Author's personal copy Mg/Ca paleothermometry in high salinity environments Babette A.A. Hoogakker a, ⁎, Gary P. Klinkhammer b , Harry Elderfield a , Eelco J. Rohling c , Chris Hayward a,d a Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, Cambridge, UK b Oregon State University, College of Oceanic and Atmospheric Sciences, Ocean. Admin. Bldg.104, Corvallis, Oregon, 97331, USA c National Oceanography Centre, University of Southampton, SO14 3ZH, Southampton, UK d School of Geosciences, Grant Institute of Earth Sciences, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK abstract article info Article history: Received 21 August 2008 Received in revised form 18 May 2009 Accepted 20 May 2009 Available online 27 June 2009 Editor: P. DeMenocal Keywords: Mg/Ca paleothermometry Red Sea salinity high Mg-calcite overgrowths conventional ICP-AES electron microprobe analysis scanning electron microscopy flow-through time resolved analysis CaCO3 supersaturation Globigerinoides ruber Planktonic foraminiferal Mg/Ca ratios have become a fundamental temperature proxy in past climate reconstructions. However, in the highly evaporative seas of the tropics and subtropics, anomalously high planktonic foraminiferal Mg/Ca ratios arise, possibly linked to high salinities. The extent to which salinity affects Mg uptake into foraminiferal calcite remains disputed. Some studies suggest only minor salinity effects, whereas others suggest a dominant role. Here, we present new data from the highly saline (N 40) Red Sea, which separate pure foraminiferal calcite from other phases. The results show that high Mg/Ca ratios (7 to 13 mmol/mol), found by conventional analysis of planktonic foraminifera from a Red Sea sediment core, are not caused by increased Mg uptake into foraminiferal calcite in a high salinity setting (e.g. beyond those predicted by culturing studies), but instead result from secondary high Mg-calcite overgrowths. The overgrowths likely formed near the sediment–seawater interface, from CaCO 3 supersaturated interstitial seawater. © 2009 Elsevier B.V. All rights reserved. 1. Introduction To fully appreciate the complexity of climate change, a thorough knowledge of past ocean hydrography and circulation is required. Heat exchange between the atmosphere and ocean takes place at the sea surface. Variations in this heat exchange play a crucial role in climate change, and it is essential that we develop reliable estimates of sea surface temperatures (SST) and salinity, which together determine ocean density (hence, circulation), as well as global ice volume/sea-level (Curry et al., 2003). Sea water temperature reconstructions based on Mg/Ca ratios in planktonic foraminiferal calcite in addition offer the potential for reconstructing depth-specific (seasonal) temperature, given that the ecology of the investigated planktonic foraminiferal species is suf- ficiently constrained. A specific advantage of Mg/Ca for reconstruct- ing sea water temperatures is that it can be paired with δ 18 O calcite measurements on the same shells, allowing its use to derive δ 18 O water (Elderfield and Ganssen, 2000), which provides critical information about global ice volume and the regional hydrological budget. Although planktonic foraminiferal Mg/Ca ratios have been widely used as a paleotemperature proxy, it has been found that the con- ventional approach of this method, that of bulk analysis of foraminifera shells after chemical cleaning, produces anomalously high Mg/Ca ratios in strongly evaporative areas of the tropics and subtropics (e.g. Greater and Little Bahama Bank, Eastern Mediterra- nean Sea, and Red Sea) (Rosenthal et al., 2000; Lear et al., 2002; Reuning et al., 2005; Ferguson et al., 2008). Recently, Ferguson et al. (2008) suggested that such anomalously high values occur because salinity may exert an important control on Mg/Ca ratios in planktonic foraminifera, with Mg/Ca ratios being 15–60% higher per salinity unit increase than estimated from pub- lished calibrations of Mg/Ca increase with temperature. Such an effect would compromise the applicability of foraminiferal Mg/Ca ratios as a paleo seawater temperature proxy. For example, Caribbean Last Glacial Maximum (LGM) temperatures could be overestimated by more than 2.5 °C (Ferguson et al., 2008). Therefore, it is essential that potential effects of salinity on Mg uptake in Earth and Planetary Science Letters 284 (2009) 583–589 ⁎ Corresponding author. E-mail address: bhoo03@esc.cam.ac.uk (B.A.A. Hoogakker). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.05.027 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl