William H. Amidon

A model for fire‐induced sediment yield by dry ravel in steep landscapes

Sediment flux from hillslopes to channels commonly increases following wildfires, with implications for the carbon cycle, river habitats, and debris-flow hazards. Although much of this material is transported via dry ravel, existing ravel models are not applicable to hillslopes with gradients greater than the angle of repose, which can constitute the majority of mountainous terrain. To fill this knowledge gap, we develop a continuity model for sediment storage by vegetation dams on steep hillslopes to predict sediment yields following wildfire. The maximum volume of sediment stored prior to wildfire is set to be a function of vegetation density, the capacity of plants to impound sediment, and the contributing hillslope area. Time is required after fire to establish vegetation and replenish hillslope sediment storage, which introduces vegetation regrowth rate, soil production rate, and fire recurrence interval as important variables that affect ravel yield. Model results for the San Gabriel Mountains, California, predict that sediment yield can increase by several orders of magnitude following fire. These results are consistent with field data of ravel yield (~30 mm per contributing area of hillslope in 5 months) we collected following the 2009 Station Fire, as well as postfire sediment flux recorded by 93 debris basins. In contrast to previous work, our model shows that heightened postfire sediment yields can be explained by a change in hillslope sediment storage independent of major changes in the soil production rate and landscape form over geomorphic timescales.

Persistent elastic behavior above a megathrust rupture patch: Nias island, West Sumatra

We quantify fore-arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible. Elevated coral microatolls, paleoreef flats, and chenier plains show that the Sumatran outer arc island of Nias has experienced a complex pattern of relatively slow long-term uplift and subsidence during the Holocene epoch. This same island rose up to 2.9 m during the Mw 8.7 Sunda megathrust rupture in 2005. The mismatch between the 2005 and Holocene uplift patterns, along with the overall low rates of Holocene deformation, reflects the dominance of elastic strain accumulation and release along this section of the Sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long-term plate convergence. The fraction of 2005 uplift that will be retained permanently is generally <4% for sites that experienced more than 0.25 m of coseismic uplift. Average uplift rates since the mid-Holocene range from 1.5 to −0.2 mm/a and are highest on the eastern coast of Nias, where coseismic uplift was nearly zero in 2005. The pattern of long-term uplift and subsidence is consistent with slow deformation of Nias along closely spaced folds in the north and trenchward dipping back thrusts in the southeast. Low Holocene tectonic uplift rates provide for excellent geomorphic and stratigraphic preservation of the mid-Holocene relative sea level high, which was under way by ∼7.3 ka and persisted until ∼2 ka.

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

Significance This paper identifies two periods of enhanced aridity that are synchronous with faunal turnovers in southern South America. Close temporal coincidence with marine climate proxies suggests a period of latest Miocene aridity was associated with a global glacial period and the expansion of C4 vegetation. We thus argue that continental aridity in the south-central Andes is associated with cold periods at high southern latitudes and propose a model to link global and regional (continental) climate via shifting of the Southern Hemisphere westerlies. This paper also provides an example of how 10Be paleo-erosion rates can be used as a climate proxy and demonstrates how 10Be and 36Cl can be combined to reduce uncertainties associated with this method. Although Earth’s climate history is best known through marine records, the corresponding continental climatic conditions drive the evolution of terrestrial life. Continental conditions during the latest Miocene are of particular interest because global faunal turnover is roughly synchronous with a period of global glaciation from ∼6.2–5.5 Ma and with the Messinian Salinity Crisis from ∼6.0–5.3 Ma. Despite the climatic and ecological significance of this period, the continental climatic conditions associated with it remain unclear. We address this question using erosion rates of ancient watersheds to constrain Mio-Pliocene climatic conditions in the south-central Andes near 30° S. Our results show two slowdowns in erosion rate, one from ∼6.1–5.2 Ma and another from 3.6 to 3.3 Ma, which we attribute to periods of continental aridity. This view is supported by synchrony with other regional proxies for aridity and with the timing of glacial ‟cold” periods as recorded by marine proxies, such as the M2 isotope excursion. We thus conclude that aridity in the south-central Andes is associated with cold periods at high southern latitudes, perhaps due to a northward migration of the Southern Hemisphere westerlies, which disrupted the South American Low Level Jet that delivers moisture to southeastern South America. Colder glacial periods, and possibly associated reductions in atmospheric CO2, thus seem to be an important driver of Mio-Pliocene ecological transitions in the central Andes. Finally, this study demonstrates that paleo-erosion rates can be a powerful proxy for ancient continental climates that lie beyond the reach of most lacustrine and glacial archives.

