Paleoceanography of the Cretaceous/Tertiary Boundary Event: Inferences from stable isotopic and other data

Abstract

We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction, whole rock, and benthic foraminifers, CaCO 3 and coarse fraction percentage determinations, and trace element (Sr) analyses on carbonate constituents across the Cretaceous/Tertiary (K/T) boundary in Deep Sea Drilling Project (DSDP) sites 47.2, 356, 384, and 577 and compare them with published results from K/T boundary sections in other DSDP sites. We used the trace element data and scanning electron microscope examination to evaluate possible diagenetic alteration and relative preservation of the samples analyzed in this study. The ∂18 O data when interpreted as isotopic paleotemperatures indicate relative stable surface water and deepwater temperatures in the late Maestrichtian followed by somewhat fluctuating temperatures in the early Paleocene. However, there is no indication of either a significant warming or cooling at or following the K/T boundary. Several sites do exhibit somewhat heavier ∂18 O values by about 0.5‰ across the boundary, which might suggest a 2‐3°C cooling at most. However, we interpret these somewhat heavier ∂18 O values as reflecting slightly better preservation of ∂18 O of carbonate constitutents in relatively clay‐rich intervals (e.g., lower diagentic potential) characteristic of the K/T boundary. The ∂13 C values of carbonate fine fraction and planktonic foraminifers indicate a major negative excursion in surface water total dissolved carbon across the boundary. The surface water and deepwater ∂13 C values from benthic foraminifers combined with calculated decreases in carbonate accumulation rates at all sites in the earliest Paleocene are consistent with a major decrease in productivity across the K/T boundary. The decrease in CaCO 3 accumulation rates is due not to increased rates of dissolution but to decreased production in surface waters. The low‐productivity episode lasted at least 1 m.y. beyond the K/T boundary crisis, suggesting that hypothesized catastrophic events alone, such as impact of an apollo object, are not the only explanation for the observed long‐term decrease in productivity. There is also an indication in the ∂13 C curves from several sites that the surface water ∂13 C decrease began in the latest Maestrichtian, suggesting that the decrease in productivity may have been independent of a hypothesized impact event.