23 research outputs found
Recommended from our members
The fluvial response to glacial-interglacial climate change in the Pacific Northwest, USA
This research focuses on the development of new techniques to explore terrestrial-ocean climate linkages along the Pacific Northwest-northeast Pacific Ocean margin. This is done by investigating river response to climate change and by unraveling this history preserved in continental margin sediments. A significant component of this work centers on developing a 40Ar-39Ar incremental heating method to fingerprint bulk fluvial sediment entering this region. Results show reproducible ages from individual rivers accounting for the majority of sediment delivered offshore. A 40Ar-39Ar detrital mixture model is developed to examine the fidelity of these results and shows that the bulk ages measured from river mouth sediments can be accurate indicators of the average age of feldspars eroded from a given catchment area.
The bulk sediment ages are combined with Nd isotopic analyses into a ternary mixing model to better understand the sources of terrigenous material delivered to offshore continental margin sites. Downcore Ar-Nd isotopic compositions can be described by three general river sediment sources proximal to the core site, the Umpqua, Rogue+Klamath, and Eel Rivers, from ~14 ka to Present. Results from the ternary model also suggest that differential contributions of eroded material plays the primary role in provenance changes seen at the core site, rather than sediment transport changes due to ocean circulation.
This research culminates in a modeling effort to examine downcore provenance changes. We develop a model that balances basin-averaged 40Ar-39Ar ages (detrital mixtures) of the contributing fluvial basins and predicts the bulk sediment value at the core site. We find that the Upper Klamath Basin (which contained pluvial Lake Modoc during Marine Isotope Stage 2) is the most influential source area that can contribute to younger bulk sediment 40Ar-39Ar ages at the core site, relative to present day values. The Eel River is also shown to have a considerable influence on changes in margin sedimentation. Combinations of increases in the sediment fluxes out of these two basins can describe the 40Ar-39Ar provenance evolution observed at the core site over the 22-14 ka time period. Overall, this new 40Ar-39Ar isotopic technique, together with the Nd isotopic system and the use of detrital mixture modeling show tremendous promise as a multi-faceted strategy to assess erosion and provenance change through the continuous history preserved in fine-grained marine sedimentary records
Recommended from our members
Erosion by rivers and transport pathways in the ocean: A provenance tool using 40Ar--39Ar incremental heating on fine-grained sediment
We use Ar-40-Ar-39 incremental heating to fingerprint bulk fluvial sediment entering the northeast Pacific Ocean, with the long-term intent of tracking sediment source and transport changes from the terrestrial system to the marine environment through time. We show reproducible age spectra from individual rivers accounting for the majority of sediment delivered to the Pacific margin. Two tests are performed to confirm the validity of the bulk sediment Ar-40-Ar-39 incremental heating measurements and to address why polymineralic sediment might yield concordant age steps. The first model tests, in light of bulk mineralogy and diffusion of Ar from silicates, whether measured K/Ca spectra (measured from Ar-39 and Ar-37, respectively) are consistent with typical values for K- and Ca-bearing minerals. Calculations show that the bulk mineralogy is reflected in the outgassing K/Ca spectra and identify plagioclase as the dominant mineral contributing to the plateau-defining portion of the age spectra. A second model predicts bulk sediment ages from integrated bedrock cooling age-area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin. Calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for. Overall, this technique shows considerable promise, not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems
Recommended from our members
Fluvial–Eolian Interactions In Sediment Routing and Sedimentary Signal Buffering: An Example From the Indus Basin and Thar Desert
Sediment production and its subsequent preservation in the marine stratigraphic record offshore of large rivers are linked by complex sediment-transfer systems. To interpret the stratigraphic record it is critical to understand how environmental signals transfer from sedimentary source regions to depositional sinks, and in particular to understand the role of buffering in obscuring climatic or tectonic signals. In dryland regions, signal buffering can include sediment cycling through linked fluvial and eolian systems. We investigate sediment-routing connectivity between the Indus River and the Thar Desert, where fluvial and eolian systems exchanged sediment over large spatial scales (hundreds of kilometers). Summer monsoon winds recycle sediment from the lower Indus River and delta northeastward, i.e., downwind and upstream, into the desert. Far-field eolian recycling of Indus