Sandbox:Creating and analysing synthetic sediment sections with R

Past environmental information is typically inferred from proxy data contained in accretionary sediments. The validity of proxy data and analysis workflows are usually assumed implicitly, with systematic tests and uncertainty estimates restricted to modern analogue studies or reduced-complexity case studies. However, a more generic and consistent approach to exploring the validity and variability of proxy functions would be to translate a sediment section into a model scenario: a “virtual twin”. Here, we introduce a conceptual framework and numerical tool set that allows the definition and analysis of synthetic sediment sections. The R package sandbox describes arbitrary stratigraphically consistent deposits by depth-dependent rules and grain-specific parameters, allowing full scalability and flexibility. Virtual samples can be taken, resulting in discrete grain mixtures with defined parameters. These samples can be virtually prepared and analysed, for example, to test hypotheses. We illustrate the concept of sandbox, explain how a sediment section can be mapped into the model and explore geochronological research questions related to the effects of sample geometry and grain-size-specific age inheritance. We summarise further application scenarios of the model framework, relevant for but not restricted to the broader geochronological community.CREDit - Chronological REference Datasets and Sites (CREDit) towards improved accuracy and precision in luminescence-based chronologie

Tectonic and climatic forcing on the Panj river system during the Quaternary

Abstract HKT-ISTP 2013 B

Late Quaternary accretion and decline of syngenetic ice-rich permafrost

The region of perennially frozen ground constitutes one quarter of the northern hemisphere landmass. Negative annual mean air temperatures and ground freezing periods exceeding ground thaw periods are the prerequisites for downward freezing of loose deposits and bedrock in non-glaciated regions. Hence, permafrost distribution and thickness on Earth are closely related to late Quaternary climate variations and ecosystem modifications. Generally, glacial stages are expected to promote permafrost accretion and ground ice formation in accumulating sediments,whereas interglacial stages lead to intense permafrost thaw and ground-ice melt. Deep freezing synchronous with ongoing sedimentation is termed as syngenetic while epigenetic freezing occurs in pre-existing deposits. Typical landforms of syngenetic permafrost are ice-wedge polygons of past tundra environments. Ice-rich silty and/or peaty deposits intersected by large ice wedges (up to several decameters in height and meters in with) build-up unique Ice Complex (IC) strata, which are aligned to mid- and late Pleistocene stadial and interstadial stages. The most prominent example for such formations is the Yedoma IC of MIS 3 interstadial age. Increasing air and ground temperatures during warm stages disturbed the thermal equilibrium at the upper permafrost boundary and subsequently led to permafrost thaw, ground-ice melt and surface subsidence. Typical permafrost degradation processes are thermokarst and thermo-erosion that result in large lake-filled basins (up to kilometers in diameter) and valley structures, respectively. The modern periglacial surface in Alaskan and East Siberian lowlands preserves Yedoma IC remnants in uplands and hills next to widely-distributed thermokarst basins since lateglacial and Holocene warming affected up to 70% of the original IC distribution on an area of more than 1,000,000 km2. The overarching climate-driven pattern of cold-stage IC permafrost accretion and warm-stage IC permafrost degradation provides, however, only a first-order approximation in understanding past permafrost dynamics. Beside long-term freezing conditions also thin snow cover and winter precipitation were required to create ice-rich permafrost such as Yedoma IC. Its dynamics are furthermore altered by on-site conditions in water supply, relief and vegetation, which promote either aggradation or degradation processes. For example, current climate warming certainly enables large-scale permafrost thaw and widespread thermokarst. But, ice-wedge growth and permafrost accretion occurs in places after local disturbance such as thermokarst lake drainage causing a change from lacustrine to palustrine environments. Repeated occupation of thermokarst basins by lakes is commonly described as thaw-lake cycle although hereditary structures, i.e. pre-existing basins promote the lacustrine refill and highlight the path-dependence of thermokarst processes and the importance of the paleo-relief. Traces of periglacial landforms preserved in permafrost deposits are indicative of the interplay between past climate and landscape settings. Besides climate control on-site periglacial morphology, hydrology and vegetation alter permafrost regimes, and are to be taken into account when interpreting late Quaternary permafrost chronologies. In summary, the completeness of certain vertical permafrost sequences depends (1) on paleo-relief that defined past accumulation and (thermo-)erosion areas, and (2) on overprints of degradation periods that erased older formations

