The arctic ocean is one of the Earth’s environments that will be most affected by climate change. As the Earth continues to warm, the Arctic Ocean will evolve into an environment that is much different from what is recognized today. Most of the animals currently living there will not be able to survive if this region warms too much. In addition, warming Arctic waters could affect ocean circulation elsewhere in the world.
The Arctic Ocean is unique in its physical properties. It has both the narrowest and the widest shelves on Earth due to glacial erosion, marine abrasion, and progradational clastics. It is a tectonically active region in which the closed-off basin of today developed in the Cretaceous (90 million years ago). The seafloor forming at the mid-ocean ridge in the Arctic is the slowest ridge system of any on Earth today.
The Lomonosov Ridge divides the ocean in half, creating the younger and deeper Eurasian (Eastern) basin and the older and larger Amerasian (Western) basin. The Amerasian Basin is divided by the Alpha Ridge into the Makarov Basin to the north, and the Canadian Basin to the south. The Alpha Ridge connects to the Mendeleev Ridge to the north. The Arctic Ocean is approximately 50 percent continental shelf, resulting in much of the sea floor being only 1,640-6,562 ft. (500-2,000 m.) below sea level.
The Arctic Ocean is the most extreme region on Earth in terms of climate and seasonality because of light and ice cover. The ocean covers an area of approximately 14,056,000 km2 and extends below the Arctic Circle in some locations. The average depth is just over 3,280 ft. (1,000 m.), with the deepest location in the Eurasian Basin at 17,881 ft. (5,450 m.) The Arctic Ocean is surrounded on all sides by land except for narrow connections to the Atlantic and Pacific Oceans. The Atlantic water flows through the Fram Strait (8,530 ft. [2,600 m.] deep), where the largest amount of water flows into the Arctic. The other is from the Pacific Ocean through the Bering Strait, a shallow connection (164 ft. [50 m.]) that has intermittently closed during glacial periods over the past few million years.
The Arctic water column is strongly stratified into three layers: the shallow, relatively fresh surface layer; the intermediate layer; and the deep salty layer. The shallow layer obtains its water predominantly from melting ice sheets, icebergs, and river runoff. Because of its low density, this layer, approximately 9.8 ft. (3 m.) deep, creates a relatively fresh water film that easily freezes. It is because of this film and the freezing temperatures that the center of the Arctic Ocean is frozen year-round. The intermediate layer receives its water from the salty waters of the North Atlantic. The deep water forms through convection and has a very slow exchange with a residence time of 450-500 years (about half that of the world ocean residence time of 1,000 years).
Large amounts of freshwater come from rivers, specifically from the Russian rivers Yenisei, Ob, and Lena, and the Canadian MacKenzie River. Together, these rivers contribute 2,000 km.3 a year of water. This influx of freshwater into an environment that is so cold creates a film of water on the surface because of the density difference and hence freezes solid. During the winter, there is 14×106 km.2 of sea ice, and about half of that in the summer. This number has decreased over the past several years, causing detrimental effects on the climate and resident species. The ice is critical for the heat exchange budget in the Arctic, as well as for sustaining life, from viruses, to polar bears, and subsistence hunters.
Because ice forms and leaves behind salt, during glacial periods the water is more saline due to an increased volume of sea ice. As the ice melts during interglacial periods, the surface layer becomes increasingly fresh, creating a deeper film of fresh water that does not mix with the rest of the water column. During this warmer period, there is more productivity in the surface layer, which allows more food and nutrients to sink to the bottom. It is this relationship between the surface layers and the deep sea that is recorded in the fossil record. The dominant surface productivity is from the phytoplankton that thrive in the summers due to 24 hours of light, and struggle to survive during the winter when there is 24-hour darkness.
MARINE LIFE
The structure of the water column influences biological productivity in the Arctic, making it unique in its biological properties. It hosts animals such as walruses, whales (belugas, narwhals, and bowheads), and seals. Other marine animals include squid, flatfish, Greenland halibut, worms, snails, crabs, shellfish, and krill. Polychaetes, crustaceans, and bivalve mollusks dominate the benthic macrofauna.