sediment is important enough to control sediment provenance at the downwind end of the desert substantially, although the proportion of Indus sediment of various ages varies regionally within the desert; dune sands in the northwestern Thar Desert resemble the late Holocene–Recent Indus delta, requiring short transport and reworking times. On smaller spatial scales (1–10 m) along fluvial channels in the northern Thar Desert, there is also stratigraphic evidence of fluvial and eolian sediment reworking from local rivers. In terms of sediment volume, we estimate that the Thar Desert could be a more substantial sedimentary store than all other known buffer regions in the Indus basin combined. Thus, since the mid-Holocene, when the desert expanded as the summer monsoon rainfall decreased, fluvial–eolian recycling has been an important but little recognized process buffering sediment flux to the ocean. Similar fluvial–eolian connectivity likely also affects sediment routing and signal transfer in other dryland regions globally.This is the publisher’s final pdf. The published article is copyrighted by the Society for Sedimentary Geology and can be found at: http://sepm.org/pages.aspx?pageid=11
Recommended from our members
The role of rock resistance and rock uplift on topographic relief and river longitudinal profiles in the coastal mountains of Oregon and a landscape-scale test for steady-state conditions
Analysis of topographic and river morphometric parameters was conducted using digital elevation models (DEMs) and field observations in order to determine the role of variable rock resistance on topographic relief, to examine how spatially and temporally variable rock uplift rates relate to river morphology, and to address the degree to which uplift and erosion are in steady-state in the actively uplifting region of the Coast Ranges and Klamath Mountains in Oregon. Four domains were differentiated based on mapped geology and topography - the northern (~45° - 46° N), central (-44° - 45° N), south-central (-43° - 44° N) and southern regions (-42° - 43° N). Bedrock control, on the range scale, is indicated through the association of higher topography with exposures of more resistant volcanic and metamorphic rocks. Lithologic changes coincide with knickpoints on river longitudinal profiles between the latitudes of 43° - 45° N, where rock uplift appears to be low. Rock type seems to be a strong control on topographic relief in these regions. However, in the southern region and less somewhat in the north, where rock uplift rates are highest, changes in lithology along river profiles do not display significant knickpoints. Uplift likely controls river profile form in the northern and southern regions. Basin hypsometric integrals and drainage density values are relatively constant in the study area except in the central region. Rivers in this region are almost exclusively alluvial - whereas most rivers in the Coast Ranges are bedrock or mixed bedrock-alluvial types. These low values in the central region, coupled with the presence of alluvial channels, suggests that the topography is expressing signals of low to no rock uplift in this region. The correspondence seen between low uplift rates and bedrock control and high uplift rates and a transparent bedrock signal suggests that an uplift rate threshold may exist. This has implications for modeling topographic evolution in tectonically-active mountain belts
A climatic trigger for a major Oligo-Miocene unconformity in the Himalayan foreland basin
Subsidence in foreland basins is modulated by the size of the flexural load, the elastic thickness of the foreland lithosphere, the nature of the sedimentary fill, and the rate of convergence. Basal forebulge unconformities are well known from these basins, but here we focus on a major unconformity in the Himalayan foreland, which removed much of the Oligocene-lower Miocene. This is a critical time in the Himalaya because it spans the period of initial rapid exhumation of high-grade metamorphic rocks and the start of motion on the Main Central Thrust. We show that the synchronous timing of unconformity and rapid Greater Himalayan exhumation may be explained by the same trigger, enhanced erosion. We estimate that accelerating erosion between 24 and 20 Ma would have removed ∼1.5 km more rock from the Greater Himalaya than would have been the case had erosion remained at the slower Paleogene rates. During this time erosion must have temporarily exceeded rock uplift rates. By transferring rock from the Himalaya to the Indian Ocean the load is reduced and the Indian Plate is flexed less, so that the depth of the foreland basin shallows to a new equilibrium state. We use a flexural model to estimate that erosional unloading could have caused uplift of 400-500 m of the basin at the range front, extending 70-150 km into the basin. Intensification of the Asian summer monsoon around the Oligo-Miocene boundary (∼24 Ma) is the most likely trigger of the stronger erosion. Similar unconformities are seen at 15-16 Ma and in the Pleistocene also, linked to faster erosion at these times. Copyright 2010 by the American Geophysical Union