Isoprenoid tetraether lipids, TEX86H and BIT in the Mediterranean Sea

The TEX86H paleothermometer based on isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) has widely been applied in various marine settings to reconstruct past sea surface temperatures (SSTs). However, it still remains uncertain how well this proxy reconstructs annual mean SSTs. Here, we assess environmental factors governing the TEX86H paleothermometer in the Mediterranean Sea, by studying the distribution of isoGDGTs in surface sediments, suspended particulate matter (SPM), and two sediment cores. A redundancy analysis using the fractional abundance of the six major isoGDGTs indicates that the sedimentary isoGDGTs are mostly influenced by three environmental factors explaining a large part (74%) of the variance in isoGDGT distribution. In order of decreasing significance, these factors are annual mean SST, continental organic matter input as indicated by the BIT index, and water depth. However, when considering only the four isoGDGTs that are used for the TEX86H proxy, water depth is the most significant parameter, explaining 63% of the variance. Indeed, a strong positive relationship between water depth and TEX86H is observed in both surface sediments and SPM from the Mediterranean Sea. This is driven by an increase in fractional abundances of GDGT-2 and crenarchaeol regio-isomer and a decrease in the fractional abundances of GDGT-1 and GDGT-3 with increasing water depth, leading to a bias to higher temperatures of TEX86H in deep-water surface sediments. The fact that the water-depth trend is also apparent in SPM suggests that this change might be due to a change in thaumarchaeotal community thriving below surface mixed-layer waters and that this signal is, at least partly, incorporated into sedimentary isoGDGTs. Interestingly, surface-sediment TEX86H values from >1000 m water depth do not show a correlation with water depth anymore and instead are correlated to annual mean SSTs. A composite deep-water TEX86H dataset of surface sediments from both the Mediterranean Sea and the Red Sea, interconnected regional restricted basins with relatively high bottom-water temperatures and high salinity, forms a distinctive correlation line, statistically distinct from that of the general global correlation. Application of this correlation on two sedimentary records from the western Mediterranean Sea covering the last deglaciation yields SSTs nearly identical to those obtained with the UK'37 paleothermometer, whereas the global calibration substantially overestimates SSTs. Our results show that the warm bias of the TEX86H proxy in the Mediterranean Sea is not due to seasonality, as previously suggested. Further research is needed to elucidate the mechanism behind the strong water depth trend of TEX86H in the Mediterranean Sea which is not apparent in open ocean settings.

基于类异戊二烯甘油二烷基甘油四醚（isoprenoid glycerol dialkyl glycerol tetraethers, isoGDGTs）的TEX86H古温度计，已广泛应用于各类海洋环境以重建过去的海表温度（sea surface temperatures, SSTs）。然而，该代用指标在重建年平均海表温度的表现仍未明确。本研究通过研究表层沉积物、悬浮颗粒物（suspended particulate matter, SPM）以及两个沉积岩芯中的isoGDGTs分布，评估了地中海地区调控TEX86H古温度计的环境因素。基于六种主要isoGDGTs分数丰度的冗余分析结果显示，沉积isoGDGTs主要受三种环境因素影响，可解释isoGDGT分布74%的方差。按影响显著性从高到低排序，这三个因素分别为年平均海表温度、由BIT指数指示的陆源有机质输入，以及水深。不过，当仅考虑TEX86H代用指标所使用的四种isoGDGTs时，水深成为最显著的参数，可解释63%的方差。研究在地中海的表层沉积物与悬浮颗粒物中，均观测到水深与TEX86H之间存在显著正相关关系。该现象源于：随着水深增加，GDGT-2与克劳索醇区域异构体（crenarchaeol regio-isomer）的分数丰度上升，而GDGT-1与GDGT-3的分数丰度下降，最终导致深水表层沉积物中的TEX86H计算温度偏高。悬浮颗粒物中同样存在水深趋势这一事实表明，这种变化可能源于在表层混合层以下繁衍的奇古菌群落组成的改变，且该信号至少部分被保存在沉积isoGDGTs中。有趣的是，水深超过1000米的表层沉积物的TEX86H值不再与水深相关，转而与年平均海表温度呈现相关性。我们将地中海与红海——二者均为区域封闭盆地，具有较高的底层水温与盐度——的表层沉积物TEX86H数据集整合，形成了一条独特的相关线，在统计学上与全球通用的校准曲线存在显著差异。将该校准曲线应用于地中海西部两条覆盖末次冰消期的沉积记录，所得海表温度结果与利用UK'37古温度计得到的结果几乎一致，而全球通用的校准方法则显著高估了海表温度。本研究结果表明，地中海地区TEX86H代用指标的暖偏差并非如此前研究所提出的那样由季节因素导致。未来仍需进一步研究，以阐明地中海地区TEX86H显著水深趋势背后的机制——这一趋势在开阔大洋环境中并未显现。