扬子区新元古代“雪球”时期古环境的分子地球化学证据

Molecular Geochemical Evidence for the Paleoenvironment of the Late Neoproterozoic “Snowball Earth” Age in the Yangtze Region

本文对采自中国南方典型剖面新元古界-下寒武统的43块岩样,包括16块冰期沉积物样品,进行了有机碳、镜质组反射率测定及可溶有机质饱和烃色谱和色谱-质谱等常规有机地球化学分析。结果显示,冰期沉积物有机碳含量比非冰期要低1-2个数量级,表明冰期海洋中仍具有一定的生物产率但严重降低,可能是由于“雪球”时期低的海水温度和冰雪覆盖严重影响了生物生存的原因。江口和南沱冰期沉积物中检测出一定量的来自叶绿素先驱物的姥鲛烷和植烷等类异戊二烯烃系列,表明“雪球”时期,扬子区古海水仍然存在微弱的透光带,局部地区没有完全被冰雪所覆盖,光合生物仍能进行一定的光合作用,在岩石中还发现了丰富的甾萜烷类生物标志化合物,也表明一些生物得以存活下来。与非冰期沉积物相比,冰期沉积物中老鲛烷＋植烷绝对含量要低1-2个数量级,表明“雪球”时期的光合作用严重减弱。分子地球化学证据表明,“雪球”时期,扬子区的古海洋并没有完全被冰雪覆盖,局部地区仍存在无冰的水体,一些生物得以存活和演化。这些经受了环境重压熬过漫漫寒冬而存活下来的生物对其后的“寒武纪生物大爆发”具有重要的意义。

A total of 43 typical Upper Neoproterozoic to Lower Cambrian sedimentary rock samples, including 16 diamictite samples, have been collected from typical stratigraphic sections in southern China, and analyzed by routine organic geochemical techniques. Almost all the rocks, but the diamictites, have middle to high TOC contents in the Cambrian and relatively lower in the Upper Neoproterozoic samples, whereas the Upper Neoproterozoic diamictites show very low TOC values, which is 1-2 orders of magnitude lower than those in most of the non-glacial sedimentary rocks, which indicate a very limited biological productivity in and a rare organic source input into the paleooceanographic environment probably due to very low sea water temperatures and the paleooceanographic water area covered by ice sheet during the ＂Snowball Earth＂ age. As the products of Chlorophyll-a, phytane and pristane are detected from all the rock samples, which implies that photosynthetic autotrophs should be the primary organisms contributive to the sedimentary organic matter （TOC） in the Yangtze paleooceanographic water area. Moreover, the absolute concentrations of phytane plus pristane range from 0.04 ng/g to 0. 17 ng/g in most non-glacial rocks, and from 0. 006 ng/g rock to 0.04 ng/g rock in diamictites. The unusual low phytane and pristane concentrations reveal a very faint photosynthetic process, a restricted euphoric zone and quite limited sunlight within the paleooceanographic water column. Thereby it can be inferred that even though the biological productivity is very low, the paleooceanographic water area could not has been entirely covered by sea ice sheet and polynyas （open water areas） should remain during the Snowball Earth age.