Dating of Caves by Cosmogenic Nuclides: Method, Possibilities, and the Siebenhengste Example

Kozmični žarki povzročajo nastanek kozmogenih nuklidov na površini zemlje in tik pod njo. Za datiranje jamskih sedimentov sta posebej pomembna 10Be in 26Al, ki nastajata v kremenu. Ko je kamen na površini in izpostavljen kozmičnemu sevanju, nastajata v stalnem razmerju. Ko ga zanese v podzemlje, se tvorba kozmogenih nuklidov ustavi, nadaljuje pa se njihov radioaktivni razpad. Ker 26 Al razpada hitreje, lahko iz razmerja med njima izračunamo čas, ki je potekel odkar je kremen pod zemljo. Metodo lahko uporabimo na sedimentih, ki so pokopani oziroma v jami med 0.1 in 5 milijoni let. Na ta način lahko v idealnih primerih izračunamo hitrost vrezovanja dolin. Z opisano metodo smo določili starost najstarejših delov sistema Siebenhengste v Švici. Najstarejši sediment je star 4.4 ± 0.6 Ma in nakazuje, da je na tem območju potekalo zakrasevanje že v pliocenu.  Cosmic rays produce nuclides at and near the Earthʼs surface. 10Be and 26Al in quartz are of particular interest for dating cave sediments. These two nuclides are produced at the surface at a fixed ratio. If the quartz is carried from the surface into a cave, the sediment is shielded from additional cosmogenic nuclide production, and the inherited 10Be and 26Al decay radioactively. Because 26Al decays more rapidly than 10Be, the ratio of these two nuclides indicates the time since the sediment was washed underground. The burial dating method can be applied to sediments in the age range of approximately 0.1 to 5 Ma. In ideal cases, we get information about valley lowering rates. If the provenance of the sediment is known, averaged erosion rates of the source area can be estimated. The oldest cave phases of the Siebenhengste system, Switzerland, were dated using cosmogenic nuclides. The oldest sediment is 4.4 ± 0.6 Ma and thus indicates Pliocene karstification of the Siebenhengste.   

Frost for the trees: Did climate increase erosion in unglaciated landscapes during the late Pleistocene?

Understanding climatic influences on the rates and mechanisms of landscape erosion is an unresolved problem in Earth science that is important for quantifying soil formation rates, sediment and solute fluxes to oceans, and atmospheric CO2 regulation by silicate weathering. Glaciated landscapes record the erosional legacy of glacial intervals through moraine deposits and U-shaped valleys, whereas more widespread unglaciated hillslopes and rivers lack obvious climate signatures, hampering mechanistic theory for how climate sets fluxes and form. Today, periglacial processes in high-elevation settings promote vigorous bedrock-to-regolith conversion and regolith transport, but the extent to which frost processes shaped vast swaths of low- to moderate-elevation terrain during past climate regimes is not well established. By combining a mechanistic frost weathering model with a regional Last Glacial Maximum (LGM) climate reconstruction derived from a paleo-Earth System Model, paleovegetation data, and a paleoerosion archive, we propose that frost-driven sediment production was pervasive during the LGM in our unglaciated Pacific Northwest study site, coincident with a 2.5 times increase in erosion relative to modern rates. Our findings provide a novel framework to quantify how climate modulates sediment production over glacial-interglacial cycles in mid-latitude unglaciated terrain

Abandonment of Unaweep Canyon (1.4–0.8 Ma), western Colorado: Effects of stream capture and anomalously rapid Pleistocene river incision

