A NEW OSMIUM ISOTOPE RECORD OF GLACIAL-INTERGLACIAL WEATHERING RESPONSE ACROSS THE PLEISTOCENE

The Quaternary period is characterized by the waxing and waning of Northern Hemisphere ice sheets that underwent a transition from 41 thousand-year (ka) cycles to 100-ka cycles during the Mid-Pleistocene, occurring around 1.2 million years (Ma) ago. In the absence of external (orbital) forcings, proposed mechanisms responsible for the MPT are generally related to cryosphere dynamics or fluctuations in the energy balance of the Earth system. One hypothesis involves large scale ice sheet stabilization, where erosion of a regolith substrate was superseded by a crystalline bed beneath the Laurentide ice sheet sometime during marine isotope stages (MIS) 16-36, this is known as the “Regolith Hypothesis”.

Osmium isotopes are an emerging geochemical tool suitable for investigating Quaternary glacial-interglacial weathering fluxes. This radiogenic isotope system is capable of investigating glacial continental weathering due to its relatively short ocean residence time (25 - 54 kyr) and also its sensitivity to variations in sediment source (e.g. Precambrian crystalline bedrock vs Mesozoic regolith).The current seawater osmium isotope record depicts a stark increase towards more radiogenic values over the last 4-5 million years, likely due to the onset of Northern Hemisphere glaciation and resultant increases in continental weathering. Here, we attempt to further deconvolve millennial scale osmium isotope variability across glacial-interglacial cycles, investigate glacial continental weathering fluxes in response to changes in glacial cyclicity of the Laurentide Ice sheet, and further interrogate the “Regolith Hypothesis”. In order to assess glacial weathering fluxes, we analyze osmium isotopes from sediment cores retrieved from the North Atlantic where sediment was likely discharged from the ice sheet. This data has allowed us to evaluate local continental weathering response to changes in glacial cyclicity during the Mid-Pleistocene.