Incorporation and subsequent diagenetic alteration of sulfur in Arctica islandica

Biogenic carbonates are important archives for reconstructing the marine sulfur cycle. However, uncertainties exist about the exact location of carbonate associated sulfate (CAS) and a possible biological control on sulfate incorporation. The behavior of CAS in biogenic carbonates during diagenetic alteration is even more poorly constrained. To investigate the mechanisms of sulfur incorporation and the effects of alteration on sulfur in biogenic carbonates, modern marine bivalve shells of Arctica islandica species were hydrothermally altered at 100 °C and 175 °C. Fluorescence microscopy, element mapping via NanoSIMS and μ-XRF, sulfur XANES analyses, and δ34S measurements were performed on the experimentally altered shell segments. Changes in elemental compositions and δ34S of sulfate in the post-alteration solutions were also determined. Results indicate clear differences between the δ34S values of the CAS (+21‰, V-CDT) that reflects ambient seawater sulfate and the organically bound sulfur that is isotopically lighter (+14.8‰, V-CDT or less). Carbonate associated sulfate is primarily incorporated in the mineral phase of the shell, whereas reduced sulfur phases are mainly found within the intrashell organic matter. Hydrothermal alteration experiments at 100 °C resulted in minimal changes of sulfur within the bivalve shells. In contrast, the 175 °C experiments triggered decomposition of intrashell organic matrices which then led to extensive diagenetic alteration in both the shell microstructure and chemistry. Changes in total concentration, speciation, and spatial distribution of sulfur reflect the diagenetic processes that occurred within the shells. Preferential incorporation of CAS in a neomorphic calcite phase with Mg/Ca ratios of 0.13-0.21 was observed. Due to its presence in both organic and inorganic phases and its multiple oxidation states with different isotopic compositions we conclude that sulfur is a useful and sensitive proxy for diagenetic alteration in biogenic aragonite.