Changes in productivity associated with algal-microbial shifts during the Early Triassic recovery of marine ecosystems

Abstract

Abstract The recovery of marine ecosystems in the aftermath of the Permian-Triassic mass extinction was accompanied by significant carbon-cycle perturbations, as reflected in large-amplitude global excursions in Lower Triassic carbonate carbon isotope records. In the present study, we generated paired carbonate carbon (δ13Ccarb), organic carbon (δ13Corg), and nitrogen (δ15N) isotope records along with molar C/N ratios for a composite section in the Chaohu area of Anhui Province (northern Yangtze Platform, South China) that spans the entire Lower Triassic. These records document concurrent changes among multiple proxies related to marine plankton community composition, productivity rates, and nutricline structure, providing unparalleled insights into changes at the base of the marine trophic web during the Early Triassic recovery interval. Changes in carbonate-organic carbon isotopic differences (Δ13Ccarb-org) and C/N ratios indicate a general shift from anoxygenic photoautotrophy to eukaryotic algal productivity during the Early Triassic. The prevalence of prokaryotic photoautotrophs in the Griesbachian to Smithian was due to frequent environmental disturbance, whereas the reestablishment of eukaryotic algae as dominant primary producers in the Spathian reflects a general amelioration of marine environments at that time. Positive δ13Ccarb excursions and brief spikes toward higher Δ13Ccarb-org and C/N ratios around the Induan-Olenekian boundary and Smithian-Spathian boundary record transient shifts toward improved conditions and temporary rebounds of algal productivity. A negative δ15N shift was associated with decreasing δ13Ccarb, indicating that marine productivity was closely linked to N-fixation intensity owing to a general nutrient-N deficiency. Synchronous fluctuations of δ13Ccarb and δ13Corg through the Olenekian reflect changes in the δ13C of oceanic dissolved inorganic carbon, but the more limited variation in δ13Corg may have been due to concurrent changes in algal-microbial assemblages and, thus, net photosynthetic fractionation of carbon isotopes.