Constraining the oceanic barium cycle with stable barium isotopes

The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope (delta Ba-137(DBA)) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang (Yangtze River). We find pronounced and systematic horizontal and depth dependent delta Ba-137(DBa) gradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3 parts per thousand), the delta Ba-137(DBa) values in the upper water column are significantly higher (+0.9 parts per thousand) than those in the deep waters (+0.5 parts per thousand). Moreover, delta Ba-137(DBa) signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and delta Ba-137(DBa) from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The subsurface delta Ba-137(DBa) distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land-sea interactions and ocean mixing processes. (C) 2015 Elsevier B.V. All rights reserved.