Sediments in arctic sea ice - entrainment, characterization and quantification

Sediments in Arctic sea ice are important for erosion and redistribution and consequently a factor for the sediment budget of the Arctic Ocean. The processes leading to the incorporation of sediments into the ice are not understood in detail yet. In the present study, experiments on the incorporation of sediments were therefore conducted in ice tanks of The Hamburg Ship Model Basin (HSVA) in winter 1996/1997, These experiments showed that on average 75 % of the artificial sea-ice sediments were located in the brine-channel system. The sediments were scavenged from the water column by frazil ice. Sediments functioning as a nucleus for the formation of frazil ice were less important for the incorporation. Filtration in grease ice during relatively calm hydrodynamic conditions was probably an effective process to enrich sediments in the ice. Wave fields did not play an important role for the incorporation of sediments into the artificial sea ice. During the expedition TRANSDRIFT III (TDIII, October 1995), different types of natural, newly-formed sea ice (grease ice, nilas and young ice) were sampled in the inner Laptev Sea at the time of freeze-up. The incorporation of sediments took place during calm meteorological conditions then. The characteristics of the clay mineral assemblages of these sedirnents served as references for sea-ice sediments which were sampled from first-year drift ice in the outer Laptev Sea and the adjacent Arctic Ocean during the POLARSTERN expedition ARK-XI/1 (July-September 1995). Based on the clay mineral assemblages, probable incorporation areas for the sedirnents in first-year drift ice could be statistically reconstructed in the inner Laptev Sea (eastern, central, and Western Laptev Sea) as well as in adjacent regions. Comparing the amounts of particulate organic carbon (POC) in sea-ice sediments and in surface sediments from the shelves of potential incorporation areas often reveals higher values in sea-ice sediments (TDIII: 3.6 %DM; ARK-XI/1: 2.3 %DM). This enrichment of POC is probably due to the incorporation process into the sea ice, as could be deducted from maceral analysis and Rock-Eval pyrolysis. Both methods were applied in the present study to particulate organic material (POM) from sea-ice sediments for the first time. It was shown that the POM of the sea-ice sediments from the Laptev Sea and the adjacent Arctic Ocean was dominated by reworked, strongly fragmented, allochthonous (terrigenous) material. This terrigenous component accounted for more than 75 % of all counted macerals. The autochthonous (marine) component was also strongly fragmented, and higher in the sediments from newly-formed sea ice (24 % of all counted macerals) as compared to first-year drift ice (17 % of all counted macerals). Average hydroge indices confirmed this pattern and were in the transition zone between kerogen types II and III (TDIII: 275 mg KW/g POC; ARK-XI/1: 200 mg KW/g POC). The sediment loads quantified in natural sea ice (TDIII: 33.6 mg/l, ARK-XI/1: 49.0 mg/l) indicated that sea-ice sediments are an important factor for the sediment budget in the Laptev Sea. In particular during the incorporation phase in autumn and early winter, about 12 % of the sediment load imported annually by rivers into the Laptev Sea can be incorporated into sea ice and redistributed during calm meteorological conditions. Single entrainment events can incorporate about 35 % of the river input into the sea ice (ca. 9 x 10**6 t) and export it via the Transpolar Drift from the Eurasian shelf to the Fram Strait.

Biogeochemical characterization of pennate diatom and Melosira arctica ice algal aggregates in the Central Arctic Ocean collected in summer 2011 and 2012

Sea-ice diatoms are known to accumulate in large aggregates in and under the sea ice including melt ponds. In the Arctic, they can contribute substantially to particle export when sinking from the ice. The role and regulation of microbial aggregation in the highly seasonal, nutrient- and light-limited Arctic sea-ice ecosystem is not yet well understood, and may vary in relation to the fate of the Arctic sea-ice cover. To elucidate the mechanism controlling the formation and export of algal aggregates from sea ice, we investigated samples taken in late summer 2011 and 2012, during two cruises to the Eurasian Basin of the Central Arctic Ocean. Dense, spherical aggregates composed mainly of pennate diatoms, and filamentous aggregates formed by Melosira arctica were found in different degradation stages, with carbon to Chlorophyll a ratios ranging from 110 to 66700, and carbon to nitrogen molar ratios of 8-35 and 9-40, respectively. Fresh sub-ice algal aggregate densities ranged between 1 and 17 aggregates/m**2, corresponding to a net primary production of 0.4-40 mg C/m**2/d, contributing 3-80% of total biomass and up to 94% of total production at a local scale. A key factor controlling buoyancy of the aggregates was light intensity, regulating photosynthetic oxygen production and flotation by gas bubbles trapped within the mucous matrix, even at low ambient nutrient concentrations. Our data was used to evaluate the factors regulating the distribution and importance of the Arctic algal aggregates as carbon source for pelagic and benthic communities.

