Geology

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Variability of rock erodibility in bedrock-floored stream channels based on abrasion mill experiments
(2015-01-01) Small, Eric E.; Blom, Tevis; Hynek, Brian M.; Hancock, Gregory S.
We quantify variations in rock erodibility, K-r, within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in Utah and basaltic bedrock in the Hawaiian Islands. Samples were taken from the thalweg and channel margins, the latter at a height that is inundated annually. For each sample, a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth. Erosion rate data from these experiments shows two things. First, the erosion rate from channel margin samples is greater than for thalweg samples, with the greatest difference observed for the rock surface that was exposed in the stream channel. Second, erosion rate decreases with depth beneath the original rock surface, by an order of magnitude in most cases. The erosion rate becomes steady at depths of 1-3 mm for channel margin samples and 0.1-0.4 mm for thalweg samples. Because only rock properties and microtopography vary throughout the sequence of mill experiments, these results suggest that K-r of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that K-r decreases over depths of similar to 1 mm. The simplest explanation for these patterns is that K-r is enhanced, at the bedrock surface and along the channel margins, due to the effects of weathering on rock strength and surface roughness. We hypothesize that a balance exists between weathering-enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg.
Marine extinction risk shaped by trait-environment interactions over 500 million years
(2015-01-01) Orzechowski, Emily A.; Finnegan, Seth; Lindberg, David R.; Lockwood, Rowan
Perhaps the most pressing issue in predicting biotic responses to present and future global change is understanding how environmental factors shape the relationship between ecological traits and extinction risk. The fossil record provides millions of years of insight into how extinction selectivity (i.e., differential extinction risk) is shaped by interactions between ecological traits and environmental conditions. Numerous paleontological studies have examined trait-based extinction selectivity; however, the extent to which these patterns are shaped by environmental conditions is poorly understood due to a lack of quantitative synthesis across studies. We conducted a meta-analysis of published studies on fossil marine bivalves and gastropods that span 458 million years to uncover how global environmental and geochemical changes covary with trait-based extinction selectivity. We focused on geographic range size and life habit (i.e., infaunal vs. epifaunal), two of the most important and commonly examined predictors of extinction selectivity. We used geochemical proxies related to global climate, as well as indicators of ocean acidification, to infer average global environmental conditions. Life-habit selectivity is weakly dependent on environmental conditions, with infaunal species relatively buffered from extinction during warmer climate states. In contrast, the odds of taxa with broad geographic ranges surviving an extinction ( > 2500km for genera, > 500km for species) are on average three times greater than narrow-ranging taxa (estimate of odds ratio: 2.8, 95% confidence interval=2.3-3.5), regardless of the prevailing global environmental conditions. The environmental independence of geographic range size extinction selectivity emphasizes the critical role of geographic range size in setting conservation priorities.
Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales Comment
(Geology, 2014-01-01) Warrick, J. A.; Milliman, J. D.; Walling, D. E.; Aalto, R. E.
Millennial-scale sustainability of the Chesapeake Bay Native American oyster fishery
(2016-01-01) Rick, Torben C.; Reeder-Myers, Leslie A.; Hofman, Courtney A.; Lockwood, Rowan; Kellogg, Lisa; Luckenbach, Mark W.; Mann, Roger; Ogburn, Matthew B.
Estuaries around the world are in a state of decline following decades or more of overfishing, pollution, and climate change. Oysters (Ostreidae), ecosystem engineers in many estuaries, influence water quality, construct habitat, and provide food for humans and wildlife. In North America's Chesapeake Bay, once-thriving eastern oyster (Crassostrea virginica) populations have declined dramatically, making their restoration and conservation extremely challenging. Here we present data on oyster size and human harvest from Chesapeake Bay archaeological sites spanning similar to 3,500 y of Native American, colonial, and historical occupation. We compare oysters from archaeological sites with Pleistocene oyster reefs that existed before human harvest, modern oyster reefs, and other records of human oyster harvest from around the world. Native American fisheries were focused on nearshore oysters and were likely harvested at a rate that was sustainable over centuries to millennia, despite changing Holocene climatic conditions and sea-level rise. These data document resilience in oyster populations under long-term Native American harvest, sea-level rise, and climate change; provide context for managing modern oyster fisheries in the Chesapeake Bay and elsewhere around the world; and demonstrate an interdisciplinary approach that can be applied broadly to other fisheries.
