INSIGHT INTO PRE-INDUSTRIAL MULTI-DECADAL CLIMATE AND OCEANOGRAPHIC VARIABILITY FROM THE GEOCHEMISTRY OF A BAHAMIAN SCLEROSPONGE (Invited Presentation)

Recently, efforts have been focused on the use of high-resolution paleoclimate proxies to reconstruct decade to century scale variability. This includes the use of tree rings and corals, though long-term (>500 yrs) reconstructions from these archives are relatively sparse in the literature and those which do exist show marked disagreement prior to the instrumental period. The skeletons of long-lived sclerosponges offer another opportunity to investigate oscillatory change. Here, a ~700 yr old sclerosponge specimen of C. nicholsoni from 133 m depth in Exuma Sound, Bahamas has been U/Th dated and micromilled at a resolution of ~2 samples per yr. The samples have been analyzed for stable C and O isotopes, as well as minor and trace element compositions in order to reconstruct oceanic temperature and salinity.

The results depict clear trends, including the ¹³C Suess Effect showing the input of light CO₂ into the ocean from the burning of fossil fuel over the last 150 yrs. Further, the temperature reconstruction indicates a warming of ~2 °C over this same interval, comparable in amplitude and small-scale variability with published sclerosponge records from the Caribbean. The reconstructions from this specimen also reveal pronounced multi-decadal cyclicity, with paleo-salinity possessing a 20-30 yr periodicity, akin to the solar cycle, over the entirety of the record. From the mid-1700s to the 1990s, the dominant periodicity in the salinity switched to ~68 yrs, consistent with the Atlantic Multi-decadal Oscillation (AMO). High salinity is associated with warm phases of the AMO and vice versa, suggesting that SST and solar variability were communicated to the sclerosponge through changes in the source region of the Salinity Maximum Waters. This record validates previous studies that indicate transitions from inter- to multi-decadal periodicities in the 1700s and supports a natural origin for the AMO through the persistence of multi-decadal trends prior to the period of anthropogenic impact. The marked long-term and regional instabilities between this and other published records imply that modern changes may have significant impacts on these modes, emphasizing the need for continued collaboration between paleoclimate and modeling communities to improve future projections of Atlantic multi-decadal variability.