Water column stratification in the western Arctic Ocean during the past climatic cycles

Dr Not, Christelle Aurelie   (Principal Investigator (PI))

Dr Thibodeau Benoit   (Co-Investigator)

24

2017-06-01

2019-05-31

44320

Water column stratification in the western Arctic Ocean during the past climatic cycles

Arctic Ocean, Mg/Ca, ostracodes, oxygen isotopes, water masses

Environmental Studies and Science,Earth Sciences

Physical Sciences (P)

201611159170

Seed Fund for PI Research – Basic Research

2016

Completed

The Arctic Ocean plays a crucial role in global climate and his at the vanguard of climate change. Since the last decade, summer sea ice cover was greatly reduced and the freshwater budget changed quite drastically (Stroeve et al., 2012, Polyakov et al., 2008). Both parameters can have serious feedback on water masses, Arctic oceanography and global climate and thus, understanding the water column stratification is crucial to foresee potential near-future climatic changes. The Arctic Ocean is highly stratified. The surface water (Polar Mixed Layer, PML) is cold and fresh and is in direct contact with the sea ice. Thus, the temperature of the PML will greatly influence the extent of the summer sea-ice melt and its reformation in autumn. Below the PML, there is the cold and salty halocline, which isolates the PML from the warmth of the underlying Arctic Intermediate Water (AIW). The AIW originates from the combination of the Atlantic Intermediate Water with water formed along the margins. The AIW is warmer and saltier water than the above water masses. At the bottom of the basin, the isolated Arctic Deep Water (ADW), colder and saltier than the other water masses, is present. The heat carried by the AIW, is responsible of the heat transport from the North Atlantic and have the potential to impact the stratification and the circulation of the Arctic Ocean. A thinner halocline, due to decreased sea-ice formation and release of brine would lead to a transfert of warmth from the AIW to the PML and thus, would enhance sea-ice melt and limit even more sea-ice formation. Higher PML temperatures have already been observed in the Canada Basin (McLaughlin et al., 2009), suggesting a warming of the halocline due to weaker stratification. Therefore, understanding the variability of the water column stratification in the Arctic Ocean is crucial to apprehend the response of sea-ice cover to global warming. Here, we propose to reconstruct the temporal variation of the intermediate and bottom water masses temperature using three sediment cores retrieved from water depth ranging 400 to 2300 meters. Using the two shallow sediment cores, we aim to reconstruct the temperature of the AIW and its possible warming during glacial periods as suggested by Cronin et al., (2012) but also is variability within interglacial periods. Moreover, these cores will allow to precisely reconstruct the lower limit of the Halocline to verify the hypothesis that it deepened during past glacial periods. Since the two cores are retrieved from 450 and 750 m of water depth, respectively, we plan to resolve the depth location of the halocline-AIW boundary. Finally, the deeper core, retrieved at a water depth of 2300m, will be used to monitored possible temperature changes of the Arctic Deep Water during glacial and interglacial periods. In summary, the aim of this project is to test the temporal variability of the Arctic Ocean stratification and the temperature of the different water masses by reconstructing the bottom water temperature using the geochemistry of ostracodes shells over the last 400 000 years. More precisely the objectives are: 1. Reconstruct the temperature of the water masses at the three coring sites using the Mg/Ca of the ostracodes shells. 2. Estimate the freshwater content of the water masses at the three coring sites using the 18O of ostracodes. 3. Determine the water depth of the boundary between the halocline and the AIW and the AIW and the ADW. 4. Reconstruct the variation of water depth boundaries of these water masses during the last 400 000years, focusing on the glacial and interglacial periods variability. Cronin, T.M., Dwyer, G.S., Farmer, J., Bauch, H.A., Spielhagen, R.F., Jakobsson, M., Nilsson, J., Briggs Jr., W.M., Stepanova, A., 2012. Deep Arctic Ocean warming during the last glacial cycle. Nature Geoscience 5, 631e634. McLaughlin, F.A., Carmack, E.C., Williams, W.J., Zimmermann, S., Shimada, K., Itoh, M., 2009. Joint effects of boundary currents and thermohaline intrusions on the warming of Atlantic water in the Canada Basin, 1993–2007. J. Geophys. Res. 114, C00A12. Polyakov, I.V., Alexeev, V.A., Belchansky, G.I., Dmitrenko, I.A., Ivanov, V.V., Kirillov, S.A., Korablev, A.A., Steele, M., Timokhov, L.A., Yashayaev, I., 2008. Arctic Ocean freshwater changes over the past 100 years and their causes. J. Clim. 21, 364–384. Stroeve, J.C., Serreze, M.C., Holland, M.M., Kay, J.E., Malanik, J., Barrett, A.P., 2012. The Arctic's rapidly shrinking sea ice cover: a research synthesis. Clim. Chang. 110, 1005–1027.