H51A-1098 0800h
Sediment Proxies of Quaternary Glacial Dynamics in the Gulf of Alaska Region
Southeastern Alaska is one setting where the depositional products of the interplay between tectonics and climate are recorded at exceptionally high temporal resolution. An important advantage of Alaska over other margins is proximity of the highest coastal mountain range on earth next to an energetic ocean with essentially no intervening basins to trap sediment. Climatic signals are therefore quickly recorded in offshore areas with little modification resulting from long transport in rivers or temporary storage in intervening sedimentary basins. Recent studies of glacial mass balances in southern Alaska reveal rapid loss in the past decade, which is ascribed to global climate change trends. However, it is not possible to place these recent changes in a larger context of regional climate change because the terrestrial record of Holocene glacial dynamics is incomplete. A recent field campaign has been initiated in the Gulf of Alaska region to examine the Late Quaternary to Modern record of glacial dynamics. We will present results of sedimentary studies on the continental margin that highlight chronological and provenance approaches used to document the dynamics of specific glacial basins in the region at decadal resolution. Specifically, a combination of environmental magnetism, U-Th-K activities, and sediment physical property measurements reveal that glacial source regions can be differentiated in marine sediments. Also, Little Ice Age glacial dynamics appear to have strongly varied over decadal time scales in conjunction with regional temperature indices. These advance-retreat cycles are manifested on the continental shelf as decadal -long pulses of sediment discharge from coastal mountains followed by equally long periods of terrestrial sediment storage and corresponding low marine sediment accumulation rates.
H51A-1099 0800h
Glacial Advances and Retreats in Tectonic Southeast Alaska During the Little Ice Age and Last Glacial Maximum: Preliminary Results from EW0408
In glacially dominated environments climate is linked directly to the earth processes of erosion and sediment deposition with some of the smallest time lags of any geological process. A cruise on the R/V Maurice Ewing that combines high-resolution seismic reflection surveys with ultra-high resolution chirp, EM1002 swath mapping, jumbo piston-coring, multicoring, multi-sensor track logging, and CTD/water sampling from Aug. to Sept., 2004, examined these processes in 30 sites throughout the fjords and continental shelf of tectonically-active southeast Alaska. Geophysical imaging of shelf depocenters and fjord sub-basins allows for tracking of Last Glacial Maximum (LGM) and Little Ice Age (LIA) ice streaming (advances) and glacial retreats. Glacial advances through ice streaming can be tracked by the presence of terminal and lateral morainal banks and extensive deposystems. We present patterns of ice streaming and the resulting sediments from the Lynn Canal/Glacier Bay systems, the Malaspina/ Hubbard glaciers, the Bering glacier, and the glaciers within Prince William Sound. Patterns of convergent and divergent ice flow result in differing deposition patterns and are sometimes reflected in modern shelf topography in features such as shelf valleys and banks. Glacial retreat can be incredibly rapid with the Glacier Bay systems retreating 100 km in < 250 years. The dominant pattern of glacial retreat both for the LIA and LGM retreats are a series of sub-basins imaged within the structurally-controlled fjords showing an episodic retreat style with the greatest volumes of sediments being deposited ice proximally, and the amount of post-glacial drape being dependent on time since the retreat. Rates of sediment deposition are extreme; for example, the second sub-basin of Muir Inlet has accumulated 65 m of paraglacial sediment in the 45 years since direct glacial influence.
H51A-1100 0800h
Quantifying Glacial Valley Widening and Deepening With Apatite (U-Th)/He Thermochronology: Coast Mountains, B.C.
