Stella C Woodard

Convection of North Pacific deep water during the early Cenozoic

The history of deep water formation and abyssal flow is poorly known but important to establish in order to develop a better understanding of changes in oceanic mass, heat, salt, and nutrient transport. North Atlantic high-latitude regions currently are the dominant deep water producers, but paleogeographic constraints, proxy interpretations, and physical models have suggested other modes for the past, such as those characterized by high-latitude Pacific sources, subtropical sources, or widespread, nonlocalized sources. Here we present new North Pacific Late Cretaceous–Paleogene Nd isotope data from fossil fish debris and detrital silicates, combined with results of coupled climate model simulations to test these hypothesized circulation modes. The data and model simulations support a circulation mode characterized by high-latitude, bipolar Pacific convection. Deep convection in the North Pacific, and likely the South Pacific, was most intense during the relatively “cool” portion of the Late Cretaceous–Paleocene and waned prior to the peak global warmth of the Early Eocene (ca. 52 Ma).

Evidence for orbital forcing of dust accumulation during the early Paleogene greenhouse

The accumulation of wind blown (eolian) dust in deep-sea sediments reflects the aridity/humidity conditions of the continental region supplying the dust, as well as the “gustiness” of the climate system. Detailed studies of Pleistocene glacial-interglacial dust fluxes suggest changes in accumulation rates corresponding to orbital variations in solar insolation (Milankovitch cycles). While the orbital cycles found in sedimentary archives of the Pleistocene are intricately related to glacial growth and decay, similar global orbital signals recognized in deep-sea sediments of early Paleogene age, the last major greenhouse interval ∼65–45 million years ago, could not have been linked to the waxing and waning of large ice sheets. Thus orbital signals recorded in early Paleogene sediments must reflect some other climate response to changes in solar insolation. To explore the potential connection between orbital forcing and the climate processes that control dust accumulation, we generated a high-resolution dust record for ∼58 Myr old sediments from Shatsky Rise (ODP Site 1209, paleolatitude ∼15°N–20°N). The dust accumulation data provide the first evidence of a correlation between dust flux to the deep sea and orbital cyclicity during the early Paleogene, indicating dust supply responded to insolation forcing during the last major interval of greenhouse climate. Furthermore, the relative amplitude of the dust flux response during the early Paleogene greenhouse was comparable to that during icehouse climates. Thus, subtle variations in solar insolation driven by changes in Earths orbit about the Sun may have had a similar impact on climate during intervals of overall warmth as they did during glacial-interglacial states.