Australia: The Land Where Time Began

A biography of the Australian continent 

Carbon Sequestration in Deep Atlantic During Last Glaciation

About 70,000 years ago there was a marked decline in concentrations of atmospheric CO₂ as the climate of the Earth descended into the last glaciation. It has been suspected that much of the carbon that was removed from the atmosphere entered the deep oceans, though evidence remains elusive of increased storage of carbon. In this paper Yu et al. report using B/Ca ratios of benthic foraminifera obtained from several sites in the Atlantic Ocean to reconstruct concentration of carbonate ions and therefore the inventory of carbon of the deep Atlantic Ocean across this transition. Their results indicate that the concentration of carbonate ions in the deep Atlantic Ocean declined by about 25 μmol/kg between about 80,000 and 65,000 years ago. It is implied by this drop that carbon inventory of the deep Atlantic Ocean increased by at least 50 Gt about the same time that the atmospheric carbon dropped by about 60 Gt. Yu et al. say this infers that based on a comparison with proxy records of deep circulation and climate model simulations the sequestration of carbon coincided with a shoaling of the Atlantic meridional overturning circulation. The conclusion was that changes in the circulation of the Atlantic Ocean may have had an important role in the reducing the concentrations of carbon dioxide during the last glaciation by increasing the amount of carbon stored in the deep Atlantic.

Carbon sequestration – reasons

Yu et al. suggest a synergy of physical and biogeochemical processes may have resulted in enhanced the storage of carbon in the deep Atlantic during MIS 4 (8,11). With regard to physical processes, it is implied by neodymium isotopes (εNd; an ocean circulation proxy) in the sediment that there was an increased contribution of southern-sourced abyssal waters rich in carbon dioxide in the deep Atlantic at the transition from MIS 5a-4 (9,39). Yu et al. suggest that the NADW-AABW boundary probably shoaled to a depth of about 2-3 km during MIS 4, and was located above major topographic ridges and seamounts (5,39). Diapycnal mixing between water masses would be weakened by such an AMOC rearrangement, enhancing stratification of the water column, which would thereby facilitate retention of sequestered carbon in the deep ocean (4,40). There was a sharp about 1ε unit increase of εNd at about70 ka in core TN057-21 that coincided exactly with a rapid about 12 μmol/kg decline in deep water [CO₃^2-]. Synchronous changes in εNd and CO₃^2- indicate a tight coupling between AMOC and cycling of carbon in the deep Atlantic during the last glaciation, as seawater [CO₃^2-] is primarily determined by DIC and ALK, both of which place direct constraints on the oceanic carbon cycle (8,11,32,34). At about 72 ka a decrease in in benthic δ¹³C of about 0.5 ‰ was previously interpreted to reflect a global carbon budget change to predate an AMOC reorganisation (9). If this δ¹³C decline was caused by carbon transfer from land biosphere (9), it would decrease concomitantly deep water [CO₃^2-] and intensify deep sea CaCO₃ dissolution, a phenomenon which is not observed in TN057-21. Yu et al. contend instead that the decline of δ¹³C might reflect processes such as air-sea isotopic exchange (42). The results from 2 Earth system models of intermediate complexity: halving the formation of NADW leads to reductions of 10-30 μmol/kg of the CO₃^2- below about 3 km in the deep Atlantic without causing deep sea anoxia, which further supports the coupling of AMOC and carbon cycling. Also, a cooler climate during MIS 4 would raise CO₂ solubility and preformed DIC in deep waters (13,43), which would enhance the sequestration of CO₂ in the deep ocean. With regard to the decreased [CO₃^2-] during MIS 4 is consistent with greater remineralisation of the water column resulting from reduced vertical mixing associated with a shoaled AMOC and a biological pump that was more efficient in the Southern Ocean during the last glacial period possibly stimulated by an increased availability of iron (10), both of which would increase respiratory DIC sequestration into the interior of the ocean and decrease concentration of atmospheric CO₂.

Overall, the calculations of Yu et al. highlight that the deep Atlantic sequestered a substantial amount of carbon during the last glaciation about 70 ka, in spite of being a relatively modest proportion of about 30 % of the volume of the deep ocean. The amount of carbon that was sequestered is quantitatively comparable to the loss of carbon from the atmosphere. It was found by this study that this large carbon sequestration was tightly coupled with changes of the AMOC. The movements of carbon between the reservoirs in the atmosphere-land biosphere-ocean system are linked intricately, and Yu et al. suggest that future studies may aim to quantify the contributions from individual sources to the increased storage in the deep ocean during glaciations.

Sources & Further reading

1. Yu, J., L. Menviel, Z. D. Jin, D. J. R. Thornalley, S. Barker, G. Marino, E. J. Rohling, Y. Cai, F. Zhang, X. Wang, Y. Dai, P. Chen and W. S. Broecker (2016). "Sequestration of carbon in the deep Atlantic during the last glaciation." Nature Geosci 9(4): 319-324.