| 1. |
- Cohen, Timothy, et al.
(författare)
-
Continental aridification and the vanishing of Australia's mega-lakes
- 2011
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 39, s. 167-170
-
Tidskriftsartikel (refereegranskat)abstract
- 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 fi nal disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridifi cation 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 peri-ods of enhanced tropical and enhanced Southern Ocean sources of water fi lling these lakes at different times during the last full glacial cycle.
|
|
| 2. |
- Abbühl, Luca, et al.
(författare)
-
Erosion rates and mechanisms of knickzone retreat inferred from Be-10 measured across strong climate gradients on the northern and central Andes Western Escarpment
- 2011
-
Ingår i: Earth Surface Processes and Landforms. - : John Wiley & Sons. - 0197-9337 .- 1096-9837. ; 36, s. 1464-1473
-
Tidskriftsartikel (refereegranskat)abstract
- escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). The Western Escarpment straddles strong north-south and east-west precipitation gradients, which we exploit in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. The Western Escarpment reflects an ongoing transient response to major tectonic perturbation in the Late Miocene when surface uplift created a steep ramp separating drainage systems above the growing escarpment from those below. Upstream of this ramp, and now forming the uplifted Meseta/Altiplano (~3000 m a.s.l.), streams remain graded to the Late Miocene base level constituted by a series of Tertiary volcanic-volcanoclastic rocks, Streams below the ramp have responded to the Late Miocene surface uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean. We find that the Tertiary units function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. Upstream of these knickzones, on the Meseta/Altiplano, 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky–1 irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky-1 to 250 mm ky-1 and correlate positively with precipitation rates, but show no significant correlation with hillslope angles or channel steepness. These relationships are explained by the presence of the cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions. In addition, valley width and the extent of dissection both appear to increase with increasing precipitation to the north, consistent with climate-forcing of landscape morphology in the deeply incised terrain beneath the escarpment edge.Since escarpment retreat and the precipitation pattern were established at least in the Miocene, we speculate that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau.
|
|
| 3. |
- Bishop, Paul, et al.
(författare)
-
‘Bottom-up’ bedrock river response to rock uplift: Unravelling the controls of landscape responses to transience.
- 2010
-
Ingår i: <em>British Society for Geomorphology Programme & Abstracts</em>. London, England, Aug 2010..
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Bedrock rivers set the boundary conditions for landscape evolution. Most recent bedrock river research has been in steady-state settings in which rock uplift is matched by landscape lowering driven by bedrock river incision and slope lowering, but more attention is now being paid to bedrock rivers is transient settings (where transience in the fluvial system is triggered by changes in the rate of rock uplift and/or by climatic oscillations). Transient responses in bedrock rivers close to base-level are dominated by ‘bottom-up’ processes. Those processes remain less well understood than the ‘top-down’ processes that are thought to be characteristic of steady state landscapes and are driven by discharges of water and sediment. Key issues in understanding rates of landscape-wide response to transience are (i) rates of knickpoint retreat to transmit a base-level fall signal through the drainage net, and (ii) rates of hillslope response once that base-level fall has passed the foot of a hillslope. Terrestrial cosmogenic nuclide (TCN) data from a transient landscape in southern Spain point to the latter being the rate-limiting control (“fast rivers, slow hillslopes”). In terms of the former, morphometric and TCN data from coastal rivers in Scotland confirm knickpoint retreat in response to glacio-isostatic rebound, whereas TCN data from higher up these rivers, above the reach affected by glacio-isostatic base-level fall, point to more diffusive bedrock channel incision, without knickpoint retreat. Determining why diffusive incision is initiated at a particular locality in those settings is difficult but in at least one case the incision is probably ‘pinned’ on resistant lithologies. A wider and more taxing issue is the relationship between ‘bottom-up’ and ‘top-down’ incisional processes and whether the former must precede, and can evolve into, the latter.
|
|
| 4. |
- Castillo, Miguel, et al.
(författare)
-
Estimating erosion rates on active bedrock channels using in situ produced 10Be: implications for landscape evolution in small transient rivers.
- 2010
-
Ingår i: <em>Geological Society of America Abstract with Programs</em>, 178678. Denver, USA, Oct-Nov 2010..
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We measured concentrations of 10Be in two small bedrock rivers catchments on the Isle of Jura (western Scotland) in order to obtain erosion rates to test models of the responses of small bedrock rivers to base-level fall. The rivers of Jura experienced an abrupt, glacioisostatic base-level fall ca. 13.5ka, triggering upstream-propagating knickpoints. 10Be concentrations were obtained in the channel bed upstream and downstream of the main knickpoint triggered by that base-level fall. The preliminary results indicate that erosion rates are slightly higher downstream of the knickpoints, reflecting incomplete accommodation of the base-level fall by knickpoint retreat, ongoing glacioisostatic uplift and/or knickpoint rotation.
|
|
| 5. |
- Cohen, T. J., et al.
