Total organic carbon, organic phosphorus, and biogenic barium fluxes as proxies for paleomarine productivity

Abstract

Although marine productivity is a key parameter in the global carbon cycle, reliable estimation of productivity in ancient marine systems has proven difficult. In this study, we evaluate the accumulation rates of three commonly used proxies for productivity from a set of primarily Quaternary sediment cores at 94 marine sites, compiled from 37 published sources. For each core, mass accumulation rates were calculated for total organic carbon (TOC), organic phosphorus (Porg), and biogenic barium (Babio). Calculated mass accumulation rates were compared to two independent estimates of modern regional primary productivity and export productivity, as well as to two potential controlling variables, bulk accumulation rate (BAR) and redox environment. BAR was found to exercise a strong control on the preservation of organic carbon. The linear regression equations relating preservation factor to BAR can be transformed to yield equations for primary and export production as a function of TOC and BAR, two variables that can be readily measured or estimated in paleomarine systems. Paleoproductivity can also be estimated from empirical relationships between elemental proxy fluxes and modern productivity rates. Although these equations do not attempt to correct for preservation, organic carbon and phosphorus (but not barium) accumulations rates were found to exhibit a systematic relationship to primary and export production. All of the paleoproductivity equations developed here have a large associated uncertainty and, so, must be regarded as yielding order-of-magnitude estimates.Relationships between proxy fluxes and BAR provide insights regarding the dominant influences on each elemental proxy. Increasing BAR exerts (1) a strong preservational effect on organic carbon that is substantially larger in oxic facies than in suboxic/anoxic facies, (2) a weak clastic-dilution effect that is observable for organic phosphorus (but not for organic carbon or biogenic barium, owing to other dominant influences on these proxies), and (3) a large negative effect on biogenic barium that is probably due to reduced uptake of barium at the sediment–water interface. These effects became evident through analysis of our globally integrated dataset; analysis of individual marine sedimentary units most commonly reveals autocorrelations between elemental proxy fluxes and BAR as a result of the latter being a factor in the calculation of the former. We conclude that organic carbon and phosphorus fluxes have considerable potential as widely useful paleoproductivity proxies, but that the applicability of biogenic barium fluxes may be limited to specific oceanic settings.