Search for signs of ancient life on Mars: expectations from hydromagnesite microbialites, Salda Lake, Turkey

Abstract

The 'White Rock', constituting a portion of what may be a lacustrine sedimentary sequence near the margin of a crater c. 90 km across in Sabaea Terra, Mars, measures 18 15 km 180-540 m high. It is re-interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an ancient evaporating crater lake. Were life to have emerged on Mars, as seems feasible, then the 'White Rock' might be expected to comprise a complex of stromatolitic mounds. Salda Gölü (Lake) in Turkey, is taken as an analogue. This enclosed lake is nearly surrounded and underlain by partially serpentinized harzburgite. Hydromagnesite stromatolites (microbialites) up to 7 m high coalesce to form a group of small islands 200 m across. The microbialites are seen to be growing near the mouth of the usually dry Salda River in the southwestern sector. Smaller developments of hydromagnesite encircle the lake and image processing of satellite data reveals a second extensive zone beneath the lake surface over a delta in the southeast. Individual columns a few centimetres high constitute bulbous mounds which are about 2 m in diameter. These columns terminate in domes a centimetre or so across. The domes are often annulated and are covered with a green biofilm a few millimetres thick comprised of cyanobactrial filaments. The columns consist of alternating fine and coarse hydromagnesite layers differentiated on a millimetric scale. The coarser layers near the surface still contain traces of the biofilm.Fossil microbialites were also discovered in the friable hydromagnesite cliffs shoreward of the main developments, though the structures of the individual microbes have not survived. Instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosity. Annular structures on their upper surfaces can be seen in places. The intervening and overlying material, also comprised of hydromagnesite, is a semi-lithified mud.Bulbous megascopic structures, separated by finer grained magnesium carbonate mudstone, within strata in the 'White Rock', would be strong evidence of a photosynthetic microbial genesis. Another deposit of white rock on the western margin of Juventae Chasma could have a similar origin.

Microbial Dolomite Precipitation under Aerobic Conditions: Results from Brejo do Espinho Lagoon (Brazil) and Culture Experiments

“…These results link bacterial precipitation of dolomite and hydromagnesite, both of which are found in natural habitats (Renaut, 1990;Warren, 1990;Russell et al, 1999;Last and Ginn, 2005).…”

Hydrochemistry and microbialites of the alkaline crater lake Alchichica, Mexico

“…The next unexpected observation is the fact that Alchichica belongs to the few lakes that produce hydromagnesite or hydromagnesite-magnesite microbialites (e.g., Renaut 1993;Braithwaite and Zedef 1994;Russell et al 1999;Power et al 2009). Alchichica's ''white'' microbialites turn out to be very similar to Holocene stromatolites from the alkaline lake Walyungup in Western Australia (Coshell et al 1998).…”

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…Hydromagnesite occurrences where groundwater is in contact with basic igneous rocks and flows into lakes include the carbonate lakes of British Columbia, Canada, the Coorongs of S. Australia, and the East African Rift Valley. A further example is in the highly Mg-rich, alkaline (pH > 9) waters of Salda Lake, Turkey, where lake drainage is rimmed on three sides by serpentinites (Russell et al 1999).…”