350 years of paleoceanographic change in the Aleutian archipelago extracted from the skeletons of a subarctic coralline alga

Numerous oceanic passes along the Aleutian Island arc connect the northern North Pacific Ocean to the Bering Sea. Seawater moving through these passes is the dominant source of water along the eastern slope of the Bering Sea, an oceanographically and ecologically significant region that is already changing in response to increasing global temperatures. However, the paucity of high-quality, high-resolution instrumental seawater temperature data extending prior to the mid-20th century prevents an understanding of how these recent changes relate to longer-term climate patterns. The long-lived crustose coralline alga Clathromorphum nereostratum is proving to be a climate archive that yields proxy records able to fill this data gap. This alga is endemic to the North Pacific/Bering Sea region and forms a dominant component of the shallow seafloor along the Aleutian Island arc. Sclerochronological and geochemical measurements of the high-Mg calcite skeleton of C. nereostratum can yield sub-annually resolved, multi-centennial reconstructions of past climatic variability. Here, we measured Mg/Ca ratios across the growth axis in multiple live-collected and subfossil specimens from two Aleutian Islands as a proxy for past surface seawater temperatures. By applying chronologies developed from U/Th dates combined with counts of annual growth rings to the Mg/Ca records, we reconstructed past seawater temperature variability extending from 1658 to 2006. In the record, the significant mode of variability shifted from interdecadal to lower frequency (>50 year) oscillations around the 1850s. In addition, the record significantly correlated with climate patterns spanning from the tropical Pacific to the Arctic Ocean, highlighting the complexity of factors driving the oceanography and climate along the Aleutian archipelago.