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.