Deep Ocean storage of heat and CO2 in the Arctic Mediterranean during the last glacial period

Abstract:

Late Pleistocene glaciations were characterized by abrupt variations in regional climates and atmospheric pCO₂, superimposed on gradual longer-term trends of increasing ice volume and decreasing atmospheric pCO₂. The ocean, as a major dynamic carbon and heat reservoir, is believed to have played a dominant role in these changes. The Fram Strait is the only deep gateway between the Arctic Ocean and the Nordic Seas and thus is a key area to study past changes in ocean circulation and the marine carbon cycle. Here, we reconstruct deep ocean temperature, δ¹⁸O, [CO₃^2-] and nutrient content in the Fram Strait during the past 35,000 years. We first explained published ambiguous differences of δ¹⁸O (>1‰) between the two benthic foraminiferal species Oridorsalis umbonatus and Cibicidoides wuellerstorfi that have been recorded from sediments dating from the last glacial maximum in the central and northern Nordic Seas. Our results indicate a thickening of Atlantic water penetrating into the northern Nordic Seas, forming a subsurface Atlantic intermediate water layer reaching to at least ~2600 m water depth during most of the late glacial period. The recirculating Atlantic layer was characterized by high [CO₃^2-] and low δ¹³C during the late glacial, and provides evidence for a Nordic Seas source to the glacial North Atlantic intermediate water flowing between 2000–3000 m depth. Furthermore, our results highlight short millennial-scale episodes of enhanced vertical mixing and possibly heat and carbon release to the atmosphere that could have contributed to abrupt changes in atmospheric pCO₂ and regional climate. In addition, evidence from previous studies suggests enhanced terrestrial carbon flux to the northern Nordic Seas at the Heinrich stadial/interstadial 2 transition (~24,000 year BP). Comparing our [CO₃^2-] and δ¹³C records with carbon cycle model results, we conclude that either the total subglacial or permafrost carbon release was low, slow, or released directly to the atmosphere.