Western Arctic Shelf-Basin Interactions (SBI) Project

THE WESTERN ARCTIC SHELF-BASIN INTERACTIONS (SBI) PROGRAM


The fundamental goal of the Shelf-Basin Interactions (SBI) program is to understand the physical and biogeochemical processes that link the arctic shelves, slopes, and deep basins within the context of global change. These processes strongly influence the biology, chemistry, and physics of the Arctic Ocean and its associated ecosystems. The Arctic marine system is linked to the global ocean and atmosphere by both physical and biogeochemical mechanisms, such as the influence that freshwater outflows from the Arctic have on North Atlantic convection and thermohaline circulation of the world ocean. Arctic Ocean waters are strongly influenced by biogeochemical processes occurring over the arctic shelves and a synoptic understanding of these processes is essential for predicting, anticipating and ameliorating the impacts of climate change. For example, carbon dioxide fluxes from sources or sinks on Arctic shelves may have direct impacts on air temperatures and sea ice coverage, with negative ramifications for local marine resources and human populations that are dependent upon them for subsistence. The SBI program will specifically focus on identifying processes that are sentinel indicators of global change, including alteration of current biogeochemical cycles.

The initial focus of the SBI program is the Western (North American) Arctic, particularly where North Pacific waters are modified by shelf processes that exert significant influences on the biology and physics of the Arctic Ocean. Prior carbon cycling work indicates that the southern Chukchi Shelf is the most biologically productive region in the marine Arctic, and that biogeochemical processes in both the water column and sediments control the use and recycling of organic carbon prior to advection into the deeper polar basins. Carbon cycling influences Arctic marine ecosystems, depending upon whether carbon enters: 1) the mixed layer (where it is available for further production), 2) the halocline (where it may be transported along lateral distances), or 3) the abyssal ocean (where it is sequestered for very long times). Since precipitation, temperature, runoff, ocean circulation, ice cover, and other physical factors directly impact the cycling of nutrients and carbon on Arctic shelves, expected global change will likely alter the biogeochemical pathways that determine the fate of shelf production. Evaluating such developments requires a significantly improved understanding of the transformations of carbon over the shelf and slope, particularly as they influence either carbon throughflow to the deep basins or deposition on the continental slopes.

From a physical oceanographic perspective, northward flowing Pacific waters are substantially modified over the shelves before entering the deeper Arctic Ocean. This has consequences for the stability of Arctic sea ice cover, vertical and horizontal mixing, internal wave dynamics, and the accessibility of inorganic nutrients to the euphotic zone. Changes in shelf-basin exchange will influence Arctic Ocean climate through feedback to the surface heat budget and freshwater cycling. The injection of shelf-modified waters into the Arctic Ocean appears to be controlled by a number of physical processes occurring over the outer shelf and the continental slope, most of which are poorly understood and some of which may have been neglected altogether by previous work. Changes in either the balance of shelf processes that modify Pacific waters, or in the slope processes that control exchange with the deep basin would likely result in major alterations of the Arctic marine environment. As examples, biological production and physical transformations on the Chukchi Sea shelf and slope likely depend in part on the quantity of freshwater transported through Bering Strait. This freshwater flux is in turn directly related to the North Pacific climatology and hydrology, and probably inversely related to nutrient concentrations. Atmospheric conditions over the Chukchi shelf have further effects upon ice and dense water production, which in turn provide a control on marine organism distributions and abundance, on materials exported from the shelf, and on the evolution of the thermohaline structure of the Arctic Ocean. Alterations in these patterns would have significant consequences upon U.S. marine transportation and resource exploitation, as well as more locally critical impacts upon Arctic residents who rely on the marine ecosystem for subsistence; other impacts may be possible on regional or even global scales.

