Organomineralization processes in freshwater stromatolites: A living example from eastern Patagonia

MURIEL PACTON, ; GABRIEL HUNGER; VINCENT MARTINUZZI; GABRIELA C. CUSMINSKY; BEATRICE BURDIN; KURT BARMETTLER; CRISOGONO VASCONCELOS ; DANIEL ARISTEGUI

Depositional Record

interntional Association of Sedimentology Wiley on line Library

Lugar: Gent; Año: 2016 vol. 1 p. 130 - 146

This article has been accepted for publication and undergone full peer review but has notbeen through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/j.2055-4877.2016.00007.xLiving stromatolites have been mostly described within shallow marine and(hyper)saline lacustrine environments. Southernmost South America lacks detailed investigations of these (organo)sedimentary buildups, particularly in regionsexperiencing extreme and variable environmental conditions. Here we report anddescribe living freshwater stromatolites in the Maquinchao region, northwesternPatagonia, Argentina. Fossil stromatolites characterized by globular and cauliflower shapes are also present in a continuous palaeoshoreline of a former lake at an altitude of 830 m, whereas their living counterparts only occur in the calm waters ofsheltered or meandering sections of the Maquinchao River. The living stromatolites and their host waters have been sampled and studied using various chemical and microscopic techniques to better constrain the environmental versus biologicalfactors controlling their development. Our results indicate that today stromatolitesonly proliferate in freshwater when Ca2+levels are high. A microscopic inspection ofthe living stromatolite mat indicates stronger photosynthetic activity in the uppergreen layer associated with crypto/microcrystalline calcite (nanoglobules) comparedto the lower beige-white biofilm. This biofilm contains more low Mg-calcite (rhombohedra) precipitates, which can form millimetre-sized aggregates in theunderlying anoxic layer. Although sulphate-reducing bacteria are living in the entire mat, they appear more abundant and widely distributed in the lower beige-whitelayer and are always associated with Mg-calcite. Low salinity and low-turbidity water along with microbial (photosynthetic andheterotrophic) activity are the most important factors promoting low-Mg calciteprecipitation in the Maquinchao Basin. These conditions are very different from those proposed for recently described lacustrine stromatolites at high altitude in the subtropical and tropical Andes as well as in Chilean Patagonia. Hence, all these observations in modern freshwater stromatolites show the importance ofgeomicrobiological studies in identifying proxies of the hydrological conditionsprevailing during their formation.