Paleoenvironments

A B S T R A C T Lipids can survive long geological intervals within sediments and provide a unique tool that allows the reconstruction of past organismic diversity and environmental conditions. The lipids discussed in this review include all substances produced by organisms that are insoluble in water but extractable by organic solvents. Lipid biomarkers refer to both functionalized biolipids as well as their hydrocarbon derivatives in geological materials that contain diverse information about biotic sources and environmental conditions. In spite of di-agenetic and catagenetic alteration, lipid biomarkers commonly preserve the hydrocarbon structure of their biotic counterparts and have been found in rocks up to 1.6 billion years in age. These features have promoted the use of lipid biomarkers in many fields, including petroleum geology, paleoclimatology, oceanography, meteorology , geobiology and environmental science. Here, we (i) review the use of lipid biomarker records for the reconstruction of environmental conditions in deep time, including climatic conditions (temperature), sedi-mentary environments (redox, salinity and chemical composition) as well as catastrophic terrestrial events (soil erosion and wildfire), and (ii) generate new insights into environmental perturbations during the Permian-Triassic transition based on investigation of lipid biomarkers. We further propose that the ratio of di-benzothiophene to phenanthrene (DBT/P) in marine carbonates may be a robust proxy for seawater sulfate concentrations in deep time. Our compiled DBT/P records show substantial variations in seawater sulfate levels through Earth history that are consistent with the results of other proxies. We discuss the future outlook for application of lipid biomarker records to deep-time environmental research.

The Pleistocene–Holocene transition was a critical time for the dispersal of human societies all over South America. People looking for places to settle had to accept high environmental variability during the colonizing process. The case studied for this paper is Patagonia (Southern South America), where the oldest dates (ca. 13–10.5 ka BP) have been obtained for the peopling in the Deseado River Basin and Magellan Basin, as well. However, two archaeological sites yielded similar dates on the western side of the Andes (Monte Verde and Tagua Tagua).Following archaeological and palaeoenvironmental data (palynological, faunal, sedimentological and glaciological information), as well as radiocarbon datings, a hypothesis about the colonization of Patagonia is presented. Analysing the ways and time of colonization for this region, it is remarkable the coincidence of these ages in the centre of the steppe and close to the Magellan Strait, even in the present Tierra del Fuego island, though the eastern Andean foothills seems to be occupied at least two millennia later. It is proposed that independent peopling entries would have occurred both through the Atlantic and Pacific facades, and that the Andean foothills were colonized much later, only when the available spaces allowed it.

Resumen El estudio de los paleosuelos, junto con la sedimentología, son herramientas útiles en la reconstrucción ambiental de los paisajes antiguos; asimismo han sido utilizados como marcadores estratigráficos para hacer correlaciones locales o para conocer tasas de sedi-mentación. Los paleosuelos más antiguos corresponden al Precámbrico, sin embargo han sido más estudiados los del Cuaternario. El objetivo del presente trabajo es dar una revisión acerca del uso y aplicaciones de los paleosuelos, con un mayor énfasis en el estudio de las calcretas pedogenéticas y presentar un estudio de caso perteneciente a la barranca Los Bonetes, Michoacán. La barranca Los Bonetes, localizada en el sur-centro de México, contiene sedimentos volcaniclásticos de edad Cretácico Tardío y los hadrosaurios más antiguos conocidos en México. Dentro de esta sucesión volcaniclástica se localizan paquetes de calcretas que, debido a sus caracte-rísticas macro-y microestructurales, como son la presencia de raíces y las texturas alfa y beta, fueron reconocidas como de origen pedogenético. Con base en los requerimientos ambientales para la formación de este tipo de paleosuelos, interpretamos que el clima en la región debió ser árido o semiárido durante los periodos de no depósito durante el Cretácico. La importancia de este estudio radica en que es uno de los pocos trabajos en México en el cual se estudian paleosuelos precuaternarios para hacer inferencias climáticas. Abstract Paleosols and sedimentological studies are useful tools in the reconstruction of past environments; they are also used as stratigraphic markers for correlation in small areas or for determining the rate of deposition. The oldest known paleosols are from the Precambrian, however the most studied are those from the Quaternary. The purpose of this work is to give a brief review of the uses and applications of paleosols, with a special emphasis on pedogenic calcretes, concluding with a case study in the Barranca Los Bonetes, Michoacán. Barranca Los Bonetes, in the south-center of Mexico, bears a volcaniclastic succession of Late Cretaceous age and the oldest hadrosaurs known in Mexico. The volcaniclastic succession contains pedogenic calcretes, identified as such by macro-and microstructural characteristics such as roots, and alpha and beta textures. Based on the environmental requirements for the formation and accumulation of calcareous paleosols, we infer an arid or semi-arid climate in the study area during the periods of non-deposition during the Cretaceous. This study is, to date, one of the few studies in Mexico in which pre-Quaternary paleosols have been used as a tool for reconstructing past environments.

Cultural and faunal remains from archaeological sites of the Rio de La Plata littoral, Argentina, have been studied, attempting to verify their primary associations and occupations during the Holocene. the sites were localized within shell beach ridges more or less parallel to the present shore and are composed of bivalves, gastropods, scarce barnacles, and sandy sediments. the thanatocoenosis found could be equivalent to a transitional area between fluviomarine (mixohaline) and oceanic (polyeuhaline) gradients, i.e., an environment with greater salinity conditions than that of the adjacent coast in the present estuary. These deposits correspond to the Las Escobas Formation (Holocene).Archaeological remains were found from the base of the shell ridge up into the overlying sediments. the hypothesis proposed here is that the human occupation is not contemporaneous with the formation of the shell ridge. the cultural sequence in this kind of site can be altered by disturbances of the stratigraphy as a consequence of environmental and geological events.

Se presenta un estudio de la variación craneofacial en muestras del noroccidente de Suramérica, distribuidas ampliamente en un sentido espacial y temporal, con el objetivo de proponer hipótesis sobre el poblamiento inicial. Se llevaron a cabo diferentes análisis (matriz-R, correlación de matrices y morfometría geométrica) para estudiar la historia y la estructura poblacional a nivel regional. Los resultados sugieren una alta diversidad morfológica, así como la existencia de patrones de estructuración espacial y temporal; también indican que ciertos eventos poblacionales y la acción de fuerzas evolutivas afectaron notablemente la diversidad craneofacial a través del tiempo. A partir de la comparación de distintos tipos de evidencia se propone un modelo sobre la forma y el tiempo del poblamiento temprano de la región y la subsecuente evolución poblacional a lo largo del Holoceno.