Relationships between bacterial-algal proliferating and mass extinction in the Late Devonian Frasnian-Famennian transition: Enlightening from carbon isotopes and molecular fossils

摘要

Studies show positive shifts of inorganic and organic carbon isotope values (δ ~(13)C_(carb) and δ ~(13)C_(kerogen)) from +0.43 (per thousand V-PDB) to +3.54 (per thousand V-PDB) and from -29.38 (per thousand V-PDB) to -24.14 (per thousand V-PDB), respectively, B~* (Ba~* = Ba/ (Al_2O_3 X 15%)) values from 0.015 to 0.144, TOC values from 0.02% to 0.21 %, V/Cr values from 0.3 to 2.0, Sr/Ba values from 3.20 to 49.50 in the Late Devonian Frasnian Upper rhenana zone to the top linguiformis zone of the Yangdi section deposited in carbonate slope facies of Guilin, Guangxi, South China, which indicates that biomass, productivity, organic carbon burial and salinity increase and that oxygenation near the boundary between sediments and waters decreases from the Late Devonian Frasnian Upper rhenana zone to the top linguiformis zone. Abundance of molecular fossils increases and normal alkanes, isoprenoid hydrocarbon, terpanes and steranes are dominated from the Late Devonian Frasnian to the bottom of Famennian, which shows that the predecessors of molecular fossils of the Frasnian-Famennian (F-F) transition are dominated by marine phytoplankton, zooplankton and benthic bacteria with no photosynthesis. Therefore, it is considered that the F-F transitional mass extinction with a multistage, selection and global synchronizing was caused by bacterial-algal proliferating, continuing deterioration of the shallow marine ecoenvironment of the middle-lower latitudes. A simple cause and effect chain can be expressed as: appearance of seed plants and multi-storied forests → enhanced chemical and biochemical weathering and pe-dogenesis → wide development of soils → increasing riverine nutrient fluxes in epicontinental sea → from superoligotrophic to eutrophic in epicontinental sea → proliferating of marine phytoplankton and zooplankton → frequent red tide and anoxia → mass extinction of shallow marine organisms in the middle-lower latitudes. It is worth notice that the factor drawdown of atmospheric P_(CO_2), climatic cooling and sea level falling caused by eutrophication, anoxia and organic carbon burial increasing may be important for the mass extinction.