Global calcite cycling constrained by sediment preservation controls

Abstract

We assess the global balance of calcite export through the water column and burial in sediments as it varies regionally. We first drive a comprehensive 1‐D model for sediment calcite preservation with globally gridded field observations and satellite‐based syntheses. We then reformulate this model into a simpler five‐parameter box model, and combine it with algorithms for surface calcite export and water column dissolution for a single expression for the vertical calcite balance. The resulting metamodel is optimized to fit the observed distributions of calcite burial flux. We quantify the degree to which calcite export, saturation state, organic carbon respiration, and lithogenic sedimentation modulate the burial of calcite. We find that 46% of burial and 88% of dissolution occurs in sediments overlain by undersaturated bottom water with sediment calcite burial strongly modulated by surface export. Relative to organic carbon export, we find surface calcite export skewed geographically toward relatively warm, oligotrophic areas dominated by small, prokaryotic phytoplankton. We assess century‐scale projected impacts of warming and acidification on calcite export, finding high sensitive to inferred saturation state controls. With respect to long‐term glacial cycling, our analysis supports the hypothesis that strong glacial abyssal stratification drives the lysocline toward much closer correspondence with the saturation horizon. Our analysis suggests that, over the transition from interglacial to glacial ocean, a resulting ∼0.029 PgC a−1decrease in deep Atlantic, Indian and Southern Ocean calcite burial leads to slow increase in ocean alkalinity until Pacific mid‐depth calcite burial increases to compensate.