186-2 Bivalve isotope geochemistry as an archive of Cretaceous climate and oceanography

Abstract:

Mollusk biostratigraphy anchors the Cretaceous timescale, providing a chronostratigraphic foundation for many studies of Cretaceous oceanic anoxic events (OAEs), stratigraphic correlation, and reconstruction of depositional environments. In many Cretaceous mudstone strata, macroinvertebrates also represent the main appreciably preserved archive for analysis using carbonate-based geochemical proxies. Here, we summarize results from a series of recent carbonate clumped isotope (Δ₄₇) analyses of bivalve fossils to test the paleothermometer’s wider application in Cretaceous strata for reconstruction of paleoclimatic and oceanographic conditions. One sample set (n=118) comprised Albian to Maastrichtian oyster specimens from strata of the Western Interior Seaway (WIS) of North America, collected from outcrops spanning from the southern to northern gateways of the seaway. Additional sample sets of Turonian and Campanian inoceramid prisms (n=32) are from two sites cored in the Indian Ocean during International Ocean Discovery Program (IODP) expeditions 369 and 392, which were located at the southern high latitudes during the Cretaceous. Results from the compilation highlight potential complicating factors particularly relevant to Cretaceous Δ₄₇ datasets. These include the potential for thermal reordering of skeletal carbonate Δ₄₇ signals during burial and possible clade-specific biases in Δ₄₇ temperatures, namely in inoceramids. Despite these potential concerns, our results collectively resolve spatial and temporal temperature gradients in the Cretaceous. The Cretaceous thermal maximum (KTM) is evident from the WIS and IODP datasets, with peak marine warmth (>30°C) in the Cenomanian to Turonian stages followed by later relative cooling through the Campanian and Maastrichtian stages. Latitudinal temperature gradients derived from the bivalve Δ₄₇ dataset of the WIS are minimal during the peak of the KTM and increase during the Campanian and Maastrichtian. Further, calculations of Cretaceous water oxygen isotope (δ¹⁸O_w) compositions from Δ₄₇ temperature and carbonate oxygen isotope data, provide geochemical evidence supporting hypotheses of mixing of distinct water masses in the WIS sourced from both the northern and southern gateways of the seaway. These findings demonstrate that Δ₄₇ analysis of properly screened bivalves can yield Cretaceous paleotemperature and δ¹⁸O_w data. Where suitably preserved specimens can be recovered, bivalve Δ₄₇ datasets have the potential to quantitatively reconstruct climatic and oceanographic responses to OAEs and the KTM, as well as test hypotheses about potential cold snaps in the Cretaceous and reconstruct local depositional conditions in mudstone strata.

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