CLUMPED ISOTOPE PALEOTEMPERATURE ANALYSIS OF LATE CRETACEOUS FORAMINIFERA FROM SOUTHEAST COASTAL TANZANIA AND THE FALKLAND PLATEAU

The clumped isotope geochemistry (Δ₄₇) of carbonate minerals can record quantitative ocean paleotemperature data. Until recently though, analytical limitations precluded analyses of small sized samples (<1 mg). Here, we report Δ₄₇ paleotemperatures (T_Δ47) from Cretaceous foraminifera at Tanzania Drilling Project (TDP) drill sites 23, 31, and 36 (~35°S paleolatitude) and Deep Sea Drilling Program Site 511 on the Falkland Plateau (~58–62°S paleolatitude). From these sites, we analyzed Turonian-Campanian (93-75– Ma) bulk planktic and benthic foraminiferal assemblages that appeared well-preserved from initial assessments of unbroken tests. Light microscope and SEM imaging of whole and dissected specimens revealed that Turonian assemblages from TDP Site 31 and DSDP Site 511 show no evidence of shell recrystallization or secondary test infilling. TDP Site 36 (Turonian) and TDP Site 23 (Campanian) specimens exhibited varying levels of secondary calcite infilling and thus may not reliably record original paleoclimate signals. Non-infilled Turonian specimens from TDP Site 31 have a mean T_Δ47 value of 27–35°C. In comparison, higher latitude paleotemperatures from DSDP Site 511 were significantly cooler with a mean T_Δ47 of 17–24°C for the late Turonian-Campanian. Although the T_Δ47 data identified a large temperature difference between Tanzania and the Falkland Plateau, expected temperature offsets between paired planktic and benthic specimens were not resolvable with the T_Δ47 data. This latter finding could be the result of an insufficient sample size of benthic specimens, or may be beyond the limits of Δ₄₇ analytical resolution on small replicate masses (~400 µg). Comparisons of Cretaceous seawater oxygen isotope compositions (δ¹⁸O_w) calculated from the T_Δ47 and δ¹⁸O_carb data, resolved mean benthic δ¹⁸O_w values that are statistically heavier than mean planktic δ¹⁸O_w values (by ~1–3‰ SMOW), which likely reflects a modest halocline at both TDP Site 31 and DSDP Site 511. This study demonstrates that Δ₄₇ analyses of foraminifera can yield independent proxy data about open ocean dynamics in the Cretaceous. However, our findings also highlight challenges in T_Δ47 analyses of microfossils in deep time, such as the need for numerous replicates, susceptibility to diagenesis, and limitations in proxy resolution.