45 research outputs found
Neogene fluvial landscape evolution in the hyperarid core of the Atacama Desert
Dating of extensive alluvial fan surfaces and fluvial features in the hyperarid core of the Atacama Desert, Chile, using cosmogenic nuclides provides unrivalled insights about the onset and variability of aridity. The predominantly hyperarid conditions help to preserve the traces of episodic climatic and/or slow tectonic change. Utilizing single clast exposure dating with cosmogenic 10Be and 21Ne, we determine the termination of episodes of enhanced fluvial erosion and deposition occurring at ~19, ~14, ~9.5âMa; large scale fluvial modification of the landscape had ceased by ~2â3âMa. The presence of clasts that record pre-Miocene exposure ages (~28âMa and ~34âMa) require stagnant landscape development during the Oligocene. Our data implies an early onset of (hyper-) aridity in the core region of the Atacama Desert, interrupted by wetter but probably still arid periods. The apparent conflict with interpretation that favour a later onset of (hyper-) aridity can be reconciled when the climatic gradients within the Atacama Desert are considered
Evidence for multiple Plio-Pleistocene lake episodes in the hyperarid Atacama Desert
Cosmogenic nuclide exposure dating of ancient shoreline terraces of the Quillagua-Llamara Soledad Lake in the central Atacama Desert of northern Chile provides new insights in the paleohydrology of the driest desert on Earth. The lake developed in a paleo-endorheic drainage system in the Central Depression prior to draining into the Pacific due to incision of the RĂo Loa canyon. The durations of lake stages were sufficiently long to form wave-erosion induced shoreline terraces on the wind-exposed slopes of former islands. Successively younger shoreline levels are preserved over an elevation range of 250 m due to progressive uplift of the islands coeval with the lake stages. Cosmogenic 10Be- and 21Ne-derived exposure ages of the shorelines reveals that the hyperarid conditions in the RĂo Loa catchment were interspersed by several pluvial stages during the Pliocene and Pleistocene, which generated a large and persistent lake in the Quillagua-Llamara basin. The exposure ages of the final lake stage provide the maximum age for the incision of the RĂo Loa canyon (274 ± 74 ka) and the subsequent breaching of the Coastal Cordillera
The timing of formation of the Douro and Tejo rivers and implications for the evolution of the landscapes of central mainland Portugal
10th International Conference on Geomorphology, Coimbra, Portugal, 12â16 Sep 2022.The formation and development of major rivers limits the overall pace of the surrounding landscape
evolution and drives sediment delivery from source to sink. The timings and rates of river incision
may be a response to external influences, such as tectonic or climate driven base-level changes, or
alternatively they may be linked to the breaching of internal thresholds, for example, drainage
capture events. The Tejo and Douro rivers (also known as Tagus and Duero rivers) each drain a
significant portion of the Iberian Peninsula and much of their courses through Portugal are typified
by v-shaped valleys that are deeply incised into the surrounding topography. Earlier work has dated
fluvial terrace deposits, mostly by luminescence techniques, but also by electron spin resonance and
cosmogenic nuclide exposure dating. This has provided constraints on the late Pleistocene histories
of the Tejo and Douro rivers, however, the timing of their transition from endorheic to exorheic is
not precisely known and whether or not their histories are linked to a common mechanism is
unclear.
This study aims to provide age constraints on the early history of the Tejo and Douro rivers, and to
examine whether and to what degree the erosion rates of low relief, granite etchplain landscapes
within the riverâs catchment areas are responding to the trunk channel incision. We focus on reaches
of the Tejo and Douro rivers located in the eastern sector of mainland Portugal. Samples were
collected for cosmogenic nuclide (10Be and 26Al) surface exposure and burial dating to date upper
fluvial terrace levels. In addition, a combination of cosmogenic nuclide exposure ages and depth
profiles in bedrock outcrops, alongside basin-wide erosion rate determinations will be used constrain
the pace of evolution of nearby granitic landscapes. Preparation of the samples for measurement is
ongoing and we will present our initial findings
The chronology of the Last Glacial Maximum and deglacial events in central Argentine Patagonia
This paper evaluates the chronology of the last glacial cycle and deglaciation in the Lago PueyrredĂłn valley of central Patagonia, 47.5° S, Argentina. The valley was a major outlet of the former Patagonian Ice Sheet and the moraines that record its fluctuations are an important proxy record of climate change in southern South America. Such moraines are well-preserved in the Lago PueyrredĂłn valley owing in part to the semi-arid environment east of the mountain front. Here, we provide the first direct chronology for the age of the âRio Blancoâ moraine system by utilizing cosmogenic-nuclide surface exposure ages. Boulders on the moraines give 10Be exposure ages that indicate the Last Glacial Maximum (LGM) maximum extent occurred by 27â25 ka. Subsequent advances occurred at 23â22 ka, 20â18 ka, and ca. 18â17 ka. Initial deglaciation began after ca. 18â17 ka and was interrupted as evidenced by the Lago Columna moraines up-valley. Subsequently the outlet glaciers occupying both the Lago PueyrredĂłn basin (Chilean name: Lago Cochrane) and the Lago Buenos Aires basin (Chilean name: Lago General Carrera) to the north, rapidly retreated more than 80 km at around 16.5â15 ka. The timing of the LGM maximum extent and the onset of deglaciation occurred broadly synchronously throughout Patagonia. Deglaciation resulted in a series of interconnected glacier-dammed lakes in the region that initially drained toward the Atlantic Ocean and later drained to the Pacific Ocean as a consequence of disintegrating ice in the Andes
Impact of CaSO4-rich soil on Miocene surface preservation and Quaternary sinuous to meandering channel forms in the hyperarid Atacama Desert
