Sources of local and regional variability in the MBT′/CBT paleotemperature proxy: Insights from a modern elevation transect across the Eastern Cordillera of Colombia
Sources of local and regional variability in the MBT′/CBT paleotemperature proxy: Insights from a modern elevation transect across the Eastern Cordillera of Colombia
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DOI：
10.1016/j.orggeochem.2014.01.022
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发表时间：
2014-04-01
影响因子：
3
通讯作者：
Mora, Andres R.
中科院分区：
文献类型：
--
作者：
Anderson, Veronica J.;Shanahan, Timothy M.;Mora, Andres R.
在文献中，通过MBT‘和CBT指数(分别是支链四醚的甲基化和环化)，brGDGT(支链甘油二烷基甘油四醚)在土壤中的分布被用作温度和pH的替代。然而，全球MBT/CBT指数与现代温度之间的经验关系存在很大的分散性，导致重建的古温度的不确定度高达+/-5.0℃。在这项研究中，我们试图使用一组新的样本来确定校准误差的几个来源的大小，这些样本跨越哥伦比亚东科迪勒拉的安第斯样带的海拔(约3000米)和温度(约16摄氏度)的大梯度。利用现场温度监测，这一赤道山地位置的实地研究提供了一个机会，以调查影响MBT‘/CBT古温度计的几个潜在混杂因素，包括气温与实际土壤温度之间的不匹配，以及土壤水分对brGDGT产量的影响。在我们研究的受限环境中，我们确定MBT‘/CBT代理的局部校准的RMSE(均方根误差)为3.0摄氏度，而最近的全球校准报告的RMSE(均方根误差)为5.0摄氏度。在我们的土壤采样点现场测量的土壤温度与内插的气象站温度数据之间的比较表明，内插方案中的误差不太可能是在MBT‘/CBT-温度关系中观测到的散布的重要因素。相反，我们得出的结论是，在安第斯山脉北部的这一校准中，3摄氏度的散布大部分来自于土壤水分和养分有效性等因素的瞬时、特定地点的变化，有几个例子表明，紧密相隔的地点之间的差异高达7.8摄氏度(距离600米，海拔50米)。此外，我们将我们的数据集与已公布的5个区域土壤高程样带进行了比较，发现MBT‘指数在降水较多的地区(如哥伦比亚)与温度有很强的相关性(R-2 0.67，0.47)，但在干旱地区与温度的相关性较弱(R-2 0.01，0.04)。相反，干旱地区的MBT‘、CBT和pH之间表现出强烈的相关关系，而高降水样带则没有这种关系。这支持了之前的发现，即在土壤水分较低的地点，brGDGT分布与温度之间的关系可能会发生变化。BrGDGT分布对温度和pH的这种异质响应可能是全球校准数据集中显著分散的主要原因。因此，我们得出的结论是，在我们的局部校准中，3℃的RMSE可能代表了基于MBT/CBT的潮湿地区温度重建误差的下限，但需要进一步的工作来可靠地确定该指标应用于古代样品的合适条件。(C)2014爱思唯尔有限公司。保留所有权利。
Distributions of brGDGTs (branched glycerol dialkyl glycerol tetraethers) in soils have been utilized in the literature as a proxy for temperature and pH via the MBT' and CBT indices (methylation of branched tetraethers and cyclization of branched tetraethers, respectively). However, there is substantial scatter in the empirical relationships between the global MBT'/CBT index and modern temperature, resulting in uncertainty as large as +/- 5.0 degrees C in reconstructed paleotemperature. In this study, we sought to determine the magnitude of several sources of calibration error using a new set of samples spanning a large gradient in elevation (ca. 3000 m) and temperature (ca. 16 degrees C) across an Andean transect in the Eastern Cordillera of Colombia. Using in situ temperature monitoring, this field study of an equatorial montane location provided an opportunity to investigate several potential confounding factors affecting the MBT'/CBT paleothermometer, including the mismatch between air temperature and actual soil temperature, and the effect of soil moisture on brGDGT production. In the well-constrained environment of our study, we determined the RMSE (root mean squared-error) for a local calibration of the MBT'/CBT proxy to be 3.0 degrees C, in contrast to the 5.0 degrees C value reported in the most recent global calibration. A comparison between in situ soil temperature measurements at our soil sampling locations and interpolated weather station temperature data indicates that errors in interpolation schemes are not likely to be a significant contributor to the observed scatter in the MBT'/CBT-temperature relationship. Rather, we conclude that the majority of the 3 degrees C scatter in this calibration for the northern Andes arises from transient, site specific variation in factors such as soil moisture and nutrient availability, with several cases showing a difference of up to 7.8 degrees C between closely spaced sites (< 600 m in distance and < 50 m in elevation apart). In addition, we compared our dataset with five published regional soil elevation transects and found that that the MBT' index correlated strongly with temperature at sites with high precipitation, such as Colombia (R-2 0.67, 0.47), but correlated weakly with temperature at arid sites (R-2 0.01, 0.04). Instead, the arid sites showed a strong relationship among MBT', CBT and pH that was absent from the higher precipitation transects. This bolsters previous findings that the relationship between brGDGT distributions and temperature may change at sites with low soil moisture. This heterogeneous response in brGDGT distributions to temperature and pH may be largely responsible for the significant scatter in the global calibration dataset. We therefore conclude that the RMSE of 3 degrees C in our local calibration may represent a lower boundary on the error in MBT'/CBT-based temperature reconstruction in moist regions, but further work is needed to reliably identify suitable conditions for the application of this proxy to ancient samples. (C) 2014 Elsevier Ltd. All rights reserved.