Using extra-terrestrial 3He to reconstruct terrigenous fluxes and their impacts on marine primary productivity and carbon burial during OAE2

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. The OAE2 is characterised by a global > 2‰ positive carbon isotope excursion (CIE) recorded in both inorganic and organic sedimentary material. This CIE is interpreted as a massive burial of ¹³C-depeleted organic carbon driven by a global decrease in seafloor oxygenation. Previous models suggested that volcanism related to Large Igneous Provinces (LIPs) increased atmospheric CO₂ concentrations, thereby increasing, through enhanced continental weathering, oceanic nutrient and primary production in superficial waters, ultimately leading to higher oxygen consumption at depth. However, the role of enhanced weathering and accelerated continental fluxes on marine primary productivity and organic carbon burial during OAE2 remains highly debated, largely due to the difficulty of reconstructing highly resolved changes in terrigenous and organic fluxes from sedimentary records. In this study, we use new extra-terrestrial ³He (³He_ET) measurements from hemipelagic marine samples from Pont d’Issole (Vocontian Basin) and Cuba (Western Interior Basin) sections to reconstruct carbonate, organic carbon and terrigenous sedimentation rates and fluxes across the OAE2.

Our results from Pont d’Issole reveal that over 70 % of the ³He in the analysed samples is extra-terrestrial in origin. Assuming a constant flux of ³He_ET-bearing Interplanetary Dust Particles, we used ³He_ET concentrationsto reconstruct relative changes in sedimentation rates at an unprecedented high resolution (every 15 cm/ 5 ka). Our preliminary results indicate constant terrigenous fluxes (ca 1.6 g/cm²/ka) across different carbonate-rich and carbonate-poor lithologies prior to and across the OAE2 onset. On the contrary, carbonate fluxes plummet (from 26 to 1 g/cm²/ka) prior the CIE. This decrease occurs exactly at the level recording a marked shift towards more radiogenic osmium isotope values, pointing to a possible volcanically triggered collapse of carbonate productivity. Sediments from the Plenus Cold Event (colder and more oxygenated period within the OAE) interval and Cenomanian-Turonian boundary show more modest but significant changes in both terrigenous and carbonate fluxes (from 0.4 to 3 g/cm²/ka and from 0.8 to 11 g/cm²/ka, respectively). Our data also show that organic matter accumulation occurred mostly as short orbitally paced pulses across the entire OAE2 interval with varying responses to changes in terrigenous fluxes. Ongoing ³He_ET analyses from Cuba will provide comparative data from a different setting that will be pivotal to determine whether the reconstructed fluxes have a local or geographically widespread significance. The implication of ³He_ET data from both locations 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.