OS44M-01
Trophic Structure on the Western Arctic Shelf: A New Paradigm for Benthic-Pelagic Coupling and Ultimate Carbon Sources?
We explored the trophic linkages and ultimate carbon sources for benthic and pelagic consumers across the western arctic shelf based on recent collections of POM, zooplankton, and benthic macrofauna. Evidence for the importance of a variety of allochthonous and autochthonous carbon sources on the arctic coast, shelf, and basin was provided by carbon and nitrogen stable isotopic signatures. Preliminary δ $^{13}$C measurements of POM reveal that values are 2-5 ppt more negative in late summer compared to spring, especially over the outer shelf and basin. Based on these results and the isotopic values of ice algae, $^{13}$C enriched ice algal carbon may contribute a significant source of POC during the spring bloom. In contrast, the unique isotopic signature of Bering Sea waters is clearly detected over the entire Chukchi Shelf and into the western Beaufort. The large depletion in $^{13}$C eastward on the Beaufort Shelf may reflect the increasing importance of terrestrially derived carbon that originates from the enormous amounts of freshwater inflow onto the Beaufort Sea coast. The depletion in $^{13}$C content of invertebrate and vertebrate consumers, which drops about 4 ppt eastward along the Alaskan Beaufort coast, may reflect the strong incorporation of terrestrial organic matter into a detrital based food web. In addition, we have documented a greater range in $^{13}$C values of consumers among several trophic levels, from 3 ppt in the northern Chukchi Sea to nearly 8 ppt in the eastern Beaufort Sea (in fauna of the same species). The increased separation in isotopic values in the Beaufort may reflect a decoupling between pelagic in situ POC production and the nearshore shelf benthos. In contrast, the relatively low range in $^{13}$C values of benthic and pelagic organisms in the western Beaufort and Chukchi are indicative of the efficient transfer of carbon between trophic levels and the tight trophic linkages characteristic of nutrient rich Anadyr waters in the Chukchi Sea.
OS44M-02
Recent Research on Arctic Benthos: Common Notions Need to be Revised
Benthic research in Arctic seas has been intensified during the past two decades, fostered by increased public awareness of the global climate significance of polar regions, technological progress and the political opening of the Russian Arctic. As the wealth of novel information gathered in these efforts has markedly enhanced our knowledge, some common scientific notions about the Arctic benthos need to be revised. In general, observational and modeling studies have clearly demonstrated that there are striking differences among the various Arctic seas in geographical and environmental setting, impact of fluvial run-off, pelagic production regime, strength of pelago-benthic coupling and, hence, food supply to the benthos. This heterogeneity impedes straight large-scale generalization of local and regional findings and strongly calls for a synoptic pan-Arctic perspective to soundly understand the general key processes regulating marine Arctic ecosystems and to accurately predict the ecological response to climate-forced changes. Moreover, field evidence has repeatedly emphasized the great significance of meso-scale features in hydrography and ice cover (marginal ice zones, polynyas, gyres) as 'hot spots' of tight pelago-benthic coupling and, hence, high benthic biomass. In contrast, the importance of terrigenic organic matter discharged through fluvial run-off as an additional benthic food source is still under debate. Much progress has been made in the scientific exploration of the central Arctic Ocean. There is now evidence that it is one order of magnitude more productive than previously thought. Therefore, the significance of shelf-basin interactions, i.e., the general importance of surplus organic carbon exported from productive shelves to the deep ocean, is being discussed yet and, hence, a major topic of ongoing and upcoming research. Another high-priority theme of current/future projects are the ecological effects of the rapid warming in the Arctic. Higher water temperatures, increased fluvial run-off and reduced ice cover will set off severe ecosystem changes propagating through all trophic levels. For instance, it is likely that there will be a shift in the relative importance of marine biota in the overall carbon and energy flux, ultimately resulting in a switch from a 'sea-ice algae-benthos' to a 'phytoplankton-zooplankton' dominance - a fundamental transformation that would surely have major repercussions for animals and people inhabiting the Arctic.
OS44M-03
Seafloor Sediment Dynamics Within the Barents Sea Marginal ice Zone
The Barents Sea marginal ice zone (MIZ) undergoes extreme intra- and inter-annual variability in connection with prevailing patterns of wind and water mass transport and is known as an area of relatively high biological productivity. Here we investigate the relationship between the MIZ and seasonal and inter-annual patterns of seafloor burial processes. Sediment profiles of radionuclide tracers (234Th, 210Pb, and 137Cs) together with sediment carbon, nitrogen and phosphorus levels are examined in three regions of different ice cover frequencies: predominantly open water (POW), ice edge zone (IEZ), and predominantly ice-covered (PIC). 210Pb-based sedimentation rates (0.16-0.31 cm/yr) and 234Th mixing rates (16.2 - 18.6 cm2/yr) were similar in both PIC and IEZ areas; sedimentation rates (0.11-0.17 cm/yr) and 234Th mixing rates (3.9 - 9.6 cm2/yr) were lowest in POW areas. Sedimentation rates also showed a clear longitudinal gradient, increasing from west-east. Organic and total carbon contents (1-3%) were highest in sediment cores collected from the medium frequency area, with sediment organic C/N ratios ranging from 7-12. Particulate organic phosphorous (max = 0.04%) and inorganic phosphorus (max = 0.2%) were lowest at POW stations. These data show a clear pattern of enhanced rates of sedimentation, mixing and nutrient preservation in sediments in the marginal ice zone. Extrapolating these results in space and time, we would expect that a northward retreating ice margin associated with longer term climate changes in the arctic will lead to a displacement in these patterns with associated consequences for communities of bottom-dwelling organisms.
