Assessing multi-site δ¹⁸O-climate calibrations of the coralline alga Clathromorphum across the high-latitude Northern Hemisphere

An increased number of climate proxy records and more refined interpretation of proxy data are crucial to improve projections of future climate at high latitudes, where internal feedbacks amplify warming and established high-resolution climate archives are especially sparse. Encrusting coralline algae are being developed as a mid- to high-latitude marine climate archive. These long-lived algae form a solid high-Mg calcite skeleton with annual growth bands similar to those of trees and tropical corals. The oxygen isotope ratio of the algal skeleton (δ¹⁸O_alg) records local environmental and climatic factors, notably sea surface temperature and seawater δ¹⁸O. Here we assess the δ¹⁸O_alg–climate relationship in diverse environments across the algal habitat range utilizing two species of coralline algae from the genus Clathromorphum. Clathromorphum is widely distributed from the cold-temperate North Atlantic and Pacific to the Arctic Ocean and has recently yielded numerous climate reconstructions of up to 650 years in length. In this study, we calibrate δ¹⁸O_alg of four specimens to gridded temperature and salinity data, the latter a proxy for seawater δ¹⁸O. These specimens were collected from a variety of algal growth environments across the high-latitude Northern Hemisphere: two specimens from the Aleutian Archipelago, one from the Canadian Arctic, and one from the Gulf of Maine. Low winter temperatures and insolation restrict the months when algae record local climate in the δ¹⁸O of their skeletons; we therefore determine these response seasons by correlating monthly temperature and salinity anomalies with annual δ¹⁸O_alg anomalies at each site. We then average gridded data over months that correlate significantly (95% confidence interval) for regression with δ¹⁸O_alg. While the timing and nature of the climate signal vary across sites, we find significant relationships between δ¹⁸O_alg and either temperature or salinity averaged over the response season at three sites. Variation in local climatology among the four sites provides a physical explanation for calibration differences, compounded by uncertainties stemming from the proxy chronology, biological variability, temporal coverage, and sparse historical climate data. This work takes an essential step toward reconstructing high-latitude marine climate patterns with coralline algal δ¹⁸O and developing algae proxy system models.