William J. Green

The geochemistry of antarctic streams and their role in the evolution of four lakes of the McMurdo dry valleys

Abstract Major ion concentrations are reported for nineteen meltwater streams in the McMurdo dry valleys, Antarctica, and for the four lakes fed by these streams. The most important solute acquisition processes for these flowing waters are dissolution of marine-derived salts, dissolution of calcite coatings derived from chemical weathering of parent rock, and direct weathering of silicates. Although Lakes Joyce, Hoare, and Fryxell have undergone significant evaporative concentration, the only simple salt which is predicted to form at present is calcite. The shallow waters of all four lakes were supersaturated with respect to this mineral, but were undersaturated with respect to other simple salts. Geochemical budgets, however, indicate that SO4, K, and Mg, in addition to Ca and HCO3, have been removed from some of these lakes over recent geological time. Bacterial sulfate reduction followed by precipitation of metal sulfides is likely to be a sink for sulfate. Sinks for Mg and K are more problematic, but may include formation of Mgcalcites and, in the case of K, reverse weathering reactions. The ages of the contemporary water columns, as indicated by chloride budgets, are relatively young, and these lakes could have evolved their chemical compositions over a few thousand years of present stream flows. It is not necessary to invoke trapped seawater or other salt sources to explain their evolution.

Geochemistry of the Onyx River (Wright Valley, Antarctica) and its role in the chemical evolution of Lake Vanda

The Onyx River (Wright Valley, Antarctica) is a dilute meltwater stream originating in the vicinity of the Wright Lower Glacier. It acquires a significant fraction of its salt content when glacial meltwaters contact Wright Valley soils at Lake Brownworth and the concentrations of all ions increase with distance along the 28-km channel down to Lake Vanda. Average millimolar concentrations of major ions at the Vanda weir during the 1980–1981 flow season were: Ca = 0.119; Mg = 0.061; Na = 0.212; K = 0.033; Q = 0.212; SO4 = 0.045; HCO3 = 0.295; and SiO2 = 0.049. Based on the flow measurements of Chinn (1982), this amounts to an annual flux (in moles) to Lake Vanda of: Ca = 0.238 × 106; Mg = 0.122 × 106; Na = 0.424 × 106; K = 0.066 × 106; Cl = 0.424 × 106; SO4 = 0.09 × 106; HCO3 = 0.59 × 106; SiO2 = 0.098 × 106. In spite of the large salt input from this source, equilibrium evaporation of Onyx River water would have resulted in early calcite deposition and in the formation of a Na-Mg-Cl-HCO3 brine rather than in the Ca-Na-Mg-Cl waters observed in Lake Vanda. The river alone could not have produced a brine having the qualitative geochemical features of the lower saline waters of Lake Vanda. It is proposed that the Vanda brine is instead the result of past ( > 1200 yrs BP) mixing events between Onyx River inflows and calcium chloride-rich deep groundwaters derived from the Don Juan Basin. The mixing model presented here shows that the Onyx River is the major contributor of K, HCO3, SO4, and (possibly) Mg found in the lake and a significant contributor (approximately one half) of the observed Na. Calcium and Cl, on the other hand, came largely from deep groundwater sources in the Don Juan Basin. All concentrations except Mg are well predicted by this model. The chemical composition of the geologically recent upper lake is explained in terms of ionic diffusion from the pre-formed brine, coupled with Onyx River inflow. Ionic ratios calculated from this latter model are in very good agreement with those observed in the lake at 35 meters.

Geochemical processes in the Lake Fryxell Basin (Victoria Land, Antarctica)

Major ion, nutrient, transition metal, and cadmium concentrations are presented for nine meltwater streams flowing into Lake Fryxell, a permanently stratified lake with an anoxic hypolimnion in Taylor Valley, Antarctica. For the major ions, stream compositions are considered in terms of dissolution of marine-derived salts and chemical weathering of local rocks. Although Lake Fryxell has undergone significant evaporative concentration, only calcite, of the simple salts, is predicted to precipitate. Geochemical budgets indicate, however, that large quantities of K, Mg, and SO4 have also been removed from the lake. Reverse weathering may be an important sink for K and Mg, although magnesium removal with calcium carbonate phases is also likely. Assuming constancy of composition over recent geologic time, all of the salts in the Fryxell water column could have been delivered under present flows in about three thousand years (chloride age).Comparison of nutrient concentrations in these meltwater streams with other flowing waters in the world reveals that the Fryxell streams are strikingly deficient in NO3-N but not PO4-P. The apparent nitrogen deficiency in Lake Fryxell itself can be attributed to the low annual stream loadings of this nutrient.Stream concentrations and loadings are also presented for Mn, Fe, Co, Ni, Cu, and Cd. ‘Dissolved’ metal concentrations correlate roughly with average crustal abundances, suggesting that chemical weathering is the major source for these elements. Vertical metal profiles within Lake Fryxell itself appear to be governed by the formation of insoluble sulfide phases, or, in the case of Mn, by MnHPO4. However, dissolved nickel levels in sulfide-bearing waters are much higher than can be explained in terms of metal-sulfide equilibria, and we suspect that significant organic complexing of this metal is occurring in the deeper waters.