Controls on intermontane basin filling, isolation and incision on the margin of the Puna Plateau, NW Argentina (similar to 23 degrees S)
(2017-01-01) Streit, Rebecca L.; Burbank, Douglas W.; Strecker, Manfred R.; Alonso, Ricardo N.; Cottle, John M.; Kylander-Clark, Andrew R. C.
Intermontane basins are illuminating stratigraphic archives of uplift, denudation and environmental conditions within the heart of actively growing mountain ranges. Commonly, however, it is difficult to determine from the sedimentary record of an individual basin whether basin formation, aggradation and dissection were controlled primarily by climatic, tectonic or lithological changes and whether these drivers were local or regional in nature. By comparing the onset of deposition, sediment-accumulation rates, incision, deformation, changes in fluvial connectivity and sediment provenance in two interrelated intermontane basins, we can identify diverse controls on basin evolution. Here, we focus on the Casa Grande basin and the adjacent Humahuaca basin along the eastern margin of the Puna Plateau in northwest Argentina. Underpinning this analysis is the robust temporal framework provided by U-Pb geochronology of multiple volcanic ashes and our new magnetostratigraphical record in the Humahuaca basin. Between 3.8 and 0.8 Ma, similar to 120 m of fluvial and lacustrine sediments accumulated in the Casa Grande basin as the rate of uplift of the Sierra Alta, the bounding range to its east, outpaced fluvial incision by the Rio Yacoraite, which presently flows eastward across the range into the Humahuaca basin. Detrital zircon provenance analysis indicates a progressive loss of fluvial connectivity from the Casa Grande basin to the downstream Humahuaca basin between 3 and 2.1 Ma, resulting in the isolation of the Casa Grande basin from 2.1 Ma to < 1.7 Ma. This episode of basin isolation is attributed to aridification due to the uplift of the ranges to the east. Enhanced aridity decreased sediment supply to the Casa Grande basin to the point that aggradation could no longer keep pace with the rate of the surface uplift at the outlet of the basin. Synchronous events in the Casa Grande and Humahuaca basins suggest that both the initial onset of deposition above unconformities at similar to 3.8 Ma and the re-establishment of fluvial connectivity at similar to 0.8 Ma were controlled by climatic and/or tectonic changes affecting both basins. Reintegration of the fluvial network allowed subsequent incision in the Humahuaca basin to propagate upstream into the Casa Grande basin.
Improving undergraduate STEM education: The efficacy of discipline-based professional development
(2018-02-01) Manduca, Cathryn A.; Iverson, Ellen R.; Luxenberg, Michael; Macdonald, R. Heather; McConnell, David A.; Mogk, David W.; Tewksbury, Barbara J.
We sought to determine whether instructional practices used by undergraduate faculty in the geosciences have shifted from traditional teacher-centered lecture toward student-engaged teaching practices and to evaluate whether the national professional development program On the Cutting Edge (hereinafter Cutting Edge) has been a contributing factor in this change. We surveyed geoscience faculty across the United States in 2004, 2009, and 2012 and asked about teaching practices as well as levels of engagement in education research, scientific research, and professional development related to teaching. We tested these self-reported survey results with direct observations of teaching using the Reformed Teaching Observation Protocol, and we conducted interviews to understand what aspects of Cutting Edge have supported change. Survey data show that teaching strategies involving active learning have become more common, that these practices are concentrated in faculty who invest in learning about teaching, and that faculty investment in learning about teaching has increased. Regression analysis shows that, after controlling for other key influences, faculty who have participated in Cutting Edge programs and who regularly use resources on the Cutting Edge website are statistically more likely to use active learning teaching strategies. Cutting Edge participants also report that learning about teaching, the availability of teaching resources, and interactions with peers have supported changes in their teaching practice. Our data suggest that even one-time participation in a workshop with peers can lead to improved teaching by supporting a combination of affective and cognitive learning outcomes.
