Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: a methodological overview and data compilation for the Last Glacial Maximum

Abstract

As part of the Multi-proxy Approach for the Reconstruction of the Glacial Ocean (MARGO) incentive, published and unpublished temperature reconstructions for the Last Glacial Maximum (LGM) based on planktonic foraminiferal Mg/Ca ratios have been synthesised and made available in an online database. Development and applications of Mg/Ca thermometry are described in order to illustrate the current state of the method. Various attempts to calibrate foraminiferal Mg/Ca ratios with temperature, including culture, trap and core-top approaches have given very consistent results although differences in methodological techniques can produce offsets between laboratories which need to be assessed and accounted for where possible. Dissolution of foraminiferal calcite at the sea-floor generally causes a lowering of Mg/Ca ratios. This effect requires further study in order to account and potentially correct for it if dissolution has occurred. Mg/Ca thermometry has advantages over other paleotemperature proxies including its use to investigate changes in the oxygen isotopic composition of seawater and the ability to reconstruct changes in the thermal structure of the water column by use of multiple species from different depth and or seasonal habitats. Presently available data are somewhat limited to low latitudes where they give fairly consistent values for the temperature difference between Late Holocene and the LGM (2-3.5 1C). Data from higher latitudes are more sparse, and suggest there may be complicating factors when comparing between multi-proxy reconstructions. r

How warm was the last interglacial? New model–data comparisons

“…The primary SST proxies are also sensitive to changes in seasonality [48,49]. SSTs derived from foraminifera Mg/Ca are known to reflect the calcification temperature of the species, which is best represented by the warm season conditions [50]. Environmental preferences of alkenone-producing algae may bias their proxy data estimates towards warmer temperatures, particularly in regions affected by both upwelling and open ocean conditions [51].…”

Chamber formation leads to Mg/Ca banding in the planktonic foraminifer Neogloboquadrina pachyderma

“…The Mg/Ca ratio in foraminiferal calcite is widely used to reconstruct seawater temperatures (Barker et al, 2005;Jonkers et al, 2010b;Lea et al, 2000;Mashiotta et al, 1999). However, Mg/Catemperature relationships are species specific and remain (largely) empirical and many studies have shown a tight biological control on Mg incorporation in foraminiferal calcite (Anand et al, 2003;Bentov and Erez, 2006;de Nooijer et al, 2014;Jonkers et al, 2012;Jonkers et al, 2013a;Lea et al, 1999;Spero et al, 2015).…”

Increased seasonality in the Western Mediterranean during the last glacial from limpet shell geochemistry

“…Assessing past seasonality during times of different boundary conditions is therefore important to our understanding of the climate system (Denton et al, 2005). Many palaeoclimate proxies, such as foraminiferal Mg/Ca ratios (Barker et al, 2005) and alkenones (Rossell-Melé et al, 2004), can be biassed to a particular season and therefore provide some seasonal information. However, very few extra-tropical records have the resolution to capture the full seasonal cycle and therefore unequivocally reveal the relationships between past seasonality and mean climate.…”