Abstract:

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 (?137BaDBa) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang ( River). We find pronounced and systematic horizontal and depth dependent ?137BaDBagradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3‰), the ?137BaDBa values in the upper water column are significantly higher (+0.9‰) than those in the deep waters (+0.5‰). Moreover, signatures are essentially constant in the entire upper 100 m, in which in the very surface waters. Combined with the decoupling of DBa concentrations and ?137BaDBa 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 ?137BaDBa 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 s and ocean mixing processes.

Figure. Vertical distributions of dissolved barium (DBa) concentrations and their stable barium isotopic compositions (?137BaDBa) in the East China Sea and the South China Sea. (a) bathymetric map showing the locations of sampling stations; (b) stations PN10, PN04, and DH13; (c) the entire water column at station KK1; (d) the upper 200 m at station KK1 (open circles) including the stable isotopic compositions of excess particulate barium (?137BaBaxs) in the upper 150 m at station A0 (solid circles).

Link to full text:

http://www.sciencedirect.com/science/article/pii/S0012821X15007219

About the author:

Dr. Zhimian Cao got his PhD from Xiamen University in 2013 and has worked as a Humboldt Postdoctoral Research Fellow at GEOMAR Helmholtz Center for Ocean Research Kiel, Germany from 2013 to 2015. He joined MEL in 2015 and his research interests include nontraditional stable isotope geochemistry and ocean carbon cycling.

References:

(1)   Cao Z., Frank M., Dai M., Grasse P., Ehlert C., 2012. Silicon isotope constraints on sources and utilization of silicic acid in the northern South China Sea. Geochimica et Cosmochimica Acta 97, 88-104. (Link: http://www.sciencedirect.com/science/article/pii/S0016703712004978)

(2)   Cao Z.*, Frank M., Dai M., 2015. Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation. Limnology and Oceanography 60, 1619-1633. (Link: http://onlinelibrary.wiley.com/doi/10.1002/lno.10124/full)