Cosmogenic-burial and U-series dating, identification of fluvial terraces and lacustrine deposits, and river-profile reconstructions show that capture of the Gunnison River by the Colorado River and abandonment of Unaweep Canyon occurred between 1.4-0.8 Ma. This event led to a rapid pulse of incision unlike any documented in the Rocky Mountains. Following abandonment of Unaweep Canyon by the ancestral Gunnison River, a wave of incision propagated upvalley rapidly through Mancos Shale at rates of ~90-440 km/Ma. The Gunnison River removed 400-500 km3 of erodible Mancos Shale and incised up to 360 m deep in 0.17-0.76 My (incision rates of ~470-2250 m/Ma). Prior to canyon abandonment, long-term (~11-1 Ma) Gunnison River incision averaged ~100 m/Ma. The wave of incision also caused the subsequent capture of the Bostwick-Shinn Park River by the ancestral Uncompahgre River ca. 0.87-0.64 Ma, at a location ~70 km upvalley from Unaweep Canyon. This event led to similarly rapid (up to ~500 m/Ma) but localized river incision. As regional river incision progressed, the juxtaposition of resistant Precambrian bedrock and erodible Mancos Shale within watersheds favored the development of significant relief between adjacent stream segments, which led to stream piracy. The response of rivers to the abandonment of Unaweep Canyon illustrates how the mode and tempo of long-term fluvial incision are punctuated by short-term geomorphic events such as stream piracy. These short-term events can trigger significant landscape changes, but the effects are more localized relative to regional climatically- or tectonically-driven events. Powered byThis is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the Geological Society of America and can be found at: http://geosphere.gsapubs.org/.Keywords: Stream piracy, Gunnison River, Knickpoint, Fluvial incision, Unaweep Canyo

The first hominin of Europe

The earliest hominin occupation of Europe is one of the most debated topics in palaeoanthropology. However, the purportedly oldest of the Early Pleistocene sites in Eurasia lack precise age control and contain stone tools rather than human fossil remains(1-5). Here we report the discovery of a human mandible associated with an assemblage of Mode 1 lithic tools and faunal remains bearing traces of hominin processing, in stratigraphic level TE9 at the site of the Sima del Elefante, Atapuerca, Spain(6-8). Level TE9 has been dated to the Early Pleistocene ( approximately 1.2 - 1.1 Myr), based on a combination of palaeomagnetism, cosmogenic nuclides and biostratigraphy. The Sima del Elefante site thus emerges as the oldest, most accurately dated record of human occupation in Europe, to our knowledge. The study of the human mandible suggests that the first settlement of Western Europe could be related to an early demographic expansion out of Africa. The new evidence, with previous findings in other Atapuerca sites ( level TD6 from Gran Dolina(9-13)), also suggests that a speciation event occurred in this extreme area of the Eurasian continent during the Early Pleistocene, initiating the hominin lineage represented by the TE9 and TD6 hominins.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62855/1/nature06815.pd

A transient search using combined human and machine classifications

Large modern surveys require efficient review of data in order to find transient sources such as supernovae, and to distinguish such sources from artefacts and noise. Much effort has been put into the development of automatic algorithms, but surveys still rely on human review of targets. This paper presents an integrated system for the identification of supernovae in data from Pan-STARRS1, combining classifications from volunteers participating in a citizen science project with those from a convolutional neural network. The unique aspect of this work is the deployment, in combination, of both human and machine classifications for near real-time discovery in an astronomical project. We show that the combination of the two methods outperforms either one used individually. This result has important implications for the future development of transient searches, especially in the era of Large Synoptic Survey Telescope and other large-throughput surveys

Pliocene−Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic 26Al and 10Be in Mammoth Cave sediments

Cosmogenic 26Al and 10Be in sediments washed into Mammoth Cave, Kentucky, record the history of 3.5 m.y. of water-table position, governed by incision and aggradation of the Green River, a tributary of the Ohio River. Upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2.3–2.4 Ma. A brief surge of river incision ca. 2 Ma was followed by river stability and cave-passage formation at a lower level. Rapid incision through 15 m of bedrock ca. 1.5 Ma was prompted by repositioning of the Ohio River to its present course along an ice-sheet margin. Renewed incision ca. 1.2 Ma and aggradation at 0.7–0.8 Ma correlate with major ice advances in the Ohio River basin. Measurements of 26Al and 10Be also indicate that sandstone-capped uplands have maintained slow erosion rates of 2–7 m/m.y. for the past 3.5 m.y., despite accelerated Pleistocene river incision rates of ∼30 m/m.y

Geomorphic process rates in the central Atacama Desert, Chile: insights from cosmogenic nuclides and implications for the onset of hyperaridity

