The Intrinsic Mass Balance Problem With Using Biogenic Barite Accumulation to Record Past Productivity

The construction of "paleoproductivity" records, especially over brief (<200,000 yr) times of major carbon cycle perturbation, has become a growing research theme in paleoceanography. These records are necessarily generated using proxies. The accumulation of euhedral barite in pelagic sediment correlates to the flux of sinking marine organic matter in many locations at present-day. Thus, records of biogenic barite have been used extensively to infer changes in past productivity. In several recent studies, euhedral barite records have been constructed at a limited number of locations, and then used to argue for regionally to globally enhanced productivity across brief events. Here we discuss straightforward results from numerical models of the marine Ba cycle to elucidate potential causes for these widespread pulses in euhedral barite accumulation. Our primary conclusion is that, irrespective of how Ba cycles within the ocean, a significant global increase in biogenic barite outputs (>10%) over appreciable time (>1000 yr) necessitates elevated Ba inputs to the ocean because Ba has a very short residence time at present-day (<10,000 yrs). High sulfate concentrations and the low Ksp for barite presumably maintain a similar residence time for much of the younger geological record. If truly global, intervals of significantly greater euhedral barite accumulation must therefore be viewed as times of increased Ba input to the ocean rather than times of elevated primary productivity. Over brief events, however, established sources of Ba to the ocean, hydrothermal waters and rivers, cannot furnish large amounts of Ba. There appear to be three general explanations for short intervals characterized by enhanced barite deposition at multiple locations: (1) decreased Ba accumulation at other locations counterbalanced the excess accumulation in examined records, (2) sulfate concentrations were much lower during these intervals, or (3) a major flux of Ba to the ocean lies outside of current models. We cannot exclude the first two possibilities with available data but suggest that methane-laden fluids escaping from the seafloor provide a very large and highly variable source of barium to deep ocean water. Changes in this source could account for certain observations.