冲绳海槽中部8.2ka以来GDGTs组成及温度重建*

异戊二烯类甘油二烷基甘油四醚脂类化合物(isoprenoid Glycerol dialkyl glycerol tetraethers, isoGDGTs)在海洋奇古菌(Thaumarchaeota)中广泛存在, 其结构对温度变化的敏感性使其成为广受欢迎的古气候与古温度重建材料。在北冰洋和西太平洋气候以及黑潮等多种因素的影响下, 冲绳海槽中部成为研究全新世以来古海洋和古气候变化的天然实验室。本文通过研究C14柱状沉积物的GDGTs组成、含量变化特征及其延伸的86个碳原子的四醚指标(tetraether index of tetraethers consisting of 86 carbons, TEX^H₈₆), 分析冲绳海槽中部的GDGTs来源, 并定量计算C14柱状沉积物记录的海洋表面温度(sea surface temperature, SST), 从而探讨8.2ka以来冲绳海槽中部古温度变化的驱动机制。通过甲烷指数和支链/异戊二烯类指标等, 我们确认isoGDGTs主要来自于氨氧化古菌, 适用于古温度重建。距今8.2ka以来, TEX^H₈₆ SST的变化范围是21.6~27.2℃。冲绳海槽中部SST主要受到西热带太平洋、低纬度冬季日晒量的影响; TEX^H₈₆指标记录的温度上升趋势与东亚夏季风强度的减弱不一致。7.4—6.6ka冷事件广泛存在于冲绳海槽的SST记录中, 但只在TEX^H₈₆数据中显示较大幅度的降低(~5℃), 我们推测可能受到Kikai-Akahoya火山灰(~7.3ka)的影响。

The source of glycerol dibiphytanyl glycerol tetraethers and temperature reconstruction since 8.2 ka in the central Okinawa Trough

The wide occurrence of isoprenoid glycerol dibiphytanyl glycerol tetraethers (isoGDGTs) and its sensitivity to temperature in marine environments proved to be suitable for paleo-climate studies. The Okinawa Trough (OT), under the control of multiple climate patterns and various sediment origins, is a natural laboratory for paleo-climate study since the Holocene. Here, we studied isoGDGTs composition and characteristics, applied outspread tetraether index of tetraethers consisting of 86 carbons (TEX^H₈₆ index), tried to identify the source of isoGDGTs, and quantificationally discussed sea surface temperature (SST) of the central OT for the past 8.2 ka BP. By calculating the Methane Index and the branched isoprenoid tetraether (BIT) indexes, we found isoGDGTs mainly came from Thaumarchaeota, and are suitable for TEX^H₈₆ appliance. TEX^H₈₆ SST of column C14 revealed a range of 21.6-27.2 °C during 8.2 ka BP. Changes in SSTs of the central OT were primarily controlled by the western tropical Pacific Ocean and low-latitude winter insolation. The warming TEX^H₈₆ SSTs in the central OT were decoupled from the descending East Asian summer monsoon intensity in the last 8.2 ka BP. The widely recorded cold event at 7.4-6.6 ka BP was magnified in GDGTs related SST. We attributed it to the well-known Kikai-Akahoya tephra (~7.3 ka BP).