Climate change and glaciation significantly influence the erosion history and topography of orogens. Glacial modification of landscapes typically occurs by valley widening and deepening. However, few studies have been able to quantify the rates at which widening and deepening occur. We integrate apatite (U-Th)/He (AHe) thermochronometry and thermo-kinematic modeling to quantify the range of glacial erosion rates in the southern Coast Mountains, British Columbia. The Coast Mountains are an ideal setting for quantifying glacial erosion rates as they have experienced substantial topographic modification by glaciers from late Miocene to present. Forty-one AHe samples were collected in a 20x20 km region covering one large and multiple smaller glaciated valleys to quantify the erosion history of this region. Samples were collected on a grid at 4 km intervals and along a vertical profile in the largest valley. Currently available ages from 22 of these samples range from 1.7-11.7 Ma and generally increase with increasing elevation. Age versus elevation along the vertical transect range from 2.4 Ma at 43 m to 11.1 Ma at 2382 m. A distinct break in slope occurs in the age-elevation plot around 1700 m corresponding to a sample age of 5 Ma. Sample ages collected in tributary valleys range in age from 4.6-11.7 Ma and are consistently older than samples at the same elevation in the main valley, which range from 1.7-11.1 Ma. We interpret the difference in AHe ages in samples at similar elevations to be the result of variable amounts of glacial valley deepening and widening. The occurrence of older ages in smaller valleys than in the main valley at the same elevation suggests significantly more glacial widening and deepening in the main valley over the last 2-5 Ma. The break in slope seen in the age versus elevation plot indicates the maximum extent of topographic modification occurs between 1700-2300 m. Preliminary estimates of long-term glacial erosion rates in this area range between 0.4-0.8 mm/yr. Work in progress involves analyzing spatial variations in cooling ages with a 3D thermo-kinematic model to quantify the preglacial topography in 3D.
H51A-1101 0800h
The Quaternary Legacy in the Organisation of Contemporary Geomorphic Processes in Forested Mountain Environments of British Columbia
Episodes of climate change are responsible for profound reorganization of rates and spatial distributions of earth surface processes. In glaciated British Columbia the series of climate changes that occurred during the Quaternary have left a landscape characterized by a peculiar hierarchy of topographic anisotropies. Currently, fluvial and hillslope processes dominate the landscape, and are slowly reshaping these anisotropies. By means of extensive channel surveys, air photo interpretation, and GIS analysis we extracted slope-area transects along the main stem of drainage basins - area ranging between 1 km2 and 50 km2 - located in three different physiographic regions of British Columbia. Results display generalized process-form disequilibrium, where topographic signatures do not match with currently active geomorphic process domains. The landscape has not recovered yet from past glacial perturbations, accordingly, the combination of glacial legacies and the effects of ongoing earth surface processes generate a more complex geomorphic picture than that described elsewhere for unglaciated mountains. This emphasizes the importance of climatic and geomorphic history in interpreting contemporary landscape dynamics.
H51A-1102 0800h
Optical dating of arroyo-system deposits: Insights into cut-fill cycles in the southern Colorado piedmont
Optical ages are presented from a range of small-scale arroyo systems found across Fort Carson and Pinon Canyon Maneuver Site in southern Colorado. Recent developments in both single-aliquot and single-grain techniques combined with more rigorous statistical methods for analysing paleodose estimates now mean that optical dating is able to provide much more accurate and precise constraints on the timing of major depositional changes in these fluvial systems. The arroyos of the region display distinct and multiple paleochannels along their side walls. Optical dating of infill deposits from within these paleochannels and from intervening alluvial fill units provides the first detailed and accurate chronology of past and present arroyo epicycle histories in the region. Resulting ages suggest that cut-fill behaviour has been active to varying degrees over much of the Holocene, particularly from the mid-Holocene onwards. Evidence also points towards a strongly episodic pattern of arroyo incision, separated by more protracted periods of stability and/or aggradation. Correlation of arroyo incision and subsequent infilling around the mid-Holocene Altithermal event in several paleochannel sequences from one arroyo system (the "Little Grand Canyon", Fort Carson) provides evidence for external climatic control on major Holocene cut-fill cycles in the region. Dating sequences from different locations within the same systems, and from different systems within the area, enables an intercomparison of arroyo epicycles at a multitude of spatial scales. The complex spatio-temporal dynamics emerging suggest that time-transgressive responses may be common both within and between the arroyos of this region. These findings imply either that basin response times to external forcing vary significantly with basin size, or that local-scale forcing mechanisms have been dominant controls on past arroyo dynamics in this region.
