Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Marine Organic Carbon Cycle and Feedbacks

Eccentricity cycles in deep-sea paleoclimate records suggest that astronomical forcing notably altered global temperatures and carbon cycle dynamics. Because changes in the distribution of insolation alone cannot explain the observed climate variability, climate-carbon cycle feedbacks must have amplified the response. However, the carbon sources and sinks operating on orbital timescales are poorly understood, especially in absence of dynamic ice sheets as during the early Cenozoic. Here, we use an Earth system model to explore the impact of astronomical forcing on the organic carbon cycle and its expression in key paleoceanographic variables, building on Vervoort et al. (2024, https://doi.org/10.1029/2023pa004826) who outlined the role of inorganic carbon cycle feedbacks. Results demonstrate that subtle changes in marine organic carbon burial, driven by nutrient (phosphate, P) availability, can produce 400-kyr cycles of negative δ¹³C excursions during periods of elevated pCO₂ and reduced CaCO₃ preservation, consistent with typical orbital variations in Paleocene records. The magnitude and phasing of the response to eccentricity forcing are determined by the balance between P release (via temperature-dependent rock weathering) and P removal (via oxygen-dependent sedimentary P retention). Because these processes are strongly influenced by the distribution of landmasses and shelves, paleogeography exerts a first-order control on the expression of astronomical cycles. We do not reproduce the high amplitude 100-kyr “hyperthermal events” of the early Eocene, but our model identifies two potential mechanisms to amplify global warming on orbital timescales: reduced organic carbon burial as well as enhanced kerogen weathering could increase CO₂ during eccentricity maxima under favorable conditions.