Studies from a variety of disciplines documentrecentchange in the northern high-latitude environment.Prompted by predictions of an amplified response oftheArctic to enhanced greenhouse forcing, we present asynthesis of these observations. Pronounced winter andspring warming over northern continents since about 1970ispartly compensated by cooling over the northern NorthAtlantic. Warming is also evident over the centralArcticOcean. There is a downward tendency in sea ice extent,attended by warming and increased areal extent of theArctic Ocean's Atlantic layer. Negative snow coveranomalies have dominated over both continents sincethelate 1980s and terrestrial precipitation has increasedsince 1900. Small Arctic glaciers have exhibitedgenerally negative mass balances. While permafrost haswarmed in Alaska and Russia, it has cooled in easternCanada. There is evidence of increased plant growth,attended by greater shrub abundance and northwardmigration of the tree line. Evidence also suggeststhatthe tundra has changed from a net sink to a net sourceofatmospheric carbon dioxide.Taken together, these results paint a reasonablycoherent picture of change, but their interpretationassignals of enhanced greenhouse warming is open todebate.Many of the environmental records are either short,areof uncertain quality, or provide limited spatialcoverage. The recent high-latitude warming is also nolarger than the interdecadal temperature range duringthis century. Nevertheless, the general patterns ofchange broadly agree with model predictions. Roughlyhalfof the pronounced recent rise in Northern Hemispherewinter temperatures reflects shifts in atmosphericcirculation. However, such changes are notinconsistentwith anthropogenic forcing and include generallypositive phases of the North Atlantic and ArcticOscillations and extratropical responses to theEl-NinoSouthern Oscillation. An anthropogenic effect is alsosuggested from interpretation of the paleoclimaterecord,which indicates that the 20th century Arctic is thewarmest of the past 400 years.

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Observational evidence of recent change in the northern high-latitude environment

We compare the volume flux divergence of Antarctic ice shelves in 2007 and 2008 with 1979 to 2010 surface accumulation and 2003 to 2008 thinning to determine their rates of melting and mass balance. Basal melt of 1325 ± 235 gigatons per year (Gt/year) exceeds a calving flux of 1089 ± 139 Gt/year, making ice-shelf melting the largest ablation process in Antarctica. The giant cold-cavity Ross, Filchner, and Ronne ice shelves covering two-thirds of the total ice-shelf area account for only 15% of net melting. Half of the meltwater comes from 10 small, warm-cavity Southeast Pacific ice shelves occupying 8% of the area. A similar high melt/area ratio is found for six East Antarctic ice shelves, implying undocumented strong ocean thermal forcing on their deep grounding lines.

/pdf/ice-shelf-melting-around-antarctica-596v47m2fr.pdf

Ice-Shelf Melting Around Antarctica

[1] Sedimentary molybdenum, [Mo]s, has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic-rich marine facies deposited under oxygen-depleted conditions A detailed analysis of [Mo]s–total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]s, while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo]s-TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction In the silled-basin environments, increasing restriction is correlated with a systematic decrease in [Mo]s/TOC ratios, from ∼45 ± 5 for Saanich Inlet to ∼45 ± 1 for the Black Sea This variation reflects control of [Mo]s by [Mo]aq, which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the “basin reservoir effect”) The temporal dynamics of this process are revealed by high-resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long-term trends toward lower [Mo]s/TOC ratios following development of water column stratification and deepwater anoxia Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo]aq availability and enhanced Mo transport to the sediment-water interface via Fe-Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (ie, more sulfidic conditions) These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (eg, silled basins) and are not valid for low-oxygen facies in open marine settings such as continent-margin upwelling systems

