Excess Barium as a Paleoproductivity Proxy: A Reevaluation

Marine barite may serve as a proxy to reconstruct past export production (Dymond, 1992). In most studies sedimentary barite accumulation is not measured directly, instead a parameter termed excess barium (Baexs), also referred to as biogenic barium, is used to estimate the barite content. Baexs is defined as the total Ba concentration in the sediment minus the Ba associated with terrigenous material. Baexs is calculated by normalization to a constant Ba/Al ratio, typically the average shale ratio. This application assumes that (1) all the Ba besides the fraction associated with terrigenous Al is in the form of barite (the phase related to productivity) (2) the Ba/Alshale is constant in space and time (3) all of the Al is associated with terrigenous matter. If these assumptions are invalidated however, this approach lead to significant errors in calculating export production rates. To test the validity of the use of Baexs as a proxy for barite we compared the Baexs in a wide range of core top sediments from different oceanic settings to the barite content in the same cores. We found that Baexs frequently overestimated the Ba fraction associated with barite and in several cases significant Baexs was measured in the cores where no barite was observed. We have also used a sequential leaching protocol (Collier and Edmond 1984) to determine Ba association with organic matter, carbonates, Fe-Mn hydroxides and silicates. While terrigenous Ba remains an important fraction, in our samples 25-95% of non-barite Ba was derived from other fractions, with Fe-Mn oxides contributing the most Ba. In addition we found that the Ba/Al ratio in the silicate fraction of our samples varied considerably from site to site. The above results suggest that at least two of the underlying assumptions for employing Baexs to reconstruct paleoproductivity are not always valid and previously published data from (Murray and Leinen 1993) indicate that the third assumption may also not hold in every oceanic regime. We conclude that using Baexs instead of direct estimates of barite accumulation rates in sediment cores may result in significant errors in estimating past productivity and this proxy should only be used when it is clear that the associated assumptions are valid throughout the cores used. Collier, R. and J. M. Edmond (1984). "The trace elements geochemistry of marine biogenic particulate matter." Progress in Oceanography 13: 113-199. Dymond, J. (1992). "Barium in deep-sea sediment: A geochemical proxy for paleoproductivity." Paleoceanography 7(2): 163-181. Murray, R. W. and M. Leinen (1993). "Chemical transport to the seafloor of the equatorial Pacific Ocean across a latitudinal transect at 135ºW: Tracking sedimentary major, trace, and rare earth element fluxes at the Equator and the Intertropical Convergence Zone." Geochimica et Cosmochimica Acta 57: 4141-4163.