Biotic vs. abiotic signatures in Late Pleistocene lacustrine microbialites of the Maquinchao Basin (Argentina)

Abstract

Identifying and further weighting the role of biotic versus abiotic processes in microbialite formation remain a big challenge. The Maquinchao basin is a closed continental basin in northwestern Patagonia, east of the Andes (Argentina). It provides an ideal site to study modern and fossil lacustrine carbonate buildups, the role of microbes in their development as biotic compound, environmental settings as abiotic stressor, and their implications in paleoenvironmental reconstructions. Presently the basin encompasses two lakes (Carri Laufquen Grande and Carri Laufquen Chica) joined by the ephemeral Maquinchao River. Fossil microbialites are preferentially found south and southwest of the largest lake. Preferential areas of development for fossil microbialites have been mapped using a high-resolution differential GPS. Outcrops are located between 820 m and 830 m, higher than actual lake levels and the Maquinchao River where living microbialites have been observed. Field data along with microscopical observations and XRD analyses have revealed a heterogeneity in both distribution and macro- morphotypes since carbonate buildups display different morphologies such as crust, columns, open flower-like, rounded and ellipsoids. Conversely, on the meso- and micro-scale they show more homogenous morphologies including laminations and shrubs. These microbial buildups are associated with basaltic substrates of variable size from pebbles to boulder. The homogeneity in meso- and micro- structures argue in favor of stable intrinsic parameters (i.e. microbial communities) whereas the variable macromorphotypes indicate changing extrinsic constraints such as steepness, energy and turbidity. The occurrence of distinctive buildups morphotypes separated in different outcrops and topographies suggest that Maquinchao microbialites are indicative of former Maquinchao big lake water levels. Thus, these microbial buildups are a very valuable proxy for water level evolution and therefore paleoenvironmental reconstructions. They can be further used to seek the causes behind the apparently random distribution of morphological types and extension of microbialites in the geological past.