A smaller glacial ocean carbon inventory?

Previous studies attempting to explain Pleistocene atmospheric CO2 variations have focused on mechanisms that transfer carbon (C) between the oceanic, atmospheric and terrestrial reservoirs, with the underlying assumption that the total C inventory in these three Earth"s surface reservoirs remained constant during glacial-interglacial cycles. Under this framework, ocean C inventory would have been marginally increased by 500-1000 GtC (1-2%) during the glacial period. Here, we show that past ocean C inventory can be revealed by reconstructed bulk ocean 14C/12C (denoted as ∆14C) and atmospheric 14C production rates with an Earth system model - cGENIE. First, we develop a bulk ocean ∆14C record that spans the last 40 ka from thousands of benthic foraminifera and deep sea coral ∆14C data with a fairly good coverage of the global seafloor. We then run cGENIE under constant pre-industrial boundary conditions, with the only forcing being atmospheric 14C production rates reconstructed by geomagnetic field intensity records and ice core record of 10Be fluxes. Under most of the 14C production scenarios, the simulated bulk ocean ∆14C are significantly lower than our composite during the Last Glacial Maximum as well as the early deglaciation. Bulk ocean ∆14C is a metric controlled by 14C production rates and ocean C inventory, with the state of ocean circulation playing a minor role. Our finding suggests either glacial 14C production was much higher and/or glacial C inventory was much lower than previously thought. Implications of both possibilities are discussed. In particular, the second possibility highlight the exchange of C and ALK between Earth"s surface and geological reservoirs as a critical missing piece in searching for a complete theory of glacial-interglacial atmospheric CO2 variability.