Dirk Dethleff

Sediments in Arctic sea ice: Implications for entrainment, transport and release

Despite the Arctic sea ice covers recognized sensitivity to environmental change, the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood. Sea ice sediment inclusions were studied in the central Arctic Ocean during the Arctic 91 expedition and in the Laptev Sea (East Siberian Arctic Region Expedition 1992). Results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central Arctic Ocean. This study documents the regional distribution and composition of particle-laden ice, investigates and evaluates processes by which sediment is incorporated into the ice cover, and identifies transport paths and probable depositional centers for the released sediment. In April 1992, sea ice in the Laptev Sea was relatively clean. The sediment occasionally observed was distributed diffusely over the entire ice column, forming turbid ice. Observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment. In the central Arctic Ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes. The surface sediment accumulation in central Arctic multi-year sea ice exceeds by far the amounts observed in first-year ice from the Laptev Sea in April 1992. Sea ice sediments are generally fine grained, although coarse sediments and stones up to 5 cm in diameter are observed. Component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles. The biogenous components, namely shells of pelecypods and benthic foraminiferal tests, point to a shallow, benthic, marine source area. Apparently, sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing. Clay mineralogy of ice-rafted sediments provides information on potential source areas. A smectite maximum in sea ice sediment samples repeatedly occurred between 81°N and 83°N along the Arctic 91 transect, indicating a rather stable and narrow smectite rich ice drift stream of the Transpolar Drift. The smectite concentrations are comparable to those found in both Laptev Sea shelf sediments and anchor ice sediments, pointing to this sea as a potential source area for sea ice sediments. In the central Arctic Ocean sea ice clay mineralogy is significantly different from deep-sea clay mineral distribution patterns. The contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms.

Contrasts in Arctic shelf sea-ice regimes and some implications: Beaufort Sea versus Laptev Sea

The winter ice-regime of the 500 km) from the mainland than in the Beaufort Sea. As a result, the annual freeze-up does not incorporate old, deep-draft ice, and with a lack of compression, such deep-draft ice is not generated in situ, as on the Beaufort Sea shelf. The Laptev Sea has as much as 1000 km of fetch at the end of summer, when freezing storms move in and large (6 m) waves can form. Also, for the first three winter months, the polynya lies inshore at a water depth of only 10 m. Turbulence and freezing are excellent conditions for sediment entrainment by frazil and anchor ice, when compared to conditions in the short-fetched Beaufort Sea. We expect entrainment to occur yearly. Different from the intensely ice-gouged Beaufort Sea shelf, hydraulic bedforms probably dominate in the Laptev Sea. Corresponding with the large volume of ice produced, more dense water is generated in the Laptev Sea, possibly accompanied by downslope sediment transport. Thermohaline convection at the midshelf polynya, together with the reduced rate of bottom disruption by ice keels, may enhance benthic productivity and permit establishment of open-shelf benthic communities which in the Beaufort Sea can thrive only in the protection of barrier islands. Indirect evidence for high benthic productivity is found in the presence of walrus, who also require year-round open water. By contrast, lack of a suitable environment restricts walrus from the Beaufort Sea, although over 700 km farther to the south. We could speculate on other consequences of the different ice regimes in the Beaufort and Laptev Seas, but these few examples serve to point out the dangers of exptrapolating from knowledge gained in the North American Arctic to other shallow Arctic shelf settings.

The Laptev Sea flaw lead - detailed investigation on ice formation and export during 1991/1992 winter season

