Leif G. Anderson

Sensitivity of the carbon cycle in the Arctic to climate change

The recent warming in the Arctic is affecting a broad spectrum of physical, ecological, and human/cultural systems that may be irreversible on century time scales and have the potential to cause rapid changes in the earth system. The response of the carbon cycle of the Arctic to changes in climate is a major issue of global concern, yet there has not been a comprehensive review of the status of the contemporary carbon cycle of the Arctic and its response to climate change. This review is designed to clarify key uncertainties and vulnerabilities in the response of the carbon cycle of the Arctic to ongoing climatic change. While it is clear that there are substantial stocks of carbon in the Arctic, there are also significant uncertainties associated with the magnitude of organic matter stocks contained in permafrost and the storage of methane hydrates beneath both subterranean and submerged permafrost of the Arctic. In the context of the global carbon cycle, this review demonstrates that the Arctic plays an important role in the global dynamics of both CO2 and CH4. Studies suggest that the Arctic has been a sink for atmospheric CO2 of between 0 and 0.8 Pg C/yr in recent decades, which is between 0% and 25% of the global net land/ocean flux during the 1990s. The Arctic is a substantial source of CH4 to the atmosphere (between 32 and 112 Tg CH4/yr), primarily because of the large area of wetlands throughout the region. Analyses to date indicate that the sensitivity of the carbon cycle of the Arctic during the remainder of the 21st century is highly uncertain. To improve the capability to assess the sensitivity of the carbon cycle of the Arctic to projected climate change, we recommend that (1) integrated regional studies be conducted to link observations of carbon dynamics to the processes that are likely to influence those dynamics, and (2) the understanding gained from these integrated studies be incorporated into both uncoupled and fully coupled carbon-climate modeling efforts. (Less)

Formation and evolution of the surface mixed layer and halocline of the Arctic Ocean

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen-Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen-Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen-Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low-salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double-diffusive convection, is trapped below the mixed layer.

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

The porewater of a sediment core taken at 6 m depth in Gullmarsfjorden, Sweden, was enriched in Fe, Mn, Co, and phosphate compared to the overlying bottom water. Yet, in situ measurements with a benthic flux-chamber, in which dissolved oxygen and pH were maintained near ambient values (regulated flux-chamber), showed that the sediment did not release any of these ions but instead removed Co, Mn, and Fe from the overlying water. In a parallel experiment, where dissolved oxygen and pH were not maintained but allowed to decrease as a result of benthic respiration, Co, Mn, Fe, and PO4 were released from the sediment. An accidental interruption of the stirring in the regulated chamber caused a pulse of dissolved Co, Mn, Fe, and PO4 to be released from the sediment. When the stirring was resumed, all four ions were again removed. The kinetics of the removal process was apparent first order with half-removal times of 3–5 days, similar to the removal kinetics of the radioactive tracers 59Fe and 54Mn from the water in a smaller chamber, run in parallel. The critical variable which controls the reactions at the sediment-water interface is the flux of oxygen from the water column into the sediment. When benthic chambers are used to measure fluxes of redox-sensitive ions, the oxygen flux must be maintained as close as possible to the actual in situ flux. If not, the measured fluxes may vary greatly in magnitude and even change direction.

Simultaneous spectrophotometric determination of nitrite and nitrate by flow injection analysis

Abstract The flow injection principle is used in the photometric determination of nitrite and nitrate with sulfanilamide and N-(1-naphthyl)ethylenediamine as reagents. An on-line copper-coated cadmium reductor reduces nitrate to nitrite. The detection limit is 0.05 μM for nitrite and 0.1 mM for nitrate at a total sample volume of 200 μM. Up to 30 samples can be analyzed per hour with a relative precision of ca. 1%.

Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation

The Atlantic and Pacific oceans provide source waters for the Arctic Ocean that can be distinguished by their differing nitrate and phosphate concentration relationships. Using these relationships, we estimate the amount of Atlantic and Pacific waters in the surface layer (top 30 m) of the Arctic Ocean. Atlantic source water is dominant in most of the Eurasian Basin and is present in significant amounts in the Makarov Basin north of the East Siberian Sea. Pacific source water is dominant in most of the Canadian Basin and is present in significant amounts in the Amundsen Basin north of Greenland. We deduce circulation patterns from the distributions of Atlantic and Pacific source waters in the surface layer of the Arctic Ocean and conclude that the flow within the surface layer differs from ice drift along the North American and European boundaries of the Polar Basin.

