Extending northwest Atlantic pH records using the boron isotope-pH proxy in two marine calcifiers in the Gulf of Maine

Branwen Williams1, Joseph Stewart2, Alan D Wanamaker3, Nina Whitney4, Brittany Jellison5, Brittany Hernandez6, Aaron Leigh Strong7 and Michele LaVigne6, (1)Claremont Colleges, Claremont, CA, United States, (2)University of Bristol, Earth Sciences, Bristol, United Kingdom, (3)Iowa State University, Department of Geological and Atmospheric Sciences, Ames, United States, (4)Iowa State University, Geological and Atmospheric Sciences, Ames, IA, United States, (5)Bowdoin College, Schiller Coastal Studies Center and Department of Biology, Brunswick, ME, United States, (6)Bowdoin College, Department of Earth and Oceanographic Science, Brunswick, ME, United States, (7)Hamilton College, Environmental Studies, Clinton, NY, United States
Increasing atmospheric CO2 concentrations are causing seawater temperatures to rise while also decreasing ocean pH and carbonate saturation. This ocean acidification (OA) threatens marine calcifiers who form a calcium carbonate skeleton. However, only recently have we been able to generate accurate and continuous records of seawater pH, hence little is known about historical variability in carbonate chemistry. In coastal Maine (USA) where local factors are further amplifying global ocean trends, OA is posing a significant threat to the shellfish industry.

Here, we use the δ11B measured in two marine calcifiers (the crustose coralline algae Clathromorphum compactum and the bivalve Arctica islandica) to reconstruct changes in seawater pH in the Gulf of Maine. We use an experimentally-derived calibration to convert δ11B in C. compactum into absolute seawater pH. In A. islandica, we reconstruct past variability in δ11B reflecting changes in seawater pH. We find broad agreement in reconstructed pH from multiple specimens of C. compactum, although a potential offset may be required to apply the experimentally-derived calibration in C. compactum to reconstruct the historical pH from wild samples. Furthermore, from all specimens of both taxa, we found significant variability in the reconstructions of historical Gulf of Maine seawater pH. These reconstructions are critical to placing modern OA in a historical context.