Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity paleotopography in the Grand Canyon region, USA

The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth’s history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (~100 km) and short (tens of kilometers) spatial wavelengths. The spatially variable thermal histories implied by these data are best explained by Proterozoic syndepositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge. The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a “Great Unconformity.”

Erosion of the Himalaya-Karakoram recorded by Indus Fan deposits since the Oligocene

The Cenozoic erosion history of the Himalaya-Karakoram, which is a function of tectonically driven uplift and monsoon climatic evolution in South Asia, remains elusive, especially prior to the Miocene. Here, we present a multiproxy geochemical and thermochronological analysis of the oldest samples available from the Arabian Sea, which we used to investigate the erosion history of the Himalayan and Karakoram orogenic system. The Indus Fan records rapid and sustained erosion of the Himalayan-Karakoram mountains from before 24 Ma (ca. 30) to ca. 16 Ma concurrent with changing provenance from the Indian (Himalayan) and Eurasian plates. Our data, combined with previous studies of younger Indus Fan deposits, indicate that the mid-to-late Cenozoic erosion history of the Himalayan-Karakoram mountains is overall consistent with a vigorous monsoonal climate from the late Oligocene to middle Miocene and with changes in global climate in the late Miocene, whereas erosion and deposition are relatively insensitive to changes in sources and rock erodibility. Although tectonic processes were active throughout, we suggest that the erosional signatures of the Himalayan-Karakoram mountains from the Indus Fan largely preserve a record of climate changes since the Oligocene.

Holocene human-environment interactions and their link to erosion in the Eastern Alps, inferred from sedaDNA

<p>Mountain ecosystems are particularly sensitive to soil erosion and it is critical to better understand how climate and human activities can be linked to erosion in the alpine environment. To explore this link across time, we combine sedimentary ancient DNA (sedaDNA) analyses with erosion proxies to assess the timing and type of animal husbandry in the Eastern Alps and its impact on hill-slope processes and vegetation. With its clear and continuous erosion history, the sediment record contained in Lake Grosssee, Switzerland, is ideally suited to resolve key stages in ecosystem change and anthropogenic land use since the last glaciation. Initial results show that sedaDNA can successfully detect key species in today’s ecosystem and suggest significant changes in mammal and plant diversity across the Holocene. DNA of domestic animals such as cattle and sheep has been the dominant in the past 4,000 years BP, which coincides with frequent and intensive erosion events in this period. Our record promises a detailed history of anthropogenic activities and terrestrial ecosystem change across the Holocene, which will help to determine how erosion, land use, and climate shape alpine ecosystems through time.</p>

Gully erosion in Northeast China -- a case study on history, erosion rates and causes

Mollisols are of major importance for food security worldwide but are increasingly degraded by soil erosion. Mollisols in Northeast China have been converted into agricultural use only recently, but gullies are widely distributed and gully erosion history, rates and causes remained unclear. We chose a study typical village to estimate initiation years and development rates of the gully systems from 1968 to 2018 by using aerial and satellite imagery. The outlet fan deposits of a large gully system were dated by Caesium-137 (137Cs) and artefacts. To verify the results, we collected information from local farmers. Gully volumes were measured by structure-from-motion technique using photos taken from an unmanned aerial vehicle. Our results showed that gully systems had already appeared on the steep slopes and along unpaved roads in 1968 and had become more complex by 2018 despite terracing and afforestation. Based on gully morphology and 137Cs, gully erosion was estimated to have started in the 1950s to 1960s when the original grassland and forest were completely converted into arable land. From 1968 to 2018, the gully density increased from 1.2 to 2.3 km km-2. The gully heads retreated at speeds from 1.5 to 2.5 m yr-1, and the soil loss from gully erosion ranged from 25.7 to 44.7 Mg yr-1 ha-1. These data demonstrate the severity of gully erosion in study region and underline the importance of appropriate countermeasures, such as maintenance of abandoned terraces under reforested land and better design and construction of roads within the arable land.

Antarctic erosion history reconstructed by Terre Adélie moraine geochronology

We report apatite fission-track and 10Be terrestrial cosmogenic nuclide (TCN) dating of 14 moraine boulders originating from inland Terre Adélie, East Antarctica. These data show cooling of the Proterozoic Terre Adélie craton at < ~120°C between 350 and 300 Ma, suggesting > 4 km temperate glacial erosion during the Late Palaeozoic Ice Age, followed by nearly null Mesozoic erosion and low glacial erosion (< 2 km) in the Cenozoic. Based on glacial flux maps, the origin of the boulders may be located ~400 km upstream. Preliminary TCN (10Be) datings of moraine boulders cluster within the last 30 ka. Cosmogenic ages from the Lacroix Nunatak suggest a main deglaciation after the Younger Dryas at c. 10 ka, while those of Cap Prud'homme mostly cluster at 0.6 ka, in agreement with an exhumation of boulders during the Little Ice Age.

Detrital zircon provenance of north Gondwana Palaeozoic sandstones from Saudi Arabia

We present the first comprehensive detrital zircon U–Pb age dataset from Palaeozoic sandstones of Saudi Arabia, which provides new insights into the erosion history of the East African Orogen and sediment recycling in northern Gondwana. Five main age populations are present in varying amounts in the zircon age spectra, with age peaks at ~625 Ma, ~775 Ma, ~980 Ma, ~1840 Ma and ~2480 Ma. Mainly igneous rocks of the Arabian–Nubian Shield are suggested to be the most prominent sources for the Ediacaran to middle Tonian zircon grains. Palaeoproterozoic and Archaean grains may be xenocrystic zircons or they have been recycled from older terrigenous sediment. A primary derivation from Palaeoproterozoic and Archaean basement is also possible, as rocks of such age occur in the vicinity. Approximately 4 % of the detrital zircons show Palaeozoic (340–541 Ma) ages. These grains are likely derived from Palaeozoic post-orogenic and anorogenic igneous rocks of NE Africa and Arabia. A few single grains gave up to Eoarchaean (3.6–4.0 Ga) ages, which are the oldest zircons yet described from Arabia and its vicinity. Their origin, however, is yet unknown. Detrital zircons with U–Pb ages of ~1.0 Ga are present in varying amounts in all of the samples and are a feature of terrigenous sediment belonging to the Gondwana super-fan system with an East African – Arabian zircon province.