The Atacama Desert is the driest and oldest desert on Earth. Despite the abundance evidence for long-term landscape stability, there are subtle signs of localised fluvial erosion and deposition since the onset of hyperaridity in the rock record. In the dry core of the Atacama Desert, pluvial episodes allowed antecedent drainage to incise into uplifting fault scarps, which in turn generated sinuous to meandering channels. Incision of ancient alluvial fan surfaces occurred during intermittent fluvial periods, albeit without signs of surface erosion. Fluvial incision during predominantly hyperarid climate periods is evident from these channels in unconsolidated alluvium. The absence of dense vegetation to provide bank stability and strength led us to investigate the potential role of regionally ubiquitous CaSO4-rich surface cover. This has enabled the preservation of Miocene surfaces and we hypothesize that it provided the required bank stability by adding strength to the upper decimetre to meter of incised alluvium to allow high sinuosity of stream channels to form during pluvial episodes in the Quaternary
Middle pleistocene glaciation in Patagonia dated by cosmogenic-nuclide measurements on outwash gravels
The well-preserved glacial record in Argentine Patagonia offers a ~ 1 Ma archive of terrestrial climate extremes in southern South America. These glacial deposits remain largely undated beyond the range of radiocarbon dating at ca. 40 ka. Dating old glacial deposits (> several 105 a) by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates. Here, we show that cobbles on outwash terraces can reliably date âoldâ glacial deposits in the Lago PueyrredĂłn valley, 47.5° S, Argentina. Favorable environmental conditions (e.g., aridity and strong winds) have enabled continuous surface exposure of cobbles and preservation of outwash terraces. The data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit. 10Be concentrations in outwash cobbles reveal a major glacial advance at ca. 260 ka, concurrent with Marine Isotope Stage 8 (MIS 8) and dust peaks in Antarctic ice cores. A 10Be concentration depth-profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca. 0.5 mm kaâ 1. We compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by ~ 100 ka; thus the oldest boulders in this area do not date closely moraine deposition. The 10Be concentration in moraine cobbles help to constrain moraine degradation rates. These data together with constraints from measured 26Al/10Be ratios suggest that all moraine boulders were likely exhumed after original deposition. We determine the local Last Glacial Maximum (LGM) occurred at ~ 27â25 ka, consistent with the maximum LGM in other parts of Patagonia
A 68âŻka precipitation record from the hyperarid core of the Atacama Desert in northern Chile
[Abstract] The Atacama Desert in northern Chile is one of the driest deserts on Earth. Hyperaridity persists at least since the Miocene and was punctuated by pluvial phases. However, very little is known about the timing, regional spread and intensities of precipitation changes. Here, we present a new precipitation record from a sedimentary sequence recovered in a tectonically blocked endorheic basin that is located in the hyperarid core of the Atacama Desert. The chronostratigraphic framework of the record is given by a multi-disciplinary dating approach, suggesting an age of ca. 68âŻka BP for the core base. The sequence consists of three sediment types, whose sedimentological and geochemical characteristics suggest different depositional processes that reflect different degrees in humidity. First, particularly fine-grained sediments with high clastic but low calcium sulfate and carbonate contents reflect a particularly dry climate with only sporadic precipitation events and fluvial supply via channel systems. Second, more coarse-grained sediments with lower clastic and higher calcium sulfate and carbonate contents reflect more moist conditions with stronger precipitation events that lead to fluvial activity not restricted to the channels but involving the slopes and plains in the catchment. Third, normally graded layers with an equally high proportion of calcium sulfate and carbonate reflect occasional high-precipitation events that caused sediment supply also from most distant parts of the catchment via severe flash floods. The sedimentary succession suggests that precipitation changes took place on orbital but also on millennial time scales. Rather moist periods occurred during most of MIS 2, several shorter periods within MIS 3 and parts of MIS 4. Comparison of the findings from the Huara record with selected climate records from continental and marine sites in South America suggests a strong precipitation heterogeneity across the Atacama. This heterogeneity is caused by pronounced differences in the dominating climate patterns and a shift from predominant summer rain in the north to winter rain in the south. Precipitation supply to the Huara clay plan is controlled by the atmospheric circulation rather than the surface temperature of the adjacent ocean
Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation
Cosmogenic nuclides: principles, concepts and applications in the earth surface sciences
This is the first book to provide a comprehensive and state-of-the-art introduction to the novel and fast-evolving topic of in-situ produced cosmogenic nuclides. It presents an accessible introduction to the theoretical foundations, with explanations of relevant concepts starting at a basic level and building in sophistication. It incorporates, and draws on, methodological discussions and advances achieved within the international CRONUS (Cosmic-Ray Produced Nuclide Systematics) networks. Practical aspects such as sampling, analytical methods and data-interpretation are discussed in detail and an essential sampling checklist is provided. The full range of cosmogenic isotopes is covered and a wide spectrum of in-situ applications are described and illustrated with specific and generic examples of exposure dating, burial dating, erosion and uplift rates and process model verification. Graduate students and experienced practitioners will find this book a vital source of information on the background concepts and practical applications in geomorphology, geography, soil-science, and geology
The Nuts and Bolts of Cosmogenic Nuclide Production
Over the last 60 years, our understanding of how cosmic rays produce cosmogenic nuclides has grown from basic physical considerations. We introduce the different types of cosmic ray particles and how their flux varies with altitude, latitude, and time. Accurately describing these variations remains a challenge for some regions when calculating production rates. We describe current and emerging computational methods for calculating production rates that address this challenge. Continuing developments in our understanding of modern and prehistoric cosmic ray fluxes and energy spectra in Earth's atmosphere and at its surface are bound to contribute in the future to more robust applications