Seafloor Sediment

Dr. Leonid Polyak is a research scientist at BPRC who was one of the principal investigators in the first complete trans-Arctic expedition that gathered samples from the sea floor. The Healy-Oden Trans-Arctic Expedition (HOTRAX) took place in 2005, and involved international collaboration, relying on the U.S. Coast Guard Cutter Healy, and the Swedish icebreaker Oden. Scientists and technicians from over 10 universities/ colleges and research facilities participated in HOTRAX.

Dr. Polyak and his research team at BPRC study geological evidence obtained from the seafloor of the Arctic Ocean, reaching back hundreds of thousands of years into the past. For paleobiological data they analyze the outer skeletons (“tests”) of microscopic floating or crawling organisms such as protists called foraminifera (“forams” for short). They also look at the chemical and physical (sedimentological) make-up of the sediment, giving them a perspective on ocean environments of the past. Also they examine the geophysical soundings that provide a view of the topography and sediment structure of the ocean floor.

In 2007, Dr. Polyak collaborated with the SwedArctic 2007 Research Programme, supported by the Swedish Polar Research Secretariat, as part of the Lomonosov Ridge off Greenland (LOMROG) expedition. This expedition continued the exploration of the Arctic seafloor

Upper left:  U.S. Coast Guard Ice-Breaker "Healy"
Upper right: Swedish ice breaker, Oden
Lower picture: Tracks of the two ships during HOTRAX '05

Paleobiological Data: Forams

These organisms fossilize well because of their shell (or “test”) that is made of calcium carbonate (CaCO3). In addition to having evidence of the species that existed, the chemistry of their shells also indicates water conditions at the time they were alive. For example, some of the forams survive better in warmer water, while others survive better in colder waters. By studying the ratio of “warm-loving” to “cold-loving” forams that died and became part of the sediments, the researchers can determine whether the water was warmer or colder during a particular time period.

(L) Planktonic foraminifer (left-coiling N. pachyderma) indicative of cold water with high ice coverage (square width approximately 300 microns)

(R) Planktonic foraminifer (right-coiling N. pachyderma) indicative of warmer water and/or less ice (square width approximately 200 microns).

Sedimentological Data: muds & dropstones

Fine sediments that were originally dumped into the ocean from rivers may travel for miles with the currents before they settle on the ocean floor. As a result, the content of the sediments gives a clue to the ocean circulation at the time the sediments were being deposited. Another type of geological evidence in polar seas is in the composition of “dropstones”. They are sediment particles, including large pieces of rocks, that were frozen into floating ice (from sea ice or as part of icebergs from a glacier--on land) . As ice travels with the currents, the surface gradually melts and rocks that were frozen in the ice drop to the ocean floor. Analyzing the amount and types of rocks that are found as dropstones helps scientists to reconstruct the currents and the ice distribution at the time the sediments were deposited.

Dr. Polyak, holding a dropstone

Geophysical Data: sonar images of the seafloor

Remote sensing of the shape and vertical structure of the seafloor can be done by various geophysical methods, notably using sound waves (sonar). HOTRAX used swath and penetrating sonar soundings to assess the bottom features. They detected areas of glacial erosion, where the seafloor was gouged by glaciers at depths up to 1,000 m. They also identified areas where grounded or calving glaciers deposited layers of poorly sorted sediment made up of mixed sizes (“diamicton”), which contrast with the normally well-sorted marine sediments.

Swath sonar records from the Northwind Ridge, ~500 miles north of Alaska, showing seafloor features formed by grounded glacier ice at water depths to ~1,000 m