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The Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state‐of‐the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system's role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures, and presence of sea ice year‐round hamper field and remotely sensed measurements. We highlight the importance of winter and under‐ice conditions in the southern WG, the role that new technology will play to overcome present‐day sampling limitations, the importance of the WG connectivity to the low‐latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East‐West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long‐term data to determine trends and will improve representation of processes for regional, Antarctic‐wide, and global modeling efforts—thereby enhancing predictions of the WG in global ocean circulation and climate.

Sea ice is an important transport vehicle for gaseous, dissolved and particulate matter in the Arctic Ocean. Due to the recently observed acceleration in sea ice drift, it has been assumed that more matter is advected by the Transpolar Drift from shallow shelf waters to the central Arctic Ocean and beyond. However, this study provides first evidence that intensified melt in the marginal zones of the Arctic Ocean interrupts the transarctic conveyor belt and has led to a reduction of the survival rates of sea ice exported from the shallow Siberian shelves (−15% per decade). As a consequence, less and less ice formed in shallow water areas (<30 m) has reached Fram Strait (−17% per decade), and more ice and ice-rafted material is released in the northern Laptev Sea and central Arctic Ocean. Decreasing survival rates of first-year ice are visible all along the Russian shelves, but significant only in the Kara Sea, East Siberian Sea and western Laptev Sea. Identified changes affect biogeochemical fluxes and ecological processes in the central Arctic: A reduced long-range transport of sea ice alters transport and redistribution of climate relevant gases, and increases accumulation of sediments and contaminates in the central Arctic Ocean, with consequences for primary production, and the biodiversity of the Arctic Ocean.

For the determination of the elemental composition of particulate organic material (POM) and its impact on the marine carbon cycle, we assembled C:N data for POM from many different sources into a single data collection for joint evaluation. The data set contains 10,200 C:N values, encompassing all major oceans and trophic levels, showing that C:N ratios are highly variable with values below the traditional Redfield ratio (C:N = 6.6) to values greatly exceeding it. On a global mean, C:N ratios of marine sinking particles from the surface water amount to 7.1 ± 0.1, and there is a systematic increase of C:N ratios with depth of 0.2 ± 0.1 units per 1000 m. The discrepancy with results from analyses of dissolved nutrient fields, yielding constant C:N ratios close to the Redfield value, can be explained by the implicit depth averaging caused by depth variations of the surfaces under consideration. Additionally, due to preferential remineralization of nitrogen compared to carbon, the C:N ratio of the dissolving component, which is seen on dissolved nutrient fields, is smaller than the C:N ratio of the remaining particles. For carbon flux estimations, elevated and depth dependent C:N ratios should be implemented in biogeochemical models to correctly represent relative strengths of downward carbon and nitrogen fluxes.

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[1] The loss of Arctic sea ice has accelerated in recent years. With the decline in sea ice cover, the Arctic Ocean biogeochemistry is undergoing unprecedented change. A key question about the changing Arctic Ocean biogeochemistry is concerning the impact of the shrinking sea ice cover on the particulate organic carbon (POC) export from the upper Arctic Ocean. Thus far, there are still very few direct measurements of POC export in the permanently ice-covered central Arctic Ocean. A further issue is that the magnitude of the POC export so far documented in this region remains controversial. During the ARK-XXII/2 expedition to the Arctic Ocean from 28 July to 7 October in 2007, we conducted a high-resolution study of POC export using 234 Th/ 238 U disequilibrium. Depth profiles of total 234 Th in the upper 200 m were collected at 36 stations in the central Arctic Ocean and its adjacent seas, i.e., the Barents Sea, the Kara Sea and the Laptev Sea. Samples were processed using a small-volume MnO 2 coprecipitation method with addition of a yield tracer, which resulted in one of the most precise 234 Th data sets ever collected. Thorium-234 deficit with respect to 238 U was found to be evident throughout the upper 100 m over the Arctic shelves. In comparison, 234 Th deficit was confined to the upper 25 m in the central Arctic Ocean. Below 25 m, secular equilibrium was approached between 234 Th and 238 U. The observed 234 Th deficit was generally associated with enhanced total chlorophyll concentrations, indicating that in situ production and export of biogenic particles are the main mechanism for 234 Th removal in the Arctic Ocean. Thorium-234-derived POC fluxes were determined with a steady state model and pump-normalized POC/ 234 Th ratios on total suspended particles collected at 100 m. Results showed enhanced POC export over the Arctic shelves. On average, POC export fluxes over the various Arctic shelves were 2.7 ± 1.7 mmol m −2 d −1 (the Barents Sea), 0.5 ± 0.8 mmol m −2 d −1 (the Kara Sea), and 2.9 ± 1.8 mmol m −2 d −1 (the Laptev Sea) respectively. In comparison, the central Arctic Ocean was characterized by the lowest POC export flux ever reported, 0.2 ± 1.0 mmol m −2 d −1 (1 standard deviation, n = 26). This value is very low compared to prior estimates and is also much lower than the POC export fluxes reported in other oligotrophic oceans. A ThE ratio ( 234 Th-derived POC export/primary production) of <6% in the central Arctic Ocean was estimated using the historical measurements of primary production. The low ThE ratio indicates that like other oligotrophic regimes, the central Arctic Ocean is characterized by low POC export relative to primary production, i.e., a tightly coupled food web. Our study strongly suggests that the current role of the central Arctic Ocean in C sequestration is still very limited. Meanwhile, this role might be altered because of global warming and future decline in sea ice cover.

Investigations of lithogenic and biogenic particle fluxes using long-term sediment traps are still very rare in the northern high latitudes and are restricted to the arctic marginal seas and sub-arctic regions. Here data on the variability of fluxes of lithogenic matter, CaCO 3 , opal, and organic carbon and biomarker composition from the central Arctic Ocean are presented for a 1-year period. The study was carried out on material obtained from a long-term mooring system equipped with two multi-sampling traps, at 150 and 1550 m depth, and deployed on the southern Lomonosov Ridge close to the Laptev Sea continental margin from September 1995 to August 1996. In addition, data from surface sediments were included in the study. Annual fluxes of lithogenic matter, CaCO 3 , opal, and particulate organic carbon were 3.9, 0.8, 2.6, and 1.5 g m À2 y À1 , respectively, in the shallow trap and 11.3, 0.5, 2.9, and 1.05 g m À2 y À1 , respectively, in the deep trap.Both the shallow and the deep trap showed significant variations in vertical flux over the year. Higher values were found from mid-July to the end of October (total mass flux of 75-130 mg m À2 d À1 in the shallow trap and 40-225 mg m À2 d À1 in the deep trap). During all other months, fluxes were fairly low in both traps (most total mass flux values o10 mg m À2 d À1 ). The interval of increased fluxes can be separated into (1) a mid-July/August maximum caused by increased primary production as documented in high abundances of marine biomarkers and diatoms and (2) a September/October maximum caused by increased influence of Lena River discharge indicated by maximum lithogenic flux and large amounts of terrigenous/fluvial biomarkers in both traps. During September/October, total mass fluxes in the deep trap were significantly higher than in the shallow trap, suggesting a lateral sediment flux at greater depth. The lithogenic flux data also support the importance of sediment input from the Laptev Sea for the sediment accumulation on the Lomonosov Ridge on geological time scales, as indicated in sedimentary records from this region. r

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