Fractional abundance and indices for brGDGTs and crenarchaeol in dust samples, surface water samples and surface sediment samples

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids produced by soil bacteria and occur in near coastal marine sediments as a result of soil organic matter input. Their abundance relative to marine-derived crenarchaeol, quantified in the BIT index, generally decreases offshore. However, in distal marine sediments, low relative amounts of brGDGTs can often still be observed. Sedimentary in situ production as well as dust input have been suggested as potential, though as yet not well constrained, sources. In this study brGDGT distributions in dust were examined and compared with those in distal marine sediments. Dust was sampled along the equatorial West African coast and brGDGTs were detected in most of the samples, albeit in low abundance. Their degree of methylation and cyclisation, expressed in the MBT' (methylation index of branched tetraethers) and DC (degree of cyclisation) indices, respectively, were comparable with those for African soils, their presumed source. Comparison of DC index values for brGDGTS in global soils, Congo deep-sea river fan sediments and dust with those of distal marine sediments clearly showed, however, that distal marine sediments had significantly higher values. This distinctive distribution is suggestive of sedimentary in situ production as a source of brGDGTs in marine sediments, rather than dust input. The presence of in situ produced brGDGTs in marine sediments means that caution should be exercised when applying the MBT'-CBT palaeothermometer to sediments with low BIT index values, i.e. < 0.1, based on our dataset.

支链甘油二烷基甘油四醚（brGDGTs）是土壤细菌合成的膜脂，可随土壤有机质输入过程进入近岸海洋沉积物中。其相对于海洋来源古菌醇（crenarchaeol）的丰度可通过BIT指数量化，通常随离岸距离增加而降低。但在远海沉积物中，仍常可观测到较低的brGDGTs相对丰度。此前研究提出沉积物原位产生与粉尘输入是其潜在来源，但二者的贡献目前尚未得到很好的限定。本研究对粉尘中的brGDGTs分布特征开展了分析，并与远海沉积物中的分布特征进行了对比。研究团队沿西非赤道沿岸采集了粉尘样品，多数样品中均检出了brGDGTs，尽管其丰度较低。其甲基化与环化程度分别通过MBT'（支链四醚甲基化指数，methylation index of branched tetraethers）与DC（环化程度，degree of cyclisation）指数表征，与作为其推测源区的非洲土壤的对应参数具有可比性。然而，对比全球土壤、刚果深海扇沉积物及粉尘中brGDGTs的DC指数值与远海沉积物的对应值后可清晰发现，远海沉积物的DC指数值显著更高。这种独特的分布特征表明，海洋沉积物中brGDGTs的主要来源为沉积物原位产生，而非粉尘输入。基于本数据集，海洋沉积物中存在原位产生的brGDGTs这一发现意味着，当对BIT指数值较低（即<0.1）的沉积物应用MBT'-CBT古温度计时，需格外谨慎。