H51A-1103 0800h
Climatic Controls on Fluvial Cut-and-Fill Cycles in Drainages with In-stream Wetlands in the Central Andes
Fluvial systems that possess in-stream wetlands, or marshes, are common in arid environments where water-tables are emergent and large discharge events uncommon. These streambeds are protected from erosion by a dense cover of hydrophyllic and phreatophytic vegetation. Along the Pacific slope of the Central Andes in northern Chile (~20°-26°S), which includes some of the driest sectors of the Atacama Desert, in-stream wetlands occur in deeply incised bedrock canyons on the Andean slope and piedmont. Over the last several years we have compiled a detailed record of late Pleistocene and Holocene vegetation changes along the Pacific slope of the Andes through the collection, analysis, and radiocarbon determination of over 180 rodent middens. Rodent middens record past changes in precipitation levels by tracking the downslope migrations of plant species into the hyperarid desert. We have also assembled a record of the cut-and-fill cycles of several fluvial systems with in-stream wetlands located at various distances (5-50 km) from the zone of ground-water recharge in the High Andes through stratigraphic mapping and the radiocarbon dating of over 100 samples of organic material within these wetlands. Combined, this well-dated record of hillslope vegetation and stream aggradation and incision allows us to assess the influence of climatic change on stream processes, including the nature of stream response, the sensitivity of different stream systems to climatic change, and the response times of streams to climate changes that vary in distance from ground-water recharge zones. The combined data set shows that in-stream wetland aggradation is directly linked to changes in climate, with aggradation occurring during wetter climatic periods when water tables are high. Incision occurs during dry climatic periods when water tables are lower and streambed sediments are no longer anchored by dense vegetation. Streams that are closer to ground-water recharge zones are more sensitive to minor changes in precipitation, whereas more distant streams with larger catchment areas appear to be less sensitive. Response times of stream incision to the initiation of drought conditions appear to be ~500 years.
H51A-1104 0800h
SOUTH AMERICAN RIVERS: THE SIGNATURE OF CLIMATE CHANGES IN HYDROLOGICAL INFLOWS
Hydrological inflows impact the whole society, since water is one of the fundamental sources of life. Even though, science has not yet achieved a complete understanding of river dynamics; characterizing and predicting water inflows is still a challenge. Our work intends to offer a new approach to river flow modeling: instead of analyzing local measurements and trying to find a relationship between them, we searched for a possible connection between inflows and global climate variables and trends. This paper describes the first results of our research through a case study with one of the more important South American rivers: Parana, which flows into Itaipu energy plant (one of the largest plants in the world). It is possible to show an impressive correlation (near 1.0) between the water inflow and some climatic variables around the world (circling the globe: Pacific, Indic, Atlantic oceans). These correlations, evaluated for different time lags, are then used to build a propagation model able to predict the river flow months ahead. Finally, it is shown that the "1970 gap" (one of the most interesting climatic characteristics) is found both in the global climatic variables and in the river flow. It is possible to identify a break-point that divides the water flow history into two very distinct quasi-stationary phenomena: pre- and post-70. Curiously, precipitation variables in the same basin do not present such a clear gap. The answer, still under research, may lie in a combination of human actions (use of the land) and changes in soil physical characteristics.
H51A-1105 0800h
Knickpoints in Fluvial Systems: Comparing Models of Basin-Wide Propagation and Initiation at Erosional Thresholds
Knickpoints, which we define morphologically as discrete negative steps in the long profiles of rivers, have been frequently associated with the dynamic adjustment of channels following a change in climate or tectonics. The danger in this process-based definition arises from the numerous circumstances, both static (e.g. substrate erodability contrasts) and dynamic (e.g. stream capture), that generate knickpoint morphologies. In addition, because changes in knickpoint form are often too slow to measure, their role as upstream propagating fronts of adjustment is most often inferred rather than observed. Most previous studies of knickpoint retreat have examined the response of a single channel to base level fall, but we propose that the timing and pattern of knickpoint distribution throughout entire fluvial networks must be characterized in order to ultimately understand landscape response times to external forcing and the history of sediment delivery to offshore basins. To explore this, we examined 236 knickpoints distributed within the fluvial network of the Waipaoa River on the North Island of New Zealand. A climatically triggered pulse of incision initiated 18,000 years ago lowered base level 50-100 m along the Waipaoa mainstem. Using field measurements, aerial photo analysis and digital elevation data, we studied the knickpoints' positions within the network. We found that ~70% of the knickpoints are located at drainage areas between 1 x 10$^{5}$ m$^{2}$ and 1 x 10$^{6}$ m$^{2}$ and more than half are $<$ 1 km upstream of tributary junctions. This observed knickpoint distribution in the Waipaoa was compared to two end-member models for knickpoint behavior. In the first model, we examined the time-evolution of a knickpoint as it propagates upstream and is distributed throughout the network at a rate that is a power law function of drainage area. In the second, we examined if knickpoints form at threshold drainage areas where their fluvial erosive potential, as determined by water and sediment flux, is incapable of incising as rapidly as downstream reaches. Though neither model addressed along-stream variability in substrate or knickpoint form, surprisingly, both models provided highly accurate fits to the ~70% of knickpoints at drainage areas $<$ 1 x 10$^{6}$ m$^{2}$. Though the field and modeled results suggest that the present positions of the 236 observed knickpoints are determined by this threshold area behavior, explaining the basin-wide time evolution of a pulse of incision will require further model refinement and field observation.
