H53E-01 13:40h
The Response of Fluvial Landscapes to Glaciation
A major consequence of climate cooling is the growth of glaciers in mountain ranges previously sculpted by fluvial and hillslope processes. Climate change and the tectonics of mountain ranges are linked if glacial erosion either alters the relief structure, or exhumes material in a different fashion from rivers. Glacial erosion carves cirques and U-shaped valleys, and cooler climates also affect hillslope processes, as freeze-thaw, rockfall, landsliding and debris flows start to dominate. The signature of glacial erosion on the landscape is readily identified from digital elevation model (DEM) analyses, including hypsometry and longitudinal profiles, and comparison with the evolution of fluvial landscapes can be made using a landscape evolution model. These techniques demonstrate that the evolution of glaciated landscapes is not a simple function of regional climate change. In smaller drainage basins in the eastern Sierra Nevada, California, glaciers have generated modest relief, and have incised the valley floor at higher elevations. In larger drainage basins, where accumulation areas are greater and the rainshadow effect is less, glaciers have carved a strikingly different morphology. There is more relief, and valley floor incision occurs at much lower elevations. The Sangre de Cristo Range, Colorado, has evolved similarly, although with pronounced asymmetry, caused by the prevailing winds from the west. Accumulation of wind-blown snow on the eastern side of the range causes much more substantial erosion and deposition of spectacular moraines. In more tectonically active regions, such as the Southern Alps of New Zealand, and the Nanga Parbat region of Pakistan, smaller glacial valley floors steepen in response to rapid rock uplift, whereas larger glaciers maintain shallow gradients despite rapid rock uplift. Hillslope processes are apparently slower than valley floor incision, at least for some period, allowing dramatic relief production and decoupling of valley floor and hillslope processes. Potential causes of the nonlinear response of basins of different sizes to regional climate cooling include the increased longevity and discharge of larger glaciers, and their more complex subglacial hydrology.
H53E-02 13:55h
Evidence for autogenic cyclicity in ephemeral stream cut-fill dynamics
Cut-fill sequences in ephemeral channels are often attributed to climate forcing. Yet it has been proposed that these sequences can also arise from internal dynamics, without the need for an external trigger. This raises the following question: under what conditions, and by what mechanisms, can such autogenic cycles occur? Intrinsic geomorphic thresholds have been cited as important controls of cut-fill cycle initiation in ephemeral streams across the American southwest, but the types of controlling geomorphic thresholds involved, and the mechanisms leading to the threshold-crossing events, are not clear. Here we use a numerical model to identify necessary and sufficient conditions for autocyclic behavior in ephemeral-channel networks. Model simulations of a hypothetical semi-arid drainage basin demonstrate the existence of alternating steady-state epicycles of aggradation and erosion without any prior changes in independent external variables. The two key intrinsic control mechanisms responsible are (1) threshold channel slope angles, and (2) upstream signal propagation following threshold-crossing events. Analysis of the process-response relationships occurring within the catchment reveals the important environmental and basin conditions that promote autocyclic behaviour, and the sorts of channel-hillslope interactions and feedbacks that are an integral part of this dynamic behavior. The results provide process-based evidence for the existence and importance of intrinsic controls on cut-fill epicycles in ephemeral systems. The implications of these findings are discussed in the context of the arroyo problem and the interpretation of field records.
H53E-03 14:10h
Massive Siliciclastic Accumulation on Slopes off Northeast Australia During the Last Sea Level Transgression: Response of an Intensified Monsoon at 10 ka?
The continental margins of southern Papua New Guinea and northeastern Australia collectively form the world's largest extant tropical mixed siliciclastic-carbonate depositional system where rivers supply large amounts of terrigenous sediment to a shelf with substantial carbonate production. Over the last 30 kyr, the flux and composition of sediment shed from these margins to surrounding slopes and basins has changed dramatically. This is not unexpected given the two dominant sediment sources and the large amplitude variations in sea level. Importantly, though, the observed accumulation of siliciclastic material deviates significantly from generic sequence stratigraphic models, at least for slopes off northeast Australia. Recent studies in this region clearly show greatly increased siliciclastic fluxes coincident with late transgression ca. 12-7 ka rather than lowstand ca. 25-18 ka. Two end-member models have been proposed to explain elevated siliciclastic discharge to the slopes during late transgression: (1) sediment was stored behind a subaerially exposed inactive barrier reef tract during lowstand, and released to the slope when the shelf was flooded; (2) sediment was not supplied during arid climates coincident with the glacial world, but intensified precipitation greatly increased river fluxes and siliciclastic supply coincident with transgression. Increased siliciclastic accumulation during late transgression has been identified offshore the Ganges dispersal system, and attributed to intensified monsoons (e.g., Weber et al., 1997; Goodbred and Kuehl, 2000). Combined oxygen isotope measurements of planktic foraminifera tests and alkenone paleotemperature measurements also suggest that intensified monsoons caused anomalously fresh surface water off the Ganges system (Kudrass et al., 2001). This study examines oxygen isotopes and Mg/Ca ratios of G. ruber (white) tests in well-dated cores offshore northeast Australia to see if increased river run-off and surface water freshening also occurred in this region during the last transgression.