OS44M-04
Sedimentation Indicators of Organic Carbon Processing in the Chukchi Sea
The Shelf-Basin Interactions (SBI) project has been a major new source of data and insights on the evolving response of Arctic ecosystems to environmental changes such as the increasing seasonal retreat of sea ice and warming ocean temperatures. However, the political division of the Chukchi Sea between Russia and the United States has limited SBI sampling to the U.S. sector, which limits direct observations of the transport of higher productivity waters in the Russian sector from Bering Strait north to the Chukchi shelf-break. The NOAA and Russian Academy of Sciences-sponsored Russian-American Long-term Census of the Arctic (RUSALCA) program has helped bridge this gap by providing opportunities for U.S. and Russian scientists to jointly sample in these higher productivity waters and sediments west of the International Dateline. We present new data on indicators of recent sedimentation that provide insights on processing of organic carbon in both the western (Russian) and eastern (U.S.) Chukchi Sea. Surface sediment activities of the particle-reactive, atmospherically-derived radioisotope $^{7}$Be (half-life 53d) indicate activities are highest in sediments immediately north of Bering Strait, and in down-slope portions of Herald and Barrow Canyons where Pacific-origin waters flow off the continental shelf. Chlorophyll concentrations in surface sediments show similar high concentrations on the shelf, with low concentrations in deep basin sediments. C/N ratios and δ$^{13}$C values of bulk organic carbon in sediments co-vary with lower C/N ratios and less depleted δ$^{13}$C values on the Russian shelf, consistent with less refractory and more readily usable, recently deposited organic materials. In down-slope and deep basin portions of the study area, these sediment indicators become more refractory, although relatively less depleted δ$^{13}$C values are observed over the entire outer continental shelf. Although this may be an indication of higher deposition of sea ice algae relative to water column production, we observed little difference in the carbon isotope composition of seston captured in floating sediment traps under ice-covered versus open-water deployments in 2004 in productive waters in Barrow Canyon. In general seston samples had significantly more depleted δ$^{13}$C values than underlying organic carbon in sediments indicating that in-sediment processing of organic materials has a significant influence on the carbon isotope ratios observed in organic matter in the sediments.
http://arctic.bio.utk.edu
OS44M-05
Spring and summer denitrification rates in the shelf and slope sediments of the western Arctic
This study examines denitrification rates in the shelf and slope sediments of the western Arctic. Recent constructions of the global nitrogen budget estimate that at least half of the world's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. Since the Arctic contains approximately 25% of the world's continental shelf, it is likely a substantial contributor to the magnitude of global sedimentary denitrification. In two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from profiles of nitrate concentrations in the shelf and slope sediments of the Chukchi and Beaufort Seas. Additionally, in the spring of 2004, denitrification rates were determined by whole core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along 4 transects going from shelf to off slope (50-3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1 mmol N/m$^{2}$/d for the shallow sediments to 0.1 mmol N/m$^{2}$/d in the deep. Rates showed little variation between the two seasons. This was surprising considering the stark dichotomy of the overlying water conditions in the Arctic from spring to summer which result in unequal amounts of organic carbon reaching the sediments. An extrapolation to the entire Chukchi and Beaufort shelves, which comprise approximately 10% of the total Arctic continental shelf, yielded a denitrification rate as high as approximately 10 Tg N/y.
OS44M-06
Experimental Degradation of Source Specific Organic Carbon and its Linkage to Sedimentary Carbon Preservation in the Western Arctic Ocean
Our recent analysis of Arctic sediments suggests that unlike other ocean basins a large fraction of organic carbon preserved in the western Arctic basin is terrestrial in origin. Sea-ice is thought to be an important vector for carbon transportation, as terrestrial organic matter trapped in land fast sea-ice from river transport and coastal erosion is redistributed to deeper waters via ice breakup and summer melting. Particulate organic matter (POM) in both surface and deep waters of the western Arctic Ocean contain very low concentrations of terrestrial organic biomarkers, in contrast to POM trapped in sea-ice and peat collected on the coast of Alaska that contains high concentrations of terrestrial sterols, long-chain saturated mono- and dicarboxylic acids, hydrocarbons, and alcohols. This dichotomy supports sea-ice as the major transport mechanism for land-derived organic carbon to the deep Arctic basins and highlights the importance of distinct recycling times for terrestrial and marine organic carbon as substrates for microbial respiration. Long-term (82-day) incubation experiments were performed where Alaskan peat, ice-rafted debris, snow debris, and ice algae were degraded in the presence of natural Arctic microbial assemblages. At in situ temperatures, ice algal lipids degraded rapidly, with rate constants equal to 20 yr$^{-1}$ for saturated short-chain fatty acids and 5.8 yr$^{-1}$ for phytol. In the peat and snow debris incubations, short- and long-chain fatty acids showed no degradation, while phytol had a rate constant less than half that for the same structures in ice algae (k=2.1 yr$^{-1}$). Despite little change in lipid concentration or distribution over the 82-day incubation, the average radiocarbon age of the organic carbon in the peat doubled. Carbohydrate carbon decreased over time and may account for the increase in average age of peat organic carbon. These experiments plus additional incubations at higher temperatures are being coupled with radiocarbon dating to evaluate the importance of remobilization and remineralization of older, typically recalcitrant carbon under climate warming scenarios.