Seasonal Mg/Ca variability of N. pachyderma (s) and G. bulloides: implications for seawater temperature reconstruction

• Published in: Earth and Planetary Science Letters
• Main Authors: Jonkers, Lukas, Jiménez-Amat, Patricia, Mortyn, P. Graham, Brummer, Geert-Jan A.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2013
• Subjects: GC Oceanography; North Atlantic
• Online Access: https://orca.cardiff.ac.uk/id/eprint/48524/; https://doi.org/10.1016/j.epsl.2013.06.019

Given the importance of high-latitude areas in the ocean–climate system, there is need for a paleothermometer that is reliable at low temperatures. Here we assess the applicability of the Mg/Ca-temperature proxy in colder waters (5–10 °C) by comparing for the first time the seasonal Mg/Ca and δ18O cycles of N. pachyderma (s) and G. bulloides using a sediment trap time-series from the northern North Atlantic. While both species show indistinguishable seasonal δ18O patterns that clearly track the near surface temperature cycle, their Mg/Ca are very different. G. bulloides Mg/Ca is high (2.0–3.1 mmol/mol), but varies in concert with the seasonal temperature cycle. The Mg/Ca of N. pachyderma (s), on the other hand, is low (1.1–1.5 mmol/mol) and shows only a very weak seasonal cycle. The δ18O patterns indicate that both species calcify in the same depth zone. Consequently, depth habitat differences cannot explain the contrasting Mg/Ca patterns. The elevated Mg/Ca in pristine G. bulloides might be due to the presence of high Mg phases that are not preserved in fossil shells. The contrasting absence of a seasonal trend in the Mg/Ca of N. pachyderma (s) confirms other studies where calcification temperatures were less well constrained. The reason for this absence is not fully known, but may include species-specific vital effects. The very different seasonal patterns of both speciesʼ Mg/Ca underscore the importance of parameters other than temperature in controlling planktonic foraminiferal Mg/Ca. Our results therefore lend further caution in the interpretation of Mg/Ca-temperature reconstructions from high northern latitudes.