Mg/Ca-Temperature Calibration of Polar Benthic foraminifera species for reconstruction of bottom water temperatures on the Antarctic shelf

Elaine M Mawbey,Katharine R Hendry,Mervyn J Greaves,Claus-Dieter Hillenbrand,Gerhard Kuhn,Charlotte L Spencer-Jones,Erin L Mcclymont,Kara J Vadman,Amelia E Shevenell,Patrycja E Jernas,James A Smith

doi:10.1016/j.gca.2020.05.027

Abstract

Benthic foraminifera Mg/Ca is a well-established bottom water temperature (BWT) proxy used in paleoclimate studies. The relationship between Mg/Ca and BWT for numerous species has been determined using core-top and culturing studies. However, the scarcity of calcareous microfossils in Antarctic shelf sediments and poorly defined calibrations at low temperatures has limited the use of the foraminiferal Mg/Ca paleothermometer in ice proximal Antarctic sediments. Here we present paired ocean temperature and modern benthic foraminifera Mg/Ca data for three species, Trifarina angulosa, Bulimina aculeata, and Globocassidulina subglobosa, but with a particular focus on Trifarina angulosa. The core-top data from several Antarctic sectors span a BWT range of −1.7 to +1.2 °C and constrain the relationship between Mg/Ca and cold temperatures. We compare our results to published lower-latitude core-top data for species in the same or related genera, and in the case of Trifarina angulosa, produce a regional calibration. The resulting regional equation for Trifarina angulosa is Temperature (°C) = (Mg/Ca −1.14 ± 0.035)/0.069 ± 0.033). Addition of our Trifarina angulosa data to the previously published Uvigerina spp. dataset provides an alternative global calibration, although some data points appear to be offset from this relationship and are discussed. Mg-temperature relationships for Bulimina aculeata and Globocassidulina subglobosa are also combined with previously published data to produce calibration equations of Temperature (°C) = (Mg/Ca-1.04 ± 0.07)/0.099 ± 0.01 and Temperature (°C) = (Mg/Ca-0.99 ± 0.03)/0.087 ± 0.01, respectively. These refined calibrations highlight the potential utility of benthic foraminifera Mg/Ca-paleothermometry for reconstructing past BWT in Antarctic margin settings.

Highlights

Satellite observations indicate that the West Antarctic Ice Sheet (WAIS) is losing mass at an accelerating rate (Rignot et al, 2019)
Mass loss is primarily driven by basal melting of ice shelves by warm Circumpolar Deep Water (CDW), which upwells at the continental shelf edge and moves across the shelf through bathymetric troughs to grounding lines (Pritchard et al, 2012)
Of particular concern is the future behaviour of ice streams draining into the Amundsen Sea Embayment (ASE), such as Pine Island and Thwaites glaciers, which contain $1.4 m of sea level-equivalent ice (Vaughan et al, 2006)

Summary

Introduction

Satellite observations indicate that the West Antarctic Ice Sheet (WAIS) is losing mass at an accelerating rate (Rignot et al, 2019). Mass loss is primarily driven by basal melting of ice shelves by warm Circumpolar Deep Water (CDW), which upwells at the continental shelf edge and moves across the shelf through bathymetric troughs to grounding lines (Pritchard et al, 2012). Predicting the future response of glaciers in this region to ongoing warming (e.g., DeConto and Pollard, 2016) requires a comprehensive understanding of the potential influence of CDW on Antarctic glaciers over coming decades and centuries (Colleoni et al, 2018). To provide context for recent observations and improve understanding of the future behaviour of ice sheets in Antarctica, detailed records of past seawater temperatures are required from Antarctica’s continental shelves (Shevenell et al, 2011; Hillenbrand et al, 2017)

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Discussion

Conclusion