Weekly Reports Polarstern
The Expedition PS100 from Tromsø to Tromsø
The cruise “Greenland ice sheet/ocean interaction and Fram Strait fluxes” (GRIFF) combines investigations of the oceanic fluxes through Fram Strait with those of the interaction between the Greenland ice sheet and the Nordic Seas. Strong temperature increase of the Arctic atmosphere and ocean and considerable changes of the Arctic fresh water budget during the past years, and in the same time the retreat of the Greenland ice sheet as well as changes in the ocean circulation are linked via complex interactions. Fram Strait constitutes the only deep gateway between the Arctic and the global ocean through which warm saline Atlantic waters flow northwards and at the same time a substantial part of the huge Arctic fresh water excess is carried southward towards the Atlantic, partly as as liquid water and partly a sea ice. In the Nordic Seas, this fresh water flux is augmented by runoff (and ice bergs) from Greenland. In deeper layers of Fram Strait, deep and bottom waters that are ventilated by polar processes exit the Arctic to be eventually incorporated in the Atlantic meridional overturning circulation. In the same time, one of the mightiest Greenland ice streams, the NEGIS (North East Greenland Ice Stream), drains into the western Fram Strait where its outlet glacier, the 79°N Glacier, is in direct contact with warm Atlantic water recirculating in Fram Strait. The Atlantic water in Fram Strait has been warming considerably during the last decades and climate models propose further warming with possible consequences for the stability of the NEGIS.
The proposed cruise will combine work on ocean circulation, geochemistry, glaciology, geodesy, geology, geophysics, mechanical engineering, biology and biochemistry of this complex system. The volume and heat fluxes through Fram Strait will be quantified by a continuation of the long-term mooring and hydrography programs. The hydrographic section work will involve the whole suite of the trace elements run by the GEOTRACES program. In combination with the oceanographic transport measurements and with the pan-Arctic GEOTRACES program carried out in 2015 (Polarstern TransArc II cruise completed 2015) this will enable us to estimate the Arctic contribution to the global trace element cycle. On the other hand, the trace elements help identifying the sources of the various water masses flowing through Fram Strait, including contributions from basal glacial melting. The oceanographic mooring array and the hydrographic and trace element surveys will be complemented for the first time by i. a meridional section in Fram Strait to investigate the recirculation of Atlantic Water and ii stations on the East Greenland shelf in order to track the flow of warm Atlantic intermediate water towards the cavity of the 79°N glacier. We will also identify melt water and quantify rates. The latter will be constrained by observations of stable noble gas isotopes.
The cruise will further support measurements of the glacial / terrestrial dynamics. Devices for glaciological and geodetic observations will be installed along the coast of Northeast Greenland and on the 79°N glacier. The glaciological program will capture the melt rates and dynamics of the ice flow. The geodetic program focuses on a re-occupation of a long-term sites to capture deformation rates of the earth crust, which will provide information of the glacial isostatic adjustment.
In order to put the modern observations near Greenland into a long-term perspective, the geology program will study the history of Northeast Greenland Ice Stream after the last glacial maximum. A particular aim is to constrain both the ice stream and ice shelf extents and thicknesses in order to determine rates of retreats and re-advances by sediment coring and acoustic seafloor surveys at key locations on the Northeast Greenland continental shelf. The ice sheet retreat cycles will then be interpreted in the context of oceanic and atmospheric conditions as well as sea level change.
The physical work will be complemented by biogeochemical and biological programmes. The biogeochemical work will focus on the characterization of water column nutrients as well as the sources of nutrients both in Fram Strait and on the shelf of Northeast Greenland. The water samples to be obtained will also serve to validate R/V Polarstern’s underway nutrient measurement system. The biological program will target the marginal ice zone of Fram Strait. Specifically this involves the quantification of abundance, biomass, biodiversity and production of ecologically important components and key species of the marine communities. Biomass and production of all important components of the food web from ice algae and phytoplankton via zooplankton to top predators such as seabirds and marine mammals will be determined synoptically. This is challenging because of the complex current regime in western Fram Strait.
In addition, a geophysical study of spreading processes at an ultraslow spreading ridge will be conducted. The aim is to find out how melts travel at segment scale along the axis of ultraslow spreading ridges and rise through the thick lithosphere. To accomplish the task, for the first time an entire segment of an ultraslow spreading ridge will be instrumented with a 180 km long network of ocean bottom seismometers in order record the earthquake activity of the ridge.
Finally investigations of R/V Polarstern’s dynamic responses due to complex ice-structure and fluid-structure interactions will made. This will require full scale vibration measurements which will be conducted aboard R/V Polarstern throughout the cruise.
R/V Polarstern will depart from Tromsö on July 18. The main work areas are Fram Strait, the shelf of Northeast Greenland including the coast, and Knipovich Ridge south of Fram Strait (Fig. 1) Since we expect the sea ice conditions to be not particularly favourable for the conduction of research on the East Greenland continental shelf in July and early August, we shall first attempt to conduct the research programmes in Fram Strait. Here moorings will be serviced along the 79°N line and an intense water column sampling program will be conducted (hydrography, trace elements, biochemistry). This shall be merged with the biological „box“ approach in the marginal ice zone of western Fram Strait. Also we will commence with the deployment of ocean bottom seismometers along Knipovich Ridge in the first half of the cruise. In the second half of the cruise, sea ice conditions should allow us to conduct our work on the ocean – glacier interaction on the shelf of Northeast Greenland. Here both mooring servicing and water column work will be conducted. This will be complemented by the geological workpackage (sediment coring, echosounder-based surveys of the bathymetry). Also the helicopter-based operations will be conducted in order to cover both the geodetic stations on the coast as well as glaciological and oceanographic aspects on the 79°N glacier. On the way back to Tromsö, where the expedition will end on September 6, the geophysical programme at Knipovich Ridge will be completed.