7-15 Trace Elements in Clam Shells: Using Ba and Mn as Proxies for Primary Production in The Gulf of Maine

Session: Undergraduate Research, Part I (Posters)

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Abstract:

Climate change is likely driving changes in the productivity of marine ecosystems.  In the Gulf of Maine (GoM), such changes are likely connected to changes in ocean circulation, nutrient delivery, and ocean stratification.   Historical records of changes in ocean productivity, for example, remote-sensed records of chlorophyll-A, are relatively limited in temporal scope. Proxy records offer a way to reconstruct past environments across significantly larger timescales. This study investigates the potential for the trace elements, Ba and Mn, within absolutely-dated Arctica islandica shells collected in the central (Seguin Island) and northern (Jonesport) GoM, to serve as proxies for past phytoplankton and redox stratification dynamics. Ba and Mn concentrations were measured at µm-scale resolution in the hinge portion of A. islandica shells, corresponding to sub-annual temporal resolution, using laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS).  Using this approach, we developed high-resolution records of environmental change at the Seguin Island (84 years; 1940-2024) and Jonesport (75 years; 1950-2024) sites. 

Across the two study sites, both Ba and Mn show a pattern of low baseline concentrations (typically 3 to 5 µmol Ba/mol Ca or 1-3 µmol Mn/mol Ca), interrupted by spikes (80-120 µmol Ba/mol Ca or 10-20 µmol Mn/mol Ca) at irregular intervals. These peaks in concentration do not coincide with the position of annual growth increments and show high synchronicity across co-eval shells, suggesting environmental rather than physiological control on these elements.  Ba peaks are thought to be caused by barite precipitation associated with sinking particulate organic matter following phytoplankton blooms.  Mn tends to correlate with Ba in our record, and higher Mn may reflect Mn release from bottom sediments when bottom waters are less oxygenated following phytoplankton blooms. Interestingly, we note fewer peaks in Ba and Mn in the most recent period, 2010-2024, compared to the earlier period 1990-2010, a finding broadly consistent with observed declining primary productivity in the GoM.  Ongoing and future work aims to compare Ba and Mn records to available instrumental records and to extend the time span of the records.

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