High-Resolution Mg/Ca and Sr/Ca Records from Protothaca staminea Mollusc Shells Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)

High resolutions records of past nearshore temperature, salinity, and nutrient enrichments at mid-latitudes would be valuable for reconstructing past changes in climate affecting coastal areas (e.g. changes in precipitation, the nature of wind-driven coastal upwelling or the El Niño-Southern Oscillation (ENSO)). We present here a potential archive of such records in growth-banded mollusc shells. The potential paleo-temperature proxy Mg/Ca, as well as Sr/Ca and stable isotopes (δ ¹⁸O, δ ¹³C) were measured in shells of the clam Protothaca staminea. This bivalve appears favorable for nearshore paleoclimate reconstructions because it occurs over a wide geographic range (Alaska to central America), grows for nearly 10 years, and is present in archeological and geological deposits. P. staminea shells were obtained from a tidal flat in Humboldt Bay, CA (40.8° N; modern), and from the Duncan's Point Cave shell midden (CA Dept. Parks and Recreation site SON-348/H) near Bodega Bay, CA (39.6° N; ~3,000 and ~9,000 cal yr BP). High-resolution (monthly to weekly) Mg/Ca and Sr/Ca measurements were made by (UV Excimer) laser-ablation inductively-coupled plasma (quadrupole) mass spectrometry (LA-ICP-MS) using a spot size of 90 μ m and sample spacing ranging from 100 to 300 μ m. The rapidity of LA-ICP-MS analysis made it possible to analyze a 4 cm-long section of shell within 1 day. Absolute scales for the Mg/Ca and Sr/Ca LA-ICP-MS records were obtained from shell powders drilled with a 0.5 or 0.7 mm burr then dissolved and analyzed by flame atomic absorbtion. Stable isotopes were measured on splits of the shell powders. The Mg/Ca record during the final four years of shell growth in the Humboldt Bay shell shows a regular seasonal pattern with highest values (3.5 mmol/mol) during summer and lowest values (2.5 mmol/mol) during winter corresponding to a nearshore climatological temperature range of 6° C. An abrupt shift in shell Mg/Ca values toward the end of each year suggests that the mean value (3.0 mmol/mol) may be biased toward warm temperatures due to a cessation in shell growth during the winter, however. In comparison, at ~3,000 and ~9,000 cal yr BP shell Mg/Ca ratios averaged 1.5 and 1.0 mmol/mol, respectively, consistent with colder nearshore temperatures relative to today. Shell Mg/Ca time series will be compared with shell δ ¹⁸O, coastal temperature, and Humboldt Bay salinity measurements. Shell Sr/Ca ratios appear to be primarily determined by temperature but show an overprint from low salinity run-off events.