The main objective of this research paper is to estimate the new-ice production in the Laptev Sea flaw lead during the 1991/1992 winter season. A one-dimensional energy balance model was applied to calculate ocean-to-atmosphere heat flux and the resulting new-ice formation over open water. For a detailed estimate of regional ice production, the flaw lead was divided into 14 sections based on the analysis of NOAA-satellite images and Russian ice charts. Opening and maintenance of the lead sections are controlled by offshore winds, whereas closing of open water is caused by onshore winds. Since the orientation of the lead varies from section to section, the same regional wind forcing can cause different local lead behavior. Model results reveal that the seasonally accumulated thickness of new ice formed in the different lead sections—under the assumption of instantaneous lateral new-ice removal from the water surface—varies from 1.3 m to 13 m over temporarily open water and may reach 20 m over permanently open water. The corresponding ice volume produced in the sections varies between 3.4 km3 and 59 km3 and amounts to 258 km3 for the entire lead. The significant regional variations in new-ice production are due to differences in (i) the number of days that a lead section is open (open-lead days), (ii) the oceanic heat loss during open-lead days, and (iii) the areal extent of the lead sections. As compared to other studies,—at least during 1991/1992 winter season—the Laptev Sea flaw lead produced between 28 and 617% more initial sea ice than the Kara, Barents, East Siberian and Chukchi leads. Despite its limited areal extent of roughly 36,000 km2, which represents only 8% of the entire Laptev Sea, the flaw lead produces about 32% of the annual shelf ice. The ice production in the flaw lead is 5.3 times higher than the remainder of the shelf (7.4 m vs. 1.4 m). Furthermore, the Laptev Sea flaw lead produces 2.6% of the ice annually formed the entire Arctic Mediterranean Sea and contributes about 9% to the volume of the Siberian branch of the Transpolar Drift Ice System. This makes the Laptev Sea flaw lead a significant producer of Arctic sea ice on local and regional scales, whereas the contribution of lead ice to the entire volume of annually formed pack in high northern latitudes amounts only to roughly 1.3%.

Modelling Siberian river runoff — implications for contaminant transport in the Arctic Ocean

Abstract This model study investigates the role of Siberian river runoff for the transport of possible river contaminants in the Arctic Ocean. Three-dimensional coupled ice-ocean-models of different horizontal resolution are applied to simulate the dispersion of river water from Ob, Yenisei and Lena. These Siberian rivers are supposed to be important sources for various contaminants. The relevant processes which are considered in this study include the dispersion of dissolved or suspended contaminants in the water column and the transport of contaminated particles, incorporated into drifting sea ice. Circulation model results from both spatial scales explain the main pathways and transit times of Siberian river water in the Arctic Ocean. Kara Sea river water clearly dominates in the Siberian branch of the Transpolar Drift, while the Lena water dominates in the Canadian branch. River water concentrations in Nares Strait, Canadian Archipelago, are similar to those in the northern Fram Strait. Special emphasis is given to the seasonal variability of the river plume in the Kara Sea. Particle tracking simulations on the regional scale illustrate that Ob and Yenisei tracers behave differently. Yenisei tracers leave the Kara Sea quite fast towards the Arctic Ocean or the Laptev Sea, but Ob tracers spread also in the southern Kara Sea, in particular at lower levels. A comparison of simulated freezing rates and particle concentrations in Siberian coastal waters suggests that during autumn, the incorporation of particles into freezing sea ice near the estuaries of Ob and Yenisei is very likely. Simulated ice trajectories, started close to the Lena river delta easily reach the multi-year Transpolar Drift within one winter. Ice trajectories from Ob and Yenisei estuaries, however, mostly drift towards the Barents Sea where the ice melts close to Svalbard. The model study confirms that contaminant transport through sediment-laden sea ice offers a short and effective pathway for pollutant transport from Siberian rivers to the Barents and Nordic Seas.