Water masses and circulation in the Eurasian basin. Results from the Oden-91 expedition

The Oden 91 North Pole expedition obtained Oceanographic measurements on four sections in the Nansen and Amundsen basins of the Eurasian Basin and in the Makarov Basin of the Canadian Basin, thereby proving the feasibility of carrying out a typical Oceanographic program using an icebreaker in the Arctic Ocean. The data show greater spatial variability in water structure and circulation than was apparent from previous data. The results show that a clear front exists between the Eurasian and Canadian basins such that upper halocline water in the Canadian Basin is almost absent from the Eurasian Basin. The lower halocline water produced in the Barents-Kara Sea region permeates much of the Eurasian Basin and flows along the continental slope into the Canadian Basin. The deeper circulation is strongly influenced by topography. Three return flows of the Atlantic layer are identified, one over the Nansen-Gakkel Ridge, one over the Lomonosov Ridge, and a third flowing from the Canadian Basin. The slight differences observed in salinity and temperature characteristics of the deeper waters of the Nansen and Amundsen basins do not lead to an obvious explanation of their origin or flow pattern.

Deep waters of the Arctic Ocean: origins and circulation

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.

Rapid, high-precision potentiometric titration of alkalinity in ocean and sediment pore waters

Abstract A system for rapid, high precision potentiometric determination of alkalinity in sea water and sediment pore water is presented. Two titration units were used: a 40 ml unit for seawater and a small volume unit for sediment pore water. Titration time was normally less than 10 minutes per sample, including sample exchange. With a 40 ml sample volume, the relative standard deviation of the alkalinity obtained in the laboratory was 0.05% and at sea 0.1 %. The small-volume system (0.5–1.5 ml) gave a precision of 0.07%. Five titration points, in two groups after the second equivalence point, were used to evaluate the equivalence volume. Results from equilibrium calculations and computer simulated alkalinity titrations show that it was possible to use a non-modified Gran function [( V 0 +v) ∗ 10 ( E Z ) ] and still achieve good accuracy and precision.

Benthic fluxes of cadmium, copper, nickel, zinc and lead in the coastal environment

Fluxes of trace metals across the sediment-water interface were measured in situ at 6 m depth in Gullmarsfjorden, Sweden, using diver-operated stirred benthic flux-chambers. These were equipped so that dissolved oxygen and pH could be maintained near ambient seawater values (regulated chamber) or be allowed to change in response to benthic respiration (unregulated chamber). In the regulated chamber, Cd, Cu, Ni, and Zn were released from the sediment at constant rates both during a winter experiment (water temperature −1 °C) and during a fall experiment (+ 10°C). During the fall experiment, fluxes (in nmol m−2 d−1) of 13 (Cd), 118 (Cu), 209 (Ni), and 1400 (Zn) were measured. In winter, the release rates were lower by factors of 5 and 10 for Cu and Ni but not significantly different for Cd and Zn. Neither release nor uptake by the sediment could be demonstrated for Pb. The pore-water in a diver-collected core was depleted in Cd, Cu, and Zn and slightly enriched in Ni and Pb, relative to the ambient seawater. There was no correspondence between fluxes calculated from porewater profiles and actually measured fluxes; nor could the fluxes be directly related to the degradation rate of organic matter. In the unregulated chamber, initial trace metal release rates were lower than in the regulated chamber. As the oxygen concentration decreased, the metal fluxes decreased as well and were ultimately reversed as sulfide began to appear in the water. The fluxes of trace metals are sensitive to the oxygen regime in the flux chamber because the solubilization of these metals, which takes place in a thin oxic layer near the sediment surface, depends on the oxygen flux across the sediment-water interface.

A carbon budget for the Arctic Ocean

We present a carbon budget for the Arctic Ocean that is based on estimates of water mass transformation and transport. The budget is constrained by conservation of mass and salt. In the model, the Eurasian and Canadian basins have been divided into five and six boxes, respectively, based on the prevailing water masses. In addition, there are three boxes representing the shelf areas. Total dissolved inorganic (C T ) and organic (TOC) carbon concentrations for the different water masses and different regions are used, together with the volume flows, to calculate the carbon transports. The carbon budget calculation shows that at present the oceanic transport into the Arctic Ocean is larger than out, that is, 3.296±0.008 Gt C yr -1 of C T transported in and 3.287±0.004 Gt C yr - transported out with the corresponding values for TOC being 0.134±0.009 Gt C yr -1 and 0.122±0.006 Gt C yr -1 , respectively. However, the outflowing waters are older than the inflowing waters and had thus been exposed to an atmosphere with lower concentration of anthropogenic carbon dioxide when entering the Arctic Ocean. When recalculating the budget to the preindustrial scenario, assuming steady state, the C T transport changes to 3.243 Gt C yr - in and 3.266 Gt C yr -1 out. To balance the preindustrial transports, assuming no change in the TOC fluxes, a direct input of atmospheric carbon dioxide of 0.011±0.014 Gt C yr -1 is required. Added to this is the burial of organic matter which is calculated as 0.013±0.010 Gt C yr -1 using a recycling efficiency of 80% [Hulth, 1995] and a new production of 0.05 Gt C yr -1 [Anderson et al., 1994]. An indirect contribution of atmospheric carbon dioxide via runoff adds 0.017±0.004 Gt C yr -1 , resulting in a preindustrial total atmospheric input of 0.041±0.018 Gt C yr -1 .