Abstract
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° 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 μmol/mol) for most of the shell growth period, which is punctuated by a significant transient enrichment (0.031–2.1 μmol/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 (r2=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.
Highlights
Mollusc bivalves grow through an incremental deposition of calcium carbonate (CaCO3) layers with most species exhibiting specific temporal marking in shell composition (Stecher et al, 1996; Chauvaud et al, 1998, 2005)
Through the observation of ([Mo]/[Ca])shell ratios along the daily striae of P. maximus, the objectives of this study are first, to evaluate ([Mo]/[Ca])shell profiles as a potential record of specific biogeochemical processes occurring at the SWI, and second to provide new confirmation of the non-conservative behaviour of Mo in coastal waters
Three live juvenile specimens of P. maximus were collected each year at the study site in the Bay of Brest, Roscanvel station (n = 3 shells per year, 1998–2004, except in 2002: n = 2), off Belle Ile in 1999 (n = 3), 2000 (n = 4) and 2001 (n = 3), off Quiberon in 2000 (n = 3) and in the Bay of Seine in 2004 (n = 3)
Summary
Mollusc bivalves grow through an incremental deposition of calcium carbonate (CaCO3) layers with most species exhibiting specific temporal marking in shell composition (Stecher et al, 1996; Chauvaud et al, 1998, 2005). Within their own exoskeleton, a chronological record of the environmental variations they have experienced during their life. Following this growth pattern, several investigations have demonstrated that variations of the historical elemental composition along the shell growth axis can be used as proxies for coastal biogeochemical processes (Dodd, 1965; Lorens and Bender, 1980; Klein et al, 1996a, b). Barats et al.: Spring Mo shell enrichment reveals diatom productivity?
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