H51A-1106 0800h
Deciphering the Role of Climate and Sea-Level Changes on Observed Decadal-Scale Variability in Salt-Marsh Sedimentation.
We are investigating the controls that climate and local oceanography exert on sedimentation patterns in 4 salt marsh-estuary complexes around Long Island, New York, USA. These systems encompass a variety of physical settings, including a range of tidal conditions, wave fetches, and human influences, but are all located within one climatic regime. Within these settings, we hypothesize that sedimentation patterns in limited-fetch, mesotidal salt marshes are influenced most strongly by sea-level changes, as the system is largely steady-state under high-energy conditions and sedimentation should track the longer-term sea-level transgression. Conversely, sedimentation in microtidal systems with large fetch should better track atmospheric forcings, because marsh-surface accretion largely occurs during episodic wind and storm events. To test this hypothesis, accretion rates (cm/yr) were determined by applying a constant-flux model to profiles of excess 210Pb, which reveals temporal variation in sedimentation. Additionally, we examined the rate of mineral sediment deposition (g/cm2/yr) and rate of organic matter accumulation (g/cm2/yr). These measures yielded a chronology of sedimentation patterns ~100 years long with a temporal resolution of 2-5 years, sufficient for resolving decadal-scale oscillations. Our proxies for sea-level change come from a variety of tide gauges; including the gauge at Battery Park, NYC which covers much of the past century, as well as local tide gauges with records spanning several decades. Proxies used for atmospheric forcings include mean annual winds for the past 50 years, storm histories and Hurrel's index of the North Atlantic Oscillation, which extends for over a century. Initial results reveal clear decadal-scale variability in marsh accretion, with variations ranging 2-3 fold about the long-term mean. These oscillations are very similar in timing and magnitude to those observed for the climate proxies and sea-level records. However, initial results reveal no significant correlation between local climate proxies and tide gauge records at the scale of this study. Therefore, we suspect that independent atmospheric and oceanic drivers of marsh sedimentation exist. The relative importance of these drivers in a particular embayment will likely depend on how characteristics such as the tidal regime, wave climate and human modifications, respond to changes in their physical forcings.
H51A-1107 0800h
Sensitivity of California River Flows to Schematic Climate Changes
Water supply in California is vulnerable to changing climate conditions, especially under global warming scenarios. Increasing human demand for water exacerbates this problem. This presentation illustrates how climate changes could affect California's water supply through a series of sensitivity studies. We performed a series of hydrological simulations over California using a number of hypothetical meteorological forcing data sets. Those data sets were created by modifying the historical meteorological data and those generated from high-resolution climate models to imitate different climate change scenarios. Different warming levels and precipitation amounts were played out to understand the effects of climate change on snow volume, seasonal timing of river flows, peak flow rates and frequency/intensity of droughts. This study is part of the ongoing climate and water research initiative in Lawrence Livermore National Laboratory to use high-resolution global and regional climate models and surface hydrology models to provide water managers with projections of the effects of climate change and variability on California hydrological cycle.
