Our goal is to extract key information from the oceanic sedimentary record to constrain variations in continental chemical and mechanical erosion rates on geologic time scales. The main constraint is the absence of continental weathering products in the sedimentary record, which gives a partial picture of exports from continental surfaces to the oceans. The scientific community in numerical modeling of climate and weathering cannot include a fundamental parameter, i.e. the dissolved fraction, in the evaluation of past global biogeochemical cycles.
Our recent work has demonstrated that marine carbonate organisms in the coastal zone record environmental physico-chemical conditions during test formation, thus highlighting the potential contribution(s) of fluvial discharges and/or submarine subsurface resurgences (El Meknassi et al., 2018, 2020; Briard et al., 2020). By applying these observations to oceanic deposits, we will be able to trace continental exports in its dissolved form, via carbonate organisms in the neritic domain. Combined with mechanical erosion rates obtained via the analysis of exported clays, it will thus be possible to quantify chemical and mechanical weathering and estimate an integrated denudation rate.
To assess the feasibility of such a scientific approach, we chose the Eocene-Oligocene transition (EOT), witnessing a major climate shift from a “green-house” to an “ice-house” climate. The chosen site is the African margin of the Equatorial Atlantic at EOT, during which detrital exports from the West African craton have been quantified (“source to sink” by seismic stratigraphy and geomorphology; Rouby et al., 2023), providing a unique context for an in-depth mineralogical and geochemical study. IODP-France gives us access to oceanic sedimentary archives.
As a first step, we have constrained paleo-ocean circulation on a global scale by studying planktonic and benthic foraminifera preserved in sediment cores around Antarctica. The aim is to constrain the effect of the geodynamic context on the circulation of different water masses and their physico-chemical state. Results obtained on foraminifera and based on seawater paleo-temperature (Mg/Ca molar ratios), redox (Ce/Ce* anomaly) and provenance (87Sr/86Sr) proxies highlight (Hodel et al., 2021, 2022):
- A gradual mixing of seawater masses between the South Atlantic and Pacific Oceans from 31 Ma to 26 Ma, and between the Indian and Pacific Oceans at 35 and 31 Ma,
- The opening of the Tasman Passage at 35 Ma, followed by the opening and deepening of the Drake Passage from 31 to 26 Ma, initiated the Antarctic Circumpolar Current (ACC),
- The earlier onset of the Oligocene glaciation (at 33.7 Ma) compared with the start of the ACC (31-26 Ma), implies that the change in atmospheric pCO2 is the triggering event for the Oligocene glaciation.
On these samples, we will use conventional geochemical and isotopic tracers (e.g. REEs, Sr, Nd) and develop new ones (e.g. Li and Cr). These elemental and isotopic compositions, characteristic of continental weathering processes, will be combined with drainage basin geometry (Reliefs & Fluxes group) and continental sediment exports preserved on the passive margin (Tectonics & Relief group), to constrain continental chemical and detrital exports and associated denudation rates. These results will then be used to constrain numerical models of continental surface and/or climatic processes to address our central question “How does climate impact, on geological time scales, the erosion and weathering dynamics of continental landforms and the exported particulate/dissolved fluxes that modify global ocean composition?”.
Involvement of young researchers: F. Hodel (post-doctoral fellow IODP-France), J. Briard and S. El Meknassi (doctoral students), C. Fériot and R. Grespan (Masters 2)
Funding: IODP-France, ED SD2E, ANR-AMOR, INSU-LEFE
Collaboration: LEGOS (UMR5566), Chrono-Environnement (UMR6249
References :
- Rouby, D., Ye, J., Chardon, D., Loparev, A., Wildman, M. Dall’Asta, M., 2023. Source-to-sink sedimentary budget of the African Equatorial Atlantic rifted margin. Geochemistry, Geophysics, Geosystems, 24, e2023GC010901.
- Hodel F., Fériot C., de Rafélis M., Dera G., Lezin C., Nardin E., Rouby D, Aretz M., Antonio P., Steinnman M., Buatier M., Lacan F., Jeandel C., Chavagnac V., 2022. Eocene-Oligocene Southwest Pacific Ocean paleoceanography: New insights from foraminifera chemistry (DSDP Site 277, Campbell Plateau). Frontiers Earth Sciences, Marine Geoscience, 10.3389/feart.2022.998237
- Hodel F., Grespan R., de Rafélis M., Dera G., Lezin C., Nardin E., Rouby D., Aretz M., Steinmann M., Buatier M., Lacan F., Jeandel C., Chavagnac V., 2021. Drake gateway opening and Antarctic Circumpolar Current onset 31 Myrs ago: The message of foraminifera and reconsideration of the Neodymium isotope record. Chemical Geology, 10.1016/j.chemgeo.2021.120171
- El Meknassi S., Dera G., De Rafélis M., Brahmi C., Lartaud F., Hodel F., Jeandel C., Menjot L., Mounic S., Henry M., Besson P., Chavagnac V., 2020. Seawater 87Sr/86Sr ratios along continental margins: Patterns and processes in open a restricted shelf domains.Chemical Geology, 10.1016/j.chemgeo.2020.119874
- Briard J., Pucéat E., Vennin E., Daëron M., Chavagnac V., Jaillet R., Merle D., de Rafélis M., 2020. Seawater paleotemperature and paleosalinity evolution in neritic environments of the Mediterranean margin during the Miocene: insights from combined isotope analysis of bivalve shells. Palaeogeography, Palaeoclimatology, Palaeoecology, 10.1016/j.palaeo.2019.109582
- El Meknassi S., Dera G., Cardone T., De Rafélis M., Chavagnac V., 2018. Sr isotope ratios of modern carbonate shells : good and bad news for chemiostratigraphy. Geology, doi: 10.1130/G45380.1