Water plays a critical role in erosion and sediment transport and this relationship is most evident in the hyperarid Atacama Desert of Northern Chile, a region characterized by erosion rates that fall to near zero and cobbles and boulders with cosmogenic nuclide concentrations indicative of exposure for many millions of years. Cosmogenic nuclide concentrations from the Atacama are used to both determine the age of hyperaridity onset and to place important constraints on rates of geomorphic processes in this uniquely arid environment. Prior determinations of cosmogenic nuclide concentrations from the Atacama Desert focus primarily on exposure ages from boulders/cobbles or erosion rates from bedrock. However, recent determinations of cosmogenic nuclide concentrations from boulders, bedrock, and sediment at the same location suggests that material from diverse sample types have different cosmogenic nuclide concentrations. Therefore, it is critical to determine which concentrations of cosmogenic nuclides are most representative of overall erosion rates from the Atacama.\ud \ud Here, concentrations of cosmogenic nuclides in more than 100 samples across two east-west transects within the central Atacama Desert (22-26°S) of Northern Chile are considered. Concentrations of 10Be and 26Al were measured in samples of bedrock, alluvial sediment, active stream sediment, and boulders within several sub-regions of the Atacama Desert, including: the western and eastern Coastal Cordillera, the inner absolute desert (including the Cerro de los Tetas), the Cordillera Domeyko, and the western flank of the Andes. This data allows detailed comparisons of cosmogenic nuclides concentrations both within diverse sample types at a given site and between major geomorphic sub-regions of the Atacama. The general pattern of cosmogenic nuclide concentrations in hyperarid environments is characterized by concentrations that are higher in boulders, moderately high in bedrock, lower in hillslope sediment, and lowest in channels that flow across the desert. At many locations in the central Atacama, boulders and bedrock have erosion rates as much as an order of magnitude slower than that of finer grained sediment at the same location, a relationship that is attributed to the fact that boulders sit above thick gypcrete soils. The hillslope to basin concentrations of cosmogenic nuclides within each sub-region, along with 26Al/10Be ratios, suggests that concentrations of 10Be in most bedrock and sediments reflects erosion rates. However, in the western Coastal Cordillera 10Be concentrations in sediment also reflects transport time.\ud \ud The overarching trend in this data set is that inferred erosion rates are lower to both the east and the west, corresponding with increases in both precipitation and erosion rates towards both the Andean Flank and in the Coastal Cordillera. This trend is previously noted for the Atacama; however, this large data set allows the first observation of the influence of other variables upon erosion rates. Most notably, another clear influence on erosion rates in the Atacama is slope. In some cases, differences in slope are enough to overcome the influence of aridity. For example, erosion rates on the flanks of the Cordillera Domeyko are faster (>1 m/Ma) than that at the crest (<0.5 m/Ma), despite the fact that the crest of this mountain range receives more moisture. Moreover, erosion rates in boulders are up to an order of magnitude slower than that of finer grained sediment. Taken together, these patterns suggest that in the extreme environment of the central Atacama erosion rates are sensitive not only to direct changes in precipitation but also to variables such as slope and soil cover. The relationship between decreased erosion and either low slope or subaerial exposure in the Atacama is potentially stronger than similar relationships documented elsewhere. These results are part of a growing body of research suggesting that even in extreme environments erosion is a complex process controlled by a multitude of variables, and where erosion rates are strongly limited by one variable, any other variable that acts to increase erosion may also have a significant effect. These new insights invoke reexamination of the global relationship between erosion and precipitation, and it is suggested that precipitation has an increasingly greater impact on erosion rates at lower values, but with some influence beginning at rates of ~1000 mm/year.\ud \ud Understanding process rates in the central Atacama is essential to interpretation of cosmogenic nuclide concentrations as the timing of the onset of hyperaridity. Previous cosmogenic nuclide studies in the Atacama have specifically targeted remnant boulders to determine the age of initial aridification and this new data set shows that the boulders do not have the same cosmogenic nuclide concentration as the sediment on which they rest. Thus, the use of boulder ages or hillslope ages for the purpose of topographic reconstruction can be problematic despite the fact that, broadly, exposure of boulders and cobbles over million-year timescales implies lack of erosion over this same period. Several aspects of these results suggest that the onset of hyperaridity in the Atacama Desert predates the Pliocene. First, a number of ages from boulders predate the Pliocene. Second, new Miocene age constraints from a surface previously identified as Pliocene was shown; the age of this surface previously formed a critical component in arguments for decreased incision rates in the late Pliocene. Third, it is shown that a site that is today very sensitive to sediment fluxes from modern storm events is at least 3 Ma in age. Finally, it is shown that most of the hillslopes in the central Atacama were exposed during the Quaternary and do not have complex exposure history; this indicates that the Atacama is not and probably never has been a frozen or static landscape