H51A-1108 0800h
Influence of Anthropogenic Alterations on Geomorphic Response to Climate Variation and Change in San Francisco Bay, Delta, and Watershed
Global warming and attendant sea-level rise may soon impact geomorphic processes in the Sacramento-San Joaquin River and San Francisco Bay Delta systems. During the past two centuries, dramatic anthropogenic changes in sediment supply and pervasive structural controls on rivers and floodplains have altered geomorphic responses to floods throughout a zone that extends upstream from tidally influenced areas to dams that regulate flow. Current geomorphic responses to floods differ from natural responses due to historical actions that concentrated the pre-disturbance multiple-channel and flood-basin system into single channels isolated by levees from increasingly developed floodplains and flood bypass channels, altered flow and sediment regimes, and caused subsidence of leveed Delta Islands. A review of historic and current geomorphic responses to floods illustrates the dominance of structural controls on geomorphic changes in the lowland part of the Sacramento-San Joaquin system. Current climate-change projections for CA suggest that the total volume of snowmelt runoff that may be shifted from spring and added to winter flows is roughly 5 maf/yr, similar to the volume currently available for flood storage in Sierra Nevadan reservoirs. Changes in timing of reservoir releases to accommodate these changes could add to either the magnitude or duration of winter flood peaks, each causing different geomorphic responses. Increased wintertime flows that accompany already large floods could increase overbank flood extent, erosion, and sedimentation, or alternatively increase the depth and strength of confined flows and increase the risk of levee failures. Runoff released from reservoirs as a relatively constant addition to winter baseflow would increase the duration of bankfull or possibly "levee-full" flows. This scenario could lead to bank and levee failure through increased saturation and seepage erosion. Projected sea level rise of 1-2 m would compound vulnerability of subsided Delta Islands to levee failure during floods and increase upstream backwater flooding. Thus, geomorphic responses to future climate variation and change will be closely tied to infrastructure and reservoir management, with survivability of infrastructure and decisions about timing, magnitude, and duration of flow releases from upstream reservoirs likely to determine the nature of those geomorphic responses.
H51A-1109 0800h
Climatic Controls on Western U.S. River Discharge: ENSO, PDO and ???
River discharge of west coast rivers is strongly tied to both ENSO and PDO. Discharge of southern Californian rivers is positively correlated with El Nino and warm PDO, whereas northwest rivers are positively correlated with La Nina and cold PDO. Central and northern Californian rivers represent a transition from southern to northern climates. These correlation coefficients, however, explain only a relatively small portion of the interannual variability. While event discharge (defined as the top 1% daily discharges over the past 50 years) reflects the above climatic signals, mean annual discharges do not. As an extreme example, mean annual discharge of the Nehalem (Oregon) and Skagit (Washington) rivers is essentially the same during El Nino and La Nina years, as well as during cold and warm PDO years. Clearly another climatic signal must be a major driver in precipitation and river runoff. We suggest that it may be coastal surface water temperature, perhaps combined with wind direction and intensity.
H51A-1110 0800h
Rapid response of a large river system to climate change, and its efficient downstream transfer of the signal: How is such close coupling of source to sink achieved?
It is generally held that the response-time of fluvial systems to climate change and the resulting flux of material to the continental margin vary significantly with basin size, such that large systems respond slowly and that signals are sharply attenuated downstream. Timescales typically discussed for basinwide fluvial responses to climate change are on the order of 10$^{5}$-10$^{7}$ years. However, reconstructions of the Ganges dispersal system through the late Quaternary reveal basinwide responses to summer monsoon strength, with sediment flux at the margin varying an order of magnitude over periods of a few millennia. This raises the question of how a massive basin like the Ganges can vary so significantly at relatively short timescales. The paper presented here discusses numerous factors involved in this response pattern, including both regional and local-scale aspects. It is suggested that influences at both scales are critical for generating the rapid, basinwide responses described for the Ganges system. Of course regional influences are necessary to generate broad-scale, large-magnitude responses, but because these forcings are imparted over a diverse geological setting (mountain to marine), local factors play a key role in linking these distinct reaches of the dispersal system. Identifying the most important of such factors, and testing them via numerical models, will be important for recognizing similar response histories in other modern and ancient dispersal systems. It is suggested here that, in general, highly seasonal sub-tropical disperal systems are the most likely to show a strong sensitivity and basin-scale response to climate change.
H51A-1111 0800h
Late Quaternary Climate and Provenance Signals of Brahmaputra River Sediments in the Sylhet Basin, Bangladesh
A high-resolution record of Brahmaputra River sediments is investigated from a 200-m borehole record from the Sylhet region in Bangladesh, a rapidly subsiding tectonic tectonic basin that preserves a nearly continuous Late Quaternary sequence. The percent abundance of major clay minerals down the core was determined by x-ray diffraction techniques. The ratio of smectite and kaolinite versus illite and chlorite was calculated as a proxy for the extent of chemical versus physical weathering. Other mineralogical studies of the Bengal Fan have found a correlation between climate and the record of clay mineralogy in the sediments (e.g. Debrabrant et al., 1996). Interestingly, this study did not yield such a clear correlation, suggesting that other factors, such as provenance or weathering, could play an important role in the preservation and hence the climatic signal as interpreted from clay mineralogy.
