The plumbing of the global biological pump

Abstract

We quantify the timescales and pathways that set the efficiency of the biological pump (the fraction of the phosphate inventory that is regenerated). We use a data-constrained phosphorus-cycling model embedded in a steady data-assimilated ocean circulation to quantify the pump’s leaks of preformed phosphate, its sources of regenerated phosphate, and the pathways with which the combined biogenic particle transport and the water circulation teleconnect different regions of the global euphotic zone. These pathways are quantified by a path density, which is the concentration of phosphate that was last utilized in a region A and that will reemerge into the euphotic zone of a region B, partitioned according to the A–to–B transit-time. Suitable integrals of this path density, computed efficiently by direct matrix inversions, yield the phosphate mass in transit, its flow rate, and its residence time in the aphotic zone. We find that a pump efficiency of (39 ± 2)% has dominant contributions from the Eastern Equatorial Pacific (25 ± 1)%, from the Southern Ocean (SO) (21 ± 1)%, and from the Eastern Equatorial Atlantic (EEqA) (12 ± 1)%. The pump’s 61% leak originates predominantly in the SO (75%) and in the SubPolar North Atlantic (17%). While the SO euphotic zone is a large leak of preformed phosphate, it is also the major receptor of phosphate reemerging from depth: The SO euphotic zone is the destination of (62 ± 6)% of the regenerated inventory and of (69 ± 5)% of the preformed inventory. The mean interior residence time of regenerated phosphate reemerging in the SO depends on where it was last utilized: 69 ± 1 years if last utilized in the SO and 500 ± 20 years if last utilized outside the SO. The transit-time distribution of the mass of regenerated phosphate last taken up in the EEqA and reemerging in the SO euphotic zone is bimodal, pointing to two distinct pathways which are quantified using the phosphate path density.

Benoît Pasquier

Research Associate

My research interests include mathematics, oceanography, and computer science.