Ba/Ca variations in the modern intertidal bean clam Donax gouldii: An upwelling proxy?

a r t i c l e i n f o The discovery and calibration of high resolution paleoceanographic proxies is necessary to extend historic climate records and to understand regional climate variability. Chemical variations of skeletal remains have emerged as an often reliable recorder of environmental conditions. Specifically, Ba/Ca ratios have been correlated to temperature, salinity, seawater Ba/Ca, and phytoplankton biomass, although, many of these relationships appear taxon-and location-specific. To assess the sub-weekly Ba/Ca variations in the intertidal shallow-burrowing bivalve Donax gouldii, specimens were collected from the Southern California Bight, skeletal growth increments were cross-dated based on tidal-driven growth patterns, and skeletal aragonite Ba/Ca was determined using laser ablation inductively coupled plasma mass spectrometry. Cross-dated growth among specimens revealed a simultaneous, large, and transient Ba/Ca peak in all shells. The timing of peak Ba/Ca was compared to a suite of locally measured physical and biological data, including temperature, salinity, density, nitrate, silicate, chlorophyll, diatom abundance, dinoflagellate abundance, and phytoplank-ton community composition. Based on cross-dated chronologies, Ba/Ca shell is significantly correlated with Chl a from six and nine days prior and nutrients (nitrate, phosphate, silicate, and nitrite) from three days prior. In this system diatom abundance was not related to Ba/Ca shell. Transiently higher seawater Ba/Ca resulting from upwelling may be reflected in peak Ba/Ca shell , however the exact mechanisms leading to population wide Ba/Ca peaks remains enigmatic. Phytoplankton primary productivity supports a diverse array of marine ecosystems and sustains an important global carbon flux from the atmosphere into the deep ocean through the export production of organic matter (Behrenfeld et al., 2006). As anthropogenic global warming and ocean acidification progressively alter the global ocean system, scientific and policy communities would benefit from a more dynamic understanding of the myriad controls on primary productivity over both time and space (Sarmiento et al., 1998). One avenue for improving this understanding is the reconstruction of spatiotemporal variations during prehistoric climate regimes, and the comparison of such " proxy-based " data to computational models (Jones et al., 2001). The concentration of barium (Ba) in seawater, marine sediments, and biogenic carbonates has long generated interest in the element's potential as a proxy for upwelling and thereby indirectly primary In the ocean, Ba typically displays a nutrient-type distribution with low concentrations in the mixed layer and increasing concentrations with depth due to remineralization and respiration (Chan et al., 1977). Higher bulk sediment Ba concentrations …