Plio-Pleistocene ocean circulation changes in the GOA and its impacts on the carbon and nitrogen cycles and the CIS development

• Published in: Paleoceanography and Paleoclimatology
• Main Authors: Sánchez Montes, M.L., Romero, O.E., Cowan, A.E., Müller, J., Moy, C.M., LLoyd, J.M., McClymont, E.L.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2022
• Subjects: Gulf of Alaska; Pacific; Ice Sheet; Alaska
• Online Access: http://dro.dur.ac.uk/36335/; http://dro.dur.ac.uk/36335/1/36335.pdf; http://dro.dur.ac.uk/36335/2/36335VoR.pdf; https://doi.org/10.1029/2021PA004341

The modern Gulf of Alaska (GOA) is a high nutrient low chlorophyll (HNLC) region, estimated to be important for nutrient cycling and CO2 exchange. Little is known of the GOA evolution over the Pliocene and Pleistocene as well as its impact on the Cordilleran Ice Sheet (CIS) development, when other evidence for changing North Pacific circulation has emerged. We analysed IODP Expedition 341 Site U1417 sediments, which extend through the Plio-Pleistocene transition (4-1.7 Ma), focussing on productivity-related biomarkers (alkenones, brassicasterol), siliceous microfossils and bulk carbon and nitrogen stable isotopes. Our results show two dominant water column regimes: one characterised by high silica and low organic matter preservation, containing microorganism remains from a mix of habitats (4-3.7 Ma) and a second characterised by low biogenic silica and increased organic matter preservation of microorganisms from dominantly open ocean habitats (3.33-3.32 Ma and 2.8-1.66 Ma). An increase of phytoplankton diversity (3.7-3.35 Ma, 3.19-2.82 Ma) characterises the two transitions of water column conditions, from oxygenated to reductive, that we attribute to a change from ocean mixing to strong stratified conditions with some occasional mixing. The biogeochemical changes in the GOA follow 400 and 100 kyr eccentricity cycles which are also reflected in changes in the CIS. We conclude that the CIS expansion created HNLC conditions in the GOA during the Mid Piacenzian Warm Period (MPWP) and the early Pleistocene. In turn, positive feedbacks increased marine productivity export, atmospheric CO2 drawdown and further CIS expansion.