Reconstructing benthic carbon oxidation rates using δ13C of benthic foraminifers

In non-bioturbated organic-rich marine sediments, the concentration and carbon isotope gradient of total dissolved carbon dioxide (∑CO2) primarily reflects the rate of carbon oxidation and CO2 diffusion within the sediment pore waters. Here, we investigate whether the calcium carbonate secreted by benthic foraminifers that inhabit discrete depths within the sediments record the pore water δ13C∑CO2 gradient and, hence, can be used as proxies to reconstruct a history of carbon oxidation change. The distribution and live abundance of Bolivina argentea s.l., Bolivina subadvena s.l. and Buliminella tenuata inhabiting the dysoxic marine sediments along the California margin is similar among sites that differ in (∑CO2) gradient by up to a factor of three. Bolivina argentea inhabits the sediments at the sediment water interface, whereas B. subadvena s.l. is most abundant at depths of between 2 and 4 mm. Buliminella tenuata is most abundant at deeper depths of between 4 and 6 mm. The δ13C of calcite precipitated by these species matches the δ13C of the pore water ∑CO2 at the depth of their maximum live abundance indicating that each species has a preferred depth for calcification. The δ13C difference between the species increases as the rate of carbon oxidation increases. These results indicate the carbon isotopic difference between these species in fossil assemblages can be used to estimate changes in the rate of carbon oxidation.