Modern Calibrations of Temperature and Nutrient Proxies for Paleoenvironmental Reconstructions in Tropical Mollusks

Molluscan shell O- and C-isotope values have been shown to be useful indicators of upwelling and freshwater input (and thus nutrient status) in nearshore marine environments, but few studies have had the accompanying long term measurements of water ?^18O (?w) and dissolved inorganic carbon (DIC) ?^13C needed to best test the fidelity of shell isotope values as environmental indicators. Previously measured seawater ?w and ?^13CDIC values collected biweekly from 2011-2012 allow us to determine if shell ?^18O and ?^13C values record temperature, ?w, ?^13CDIC, and ultimately nutrient status in tropical waters. Conus, Vasum, and Strombus shell ?^18O values reflect predicted seasonal upwelling and freshening signals in the Pacific and seasonal freshening with minimal upwelling in the Caribbean. Both Pacific and Caribbean shell ?^13C profiles show cyclicity, but only Conus samples from Veracruz Beach (Pacific) record seasonal changes in ?13CDIC. This observation likely results from: (1) more distinct seasonal ?^13CDIC variation in Pacific waters compared with Caribbean waters and (2) greater availability of metabolic CO2 for shell growth in Strombus, which we hypothesize is related to greater activity associated with an herbivorous feeding habit. To examine N-isotope ratios (^15N/^14N) in tissues and shell organic matrix of bivalves as a proxy for natural and anthropogenic nutrient fluxes in coastal environments, Pinctada imbricata, Isognomon alatus, and Brachidontes exustus bivalves were live-collected and analyzed from eight sites in Bocas del Toro, Panama. Sites include a variety of coastal environments, including more urbanized, uninhabited, riverine, and oceanic sites. At all sites there is no single dominant source of organic matter contributing to bivalve ?^15N and ?^13C values. Bivalve ?^15N and ?^13C values likely represent a mixture of mangrove and seagrass N and C, although terrestrial sources cannot be ruled out. Despite hydrographic differences between end-members, we see minimal ?^15N and ?^13C difference between bivalves from the river-influenced Rio Guarumo site and those from the oceanic Escudo de Veraguas site, with no evidence for N from open-ocean phytoplankton in the latter. Lastly, ?^15N values of tissue and shell organic matrix correlate significantly for pterioideans P. imbricata and I. alatus. Thus for these species, N isotope studies of historical and fossil shells may provide records of ecology of past environments.