Introduction

The third leg of the Beringia 2005 research expedition consisted of a joint crossing of the central Arctic Ocean involving Swedish icebreaker Oden and U.S. Coast Guard Cutter Healy. This complete Arctic Ocean transect, also named Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX’05), was only the second in history made by surface vessels; the first involved the U.S. Coast Guard Cutter Polar Sea and Canadian icebreaker Louis St. Laurent in 1994. Due to the ambitious scientific agenda the two icebreakers Healy and Oden did not reach the North Pole until 12^th September. This North Pole visit was later in the year than for any previous non-nuclear surface vessels. The scientific program was byand-large divided between the two ships such that oceanography was carried out from the Oden and geology/geophysics onboard the Healy. This cruise report concerns the geological program involving 11 shipboard scientists from four countries.

Healy started the HOTRAX’05 transect on 5^th August from Dutch Harbor, Alaska, and reached the first working area, the Chukchi Borderland, off the Alaskan margin, on 9^th  August (figure 1). On 20th August the third leg of Beringia 2005 with Oden commenced from Barrow, Alaska, and the oceanographic program started a transect across the Canada Basin. A rendezvous date for the two ships was agreed upon at 84°N, 145°W. Subsequent to this rendezvous location on the Alpha Ridge the ships would come together and planned to depart from the North Pole in time to avoid the onset of refreeze and adverse winter conditions.

Scientific goals

The overall scientific goals for the HOTRAX’05 geological program can be summarized in five points:

1. Linking Amerasian and Eurasian marine sedimentary records to establish a pan-Arctic Quaternary stratigraphy across the entire Arctic Ocean.
2. Determine a palaeoclimate/palaeoceanography record from key areas across the Arctic Ocean, such as the Chukchi Borderland, the Mendeleyev and Alpha Ridge complex, the Lomonosov Ridge, the Gakkel Ridge and the Yermak Plateau.
3. Map glacial sea floor features along the transect to study the Arctic Ocean glacial history
4. Determine the source and extent of dirty ice in the central Arctic Ocean.
5. Map an area of the central Lomonosov Ridge – in collaboration with the oceanographers onboard the Oden – where deep-water exchange may take place between the Amundsen and Makarov basins.

To achieve these goals sediment cores from key locations along the transect were collected and geophysical measurements, using Healy’s hull mounted chirp sonar subbottom profiler and multibeam swath bathymetric system, were carried out. The coring locations included potential high sedimentation areas, or drifts, as well as areas where earlier cores were taken and used to construct the first widely applied stratigraphic model in the central Arctic Ocean. In the Amerasian part the majority of previous cores were collected during the era of drifting ice islands and most of them are only a few meters long (e.g. Jackson et al., 1985). These cores were the basis for the development of the widely used stratigraphic model, which indicated sedimentation rates on the order of mm/ky (e.g. Clark et al., 1980). Recent studies from the Eurasian Arctic, including the results from the drilling on the Lomonosov Ridge in 2004 (Backman et al., 2005), have concluded much higher sedimentation rates on the order of cm/kyr (e.g. Jakobsson et al., 2000). The cores from the 2005 transect will be studied to develop a pan-Arctic stratigraphy in order to test these two alternative existing stratigraphic models: very slow deposition (≈ 0.1 cm/kyr) versus moderately fast accumulation (1-2 cm/kyr). First with this information established can palaeoclimate studies investigate the history of climate change archived in the central Arctic Ocean cores.

The objective of studying the Arctic Ocean glacial history is centered on the scientific question: Have immense ice shelves existed in the Arctic Ocean some time during the past glacial times? This question has emerged from the widespread presence of glacially generated features on the Arctic seafloor, for example the glacial erosion at 1 km water depth on the Lomonosov Ridge first mapped by Oden in 1996 (Jakobsson, 1999, Polyak et al., 2001). Healy’s chirp sonar and multibeam provided excellent tools to investigate the highs along the transect for traces of glacial activity.

Field operations and results

The sea ice conditions in the Chukchi Borderland and Mendeleyev Ridge area were light, and Healy encountered one of the most ice-free conditions for early August in this part of the Arctic ever recorded. As a consequence high-quality geophysical mapping could be performed without the large disturbances heavy ice breaking causes due to the noise it creates. However, conditions quickly changed to tougher sea ice with 9/10 cover over the Alpha Ridge and Makarov Basin. When Oden and Healy reached the Lomonosov Ridge, large open lead systems made it possible to carry out the planned multibeam mapping of the central ridge near 88°25’N where deep-water exchange has been postulated to take place. After mapping and coring the Lomonosov Ridge, ice conditions drastically changed to 10/10 cover and large patches of multiyear flows were encountered. In fact, the conditions after the North Pole were by far the hardest Healy faced during the Arctic Ocean transect, and the two icebreakers assisted each other until the pack ice margin was reached northwest of Franz Josef Land. Our goal of surveying the Langseth Ridge, an obstacle of the Gakkel Ridge presumably less than 400 m deep, could not be completed due to harsh ice conditions.

