The isotopic composition of modern 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 modern 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 (δ34S = + 20.9 ± 0.13‰; n = 131; versus V-CDT) and the summer campaign (δ34S = + 21.1 ± 0.13‰; n = 59). The mean composition of North Sea water sulfate of δ34S = + 21‰ is within 0.2‰ of δ34S values found for essentially unpolluted coastal waters collected at different stations worldwide (Atlantic and Pacific Ocean) and the water column of the modern open ocean.