Silicon redistribution and promoted low-latitude diatom growth after Atlantic circulation recovered from deglacial meltwater discharge

Abstract. How the marine silicon cycle modulates the biological carbon pump and contributes to glacial–interglacial variations in atmospheric CO2 remains unresolved. Proposed mechanisms include Southern Ocean silicic acid leakage, stimulating low-latitude diatom productivity either during the Last Glacial Maximum (LGM) or at the onset of deglaciation, and deglacial silicon redistribution from the abyss to the surface. To test these hypotheses, we performed two equilibrium simulations (pre-industrial and LGM), one LGM sensitivity simulation with pre-industrial dust deposition and two freshwater perturbation experiments under LGM conditions with the Earth system model AWI-ESM2.1, in which the diatom silicon-to-nitrogen (Si:N) uptake ratio varies with the nutrient limitation state. Glacial dust deposition enriches Southern Ocean surface waters and southern-sourced intermediate waters in silicon relative to nitrogen, but strengthened stratification traps this reservoir, preventing it from fuelling low-latitude diatom growth. We identify two mechanisms that release silicon to low-latitude surface waters: (1) the recovery of the Atlantic Meridional Overturning Circulation (AMOC) after the freshwater perturbation strengthens intermediate-water formation, upwells the leaked silicon and drives short-lived but pronounced opal export peaks in the tropics; (2) continued warming and sea-ice retreat mobilise the deep glacial silicon pool, sustaining a more persistent enhancement. A longer freshwater perturbation suppresses the first mechanism. Despite some discrepancies in timing, our results reconcile previous hypotheses within a unified framework, defining circulation recovery as the critical trigger that links freshwater forcing to sedimentary opal records.