Browsing by Author "Jansen, JD"

Now showing 1 - 14 of 14
  • No Thumbnail Available
    Item
    Climate variability over the last 35,000 years recorded in marine and terrestrial archives in the Australian region: an OZ-INTIMATE compilation
    (Elsevier Science Ltd., 2013-08-15) Reeves, JM; Barrows, TT; Cohen, TJ; Kiem, AS; Bostock, HC; Fitzsimmons, KE; Jansen, JD; Kemp, J; Krause, C; Phipps, SJ; Petherick, LM
    The Australian region spans some 600 of latitude and 500 of longitude and displays considerable regional climate variability both today and during the Late Quaternary. A synthesis of marine and terrestrial climate records, combining findings from the Southern Ocean, temperate, tropical and arid zones, identifies a complex response of climate proxies to a background of changing boundary conditions over the last 35,000 years. Climate drivers include the seasonal timing of insolation, greenhouse gas content of the atmosphere, sea level rise and ocean and atmospheric circulation changes. Our compilation finds few climatic events that could be used to construct a climate event stratigraphy for the entire region, limiting the usefulness of this approach. Instead we have taken a spatial approach, looking to discern the patterns of change across the continent. The data identify the clearest and most synchronous climatic response at the time of the Last Glacial Maximum (LGM) (21 +/- 3 ka), with unambiguous cooling recorded in the ocean, and evidence of glaciation in the highlands of tropical New Guinea, southeast Australia and Tasmania. Many terrestrial records suggest drier conditions, but with the timing of inferred snowmelt, and changes to the rainfall/runoff relationships, driving higher river discharge at the LGM. In contrast, the deglaciation is a time of considerable south-east to north-west variation across the region. Warming was underway in all regions by 17 ka. Post-glacial sea level rise and its associated regional impacts have played an important role in determining the magnitude and timing of climate response in the north-west of the continent in contrast to the southern latitudes. No evidence for cooling during the Younger Dryas chronozone is evident in the region, but the Antarctic cold reversal clearly occurs south of Australia. The Holocene period is a time of considerable climate variability associated with an intense monsoon in the tropics early in the Holocene, giving way to a weakened monsoon and an increasingly El Nino-dominated ENSO to the present. The influence of ENSO is evident throughout the southeast of Australia, but not the southwest. This climate history provides a template from which to assess the regionality of climate events across Australia and make comparisons beyond our region.© 2013, Elsevier Ltd.
  • No Thumbnail Available
    Item
    Continental aridification and the vanishing of Australia's megalakes
    (Geological Society of America, 2011-02) Cohen, TJ; Nanson, GC; Jansen, JD; Jones, BG; Jacobs, Z; Treble, PC; Price, DM; May, JH; Smith, AM; Ayliffe, LK; Hellstrom, JC
    The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle. © 2011, Geological Society of America
  • Thumbnail Image
    Item
    Deglaciation of Fennoscandia
    (Elsevier, 2016-09-01) Stroeven, AP; Hättestrand, C; Kleman, J; Heyman, J; Fabel, D; Fredin, O; Goodfellow, BW; Harbor, JM; Jansen, JD; Olsen, L; Caffee, MW; Fink, D; Lundqvist, J; Rosqvist, GC; Strömberg, B; Jansson, KN
    To provide a new reconstruction of the deglaciation of the Fennoscandian Ice Sheet, in the form of calendar-year time-slices, which are particularly useful for ice sheet modelling, we have compiled and synthesized published geomorphological data for eskers, ice-marginal formations, lineations, marginal meltwater channels, striae, ice-dammed lakes, and geochronological data from radiocarbon, varve, optically-stimulated luminescence, and cosmogenic nuclide dating. This is summarized as a deglaciation map of the Fennoscandian Ice Sheet with isochrons marking every 1000 years between 22 and 13 cal kyr BP and every hundred years between 11.6 and final ice decay after 9.7 cal kyr BP. Deglaciation patterns vary across the Fennoscandian Ice Sheet domain, reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins. For example, the ice sheet margin in the high-precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice-marginal deposits in many fjords and coastal valleys. Retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern Sweden and rapid retreat of the calving ice margin in the Baltic Basin. Our reconstruction is consistent with much of the published research. However, the synthesis of a large amount of existing and new data support refined reconstructions in some areas. For example, the LGM extent of the ice sheet in northwestern Russia was located far east and it occurred at a later time than the rest of the ice sheet, at around 17–15 cal kyr BP. We also propose a slightly different chronology of moraine formation over southern Sweden based on improved correlations of moraine segments using new LiDAR data and tying the timing of moraine formation to Greenland ice core cold stages. Retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet, with the lowest rates on the high-elevation and maritime Norwegian margin. Retreat rates compared to the climatic information provided by the Greenland ice core record show a general correspondence between retreat rate and climatic forcing, although a close match between retreat rate and climate is unlikely because of other controls, such as topography and marine versus terrestrial margins. Overall, the time slice reconstructions of Fennoscandian Ice Sheet deglaciation from 22 to 9.7 cal kyr BP provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the Fennoscandian Ice Sheet, and are an important resource for testing and refining ice sheet models. © 2015 The Authors. Under a Creative Commons license.
