Barium in seawater: An isotopic perspective

Pelagic barite precipitation and dissolution drives large gradients in the barium isotope composition of seawater. Since pelagic barite precipitation is associated with the microbial oxidation of particulate organic matter, marine barium isotope variations are fundamentally connected to the biogeochemical cycles of carbon and nutrients. Despite this important connection, recent interest in marine barium isotope chemistry has revealed several uncertainties concerning the inputs, internal cycling, and burial of barium from the oceans. Specifically: global barium inputs from rivers are too heavy to close the isotopic budget of seawater; there is a mismatch in the magnitude of barium isotope fractionation between barite precipitated in the laboratory compared to field settings; and, the long-term fidelity of barium isotopes preserved in sedimentary barite is unconstrained. In this talk, I review recent progress toward addressing these uncertainties by focusing on three case studies examining barium isotope cycling in estuaries, during barite formation, and in sedimentary systems undergoing recrystallization. In addition to reducing uncertainties in these systems, results from these case studies offer broader insights into the behavior of barium in seawater. These insights are then explored in the context of applying barium isotopes to study Earth history, with applications ranging from constraining past biogeochemical cycles to identifying the origin of enigmatic barites.