Cenozoic sedimentation rates and provenance variations in the South Pacific Gyre

Pelagic clays are traditionally difficult to date due to the scarcity of biogenic deposition and the prevalence of homogenous, altered, very fine-grain sediment. Integrated Ocean Drilling Program Expedition 329 recovered completely oxic, brown, pelagic zeolitic metalliferous clay from the South Pacific Gyre (SPG). Despite post-depositional alteration, the sediment retains enough of its original chemical signature to track downcore changes in provenance. Of particular interest is the cosmogenic portion of the sediment (e.g., Co), which we use to determine sedimentation rates and explore the paleoceanographic implications of changing sedimentation in the SPG throughout the Cenozoic. Under the assumption that the flux of extraterrestrial cosmogenic dust is spatially and temporally constant (or can at least be constrained), the concentration of cosmic dust in the sediment is inversely proportional to the sedimentation rate. Previous studies have used this premise to successfully date pelagic clays and Fe-Mn crusts, using Co as a proxy for cosmic dust deposition. The SPG has the slowest marine sedimentation rates in the world (as low as 0.1 m/Myr) and subsequently has the highest concentration of cosmic dust in the seafloor, making it an ideal region to apply this techniques. Building upon Zhou and Kyte (1992, Paleocean., 7, 441-465) at the single location of DSDP Site 596, we are dating SPG pelagic clays and identifying provenance variations throughout the SPG using a combined analytical and statistical approach. We analyze bulk sediment from Exp. 329 for a wide suite of major, trace, and rare earth elements by ICP-ES/MS. We apply multivariate statistical methods to quantify the contributions through time from various sources, aiming to distinguish a cosmogenic component. From the cosmogenic abundances, we produce a high-resolution record of instantaneous sedimentation rates of SPG pelagic clays during the Cenozoic. Our preliminary constant cosmogenic Co models show variations in pelagic clay sedimentation rates between ~10 to 170 cm/Myr at Site U1366. Site U1370 is positioned closer to the edge of the gyre and more downwind of Australia than Site U1366 and, as expected, yields overall higher sedimentation rates ranging from ~30 cm/Myr at the beginning of the Cenozoic to more recent rates of ~250 cm/Myr. A gradual increase in sedimentation rate throughout the Cenozoic is seen in the pelagic clay portions of multiple Exp. 329 Sites (U1365, U1366, U1369, and U1370) across the SPG. Additional refinements to our mixing models will examine the relative importance of factors contributing to this sedimentation increase, including the aridification of Australia and Antarctica, increased volcanism, and the reorganization of ocean currents caused by the opening of the Drake Passage.