Entrainment and export of Laptev Sea ice sediments, Siberian Arctic

Sea ice sediments (SIS; sediments rafted by ice) and shelf surface deposits from the Siberian Laptev Sea were analyzed to study turbulent entrainment processes. This study focused on the silt and clay fractions, on average representing 63.4% and 30.4% (total: 93.7%) of the bulk SIS, respectively. Generally, the SIS contains 50% more silt than the shelf deposits, while the clay percentages in both sample groups are similar. Sand-sized particles are less abundant in SIS than in bottom sediment. Although SIS is clearly enriched in silt, no evidence was found for preferential ice entrainment of any silt subfraction (coarse, medium, fine). Statistical tests of SIS and shelf silt parameters, and the similar clay mineral compositions of both sample groups, provide evidence for entrainment of Laptev bottom deposits into newly forming ice. The entrainment is related to the mechanism known as suspension freezing. Accordingly, coarser clasts (sand) and plant debris may be lifted upward by buoyant anchor ice, while coarse silt particles are supposed to be transported to the surface by rising frazil aggregates. Additionally, turbulent interaction of convergent Langmuir circulation and near-surface frazil ice streaks is proposed to support ice entrainment of the finest particles >6.4 phi (<12 μm). Entrainment and export of SIS from the Laptev Sea were assessed for ten individual flaw leads (extended narrow polynyas separating fast and drifting ice) using particle concentrations in ice samples and new ice formation rates. The results show that roughly 20 Mt of SIS can be annually exported from the Laptev Sea by lead ice only. Thus formation and export of sediment-laden lead ice may play a major geological role in Siberian Arctic shelf erosion and coastal retreat.

Anthropogenic radioactivity in the Arctic Ocean } review of the results from the joint German project

The paper presents the results of the joint project carried out in Germany in order to assess the consequences in the marine environment from the dumping of nuclear wastes in the Kara and Barents Seas. The project consisted of experimental work on measurements of radionuclides in samples from the Arctic marine environment and numerical modelling of the potential pathways and dispersion of contaminants in the Arctic Ocean. Water and sediment samples were collected for determination of radionuclide such as 137Cs, 90Sr, 239 + 240Pu, 238Pu, and 241Am and various organic micropollutants. In addition, a few water and numerous surface sediment samples collected in the Kara Sea and from the Kola peninsula were taken by Russian colleagues and analysed for artificial radionuclide by the BSH laboratory. The role of transport by sea ice from the Kara Sea into the Arctic Ocean was assessed by a small subgroup at GEOMAR. This transport process might be considered as a rapid contribution due to entrainment of contaminated sediments into sea ice, following export from the Kara Sea into the transpolar ice drift and subsequent release in the Atlantic Ocean in the area of the East Greenland Current. Numerical modelling of dispersion of pollutants from the Kara and Barents Seas was carried out both on a local scale for the Barents and Kara Seas and for long range dispersion into the Arctic and Atlantic Oceans. Three-dimensional baroclinic circulation models were applied to trace the transport of pollutants. Experimental results were used to validate the model results such as the discharges from the nuclear reprocessing plant at Sellafield and subsequent contamination of the North Sea up the Arctic Seas.

Storm-generated sediment deposition on rocky shores: Simulating burial effects on the physiology and morphology of Saccharina latissima sporophytes

Abstract Kelps inhabiting wave-exposed coasts are frequently exposed to disturbance such as storm-generated sediment deposition on blades which can be resuspended and removed by local hydrodynamic processes. However, in extreme cases, whole thalli can be buried under sediment for a period of time. The objectives of the present study are to determine the proximate effects of long-term sediment burial on kelp physiology and to confirm whether transient sediment load have functional significance in mitigating the negative effects of saturating light intensity normally encountered in the field. We simulated sediment burial to evaluate its impact on the vitality, in terms of photosystem II (PSII) function, pigments and morphology, of Saccharina latissima on different time scales. The effect was compared to the negative controls, kelps without sediment cover exposed to the whole radiation treatment. Results of our study showed that short-term burial under different sediment types (gravel, sand, and silt and clay) has no negative effect on the physiology and morphology of S. latissima, whereas sediment-free algal discs exposed to high PAR and UVR were bleached and photoinhibited. Conversely, long-term burial under silt and clay showed adverse smothering effect leading to bleaching, loss of PSII function and tissue decay. We observed that burial under gravel and sand (up to a period of 7 days), e.g. after storm slowdown and settlement of sediment particles, showed some protective function in mitigating the negative effect of photoinhibiting high light intensities naturally encountered by most intertidal and upper subtidal macroalgae.

