Radium in Arctic surface water

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8-1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

Physical oceanography, nutrients, and d18O measured on water bottle samples during POLARSTERN cruise ARK-XXII/2

Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and d18O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and d18O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO*) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO*-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin. Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30 to 50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift. The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32 to 34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf's bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30 to 32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.

Helicopter-borne sea ice thickness measurements during cruises ARK-XX/2 and ARK-XXII/2 (SPACE) in the Arctic Ocean

Helicopter-borne electromagnetic sea ice thickness measurements were performed over the Transpolar Drift in late summers of 2001, 2004, and 2007, continuing ground-based measurements since 1991. These show an ongoing reduction of modal and mean ice thicknesses in the region of the North Pole of up to 53 and 44%, respectively, since 2001. A buoy derived ice age model showed that the thinning was mainly due to a regime shift from predominantly multi- and second-year ice in earlier years to first-year ice in 2007, which had modal and mean summer thicknesses of 0.9 and 1.27 m. Measurements of second-year ice which still persisted at the North Pole in April 2007 indicate a reduction of late-summer second-year modal and mean ice thicknesses since 2001 of 20 and 25% to 1.65 and 1.81 m, respectively. The regime shift to younger and thinner ice could soon result in an ice free North Pole during summer.