Tracking the 10Be-26Al source-area signal in sediment-routing systems of arid central Australia

Sediment-routing systems continuously transfer information and mass from eroding source areas to depositional sinks. Understanding how these systems alter environmental signals is critical when it comes to inferring source-area properties from the sedimentary record. We measure cosmogenic 10Be and 26Al along three large sediment-routing systems ( ∼  100 000 km2) in central Australia with the aim of tracking downstream variations in 10Be–26Al inventories and identifying the factors responsible for these variations. By comparing 56 new cosmogenic 10Be and 26Al measurements in stream sediments with matching data (n =  55) from source areas, we show that 10Be–26Al inventories in hillslope bedrock and soils set the benchmark for relative downstream modifications. Lithology is the primary determinant of erosion-rate variations in source areas and despite sediment mixing over hundreds of kilometres downstream, a distinct lithological signal is retained. Post-orogenic ranges yield catchment erosion rates of  ∼  6–11 m Myr−1 and silcrete-dominant areas erode as slow as  ∼  0.2 m Myr−1. 10Be–26Al inventories in stream sediments indicate that cumulative-burial terms increase downstream to mostly  ∼  400–800 kyr and up to  ∼  1.1 Myr. The magnitude of the burial signal correlates with increasing sediment cover downstream and reflects assimilation from storages with long exposure histories, such as alluvial fans, desert pavements, alluvial plains, and aeolian dunes. We propose that the tendency for large alluvial rivers to mask their 10Be–26Al source-area signal differs according to geomorphic setting. Signal preservation is favoured by (i) high sediment supply rates, (ii) high mean runoff, and (iii) a thick sedimentary basin pile. Conversely, signal masking prevails in landscapes of (i) low sediment supply and (ii) juxtaposition of sediment storages with notably different exposure histories.Financial support was provided by an Australian Research Council grant (DP130104023) to Gerald Nanson and John D. Jansen, by a GeoQuEST Research Centre grant to John D. Jansen and Alexandru T. Codilean, a Marie Skłodowska-Curie Fellowship to John D. Jansen, and by the Centre for Accelerator Science at ANSTO through the National Collaborative Research Infrastructure Strategy. Martin Struck received an International Postgraduate Tuition Award provided by UOW and a matching scholarship funded by UOW and ANSTO

Cosmogenic 21Ne and 10Be reveal a more than 2 Ma alluvial fan flanking the Cape Mountains, South Africa

Cosmogenic 21Ne and 10Be analyses across a subhorizontal fossil alluvial fan flanking the Cape Mountains near Laingsburg (Western Cape, South Africa) indicate a formation age for this fan of at least 2 Ma. Maximum erosion rates obtained from samples of quartzite ridges protruding through the fan alluvium deposits are very low (<~0.4 m/Ma) and analytically indistinguishable from those derived from pebbles from the fan surface (<~1.5 m/Ma). Our results are consistent with the low denudation rates obtained from other cosmogenic nuclide analyses and predicted by apatite fission-track and offshore sedimentation analyses for southern Africa during the Cenozoic. They integrate already existing substantial evidence to show that relative tectonic stability has prevailed across southern Africa throughout this period, and that the present-day large-scale topography is largely inherited from the Cretaceous and has been only minimally resculptured by denudation during the Cenozoic

Cosmogenic \u3csup\u3e21\u3c/sup\u3eNe and \u3csup\u3e10\u3c/sup\u3eBe reveal a more than 2 Ma alluvial fan flanking the Cape Mountains, South Africa

Cosmogenic 21Ne and 10Be analyses across a subhorizontal fossil alluvial fan flanking the Cape Mountains near Laingsburg (Western Cape, South Africa) indicate a formation age for this fan of at least 2 Ma. Maximum erosion rates obtained from samples of quartzite ridges protruding through the fan alluvium deposits are very low (\u3c~0.4 m/Ma) and analytically indistinguishable from those derived from pebbles from the fan surface (\u3c~1.5 m/Ma). Our results are consistent with the low denudation rates obtained from other cosmogenic nuclide analyses and predicted by apatite fission-track and offshore sedimentation analyses for southern Africa during the Cenozoic. They integrate already existing substantial evidence to show that relative tectonic stability has prevailed across southern Africa throughout this period, and that the present-day large-scale topography is largely inherited from the Cretaceous and has been only minimally resculptured by denudation during the Cenozoic

Million-year lag times in a post-orogenic sediment conveyor

Understanding how sediment transport and storage will delay, attenuate, and even erase the erosional signal of tectonic and climatic forcings has bearing on our ability to read and interpret the geologic record effectively. Here, we estimate sediment transit times in Australia’s largest river system, the Murray-Darling basin, by measuring downstream changes in cosmogenic 26Al/10Be/14C ratios in modern river sediment. Results show that the sediments have experienced multiple episodes of burial and reexposure, with cumulative lag times exceeding 1 Ma in the downstream reaches of the Murray and Darling rivers. Combined with low sediment supply rates and old sediment blanketing the landscape, we posit that sediment recycling in the Murray-Darling is an important and ongoing process that will substantially delay and alter signals of external environmental forcing transmitted from the sediment’s hinterland.ISSN:2375-254

Rapid ice sheet response to deglacial and Holocene paleoenvironmental changes in eastern Prydz Bay, East Antarctica

Prydz Bay lies at the terminus of one of East Antarctica's largest glacial systems and is a key region for understanding the response of the ice sheet to past and future environmental changes. In this study, we explore the dynamics and paleo-geometry of the ice sheet in eastern Prydz Bay, using a combination of bathymetric features on the seafloor to delineate past flow patterns, and cosmogenic nuclide dating of glacial deposits on land to constrain ice sheet terminus chronologies. Large streamlined bedforms on the sea floor record the existence of a primary ice stream in Svenner Channel, collecting ice from multiple tributary ice streams and discharging into the main trunk stream of the Lambert Glacier-Amery Ice Shelf system in Prydz Channel. The location and orientation of the north-eastern tributary to this ice stream also provide evidence for a substantial independent ice dome on the outer shelf at Four Ladies Bank. Exposure of ice-free areas at outer Rauer Group and Vestfold Hills indicates that grounding line retreat across eastern Prydz Bay was largely complete by ∼14 ka BP, and the ice margin had retreated to within ∼1 km of its present position by ∼10 ka BP. Onshore moraines record periods of ice margin retreat during the middle (∼6 ka BP) and very late (∼0.5 ka BP) Holocene coinciding with local warm periods. Subsidence recorded in modern Global Navigation Satellite System (GNSS) observations suggests the regional ice sheet was smaller than at present in the period between the middle and very late Holocene advances. The unusually dynamic ice sheet behaviour in this area is attributed to the smooth, reverse-slope bed characteristics of eastern Prydz Bay, which enabled rapid retreat of the ice sheet margin several hundred kilometres during deglacial events

Surface process models and the links between tectonics and topography

Advances in the theoretical understanding of large-scale tectonic and surface processes, along with a rapid growth of computing technology, have stimulated interest in the use of numerical surface process models (SPMs) of long-term landscape evolution, especially in relation to the links between tectonics and topography. Because of these advances and possibilities and because SPMs continue to play an important part in recent geological, geomorphological, thermochronological and other geosciences research, the models warrant review and assessment. This review summarizes and evaluates the important issues concerning SPMs of long-term landscape evolution that have been addressed only in a passing way by previous authors. The issues reviewed here are: (1) the formulation of the ‘laws’ that represent fluvial and hillslope processes in SPMs; (2) the implementation of the various algorithms on numerical grids; (3) model parameterization and calibration; and (4) model testing