. (1995) reported planktonic foraminiferal  values as light as -4.39 per mil (PDB). Assuming a range of reasonable local water estimates (-1.2 to -1.75 per mil SMOW), the data reported by Huber et al. indicate upper ocean waters as warm as 28-30°C at ~60°S paleolatitude in the Late Turonian. In the modern ocean, such temperatures are typical of the western Pacific tropical warm pool. In the geologic record, such temperatures are currently unsupported by other data from equivalent latitudes in either hemisphere for any time period.
We have obtained a detailed record of planktonic and benthic isotope values across the negative shift reported by Huber et al. (1995). The planktonic foraminiferal and calcareous nannofossil assemblages include cosmopolitan species that are associated with normal, open marine depositional settings. The foraminifera exhibit excellent preservation. Shells are translucent and chambers are hollow. Surface ornamentation is preserved. No overgrowths or infilling material is present. Primary wall microstructure is preserved, with distinct shell layering and wall pores clearly visible. A Late Turonian age is assigned to these samples based on co-occurrence of the calcareous nannofossil Kamptnerius magnificum and the planktonic foraminifera Marginotruncana marginata (=Globotruncana bulloides of Huber, 1995) and Praeglobotruncana stephani. Strontium ratios indicate that the 511 event may be age equivalent with very depleted planktonic forams from tropical Atlantic Site 144 (5°N paleolatitude; Wilson et al., 2002).
The new isotope data are consistent with those published earlier. There is an abrupt change in planktonic at 412.18 mbsf where planktonic oxygen values decrease by ~2 per mil. M. marginata yielded the lightest measurement (-4.66 per mil PDB). Benthic values show little change through the study interval, which makes it appear that the event is one of surface "freshening." Reasonable mixing ratio calculations show that, assuming no change in freshwater , the local freshwater component would have had to increase from 5% before the event to 27% during. In that case, a salinity decrease of ~7 psu would be expected. However, foram and nannofossil assemblage changes indicate, if anything, a change to more normal marine conditions within the depleted interval.
A decrease in freshwater also does not appear to be a plausible mechanism: it would entail at least a 40 per mil decrease, to values like that of modern central Antarctic snowfall. In fact, if some warming accompanied the Site 511 event (a change supported by foram and nannofossil assemblage changes), the high latitude precipitation would have become less depleted, as indicated by climate models that include water isotope tracers. The comparison of ocean circulation model results and bottom water temperatures inferred from benthic indicates that the 511 event is unlikely to have resulted from a surface water temperature increase to 32°C (estimate based on M. marginata ). The lack of evidence of a salinity decrease, detrital influx or benthic change argue against this being a glacial meltwater record.
We are currently unable to explain how surface waters over Falkland Plateau became highly 18O-depleted. The paradoxes presented by the Site 511 data require that this interval be sampled at other locations outside the high latitude South Atlantic basin and that multiple-proxy techniques be applied to these new Late Turonian sections when they are found. |