Spring molybdenum enrichment in scallop shells: a potential tracer of diatom productivity in temperate coastal environments (Brittany, NW France)

Abstract : Skeletal molybdenum/calcium ([Mo]/[Ca])(shell) ratios were examined in shells of the Great Scallop Pecten maximus collected in temperate coastal environments of Western Europe (42 to 49 degrees N). These ratios were determined by quantitative LA-ICP-MS analyses of daily striae taken every third day (i.e. a temporal resolution of 3 days) in 36 flat valves (2-years old; 3 shells/year). Variations of ([Mo]/[Ca])(shell) ratios were significant and reproducible for scallops from the same population, from different years (1998-2004) and temperate coastal locations (NW France). The [Mo]/[Ca])(shell) ratios exhibit typical profiles characterized by a background content, below the detection limit for this method (< 0.003 mu mol/mol) for most of the shell growth period, which is punctuated by a significant transient enrichment (0.031-2.1 mu mol/mol) mainly occurring from May to June. The Bay of Brest (France) was investigated in particular because of its long term observations on scallop communities, environmental variables, and high resolution analyses of dissolved Mo in bottom seawater in 2000. In 2000, dissolved Mo exhibited a significant increase in concentration just preceding the maximum ([Mo]/[Ca])(shell) ratio. Both the intense monitoring survey in 2000 and over the 7-year period indicates that the ([Mo]/[Ca])(shell) maximum is directly influenced by spring changes of environmental conditions at the sediment water interface (SWI), occurring subsequent to the intense and periodic spring bloom. Spring maxima of ([Mo]/[Ca])(shell) ratios are closely correlated to the extent of silicic acid and nitrate depletion in seawater between winter and late spring (r(2)=0.878 and 0.780, p < 0.05, n=6) that reflects diatom uptake and productivity in the Bay of Brest. The Mo inputs in bottom waters and subsequent shell enrichment are thus suggested to be directly or indirectly influenced by such biogenic material input at the SWI. The [Mo]/[Ca])(shell) records thus reveal unexpected biogeochemical cycles of Mo influenced by coastal spring productivity, faithfully recorded in scallop shells.