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 6C 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 d 18 O shell 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 6C 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 6C 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.

Refining Estimates for the Season of Shellfish Collection on the Pacific Northwest Coast: Applying High‐resolution Stable Oxygen Isotope Analysis and Sclerochronology

“…The results of the studies by Gillikin et al . (2005) and Hallmann et al . (2009, 2011, 2012), as well as the present study, provide a strong foundation to examine the variability observed in region‐specific growth rates, the influence of seasonal freshwater influxes and seasonality in the Namu region.…”

“…Even though the coast of BC is subject to seasonal freshwater influxes, previous studies have argued that growth rates of this species are controlled more by temperature than by freshwater (Hallmann et al . 2009, 2011), and the changes in δ 18 O shell are strongly controlled by seasonal influxes of freshwater.…”

“…For both microgrowth pattern and stable oxygen isotope analysis, methods followed those of Schöne et al . (2005a,b) and Hallmann et al . (2009, 2011, 2012).…”

“…2009). These studies have also demonstrated a distinct regional variability in shell growth rates (Hallmann et al . 2009, 2011, 2012).…”

“…More recent sclerochronological and stable isotope studies from the north and south coasts of British Columbia (BC) and Alaska have demonstrated that growth line analysis alone cannot distinguish winter lines from disturbance lines (Clark 1974) caused by stress, predation, spawning or freshwater influxes (Hallmann et al 2009). These studies have also demonstrated a distinct regional variability in shell growth rates (Hallmann et al 2009(Hallmann et al , 2011(Hallmann et al , 2012. Therefore, prior to determining the season of shellfish collection, the region-specific growth rates of the species in question for both modern and archaeological specimens must be determined.…”