Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.
Geobiology. 2017 Sep;15(5):641-663. doi: 10.1111/gbi.12245. Epub 2017 Jul 10.
While numerous studies have examined modern hypersaline ecosystems, their equivalents in the geologic past, particularly in the Precambrian, are poorly understood. In this study, biomarkers from ~820 million year (Ma)-old evaporites from the Gillen Formation of the mid-Neoproterozoic Bitter Springs Group, central Australia, are investigated to elucidate the antiquity and paleoecology of halophiles. The sediments were composed of alternating laminae of dolomitized microbial mats and up to 90% anhydrite. Solvent extraction of these samples yielded thermally well-preserved hydrocarbon biomarkers. The regularly branched C isoprenoid 2,6,10,14,18-pentamethylicosane, the tail-to-tail linked C isoprenoid squalane, and breakdown products of the head-to-head linked C isoprenoid biphytane, were particularly abundant in the most anhydrite-rich sediments and mark the oldest current evidence for halophilic archaea. Linear correlations between isoprenoid concentrations (normalized to n-alkanes) and the anhydrite/dolomite ratio reveal microbial consortia that fluctuated with changing salinity levels. Halophilic archaea were the dominant organisms during periods of high salinity and gypsum precipitation, while bacteria were prevalent during stages of carbonate formation. The irregularly branched C isoprenoid 2,6,10,15,19-pentamethylicosane (PMI), with a central tail-to-tail link, was also abundant during periods of elevated salinity, highlighting the activity of methanogens. By contrast, the irregularly branched C isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane) was more common in dolomite-rich facies, revealing that an alternate group of archaea was active during less saline periods. Elevated concentrations of isotopically depleted heptadecane (n-C ) revealed the presence of cyanobacteria under all salinity regimes. The combination of biomarkers in the mid-Neoproterozoic Gillen Formation resembles lipid compositions from modern hypersaline cyanobacterial mats, pointing to a community composition that remained broadly constant since at least the Neoproterozoic. However, as a major contrast to most modern hypersaline environments, the Gillen evaporites did not yield any evidence for algae or other eukaryotes.
虽然有许多研究考察了现代高盐环境，但对于地质历史时期的类似环境，尤其是前寒武纪的类似环境，人们了解甚少。在这项研究中，对来自澳大利亚中部中新元古代苦泉群中 8.2 亿年前的蒸发岩（Gillen 组）的生物标志物进行了研究，以阐明嗜盐生物的古老性和古生态学。这些沉积物由微生物纹层组成，微生物纹层交替出现，纹层之间为白云石化微生物席和高达 90%的硬石膏。对这些样品进行溶剂萃取，得到了热稳定性好的烃类生物标志物。规则分支的 C 异戊二烯 2,6,10,14,18-五甲基二十烷、尾对尾连接的 C 异戊二烯鲨烷和头对头连接的 C 异戊二烯双萜烷的断裂产物在最富含硬石膏的沉积物中特别丰富，这标志着嗜盐古菌的最古老的现代证据。异戊二烯浓度（标准化为正构烷烃）与硬石膏/白云石比值之间的线性相关性表明，微生物群落在盐度变化时会发生波动。嗜盐古菌是高盐和石膏沉淀时期的主要生物，而细菌则在碳酸盐形成阶段占优势。在盐度升高的时期，不规则分支的 C 异戊二烯 2,6,10,15,19-五甲基二十烷（PMI）也很丰富，其中心有一个尾对尾连接，这突出了产甲烷菌的活动。相比之下，不规则分支的 C 异戊二烯 2,6,11,15-四甲基十六烷（克罗烷）在白云岩相更为常见，表明在低盐度时期，另一组古菌更为活跃。正十七烷（n-C ）的同位素丰度降低表明，在所有盐度条件下都存在蓝细菌。中新元古代 Gillen 组的生物标志物组合类似于现代高盐蓝细菌席的脂质组成，表明自新元古代以来，群落组成基本保持不变。然而，与大多数现代高盐环境的主要区别是，Gillen 蒸发岩没有产生任何藻类或其他真核生物的证据。
