Carbon Cycle Responses to Changes in Weathering and the Long-Term Fate of Stable Carbon Isotopes

The causes of CO2 variations over the past million years remain poorly understood. Imbalances between the input of elements from rock weathering and their removal from the atmosphere-ocean-biosphere system to the lithosphere likely contributed to reconstructed changes. We employ the Bern3D model to investigate carbon-climate responses to step-changes in the weathering input of phosphorus, alkalinity, carbon, and carbon isotope ratio (d(13)C) in simulations extending up to 600,000 years. CO2 and climate approach a new equilibrium within a few ten thousand years, whereas equilibrium is established after several hundred thousand years for d(13)C. These timescales represent a challenge for the initialization of sediment-enabled models and unintended drifts may be larger than forced signals in simulations of the last glacial-interglacial cycle. Changes in dissolved CO2 change isotopic fractionation during marine photosynthesis. This causes distinct spatio-temporal perturbations in d(13)C and affects the burial flux of C-13. We force a cost-efficient emulator, based on the Bern3D results, with contrasting literature-based weathering histories over the last 800 thousand years. Glacial-interglacial amplitudes of up to 30 ppm in CO2, 0.05%0 in d(13)C, and similar to 15 mmol m-3 in deep ocean CO32- are emulated for changes in carbonate rock weathering. Plausible input from the decomposition of organic matter on shelves causes variations of up to 10 ppm in CO2, 0.09%0 in d(13)C, and 5 mmol m(-3) in CO32- , highlighting the non-negligible effect of weathering-burial imbalances.Plain Language Summary Data from ice cores and marine sediments document large changes in atmospheric carbon dioxide (CO2) and climate during the past million years. Carbon isotopes and other proxies can help to understand underlying processes. In this study, we investigate Earth's response to plausible changes in the input of carbon and other elements from the weathering of rocks or the decomposition of previously accumulated organic matter with the help of a computer model. Results show significant variations in CO2, carbon isotopes, marine chemistry, marine biological productivity, and burial fluxes of biogenic particles to the lithosphere. The adjustment time to changes in input flux is several ten thousand years for CO2 and climate, and several hundred thousand years for carbon isotopes. As it is computationally challenging to simulate such long time periods with complex models, we used our results to build an emulator. Such emulators, representing the responses of spatially resolved and process-based models, are useful for studies addressing Earth's history over many millions of years. Simulating a million years with the emulator takes seconds, whereas it takes about 4 months with the complex model. In conclusion, our work highlights the role of weathering fluxes and their possible contribution to past climate-carbon cycle swings.