Isotopic and biogenic silica evidence for eutrophic conditions in South China during the Frasnian-Famennian biotic crisis

Abstract

The Late Devonian Frasnian-Famennian (F F) transition at ca. 367 Ma was a key event in Phanerozoic Eon involving a massive biotic crisis, which coincided with a positive carbon isotope anomaly and large climate changes associated with eustatic fluctuations. The details underpinning the initiation of the mass extinction, however, remain problematic. To better understand this scientific issue, we conducted a time series of biogeochemical studies that included C N isotopes and biogenic silica (BSi) from the F F transition in the Yangdi section, South China. The carbon isotope data accurately constrain the Upper Kellwasser Horizon (UKH) and Lower Kellwasser Horizon (LKH) in this section. A short-term decrease in carbon isotope fractionation in the F F transition indicates a decreased p CO 2 level in the LKU and UKH, while the increased fractionation in the uppermost UKH was caused by the rapidly negative δ 13 C org excursion, likely as the result of enhanced inputs from chemoautotrophs. On the other hand, the nitrogen isotope data suggest that the surface water had limited availability of dissolved N before and after the F F boundary due to enhanced benthic denitrification; these conditions led to a supply of external nitrogen that was likely driven by persistent diazotrophy. Increased sedimentary δ 15 N (up to +7.3‰) during the UKH suggests a rapidly amplified nitrate pool associated with water column denitrification, implying intensification of marine productivity and expansion of dysoxic-anoxic conditions linked to intensified oceanic circulation. In addition, BSi in sediments has the potential to evaluate historic changes in primary productivity of oceans and the maxima of BSi mass values (up to 1.96%) are clearly documented in the upper part of the UKH. Insofar as the elevated abundance of biolimited nitrogen coincided with strongly increased marine phytoplankton productivity at the F F boundary, we speculate that the nutrient-driven eutrophic conditions in surface waters associated with attendant redox changes may have been the main killing mechanism in the mass extinction. • The cooling event in the UKH coincided with declined △ 13 C and elevated δ 15 N. • We attribute the low δ 15 N values to enhanced microbial nitrogen fixation. • Sedimentary δ 15 N increases are indicative of an amplified N pool. • Siliceous organism blooms are consistent with positive δ 13 C carb and δ 15 N shifts. • The F F crisis was likely the result of eutrophication in shallow water.