THE INFLUENCE OF A DYNAMIC ANTARCTIC ICE SHEET ON SILICATE WEATHERING AND BIOGENIC OPAL PRODUCTION IN THE ROSS SEA, ANTARCTICA DURING THE MIOCENE CLIMATIC OPTIMUM

The Miocene Climatic Optimum (MCO, ~17-14.5 Ma) was an interval of global warmth that interrupted Cenozoic cooling, which immediately preceded the global cooling and ice growth associated with the middle Miocene Climatic Transition (~14 Ma). Evidence from deep-sea geochemical records indicate that the onset of the MCO was manifested by deep-sea warming and/or ice volume decrease as well as a carbon cycle perturbation. However, the driving mechanisms and high-latitude processes associated with the MCO are not well understood.

International Ocean Discovery Program Expedition 374 to the Ross Sea, Antarctica recovered a lower Miocene to recent sedimentary sequence from continental shelf Site U1521. Lithostratigraphic Unit III (85.34-209.17m CSF-A) consists of well-preserved early to middle Miocene diatom-bearing/-rich mudstone to diatomite sequence that indicates open marine conditions from 16.1 to 15.5 Ma. Our high temporal resolution (~5 kyr) record of biogenic opal concentration reveals a significant increase from ~14% to ~50% in the transition from Unit IV to Unit III. Biogenic opal values remain high (30 to 62%) throughout Unit III, indicating high marine primary productivity. The shipboard age model indicates the presence of cyclic variations in opal that suggests orbital-scale (~100 kyr) cyclicity in ice-proximal oceanic conditions. Increased opal flux in Unit III may reflect 1) increased silicate weathering, 2) a change in the source of dissolved silica and other nutrients to the Ross Sea, or 3) both. Silicon isotope (δ³⁰Si) values of the biogenic opal are constant (~1.3‰) across the Unit IV/III transition and do not respond to conditions that drove the opal production increase. Throughout Unit III, there is a strong antiphased relationship between the δ³⁰Si and biogenic opal concentrations. We interpret the high-amplitude fluctuations in biogenic opal and δ³⁰Si as changes in the amount of dissolved silicon supply or restriction of limiting nutrients (e.g. Fe) on orbital timescales during the MCO.

Our data provide the first constraints on biogenic opal production and burial in the Ross Sea during the MCO. Enhanced silicate weathering as a result of a dynamic Antarctic ice sheet and the supply of nutrients and dissolved silica to the coastal Antarctic waters may be a major driver of global climate change during the MCO and subsequent middle Miocene Climate Transition.