Nadine Hallmann

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

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 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/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 18 O shell -derived temperature, but the strength of this relationship was weak ( r 2 p 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 jokulhlaups (subglacial outburst floods). Further works, including experimental studies, are needed to test these hypotheses.

AN IMPROVED UNDERSTANDING OF THE ALASKA COASTAL CURRENT: THE APPLICATION OF A BIVALVE GROWTH-TEMPERATURE MODEL TO RECONSTRUCT FRESHWATER-INFLUENCED PALEOENVIRONMENTS

Abstract Shells of intertidal bivalve mollusks contain sub-seasonally to interannually resolved records of temperature and salinity variations in coastal settings. Such data are essential to understand changing land-sea interactions through time, specifically atmospheric (precipitation rate, glacial meltwater, river discharge) and oceanographic circulation patterns; however, independent temperature and salinity proxies are currently not available. We established a model for reconstructing daily water temperatures with an average standard error of ∼1.3 °C based on variations in the width of lunar daily growth increments of Saxidomus gigantea from southwestern Alaska, United States. Temperature explains 70% of the variability in shell growth. When used in conjunction with stable oxygen isotope data, this approach can also be used to identify changes in past seawater salinity. This study provides a better understanding of the hydrological changes related to the Alaska Coastal Current (ACC). In combination with δ18Oshell values, increment-derived temperatures were used to estimate salinity changes with an average error of 1.4 ± 1.1 PSU. Our model was calibrated and tested with modern shells and then applied to archaeological specimens. As derived from the model, the time interval of 988–1447 cal yr BP was characterized by ∼1–2 °C colder and much drier (2–5 PSU) summers. During that time, the ACC was likely flowing much more slowly than at present. In contrast, between 599–1014 cal yr BP, the Aleutian low may have been stronger, which resulted in a 3 °C temperature decrease during summers and 1–2 PSU fresher conditions than today; the ACC was probably flowing more quickly at that time. The shell growth–temperature model can be used to estimate seasonal to interannual salinity and temperature changes in freshwater-influenced environments through time.

Seasonality and Intensity of Shellfish Harvesting on the North Coast of British Columbia

ABSTRACT Biogeochemical and growth increment analyses show contrasting seasonal patterns of butter clam collection and rates of harvest intensity between archaeological shell midden sites from the Dundas Islands archipelago and the mainland coast in Prince Rupert Harbour, northern British Columbia. Growth increment analysis shows more intensive clam harvest in the Dundas Islands in comparison to the residential sites in Prince Rupert Harbour. Stable oxygen isotope analysis shows multi-seasonal collection of clams in the Dundas Islands and a more seasonally specific emphasis in Prince Rupert Harbour. Comparison of these results to those of similar studies in the Namu region on the central coast of British Columbia provides a basis for broader regional understanding of variation in shellfish harvesting intensity and seasonality on the Pacific Northwest Coast.