  • Thumbnail Image
    Item
    Erosion and the sediment conveyor in central Australia
    (Geological Society of Australia, 2016-02-29) Jansen, JD; Chappell, J; Struck, M; Eccleshall, SV; Fujioka, T; Codilean, AT; Fülöp, RH; Fink, D; Cohen, TJ; Nanson, GC
    Why are the Neogene sedimentary fills across central Australia generally thin and discontinuous? One long-standing explanation is that sluggish tectonism and intensified aridity have combined to suppress rates of erosion and sediment production yielding a landscape crowded with inherited, preMiocene forms. Quantifying rates of sediment production, residence time and transport is possible with numerous methods, but the recent growth of cosmogenic nuclide (CN) analysis has provided unprecedented quantitative insights to rates of landscape evolution. Measurements of in situ produced cosmogenic 10Be and 26Al integrate rates of surface processes over million-year timescales—the last part of the Neogene in which aridity has strengthened across the continental interior. We present a compilation of ~600 published and unpublished 10Be and 26Al measurements from central Australia with a focus on the Neogene Eyre Basin and its periphery. Outlying and inlying bedrock uplands serve as engines of sediment production via erosion of bedrock. Surrounding the bedrock outcrops are vast sediment conveyors of varying efficiency and tempo: hillslopes, pediments, and alluvial fans are interim storage/burial zones for sediment in transit to the network of low-gradient rivers, dunes, and playas towards base level. Interactions between fluvial and aeolian processes are especially pertinent to sediment flux in the Eyre Basin. Major rivers such as the Cooper and Finke traverse dunefields in their lower reaches where quantities of alluvia are recirculated into dunes and vice versa. Tracking the trajectories of sediment from source-to-sink (including aeolian recirculation) remains a major challenge, but is central to unravelling the sedimentary dynamics of central Australia's Neogene basins. Based on the CN compilation we estimate 1) spatially averaged erosion rates at the scale of a hillslope or river catchment; 2) pointbased erosion rates on bedrock surfaces; 3) residence time of sediment in hillslope regolith and alluvial fans; and 4) cumulative burial history of sediments in transit. Catchment-scale erosion rates (n~100) are consistently low (<10 m/Myr) and include some of the lowest rates ever measured (~0.3 m/Myr); however, a small group of catchments in the Flinders Ras yield higher erosion rates (~30–60 m/Myr). Bedrock hillslopes (n~200) tend to erode even slower (<5 m/Myr), with a subset of Flinders Ras sites again being the exception (~10–30 m/Myr) and suggesting the influence of recent tectonism. Several CN depth-profiles measured on hillslopes and alluvial fans indicate sediment residence times >0.5 Myr, and high-resolution sampling along three hillslopes with differing morphology (linear, convex, and concave) reveals major variations in sediment production and transport rates that hint at the long-term evolution. In the rivers, fluvial sediments show a weak tendency to increase cumulative burial history downstream (1–2 Myr), consistent with the expanding accommodation space for storage and burial. Dune sediments sampled in the Simpson and Tirari dunefields (n~16) contain cumulative burial histories (up to 1.5 Myr) similar to that of the intersecting rivers. This points to an intimate mix of fluvial and aeolian processes in areas approaching base level. Curiously, these sediments occur in the lowest part of the continent and contain the longest histories of cumulative burial, yet do not form part of the thickest sedimentary fills in the Eyre Basin.