Late Quaternary accretion and decline of syngenetic ice-rich permafrost

The region of perennially frozen ground constitutes one quarter of the northern hemisphere landmass. Negative annual mean air temperatures and ground freezing periods exceeding ground thaw periods are the prerequisites for downward freezing of loose deposits and bedrock in non-glaciated regions. Hence, permafrost distribution and thickness on Earth are closely related to late Quaternary climate variations and ecosystem modifications. Generally, glacial stages are expected to promote permafrost accretion and ground ice formation in accumulating sediments,whereas interglacial stages lead to intense permafrost thaw and ground-ice melt. Deep freezing synchronous with ongoing sedimentation is termed as syngenetic while epigenetic freezing occurs in pre-existing deposits. Typical landforms of syngenetic permafrost are ice-wedge polygons of past tundra environments. Ice-rich silty and/or peaty deposits intersected by large ice wedges (up to several decameters in height and meters in with) build-up unique Ice Complex (IC) strata, which are aligned to mid- and late Pleistocene stadial and interstadial stages. The most prominent example for such formations is the Yedoma IC of MIS 3 interstadial age. Increasing air and ground temperatures during warm stages disturbed the thermal equilibrium at the upper permafrost boundary and subsequently led to permafrost thaw, ground-ice melt and surface subsidence. Typical permafrost degradation processes are thermokarst and thermo-erosion that result in large lake-filled basins (up to kilometers in diameter) and valley structures, respectively. The modern periglacial surface in Alaskan and East Siberian lowlands preserves Yedoma IC remnants in uplands and hills next to widely-distributed thermokarst basins since lateglacial and Holocene warming affected up to 70% of the original IC distribution on an area of more than 1,000,000 km2. The overarching climate-driven pattern of cold-stage IC permafrost accretion and warm-stage IC permafrost degradation provides, however, only a first-order approximation in understanding past permafrost dynamics. Beside long-term freezing conditions also thin snow cover and winter precipitation were required to create ice-rich permafrost such as Yedoma IC. Its dynamics are furthermore altered by on-site conditions in water supply, relief and vegetation, which promote either aggradation or degradation processes. For example, current climate warming certainly enables large-scale permafrost thaw and widespread thermokarst. But, ice-wedge growth and permafrost accretion occurs in places after local disturbance such as thermokarst lake drainage causing a change from lacustrine to palustrine environments. Repeated occupation of thermokarst basins by lakes is commonly described as thaw-lake cycle although hereditary structures, i.e. pre-existing basins promote the lacustrine refill and highlight the path-dependence of thermokarst processes and the importance of the paleo-relief. Traces of periglacial landforms preserved in permafrost deposits are indicative of the interplay between past climate and landscape settings. Besides climate control on-site periglacial morphology, hydrology and vegetation alter permafrost regimes, and are to be taken into account when interpreting late Quaternary permafrost chronologies. In summary, the completeness of certain vertical permafrost sequences depends (1) on paleo-relief that defined past accumulation and (thermo-)erosion areas, and (2) on overprints of degradation periods that erased older formations

Late Quaternary records from the Chatanika River valley near Fairbanks (Alaska).

