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

Abstract

Living 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 regions experiencing extreme and variable environmental conditions. Here, we report and describe living freshwater stromatolites in the Maquinchao region, north-western Patagonia, 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 of sheltered 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 biological factors controlling their development. Our results indicate that today stromatolites only proliferate in freshwater when Ca 2+ levels are high. A microscopic inspection of the living stromatolite mat indicates stronger photosynthetic activity in the upper green layer associated with crypto/ microcrystalline calcite (nanoglobules) compared to the lower beige-white biofilm. This biofilm contains more low-Mg calcite (rhombohedra) precipitates, which can form millimetre-sized aggregates in the underlying anoxic layer. Although sulphate-reducing bacteria are living in the entire mat, they appear more abundant and widely distributed in the lower beige-white layer and are always associated with Mg calcite. Low salinity and low-turbidity water along with microbial (photosynthetic and heterotrophic) activity are the most important factors promoting low-Mg calcite precipitation 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 of geomicrobiological studies in identifying proxies of the hydrological conditions prevailing during their formation.

Growth morphologies and plausible stressors ruling the formation of Late Pleistocene lacustrine carbonate buildups in the Maquinchao Basin (Argentina)

“…The XRD and EDS analyses have confirmed that low‐Mg calcite is the main mineral phase of the different morphologies and meso‐structures of the buildups as previously observed by Pacton et al () in the modern stromatolites.…”

“…Pacton et al () have discovered living microbial mats containing low‐Mg calcite, the same mineralogy as in the fossil carbonate buildups. They are only present in the Maquinchao River and absent in both lakes.…”

“…Despite the presence of basalt boulders and pebbles over the entire basin, no outcrops have been observed on the west part of CLG, around CLC, or below 815 m (Figure ). Additionally, no living microbial mats have been spotted close to the actual flat shores of CLG and CLC despite the abundant basalt pebbles (Pacton et al, ). It thus appears that in addition to the presence of hard basalt substrates of different sizes (Ginsburg & Planavsky, ) several other concurrent factors within the basin are conditioning the distribution and development of these carbonate buildups.…”

“…brackish to freshwater; Pacton et al, ). Furthermore, during the former Maquinchao Lake high‐stand that characterized the last deglaciation (Cartwright et al, ; Galloway et al, ) the chemistry of the water was most probably less saline than today and closer to the actual Maquinchao River chemistry allowing preferential conditions for the growth of carbonate buildups (Pacton et al, ).…”

“…In the Maquinchao Basin, located in northern Patagonia (Argentina) carbonate constructions have been described as tufa (Cartwright et al, 2011;Galloway, Markgraf, & Bradbury, 1988) and stromatolites (Pacton et al, 2015;Tatur et al, 2002). Previous studies in the area include the evolution of the present lakes through time using seismic profiles and sedimentary cores, ostracods auto-ecology and their stable isotope compositions (Ariztegui, Anselmetti, Gilli, & Waldmann, 2008;Ariztegui, Anselmetti, Kelts, Seltzer, & D'Agostino, 2001;Coviaga et al, 2018;Cusminsky et al, 2011;Schwalb, Burns, Cusminsky, Kelts, & Markgraf, 2002;Whatley & Cusminsky, 1999), lake-level variations (Cartwright et al, 2011;Galloway et al, 1988;Tatur et al, 2002) and the formation of modern microbialites (Pacton et al, 2015). This contribution focuses on the Pleistocene carbonate buildups and their occurrence in the Maquinchao Basin.…”