Gulf of Maine shells reveal changes in seawater temperature seasonality during the Medieval Climate Anomaly and the Little Ice Age

In this study, we use subannually resolved oxygen isotope values of fossil (dead-collected) and modern (live-caught) bivalve shells (Arctica islandica L.) from the northwestern Atlantic (Gulf of Maine, USA) to reconstruct past seasonal changes in seawater temperature. Our results indicate decreased seasonal temperature amplitude of about 1.6 °C (or ∼ 21%) during Medieval times (ca. AD 1033–1062) compared to shells from the early Little Ice Age (ca. AD 1321–1391) and during the late 19th century (AD 1864–1886). Additionally, seasonal oxygen isotope data suggest that summers were cooler and winters were warmer in the Gulf of Maine during the 11th century compared to summers and winters in the 14th century and the late 19th century. The inferred decreased seasonality during Medieval times likely resulted from increased stratification of the coastal waters due to warmer seawater temperatures. As seawater cooled during the Little Ice Age, we suggest that increased vertical mixing of the coastal surface waters was a major driving factor for the observed increase in the amplitude of the seasonal seawater temperature cycle.