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Limnology....The Long Version

LimnologyLittle is known about limnological conditions in subglacial lakes. Knowledge of physical, chemical, and microbiological processes within “lakes” will be essential for detecting the presence of life, understanding the associated biogeochemistry and deciphering sedimentary records of lake histories. There is a range in lake sizes, morphologies, and geological settings that reflects a spectrum of ages, environmental conditions, and habitats. Model predictions of circulation and analyses of accreted lake water have been used to speculate about subglacial lake limnology in general and Lake Vostok’s chemical environment in particular.

Physical models assessing the likelihood of water circulation in Lake Vostok have been developed by Wüest and Carmack (2000), Williams (2001) and Mayer et al. (2003) and recently reviewed by Siegert (2005). Lake Vostok circulation is predicted to be a consequence of differences in the pressure melting point between the north and south ends of Lake Vostok. In a freshwater (i.e. low ionic strength) lake, geothermal heating warms bottom waters to a temperature higher than that of the upper layers. Water density decreases with increasing temperature resulting in an unstable water column leading to convective circulation where cold melt water sinks and water warmed by geothermal heat ascends (Wüest & Carmack 2000). Conversely, if the lake is slightly saline the fresh glacier melt water will be buoyant relative to bulk lake water and the northern melt water would spread southward and upward (Souchez et al. 2000). If the horizontal salinity gradient is great enough to compensate for geothermal warming, water would move into regions of progressively lower pressure displacing lake water in the south. The cold northern water eventually refreezes onto the ice sheet base some distance from where it first melted. In this case, a conveyor of fresh cool melt water migrates from north to south beneath the ice sheet causing displacement of warmer dense lake water from the south to the north. If the lakes exhibit vertical chemical gradients a stratified upper layer of cold fresh water would be predicted to overlie a deeper layer of warm saline water.

Several authors have discussed the geochemistry of Lake Vostok as inferred from the analysis of proxies in accreted lake ice. Accreted ice is ice frozen onto the base of the ice sheet as it moves across the lake surface. As described above, this process is the result of a differential in the thickness of the overlying ice that establishes a melt/freeze cycle within a lake basin. The freezing process imparts distinct stable oxygen and hydrogen isotopic compositions to accretion ice water that is significantly different than the overlying glacial ice (Jouzel et al, 1999; Souchez et al, 2003). Two types of accretion ice have been recovered from the Vostok borehole: (i) ice with sediment/particle inclusions and (ii) ice with no visible inclusions (e.g. Jouzel et al, 1999, Souchez et al, 2003). It is thought that the ice with no visible inclusions is water from the deeper portions of the lake while the ice containing visible inclusions may have entrained material as it intersected the bedrock upstream of Vostok Station (Jouzel et al, 1999). Smaller inclusions consist of mostly fine clays, quartz aggregates and various salts. Dating suggests a Precambrian origin for the inclusions consistent with a continental shield source (Delmonte et al, 2004).

Gas concentrations and isotopic ratios in accreted ice lend clues to the origins of Lake Vostok water. Helium isotope ratios in accretion ice are lower than glacier ice (Jean Baptiste et al, 2001) suggesting an input of radiogenic helium from the degassing of deep faults (Bulat et al, 2004). In contrast, 3He concentrations suggest little if any hydrothermal input to Lake Vostok (Jean Baptiste et al, 2001). Seismic activity near Vostok station may contribute heat and chemicals to the lake but this conjecture remains speculative (Studinger et al, 2003; Bulat et al, 2004).

