Changes In Marine Biogeochemical Cycles And Redox Conditions During The Devonian-Carboniferous Mass Extinction Boundary

The end-Famennian?Late Devonian?biotic crisis,also known as the Hangenberg Event,eliminated²1%of marine invertebrate genera and¹6%of families,making it approximately one of the largest mass extinction of the Phanerozoic.The extinction interval features a layer of black shale called the Hangenberg Black Shale?HBS?.A wide diversity of marine biota was effected by the global Hangenberg Event,especially for ammonoids,trilobites,conodonts,stromatoporoids,corals,and ostracodes.However,the mechanism for Hangenberg Event remains elusive.The changes in the marine biogeochemical cycles and redox conditions during the D-C transition are not clear,which contrains the understanding of the mechanism for the Hangenberg Event.The three D-C boundary sections investigated in this study are located on the South China Craton,which was situated in the equatorial eastern Tethys Ocean during the D-C transition.At that time,the southern?paleo-eastern?margin of the South China Craton consisted of a large marine embayment,comprising many shallow carbonate platforms?the Xianggui-Qiangui platform complex?and deepwater troughs?of which the Youjiang Basin was the largest?.The more proximal platforms were attached to the southern margin of the South China Craton,whereas the more distal platforms were isolated from land areas by intervening troughs.The three sections are?1?the Long'an section?23°10?34.8?N,107°27?48.0?E?in Dujie village of Long'an County in Guangxi Zhuang Autonomous Region that was deposited on the Nanning isolated carbonate platform,?2?the Qilinzhai section?25°49?04?N,107°29?26?E?in Dushan County of Guizhou Province that located on the margin of the Qiangui attached carbonate platform,and?3?the Malanbian section?27°22?48.8?N,111°34?2.4?E?in Xinshao County of Hunnan Province that was situated at the Xianggui platform.An improved understanding of the contemporaneous carbon-nitrogen-sulfur cycle(?¹³C_carb,?¹⁵N_bulk,and?³⁴S_CAS)and redox conditions?I/Ca?in three sections can provide insights on marine environmental changes that may have a bearing on this biotic crisis.High-resolution studies of carbonate isotope(?¹³C_carb)and organic carbon isotope(?¹³C_org)profiles show a major perturbation of marine carbon cycle during D-C boundary.A positive excursion of?¹³C_carbarb profile was documented in Long'an Section?from-1.6 to+2.5‰?,Qilinzhai Section?from+3.0 to+4.4‰?and Malanbian section?from-1.6 to+3.3‰?.The Hangenberg carbon isotope excursion?HICE?is characterized by a positive excursion of 4-5‰.Similarly,the?¹³C_orgrg compositons recorded a positive shift from-25.8 to-22.6‰in Long'an section,from-26.1 to-25.0‰in Qilinzhai Section,and from-28.0 to-25.6‰in Malanbian Section.The variation in?¹³C_orgrg profiles reveal a positive shift of 3-4‰.The positive excursions of?¹³C_carb and?¹³C_orgrg profiles are ascribed to enhanced burial of organic carbon.Enhanced burial of organic carbon was resulted from some combination of increased productivity and increased organic matter preservation,which can potentially be linked to changes in global-ocean circulation.The global climatic cooling tends to accentuate equator-to-pole temperature gradients,leading to intensified oceanic circulation.Well-documented climatic cooling during the end-Devonian is thus likely to have set the stage for intensified oceanic circulation.This process can enhance the return of deepwater nutrients to the ocean-surface layer,stimulating marine productivity.The increased marine productivity led to expanded anoxia,which provided the possibility for the increased organic matter preservation.High-resolution studies of nitrogen isotope(?¹⁵N_bulk)profiles show a major perturbation of marine nitrogen cycle during D-C boundary.The?¹⁵N_bulkulk values decreased from+4.6 to⁰‰in Long'an Section,from+6.1 to+3.5‰in Qilinzhai Section,and from+4.5 to+1.9‰.These records shows that the Hangenberg nitrogen isotope excursion?HINE?was marked by a negative shift of 3-4‰within the Middle-Upper Si.praesulcata Zone,which was attributed to enhanced microbial nitrogen fixation that,in turn,was a result of enhanced denitrification and anammox.High-resolution sulfur isotopic compositions(?³⁴S_CAS)of carbonate-associated sulfate?CAS?show a major perturbation of marine sulfur cycle during D-C boundary.In Long'an Section,the sharp negative excursion of?³⁴S_CASAS occurred from+19.0‰in the upper Lower Si.praesulcata Zone to+13.0‰in the middle Upper Si.praesulcata Zone,followed by a recovery to+20.3‰in the uppermost Upper Si.praesulcata Zone,representing a record of the full Hangenberg sulfur isotope excursion?HISE?.In Qilinzhai Section,the?³⁴S_CASAS profile exhibits a positive excursion from+12.1‰to+19.0‰in the Upper Si.praesulcata Zone,which represents approximately the upper half of the HISE.The negative?³⁴S_CAS shift was caused by oxidation of ³⁴S-depleted H₂S as the enhanced anoxia stimulated microbial sulfate reduction,increasing the production of H₂S.High-temporal-resolution I/Ca ratio?as a novel redox proxy?of shallow water carbonates were analyzed in Long'an and Malanbian sections of South China.The results show that I/Ca values underwent a distinct decline in the Middle and Upper Si.praesulcata conodont zones,which was documented by the excursion from 1.8 to 0?mol/mol at Long'an and from 2.0 to 0?mol/mol at Malanbian.These results suggest that oxygen minimum zone?OMZ?expanded and reached shallow water environments in the end Devonian.This study,which is based on the Long'an,Qilinzhai and Malanbian sections in South China,focuses on the high-resolution chemostratigraphic analysis of the?¹³C_carb,?¹³C_org,?¹⁵N_bulk,?³⁴S_CAS and I/Ca profiles during the D-C transition.The perturbation of marine carbon-nitrogen-sulfur cycles are linked to the intensified anoxia and euxinic deep-water.Furthermore,the decreased I/Ca compositions are the result of upward incursion of OMZ into the ocean-surface layer.It corroborates the notion that oxygen deficiency/expansion of the OMZ into shallow water was a direct mechanism inducing the end-Devonian mass extinction.