Stable Isotopic Values of Fish Remains and Inoceramids in Late Cretaceous Black Shales on Demerara Rise
Abstract
Stable isotopic measurements of exquisitely preserved foraminifera (those with radial hyaline microstructure preserved and no shell infilling) have galvanized investigation of ancient sea surface temperatures. Intervals within the `black shale' facies on Demerara Rise exhibit such preservation and suggest contemporary tropical sea surface temperatures at least as warm, if not several degrees warmer, than modern tropical surface waters. Further, these data indicate a 5-10 degrees C temperature range across the planktic realm. Unfortunately, exquisite preservation is present in only some intervals on Demerara Rise and benthic taxa are rare to absent throughout. To try to generate a more complete stratigraphic record of paleotemperatures and a fuller characterization of conditions through the water column, we are measuring stable isotopic values of phosphate oxygen in fish remains, carbonate carbon and oxygen in inoceramids, and organic carbon in organic material within inoceramids shells. Preliminary results for the fish debris range from 19.5 to 21.0 per mil V-SMOW suggesting paleotemperatures of ~20 degrees C. The estimated temperature range overlaps the cooler planktic foraminifera estimates suggesting the Demerara Rise fish analyzed lived at or near the thermocline. The taxonomic resolution possible with fish remains is limited, but the relative resistance of phosphate oxygen to diagenetic alteration should allow us to track temperature changes across intervals where foraminifera preservation deteriorates. Inoceramid values, on the other hand, yield problematically warm benthic temperature estimates. Inoceramid oxygen isotopic values average -2.9 per mil V-PDB and are as low as -3.3 per mil. Paleotemperatures estimated from these values are 4-6 degrees C warmer than estimated thermocline temperatures (but 6-8 degrees C cooler than the warmest surface temperature estimates). Stratigraphic coverage is currently too sparse to critically evaluate this observation. Finally, inoceramid carbonate-carbon values are comparable to values in planktic foraminifera (~1 per mil V-PDB) but shell organic carbon values (~-25 per mil V-PDB) provide no indication of unusual metabolism in these bivalves.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFMPP42C..04M
- Keywords:
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- 3030 Micropaleontology;
- 4267 Paleoceanography;
- 4825 Geochemistry;
- 4870 Stable isotopes