FACTORS CONTROLLING OSMIUM RESIDENCE TIME AND IMPLICATIONS FOR ISOTOPIC SIGNAL DURING OAE2

Osmium (Os) isotope geochemistry has become a reliable proxy for the identification of catastrophic volcanic and extraterrestrial impact events in the geologic past. This proxy is based on the ¹⁸⁷Os/¹⁸⁸Os ratio of marine organic rich sediments in current and past times. Despite major ¹⁸⁷Os/¹⁸⁸Os anomalies being identified during intervals of increased mantle/extraterrestrial input, the limited constraints on varying inputs and outputs of marine Os has made the quantification of its global ocean residence time over geologic time periods difficult. The marine Os cycle is primarily controlled by changes in its main input fluxes (continental weathering, mantle/extraterrestrial material) and output fluxes (oxic, anoxic and euxinic sediments). In this work we use a forward box model of the marine Os cycle to assess the processes (inputs/outputs) that controlled the occurrence of a ~150 kyr global negative anomaly in the seawater ¹⁸⁷Os/¹⁸⁸Os during Oceanic Anoxic Event 2 (OAE2). The model suggests that the rapid decrease in ¹⁸⁷Os/¹⁸⁸Os is only achieved when the unradiogenic (mantle/extraterrestrial) input is >100,000 times the modern input. It also suggests that the subsequent ~400 kyr recovery of the ¹⁸⁷Os/¹⁸⁸Os values is achieved when the main radiogenic input (continental weathering) is kept at > 10,000 times the modern input during the late stages of OAE2. The enhanced input from continental weathering is consistent with OEA2 Li isotope profiles. Calculation of the main inputs and outputs suggest that the Os residence time during OAE2 was much longer than the present. The model also suggests that the recovery of the ¹⁸⁷Os/¹⁸⁸Os values to those of pre OAE2 is achieved when oxic/suboxic marine sedimentary environments account for 99 % of the Os output and was ~20x the modern output. This implies that the Os residence time was most likely dynamic throughout OAE2 and much longer than the present.