Neogloboquadrina pachyderma (dex. and sin.) Mg/Ca and δ18O records from the Norwegian Sea

Abstract

Magnesium/calcium (Mg/Ca) records based on N. pachyderma (dex.) and N. pachyderma (sin.) from the last 1200 years have been retrieved from high-resolution cores in the Norwegian Sea. Comparing temperatures inferred from Mg/Ca and the oxygen isotopic composition of calcite (δ18OC) with instrumental temperature records from Ocean Weather Ship Mike suggests that N. pachyderma calcifies during the summer season with an offset to oxygen isotope equilibrium of 0.6%. In this region, summer temperatures below the thermocline are related to the winter season ventilation and breakdown of the seasonal thermocline; hence the deeper dwelling N. pachyderma (sin.) provides a winter season signal. Down-core N. pachyderma (dex.) Mg/Ca temperatures display larger variance than observed in δ18OC temperatures record derived from the same morphotype. The smaller variance in the δ18OC N. pachyderma (dex.) temperatures is probably linked to salinity changes in the upper 50 m of the water column. The Mg/Ca temperature and δ18OC records are used to reconstruct changes in the oxygen isotopic composition of water (δ18Ow) during the last 1200 years. The reconstructed δ18OW records show variation within a realistic range, allowing the influence of other water masses than those present at the site today, and suggest that the Mg/Ca signal in morphotypes of N. pachyderma in the Norwegian Sea responds to changes in climatic parameters. © 2006 by the American Geophysical Union.