Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

• Main Authors: Parker, W.G., Yanes, Y., Hernández, E.M., Surge, D.
• Other Authors: University of North Carolina at Chapel Hill
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Phorcus atratus; Holocene; Stable isotopes; Medieval climate anomaly; North Atlantic; Paleoclimatology; North Atlantic oscillation
• Online Access: https://doi.org/10.17615/q25x-cz56; https://cdr.lib.unc.edu/downloads/2v23w466t?file=thumbnail; https://cdr.lib.unc.edu/downloads/2v23w466t

The Medieval Climate Anomaly (MCA; 900–1300 AD) was the most recent period of pre-industrial climatic warming in the northern hemisphere, and thus estimations of MCA signals can illuminate possible impacts of anthropogenic climate change. Current high-resolution MCA climate signals are restricted to mid- and high-latitude regions, which confounds inferences of how the MCA impacted some global/hemispheric climate mechanisms (e.g. North Atlantic Oscillation; NAO). To address this knowledge gap, we estimate seasonally-resolved sea surface temperatures (SSTs) from the oxygen isotope composition (δ18O) of serially sampled Phorcus atratus shells from archaeological sites spanning the MCA in the Canary Islands. Twelve archaeological and six modern P. atratus shells were analyzed, and archaeological shells were dated using carbonate-target radiocarbon dating. SSTs were estimated using the published aragonite-water equilibrium fractionation equation. Modern shells showed a mean SST of 20.0 ± 1.5 °C, with a seasonal amplitude of 5.3 ± 0.9 °C. Archaeological shells exhibited a mean SST of 18.2 ± 0.7 °C, with a mean seasonal amplitude of 5.5 ± 1.0 °C. Thus, shells that span the MCA in the Canary Islands recorded SSTs that were significantly cooler than the modern (P <.05), contrasting with warming estimates and model predictions elsewhere in the Northern Hemisphere. We propose that the observed cooling resulted from increased upwelling in NW Africa due to a strengthening of the prevailing westerlies and coastal winds along the African shoreline. The intensified upwelling scenario during the MCA is partially supported by in-situ carbon isotope data (δ13C) retrieved from the archaeological shells, which was compared to the δ13C values of modern shells and dissolved inorganic carbon in the ambient seawater. These results are consistent with other low-latitude temperature/precipitation anomalies associated with a positive NAO mode, suggesting a transition to a positive NAO index during the middle and late MCA that possibly ...