(författare)
-
A pluvial episode identified in arid Australia during the Medieval Climatic Anomaly
- 2012
-
Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 56, s. 167-171
-
Tidskriftsartikel (refereegranskat)abstract
- Optically stimulated luminescence (OSL) ages from a relict shoreline on Lake Callabonna record a major pluvial episode in southern central Australia between 1050 70 and 1100 60 Common Era (CE), within the Medieval Climatic Anomaly (MCA). During this pluvial interval Lake Callabonna filled to 10e12 times the volume of the largest historical filling (1974) and reached maximum depths of 4e5 m, compared to the 0.5e1.0 m achieved today. Until now there has been no direct evidence for the MCA in the arid interior of Australia. A multi-proxy, analogue-based atmospheric circulation reconstruction indicates that the pluvial episode was associated with an anomalous meridional atmospheric circulation pattern over the Southern extratropics, with high sea-level pressure ridges in the central Indian Ocean and Tasman Sea, and a trough extending from the Southern Ocean into central Australia. A major decline in the mobility of the Australian aboriginal hunter-gatherer coincides with this MCA period, in southern central Australia.
|
|
| 6. |
- Cohen, Timothy, et al.
(författare)
-
Late Quaternary mega-lakes fed by the northern and southern river systems of central Australia : varying moisture sources and increased continental aridity
- 2012
-
Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182 .- 1872-616X. ; 356:Special Issue, s. 89-108
-
Tidskriftsartikel (refereegranskat)abstract
- Optically stimulated and thermoluminescence ages from relict shorelines, along with accelerator mass spectrometer 14C 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.
|
|
| 7. |
|
|
| 8. |
- Cohen, Timothy, et al.
(författare)
-
Late Quaternary mega-lakes of central Australia: varying moisture sources and increased continental aridity.
- 2010
-
Ingår i: <em>EOS Transactions, </em><em>American Geophysical Union</em>, PP31B-1624. San Francisco, USA, Dec. 2010.. - Washington DC : American Geophysical Union.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Optically stimulated (OSL) and thermoluminescence (TL) ages from relict shorelines, along with accelerator mass spectrometer (AMS) 14C ages from freshwater molluscs reveal a record of mega-lake phases over the late Quaternary for Lake Mega-Frome, situated in the arid zone of South Australia. Additional lacustrine, palynological and terrestrial proxies are used to reconstruct a record that extends as far back as 105 ka and demonstrates that Mega-Frome was last joined to the adjacent mega-Eyre at 50 - 47 ka forming the largest palaeolake on the Australian continent. This time interval for hydrological decline coincides with the arrival of humans on the Australian continent and the demise of the mega-fauna. Since then, Mega-Frome has filled independently at 33 - 31 ka (Heinrich event 3) and at the termination of the Last Glacial Maximum (LGM) to depths far in excess of those seen today (up to 15 - 20 times modern lake volumes). Further evidence of subsequent lake-filling episodes is recorded immediately prior to the Younger Dryas, the mid Holocene and during the medieval climatic anomaly. We present evidence for multiple moisture sources over this time period with lake phases being driven by either an enhanced Southern Ocean circulation or an enhanced tropical moisture source or a combination of both. We show however, that southern hemisphere summer insolation maxima (monsoon proxy) is a poor predictor for past palaeo-lake filling episodes. This is the first palaeohydrological record for a large area of southern semi-arid Australia indicating a progressive shift to more arid conditions throughout the last glacial cycle
|
|
| 9. |
|
|
| 10. |
- Jansen, John, 1967-
(författare)
-
Contrasting effects of transience and substrate erodibility on downstream variations in bedrock channel geometry and erosional capacity
- 2010
-
Ingår i: <em>Geophysical Research Abstracts</em>, 11, 11770, European Geosciences Union General Assembly, Vienna, Austria, May 2010..
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Channel geometry directly influences the erosional capacity of rivers incising bedrock and so is key to how fluvial erosion drives landscape evolution; however, widely divergent relationships among river incision rate, channel width, and channel slope, have been observed in different settings. Based on a large empirical dataset of channel width and slope (n6000) we identify regression-based scaling relationships in four mixed bedrock-alluvial rivers in western Scotland that are subject to spatial variations in fluvial incision rate and substrate erodibility. We find that stream power per unit area (reach-averaged) increases by up to a factor of 10 downstream of actively retreating bedrock knickpoints, and that channel narrowing is the dominant mechanism for raising erosional capacity under transient conditions. By contrast when it comes to substrate erodibility we find that bedrock channel width is far less sensitive than channel slope. Resistant outcrops (quartzites) steepen bedrock channels by a factor of 1.5 to 6.0 and, in terms of stream power per unit area, channels increase their erosional capacity by a factor of 2.7 to 3.5. Yet only 4 to 13 % of this increase is due to channel narrowing. Channel width scales with discharge as W = 3.32 Qˆ0.33, irrespective of lithology. Our results are consistent with the view that W-Q scaling is similar in all single-thread rivers subject to transport-limited conditions, but for increasingly sediment supply-limited settings such as western Scotland, erosional thresholds at the channel boundary exert a strong control on bedrock channel width.
|
|
