Linking changes in productivity and nutrient distribution in the Labrador Sea to ocean-atmosphere climate dynamics from the Glacial to today
Ocean Frontier Institute (OFI), Postdoctoral Fellow (PDF)
Dr. Kristin Doering
Supervisors: Prof. Dr. Stephanie Kienast
Prof. Dr. Martin Frank
In the present and past ocean nutrient distribution and utilization in high latitudes are closely coupled to ocean-atmosphere dynamics and strongly influence the nutrient supply to the thermocline of low latitudes and the deep ocean. The Labrador Sea (LS) is a key area for such climatic and oceanographic interactions in the Northern Hemisphere as it links the Arctic Ocean and the North Atlantic Ocean, and is one of the major source areas for the formation of North Atlantic Deep Water (NADW) (e.g. Clarke and Gascard, 1983). In the LS hydrographic and biogeochemical conditions are sensitive to past and present climate variability including major melt-water events. For instance, accelerated warming and melting of Arctic sea-ice have been shown to significantly increase phytoplankton productivity over the last decades, while cold phases such as the Little Ice Age (~1450-1850 AD) were characterized by reduced primary productivity (Chan et al., 2017) in this region. Silicic acid is a major nutrient controlling productivity in the subpolar North Atlantic. It is an essential macronutrient for diatoms, which are the dominant phytoplankton group today during spring blooms and are responsible for a significant portion of marine carbon export production. Furthermore, the deepwater (mainly Labrador Sea Water, LSW), which is injected into the mid-depth circulation system, is critical for ventilating and renewing the water masses of the interior North Atlantic Ocean (Kieke and Yashayaev, 2015). Especially during the last deglaciation and the early to Mid-Holocene, deep water formation in the Labrador Sea is thought to have been perturbed by melt-water discharge events from the Laurentian ice sheet, which reduced the strength of the Atlantic Meridional Overturning Circulation (AMOC) (Hillaire-Marcel et al., 2007; Hoffman et al., 2012).
We propose to reconstruct deglacial to Holocene changes in primary productivity (PP) and nutrient cycling in relation to melt-water events and sea-ice extent along the Labrador shelf. We apply coupled analysis of stable silicon and nitrogen isotope compositions, which enable us to gain insight into the past nutrient distribution and nutrient uptake by different phytoplankton groups in relation to changes in the dominant water masses. Furthermore, we will analyze stable silicon isotope signatures of organisms living at different depths (diatoms, radiolaria, and sponges) to study changes in the nutrient inventory of intermediate and deep water masses, in relation to LSW formation and circulation since the last glacial.
References:
Chan, P., Halfar, J., Adey, W., Hetzinger, S., Zack, T., Moore, G.W.K., Wortmann, U.G., Williams, B., Hou, A., 2017. Multicentennial record of Labrador Sea primary productivity and sea-ice variability archived in coralline algal barium. Nat Comms 8, 1–10. doi:10.1038/ncomms15543
Clarke, R.A., Gascard, J.-C., 1983. The Formation of Labrador Sea Water. Part I: Large-Scale Processes. Journal of Physical Oceanography 13, 1764–1778.
Kieke, D., Yashayaev, I., 2015. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light of international partnership and collaboration. Progress in Oceanography 132, 220–232. doi:10.1016/j.pocean.2014.12.010
Hillaire-Marcel, C., de Vernal, A., Piper, D.J.W., 2007. Lake Agassiz Final drainage event in the northwest North Atlantic. Geophys. Res. Lett. 34, 344–6. doi:10.1029/2007GL030396
Hoffman, J.S., Carlson, A.E., Winsor, K., Klinkhammer, G.P., LeGrande, A.N., Andrews, J.T., Strasser, J.C., 2012. Linking the 8.2 ka event and its freshwater forcing in the Labrador Sea. Geophys. Res. Lett. 39. doi:10.1029/2012GL053047