(Appendix A) Dissolved manganese concentrations during POLARSTERN Cruise ARK-XXII/2

A total of 773 samples were analysed for dissolved manganese (Mn) in the Arctic Ocean aboard R.V. Polarstern during expedition ARK XXII/2 from 28 July until 07 October 2007 from Tromsø (Norway) to Bremerhaven. Concentrations of Mn were elevated in the surface layer with concentrations of up to 6 nM over the deep Basins and over 20 nM in the Laptev Sea. The general distribution of Mn through the water column is consistent with previous studies, but there are differences in the absolute concentrations that are most likely related to differences in sample area, sampling and filtration. The elevated concentrations of Mn in the surface layer are related to fresh water input. This was visible in the strong negative correlations observed between dissolved Mn and salinity. The correlation between Mn and salinity and the correlation between Mn and the quasi conservative trace water mass tracer PO4*, showed fluvial and melt water input and the Pacific and Atlantic origin of the surface waters. A large portion of the Mn delivered by the Arctic rivers is removed in the shelf seas and does not pass into the central basins. Most likely a benthic flux is at the origin of the elevated concentrations of Mn near the sediments in the Barents and Kara Seas. These elevated concentrations of Mn apparently affected the deep basins as well, as maxima in the concentrations of Mn were observed that corresponded with lowered transmission over the continental slope. A maximum in the concentration of Mn in the deep basin corresponded with anomalies in light transmission, potential temperature and dissolved iron, confirming the hydrothermal origin. The hydrothermal plume was observed throughout the Nansen Basin and over the deep Gakkel Ridge around 2500 m depth and a smaller plume was observed around 3200 m. The concentration of Mn at the Mn maximum around 2500 m depth decreased exponentially, consistent with a first order scavenging model. The concentrations of Mn were extremely low in the deep Makarov Basin (~0.05 nM) and slightly higher in the Eurasian Basin (~0.1 nM) outside the influence of the hydrothermal activity.

Dissolved aluminium measured on water bottle samples during POLARSTERN cruise ARK-XXII/2 (SPACE)

Concentrations of dissolved (0.2 µm filtered) aluminium (Al) have been determined for the first time in the Eurasian part of the Arctic Ocean over the entire water column during expedition ARK XXII/2 aboard R.V. Polarstern (2007). An unprecedented number of 666 samples was analysed for 44 stations along 5 ocean transects. Dissolved Al in surface layer water (SLW) was very low, close to 1 nM, with lowest SLW concentrations towards the Canadian part of the Arctic Ocean and higher values adjacent to and in the shelf seas. The low SLW concentrations indicate no or little influence from aeolian dust input. Dissolved Al showed a nutrient-type increase with depth up to 28 nM, but large differences existed between the different deep Arctic basins. The differences in concentrations of Al between water masses and basins could largely be related to the different origins of the water masses. In the SLW and intermediate water layers, Atlantic and Pacific inflows were of importance. Deep shelf convection appeared to influence the Al distribution in the deep Eurasian Basin. The Al distribution of the deep Makarov Basin provides evidence for Eurasian Basin water inflow into the deep Makarov Basin. A strong correlation between Al and Silicon (Si) was observed in all basins. This correlation and the nutrient-like profile indicate a strong biological influence on the cycling and distribution of Al. The biological influence can be direct by the incorporation of Al in biogenic silica, indirect by preferential scavenging of Al onto biogenic siliceous particles, or by a combination of both processes. From the slope of the overall Al-Si relationship in the intermediate water layer (AIDW; ~ 200-2000 m depth), an Al/Si ratio of 2.2 atoms Al per 1000 atoms Si was derived. This ratio is consistent with the range of previously reported Al/Si uptake ratio in biogenic opal frustules of diatoms. In the deepest waters (>2000 m depth) a steeper slope of the Al-Si relationship of 7.4 to 13 atoms Al per 1000 atoms Si likely results from entrainment of cold shelf water into the deep basins, carrying the signal of dissolution of terrigenous particles with a much higher Al:Si ratio of crustal abundance. Only a small enrichment with such crustal Al and Si component may readily account for the higher Al:Si slope in the deepest waters.

