SYNCHRONOUS BARIUM PEAKS IN HIGH-RESOLUTION PROFILES OF CALCITE AND ARAGONITE MARINE BIVALVE SHELLS

Biogenic carbonates have become a vital tool in paleoclimate reconstruction. Both stable isotopic and elemental signals recorded in the skeletons of marine organisms have been shown to be robust proxies of environmental conditions. Most high resolution barium/calcium profiles of bivalve shells show a characteristic flat background signal periodically interrupted with sharp peaks. A recent study has shown that the background Ba/Ca ratios are a good proxy of seawater Ba/Ca ratios, which can indirectly be used as a relative salinity proxy in estuaries (Gillikin et al. 2006, GCA 70:395). The sharp episodic peaks, however, remain a mystery. The main hypothesis is that barite formed in decaying phytoplankton flocs is ingested by the bivalves and sequestered in the shell (Stecher et al. 1996, GCA 60:3445). In this study we test if the sharp episodic peaks are in fact an environmental signal by determining if they are synchronous between different individuals growing at the same location, then see if they match or shortly follow phytoplankton blooms. Two aragonitic Saxidomus giganteus shells from Puget Sound (WA, USA) show a remarkable similarity, with very narrow and sharp peaks being reproduced in both shells. This clearly indicates an environmental forcing, but the peaks sometimes precede chlorophyll a peaks indicating there is no causal relationship. However, this could be critiqued due to potential time base reconstruction errors when matching shell data to environmental data. Therefore, we also analyzed calcitic Pecten maximus shells from the Bay of Brest (France) which contain clear daily external growth marks allowing precise matching between shell and environmental data. The four shells also had remarkably synchronous Ba/Ca peaks, again indicating an environmental control. The Ba/Ca peaks, however, start about 40 days after the crash of the phytoplankton bloom. This is clearly too long after the bloom as sedimentation of phytoplankton should be rather rapid. In summary, this study suggests that Ba/Ca peaks in bivalve shells are caused by an environmental forcing regardless of mineralogy, but the link with phytoplankton productivity is dubious.