Interaction of outburst floods with basaltic aquifers on the Snake River Plain: Implications for Martian canyons

Idaho’s Snake River Plain is underlain by a young sequence of basaltic lava fl ows that house one of the most conductive aquifers in the world and have been sculpted by at least three megafl oods in the last ~100 k.y. The timing and routing of these fl oods, and their interaction with the underlying aquifer, have taken on renewed signifi cance because they have carved amphitheater-headed dry canyons analogous to those found on Mars. In this study, we use cosmogenic 3 He and 21 Ne dating of fl ood-deposited boulders to show that the Big Lost River and Bonne ville fl oods were closely spaced in time at ca. 22.3 and ca. 17.5 ka, respectively. Most of the dry canyons record signifi cant erosion during the Big Lost River fl ood, despite its much smaller magnitude than the later Bonne ville fl ood. We explain this puzzling observation by proposing a composite erosion model in which erosion during the Big Lost River fl ood was partially accomplished by routing of fl oodwaters through the Snake River Plain aquifer. Topographic analysis shows that Big Lost River fl oodwaters ponded in the Terreton Basin, infi ltrated into the aquifer, and likely emerged as return fl ow in watersheds upstream of the dry canyons. We propose that sustained and focused erosion associated with return fl ow over months to years could explain the unique morphology of some dry canyons. Such a model also explains why most dry canyons are coincident with springs and surface watersheds, and it may provide a model for the way in which morphologically similar canyons evolved on Mars.

Holocene volcanism of the upper McKenzie River catchment, central Oregon Cascades, USA

To assess the complexity of eruptive activity within mafic volcanic fields, we present a detailed geologic investigation of Holocene volcanism in the upper McKenzie River catchment in the central Oregon Cascades, United States. We focus on the Sand Mountain volcanic field, which covers 76 km 2 and consists of 23 vents, associated tephra deposits, and lava fields. We find that the Sand Mountain volcanic field was active for a few decades around 3 ka and involved at least 13 eruptive units. Despite the small total volume erupted (∼1 km 3 dense rock equivalent [DRE]), Sand Mountain volcanic field lava geochemistry indicates that erupted magmas were derived from at least two, and likely three, different magma sources. Single units erupted from one or more vents, and field data provide evidence of both vent migration and reoccupation. Overall, our study shows that mafic volcanism was clustered in space and time, involved both explosive and effusive behavior, and tapped several magma sources. These observations provide important insights on possible future hazards from mafic volcanism in the central Oregon Cascades.

Mass spectrometric ^3He measurement in ^4He-rich phases: Techniques and limitations for cosmogenic ^3He dating of zircon, apatite, and titanite

Recent calibration studies have expanded the range of target minerals suitable for cosmogenic ^3He dating to include U and Th-rich phases such as zircon, apatite, and titanite. These minerals often contain large amounts of radiogenic ^4He that present several analytical challenges for precise and accurate ^3He determinations. In this paper we document the abundance sensitivity and changes in the absolute sensitivity and time evolution of the ^3He signal over a wide range of ^4He pressures in a MAP 215-50 noble gas mass spectrometer. Large (>50%) decreases in sensitivity with ^4He amount arising from space charge effects were observed but can be corrected for using an isotope dilution–like technique in which ^3He spike is added to a sample midway through the mass spectrometric analysis. Large amounts of ^4He also cause the time evolution of the ^3He signal to become steeper, degrading precision of the initial peak height determination from the intercept. Taken together we find that these effects preclude reliable analysis of samples with ^4He > 1 μmol and that ^3He/^4He ratios of greater than ~5 × 10^(−10) are required to routinely measure ^3He to better than 20% precision. We present some general considerations by which to assess the probability of success of measuring cosmogenic ^3He in these phases as a function of elevation, exposure age, and helium cooling age.