The isotopic composition of modem seawater sulfate:: I.: Coastal waters with special regard to the North Sea

The sulfur isotopic composition of dissolved seawater sulfate is a basic parameter in quantification of fluxes in the global sulfur cycle. It has changed during Earth history but is generally assumed to be constant at a given time due to the long residence time of sulfate in the ocean. Past measurements in the modem ocean, however, have shown an unexpected variability. Since the known marine sedimentary record essentially reflects continental margin processes, we have investigated for the first time the isotopic composition of dissolved sulfate of a marginal sea, the North Sea by means of stable-isotope-ratio-monitoring mass spectrometry. Sampling took place in the water column at 30 stations during two seasons (early spring 1996 and summer 1997). Stable isotope results are compared to measurements of salinity, temperature, and phytoplankton biomass. Except for one profile, the isotopic composition was found to be essentially constant at both the spring (delta S-34 =+20.9 +/- 0.13 parts per thousand; n = 13 1; versus V-CDT) and the summer campaign (delta S-34 =+21.1 +/- 0.13 parts per thousand; n=59). The mean composition of North Sea water sulfate of delta S-34 =+21 parts per thousand is within 0.2 parts per thousand of delta S-34 values found for essentially unpolluted coastal waters collected at different stations worldwide (Atlantic and Pacific Ocean) and the water column of the modem open ocean. (c) 2006 Elsevier B.V. All rights reserved.