Re-organization of ocean circulation during the Miocene transition from Greenhouse to Icehouse: The role of the Indian Ocean
Final Report Abstract
We tested the overarching hypothesis that the boundary conditions for the “modern” global oceanatmosphere circulation were established during the middle-late Miocene in a stepwise manner, involving threshold changes in inter-ocean heat and carbon transfer. To achieve this goal, we created a continuous, high resolution, orbitally tuned deep water paleoceanographic (benthic stable isotope) time series between 18 and 8 Ma for the middle to late Miocene interval at International Ocean Discovery Program (IODP) Expedition 353 Site U1443 at the northern end of the Ninetyeast Ridge in the equatorial Indian Ocean. Our results suggest that the bottom water saturation state in Indian Ocean intermediate-deep waters experienced a maximum at the end of the Middle Miocene Climatic Transition and decreased stepwise during the late Miocene in line with declining atmospheric pCO2 levels, global cooling and cryosphere expansion. The complete record of Site U1443 is ideally suited to assess changes in inter-ocean heat and carbon gradients and, thus, to evaluate the global extent of large-scale deep ocean δ13C changes and their phase relationship to benthic δ18O fluctuations. In particular, our results suggest that changes in monsoonal discharge of carbon and alkalinity from the Asian continent into the equatorial Indian Ocean played a crucial role in the global marine carbon cycle during the Middle Miocene Climatic Transition and the ensuing late Miocene stepwise cooling. These changes in transfer and storage of carbon, alkalinity, nutrients and heat mediated the transition between different equilibrium states of the Miocene ocean/climate system (Miocene Climatic Optimum 16.9-14.7 Ma, Middle Miocene Climatic Transition 14.7-13.8 Ma, Carbonate Crash 13.2-8.6 Ma, and Biogenic Bloom <8.6 Ma). In the course of the project, we encountered a surprisingly intense effect of carbonate dissolution at an equatorial Indian Ocean location in water depth shallower than 3000 m, well away from major upwelling zones. These carbonate dissolution events occurred at orbital eccentricity maxima during the Miocene Climatic Optimum. Carbonate dissolution in the equatorial Indian Ocean intensified and was not restricted to eccentricity maxima during the Carbonate Crash interval. The unexpected rare occurrence of benthic foraminifera and the possible effect of these strong carbonate dissolution events on the test geochemistry within these intervals restricted Mg/Ca analyses as a deep water temperature proxy. However, we now consider exploring the effect of these carbonate dissolution events on benthic foraminiferal test geochemistry in the near future, when a next generation ICP-OES will be available at our institute that allows reliable analysis of smaller samples.
Publications
-
2019. The middle to late Miocene “Carbonate Crash” in the equatorial Indian Ocean. Paleoceanography and Paleoclimatology 34, 813-832
Lübbers, J., Kuhnt, W., Holbourn A.E., Bolton, C.T., Gray, E., Usui, Y., Kochhann, K.G.D., Beil, S., Andersen, N.
