Mikhail N. Grigoriev

Modern terrigenous organic carbon input to the Arctic Ocean

The main objective of this chapter is to assess recent fluxes of terrigenous dissolved and particulate organic carbon into the Arctic Ocean. The most important terrigenous sources of organic matter (OM) in the ocean are (1) river and groundwater discharge, (2) coastal erosion, (3) sea-ice input and (4) aeolian material fluxes. The organic carbon fluxes of each of these pathways will be considered separately and a evaluation will be made of their roles in the total balance of OM input to the ocean.

Nearshore arctic subsea permafrost in transition

Models and geophysical data indicate that large areas of the Arctic shelves, as a result of their exposure during the Last Glacial Maximum, are thought to be almost entirely underlain by subsea permafrost from the coastline down to a water depth of about 100 meters. Subsea permafrost is still poorly understood, due mainly to the lack of direct observations. However, it is known to contain gas hydrates, a solid phase composed of water and gases that formed under low-temperature, high-pressure conditions. Large volumes of methane in gas hydrate form can be stored within or below the subsea permafrost, and the stability of this gas hydrate zone is sustained by the existence of permafrost. Degradation of subsea permafrost and the consequent destabilization of gas hydrates could significantly if not dramatically increase the flux of methane, a potent greenhouse gas, to the atmosphere.

The Influence of Cryogenic Processes on the Erosional Arctic Shoreface

Abstract Coastal dynamics and shoreface relief in ice-free seas are a function of hydrodynamic interactions between the sea and bottom sediments. In the Arctic, additional, cryogenic factors such as permafrost and the action of sea ice influence coastal processes. The goal of our paper is to assess this influence, mainly on the profile shape. Mathematical analyses of the shape of 63 shoreface profiles from the Laptev, Beaufort, and Chukchi Seas were carried out. The shapes of Arctic shoreface profiles and those of ice-free seas are compared. We found that large ice and silt content in perennially frozen sediments composing Arctic coasts favor their erosion. Sea ice plays an important role in sediment transport on the shoreface and correspondingly changes shoreface relief significantly. Some effects of ice intensify coastal erosion considerably, but others play a protective role. The overall influence of cryogenic processes on Arctic coasts composed of loose sediments is seen in that the average rate of coastal retreat is larger than in the temperate environments, even though Arctic coasts are protected by a continuous ice cover most of the year. The shape of the shoreface profile in the Arctic does not differ from that in ice-free seas, and is satisfactorily described by the Bruun/Dean equilibrium profile equation. The explanation of this fact is that all changes of the profile shape, caused by cryogenic processes, are short lived and quickly eliminated by wave action.

Recent changes in shelf hydrography in the Siberian Arctic: Potential for subsea permafrost instability

Summer hydrographic data (1920–2009) show a dramatic warming of the bottom water layer over the eastern Siberian shelf coastal zone (<10 m depth), since the mid-1980s, by 2.1°C. We attribute this warming to changes in the Arctic atmosphere. The enhanced summer cyclonicity results in warmer air temperatures and a reduction in ice extent, mainly through thermodynamic melting. This leads to a lengthening of the summer open-water season and to more solar heating of the water column. The permafrost modeling indicates, however, that a significant change in the permafrost depth lags behind the imposed changes in surface temperature, and after 25 years of summer seafloor warming (as observed from 1985 to 2009), the upper boundary of permafrost deepens only by ∼1 m. Thus, the observed increase in temperature does not lead to a destabilization of methane-bearing subsea permafrost or to an increase in methane emission. The CH4 supersaturation, recently reported from the eastern Siberian shelf, is believed to be the result of the degradation of subsea permafrost that is due to the long-lasting warming initiated by permafrost submergence about 8000 years ago rather than from those triggered by recent Arctic climate changes. A significant degradation of subsea permafrost is expected to be detectable at the beginning of the next millennium. Until that time, the simulated permafrost table shows a deepening down to ∼70 m below the seafloor that is considered to be important for the stability of the subsea permafrost and the permafrost-related gas hydrate stability zone.

Methane oxidation following submarine permafrost degradation: Measurements from a central Laptev Sea shelf borehole

Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Coastal erosion dynamics on the permafrost-dominated Bykovsky Peninsula, north Siberia, 1951–2006

This study investigates the rate of erosion during the 1951–2006 period on the Bykovsky Peninsula, located north-east of the harbour town of Tiksi, north Siberia. Its coastline, which is characterized by the presence of ice-rich sediment (Ice Complex) and the vicinity of the Lena River Delta, retreated at a mean rate of 0.59 m/yr between 1951 and 2006. Total erosion ranged from 434 m of erosion to 92 m of accretion during these 56 years and exhibited large variability (σ = 45.4). Ninety-seven percent of the rates observed were less than 2 m/yr and 81.6% were less than 1 m/yr. No significant trend in erosion could be recorded despite the study of five temporal subperiods within 1951–2006. Erosion modes and rates actually appear to be strongly dependant on the nature of the backshore material, erosion being stronger along low-lying coastal stretches affected by past or current thermokarst activity. The juxtaposition of wind records monitored at the town of Tiksi and erosion records yielded no significant relationship despite strong record amplitude for both data sets. We explain this poor relationship by the only rough incorporation of sea-ice cover in our storm extraction algorithm, the use of land-based wind records vs. offshore winds, the proximity of the peninsula to the Lena River Delta freshwater and sediment plume and the local topographical constraints on wave development.