  • No Thumbnail Available
    Item
    Extensive glaciation in Transbaikalia, Siberia, at the Last Glacial Maximum
    (Elsevier, 2016-01-15) Margold, M; Jansen, JD; Gurinov, AL; Codilean, AT; Fink, D; Preusser, F; Reznichenko, NV; Mifsud, C
    Successively smaller glacial extents have been proposed for continental Eurasia during the stadials of the last glacial period leading up to the Last Glacial Maximum (LGM). At the same time the large mountainous region east of Lake Baikal, Transbaikalia, has remained unexplored in terms of glacial chronology despite clear geomorphological evidence of substantial past glaciations. We have applied cosmogenic 10Be exposure dating and optically stimulated luminescence to establish the first quantitative glacial chronology for this region. Based on eighteen exposure ages from five moraine complexes, we propose that large mountain ice fields existed in the Kodar and Udokan mountains during Oxygen Isotope Stage 2, commensurate with the global LGM. These ice fields fed valley glaciers (>100 km in length) reaching down to the Chara Depression between the Kodar and Udokan mountains and to the valley of the Vitim River northwest of the Kodar Mountains. Two of the investigated moraines date to the Late Glacial, but indications of incomplete exposure among some of the sampled boulders obscure the specific details of the post-LGM glacial history. In addition to the LGM ice fields in the highest mountains of Transbaikalia, we report geomorphological evidence of a much more extensive, ice-cap type glaciation at a time that is yet to be firmly resolved. © 2015 Elsevier Ltd.
  • No Thumbnail Available
    Item
    Geomorphic imprint of dynamic topography and intraplate tectonism in central Australia
    (Copernicus GmbH, 2020-05-04) Jansen, JD; Sandiford, M; Fujioka, T; Cohen, TJ; Struck, M; Anderson, SP; Anderson, RS; Egholm, DL
    The mantle convection accompanying plate motion causes vertical movements of up to a few hundred metres at Earth’s surface over wavelengths of 102–103 km. This dynamic topography appears to come and go at ~ 1–10 Myr timescales in areas that are often well away from plate margins, although its spatial and temporal characteristics are subject to ongoing debate. Since such motions are small and transient, discriminating convective signals from other drivers of relief generation and/or sediment dispersal remains tricky. An outstanding challenge is to detect these elusive, transient undulations from a tell-tale geomorphic imprint preserved in either drainage patterns or the stratigraphic record. In the intra-plate setting of central Australia, a 30 km long sinuous gorge is developed where the major regional drainage, Finke River, dissects a band of low hills. Remarkably, this gorge is intertwined with an abandoned and less deeply incised gorge that forms hanging junctions and shares similar width and sinuosity. This unusual overprinting of the two gorges remains unexplained. With an aim to investigate the history of the intertwined gorges, we measured cosmogenic 10Be and 26Al in fluvial gravels stored in the palaeovalley cutoffs. The gravels are remnants of major alluviation episodes that we surmise result from ongoing vertical motions associated with dynamic topography. We use a Markov chain Monte Carlo-based inversion model to test two hypotheses to explain the nuclide inventory contained within the stored fluvial gravels. In the first case, rapid alluviation and erosion since 1 Ma preserves the nuclide memory of the source area; in the second, the nuclide memory is erased during long-term fluvial storage (> 5 Myr) and is restored during exhumation of the palaeovalley gravel-pile. The two hypotheses are therefore limiting-case scenarios that constrain overall fast versus slow landscape evolution, respectively. Our model results suggest that long-term burial decouples the source-area signal from nuclide abundances measured in the palaeovalley gravels. This casts events into a Miocene timescale. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 Licence.
