A reduced estimate of the strength of the ocean's biological carbon pump

TLDR

The ocean’s biological carbon pump, driven by sinking of small organic particles, has been estimated using various methods that produce a wide range of values, highlighting incomplete knowledge of this major carbon flux. The study aims to use a thorium isotope tracer to directly estimate particulate organic carbon export. The authors compiled a large database of thorium‑derived export measurements and extrapolated them globally by correlating with satellite sea surface temperature fields. The thorium‑based estimates show export efficiency significantly lower than f‑ratio estimates, yielding a global integrated carbon export of about 5 GtC yr⁻¹, below most current estimates.

Abstract

[1] A major term in the global carbon cycle is the ocean's biological carbon pump which is dominated by sinking of small organic particles from the surface ocean to its interior. Several different approaches to estimating the magnitude of the pump have been used, yielding a large range of estimates. Here, we use an alternative methodology, a thorium isotope tracer, that provides direct estimates of particulate organic carbon export. A large database of thorium-derived export measurements was compiled and extrapolated to the global scale by correlation with satellite sea surface temperature fields. Our estimates of export efficiency are significantly lower than those derived from the f-ratio, and we estimate global integrated carbon export as ∼5 GtC yr−1, lower than most current estimates. The lack of consensus amongst different methodologies on the strength of the biological carbon pump emphasises that our knowledge of a major planetary carbon flux remains incomplete.