Rare earth elements in the water column of Lake Vanda, McMurdo dry valleys, Antarctica

Abstract We present data on the composition of water from Lake Vanda, Antarctica. Vanda and other lakes in the McMurdo Dry Valleys of Antarctica are characterized by closed basins, permanent ice covers, and deep saline waters. The meromictic lakes provide model systems for the study of trace metal cycling owing to their pristine nature and the relative simplicity of their biogeochemical systems. Lake Vanda, in the Wright Valley, is supplied by a single input, the Onyx River, and has no output. Water input to the lake is balanced by sublimation of the nearly permanent ice cap that is broken only near the shoreline during the austral summer. The water column is characterized by an inverse thermal stratification of anoxic warm hypersaline water underlying cold oxic freshwater. Water collected under trace-element clean conditions was analyzed for its dissolved and total rare earth element (REE) concentrations by inductively coupled plasma mass spectrometry. Depth profiles are characterized by low dissolved REE concentrations (La, Ce,

The cycling of nutrients in a closed-basin antarctic lake: Lake Vanda

Lake Vanda is a permanently ice covered, meromictic, closed basin lake, located in the Dry Valley region of Southern Victoria Land, Antarctica. A unique feature of the lake water column structure is that the bottom lake waters exist as a natural diffusion cell. The diffusive nature of these waters allows rates of sulfate reduction, nitrification and denitrification to be calculated from nutrient concentration gradients. Calculation reveals that sulfate reduction is by far the most important anoxic process acting to oxidize organic material. In addition, rate calculations reveal that bottom water nutrient profiles are in steady state. One argument in support of this conclusion is that the calculated rate of nitrification balances the flux of ammonia from the anoxic lake waters. The flux of phosphorus from the reducing waters is several times less than would be predicted from the nitrogen and phosphorus content of decomposing lake seston. Solubility calculations show that phosphorus may be actively removed at depth in Lake Vanda by the formation of hydroxyapatite. It is found that estimated rates of nitrogen and phosphorus removal in the bottom lake waters and sediments roughly balance the riverine input flux. This suggests that throughout the lake a nutrient steady state may exist, and that the anoxic zone may be the most important loci for nutrient removal. Finally, the ratio of nitrogen to phosphorus entering Lake Vanda by riverine input is less than the ‘Redfield’ ratio of 16/1; in contrast to the lake waters which are strongly phosphorus limited at all depths. This curious aspect of the lakes nutrient chemistry is explained by the presence of preformed nitrogen, which has been concentrated in the deep brine due to several episodes of evaporative concentration.

Distribution of major, minor and trace elements in lake environments of Antarctica

The concentrations of major, minor and trace metals were measured in water samples collected from five shallow Antarctic lakes (Carezza, Edmonson Point (No 14 and 15a), Inexpressible Island and Tarn Flat) found in Terra Nova Bay (northern Victoria Land, Antarctica) during the Italian Expeditions of 1993-2001. The total concentrations of a large suite of elements (Al, As, Ba, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Gd, K, La, Li, Mg, Mn, Mo, Na, Nd, Ni, Pb, Pr, Rb, Sc, Si, Sr, Ta, Ti, U, V, Y, W, Zn and Zr) were determined using spectroscopic techniques (ICP-AES, GF-AAS and ICP-MS). The results are similar to those obtained for the freshwater lakes of the Larsemann Hills, East Antarctica, and for the McMurdo Dry Valleys. Principal Component Analysis (PCA) and Cluster Analysis (CA) were performed to identify groups of samples with similar characteristics and to find correlations between the variables. The variability observed within the water samples is closely connected to the sea spray input; hence, it is primarily a consequence of geographical and meteorological factors, such as distance from the ocean and time of year. The trace element levels, in particular those of heavy metals, are very low, suggesting an origin from natural sources rather than from anthropogenic contamination.

