We started this week with the northernmost station of our cruise on the western slope of the Yermak Plateau. Although we were eventually able to pass the magic 82˚ North on our transit onto the Yermak Plateau, our sixth ice station has not been located at the western slope of the plateau.
Roughly around 82°13‘N we got another taste of the strength of the ice, and a progress downslope was thus impossible. We had to leave behind the idea to take advantage of the floe drift force to reach the 1000 m water depth due to changing winds which let us drift to the south instead of the southwest as we had anticipated from previous drift experiences. Originally, we planned for this region in order to carry out geological coring. Fortunately, the geology team was able to locate similar sediments on the eastern slope of the plateau and was thus able to retrieve two long sediment cores. The Geology Group on this expedition is composed of nine people, representing six different research institutes in five different countries. Our international team consists of real Polarstern veterans as well as newcomers. The different research fields we are covering are just as diverse as our team. Our work includes mapping of the seafloor using the ship’s bathymetry system (ATLAS Hydrosweep DS3) and the sediment profiling Parasound Hydromap. The ATLAS Hydrosweep has been running since the beginning of our cruise, providing information about the seafloor topography. It scans the ground by sending a fan of acoustic signals to the seafloor and measuring the time they take to return to the ship. From the travel time of the individual signals, the water depth is calculated, resulting in a several kilometer wide strip portraying the bathymetry along our cruise track. Each newly mapped square metre provides valuable information that is fed into global data sets and is used to continuously improve the bathymetric chart of the Arctic Ocean. Data from the Parasound system provides us with insights into the stratification and deposition of subsurface sediments. Ice sheets and icebergs often generate so-called “plough marks” on the seafloor that can be filled with sediments. Such marks that have been found north of Svalbard (Figure 1) and on the Yermak-Plateau (Figure 1), and provide information on the drift direction, extent, and thickness of large ice masses that existed in the Arctic during past glacial periods.
Information provided by the Parasound on the type and stratification of sediment deposits is critical to identify appropriate sediments to reliably reconstruct climatic and environmental conditions of the past. Once a good location is identified, subsurface sediments are sampled with a variety of tools, all driven by the very simple principle of gravity. Tubes or boxes are pushed into the ocean floor by lead weights. However, gravity alone is not sufficient for a successful retrieval of sediment cores: At day and at night scientists can rely on the readiness of the well-trained deck crew (Figure 2). The Giant Box Corer and Multicorer (Figure 3) penetrate approximately 50 cm into the sediment and are ideally suited to preserve a undisturbed sediment surface. On this expedition, the Multicorer is additionally equipped with an online video system that transfers high-resolution pictures from the seafloor to the ship via glass fibre cable (Figure 4).
When using such modern deep sea technology, the support of the experienced crew is invaluable (Figure 5). For longer climate reconstructions, in our case the last 100,000 to 200,000 years, Gravity and Kastenlot Corers are used. If successful, they can provide more than 10 m long sediment cores. As proof, a 9 m long Kastenlot core was retrieved already a week ago from the northeastern Yermak Plateau. According to preliminary analyses onboard, it may hold information of the last 50,000 to 150,000 years of climate history (Figure 6).
In the wet lab, the recovered sediments are sampled (Figure 7) and are placed in cold storage, for later analyses of various physical, chemical or micropaleontological parameters. Already at the sampling stage, good preparation and organisation are of the essence: Sediment samples that are going to be analysed for certain organic parameters, for example, must not be in contact with any plastic material (Figure 8). Other samples need to be frozen immediately to prevent specific minerals from oxidizing in the air.
Expertise within the Geology Group covers a range of analytical methods. All these different methods will enable them to trace specific environmental conditions in the geological past - for example, how densely the surface of the Arctic Ocean was covered with sea ice over the last glacial cycle, in which direction the ice was drifting, or how much food arrived at the sea floor to support benthic organisms. The changing drift direction of sea ice over the past millennia can be reconstructed using a variety of methods. Most of these depend on a detailed knowledge of the geology of the different land areas surrounding the Arctic Ocean. In this way, certain minerals or chemical elements found in the sediment cores are related to particular rock formations on land, and the drift direction small sediment particles transported with the sea ice across the ocean can be reconstructed.