  • No Thumbnail Available
    Item
    Lake Quaternary mega-lakes fed by the northern and southern river systems of central Australia: varying moisture sources and increased continental aridity
    (Elsevier Science BV, 2012-10-15) Cohen, TJ; Nanson, GC; Jansen, JD; Jones, BG; Jacobs, Z; Larsen, JR; May, JH; Treble, PC; Price, DM; Smith, AM
    Optically stimulated and thermoluminescence ages from relict shorelines, along with accelerator mass spectrometer C-14 ages from freshwater molluscs reveal a record of variable moisture sources supplied by northern and southern river systems to Lake Mega-frome in southern central Australia during the late Quaternary. Additional lacustrine, palynological and terrestrial proxies are used to reconstruct a record that extends back to 105 ka, confirming that Lakes Mega-frome and Mega-Eyre were joined to create the largest system of palaeolakes on the Australian continent as recently as 50-47 ka. The palaeohydrological record indicates a progressive shift to more arid conditions, with marked drying after 45 ka. Subsequently, lake Mega-Frome has filled independently at 33-31 ka and at the termination of the Last Glacial Maximum to volumes some 40 times those of today. Further sequentially declining filling episodes (to volumes 25-10 those of today) occurred immediately prior to the Younger Dryas stadial, in the mid Holocene and during the medieval climatic anomaly. Southern hemisphere summer insolation maxima are a poor predictor of palaeolake-filling episodes. An examination of multiple active moisture sources suggests that palaeolake phases were driven independently of insolation and at times by some combination of enhanced Southern Ocean circulation and strengthened tropical moisture sources. © 2012, Elsevier Ltd.
  • No Thumbnail Available
    Item
    Late quaternary palaeoenvironmental change in the Australian drylands
    (Elsevier, 2013-08-15) Fitzsimmons, KE; Cohen, TJ; Hesse, PP; Jansen, JD; Nanson, GC; May, JH; Barrows, TT; Haberlah, D; Hilgers, A; Kelly, T; Larsen, JR; Lomax, J; Treble, PC
    In this paper we synthesise existing palaeoenvironmental data from the arid and semi-arid interior of the Australian continent for the period 40–0 ka. Moisture is the predominant variable controlling environmental change in the arid zone. Landscapes in this region respond more noticeably to changes in precipitation than to temperature. Depending on their location, arid zone records broadly respond to tropical monsoon-influenced climate regimes, the temperate latitude westerly systems, or a combination of both. The timing and extent of relatively arid and humid phases vary across the continent, in particular between the westerly wind-controlled temperate latitudes, and the interior and north which are influenced by tropically sourced precipitation. Relatively humid phases in the Murray-Darling Basin on the semi-arid margins, which were characterised by large rivers most likely fed by snow melt, prevailed from 40 ka to the Last Glacial Maximum (LGM), and from the deglacial to the mid Holocene. By contrast, the Lake Eyre basin in central Australia remained relatively dry throughout the last 40 ka, with lake high stands at Lake Frome around 35–30 ka, and parts of the deglacial period and the mid-Holocene. The LGM was characterised by widespread relative aridity and colder conditions, as evidenced by extensive desert dune activity and dust transport, lake level fall, and reduced but episodic fluvial activity. The climate of the deglacial period was spatially divergent. The southern part of the continent experienced a brief humid phase around ∼17–15 ka, followed by increased dune activity around ∼14–10 ka. This contrasts with the post-LGM persistence of arid conditions in the north, associated with a lapsed monsoon and reflected in lake level lows and reduced fluvial activity, followed by intensification of the monsoon and increasingly effective precipitation from ∼14 ka. Palaeoenvironmental change during the Holocene was also spatially variable. The early to mid-Holocene was, however, generally characterised by moderately humid conditions, demonstrated by lake level rise, source-bordering dune activity, and speleothem growth, persisting at different times across the continent. Increasingly arid conditions developed into the late Holocene, particularly in the central arid zone. © 2012 Elsevier Ltd.
