Spatial variation in soil active-layer geochemistry across hydrologic margins in polar desert ecosystems

Abstract

Abstract. Polar deserts are characterized by severe spatialtemporal limitations of liquid water. In soil active layers of the Antarctic Dry Valleys, liquid water is infrequently available over most of the arid terrestrial landscape. However, soils on the margins of glacial melt-water streams and lakes are visibly wet during the brief Austral summer when temperatures permit the existence of liquid water. We examined the role of these hydrologic margins as preferential zones for the transformation and transport of nutrient elements and solutes in an environment where geochemical weathering and biological activity is strictly limited by the dearth of liquid water. We report on hydropedological investigations of aquatic-terrestrial transition zones adjacent to 11 stream and lake systems in the Antarctic Dry Valleys. Our results show that wetted zones extended 1-11 m from the edges of lotic and lentic systems. While capillary demand and surface evaporation drive a one-way flux of water through these zones, the scale of these transition zones is determined by the topography and physical characteristics of the surrounding soils. Nutrient concentrations and fluxes appear to be influenced by both the hydrology and microbialmediated biogeochemical processes. Salt concentrations are enriched near the distal boundary of the wetted fronts due to evapo-concentration of pore water in lake margin soils, while organic matter, ammonium and phosphate concentrations are highest in stream channel sediments where potential for biological activity is greatest. Thus, in the Antarctic Dry Valleys, intermittently wet soils on the margins of streams and lakes are important zones of both geochemical cycling and biological activity.

Patterns and Processes of Salt Efflorescences in the McMurdo region, Antarctica

“…With the exception of the Miers lake sample (which was most likely formed by "evapoconcentration" whereby lake water evaporates near the soil surface and concentrates lake-derived salts in the upper soil column [Barrett et al, 2009]), the salts rich in bicarbonate/carbonate are found in the eastern (coastal) region of TV near streams. Bicarbonate/carbonate salts are not found west of the Fryxell basin, which is consistent with the findings of Claridge and Campbell (1977) because a lower pH in soils westward is attributed to the accumulation of alkaline marine aerosols near the coast and acidic aerosols near the East Antarctic Ice Sheet.…”

Soluble salt accumulations in Taylor Valley, Antarctica: Implications for paleolakes and Ross Sea Ice Sheet dynamics

“…In a process that has been termed "evapoconcentration," this lake water evaporates near the soil surface, concentrating salts dissolved in the lake water into the upper soil . Soluble salts in soils influenced by evapoconcentration have been sampled by Barrett et al [2009] to 10 cm depth. Clconcentrations in these soils average about 100 mmol Clkg -1 .…”

“…Clconcentrations in these soils average about 100 mmol Clkg -1 . Using this Clconcentration and an average bulk density of about 1.65 g cm -3 as measured by Barrett et al [2009], the average Clcontent calculated from equation (2) is 16.5 mol Clm -2 . This value is relatively high and is similar to Clcontents measured in Bonney Basin soils, older soils near the Nussbaum Riegel, and soils below approximately 120 m elevation in eastern Taylor Valley.…”

“…These lower salt concentrations are likely caused by the lower solute concentrations in streams relative to lakes [Lyons et al, 1998] and the instability of stream-wetted margins because of variable water fluxes Northcott et al, 2009]. Wetted stream margins sampled by Barrett et al [2009] have Clconcentrations averaging about 5 mmol kg -1 . The Clcontent of these soils calculated in equation (2) for a bulk density of 1.65 g cm -3 , as above for wetted lake margins, is 0.5 mol Clm -2 .…”

“…Bockheim et al [2008a] analyzed the distribution of soluble salts in soils, primarily in western Taylor Valley, and concluded that soluble salt accumulations are influenced primarily by soil age, not inundation by paleolakes. This conclusion was based on the interpretation that soils inundated by lakes are leached of soluble salts; however, Barrett et al [2009] showed that salts can accumulate near wetted lake margins as lake water moves along water potential gradients towards an evaporation zone near the soil surface. Barrett et al [2010] studied salt accumulations near Lake Fryxell, Lake Hoare, and Lake Bonney by measuring the electroconductivity of soil-water mixtures and found evidence of salt accumulation along paleolake margins in all three areas.…”