Ponding of Water in the Crater

The hydrologic depression within the impact structure caused by excavation is likely to fill with water, forming pond(s) and lake(s). This may not always be the case. Small impacts in arid desert environments, for example, can form a crater devoid of water, but these are rarer [3]. A diversity of impact structures contain lakes today, some of them many millions of years after impact, demonstrating that the re-arrangement of the hydrologic system is by no means a transient phenomenon from the point of view of the longevity of the resulting microbial habitat.

The formation of the lake may overlap with the hydrothermal phase. In the Boltysh impact structure in Russia, hydrothermally altered sediments have been found in the lower parts of the stratigraphic sequence [14], suggesting that like volcanic calderas, there may have been an interaction of the lake with the postimpact hydrothermal system. In this case microbial communities within the lake obtain the advantage of hydrothermal warming of the lake waters and possibly an improved availability of nutrients caused by convective mixing of the lake waters.

11.3.1 Present-Day Ponding

The microbiota within the lake will be influenced by the climatic regimen in which the impact occurs. As the crater depression will remain in place for millions of years, the lake biota will be influenced by long-term changes in the planetary climate. Thus, generalized comments on the microbial content of impact lakes are not possible. It depends both on time and location and indeed, on the scale of the impact, which will influence the size of the resulting water bodies.

A number of impact lakes have been examined and their present-day microbial ecosystems characterized, although infrequently these studies have been related to post-impact colonization. To provide contrast we will discuss two intra-crater lakes in two very different climates. First, Lake El'gygytgyn in northeastern Siberia, a crater-lake characteristic of a tundra biome and second, the intra-crater lake of the Tswaing impact structure in South Africa, a lake typical of highveld summer rainfall zone. The purpose of this brief review is to illustrate the diverse physical and chemical environments associated with intra-crater bodies of water.

Lake El'gygytgyn is a 12 km-diameter lake situated within a complex impact structure formed ~ 3.6 Ma ago [15] (Fig. 11.3 left). The total diameter of the crater is ~18 km with a well-defined rim. The crater-lake is fed by a catchment area of just under 300 km2 and the major outflow from the lake is a river that runs from the south of the crater.

The lake is situated in a dry, cold, tundra biome above the tree line, with less than 400 mm of rain each year. At 67°10'N and 172°05'E, the crater-lake experiences frigid winters with temperatures below -30°C in January and rising to a maximum of 8°C in July. Because of these low temperatures the lake is ice-covered for most of the year.

Because of its biologically depauperate setting and extreme weather conditions, the lake has low organic carbon concentrations. The Secchi disk depth is 19 m (a Secchi disk is a 20 cm-diameter disk with alternating black and white quadrants. It is lowered into a lake until the observer can no longer see it. This depth of disappearance is called the Secchi disk depth). Recent studies of the lake show the physical and chemical characteristics to be quite constant through its depth [15]. The temperature through the 175 m deep lake is approximately 3.3°C, with a pH just less than 6.8. The oxygen concentration is 12 mg l-1 through most of the lake. Below 140 m it becomes slightly more anoxic. At the bottom of the lake oxygen concentrations are about 10 mg l-1.

The microbiological study of this lake is limited to the diatom flora. A diverse diatom flora is recorded. 113 diatom taxa were found in the lake, although one species, Cyclotella sp. dominated the flora, accounting for over 95% of the planktic community [15].

The near constancy of the physical and chemical conditions in the lake suggests complete mixing, probably caused by the high winds of this extreme location. Because the electrical conductivity is low (12 |S cm-1), it is suggested that the lake is mainly supplied by meltwater from snow and ice. This would also explain the high Secchi disk depth and the apparent lack of organics. In summary, therefore, this lake is a cold, mixed, nutrient-poor environment [15].

Fig 11.3 The 12 km-wide El'gygytgyn crater-lake in Siberia within the 18-km diameter complex crater (left) and the much smaller Tswaing crater-lake in the 1.13 km-diameter simple crater in South Africa (right).

By contrast, the Tswaing crater at 25°24'S, 28°04'E is a very different limnetic environment [16-18] (Fig. 11.3 right). The intra-crater lake is set within a simple 1.13 km-diameter impact structure formed about 200,000 years ago. The crater-lake is set below the treeline in a region of vegetation of the South African bushveld. The crater is heavily colonized by vegetation such as Acacia spp. By contrast to the tundra environment of El'gygytgyn, the rainfall is 400-750 mm a year. Mean summer temperatures vary between 14.2 and 35.2°C in December and January (summer) and between 3.6 and 15.6°C in June and July (winter). Unlike El'gygytgyn, the lake is never ice-covered. Because of the rich vegetation and warm temperatures, organic and nutrient inputs from the soil are high. The Secchi disk depth of the lake is a mere 7 cm (contrast this to 19 m for El'gygytgyn).

Unlike El'gygytgyn, the lake is highly stratified, even though it is only 0.49 m deep at its lowest point and up to 2.85 m deep at high water in winter. Temperatures at the surface can exceed 35°C, but drop to 25°C at the bottom of the lake. The lake is hypersaline, with a salinity of 10 % at the top increasing to >35% at the bottom of the lake. This is partly caused by the warm climate and the high rates of evaporation. The saltpan has been classified as a Na-Cl-CO3-type brine. The pH was found to vary from 8.8 at the surface to 9.2 at the bottom with some variation up to 10.4 in between. Oxygen concentrations also varied considerably from ~20 mg l-1 near the surface to < 0.1 mg l-1 below 30 cm and they varied depending on the time of day. The electrical conductivity, because of the high salinity, is several orders of magnitude higher than in El'gygytgyn.

