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

Abstract

[1] 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/Ca shell ranged between 7.00 and 11.12 mmol mol À1 and presented well-marked seasonal cycles with minimum values recorded at the annual growth lines; a general pattern was a progressive increase in Li/Ca shell from March to May, followed by a plateau in June and a decrease down to minimum values in July-August. Li/Ca shell was correlated with d 18O shell -derived temperature, but the strength of this relationship was weak (r 2 < 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/Ca shell 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/Ca shell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns. Abrupt decreases of Li/Ca shell may, in turn, help identify growth retardations due to harsh environmental conditions. Alternatively, if Li/Ca shell 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.

Li/Ca enrichments in great scallop shells (Pecten maximus) and their relationship with phytoplankton blooms

“…It is clear from our results that seawater temperature in the bay of Brest was not the primary factor explaining variations of Li/Ca shell between 1999 and 2007. Therefore, we conclude that calcification temperature has only a weak positive influence on Li incorporation in Pecten maximus shell calcite precipitated between 8 and 18°C, thus confirming observations by Thébault et al (2009b) on juvenile Arctica islandica.…”

“…Surprisingly, these results are in contradiction with thermodynamic calculations stating that Li content in calcium carbonate should increase with increasing temperature (Hall and Chan, 2004). Conversely, recent studies on foraminifera and aragonitic bivalve shells found a positive dependance of Li/Ca shell on temperature (Hendry et al, 2009;Thébault et al, 2009b). Simple and multiple linear regressions performed on the whole dataset (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) indicated that seawater temperature alone explained only 6.2% of the Li/Ca shell variability (r 2 = 0.062; p = 0.004; slope = 2.73; Table 2).…”

“…Most studies on Li/Ca ratio in biocarbonates dealt with foraminifera (Delaney et al, 1985;Delaney and Boyle, 1986;Hall and Chan, 2004;Marriott et al, 2004b;Hathorne and James, 2006;Lear and Rosenthal, 2006;Hendry et al, 2009;Lear et al, 2010), and to a lesser extent with corals (Marriott et al, 2004a;Montagna et al, 2006) and brachiopods (Delaney et al, 1989). The only known study on Li/Ca ratio in bivalve mollusc shells was conducted on juvenile Arctica islandica shells from northeast Iceland (Thébault et al, 2009b). Given the very low inter-individual variability in Li/Ca shell for a given season of growth ( Figures 5 and 6), it is likely that this ratio responds either to variations of one (or several) environmental parameter(s), and/or to variations of a physiological process synchronized within a given population.…”

“…In their study on Li/Ca shell in juvenile Arctica islandica from northeast Iceland, Thébault et al (2009b) observed that seasonal variations of the closest river discharge roughly followed the same pattern as Li/Ca shell , with maximum values in June. A direct relationship was highlighted by Gislason et al (2009) between river discharge and mechanical weathering of Icelandic basaltic rocks that have a high Li content.…”

“…Most studies dealt with foraminifera where Li/Ca ratio was suggested to be a proxy either for temperature, for Li/Ca ratio in seawater, or for oceanic carbonate ion concentration (Delaney et al, 1985;Hall and Chan, 2004;Marriott et al, 2004b;Hathorne and James, 2006). The only known study dealing with Li/Ca in bivalves was performed on aragonitic shells of the ocean quahog Arctica islandica (Thébault et al, 2009b). It was suggested that calcification rate and/or river inputs of Li-rich silicate particles were likely the main factors controlling incorporation of Li in shell aragonite.…”