Perennially-frozen deposits are considered as excellent paleoenvironmental archives similar to lacustrine, deep marine, and glacier records because of the long-term and good preservation of fossil records under stable permafrost conditions. A permafrost tunnel in the Vault Creek Valley (Chatanika River Valley, near Fairbanks) exposes a sequence of frozen deposits and ground ice that provides a comprehensive set of proxies to reconstruct the late Quaternary environmental history of Interior Alaska. The multi-proxy approach includes different dating techniques (radiocarbon-accelerator mass spectrometry [AMS 14C], optically stimulated luminescence [OSL], thorium/uranium radioisotope disequilibria [230Th/U]), as well as methods of sedimentology, paleoecology, hydrochemistry, and stable isotope geochemistry of ground ice. The studied sequence consists of 36-m-thick late Quaternary deposits above schistose bedrock. Main portions of the sequence accumulated during the early and middle Wisconsin periods. The lowermost unit A consists of about 9-m-thick ice-bonded fluvial gravels with sand and peat lenses. A late Sangamon (MIS 5a) age of unit A is assumed. Spruce forest with birch, larch, and some shrubby alder dominated the vegetation. High presence of Sphagnum spores and Cyperaceae pollen points to mires in the Vault Creek Valley. The overlying unit B consists of 10-m-thick alternating fluvial gravels, loess-like silt, and sand layers, penetrated by small ice wedges. OSL dates support a stadial early Wisconsin (MIS 4) age of unit B. Pollen and plant macrofossil data point to spruce forests with some birch interspersed with wetlands around the site. The following unit C is composed of 15-m-thick ice-rich loess-like and organic-rich silt with fossil bones and large ice wedges. Unit C formed during the interstadial mid-Wisconsin (MIS 3) and stadial late Wisconsin (MIS 2) as indicated by radiocarbon ages. Post-depositional slope processes significantly deformed both, ground ice and sediments of unit C. Pollen data show that spruce forests and wetlands dominated the area. The macrofossil remains of Picea, Larix, and Alnus incana ssp. tenuifolia also prove the existence of boreal coniferous forests during the mid-Wisconsin interstadial, which were replaced by treeless tundra-steppe vegetation during the late Wisconsin stadial. Unit C is discordantly overlain by the 2-m-thick late Holocene deposits of unit D. The pollen record of unit D indicates boreal forest vegetation similar to the modern one. The permafrost record from the Vault Creek tunnel reflects more than 90 ka of periglacial landscape dynamics triggered by fluvial and eolian accumulation, and formation of ice-wedge polygons and post-depositional deformation by slope processes. The record represents a typical Wisconsin valley-bottom facies in Central Alaska

Ice Complex chronologies and environments in western Beringia

Polygon tundra with tundra-steppe vegetation cover and growing syngenetic ice-wedge nets evolved during stadial and interstadial periods of the late Quaternary in non-glaciated Beringia. The depositional relict of such environments is called Ice Complex (IC; ледовый комплекс [ledovyi kompleks] in Russian) permafrost. The IC archives preserve information of past periglacial and climate landscape conditions of mid- and late Pleistocene Beringian environments. In certain locations of the East Siberian Arctic, IC remnants of different age and extent are known. While using IC deposits as archives of palaeo-landscape and palaeo-environmental dynamics, summer and winter conditions over large time-scales are detectable. Commonly applied summer proxy include palaeontological proxy such as pollen, plant macrofossils, insect fossils and, most prominent, mammal fossils of the Mammoth fauna, while geochemical and stable isotope properties of ground ice allow for insights into freezing and winter conditions. IC chronologies are challenging because the deposition and post-sedimentary preservation of ice-rich permafrost are triggered by palaeo-relief settings and related processes as well as by the intensity of thermokarst. This complicates geochronological interpretations, as representatives of consecutive late Quaternary periods may be found at laterally different positions and altitudes in coastal and riverine exposures. Shifts between permafrost aggradation and degradation over time frequently cause gaps in sequences. Furthermore, numerical dating of IC mainly includes different approaches such as radiocarbon (14C) dating of organic material, infrared and optically-stimulated luminescence (IRSL, OSL) dating on feldspar and quartz grains, radioisotope disequilibria of thorium-230 to uranium-234 (230Th/U) dating of peat, and chlorine-36 to chloride ratios (36Cl/Cl) of ground ice. The application of various geochronologic methods to cover the age intervals of certain IC deposits implies that different permafrost components (organic, mineralic, ice) as well as different geochemical and physical properties have to be employed. At the southern coast of Bol'shoy Lyakhovsky Island at least four distinct IC strata were previously described and dated, which cover among the longest time interval of late Quaternary terrestrial permafrost deposition in East Siberia; starting about 200 kyr ago. With this contribution we seek to present and discuss our current understanding of IC chronologies preserved on the New Siberian Archipelago including MIS2 Yedoma (Sartan) IC, MIS3 Yedoma (Molotkov) IC, MIS5 Buchchagy IC, and MIS7a Yukagir IC. Geocryological and palaeo-environmental proxy data highlight past periglacial landscape and deposition processes to deduce past climate conditions and Beringian palaeo-ecological settings and dynamics

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements