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Distributions of 5- and 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) in East African lake sediment: Effects of temperature, pH, and new lacustrine paleotemperature calibrations
Organic Geochemistry ( IF 3 ) Pub Date : 2018-03-01 , DOI: 10.1016/j.orggeochem.2017.12.003
James M. Russell , Ellen C. Hopmans , Shannon E. Loomis , Jie Liang , Jaap S. Sinninghe Damsté

Abstract The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, peats, and lake sediments has been shown to correlate with mean annual air temperature (MAAT) and has provided valuable new climate reconstructions. Here we use an improved chromatographic method to quantify the fractional abundances of 5- and 6-methyl isomers in surface sediments from 65 East African lakes spanning temperatures 1.6–26.8 °C, and investigate the relationships between these fractional abundances and temperature, lake pH, and other environmental variables. We find that temperature exerts a strong control on brGDGT distributions, including the relative abundances of 5- and 6-methyl isomers, whereas other environmental variables, including lake pH, are weakly correlated to the fractional abundances of the brGDGTs. The distributions of brGDGTs in our lake sediments differ from those of soils and peats, leading to temperature offsets if soil- and peat-based brGDGT temperature calibrations are applied. We develop new calibrations for MAAT for use in lake sediment based upon the MBT′5Me and Index 1 ratios, as well as a multivariate regression of brGDGT fractional abundances on temperature using stepwise forward selection. We obtain root mean square errors (RMSE) between ∼ 2.1 and 2.5 °C for these calibrations, highlighting the potential for brGDGTs to provide precise temperature reconstructions using lake sediment cores. Calibrations for lake pH perform more poorly, likely due to weak correlations between pH and brGDGT distributions in East African lakes. These results indicate that quantification of 5- and 6-methyl isomers separately in lake sediment can improve paleoclimatic reconstructions.

中文翻译:

东非湖泊沉积物中 5- 和 6- 甲基支化甘油二烷基甘油四醚 (brGDGT) 的分布:温度、pH 值和新湖相古温度校准的影响

摘要 支化甘油二烷基甘油四醚 (brGDGTs) 在土壤、泥炭和湖泊沉积物中的分布已被证明与年平均气温 (MAAT) 相关,并提供了有价值的新气候重建。在这里,我们使用改进的色谱方法来量化来自 65 个东非湖泊的表层沉积物中 5- 和 6- 甲基异构体的丰度分数,这些湖泊的温度跨度为 1.6-26.8 °C,并研究了这些分数丰度与温度、湖泊 pH 值、和其他环境变量。我们发现温度对 brGDGT 分布有很强的控制,包括 5- 和 6- 甲基异构体的相对丰度,而其他环境变量,包括湖泊 pH 值,与 brGDGT 的丰度分数相关性较弱。我们湖泊沉积物中 brGDGT 的分布与土壤和泥炭的分布不同,如果应用基于土壤和泥炭的 brGDGT 温度校准,则会导致温度偏移。我们基于 MBT'5Me 和 Index 1 比率开发了用于湖泊沉积物的 MAAT 的新校准,以及使用逐步正向选择的 brGDGT 分数丰度对温度的多元回归。对于这些校准,我们获得了介于 ~ 2.1 和 2.5 °C 之间的均方根误差 (RMSE),突出了 brGDGT 使用湖泊沉积物核心提供精确温度重建的潜力。湖泊 pH 值的校准效果较差,可能是由于东非湖泊中 pH 值和 brGDGT 分布之间的相关性较弱。
更新日期:2018-03-01
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