Organic Carbon Remineralization and Calcium Carbonate Production Rates in the Red Sea Computed From Oxygen and Alkalinity Utilizations

Abstract

Organic carbon remineralization rate (OCRR) and the calcium carbonate production rate (CCPR) are influential variables on the efficiency of the biological carbon pump (BCP) but are not well understood in Red Sea. We used historical cruise data of carbonate chemistry, oxygen, and transient tracers from five locations along the north–south central axis of the Red Sea to estimate OCRR and CCPR from tracer-based water mean ages (Γ), apparent oxygen utilization (AOU), and alkalinity utilization (AU). This resulted in the first basin-wide and depth-resolving (100–1,000 m) OCRR and CCPR estimates. Spatial distributions for Γ, AOU, and AU were strongly influenced by the large-scale circulation and showed maxima intermediate depths (400–500 m). Conversely, OCRR and CCPR showed no statistically significant latitudinal differences and peaked (6.5 ± 4.3 and 11.9 ± 4.6 mmol C m⁻³ yr⁻¹, respectively) at 100-m depth, which decreased to nearly constant values (3.8 ± 0.7 and 1.4 ± 0.3 mmol C m⁻³ yr⁻¹, respectively) at 300 m and deeper. By depth-integrating CCPR, we estimated annual calcium carbonate production (CCP) of (0.8 ± 0.3) × 10¹² mol, or 0.6% of global ocean production, in the Red Sea, which has only 0.12% of the world ocean area. High correlation between AU and Γ indicated in situ alkalinity removal taking place also in subsurface and deep waters, probably due to chemical precipitation, which has been previously reported for the area. CCP-induced AU affects the carbonate chemistry in the Red Sea water column, and we hypothesize that it also impacts that of the Gulf of Aden through the outflowing Red Sea Outflow Water.