Investigation of Li/Ca variations in aragonitic shells of the ocean quahog Arctica islandica, northeast Iceland

Abstract : Interannual and intra-annual variations in lithium-to-calcium ratio were investigated with high temporal resolution in the aragonitic outer shell layer of juvenile Arctica islandica (Mollusca; Bivalvia) collected alive in 2006 off northeast Iceland. Li/Cashell ranged between 7.00 and 11.12 µmol mol-1 and presented well-marked seasonal cycles with minimum values recorded at the annual growth lines; a general pattern was a progressive increase in Li/Cashell from March to May, followed by a plateau in June and a decrease down to minimum values in July-August. Li/Cashell was correlated with d18Oshell-derived temperature, but the strength of this relationship was weak (r2 < 0.25 and p < 0.05). It covaried significantly with microgrowth increment width and with the discharge from one of the closest rivers. Seasonal variations of Li/Cashell in A. islandica may most likely be explained (1) by calcification rate and/or (2) by significant river inputs of Li-rich silicate particles flowing to the sea as soon as snow melts. In the first case, Li/Cashell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns. Abrupt decreases of Li/Cashell may, in turn, help identify growth retardations due to harsh environmental conditions. Alternatively, if Li/Cashell variations are linked to particulate Li inputs by rivers, this could be a new proxy for the intensity of mechanical weathering of Icelandic basalts, with interesting perspectives for the reconstruction of frequency and intensity of past jökulhlaups (subglacial outburst floods). Further works, including experimental studies, are needed to test these hypotheses.
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Julien Thébault, Bernd R. Schöne, Nadine Hallmann, Matthias Barth, Elizabeth V. Nunn. Investigation of Li/Ca variations in aragonitic shells of the ocean quahog Arctica islandica, northeast Iceland. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2009, 10 (12), pp.Q12008. ⟨10.1029/2009GC002789⟩. ⟨hal-00449411⟩

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