Coring

The piston coring was conducted using a system designed at Woods Hole Oceanographic Institute with components from both this and the Oregon State University coring equipment. The core head weight could be adjusted, but the maximum weight of approx. 2.7 tons was used for all coring sites except for some sites during the beginning of the cruise. The maximum feasible core length that can be safely rigged on Healy is 22 m, and this requires extrusion of the core liner, largely over the ship’s rail at the end of the core barrel. During the HOTRAX’05 transect a maximum of 18 m was rigged due to the cold weather – necessitating rapid removal of the PVC liner – and the stiffness of the central Arctic Ocean sediments. Subbottom chirp data were used to determine the probable stiffness of the bottom sediments and the core barrel length to be rigged.

The geological program retrieved 21 piston cores from the complete transect across the central Arctic Ocean. Together with the 8 piston cores collected two months earlier in the season during Healy’s first leg, also constituting a part of the HOTRAX’05 geological program, the cores averaged nearly 12 m in length. These cores provide a critically needed sample cache for both a pan-Arctic stratigraphy and a long-awaited palaeoclimate record. A Multi Sensor Core Logger (MSCL) from Stockholm University measuring sediment physical properties (Density, magnetic susceptibility and sound velocity) was brought onboard Healy and all retrieved cores were immediately logged after retrieval. The long cores obtained along the Mendeleyev and Alpha Ridge show excellent preliminary correlations from the core logs and lithological description. A pronounced increase in sedimentation rates can be followed from the Alpha Ridge to the southern Mendeleyev Ridge where the present sea ice summer margin is approximately located. In the longer cores as many as 80 distinct cycles were described based on color changes and texture (figure 2). These cycles are previously described in various areas of the Arctic Ocean and have been attributed to glacialinterglacial fluctuations (e.g. Jakobsson et al., 2000). The processes behind this clearly visible sediment cyclicity are not fully understood and will be one of the main scientific targets for the post-cruise scientific work.

Multibeam mapping and chirp sonar profiling

Healy is equipped with a 12 kHz Seabeam 2112 multibeam bathymetric sonar. It has 151 acoustic beams and is capable of mapping an area underneath the ship of about 2.5–3.5 times the water depth. This system is analogous to using 151 echo sounders simultaneously to create a 3D image of the seafloor underneath the ship as it progresses along its track. Multibeam bathymetry was collected continuously during the nearly two month long cruise except for one day due to acquisition software problems. However the ice conditions greatly affected the quality of the acquired bathymetric data. During heavy ice breaking, noise and possibly also ice gliding underneath the hull of the ship occasionally caused complete data loss. The icebreaker also has a Knudsen 320B/R dual frequency (3.5 kHz and 12 kHz) chirp sonar subbottom profiler. Only the low frequency channel was used during the cruise since the sonar’s main function was to produce subbottom information and not optimal bathymetry. About 50–100 m of subbottom penetration was commonly achieved in the Arctic Ocean sediments.

An almost complete profile along the entire Northwind Ridge was accomplished and glacial erosion was mapped with the multibeam and subbottom profiler to depths of nearly 1 000 meters, or more than 200 meters deeper than previously mapped in this area (Polyak et al., 2001). Some of these glacial features were mapped for the first time, greatly expanding the extent of grounded ice in this area (figure 3). Important for the Arctic Ocean glacial history was the finding of undisturbed seabed on the Mendeleyev Ridge where the shallowest mapped area was slightly deeper than 800 m. This implies that this part of the Arctic Ocean never was reached by glacier ice as thick as that over the Lomonosov Ridge or the Chukchi Borderland (e.g. Jakobsson 1999, Polyak et al., 2001).

The oceanographic measurements from the Swedish icebreaker Oden in combination with the multibeam mapping, chirp sonar profiling and coring from the Healy will help to resolve the question of the deep-water exchange between Arctic basins. The acquired data shows that the deepest passage in the Lomonosov Ridge, located on the Makarov Basin side of an intra basin in the ridge, is shallower than shown on both the International Bathymetric Chart of the Arctic Ocean (IBCAO) and the Russian bathymetric map published in 1999 by the Head Department Navigation and Oceanography (HDNO). The new maximum depth for the passage near 88°25’N in the Lomonosov Ridge is near 1 900 m instead of deeper than 2 400 m. How important this passage is for the deep water exchange remains to be resolved through a post-cruise cross-disciplinary collaboration between the scientists that were onboard the two icebreakers.

We thank Captain Dan Oliver, Captain Tomas Årnell and the crews of the USCGC Healy and icebreaker Oden for making the science operations possible.