Trace metals in Antarctic clam shells record the chemical dynamics of changing sea ice conditions.

The dynamics of freeze and thaw events in Antarctic sea ice impart chemical changes in the underlying sea water. Trace metals in sea ice and accumulated through deposition of dust are released into sea water as sea ice breaks up in spring. Clams, such as Laternula elliptica incorporate a record of these and associated chemical dynamics in their carbonate shells. In 2012, we collected samples of L. elliptica from three sites along a sea ice persistence gradient in McMurdo Sound, Ross Sea Antarctica. Concentrations of trace metals in the chondrophore of each shell were measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Ablations transected annual growth increments creating time series ranging in length from 13 to 25 years. An eight-year time period of persistent sea ice, associated with presence of the B-15 and C-19 icebergs at the entrance of McMurdo Sound, was clearly resolved in the trace element time series. Conservative trace metals (Sr, Ba) were found at higher concentrations and highly scavenged elements (Pb, Cu) were found at lower concentrations at sites with more persistent sea ice and during the eight-year period of iceberg-influenced sea ice persistence. Bioactive trace metals (Fe, Ni) were found in higher concentrations during ice free conditions, associated with a period of high pelagic productivity. Our results provide an important case study for understanding the chemical signature of changing sea ice dynamics as reflected in bivalve shell material under a changing Antarctic climate.