Barium measured on water bottle samples during POLARSTERN cruise ARK-XXII/2 (SPACE)

Dissolved barium has been shown to have the potential to distinguish Eurasian from North American (NA) river runoff. As part of the ARK-XXII/2 Polarstern expedition in summer 2007, Ba was analyzed in the Barents, Kara, Laptev seas, and the Eurasian Basins as well as the Makarov Basin up to the Alpha and Mendeleyev Ridges. By combining salinity, d18O and initial phosphate corrected for mineralization with oxygen (PO4*) or N/P ratios we identified the water mass fractions of meteoric water, sea ice meltwater, and marine waters of Atlantic as well as Pacific origin in the upper water column. In all basins inside the lower halocline layer and the Arctic intermediate waters we find Ba concentrations close to those of the Fram Strait branch of the lower halocline (41-45 nM), reflecting the composition of the incoming Atlantic water. A layer of upper halocline water (UHW) with higher Ba concentrations (45-55 nM) is identified in the Makarov Basin. Atop of the UHW, the Surface Mixed Layer (SML), including the summer and winter mixed layers, has high concentrations of Ba (58-67 nM). In the SML of the investigated area of the central Arctic the meteoric fraction can be identified by assuming a conservative behavior of Ba to be primarily of Eurasian river origin. However, in productive coastal regions biological removal compromises the use of Ba to distinguish between Eurasian and NA rivers. As a consequence, the NA river water fraction is underestimated in productive surface waters or waters that have passed a productive region, whereas this fraction is overestimated in subsurface waters containing remineralised Ba, particularly when these waters have passed productive shelf regions. Especially in the Laptev Sea and small regions in the Barents Sea, Ba concentrations are low in surface waters. In the Laptev Sea exceptionally high Ba concentrations in shelf bottom waters indicate that Ba is removed from surface waters to deep waters by biological activity enhanced by increasing ice-free conditions as well as by scavenging by organic matter of terrestrial origin. We interpret high Ba concentrations in the UHW of the Makarov Basin to result from enrichment by remineralisation in bottom waters on the shelf of the Chukchi Sea and therefore the calculated NA runoff is an artefact. We conclude that no NA runoff can be demonstrated unequivocally anywhere during our expedition with the set of tracers considered here. Small contributions of NA runoff may have been masked by Ba depletion and could only be resolved by supportive tracers on the uptake history. We thus suggest that Ba has to be used with care as it can put limits but not yield quantitative water mass distributions. Only if the extra Ba inputs exceed the cumulative biological uptake the signal can be unequivocally attributed to NA runoff.

Dissolved iron measurements from 32 stations in the Central Arctic Ocean and Arctic Shelf Sea during POLARSTERN expedition ARK-XXII/2

Concentrations of dissolved (<0.2 µm) Fe (DFe) in the Arctic shelf seas and in the surface waters of the central Arctic Ocean are presented. In the Barents and Kara seas, near-surface DFe minima indicate depletion of DFe by phytoplankton growth. Below the surface, lower DFe concentrations in the Kara Sea (~0.4-0.6 nM) than in the Barents Sea (~0.6-0.8 nM) likely reflect scavenging removal or biological depletion of DFe. Very high DFe concentrations (>10 nM) in the bottom waters of the Laptev Sea shelf may be attributed to either sediment resuspension, sinking of brine or regeneration of DFe in the lower layers. A significant correlation (R2 = 0.60) between salinity and DFe is observed. Using d18O, salinity, nutrients and total alkalinity data, the main source for the high (>2 nM) DFe concentrations in the Amundsen and Makarov Basins is identified as (Eurasian) river water, transported with the Transpolar Drift (TPD). On the North American side of the TPD, the DFe concentrations are low (<0.8 nM) and variations are determined by the effects of sea-ice meltwater, biological depletion and remineralization and scavenging in halocline waters from the shelf. This distribution pattern of DFe is also supported by the ratio between unfiltered and dissolved Fe (high (>4) above the shelf and low (<4) off the shelf).