Traces and remains of various biota in Arctic sediments also holds a wealth of information. Sea ice and temperature reconstructions of surface waters, for example, are based on the analysis of specific organic substances in the sediments (biomarkers) that need to be quantified following an elaborate extraction procedure. One of these indicators is the TEX86 index, but its application in the Arctic Ocean has not been fully established yet – here the studies of the Geology team can make a decisive contribution. A classical archive for environmental and climate reconstructions are tests of single-celled organisms like foraminifera that dwell in the water column or at the sea floor. Their shells can be composed of calcium carbonate or sediment particles. Their distribution, presence or absence of certain species, and the chemical composition of their tests are used to decipher past distribution of different Arctic water masses, their carbonate chemistry, pH or the bioproductivity of the overlying surface waters. An additional advantage of foraminifera lies in their value to determine the ages of the sediments.. Without accurate age models, reconstructions of environmental parameters cannot be put into a temporal context, and connected with other global paleoenvironmental records. Age information can be extracted in different ways. On one hand, the occurrence of certain foraminifera species in defined geological periods can serve as an age indicator for the recovered sediments. On the other hand, foraminifera tests can be precisely dated using the radiocarbon isotope method; this methods works well until about 45,000 years before present, but not in older sediments.
First hints regarding the age of the newly recovered sediments is also derived from geophysical analyses conducted directly onboard before the sediment cores are opened and sampled. The Multi Sensor Core Logger (Figure 9) performs non-destructive measurements of the magnetic susceptibility and bulk density of the sediments at a very high spatial resolution (every 0.5 cm). These data sets are correlated to other sediment records from the regions (collected on past expeditions) that already have established age models.
Members of the Geology Group also study the degradation of organic material – remains of dead plants and animals – within the sea floor. These degradation processes can recycle essential nutrients back into the sea water, but they can also alter the physical and chemical properties of the sediments long after their deposition. These early diagenetic processes leave characteristic traces in the sedimentary pore waters, which are extracted from the sediments using rhizones – “artificial roots” – and partly analysed directly onboard (Figure 10).
Furthermore, a distinct research project of the Geology Group during this expedition is the attempt to directly correlate the geological information locked in the sediments (so-called proxy parameters) with the biological and physical properties of the overlying water masses. This proxy calibration not only requires sampling and analysis of the surface and deeper sediment layers, but also of the sea water, plankton community, and suspended particles. This is the only way to establish that, for instance, a specific chemical composition of a microfossil (for example, a foraminifer) shell reflects a clearly defined temperature, carbonate chemistry or pH value of the sea water it grew in. Establishing such qualitative and quantitative relationships is crucial for a correct interpretation of the environmental information encrypted within the sedimentary record. Therefore, the Geology Group not only works on sea floor deposits, but are actively involved in the ice stations where ice cores are taken and nets are used to fish for living foraminifera under the ice (Figure 11). The water column is also being sampled to compare its current chemical and physical parameters with those of the surface sediments at the same location. In addition, multi-nets are used to document the plankton assemblages at specific depths within the modern water column (Figure 12). Finally, team members study surface sediments hosting specific foraminifera that are kept alive and, back on shore, are cultivated in special high-pressure aquaria. In this sense, the research approach of the TRANSSIZ Geology Group reaches far beyond a “classical” geological working program. The ultimate goal will be to gather valuable data from the past and use them to contribute to a systematic understanding of a changing Arctic Ocean.
During our 7th ice station on the eastern Yermak Plateau we could already see increasing snow melt on the surface of the ice floe. Darker patches on the floes indicate where melt ponds might soon evolve. Despite these signs of melting we were confronted again with the force of the ice. Two gigantic ice floes with a much stronger drift pattern than the small floes north and south of our ice station resulted in a jamming of large and small ice floes. Since this turmoil of closing up floes moved towards our ice station we could unfortunately not complete our station but had to leave earlier in order to not being trapped in the ice.
Meanwhile we shifted further south and now also completed our 8th and last ice station. This last ice station happened to be in an interesting area with strong melting of the ice floes seen from the bottom of the floe, while on top a rather intact snow layer was still visible. Usually in the Artic the ice melts from the top, meaning the snow melts first. This eventually leads to the well-known melt ponds, typical for Arctic sea ice. Here we were in an area were the relative warm Atlantic Water mixes with the cold polar water, and with our detailed measurements of water mass mixing we will eventually be able to address the importance of the Atlantic Water layer for enhanced sea-ice melting. In summary, during our 8th ice station, process studies about productivity, the dynamics of the ecosystem and the biogeochemical studies provided valuable insights into the local differences of the Arctic marine ecosystems. One exciting part of this expedition will be now to link the present observations with the geological history to better understand the transition of sea ice and the impact of Atlantic Water within the Arctic Ocean.
Sadly we need to get accustomed that the exciting voyage will soon come to an end. However the sun and increasing temperatures today let us feel the start of the summer season even here, high up in the North.
Ilka Peeken with contributions by Christian März, Jens Mathiessen, Matt O’Regan, Clara Stolle, Kirstin Werner and Jutta Wollenburg