Coastal changes in the Arctic

Abstract The arctic environment is changing: air temperatures, major river discharges and open water season length have increased, and storm intensities and tracks are changing. Thirteen quantitative studies of the rates of coastline position change throughout the Arctic show that recently observed environmental changes have not led to ubiquitously or continuously increasing coastal erosion rates, which currently range between 0 and 2 m/yr when averaged for the arctic shelf seas. Current data is probably insufficient, both spatially and temporally, however, to capture change at decadal to sub-decadal time scales. In this context, we describe the current understanding of arctic coastal geomorphodynamics with an emphasis on erosional regimes of coasts with ice-rich sedimentary deposits in the Laptev, East Siberian and Beaufort seas, where local coastal erosion can exceed 20 m/yr. We also examine coasts with lithified (rocky) substrates where geomorphodynamics are intensified by rapid glacial retreat. Coastlines of Svalbard, Greenland and the Canadian Archipelago are less frequently studied than ice-rich continental coasts of North America and Siberia, and studies often focus on coastal sections composed of unlithified material. As air temperature and sea ice duration and extent change, longer thaw and wave seasons will intensify coastal dynamics in the Arctic.

Ground penetrating radar and shallow seismic-stratigraphic and permafrost investigations of Lake Nikolay, Delta Lena, Arctic Siberia

The River Lena draining to the Siberian Arctic is considered to be the main sediment source for the Laptev Sea. Nevertheless, for the delta of River Lena occupying an area of 28,000 km2, there are many unsolved questions concerning the environmental and sedimentation history, genesis of deposits, permafrost distribution, and climatic records in the area especially for the second sandy terrace deposits in western Delta Lena. To survey the stratigraphic sequence of Lake Nikolay and its surrounding which is representative for the western Delta Lena area, radio-echo sounding (RES) with 25 MHz and 100 MHz antennae on the frozen lake and on land have been carried out in connection with drilling activities. RES measurements of the lake sediments are complemented by shallow seismic and reveal subsurface stratigraphy of the lake sediments and the bordering permafrost deposits.

Anaerobic methanotrophic communities thrive in deep submarine permafrost

Thawing submarine permafrost is a source of methane to the subsurface biosphere. Methane oxidation in submarine permafrost sediments has been proposed, but the responsible microorganisms remain uncharacterized. We analyzed archaeal communities and identified distinct anaerobic methanotrophic assemblages of marine and terrestrial origin (ANME-2a/b, ANME-2d) both in frozen and completely thawed submarine permafrost sediments. Besides archaea potentially involved in anaerobic oxidation of methane (AOM) we found a large diversity of archaea mainly belonging to Bathyarchaeota, Thaumarchaeota, and Euryarchaeota. Methane concentrations and δ13C-methane signatures distinguish horizons of potential AOM coupled either to sulfate reduction in a sulfate-methane transition zone (SMTZ) or to the reduction of other electron acceptors, such as iron, manganese or nitrate. Analysis of functional marker genes (mcrA) and fluorescence in situ hybridization (FISH) corroborate potential activity of AOM communities in submarine permafrost sediments at low temperatures. Modeled potential AOM consumes 72–100% of submarine permafrost methane and up to 1.2 Tg of carbon per year for the total expected area of submarine permafrost. This is comparable with AOM habitats such as cold seeps. We thus propose that AOM is active where submarine permafrost thaws, which should be included in global methane budgets.

The development of permafrost bacterial communities under submarine conditions

Submarine permafrost is more vulnerable to thawing than permafrost on land. Besides increased heat transfer from the ocean water, the penetration of salt lowers the freezing temperature and accelerates permafrost degradation. Microbial communities in thawing permafrost are expected to be stimulated by warming but how they develop under submarine conditions is completely unknown. We used the unique records of two submarine permafrost cores from the Laptev Sea on the East Siberian Arctic Shelf, inundated about 540 and 2500 years ago, to trace how bacterial communities develop depending on duration of the marine influence and pore water chemistry. Combined with geochemical analysis, we quantified total cell numbers and bacterial gene copies, and determined the community structure of bacteria using deep sequencing of the bacterial 16S rRNA gene. We show that submarine permafrost is an extreme habitat for microbial life deep below the seafloor with changing thermal and chemical conditions. Pore water chemistry revealed different pore water units reflecting the degree of marine influence and stages of permafrost thaw. Millennia after inundation by sea water, bacteria stratify into communities in permafrost, marine-affected permafrost, and seabed sediments. In contrast to pore water chemistry, the development of bacterial community structure, diversity and abundance in submarine permafrost appears site-specific, showing that both sedimentation and permafrost thaw histories strongly affect bacteria. Finally, highest microbial abundance was observed in the ice-bonded seawater unaffected but warmed permafrost of the longer inundated core, suggesting that permafrost bacterial communities exposed to submarine conditions start to proliferate millennia after warming.