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

Abstract

In a seminal paper regarding the mechanisms of carbonate stromatolite formation, Ginsburg (1991, Controversies in Modern Geology, pp. 25–36) emphasized the need to question the relative role of microbes versus environment in their formation. The Maquinchao Basin is a continental lacustrine system in southern Argentina. It provides an ideal site to study carbonate buildups, the role of microbes and environmental stressors in their development and their implications in palaeoenvironmental reconstructions. Presently, the basin encompasses two lakes (Carri Laufquen Grande and Carri Laufquen Chica) joined by the ephemeral Maquinchao River. Fossil microbialites are found south and southwest of the largest lake. Preferential areas of development for fossil microbialites have been mapped using a high‐resolution differential Global Positioning System. Outcrops are located between 820 and 830 m elevation, higher than actual lake levels and the Maquinchao River where living microbialites have been observed. Field data along with microscopical observations and X‐ray diffraction 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 homogeneous 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 favour of stable intrinsic parameters (i.e. microbial communities) whereas the variable macro‐morphotypes indicate changing extrinsic constraints such as steepness, energy and turbidity. The occurrence of distinctive morphotypes in buildups separated by outcrop and topography suggest that the Maquinchao microbialites are indicative of a former larger lake. Thus, the Maquinchao microbial buildups are a valuable proxy for water‐level evolution and therefore palaeoenvironmental reconstructions. They can be further used to interpret the apparently random distribution of morphological types and extension of microbialites in the geological past.

Tracking Organomineralization Processes from Living Microbial Mats to Fossil Microbialites

“…It is composed nowadays of two main lakes, connected by the ephemeral Maquinchao River, partially sustained by groundwater. Fossil microbialites attest to a former higher water level in the basin [34,36]. Living and mineralizing mats were observed in 2011 in several ponds of the Maquinchao River [35].…”

“…Microfabrics of the Pleistocene microbialites of the Maquinchao Basin have been investigated [34], thus allowing to make parallels between Pleistocene fossils and modern crusts. Although less abundant, similar elongated pores to those observed in the modern crusts averaging 8-µm diameters are also observed in the Pleistocene samples of the Maquinchao Basin [34].…”

“…Microfabrics of the Pleistocene microbialites of the Maquinchao Basin have been investigated [34], thus allowing to make parallels between Pleistocene fossils and modern crusts. Although less abundant, similar elongated pores to those observed in the modern crusts averaging 8-µm diameters are also observed in the Pleistocene samples of the Maquinchao Basin [34]. Additionally, the cathodoluminescence inspection of these Pleistocene buildups has revealed micrometric (1-2-µm)-sized microstructures of rounded or more irregular shapes [34].…”

“…The Maquinchao Basin in Northwestern Patagonia, Argentina, provides a unique opportunity to explore the role of different aspects and stages of mineralization ( Figure 1A). The basin contains Pleistocene microbialites associated with a former large lake [34], along with living mats precipitating carbonates [35]. The Maquinchao Basin is a closed endoreic basin located in the Argentinian province of Rio Negro.…”

Preface: Celebration of the Career of Robert Nathan Ginsburg

“…Another theme dear to Bob’s heart was the study of stromatolites. The papers of Suosaari, Reid, and Andres () and Eymard et al () update his important 1991 paper (Ginsburg, ) which summarized the current ideas about stromatolites including controversies of definition, whether ancient stromatolites should be interpreted as sedimentary structures and mechanisms of carbonate fixation. Throughout his career, Bob was extremely interested in the formation of carbonate buildups and papers by Gischler and Hudson (), Counts et al (), Meeder and Harlem (), and McNeill, Pederson, Brownlow, Klaus, and Blackwelder () cover this topic, drawing on examples from the Pleistocene Miami Limestone, reefs and platforms from the Caribbean and Pacific and Indian Oceans.…”