Sensitivity of the Western Arctic to climate change has direct consequences for human society, perhaps most obviously shown in such physical conditions as the extent of the ice cover. Higher trophic-level animals, particularly marine mammals and seabirds, migrate between the Pacific and the Arctic, and Native Iņupiat communities have traditionally depended on harvests from these migrations. As another example, prior work indicates a large fraction of the carbon, nutrients, and lower trophic-level organisms on the Western Arctic shelves is exported offshore to the central Arctic Ocean, while another fraction is recycled on the shelves. The export of carbon and nutrients is important to the Arctic Ocean ecosystem, where heterotrophic production (including marine mammals) may exceed autotrophic production. Dissolved organic carbon added by runoff, as well as inorganic carbon, are also involved in the shelf-basin exchange. Hence the present Arctic Ocean ecosystem and its cycling of organic carbon depends upon export fluxes from the surrounding shelves. The Chukchi shelf is among the most important in this regard, both because the biological production there ranks as one of the highest in the world, and also because the Pacific throughflow is substantial. Indeed, the latter is a pivotal component of the global hydrologic cycle.

Recent paleoceanographic studies under SBI Phase I infer that the Chukchi Sea has been subjected to large temperature variations during the last 6 yrs., which were not recorded in the GISP/Vostok ice cores. Contemporary studies are demonstrating declining faunal biomass and changing community structure on the Bering and Chukchi shelves. This variability suggests that the Western Arctic is a region with considerable sensitivity to large-scale biological and climatic change. We note that high-resolution paleo-records can provide an important understanding of this interactive system and its variability by indicating which important variables change during dramatic past climatic events and how they are related to global climate recorded elsewhere. In addition, better understanding of the modern environment will aid the interpretation of the paleo-record, thereby adding to our understanding of the past. An additional benefit will be to provide a paradigm for a changing world by linking the paleoclimate proxy-record with contemporary knowledge of seasonal and interannual process variability in the Arctic.

The SBI Program will focus on:

1. Physical modifications of North Pacific and other waters on the Chukchi shelf and slope, since these modifications subsequently affect both exchange across the continental slope and the thermohaline structure and circulation of the Arctic Ocean.

2. Biogeochemical modifications of North Pacific and other waters over the Chukchi and Beaufort shelf and slope areas, with an emphasis on carbon, nutrients, and key organisms that represent a suite of trophic levels.

3. Comparative studies over the wide Chukchi and narrow Beaufort shelves and adjacent slopes to facilitate extrapolation of the Western Arctic work to a Pan-Arctic perspective. Modeling of shelf-basin exchange processes and their sensitivity to global change will be an important methodology in this extrapolation.

Because of the markedly different summer and winter regimes in the Arctic, as well as the remarkable year-to-year variability, seasonal and interannual time series are required to identify the processes that control the functioning of the Arctic ecosystem. Therefore SBI will undertake time series studies to:

4. Quantify the major physical processes over the shelf and slope and their variability, including resolution of smaller scale (< 30 km) physical exchange processes along the shelf break and slope, since these appear critical to the disposition within the Arctic Ocean of mass, heat, salt, carbon and nutrients, from the shelves.

5. Quantify the major biological and chemical processes over the shelf and slope and their variability, since the timing of biological rate processes relative to physical transport processes is critical to understanding ecosystem dynamics.

6. Quantify the export of mass, heat, salt, carbon and nutrients into the Arctic Ocean and their variability.

In summary, the goal of the SBI program is to provide a clear understanding of the physical and biogeochemical connections between the Arctic shelves, slopes, and deep basins that could be influenced by global change. The physical and chemical nature of seawater transiting the Western Arctic and entering the world ocean is defined by the physical and biogeochemical processes occurring over the large Western Arctic shelves. The current Arctic Ocean system and the cycling of carbon depends upon carbon export fluxes from the surrounding shelves. Changes in either the balance of shelf processes that modify the Pacific waters, or in the slope processes that control exchange with the deep basin, will likely result in major alterations of the Arctic marine environment. These alterations would also have significant societal consequences on local, regional, and national levels in areas as diverse as marine transporation, resource use, and climatic change. An understanding of these processes is therefore essential. This new understanding will in turn allow realistic assessment both of the potential responses of the Arctic system to global change, and of the role of these interactive processes on the global climate system.