Modeling the global oceanic barium cycle and implications for paleoceanographic proxies

Abstract

Barium (Ba) abundance is widely utilized as a proxy for deep ocean nutrients and organic carbon export. However, significant uncertainties remain regarding key aspects of the marine Ba cycle, including the primary Ba source to barite, the depths and rates of barite formation and dissolution, mechanisms linking the dissolved Ba and silicon cycles, and regional variability in the barium-to-organic matter ratio of exported particulates. Here, we integrate dissolved Ba observations from GEOTRACES with a model of barite saturation state and employ a mechanistic framework to constrain the major biogeochemical processes governing dissolved Ba distributions. Our results indicate that seawater, rather than organic matter, is the primary Ba source for pelagic barite, with 90 % of precipitation occurring above 900 m. Approximately two-thirds of pelagic barite dissolution occurs in the water column, while the remainder takes place at the seafloor. Dissolution rates appear independent of ambient saturation state, with the best model–data fit achieved when dissolution is treated as a constant-rate process. The similarities between marine Ba and silicon distributions are primarily driven by ocean circulation rather than biological uptake or similarities in the remineralization length scales of barite and opal. Furthermore, our model predicts significant spatial variability in barium-to-organic matter ratios of sinking particulates, challenging the use of Ba as a quantitative proxy for export productivity. Overall, this study provides new insights into the modern marine Ba cycle, highlighting the key role of pelagic barite in regulating dissolved Ba distributions and elucidating the processes that control barite formation and dissolution.