Late Cenozoic landscape evolution along the Ailao Shan Shear Zone, SE Tibetan Plateau: evidence from fluvial longitudinal profiles and cosmogenic erosion rates

In tectonically active regions, geomorphic features, such as catchment slopes, terraces, and river profiles can be interpreted in the context of tectonic and climatic forcing; however, distinguishing tectonic impacts from other factors such as pre-existing geologic complexities and climate changes is challenging. We use fluvial longitudinal profiles, catchment slopes, and catchment mean erosion rates derived from in-situ cosmogenic Be and Al to examine the late Cenozoic landscape evolution of the Ailao Shan Shear Zone (ASSZ) in the southeastern Tibetan Plateau. The trunk stream of the Red River, flowing along the eastern side of the shear zone, consists of three sections with distinct channel parameters, separated by knickzones (the Midu, Ejia, and Nansha sections from NW to SE). Tributaries to the Red River within the Ailao Shan Shear Zone in the Ejia and Nansha sections consistently display two channel segments (upper low-gradient and middle steep channel segments); a third set of lower, less steep channel segments are identified only along the tributaries in the Nansha section. Catchment mean erosion rates contrast sharply along strike: ca. 300 m/Myr in the Ejia section and ca. 100 m/Myr in the Nansha section. Collectively, our results provide strong evidence that: 1) two waves of incision induced by pulsed and declining regional uplift are propagating up the Red River shaping the background pattern of river incision; 2) vertical fault displacements, river reorganization and additional factors locally affect river profile morphology. Normalized steepness indices (k), catchment slopes, and knickzone distribution vary systematically along the Ailao Shan Shear Zone, indicating long-wavelength regional surface uplift during plateau growth in the middle-late Miocene, which points to a tectonic model involving crustal thickening and diffuse or continuous deformation in the southeastern margin of the Tibetan Plateau

²⁶Al/¹⁰Be Burial Dating of Xujiayao-Houjiayao Site in Nihewan Basin, Northern China

<div><p>The Xujiayao-Houjiayao site in Nihewan Basin is among the most important Paleolithic sites in China for having provided a rich collection of hominin and mammalian fossils and lithic artifacts. Based on biostratigraphical correlation and exploratory results from a variety of dating methods, the site has been widely accepted as early Upper Pleistocene in time. However, more recent paleomagnetic analyses assigned a much older age of ∼500 ka (thousand years). This paper reports the application of ²⁶Al/¹⁰Be burial dating as an independent check. Two quartz samples from a lower cultural horizon give a weighted mean age of 0.24 ± 0.05 Ma (million years, 1σ). The site is thus younger than 340 ka at 95% confidence, which is at variance with the previous paleomagnetic results. On the other hand, our result suggests an age of older than 140 ka for the site’s lower cultural deposits, which is consistent with recent post-infrared infrared stimulated luminescence (pIR-IRSL) dating at 160–220 ka.</p></div

Isochron Al-26/Be-10 burial dating of Xihoudu: Evidence for the earliest human settlement in northern China

The mainstream school of human evolution posits that Homo erectus was the earliest species to leave Africa at similar to 1.85 million years (Ma) ago. Recent discoveries from the Shangchen loess-palaeosol sequence near the Lantian hominid site in northern China, however, show lithic artifacts up to 2.12 Ma, pre-dating the fossil record of H. erectus. Here we apply the recently established isochron Al-26/Be-10 burial dating, whose reliability has been verified by intercomparison with Ar-40/Ar-39 dating, to two locations at the Palaeolithic site of Xihoudu similar to 100 km east of Shangchen. The results show that the stone tools found within river gravels there are 2.43 +/- 0.06 Ma (1 sigma) old, making them the earliest radioisotopically dated evidence for human occupation in Eurasia. The new date is supported by relative dating of the associated fauna, by consistent simple burial ages of quartz sand from deep cores, and by agreement between the two isochrons. Although the validity of the Xihoudu artifacts has been previously debated because of marked fluvial abrasion, there is clear evidence of intentional flaking. In particular, in addition to two cores with small amounts of cortex, a number of flakes lack original rock cortex, which indicate multiple and sustained removals that do not occur naturally. The great antiquity of the Xihoudu artifacts renders firm support to an earlier "Out of Africa I" that approaches the emergence of the genus Homo and the Oldowan industry. (C) 2020 Elsevier Masson SAS. All rights reserved