Fram Strait sea-ice sediment provinces based on silt and clay compositions identify Siberian Kara and Laptev seas as main source regions

Fram Strait sea-ice sediments (SIS) contain on average more than 94% silt and clay. Both fractions were compared with bottom deposits of the Kara and Laptev seas to identify shelf sources of fine-grained Arctic SIS. Based on silt granulometry and clay mineral assemblages we determined Fram Strait SIS provinces. Western Fram Strait SIS has medium to fine silt compositions, whereas eastern Fram Strait SIS is enriched in fine silt. Western Fram Strait SIS clays (low smectite/high illite) were statistically grouped with eastern Laptev shelf deposits, and are similar to East Siberian and North American shelf sources. Eastern Fram Strait SIS clays (high smectite/low illite) cluster with shelf deposits of the western Laptev Sea and the Kara Sea. We conclude that western Fram Strait pack ice consisted of a mixture of floes from the Laptev Sea and sources farther to the east during the 1997 and 1999 sampling periods. Eastern Fram Strait ice originated from sources towards the Kara Sea. There was an average annual flux of ca. 158 Tg (Mt) SIS export through Fram Strait during the late 1990s. We expect no qualitative changes in the SIS entrainment process (“suspension freezing”) with decreasing Arctic ice cover, although the process may increase through larger fetch. SIS incorporation and flux will be enhanced with increasing shelf open water during winter freezing, and with the current acceleration of the Transpolar Drift, but we speculate that the transport of SIS towards Fram Strait will be seasonally truncated with the onset of ice-free summers in the Arctic Ocean.

Transient sediment load on blades of Arctic Saccharina latissima can mitigate UV radiation effect on photosynthesis

We studied the short-term impact of sediment load on the photosynthetic performance of Saccharina latissima sporophytes exposed to ultraviolet radiation (UVR). The algae were collected from different sediment-influenced environments in Svalbard in August 2007. Initial optimum quantum yield (Fv/Fm) of sediment-covered sporophytes was significantly higher compared to sediment-free sporophytes. Experimental sediment coating on blade discs had a photoprotective function by screening out 92% of the weighted UV-B (UVery) treatment. No UVR-induced photoinhibition was observed in sediment-coated blade discs while sediment removal caused a reduction in Fv/Fm not only after 12-h UVR exposure but also after 6-h recovery in low white light compared to the initial value. Thus, sediment coating has a short-term functional significance in mitigating the negative effect of UVR on photosynthesis of an important kelp species and set a baseline for further studies.

Transport of radionuclides by sea-ice and dense-water formed in western Kara Sea flaw leads

A transport assessment of particle-bound and dissolved artificial radionuclides (137Cs and 239,240Pu) by sea-ice and dense-water formed in western Kara Sea flaw leads close to the Novaya Zemlya dumping sites is presented in this study. We both performed a “best estimate” based on available data, and a “maximum assessment” relying on simulated constant releases of 1 TBq 137Cs and 239,240Pu from individual dumping bays. The estimates are based on a combination of (i) the content of particulate matter in sea-ice; (ii) analytical data and numerical simulations of radionuclide concentrations in shelf surface deposits, suspended particulate matter (SPM), and the dissolved phase; and (iii) estimates of lead-ice and dense-water formation rates as well as modeling results of local ice drift pathways. In the “best estimate” case, 2.90 GBq 137Cs and 0.51 GBq 239,240Pu attached to sea-ice sediments can be exported from the lead areas toward the central Arctic basin. The radionuclide burden of the annually formed dense lead water in the “best estimate” amounts to 4.68 TBq 137Cs and 0.014 TBq 239,240Pu. In the “maximum assessment”, potential export-rates of ice-particle bound 137Cs and 239,240Pu toward the central Arctic would amount to 0.64 and 0.16 TBq, respectively. As much as ≈900 TBq 137Cs and ≈6.75 TBq 239,240Pu could be annually taken up by 34.75 dense-water rejected in the lead area. Assuming the (unlikely) instantaneous release of the total 137Cs and 239,240Pu inventories (≈1 PBq and 10 TBq, respectively) from the Novaya Zemlya dumping sites into the dissolved phase, the dense lead water locally formed during one winter season could take up ≈90% of the Cs and ≈68% of the Pu released.