H51A-1112 0800h
Climate-induced variations in the sourcing and weathering of fluvial sediments: Deltaic records from the monsoon-forced Ganges and Brahmaputra dispersal systems
The Ganges-Brahmaputra river system is among the largest in the world and has been shown to respond rapidly to millennial-scale shifts in South Asian Monsoon climate. This response is seen in an order of magnitude shift in both river discharge and sediment deposition from the modern mean throughout the Holocene. The tectonic dynamics of the basin and the high sediment loads have led to thick, high temporal resolution sediment sequences deposited on the delta during this time. Drawing on the unique chemical signatures of the major Himalyan lithologies drained by these rivers, other works have carried out provenance studies on sediments from the Bengal Fan looking for changes over the last glacial-interglacial cycle. These studies show that there has not been significant change in provenance on this time scale, so we examined sediments from the deltaic sequences using the same method of Sr isotope composition. From several long (100m-180m) boreholes from around the delta, we were able to sample specific, separate sub-basins of Ganges and Brahmaputra influence, as well as regions where this influence was mixed. Throughout the Holocene we find no significant change in downcore Sr ratios. However, our results indicate that there is a large departure from the mean $^{87}$Sr/$^{86}$Sr at the base of the Holocene Brahmaputra sequence presumed to date to the last glacial maximum. Initial clay mineralogy and major element data suggest no change in extent of weathering of these sediments, implying a shift in provenance as the cause of this excursion. Early results also indicate unique Sr isotopic compositions between the two rivers, which can be used to delineate specific source areas for each river as well as the delta as a whole. Also noted has been an increase in weathering extent of Ganges deposits over the past 3000 years. Further work on the provenance of these sediments will include studies of the sedimentology, mineralogy, and Nd and Ar isotopic compositions. Clay mineralogy and major element data will continue to be used to reconstruct the weathering history of these sediments as well. With these added data, we expect in the near future to reconstruct the sourcing, weathering, and transport history of the sediments delivered to each drainage basin during this period of significant and rapid climatic change.
H51A-1113 0800h
Regolith Thickness Instability and the Formation of Tors in Arid, Granitic Environments
We previously developed a model that incorporates a feedback between bedrock weathering and physical erosion (stream flow and diffusive processes) to explain the origin and maintenance of the laterally extensive (km's), nearly uniformly thin regolith blanket that characterizes pediments in arid regions such as the southwestern United States. Specifically, physical erosion controls pediment form by modifying regolith thickness, which, in turn, regulates bedrock weathering rates (Anderson, 2002). This relationship yields coupled bedrock and alluvial surfaces that lower together through time. Hydrogeochemical considerations and field observations in arid, granitic environments suggest that the relationship between weathering rates and regolith thickness exhibits a maximum for a finite thickness of cover. Our model offers an explanation for the isolated bedrock knobs (tors/inselbergs) that often punctuate otherwise smooth pediments. These features may arise as a consequence of stochasticity in rainfall and sediment transport conditions driven by fluctuations in climate, combined with the "peaked" nature of our weathering relationship. Climate fluctuations, which may manifest themselves as periods of higher effective moisture (decadal to millennial time scales) or changes in local base levels, may invoke a transition in which mantled surfaces lower at rates exceeding the bare-bedrock weathering rate. (Typically, boundary conditions and geometrical constraints restrict mantled surfaces to lower at rates below the bare-bedrock weathering rate in our model.) With a mantled surface lowering at such a rate, the form of the weathering-rate curve predicts that the pediment will be susceptible to an instability that will lead to bare-bedrock; regolith thickness that falls below a threshold value will approach a stable attractor at zero thickness. If this instability is triggered in a spatially heterogeneous pattern, perhaps as runoff from an intense rainfall event interacts with heterogeneities in regolith thickness and surface topography, a tor field will tend to develop. Incipient tors may then grow due to accelerated denudation on mantled surfaces compared to bare rock surfaces. As the tors grow in height, they will also tend to develop the steep sides and angular junctions with the surrounding pediment that characterize classic tor fields such as those found on the flanks of Cima Dome or in Joshua Tree National Park in the Mojave Desert of Southern California. Subsequent shifts in climate or local base level that cause the sediment surface to lower at a rate less than the bare-bedrock weathering rate will lead to a progressive decrease in tor height, ultimately leading to their disappearance. Tors in these environments thus represent possibly transient features related to fluctuations in climate or local transport conditions.
