Metrics that matter for assessing the ocean biological carbon pump

Ken O. Buesselera,1, Philip W. Boydb, Erin E. Blackc,d, and David A. Siegele

Edited by David M. Karl, University of Hawaii at Manoa, Honolulu, HI, and approved March 10, 2020 (received for review December30, 2019)

PNAS first published April 6, 2020 https://doi.org/10.1073/pnas.1918114117

 

The biological carbon pump (BCP) comprises wide-ranging processes that set carbon supply, consumption,

and storage in the oceans’ interior. It is becoming increasingly evident that small changes in the

efficiency of the BCP can significantly alter ocean carbon sequestration and, thus, atmospheric CO2 and

climate, as well as the functioning of midwater ecosystems. Earth system models, including those used

by the United Nation’s Intergovernmental Panel on Climate Change, most often assess POC (particulate

organic carbon) flux into the ocean interior at a fixed reference depth. The extrapolation of these fluxes

to other depths, which defines the BCP efficiencies, is often executed using an idealized and empirically

based flux-vs.-depth relationship, often referred to as the “Martin curve.” We use a new compilation of

POC fluxes in the upper ocean to reveal very different patterns in BCP efficiencies depending upon

whether the fluxes are assessed at a fixed reference depth or relative to the depth of the sunlit euphotic

zone (Ez). We find that the fixed-depth approach underestimates BCP efficiencies when the Ez is shallow,

and vice versa. This adjustment alters regional assessments of BCP efficiencies as well as global

carbon budgets and the interpretation of prior BCP studies. With several international studies recently

underway to study the ocean BCP, there are new and unique opportunities to improve our understanding

of the mechanistic controls on BCP efficiencies. However, we will only be able to compare results

between studies if we use a common set of Ez-based metrics.