H53E-04 14:25h
Unraveling Forcing Factors of Shallow Shelf Stratigraphy With Numerical Modeling
Shallow shelf stratigraphy is controlled by a series of geological boundary conditions; sediment supply, paleo-bathymetry, sea level, oceanic reworking processes, and tectonics. Geological boundary conditions evolve non-linearly over time; sudden changes in the rate of sea level change or abrupt climate changes dictate the stratigraphic evolution. We use numerical modeling to unravel the influence of different forcing factors on the stratigraphy. Our numerical model, SedFlux, is a two-dimensional process-based stratigraphic model. Time-varying sediment fluxes drive floodplain sedimentation and coarse bedload deposition by fluvio-deltaic processes. The suspended sediment carried by the river is dispersed in the ocean through hyper- and hypopycnal plumes. Seafloor sediment is reworked by ocean storms, failures and subsequent transport as sediment gravity flows (turbidity currents or debris flows). SedFlux records the thickness of the deposited sediment and the grain size preserved over time along a longitudinal profile. Simulations of the New Jersey shallow margin over the last 40 ka serve as an example of the unraveling of different forcing factors. Initially, the shelf is exposed and Arctic conditions prevail in the drainage areas, subsequently the high sediment supply from the rivers draining the melting Laurentide Ice Sheet is a dominant impact on the shelf stratigraphy. Over the last 10 ka the shelf becomes sediment-starved and storm-dominated. Shallow seismic data have been used to validate the large-scale deposited thicknesses of the SedFlux `base-case' prediction. It was found that the range in thickness predictions is comparable to the lateral variations reconstructed from the seismic data. In addition, we quantitatively compared SedFlux predictions with a data set of 98 seafloor grab samples taken on the New Jersey shelf between 50 and 150 m water depth. The subtle fining trend towards deeper water is evident from both observed and predicted data. However, SedFlux shows a larger component of fine sand, especially in shallower water. Twenty sensitivity tests quantify the significant impact of uncertainty in the forcing factors. Estimates of meltwater pulses (and grain-size distributions carried by the rivers at those times) and paleo-storm climate were found to have a wide range and strongly impact the predicted stratigraphy. The sensitivity experiments form a tool to quantify the relative effects of the forcing factors. We advocate an approach which couples a standard deviation based on a series of sensitivity experiments to a predicted `base-case' value. In that way the stratigraphic variability caused by ranges in the boundary conditions is evident for later users.
H53E-05 14:40h
Long-Term Glacial Erosion Rates and Pre-Glacial Topography in Southwest British Columbia
The hypothesis that Late Cenozoic climate change increased the topographic relief of mountain ranges relies on the assumption that alpine glaciers are more efficient at eroding valley bottoms than ridge crests. Although theoretical and field studies have made advances in quantifying glacial erosion processes, rigorous tests of this hypothesis have been limited by uncertainties in long-term glacial erosion rates and pre-glacial topographic relief. Here we interpret long-term ($>$10$^{6}$ yr) glacial erosion rates and pre-glacial topographic relief in the southern Coast Mountains, British Columbia, using apatite (U-Th)/He and apatite fission track cooling ages and a thermal-kinematic numerical model. Twenty-six new apatite (U-Th)/He samples were collected along two 60 km long transects that cross the glacially sculpted topography of Mount Waddington. Samples were collected between elevations of 0 and 4000 m, with a subset of samples along each transect collected at a constant elevation of 1600 m. Apatite (U-Th)/He ages range between 1.5 and 14.1 Myr. Two patterns are present in the spatial distribution of ages. First, ages generally increase in elevation with some key exceptions due to glacial erosion discussed later. Second, samples collected on the 1600 m contour on each transect vary systematically between ~1.5-8 Myr BP and are youngest near the high topography but are noticeably shifted to the west of the high topography of Mount Waddington. We interpreted the apatite (U-Th)/He ages using a coupled 3D thermal-kinematic model. The model was used to: (1) predict apatite He sample ages across the present-day topography for variations in model parameters, (2) identify the best-fit model simulations with a statistical comparison, (3) calculate the difference in ages (age anomaly) between model predicted and measured ages, and (4) calculate the change in topography that could produce the observed age anomaly. In regions like the Coast Mountains where glacial erosion is pervasive and sample ages are younger than the onset of glaciation then the calculated change in topography (step 4) is a proxy for the magnitude of glacial erosion. Calculated glacial erosion rates across two 60 km long transects range between ~0.4 and 0.8 mm/yr. Calculated preglacial topographic relief suggests massive valley widening resulted in a 16 km migration of the peak elevation. Topographic relief increased by +1600 to +2200 m along one transect and -200 to +900 m along the other. We find that glacial erosion magnitudes are variable over 60 km distance and that long-term glacial erosion rates are an order of magnitude lower than short-term ($<$10$^{4}$ yr) rates.
