Eccentricity Rhythms in the Oligocene-Miocene Carbon Cycle Regulated by Weathering and Carbonate Burial

Orbital eccentricity exerts only a minor influence on annual mean insolation, yet pronounced eccentricity-paced variability is ubiquitously recorded in Cenozoic deep-ocean carbon isotope and carbonate system proxies. The physical mechanism linking eccentricity forcing to global carbon cycle dynamics, particularly during greenhouse climate states lacking large Northern Hemisphere ice sheets, remains unresolved. Here we present high-resolution geochemical reconstructions from Oligocene–Miocene deep-sea sedimentary archives integrated with carbon cycle modeling to identify the processes transmitting eccentricity forcing into the marine carbon reservoir. Our multiproxy dataset includes benthic foraminiferal δ¹³C and δ¹⁸O records together with indicators of deep-ocean carbonate chemistry that constrain changes in carbonate ion concentration and carbonate burial dynamics. Spectral analyses reveal robust 405-kyr and ~100-kyr eccentricity signals in carbonate system proxies that are phase-locked with benthic δ¹³C and δ¹⁸O variability. Notably, intervals of eccentricity maxima are associated with systematic reductions in deep-ocean carbonate saturation state and coherent shifts in carbonate accumulation patterns, indicating large-scale reorganization of the marine carbonate burial system rather than simple temperature-driven solubility effects. We propose a mechanism in which eccentricity forcing is amplified through low-latitude hydrological processes under warm climate boundary conditions. During eccentricity maxima, enhanced seasonal insolation contrast intensifies tropical monsoon systems and strengthens the low-latitude hydrological cycle. Increased monsoon precipitation accelerates continental silicate weathering and enhances fluvial delivery of dissolved inorganic carbon (DIC) and alkalinity to the ocean. The augmented alkalinity flux modifies ocean carbonate chemistry and promotes redistribution of carbonate burial from deep-sea basins toward continental shelves. This burial reorganization lowers deep-ocean carbonate ion concentration and drives synchronous variations in marine δ¹³C, thereby producing the observed in-phase eccentricity pacing of carbon isotope and carbonate system records. Carbon cycle box-model simulations and sensitivity experiments support this interpretation. Enhanced weathering and alkalinity inputs reproduce both the amplitude and phase relationships observed between δ¹³C and carbonate saturation proxies. Model results further demonstrate that the efficiency of eccentricity transmission to the carbon cycle is strongly climate-state dependent: under greenhouse conditions characterized by elevated atmospheric CO₂ and intensified hydrological cycling, low-latitude hydroclimate acts as a dominant amplifier of eccentricity forcing. In contrast, during icehouse states, cryospheric feedbacks may exert greater control over long-period orbital signals. Our results reconcile the apparent paradox of strong eccentricity expression in Cenozoic carbon cycle archives despite weak direct radiative forcing. Rather than operating through high-latitude ice-volume modulation alone, eccentricity forcing can influence global biogeochemical cycles via tropical hydroclimate–weathering–carbonate burial coupling. This mechanism provides a process-based explanation for the long-observed in-phase behavior of δ¹³C and δ¹⁸O at eccentricity timescales and highlights the central role of low-latitude hydrological dynamics in regulating marine carbon reservoirs during warm climate regimes. By integrating geochemical proxies with mechanistic modeling, this study expands the classical Milankovitch framework to include state-dependent amplification through the tropical hydrological cycle. The findings underscore that orbital forcing is transmitted through different dominant feedback pathways depending on background climate conditions, offering new insights into the coupled evolution of climate and carbon cycles throughout the Cenozoic and informing projections of hydrological–carbon feedbacks under future warming scenarios.