Many microscopic animals such as zooplankton, diatoms, copepods, and foraminifera also live in the freezing waters of the Arctic water column (plank-tonic) and on the sea floor (benthic). The most important primary producers are phytoplankton. However, the growing season is restricted because of the short summer season, low light angles, and the snow and ice cover. The season is between April and September with a single peak, June to July. With the retreating ice on the shelves during these summer months, an algal bloom is possible near shore.
The Arctic deep sea has been widely ignored because of the thick sea ice. Initial studies show little diversity in this area, with a dominance of deposit feeders. Dominant animals include poly-chaetes, crustaceans, and bivalve mollusks. Occasional tunicates, sponges, cnidarians, ophiuroids, and several species of worms have also been found. Approximately 350-400 species have been found so far in the deep central Arctic Ocean. The long-believed hypothesis that species diversity decreases with higher latitudes is being reconsidered as more species are discovered in the central Arctic. As more research is conducted in this region, the known species diversity is increasing, discounting this outdated theory.
In the Arctic, as in all high latitudes, the food cascading to the bottom of the is the limiting factor for the benthos, not temperature, as the benthos organisms in the Arctic are adapted to the freezing temperatures. However, they need food for survival and can only obtain it from organic matter produced in the surface waters. In the shallow shelf areas during the ice-free periods, particle transport is abundant. The benthos, therefore, play an important role in the system and production of the Arctic waters.
Many small but important forms of life underneath the ice of the Arctic Ocean still have not been adequately studied.
Although collection of the pelagic organisms has gone on for over a century, many of the taxa are understudied. Only the larger organisms, mostly near shore, are well understood because of their ease of collection. Smaller taxa, deep-water organisms, and the gelatinous forms have been missed by current sampling techniques, therefore, scientists know little about these organisms.
Macro- and megafauna have received the most attention in the Arctic waters, while the meiofauna and microbial communities have been persistently ignored. This is, more often than not, because of sampling techniques and the ease of capture in the ice-filled environment. However, in terms of the quantity of organisms, there are more microbial animals per square meter than there are megafauna. There are still many species of microscopic organisms that have not been identified and further study of the microbial world will enhance knowledge of the Arctic system.
Polar bears also venture into the frozen waters, using floating icebergs as islands on which to rest before venturing onward. With the warming of the Earth and Arctic climate change, these icebergs are not as prevalent as they once were, and the polar bears are having difficulty without these resting places. In addition to the deteriorating climate, there are still many mysteries about marine life, both macro- and microscopic, and how the interaction of these many species sustains an environment that allows life to flourish despite the harsh conditions.
Sea ice
The Arctic sea ice contains its own unique biodiversity with many endemic species. Specialized sympagic (ice-associated) communities live in brine pockets on top of the ice and in the ice-water interface. Flagellated protists, diatoms, and ice algae account for most of the primary productivity in the ice community. Protozoan and metazoan organisms (in particular turbellarians, nematodes, crustaceans, and rotifers), make up a portion of the ice community. Larvae and juveniles of some benthic animals migrate seasonally into the ice to feed on the algae in shallow waters. All of the organisms that live on and around the ice play a vital role in feeding the benthos. The climate cycle of melting and freezing causes these organisms to fall to the sea floor, creating food for the organisms living on the bottom.
The sediment record left by the sea ice can be used to reconstruct glacial/interglacial cycles. When sea ice and icebergs melt, they drop the ice rafted debris (IRD) that was contained within the ice as it scraped across the land and into the ocean. Sediment with abundant foraminifera and large sediment clasts is deposited during interglacial (interstadial) periods. Sediment that is mostly fine-grained, with few to no foraminifera, is deposited during glacial (sta-dial) periods. However, the lack of foraminifera in a sediment sample can also be a result of low surface salinity, a decrease in nutrients, dissolution of tests, a high sedimentation rate, or a thick layer of sea ice, causing difficulty in reconstructing glacial cycles.
