Mg/Ca-temperature proxy in benthic foraminifera from the Florida Strait

Over the past decade, the ratio of Mg to Ca in foraminiferal tests has emerged as a valuable paleotemperature proxy. However, large uncertainties remain in the relationships between benthic foraminiferal Mg/Ca and temperature. Mg/Ca was measured in benthic foraminifera from 31 high-quality multicore tops collected in the Florida Straits, spanning a temperature range of 5.8° to 18.6°C. New calibrations are presented for Uvigerina peregrina, Planulina ariminensis, Planulina foveolata, and Hoeglundina elegans. The Mg/Ca values and temperature sensitivities vary among species, but all species exhibit a positive correlation that decreases in slope at higher temperatures. The decrease in the sensitivity of Mg/Ca to temperature may potentially be explained by Mg/Ca suppression at high carbonate ion concentrations. It is suggested that a carbonate ion influence on Mg/Ca may be adjusted for by dividing Mg/Ca by Li/Ca. The Mg/Li ratio displays stronger correlations to temperature, with up to 90% of variance explained, than Mg/Ca alone. These new calibrations are tested on several Last Glacial Maximum (LGM) samples from the Florida Straits. LGM temperatures reconstructed from Mg/Ca and Mg/Li are generally more scattered than core top measurements and may be contaminated by high-Mg overgrowths. The potential for Mg/Ca and Mg/Li as temperature proxies warrants further testing.近十年来，有孔虫壳体中的镁钙比（Mg/Ca）已成为极具应用价值的古温度代用指标。然而，底栖有孔虫Mg/Ca与温度之间的关系仍存在较大不确定性。本研究对佛罗里达海峡采集的31根高质量多管岩芯表层样中的底栖有孔虫进行了Mg/Ca比值测定，其对应温度区间为5.8℃至18.6℃。本研究针对异地冀地虫（Uvigerina peregrina）、阿里米平旋虫（Planulina ariminensis）、凹坑平旋虫（Planulina foveolata）与优雅霍氏虫（Hoeglundina elegans）建立了全新的温度校准模型。不同物种的Mg/Ca比值与温度敏感性存在显著差异，但所有物种均呈现正相关关系，且温度敏感性的斜率随温度升高而逐渐降低。Mg/Ca对温度的敏感性随温度升高而降低这一现象，或可通过高碳酸根离子浓度下的Mg/Ca抑制效应得到解释。研究表明，若将Mg/Ca比值除以锂钙比（Li/Ca），即可校正碳酸根离子对Mg/Ca比值的影响。相较于单一的Mg/Ca比值，镁锂比（Mg/Li）与温度的相关性更强，其方差解释率最高可达90%。本研究利用佛罗里达海峡的多个末次冰盛期（Last Glacial Maximum, LGM）样品对上述新校准模型进行了验证。通过Mg/Ca与Mg/Li比值重建的末次冰盛期温度，其离散程度普遍高于岩芯表层样的测定结果，且可能受到高镁次生包裹体的污染。Mg/Ca与Mg/Li比值作为古温度代用指标的应用潜力，仍有待进一步的实验验证。