Discussing the dating of ferruginous duricrusts: promises from mineralogy of supergene minerals with non-destructive microsampling

&lt;p&gt;Ferruginous duricrusts record a part of the Earth&amp;#8217;s geodynamical and climatic history in tropical area, because they can be formed over a wide geologic period. However, the events and processes related to their formation, transformation and distribution are still obscure. This is mainly due to the complexity arising from their finely divided and polycrystalline nature together with the coexistence of various generations of supergene minerals, such as iron and aluminum oxides, oxyhydroxides or hydroxides&amp;#160;(e.g. goethite, hematite and gibbsite) and kaolinite, even at microscopic scale. Classical mineralogical investigations are often realized using powders samples, which hinders subsequent analyses on the same sample, such as SEM or (U-Th)/He dating. Thus, the aim of this study was to propose a new way to investigate the mineralogy of supergene ferruginous samples on micrometric grains that will be analyzed by (U-Th)/He dating method. Prior to this analysis, we first compare the X-ray diffraction data of grains and small amounts of powders looking to reveal the mineralogical composition of populations of secondary minerals of a ferruginous duricrust by taking into account the heterogeneity of the material. Samples were collected from a ferruginous duricrust with pisolitic structure developed over epiclastic conglomerates and sandstones deposited by alluvial fan and fluvial streams from the Upper Cretaceous at the western Minas Gerais state (Brazil). The geomorphology of the study area is delineated by remnants of paleosurface (up to 1,000 m a.s.l.), which comprises an important record of long-term Brazilian continental history.Macroscopic facies recognized on duricrusts sections were described, which allowed the identification of various populations of secondary minerals. After this detailed description, grains (size &lt; 0.5 mm) were collected and powder samples of each population were prepared by crushing. Overall, the results point out that the grain and powder samples could be used to identify mineralogical composition at fine resolution of secondary minerals from ferruginous duricrusts. In addition, XRD results are similar for both types of sample preparation, however the &lt; 0.5 mm grain samples are more advantageous because they are not destructive and thus allow to get a finer description of the mineralogy of different populations and can subsequently be used for e.g. (U-Th)/He dating, providing critical information for interpreting and discussing the ages of iron oxides.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Grant:&lt;/strong&gt; 19/10708-7; 17/22292-4; 17/20788-2, S&amp;#227;o Paulo Research Foundation (FAPESP)&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Allard, T., Gautheron, C., Riffel, S.B., Balan, E., Soares, B.F., Pinna-Jamme, R., Derycke, A., Morin G., Bueno, G.T., Nascimento, N., 2018. Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia. Chemical Geology 479, 136-150.&lt;/p&gt;&lt;p&gt;Monteiro, H.S., Vasconcelos, P.M.P., Farley, K.A., Spier, C.A., Mello, C.L., 2014. (U-Th)/He geochronology of goethite and the origin and evolution of cangas. Geochim. Cosmochim. Acta&amp;#160; 131, 267&amp;#8211;289.&lt;/p&gt;&lt;p&gt;Vasconcelos,&amp;#160;P.M., Heim,&amp;#160;J.A., Farley,&amp;#160;K.A., Monteiro,&amp;#160;H.S., Waltenberg, K., 2013. &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar and (U&amp;#8211;Th)/He -&amp;#160;&lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He geochronology of landscape evolution and channel irondeposit genesis at LynnPeak, Western, Australia. Geochim. Cosmochim. Acta&amp;#160;117,&amp;#160;283-312.&lt;/p&gt;

Constraints from cosmogenic nuclides on the glaciation and erosion history of Dove Bugt, northeast Greenland

The intricate interplay between subglacial topography and ice-sheet dynamics is key to the evolution of large ice sheets, but in Greenland as elsewhere the effects of long-term glacial history on landscape evolution remain poorly constrained. Here we measure abundances of cosmogenic 10Be and 26Al in bedrock and transported boulders to unveil the glaciation and erosion history of Dove Bugt, northeast Greenland. In agreement with studies of west Greenland, we find that apparent exposure ages increase with elevation from 9 ka to 13 ka in low-lying valleys to 21 ka to 204 ka on high-elevation, blockfield-covered plateaus. We employ a Markov chain Monte Carlo inversion framework to constrain the probability of various erosion histories, and we quantify the residence time of samples within the upper 2 m of the bedrock subsurface—a measure defined as the cosmogenic nuclide memory. This cosmogenic nuclide memory exceeds 600 ka on the highest plateaus but is limited to less than 500 ka in most other high-elevation samples and to less than 100 ka at low-elevations. Our results define maximum limits for the fraction of ice cover during the past 1 Ma to ∼70% on the Store Koldewey peaks and ∼90% farther inland at Pusterdal, respectively. Minimum limits to ice cover, however, cannot be reliably constrained by the data. Finally, we propose that limited erosion on the highest plateaus of Store Koldewey since 0.6–1.0 Ma indicates a minimum age for fjord-plateau formation within this area of northeast Greenland.