Numerical simulations of the effects of astronomical forcing on nutrient supply and oxygen levels during the Devonian

Abstract

Declining oxygen levels in the ocean since the middle of the 20th century have been linked to increasing temperatures, CO2 concentrations, and nutrient inputs. In the geological past, numerous oceanic anoxic events have occurred under similar conditions. These events, during which dissolved oxygen in the ocean drop to potentially harmful levels, can have serious consequences for marine life and can also alter the geochemistry of the ocean.Specifically, we focus here on the Devonian (419 and 359 Ma), a warmer-than-present geological period. The sixty million years Devonian stage was the theatre of at least 29 identified anoxic events (Becker et al., 2020), marked most of the time by the deposition of black shales, associated with carbon isotopic excursion. It is understood that concurrent trends in CO2 and silicate weathering during the Devonian period have generated a context prone to ocean anoxia. On the other hand, there is growing evidence that their periodic recurrences in sedimentary records may have been influenced by astronomical forcing, such as changes in Earth's axis rotation and orbit geometry (De Vleeschouwer et al., 2017; Da Silva et al. 2020)In the umbrella project WarmAnoxia, we combine climate models and geological observations to explore and test proposals linking astronomical forcing to Devonian anoxia. Through this presentation, we focus specifically on the hypothesis that astronomical forcing influenced precipitation and temperature patterns in a way that significantly modified soil weathering dynamics, with enough effects on nutrient fluxes toward the ocean to promote oceanic anoxia.To test this proposal, we performed 81 experiments with the global atmosphere-slab model HadSM3. Experiments have been designed to span the range of astronomical forcing and CO2 concentrations experienced during the Devonian. The output was used to calibrate an emulator. With the latter, we estimate the transient evolutions of temperature and precipitation over 5 million-year periods, for which we assumed both simplified and realistic astronomical forcing scenarios. In turn, these transient evolutions force the GEOCLIM model (Maffre et al. 2022), which simulates soil dynamics, estimates nutrient fluxes from the continents to the oceans, and the response on the oceanic chemistry and atmospheric oxygen levels.