/pdf/mo-total-organic-carbon-covariation-in-modern-anoxic-marine-2do2lpkttq.pdf

Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions

Benthic O2 availability regulates many important biogeochemical processes and has crucial implications for the biology and ecology of benthic communities. Further, the benthic O2 exchange rate represents the most widely used proxy for quantifying mineralization and primary production of marine sediments. Consequently, numerous researchers have investigated the benthic O2 dynamics in a wide range of environments. On the basis of case studies from abyssal sediments to microbial phototrophic communities I hereby try to review the current status on what we know about controls that interrelate with the O2 dynamics of marine sediments. This includes factors like: sedimentation rates, bottom water O2 concentrations, diffusive boundary layers, fauna activity, light, temperature, and sediment permeability. The investigation of benthic O2 dynamics represents a challenge in resolving variations on temporal and spatial scales covering several orders of magnitude. Such an effort requires the use of several complementary measuring techniques and modeling approaches. Recent technical developments (improved chamber approaches, O2 optodes, eddy-correlation, benthic observatories) and advances in diagenetic modeling have facilitated our abilities to resolve and interpret benthic O2 dynamics. However, all approaches have limitations and caveats that must be carefully evaluated during data interpretation. Much has been learned during the last decades but there are still many unanswered questions that need to be addressed in order to fully understand benthic O2 dynamics and the role of sediments for marine carbon cycling.

/pdf/oxygen-dynamics-of-marine-sediments-2ruizssndk.pdf

Oxygen dynamics of marine sediments

Recent climate change in the Arctic and its impact on contaminant pathways and interpretation of temporal trend data.

On the Intermediate Depth Waters of the Arctic Ocean

The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface

Simultaneous spectrophotometric determination of nitrite and nitrate by flow injection analysis

The suitability of total carbonate production instead of oxygen consumption as a measure of benthic respiration has been investigated. In situ fluxes of total carbonate, oxygen, calcium, total alkalinity, nutrients, and sulfide across the sediment-water interface were measured in diver-operated benthic flux chambers. Two chambers were run in parallel to test the influence of oxygen and pH levels on total carbonate production. In one, oxygen and pH were kept constant near ambient levels; in the other, benthic respiration was allowed to deplete oxygen and lower the pH. We found that the flux of total carbonate, corrected for CaCO3 precipitation/dissolution, is a suitable measure of benthic mineralization in sediments where methane production can be neglected. The production rate of total carbonate was not seriously affected as long as the oxygen concentration remained > 100 µM.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1986.31.2.0319

Benthic respiration measured by total carbonate production

The Oden 91 Expedition has provided a data set that makes it possible to deduce more detailed ideas regarding the origin and circulation of waters in the Arctic Ocean. The three possible sources for the deep water of the Arctic Ocean are: (1) density flows down the continental slope triggered by brine enhanced waters formed on the continental shelves but consisting primarily of waters entrained from the Atlantic and intermediate layers; (2) inflow of Atlantic Water over the Barents Sea shelf that has experienced a density increase by cooling and freezing in that sea and then sinks with little entrainment down the St Anna Trough into deep layers of the Arctic Ocean; and (3) the inflow of Norwegian Sea Deep Water through Fram Strait. Of these three sources, the first appears to contribute the most to the Arctic Ocean deep water and the third the least. The Eurasian Basin communicates with the Canadian Basin through a boundary current that enters the Canadian Basin north of Siberia and leaves it north of Greenland. The fact that both the temperature and salinity are higher in the Canadian Basin than in the Eurasian Basin at levels above as well as below the sill depth of the Lomonosov Ridge indicates that slope convection is active in the Canadian Basin. The deepest layers have constant salinity, but show a weak temperature decrease towards the bottom. This suggests that these layers of the Canadian Basin are not primarily renewed by convection down the continental slope but by a spill over of Eurasian Basin Deep Water across the central part of the Lomonosov Ridge. A model which incorporates density flows triggered by high salinity shelf water and water overflowing the Lomonosov Ridge flom the Eurasian Basin is applied to reproduce the observed profiles of the Canadian Basin and to establish the relative importance of these two sources. By incorporation of I4C profiles into the model, estimates of the exchange rates of water in these layers can be made.

Leif G. Anderson

Papers

On the Intermediate Depth Waters of the Arctic Ocean

The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface

Simultaneous spectrophotometric determination of nitrite and nitrate by flow injection analysis

Benthic respiration measured by total carbonate production

Deep waters of the Arctic Ocean: origins and circulation