Evaluation of the distributions of hydroxylated isoprenoidal GDGTs in Holocene Baltic Sea sediments for reconstruction of sea surface temperature: The effect of changing salinity
收藏资源简介：
Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if their OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT proxies for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH? indices, were tested using a Thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22?C, and 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise of temperature, but their absolute values were not always in line with existing marine core top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH? index shows only a linear correlation with SST when the sediments from the Bothnian Sea and Bay area, with a reduced salinity and increased lateral sediment influx, are omitted from the dataset. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH? index shows a good correlation with the TEX_86^( L), an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH? (and RI-OH) index records far too high values, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano?s water column. In the Littorina Sea, the Ancylus Lake and the Yoldia Lake phases of the Baltic basin, the record of the RI-OH? index, thus, most likely reflects both changes in temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH? (and RI-OH) indices substantially and this should be considered when applying these proxies in other settings.
由海洋与淡水奇古菌（Thaumarchaea）产生的羟基化甘油二植烷醇甘油四醚（Hydroxylated glycerol dibiphytanyl glycerol tetraethers, OH-GDGTs），现已愈发广泛地应用于古海表温度（sea surface temperature, SST）的重建工作。该类化合物广泛分布于现代波罗的海海域，但在全新世（Holocene）盐度发生显著变化的波罗的海区域，其OH-GDGTs能否用于评估古海表温度，目前仍不明确。 本研究选取三种常用于海表温度重建的OH-GDGTs代用指标，即OH-GDGT%、RI-OH及RI-OH？指标，通过两组实验开展验证：其一为从波罗的海富集培养的奇古菌菌株，分别在4℃与22℃条件下进行培养；其二为采集自波罗的海及邻近斯卡格拉克海峡的12个表层沉积物样品。 培养实验结果显示，三种指标均随温度升高呈现出预期的响应趋势，但其绝对数值并非始终与已有的海洋岩芯表层校准结果相符，尤以OH-GDGT%指标最为显著。在两种基于OH-GDGTs分布的代用指标中，RI-OH指标并未随年平均海表温度的升高而增加；而RI-OH？指标仅在剔除盐度降低且侧向沉积物输入增加的波的尼亚海与波的尼亚湾区域的沉积物样品后，才与海表温度呈现出线性相关关系。 本研究还对阿尔科纳盆地与哥特兰盆地的两处全新世沉积记录展开了分析，其中哥特兰盆地的记录具备高分辨率特征。在波罗的海的半咸水阶段（即利托尔内亚海期），RI-OH？指标与波罗的海已成熟的温度代用指标TEX₈₆^L呈现出良好的相关性，可用于识别关键气候事件。然而在前序的安基卢斯湖期，RI-OH？（及RI-OH）指标记录的数值显著偏高，导致估算的海表温度出现异常偏高的情况。 这一现象可能源于淡水奇古菌对其OH-GDGTs的调控机制存在差异，正如卢加诺湖水体中奇古菌的相关研究所示。在波罗的海盆的利托尔内亚海期、安基卢斯湖期和约尔迪亚湖期，RI-OH？指标的记录很可能同时反映了温度与盐度的变化。总体而言，本研究表明盐度降低会显著提升RI-OH？（及RI-OH）指标的数值，在其他环境中应用此类代用指标时，应充分考虑这一影响因素。
