Mid-latitude shelf seas: a NW European perspective on the seasonal dynamics of temperature, salinity and oxygen isotopes

Pronounced seasonal variability, particularly in the surface ocean heat flux, imparts an important control that drives thermal stratification of the tide-dominated middle-and high-latitude shelf seas. Bottom water temperature and salinity data, resolved on a grid 20’ latitude by 30’ longitude, were combined with a regional synthesis of the salinity: δ18O relationship in order to generate a spatial and temporal understanding of oxygen isotopes in seawater around the shelf seas of NW Europe. The data are expressed according to equilibrium calcite (δ18OEq.calcite) and, in the shallow mixed water column, exhibit large seasonal changes that are primarily driven by bottom water temperature. Annual bottom water temperature varies from 3°C to 17°C in the southern North Sea, generating a seasonal δ18OEq.calcite signal of up to 3.2%. The amplitude of the seasonal d18OEq.calcite signal is significantly damped (0.1 - 0.2o/oo) in deeper, thermally stratified shelf waters. Maps of the monthly distribution δ18OEq.calcite provide the first systematic overview of the spatial and temporal changes on the NW European shelf and highlight the importance of understanding seasonal growth on the incorporation of geochemical signatures into marine organisms.