Sharon Bywater-Reyes

Effect of late Cenozoic aridification on sedimentation in the Eastern Cordillera of northwest Argentina (Angastaco basin)

This study evaluates the effect of climate on facies, grain size, and sedimentation rates using sedimentology, geochronology, and stable isotope geochemistry for Miocene–Pliocene deposits in the Angastaco basin (Eastern Cordillera, northwest Argentina). U-Pb zircon data from ash layers constrain the transition between the finer grained fluvial-lacustrine Palo Pintado Formation and the coarser grained fluvial-alluvial San Felipe Formation to ca. 5.2 Ma and the first deposition of sediment derived from the present-day orographic barrier to ca. 4 Ma. δ 13 C values from pedogenic carbonate nodules range from −15.4‰ to −10.2‰ for the Palo Pintado Formation and from −9.5‰ to −8.2‰ for the San Felipe Formation; this can be best explained by increased, sustained aridity since ca. 5 Ma. The δ 18 O values range from −9.6‰ to −5.9‰ for the Palo Pintado Formation and from −6.1‰ to −5.2‰ for the San Felipe Formation, corroborating this interpretation. The shift toward more arid conditions correlates with a significant increase in grain size but no significant change in sedimentation rate. Because aridity precedes the development of an orographic effect, we interpret the grain size increase in the Angastaco basin since ca. 5 Ma to be a response of the sedimentary system to aridification resulting from regional climate change.

Uplift of the Central Andes of NW Argentina associated with upper crustal shortening, revealed by multiproxy isotopic analyses

This study contributes to the uplift history of the Andes, which has received increasing attention in recent years because of its implications for geodynamic models and climate feedbacks. Shortening resulting in crustal thickening and removal of gravitationally unstable mantle lithosphere has been proposed to control deformation and uplift of Cordillera-type orogenic systems such as the Puna Plateau of the central Andes and its eastern margin, the Eastern Cordillera. We present new clumped isotope (Δ47), δ18O, and δ2Η data from carbonate nodules, marlstone, spring deposits, and volcanic ashes from the Puna Plateau and Eastern Cordillera of NW Argentina. When combined with other geological evidence, our data indicate that the Puna Plateau was near its present elevation since at least ~10 Ma, whereas the Eastern Cordillera rose ~1.5 km between ~14 and ~7 Ma. This history of uplift correlates with active shortening in the Eastern Cordillera and with incorporation of a regional foreland into the propagating orogenic wedge. Our study suggests that the elevation of the Puna Plateau changed little during the Miocene-Pliocene, whereas the margin experienced significant uplift associated with active deformation and crustal thickening.

Multiscale influence of woody riparian vegetation on fluvial topography quantified with ground‐based and airborne lidar

Coupling between riparian vegetation and river processes can result in the coevolution of plant communities and channel morphology. Quantifying biotic-abiotic interactions remains difficult because of the challenges in making and analyzing appropriately scaled observations. We measure the influence of woody vegetation on channel topography at the patch and reach scales in a sand-bedded, dryland river system (Santa Maria River, Arizona) with native Populus and invasive Tamarix. At the patch scale, we use ground-based LiDAR to relate plant morphology to “tail bars” formed in the lee of vegetation. We find vegetation roughness density (λf) to most influence tail bar shape and size, suggesting coherent flow structures associated with roughness density are responsible for sediment deposition at this scale. Using airborne LiDAR, we test whether relationships between topography and vegetation morphology observed at the patch scale are persistent at the reach scale. We find elevation of the channel (relative to the local mean) covaries with a metric of vegetation density, indicating analogous influences of vegetation density on topography across spatial scales. While these results are expected, our approach provides insight regarding interactions between woody riparian vegetation and channel topography at multiple scales, and a means to quantify such interactions for use in other field settings.

Relative Influence of Landscape Variables and Discharge on Suspended Sediment Yields in Temperate Mountain Catchments

Suspended sediment is an important regulator of stream habitat quality but notoriously difficult to predict and regulate. This difficulty arises because of high natural variability in suspended sediment yield in space and time. Here we quantified associations between suspended sediment yields and discharge, watershed setting (i.e., physiography and lithology), and disturbance history for 10 temperate mountain watersheds (8.5–6,242 ha) in the U.S. Pacific Northwest (H.J. Andrews Long-Term Ecological Research, LTER) over an ~60-year period. Annual suspended sediment yields varied almost 4 orders of magnitude across space and time. A linear mixed effects model indicated that much of the variation in yields could be explained by the random effect of site (conditional R = 0.74) with additional variation explained by the fixed effects (marginal R = 0.67) of cumulative annual discharge (p < 0.001) and the variability (standard deviation) of watershed slope (p< 0.001). Two annual sediment yield data points were model outliers, that each occurred within a decade after forest management activities and a large-magnitude storm event at sites with high variability of catchment slope. Other sites had low sediment yields for a range of conditions, including management or flood disturbance. Taken together, our study shows that watersheds with high slope variability have higher suspended sediment yields and may be more vulnerable to increases in sediment yields following disturbances.