H51A-1114 0800h
Short-lived Lake(s) on the Late Wisconsin Margin of the Laurentide Ice Sheet, Musselshell Basin, Montana
Glacial Lake Musselshell is the middle link in a chain of lakes that formed along the Pleistocene Laurentide ice margin in central Montana. It was first recognized because scores of glacially-transported boulders from the Canadian Shield are found in the Musselshell River basin, yet there is no evidence that the Laurentide ice sheet advanced that far south. For a century, the ice-rafted boulders remained the only physical evidence associated with the lake. No other features typical of other large, ephemeral lakes - varved lacustrine sediment, inflow deltas, or lake shorelines - have been identified for Lake Musselshell. A sequence of nine river terraces and more than 100 previously located boulders provided the opportunity to place Lake Musselshell, and the corresponding Laurentide ice margin, in the context of regional and global chronologies. Terrace gradient and provenance, surface exposure ages of ice-rafted boulders, and identification of additional lake-related features were the most useful tools for establishing the extent and timing of Lake Musselshell. Lake Musselshell probably existed as one or more short-lived stage(s) that reached a maximum altitude of approximately 920 m. The absence of varves, deltas and shorelines suggests against one or more stable levels. Deposits of sheet-like silt and fine sand are interpreted as slackwater sediment from one or more short-lived lakes. The lake(s) drained under or in front of the ice sheet, down the modern Missouri River channel. Strong evidence was found that Lake Musselshell existed during the Late Wisconsin stage. Twenty-seven Be-10 surface exposure ages from ice-rafted boulders are all Late Wisconsin and younger (5.2-21.7 ka). Canadian Shield gravel occurs only in the lowest (probably Late Wisconsin) Pleistocene terrace. Additionally, upstream convergence of the Musselshell River terraces implies that displacement of the Missouri River by the Laurentide ice sheet occurred only recently (possibly Late Wisconsin). Pre-Late Wisconsin glacial advances into central Montana cannot be ruled out. Older deposits may be buried, removed or modified by erosion. However, the ice-rafted boulders and glacially-derived alluvium in the Musselshell basin are probably Late Wisconsin in age. Therefore, the Late Wisconsin Laurentide ice sheet may have been the most extensive Pleistocene ice sheet in central Montana.
H51A-1115 0800h
On the dynamics of soil moisture vegetation and erosion: Implications of stochastic climate forcing
Landscapes are observable manifestations of dynamic interactions between climatic, hydrologic, geomorphic, and ecosystem processes. As such, understanding landscape system response to fluctuations and changes in climatic forcing is necessary to predict impacts of future climate change on landscapes, and interpret geological records as indicators of past climate. We develop a simple stochastic model for climate, soil moisture, vegetation and runoff erosion dynamics driven by the Poisson pulse rainfall model. In the model, runoff generation, moisture losses due to drainage and evapotranspiration, and vegetation growth and mortality are related to vegetation cover and soil moisture state. We apply this model to investigate the sensitivity of soil moisture, grass cover and erosion potential to rainfall variability (Rvar) and mean annual precipitation (MAP). In general, under fixed MAP and rainfall rate, both soil moisture and grass cover increase, reach a maximum, and then decrease as Rvar increases. Erosion potential tends to increase with reduced vegetation cover. Analysis of existing data suggests power-law dependence between both Rvar and interstorm period with MAP. When climate is characterized in this manner, the model shows both long-term mean soil moisture and vegetation cover increase with increasing MAP. Erosion potential, however, initially increases with increasing MAP, but reaches a peak and subsequently decreases as MAP grows large. Similar behavior between measured sediment yields and MAP have been reported in the literature for a range of climatic conditions. We find that the degree of nonlinear dependence between Rvar and MAP exerts an important control on the shape of the relationship between erosion potential and MAP. Results underscore the importance of coupled soil, vegetation and climate dynamics on erosion rates.