Reconstructing Components of the Atlantic Meridional Overturning Circulation Over the Last 300 Years Using Multiple High Resolution Geochemical Proxies Measured in Arctica Islandica Shells

Due to the short length of the instrumental records available, palaeoceanographic reconstructions of components of the Atlantic Meridional Overturning Circulation (AMOC) are critical in understanding the variability of this important climate parameter. All palaeoceanographic records that extend until present day suggest recent weakening of the AMOC. However, the timing of this weakening varies, from a commencement at the end of the Little Ice Age (c. 1850) to one around the mid-twentieth century. Therefore, additional reconstructions of the AMOC are warranted in order to broaden our understanding of AMOC behavior in the last millennia.

Here we present a 300 year reconstruction of relative bottom current AMOC strength by reconstructing water properties in the Gulf of Maine. These water properties have been shown to be impacted by the strength of the Deep Western Boundary Current (DWBC), a component of the AMOC. The DWBC dictates the position of major ocean currents, including the Gulf Stream, relative to the entrance to the Gulf of Maine and thereby influences the proportion of different-sourced water masses in the region. Among other differences, these water masses vary in their temperature (as measured by oxygen isotopes - δ¹⁸O), nitrogen isotope (δ¹⁵N) composition, and radiocarbon (Δ¹⁴C) signature. We have measured these geochemical proxies in Arctica islandica shells collected from the western Gulf of Maine.

δ¹⁸O values suggest that the Gulf of Maine has been warming since 1890. Strong similarities in trend between this record and the δ¹⁵N and Δ¹⁴C records suggest that this warming is in part related to an increasing proportion of Gulf Stream derived waters in the region, suggesting a weakening AMOC. The timing of this weakening is similar to several other high resolution AMOC proxies in the western North Atlantic. Short but strong correlations with the AMOC RAPID array and the Florida Current instrumental record confirm a relationship between these geochemical proxies and AMOC variability.

All three geochemical proxies suggest that the AMOC strengthened out of the Little Ice Age prior to the weakening at the end of the 19^th century. This finding does not agree with other high resolution records of AMOC variability in the area and suggests that further investigation into AMOC behavior out of the Little Ice Age is needed.