The very different chemical and physical regimen to be found in the hypersaline Tswaing crater compared to El'gygytgyn drives a very different microbial composition. The lake is inhabited by cyanobacterial benthic microbial mats that grow around the edges of the lake. A rich diversity of planktonic algae and bacteria inhabit the surface waters where light is available. A 5-6 cm deep plate of photosynthetic bacteria (Chlorobium sp.) is formed just below the zone of maximum temperature [16]. The lake also hosts a diatom flora [19]. Eleven species of diatom were examined, but in many of the samples Nitzschia sp. comprised over 70 % of the diatom population.

In view of the very different climatic regimens, the differences in the chemical, physical and thus biotic conditions between the intra-crater lakes at El'gygytgyn and Tswaing are not surprising (similar differences would be seen in lakes within depressions caused by other agents such as glacial scouring). However, they demonstrate to the reader unfamiliar with the microbiota of lakes that the ponding of water within a crater depression can lead to very different microbiological consequences depending on the setting, despite the fact that the mechanism of formation (formation of a hydrologic depression by impact excavation) is the same [3].

In some cases the chemistry of the intra-crater materials can influence lake chemistry. Lim and Douglas [20] report that the high sulfate concentrations of two lakes within the Haughton structure are caused by the underlying melt rocks. Their data suggest that intra-crater lakes cannot simply be considered as ponding associated with a large hole in the ground, but that impact-processing may influence the subsequent chemical interactions between the intra-crater materials and intra-crater bodies of water. Impact crater lakes must therefore be examined within the context of knowledge of the effects of impact on the target materials.

In some craters the rate of evaporation in desert climates may be so high as to transform the intra-crater lake into a playa deposit, examples being Wolfe Creek crater in the Australian desert and Monturaqui in the Atacama desert of Chile.

The colonization of Martian impact crater lakes by a biota is completely speculative. Whilst there is no climate on Earth that can truly be described as 'analogous' to Mars, the ultra-oligotrophic, ice-covered lake of El'gygytgyn with its poor nutrient regimen and its low organic content represents the type of nutrient and physical environment that would be close to that expected in early

Martian lakes, albeit they would be expected to also be anoxic and high in ultraviolet radiation, because of the lack of an ozone shield on Mars.

11.3.2 The Fossil Record

In the previous section we showed the great contrast in the physical and chemical characteristics and thus, micro-biota that can exist in even present-day intra-crater lakes. Perhaps the most exobiologically relevant link between impact structures and Mars is the search for fossil life within intra-crater lake sedimentary deposits.

The coring of intra-crater lake sediments on Earth has revealed remnants of past microbial communities. An 89.95 m core was removed from the Tswaing impact crater in South Africa [21]. Extending from the floor of the lake through to the fractured basement granite, the 200,000 years of sequence could be divided into a number of well-defined parts. The top 34 m consisted of evaporites and muds, similar to the sediments being deposited today, with a high concentration of NaCl (halite). Below 30 m to the base of the sequence the core is dominated by calcium carbonate and within this sequence is abundant biological debris. Two segments within the core record a drop in pollen concentrations, suggesting xeric, desert-like, periods and illustrating how the record in impact lake cores can be used to derive a picture of post-impact climatic changes since the beginning of deposition. Other craters have been examined for their sedimentary records, notable examples being the Haughton impact structure and the Ries structure [22, 23].

In a previous publication we used the published information on the fossil record of microbial life in impact structures to postulate a sequence for the exobiological study of impact craters on Mars [3]. First, the crater rim might be examined for macroscopic signs of fossil life. An obvious analogy here is the fossil bioherms associated with the Ries crater in Germany [24] and with the central uplift of its postulated twin, the Steinheim crater. These outcrops of visibly laminated structures would present the most obvious signs of life in a post-impact lake. The bioherms are associated with the edges of the crater where they had a surface on which to grow, but light levels were sufficient for photosynthesis. In the absence of such biota, the surface of the playa/sedimentary deposits might be examined for signs of life. In the case of Martian craters, the interface between the dust layer from the Martian regolith and the beginning of sedimentary sequences could be examined for microscopic signatures of life. The terrestrial analogy here would be the surfaces of sediments such as those found in the Haughton structure in the high arctic. In the absence of a signature here, then the third step is to drill into the sedimentary deposits and seek signs of life buried within the deposits and fossilized. Analogues here include the drilling efforts into a diversity of craters, including the Ries and Lappajarvi structures, whose cores have revealed the presence of pre- and post-impact micro-fossils [e.g. 25]. Finally, deep drilling into the sediments to collect a complete core from the earliest stages of post-impact colonization might be attempted. In the case of Martian impact structures, the phase of liquid water availability might be transient and so drilling to the interface between the first sedimentary deposits and the crater floor may yield the most trustworthy answer to the question of whether microbial life used the post-impact lake habitat.

0 0

Post a comment