  • Thumbnail Image
    Item
    Long-term waterfall dynamics in monsoonal Australia based on cosmogenic Be-10
    (AMS-13 The Thirteenth International Conference on Accelerator Mass Spectrometry, 2014-08-24) Fujioka, T; May, JH; Fink, D; Nanson, GC; Jansen, JD; Codilean, AT
    Extensive plateaus, arrays of escarpments and a variety of waterfalls are iconic to northern Australia. How old and stable are these features ? Tectonically, northern Australia has been quiescent during the Quaternary. Rainfall is highly seasonal and dominated by the summer monsoon. In this setting, regional landscape dynamics should be strongly afected by uctuations in monsoon and the associated uvial processes. Here, we examine timescales and processes of waterfall evolution in northern Australia. Situated in the Kimberley sandstone plateau, Durack Falls comprise a series of 1-3 m falls, while Bindoola Fall is a large 15 m fall. Surprise Creek, 100 km south of Darwin, has three 3-5 m waterfalls with deep plunge pools developed at the edge of a quartzite plateau. Over 30 samples were collected from bedrock straths up- and downstream of the waterfalls and on their headwall. Their 10Be exposure ages (assuming zero erosion) reveal contrasting results. While two waterfalls in the Kimberley show relatively young, variable ages (15-110 ka for Durack and 11-57 ka for Bindoola), Surprise Creek indicates old, but uniform ages (94-160 ka). Out-of-channel,undisturbed bedrock exhibits consistently high 10Be equivalent to steady-state erosion rates of 2-5 mm/ka, in agreement with typical bedrock erosion rates observed across Australia. Based on these data, we here present a model to evaluate process and rates of waterfalls formation, and discuss the controlling factors.
  • No Thumbnail Available
    Item
    Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic 10Be
    (Elesevier Science BV, 2013-03-15) Jansen, JD; Nanson, GC; Cohen, TJ; Fujioka, T; Fabel, D; Larsen, JR; Codilean, AT; Price, DM; Bowman, HH; May, JH; Gliganic, LA
    Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia's continental depocentre at Lake Eyre. We apply cosmogenic Be-10 exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4 +/- 6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than similar to 112-121 ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since similar to 270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period similar to 75-55 ka Cooper Ck changed from a bedload-dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline-anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term. © 2013, Elsevier Ltd.
  • No Thumbnail Available
    Item
    Repeated megafloods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years
    (Elsevier, 2018-05-01) Margold, M; Jansen, JD; Codilean, AT; Preusser, F; Gurinov, AL; Fujioka, T; Fink, D
    Cataclysmic outburst floods transformed landscapes and caused abrupt climate change during the last deglaciation. Whether such events have also characterized previous deglaciations is not known. Arctic marine cores hint at megafloods prior to Oxygen Isotope Stage (OIS) 2, but the overprint of successive glaciations means that geomorphological traces of ancient floods remain scarce in Eurasia and North America. Here we present the first well-constrained terrestrial megaflood record to be linked with Arctic archives. Based on cosmogenic-nuclide exposure dating and optically stimulated luminescence dating applied to glacial-lake sediments, a 300-m deep bedrock spillway, and giant eddy-bars > 200-m high, we reconstruct a history of cataclysmic outburst floods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000-years. Three megafloods have reflected the rhythm of Eurasian glaciations, leaving traces that stretch more than 3500 km to the Lena Delta. The first flood was coincident with deglaciation from OIS-4 and the largest meltwater spike in Arctic marine-cores within the past 100,000 years (isotope-event 3.31 at 55.5 ka). The second flood marked the lead up to the local Last Glacial Maximum, and the third flood occurred during the last deglaciation. This final 3000 km3 megaflood stands as one of the largest freshwater floods ever documented, with peak discharge of 4.0–6.5 million m3s−1, mean flow depths of 120–150 m, and average flow velocities up to 21 m s−1. © 2018 Elsevier Ltd.