H53E-06 INVITED 14:55h
Effects of Climate on Long-term Rates of Physical Erosion and Chemical Weathering: Evidence from Cosmogenic Nuclides and Geochemical Mass Balance
Cosmogenic nuclides such as $^{10}$Be and $^{26}$Al have recently become important tools for measuring long-term denudation rates. We have recently shown how cosmogenic nuclide measurements of denudation fluxes can be partitioned into their physical and chemical components, using the enrichment of insoluble tracers in regolith relative to its parent rock. We used these methods to measure long-term rates of physical erosion and chemical weathering for 42 sites, encompassing widely varying climates and denudation rates. Across these sites, mean annual temperatures vary from 2 to 25 $\deg$C, average annual precipitation spans a 20-fold range (from 22 to 420 cm/yr), and denudation rates vary by 32-fold (from 23 to 755 t km$^{-2}$ yr$^{-2}$). Our measurements show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. The relative importance of chemical weathering can be quantified using the "Weathering Intensity Factor" (WIF), the ratio of the chemical weathering rate to the physical erosion rate. Over 60 percent of the variance in WIF's can be explained by a simple Arrhenius-like relationship based on mean annual temperature and average annual precipitation. The temperature-dependence of WIF is roughly half of what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of short-term average weathering rates from the field. Our results imply that the strength of climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated granitic terrain similar to our study sites.
H53E-07 INVITED 15:10h
Climate, weathering, transport and sedimentary record during Himalayan erosion
The Himalayan system is one where climate and tectonic both play dominant controls on erosion and sedimentary fluxes. Deciphering climatic changes from the sedimentary record are however not straightforward and the study of the modern erosion system is the main source of interpretation. The comparison of the Brahmaputra and Ganga rivers is very interesting as they reflect very contrasted climatic conditions. Sediments of the Brahmaputra show apparent signals of lower weathering than those of the Ganga. Clay assemblages of the Brahmaputra contain no smectite and some kaolinite whereas those of the Ganga are rich in those secondary phases. Similarly, chemical indexes of alteration are lower for the Brahmaputra than for the Ganga. Such weathering indices tend to suggest that weathering hence climate is more active and humid over the Gangetic basin than the Brahmaputra. However, this led to a complete misinterpretation of the reality. The monsoon is much more intense over Eastern than Western Himalaya as shown for instance by a much higher runoff for the Brahmaputra than the Ganga. Based on modern dissolved chemistry of both rivers, chemical erosion of silicates and carbonates of the Brahmaputra is about double of that of the Ganga. It is important to take into account his apparent contradiction between river sediment and dissolved element budget when interpreting the sedimentary record in term of climate conditions. Because Himalaya is a system where physical erosion is by far the dominant process particularly on it's eastern side, the sediments of the Brahmaputra appears less matured than those of the Ganga. For the Ganga, lower precipitation level induces lower physical erosion flux and longer residence time in the floodplain; then relatively arid condition in the floodplain generates smectite. On the contrary, Brahmaputra climate favours fast transport of particles and reduced processing in the floodplain. Sediment transport processes generate a second source of uncertainty. Mineralogical sorting due to the transport dynamics induces geochemical differences. Finer grain sediments appear with higher level of weathering than coarser fractions of the same sedimentary flux. We report this in the river system as well as in the sedimentary record.
H53E-08 15:25h
Marine Sedimentary Record of Cenozoic Monsoon Intensity Preserved in the Asian Marginal Seas
Climate models predict that the intensity of the Asian monsoon is largely controlled by the elevation of the Tibetan Plateau. However, the elevation of Tibet and the erosional response to monsoon intensification is contentious. Compilation of seismic data from the marginal seas of East Asia now shows that sedimentation rates sharply increased across the region, and especially in the Red River catchment after around 33 Ma and continued to increase until around 11 Ma. The dating of potassic magmatism and strike-slip faulting on the eastern flanks of the Tibetan Plateau starting at 35-30 Ma suggests that this tectonism may accompany a period of accelerating surface uplift in Tibet. At present few climate records exist for this time period, though the monsoon might be expected to have strengthened in stages since that time. Most notably new data from ODP Site 1148 in the northern South China Sea show a sharp increase in clastic and carbonate mass accumulation rates associated with a change in clay mineralogy to greater illite dominance (physical weathering) starting around 17.2 Ma. I interpret this to reflect a change to a more erosive, more monsoonal climate starting at that time in southern China. Falling sedimentation rates at 11-8 Ma and lasting until 4 Ma indicate drier conditions across Asia at that time, also recorded by strong eolian sedimentation in the North Pacific, slower sedimentation on the Indus and Bengal Fans, and change to a more smectite dominated mineralogy on both these fans, indicating a less erosive environment. Only in the Mekong does erosion increase after 8 Ma, driven by local tectonic uplift in the Vietnamese Highlands. Faster sedimentation and a return to an illite dominated mineralogy after 4 Ma accompanies a series of proxies that indicate a stronger summer monsoon at that time, and a return to faster erosion driven by rains in East and South Asia. The evidence shows a broad positive correlation between monsoon strength and erosion over long periods of geological time. Pulses of sediment caused by faster erosion do not appear to be strongly dampened over time scales >100 kyr by sequestering in continental sedimentary basins, but mostly reach the continental margins of the South China Sea.
http://www.whoi.edu/pclift/asia_erosion.html