Using extraterrestrial 3He to reconstruct terrigenous fluxes and their impacts on marine primary productivity and carbon burial during OAE2 in the Vocontian Basin
Résumé
The Mesozoic was punctuated by several Oceanic Anoxic Events (OAE) characterised by widespread black shale deposition and global carbon cycle perturbations. Among them, the OAE2 spanning the Cenomanian-Turonian boundary (~ 94 Ma), has been identified as one of the most severe OAE of the Mesozoic. Lasting ~600 ka, this event is marked by a global > 2‰ positive excursion in the 13C signature of both carbonates and organic matter, interpreted as reflecting a massive burial of isotopically light organic carbon driven by a deoxygenation of bottom seawater. Previous models suggested that volcanism related to Large Igneous Provinces (LIPs) increased atmospheric and oceanic CO2 concentrations, thereby increasing nutrient input to the ocean and marine primary production and decreasing seawater oxygenation. This volcanism is also known for disturbing climate and especially increasing precipitations and weathering. However, the respective roles of weathering and detrital fluxes on marine primary productivity and organic carbon burial during OAE2 remains highly debated, mainly due to the lack of highly resolved chronologies. In this study, we perform new extra-terrestrial 3He (3HeET) measurements from hemipelagic marine samples from Pont d’Issole (Vocontian Basin) to reconstruct carbonate, organic carbon and terrigenous sedimentation rates and fluxes across the OAE2. Combined with organic matter content, trace metals concentrations and nannofossils fluxes, this method allows us to trace sources and variations of each of the three sedimentary components during the event. Our results reveal that over 70 % of the 3He in the analysed samples is extraterrestrial in origin. Assuming a constant flux of Interplanetary Dust Particles, the 3HeET concentrations allowed us to reconstruct relative changes in sedimentation rates at an unprecedented high resolution (every 15 cm). Our preliminary results suggest changes in the variability of terrigenous and carbonate depositional fluxes across OAE2. Indeed, pre-OAE2 and onset sediments are characterised by constant terrigenous input across different carbonate-rich and carbonate-poor lithologies while total sedimentation plummets (from 17 to 2 g/cm²/ka). The beginning of the event was thus marked by a collapse of carbonate net deposition. On the contrary, sediments from the Plenus Cold Event (colder and more oxygenated period within the OAE) and from the Cenomanian-Turonian boundary show variations of both terrigenous and carbonate fluxes (from 0.2 to 2 g/cm²/ka and from 0.5 to 8 g/cm²/ka respectively). In both cases, reconstructed organic matter accumulation rates peaked during short-lived pulses. The implication of our new nannofossil and 3HeET data will be discussed in the context of the debated role of increased detrital input on marine primary productivity and organic carbon burial during the OAE2.