Sea-Air CO2 Exchange in the SW Iberian Upwelling System during Two Contrasting Climate Cycles: 860–780 ka and 630–520 ka

Gloria Martin-Garcia

doi:10.3390/geosciences8120454

Abstract

Analysis of planktonic and benthic foraminifers’ accumulation rates from the Iberian margin reveal a substantial change in the biogenic ocean-atmosphere CO2 exchange during the Mid-Pleistocene Transition (MPT; ~800–650 ka from present). Such changes resulted from the major reorganisations in both surface and deep-water circulation that occurred in the North Atlantic at the time, and affected the behaviour of this upwelling region as a CO2 uptake/release area during climate cycles before and after the MPT. During Marine Isotope Stages (MIS) 21-MIS 20 (860–780 ka), this margin acted mostly as an uptake area during interglacials and early glacials. During glacial maxima and terminations it would be neutral because, although surface production and export were very low, carbon storage occurred at the seafloor. During MIS 15-MIS 14 (630–520 ka), the pattern was the opposite, and the Iberian margin worked as a neutral, or as a source area during most interglacials, while during glacials it acted as an important uptake area. Present findings support the idea that glacial/interglacial atmospheric pCO2 oscillations are partly driven by alterations in the meridional overturning circulation that results in substantial variations of the biological pump, and carbon sequestration rate, in some high-productivity regions.

Highlights

Variations in biological oceanic processes modify the atmospheric carbon sequestration rate, which has direct implications on climate change
The Portugal Current (PC), which is the southward branch of the North Atlantic Current (NAC), dominates the surface circulation in the western Iberian margin [35] (Figure 1)
One of the most widely accepted explanations for the glacial/interglacial oscillation of the atmospheric pCO2 is an enhancement of the biological pump during glacials, which would result from an increased supply of land-derived nutrients [56,57]

Summary

Introduction

Variations in biological oceanic processes modify the atmospheric carbon sequestration rate, which has direct implications on climate change. Carbon sequestration begins with primary production, for example, [1], which occurs in the surface of the oceans but fuels heterotrophic activity from the surface to the seabed. Around 5–15% of the carbon is exported to the depths through mixing, advection, diffusion, passive sinking, and active transport [2,3]. The zooplankton have a significant impact on this biological pump [4]. They consume organic matter and form a biomass that, by vertical migration, will be transported to the aphotic zone, and eventually will end up in the seabed. Vertically migrating zooplankton feed in surface waters but excrete at depth, actively transporting organic material to the meso- and bathypelagic zones. The zooplankton are responsible for up to 70% of the total organic particulate carbon (POC) exports [5,6,7,8]

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