The history of the southern central Andes, including the world’s second largest plateau and adjacent intermontane basins and ranges of the Eastern Cordillera and the northern Sierras Pampeanas of Argentina and Bolivia, impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the development of orographic barriers, and their influence on erosion and landscape evolution at various timescales. Protracted aridity in the orogen interior has facilitated the creation and maintenance of the Puna-Altiplano plateau. Contraction and range uplift, filling of basins, and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior, which caused the eastward migration of deformation into the foreland and successive aridification. The uplift of the Andean orogen has also had a far-reaching influence on atmospheric and moisture-transport patterns in South America. This is documented by the onset of humid climate conditions on the eastern side of the Andes in late Miocene time, which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen, and changes in tectonic processes in the Andean orogenic wedge.

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Tectonics and Climate of the Southern Central Andes

A detailed record of the early-Earth sulfur (S) cycle is chronicled by the S isotope values of sulfide and sulfate preserved in the rock record. Interpretation of this record rests on our understanding of sulfur cycling in modern systems, experiments, and the resulting isotopic signatures. Very large fractionations in δ34S of ≥70‰ are commonly measured between sulfide and sulfate in modern systems and in ancient sediments. Theoretical calculations suggest that sulfate-reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide, although they have only been demonstrated to generate fractionations up to 48‰. Here we report the first direct determination of 60‰–70‰ fractionations by natural populations of sulfate reducers. These high fractionations occur under the relatively low-sulfate conditions (1.1–2 mM) of meromictic Lago di Cadagno in Switzerland. The major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction, with little evidence for modification by further oxidative sulfur cycling. These observations help us to constrain the evolution of seawater sulfate concentrations.

High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1028&context=geosciencefacpub

Hydrologic variation during the last 170,000 years in the southern hemisphere tropics of South America

A large fraction of the vital groundwater in the Atacama Desert of northern Chile is likely composed of “fossil” or “ancient” reserves that receive little or no recharge in today's hyperarid climate. Here, we present evidence for latest Pleistocene perennial streamflow in canyons from the hyperarid core of the Atacama Desert in northern Chile. Fluvial terraces in the Pampa del Tamarugal (PdT) basin (21°S) contain widespread fossil wood, in situ roots, and well preserved leaf litter deposits indicative of perennial surface flow currently absent in these channels. Nineteen radiocarbon dates on these deposits from four separate drainages within this endorheic basin indicate ages from 16,380 to 13,740 cal yr BP, synchronous with paleolake Tauca on the Bolivian Altiplano and other regional evidence for wetter conditions during the latest Pleistocene. Groundwater-fed riparian ecosystems and associated fluvial deposits abound today in the absence of direct rainfall in northern Atacama canyons with perennial discharge. Our relict riparian ecosystems from the PdT basin are indicative of conditions similar to these northern canyons. Given that discharge was higher than present during this time, we propose that these deposits represent the most important groundwater recharge events of the last 18,000 years. A lesser recharge event occurred during the Holocene, when phreatophytic trees also grew in these drainages between 1,070 and 700 cal yr BP, during the Medieval Climatic Anomaly. Taken together, our evidence lends further support for gradient changes in the equatorial Pacific as a major driver of hydrologic change in the Atacama on both centennial and millennial time scales. 

central Andes
hyperaridity
riparian ecosystems
late Quaternary
fluvial terraces

Perennial stream discharge in the hyperarid Atacama Desert of northern Chile during the latest Pleistocene

The landscape of the hyperarid Atacama Desert in northern Chile records extremely slow change on Earth’s surface Disputed ages for the onset of hyperaridity range from the late Paleogene through the Pleistocene A long-term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata For this setting, the primary proxies for climate state are the mineralogical and chemical composition of soil, which varies across a precipitation gradient, and the landforms and deposits of alluvial fans During the most recent ∼15 million years, five climate-related landscape stages are resolved for the Pampa del Tamarugal sedimentary basin, with each successively younger stage inset lower in the local topography than its predecessor The earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi-arid climate, ca 14–12 Ma The younger four landscape stages generated a composite long-lasting exposure surface Predominantly hyperarid conditions have persisted since ca 12 ± 1 Ma, during which four intervals of arid to semi-arid climate occurred Each wet interval was short lived, a million years or less, whereas some of the hyperarid periods were lengthy, 1–5 my The hyperarid intervals were roughly 11–55 Ma, 45–4 Ma, 36–26 Ma, 22–1 Ma, and repeated intervals during the last 1 my The onset of hyperaridity ca 12 Ma likely reflects the growth of the Andes Mountains above a climate threshold In contrast, sea surface temperature variability likely has controlled Atacama paleoclimate changes since the late Miocene

Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile

Dated subsurface cores, 40 m, 100 m, and 200 m in length, from the Salar de Atacama, Chile, record changes in climate and tectonics over the past 325 ka. These cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change. The same basic facies, efflorescent halite crust and cement (subaerial halite), and chevron halite (saline lake) occur in each core. The dominant facies is efflorescent halite formed in a subaerial environment like that in the Atacama basin today. Thick, well-preserved efflorescent crusts in the Salar de Atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines. Similar pedogenically formed evaporites may be pre served in the geologic record. The N-S trending Salar Fault System has downdropped the eastern block of the Salar de Atacama (core 2002) relative to the western block (cores 2005 and 2031). Relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium-series ages from core 2005. Major faulting along the Salar Fault System occurred between 16.5 and 5.4 ka, with offset of 26.5 m and faulting rates of 2.4 m/kyr. Aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting. Although stratigraphic units in core 2002 shifted down > 30 m over the past 60 ka, the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores. Faulting along the Salar Fault System, despite significant offset, did not alter the basic stratigraphic record. Temporal completeness of the stratigraphic record, within the limits of the age dates and their errors of greater than ± 3 ka, was examined qualitatively by comparing long-term sedimentation rates (0.6-0.9 m/kyr) and shorter-term sedimentation rates (0.3-3.6 m/kyr). The similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of Salar de Atacama sediments, at least at the 10 kyr scale. Although hiatuses, long periods of subaerial exposure, and dissolution of evaporites may have occurred, the main climate fluctuations at Salar de Atacama over the last 100 ka are recorded in all three subsurface cores. Paleoclimate records obtained from such cores are therefore representative of basin-wide climate change and are valid for regional paleoclimate reconstructions.

Matthew C. Hein

Papers

An Assessment of Stratigraphic Completeness in Climate-Sensitive Closed-Basin Lake Sediments: Salar de Atacama, Chile