  • Thumbnail Image
    Item
    Sediment transport dynamics in central Australian low-gradient landscapes quantified with cosmogenic nuclides
    (Science Council of Japan, 2015-07-26) Struck, M; Jansen, JD; Codilean, AT; Fujioka, T; Fink, D; Kotevski, S
    Erosion and sediment routing are key to understanding landscape evolution. In this regard, low-gradient landscapes have remained effectively unstudied in spite of their vast global extent, whereas steep mountain regions have been the focus of most research efforts. Sediment transport and storage is widely thought to occur on much longer timescales in regions of low relief relative to their steep counterparts. Here we apply in situ-produced cosmogenic nuclides to examine the sediment transport and storage history of a low-gradient catchment (Peake River) in arid central Australia. The catchment covers 27,300 km2 with a total relief of 394 m and a mean slope of 32 ± 29 m/km (mean ± 1σ, calculated from 1 arc-sec SRTM). Previous studies in central Australia have focused mainly upon local measurements of landscape lowering and bedrock erosion; however, to better understand the processes shaping these landscapes, we adopt a source-to-sink approach coupling bedrock and hillslope colluvium measurements of cosmogenic nuclides with basin-wide measurements in fluvial sediment. Variation in concentrations and ratios of 10Be and 26Al in sediment provides insights to residence times and burial history as grains are transmitted through the bedrock-hillslope-stream sediment conveyor. Our preliminary results reveal basin-wide erosion rates ranging from 0.2 to 5.8 m/Myr (weighted mean = 0.41 ± 0.03 m/Myr), which are among the lowest rates ever measured. We discuss the sediment dynamics of flat landscapes, emphasizing the contrast with steeper settings. Copyright © 2015, XIX INQUA Congress LOC
  • No Thumbnail Available
    Item
    Soil production and transport on postorogenic desert hillslopes quantified with 10Be and 26Al
    (GeoScienceWorld, 2018-01-02) Struck, M; Jansen, JD; Fujioka, T; Codilean, AT; Fink, D; Egholm, DL; Fülöp, RH; Wilcken, KM; Kotevski, S
    Hillslopes stand at the top of the geomorphic conveyor belt that produces and transports mass throughout landscapes. Quantification of the tempo of hillslope evolution is key to identifying primary sediment production and understanding how surface processes shape topography. We measured cosmogenic 10Be and 26Al on three desert hillslopes in postorogenic central Australia and quantified their soil dynamics and evolution. We found that hillslope morphology is governed by lithological factors, and differing nuclide abundances reflect the main sediment transport processes. Slope wash is widespread, and shrink-swell soil processes drive downslope creep and upward migration of gravels detached from underlying bedrock. We applied Monte Carlo–based inversion modeling to reconstruct soil production and the exhumation histories of stony mantle gravels. Underlying silty soils derive from eolian dust inputs dating to at least 0.2 Ma and possibly more than 1 Ma, in line with intensified aridity. Exposed bedrock erodes at ∼0.2–7 m/m.y., and under soil, it erodes at maximum rates of <0.1 m/m.y. up to 10 m/m.y. Accordingly, particles spend 2–6 m.y. or more in the upper 0.6 m of the bedrock column and an additional ∼0.2–2 m.y. or more within hillslope soils. Such long periods near the surface result in surface particles acquiring inherently low 26Al/10Be ratios. Bedrock erodibility underpins regional variations in erosion rate, and the slow tempo of hillslope evolution is largely independent of base level. This suggests a distinctive top-down evolution among postorogenic hillslopes set by authigenic rates of sediment production, rather than by fluvial incision as in tectonically active settings. © 2021 Geological Society of America
  • Thumbnail Image
    Item
    Tracking the 10Be–26AI source-area signal in sediment-routing systems of arid central Australia
    (European Geosciences Union, 2018-05-07) Struck, M; Jansen, JD; Fujioka, T; Codilean, AT; Fink, D; Fülöp, RH; Wilcken, KM; Price, DM; Kotevski, S; Fifield, LK; Chappell, J
    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. © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 Licence