2 research outputs found
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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
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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