Mn, Fe, Cu and Cd distributions and residence times in closed basin Lake Vanda (Wright Valley, Antarctica)

Mn, Fe, Cu, and Cd concentrations are reported for Lake Vanda, a closed-basin, meromictic, Antarctic lake and for its single major inflow, the Onyx River. Trace metal distributions in the upper lake and annual metal fluxes from the Onyx River were used to estimate chemical residence times in the mixed zone above the chemocline. Residence times, based on total metal loads, were 9.4 years for Mn; 1.4 years for iron; 174 years for copper; and 82 years for cadmium. Controls on the steady state concentrations of metals in this system are likely to include: particle settling of Fe and Mn; scavenging of minor elements on metal oxide surfaces; sulfide precipitation from the anoxic brine; and possibly uptake of metals on the surface of benthic algal mats. Model calculations show that metal removal by sinking phytoplankton can account for only a small fraction of the annual loss.

210Pb and stable lead through the redox transition zone of an Antarctic lake

Despite fully aerobic waters, below 51 m depth in Lake Vanda relatively low Eh values, combined with a lowering of pH, lead to the dissolution of manganese oxides, and the accumulation of dissolved Mn. Just above 61 m oxygen is depleted, and Fe2+ accumulates. Just below 61 m sulfide accumulates as a result of sulfate reduction. We measured the depth distributions of dissolved and particulate stable lead, 210Pb, and 226Ra, to understand the dynamics of lead cycling through the redox transition zone. Our results indicate that stable lead is released from dissolving manganese oxides in the region between 51 to 61 m, resulting in a dissolved lead maximum at 59 m. Concentration profiles show that dissolved lead diffuses to the top of the zone of aerobic Mn reduction (AMR), and is sequestered onto newly formed manganese oxides. These oxides settle and dissolve, releasing lead back to solution. Lead also diffuses downward into the anoxic-sulfidic waters for permanent removal as insoluble PbS phases. Despite net release of stable lead into solution in the AMR zone, 210Pb and 226Ra results demonstrate active removal of 210Pb at all depths. Removal is fastest at the top of the AMR zone and in the anoxic zone. In the middle of the AMR zone scavenging is less active, though significant. Model calculations demonstrate that scavenging onto particles is the most important removal pathway for stable lead from the AMR zone. Finally, similar timescales of removal are found for 210Pb and stable lead, showing that in this environment, the 210Pb “clock” may be applied to understanding rates of stable lead cycling.

Metal dynamics in Lake Vanda (Wright Valley, Antarctica).

Data are reported for Mn, Fe, Co, Ni, Cu and Cd in the Onyx River, and for Mn, Co, Ni, Cu and Cd in Lake Vanda, a closed-basin Antarctic lake. Oxic water concentrations for Co, Ni, Cu and Cd were quite low and approximate pelagic ocean values. Scavenging of these metals by sinking particles is strongly indicated. Deep-lake profiles reveal a sharp peak in the concentrations of Mn, Fe and Co at the oxic-anoxic boundary at 60 m. Maxima for Ni, Cu and Cd occur higher in the water column, in the vicinity of a Mn submaximum, suggesting early release of these metals from sinking manganese oxide-coated particles. A rough steady-state model leads to the conclusion that there is a large downward flux of Mn into the deep lake and that this flux is sufficient to explain the annual loss of Co, Ni, Cu and Cd. A pronounced geochemical separation between Fe and Mn apparently occurs in this system--Fe being best lost in near-shore environments and Mn being lost in deeper waters. Comparison of metal residence times in Lake Vanda with those in the oceans shows that in both systems Mn, Fe and Co are much more reactive than Ni, Cu and Cd. Energetically favorable inclusion of the more highly charged metals, Mn(IV), Fe(III) and Co(III), into oxide-based lattices is a plausible explanation.

The Residence times of eight trace metals in a closed-basin Antarctic Lake: Lake Hoare

Water column and stream measurements of eight trace metals are presented for the Lake Hoare system. With the exception of Cd, metals showed little tendency to accumulate in the upper reservoir (> 24 m) of this closed-basin lake. Residence time trends for trace and major elements in this system were comparable to those in the oceans. It is concluded that particle reactive elements will behave in a similar manner from one closed aquatic system to another, and will tend to undergo rapid removal in comparison with the major elements. Of the eight metals studied, the adjacent transition series elements Mn, Fe, and Co had the shortest residence times.