Abstract Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are abundant in lake sediments and can potentially be used as proxies for evaluating past environmental conditions. The degree of methylation of brGDGTs has a strong relationship with the ambient temperature, and numerous brGDGT temperature reconstructions have been generated ranging from the tropics to high latitudes and over a variety of timescales. However, this proxy still requires calibration and validation in Arctic settings, which are among the most climatically sensitive regions on Earth. Here, we investigate Lake 578, a dimictic lake located in southern Greenland, and examine brGDGTs in a sediment core, surface sediments, catchment soils, and settling particulate matter (SPM). Our results suggest that sedimentary brGDGTs are mainly produced in the water column, and the SPM MBT′5ME (Methylation of Branched Tetraethers) values are significantly correlated with depth-integrated summer water temperature. Here, we provide the first lake water temperature-brGDGT calibration based on the MBT′5ME index, with an improved root mean square error (RMSE) of 0.52 °C. Like previous studies, we find that watershed soils differ in brGDGT distributions from those of the SPM or lake sediments, and we conclude that most brGDGT production in Lake 578 is occurring in situ. In the sediment core, we observe a significant cooling trend in the uppermost 7 cm, but this result is unrealistic because it conflicts with the instrumental temperature record from this region. This “core-top cooling” is associated with increasing relative abundances of brGDGT-IIa and IIIa, and with decreasing brGDGT-Ia, and we speculate that it may be driven by diagenetic effects or changing microbial communities in the sediment subsurface. The anomalous “core-top cooling” phenomenon, which is noted in numerous other published lacustrine brGDGT records, highlights the need for further studies of brGDGTs in modern and recent lake sediments. Constraining the source organisms of brGDGTs and better understanding the relationship between brGDGTs and environmental factors are essential, prior to their use in paleoclimate reconstructions.

中文翻译：

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格陵兰岛南部 578 湖原位分支 GDGT 校准的开发

摘要支链甘油二烷基甘油四醚 (brGDGTs) 在湖泊沉积物中含量丰富，可作为评估过去环境条件的替代物。brGDGTs 的甲基化程度与环境温度有很强的关系，从热带到高纬度地区，在各种时间尺度上，已经产生了许多 brGDGT 温度重建。然而，该代理仍需要在北极环境中进行校准和验证，北极环境是地球上气候最敏感的地区之一。在这里，我们调查了位于格陵兰岛南部的双晶湖 578 湖，并检查了沉积物核心、表层沉积物、集水土壤和沉降颗粒物质 (SPM) 中的 brGDGT。我们的结果表明沉积 brGDGTs 主要在水体中产生，SPM MBT'5ME（支链四醚甲基化）值与夏季水温深度积分显着相关。在这里，我们提供了第一个基于 MBT'5ME 指数的湖水温度-brGDGT 校准，均方根误差 (RMSE) 改进为 0.52 °C。与之前的研究一样，我们发现流域土壤的 brGDGT 分布与 SPM 或湖泊沉积物的分布不同，我们得出结论，578 湖中的大多数 brGDGT 生产发生在原位。在沉积岩芯中，我们观察到最上部 7 cm 处有明显的降温趋势，但该结果与该区域的仪器温度记录相冲突，因此不切实际。这种“核心顶部冷却”与 brGDGT-IIa 和 IIIa 的相对丰度增加以及 brGDGT-Ia 的减少有关，我们推测这可能是由成岩作用或沉积物地下微生物群落的变化驱动的。在许多其他已发表的湖泊 brGDGT 记录中注意到了异常的“芯顶冷却”现象，突出了对现代和最近湖泊沉积物中 brGDGT 进行进一步研究的必要性。在将 brGDGTs 用于古气候重建之前，限制 brGDGTs 的源生物并更好地理解 brGDGTs 与环境因素之间的关系是必不可少的。