The perennial sea ice is increasingly thinning, and there is a seasonal hole in the ozone layer over the North Pole. The thinning sea ice is having an effect on global albedo (amount of sunlight absorbed versus reflected back out of the Earth’s atmosphere). Ice reflects more sunlight than it absorbs, so it has a cooling effect on the Earth. Without this ice to reflect the sun’s rays, more heat will be absorbed, causing the Earth to warm. This causes a positive feedback that will continue to warm the earth, melting more sea ice. This is a concern because the added fresh water to the global marine system could cause a shutdown of the thermohaline circulation through the connection between the Arctic and Atlantic Oceans.
PROBLEMS WITH THE ARCTIC RECORD
The Arctic contains many records of past environments that scientists have used to recreate past climates. Bottom water temperatures can be inferred from calcite pseudomorphs such as ikaite (CaCO36H2O), an organic-rich mineral that accumulates in the sediment. Biomarkers such as algae, dino-flagellates, diatoms, and foraminifera are used for their stable isotopes to infer temperature and salinity over the past several million years. The 18O/16O values in these inorganic carbonates reflect sea ice and ice sheet variations through time. These can, in turn, be used to model glacial/interglacial periods. 13C/12C values in planktonic organisms reflect variation in productivity or sediment carbon flux from the surrounding continents.
The unique isolation of the Arctic Ocean may be a factor in maximizing the amplitude of some environmental signals in the sediment record. The water column is more strongly stratified than in most other parts of the world. However, throughout history, as the passageways between the Arctic Ocean and the Pacific and Atlantic Oceans opened and closed, this record was modified. Deep ocean waves may have also modified the sedimentary record. As water currents flow around the ridges on the ocean floor they move the sediment and replace it with sediment from other parts of the sea floor. Bioturbation also plays a role in the modification of the sediment record.
Modification of the sediment in the Arctic Ocean creates even more difficulty in reconstructing the history of this basin. The Arctic has extremely low sedimentation rates, so any disruption in the record could potentially create huge problems with dating and isotope analysis. The sedimentation rate is low in the Arctic for several reasons: the major reason is due to the presence of sea ice. It prevents most, if not all, sediment from settling onto the sea floor anywhere that is covered in ice. This, however, is one way that scientists can reconstruct glacial/interglacial cycles. At the beginning of every interglacial cycle, more sediment accumulates because it is dropped from icebergs and sea ice through melting. This can also be seen in the additional sediment discharge from the Arctic rivers.
Collecting enough material for stable isotope and trace element analysis can also be difficult, especially because during glacial periods when sedimentation and productivity are extremely low, there may not be any biological material to collect. During interglacial periods, there is typically enough material; however, in the event of a melt-water pulse or extreme melting, the amount of sediment is much higher than the number of organisms. This makes picking out acceptable specimens difficult. Dissolution is another problem in the Arctic Ocean. In many locations, any cal-cite will have dissolved before reaching the sea floor without leaving a record to what was present at one time. This also makes comparison across the ocean difficult because dissolution takes places at different times in different areas, creating hiatuses that only appear in certain sediment cores. This is usually caught when material is dated, but the dating of some cores is impossible.
Yet another problem with the oceanic record in the Arctic is the difficulty in getting to it. The field season is short, typically from late July to early October, because of sea ice. Even during this time, an icebreaker is needed to clear a path for ships to pass through, making it expensive and dangerous. Even with an icebreaker, the ice in the central Arctic is too thick to break through, and, therefore, almost impossible to study. Political boundaries also play a role in the scientific study of the Arctic. Seafloor sampling and imaging, especially on the Russian shelves, may be unwanted by foreign governments. This is especially true in areas with natural resources and suspected or known illegal dumping.
As knowledge of this region expands, scientists will gain insight into the dynamics of the past and present world climates. This information can then be used to model future climate changes that will aid in the prediction of how life will need to respond to the changing global climate. However, despite the importance of this region, the Arctic Ocean is one of the least studied places on Earth. Due to the harsh environment and perennial sea ice, this location is difficult to access. The knowledge obtained from sampling is also biased toward effort spent on certain taxa, specific regions, and techniques used for the acquisition of this data. The shallow Arctic shelves are the most studied, especially during the summer months when the sea ice melts back. The Canadian Basin is the least studied because it is covered in ice year round. There is much more that needs to be studied in this complex system of ice, water, land, and life.
