High frequency Barium profiles in shells of the Great Scallop Pecten maximus: a methodical long-term and multi-site survey in Western Europe
Abstract. Skeletal barium/calcium ([Ba]/[Ca])shell ratios were measured every third daily striae in 39 flat valves of the Great Scallop Pecten maximus collected in temperate coastal environments of Western Europe. A methodical evaluation of the ([Ba]/[Ca])shell ratio was performed for the first time and demonstrates that ([Ba]/[Ca])shell profiles are reproducible for several scallop individuals from the same population (2-year old; 3 shells/year), over a 7-year period (1998–2004), and from different coastal environments in France (42–49° N). As previously determined in the shells of other bivalve species, ([Ba]/[Ca])shell profiles generally exhibited a background ratio punctuated by two transient maxima occurring in early and late summer. Background partition coefficient (DBa=0.11±0.03, in 2000) was similar to that previously reported in P. maximus shells, suggesting a direct shell uptake of dissolved seawater Ba (Gillikin et al., 2008). The 7-year survey in the Bay of Brest of the high frequency ([Ba]/[Ca])shell profiles in the scallop's shell was exploited to better constrain both the occurrence and the amplitude of the summer Ba relative enrichments as influenced by environmental processes. Seawater Ba contents in 2000 underlined significant particulate Ba inputs at the sediment water interface (SWI) during ([Ba]/[Ca])shell peak events. These Ba inputs are thus suggested to be subsequently induced by a pelagic biogenic process, which mainly occurs under summer post-bloom conditions in relationship to the cycling of particulate organic matter and associated Ba. The long term survey reveals that such pelagic Ba cycling processes are responsible for particulate Ba inputs to the sediment water interface (SWI). Subsequent indirect Ba uptake by the bivalve results in higher ([Ba]/[Ca])shell ratios, in that archived Ba within the shell cannot be used as a direct paleo productivity tracer. Our methodical approach, based on a multi-year and multi-site-survey of ([Ba]/[Ca])shell ratio in Scallop bivalves, allows us to establish the potential application of such high frequency archives for further biogeochemical and ecological investigations of bivalves in the coastal environment.