Gas hydrate is suspected of playing a role in establishing the physical, chemical, and biological characteristics of subglacial lakes. Atmospheric air, captured in ice sheets, occurs exclusively as gas hydrate at an ice thickness of a few kilometers (Hondoh, 1996; Uchida etal, 1994). In large subglacial lakes, such as Lake Vostok, with a geometry that favors the establishment of a melt/freeze cycle, the melting of the ice sheet would release air hydrate to the water. Accretion ice is nearly gas-free relative to overlying glacial ice due to the exclusion of gas and salt during refreezing (Jouzel et al. 1999). This exclusion would lead to increased dissolved gas concentrations in Lake Vostok (McKay et al. 2003). Dissolved oxygen concentrations are predicted to be many times higher than air-equilibrated water (McKay et al. 2003). Within 400,000 years, gas concentrations in the lake are predicted to reach levels that favor the formation of gas hydrate. Gas hydrate formed in the lake would float to the surface of the lake if formed from air or sink if it contained significant CO2. These estimates of Lake Vostok water dissolved oxygen concentrations do not consider the effects of removal processes (McKay et al. 2003; Lipenkov et al. 2001). Metabolic consumption of oxygen in bottom waters and/or sediments can reduce oxygen levels and balance the accumulation of gases due to gas hydrate disassociation (e.g., McKay et al. 2003).

Dissolved organic carbon (DOC) plays a key role in ecosystem carbon cycling owing to its role as an energy and carbon source for heterotrophic organisms. DOC levels in Lake Vostok accretion ice are low implying that DOC levels in surface lake waters are low as well. However, little is known about the partitioning coefficients of DOC as water freezes so the actual DOC levels in the lake remain unknown (for further details see the next section on Microbial Life, Evolution, and Adaptations).

A more complete description of the limnology and biogeochemical processes at work in subglacial environments is fundamental to understanding subglacial environments as systems. Scientific questions related to limnology and biogeochemistry will be answered with a combination of models, field observations, in situ experiments and monitoring, and laboratory measurements (see Inset). Refined models of circulation will require detailed knowledge of basin topography and water density. The equations of state used to compute density from salinity and temperature data in seawater may not apply to subglacial lakes if the ion ratios differ significantly from seawater. It will be important to test the models using natural tracers (e.g. isotopic compositions of Be, He, rare earth elements, etc). Given the potential freeze/thaw nature of some of these systems, it will also be important to understand the dynamics of salt rejection and partitioning that occurs at the ice-water interface. The potentially low kinetic energy of these systems in concert with steep-sided basins may lead to vertical density stratification that inhibits mixing of near-surface and near-bottom lake waters. In such cases, thermohaline convection may be an important vertical transport mechanism. In order to understand the density structure of the lakes, detailed profiles of the ionic content of the water column are needed. If there is a geothermal source of heat and chemicals in some subglacial environments, it will play an important role in establishing buoyancy driven circulation and determining the ionic composition of lake waters. Critical to understanding subglacial lake circulation patterns is the possibility of episodic or continuous hydraulic connectivity between lake systems. Outbursts of subglacial melt waters would “reset” water density gradients homogenizing the physical, chemical, and biological environment. It is critically important to know if these environments are “open” or “closed” systems, i.e., is there exchange among lakes within a certain geographical area and on what timeframes. To model circulation; construct water, heat, and biochemical budgets; and estimate the age of the water, the basin topography, salinity, temperature, density, and current distributions must be accurately known.

Biogeochemical studies will provide important data documenting the presence and type of metabolic activity. Metabolic reactions require a number of essential elements. The amount and distribution of nutritive elements (e.g., nitrogen, phosphorus, sulfur, iron and others) in subglacial environments will provide fundamental constraints on biological productivity and suggest how microorganisms meet their metabolic needs. It will be essential to document the spatial and temporal distribution of various elements in the water column and sediments to understand their cycling. Information on potentially toxic elements (i.e., oxygen radicals) will lend clues to the habitability of subglacial environments. Characterization of the water column will include profiles of salinity, temperature, density, dissolved O2, the CO2 system, pH, Eh, and characterization of dissolved and particulate organic matter. Observatories with strings of sensors will be deployed in water columns to detect temporal changes in water properties. Discrete samples taken in profile within the water column will be analyzed for biogenic gases such as N2O, H2S, N2, and CH4; inorganic N (NH4+, NO2-, N2O); iron; soluble reactive phosphorus; dissolved organic carbon; dissolved inorganic carbon; major cations and anions; particulate carbon and nitrogen; microorganism; and 16S and 18S r-RNA gene microbial diversity. Eventually, these same measurements will be performed on sediment samples, including vertical profiles, within sedimentary columns to establish the presence of chemical gradients that support biological activity.


 

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