HYDROMAGNESITE STROMATOLITES AND SEDIMENTS IN AN ALKALINE LAKE, SALDA-GOLU, TURKEY

Salda Lake in SW Turkey is a highly alkaline (pH > 9) water body with waters enriched in magnesium, Microbial stromatolites along shorelines contain a microflora of diatoms and cyanobacteria with extensive asso ciated biofilms. Together these are responsible for precipitation of h ydromagnesite, although distribution is not congruent with either cell s or films. Deposition has continued over several thousand years and i s independent of both lake volume and the general concentration of lak e waters. The lake is rimmed on three sides by serpentinites and on th e fourth by dolomite, The bulk of the water entering the lake is meteo ric, fed via extensive gravelly alluvial fan deltas (with predominantl y serpentinite pebbles) whose total areas and catchments exceed that o f the lake. These waters are significantly cooler than surface waters of the lake, and there is no evidence for derivation from hot springs, The magnesium is thought to have been leached from the gravels. Exten sive areas of recent sediments accumulating on shorelines have apparen tly formed by mechanical breakdown of microbialites. Similar deposits form conspicuous terraces in two areas and point to the longterm effec tiveness of these processes, Terrace deposits are modified by a variet y of subaerial processes, including growth of hydromagnesite cements. No microbial structures are preserved in these, and it is presumed tha t they are destroyed during reworking. Fine-grained hydromagnesite sed iment derived from shorelines and from terraces is suspended in the wa ter column during storms and is probably redeposited as turbidites in the deeper parts of the lake, Sediments of this kind may be a useful a nalogue for some ancient and economically important stratiform magnesi te deposits.