Asymmetric Hillslope Erosion Following Wildfire in Fourmile Canyon, Colorado
(2018-07-01) Abrahams, Edward R.; Kaste, James M.; Ouimet, William; Dethier, David P.
Infrequent, high-magnitude events cause a disproportionate amount of sediment transport on steep hillslopes, but few quantitative data are available that capture these processes. Here we study the influence of wildfire and hillslope aspect on soil erosion in Fourmile Canyon, Colorado. This region experienced the Fourmile Fire of 2010, strong summer convective storms in 2011 and 2012, and extreme flooding in September 2013. We sampled soils shortly after these events and use fallout radionuclides to trace erosion on polar- and equatorial-facing burned slopes and on a polar-facing unburned slope. Because these radionuclides are concentrated in the upper decimeter of soil, soil inventories are sensitive to erosion by surface runoff. The polar-facing burned slope had significantly lower cesium-137 (137Cs) and lead-210 (210Pb) inventories (p < 0.05) than either the polar-facing unburned slope or equatorial-facing burned slope. Local slope magnitude does not appear to control the erosional response to wildfire, as relatively gently sloping (~20%) polar-facing positions were severely eroded in the most intensively burned area. Field evidence and soil profile analyses indicate up to 4 cm of local soil erosion on the polar-facing burned slope, but radionuclide mass balance indicates that much of this was trapped nearby. Using a 137Cs-based erosion model, we find that the burned polar-facing slope had a net mean sediment loss of 2 mm (~1 kg m−2) over a one to three year period, which is one to two orders of magnitude higher than longer-term erosion rates reported for this region. In this part of the Colorado Front Range, strong hillslope asymmetry controls soil moisture and vegetation; polar-facing slopes support significantly denser pine and fir stands, which fuels more intense wildfires. We conclude that polar-facing slopes experience the most severe surface erosion following wildfires in this region, indicating that landscape-scale aridity can control the geomorphic response of hillslopes to wildfires. Copyright © 2018 John Wiley & Sons, Ltd.
Livelihood responses to mangrove deforestation in the northern provinces of Ecuador
(2013-01-01) Hamilton, Stuart E.
Mangrove forests worldwide are under threat. Ecuador is no exception to this trend, with substantial mangrove deforestation across almost all regions. This paper synthesizes a literature review of Ecuadorian mangroves, a remote sensing analysis of the past and present extent of mangrove forests conducted for another paper, and ethnographic field research conducted in the major estuaries of northern Ecuador to present the role of mangrove wetlands in supporting local livelihoods in Ecuador's coastal communities. This paper takes a macro-micro approach, examining the global questions of mangroves and then discussing the micro situation of mangroves in Ecuador before moving onto estuarine specific profiles. All the major mangrove regions of northern Ecuador are examined with a particular emphasis on deforestation / reforestation trends, the estuarine specific forces driving and responding to these trends, as well as the livelihood response of the impacted communities. The research relies on the most current estimates of mangrove forests as well as historic calculations of mangrove area.
Estimating total horizontal aeolian flux within shrub-invaded groundwater-dependent meadows using empirical and mechanistic models
(2013-01-01) Vest, Kimberly R.; Elmore, Andrew J.; Kaste, James M.; Okin, Gregory S.
Wind erosion is a significant environmental problem that removes soil resources from sensitive ecosystems and contributes to air pollution. In regions of shallow groundwater, friable (puffy) soils are maintained through capillary action, surface evaporation of solute-rich soil moisture, and protection from mobilization by groundwater-dependent grasses and shrubs. When a reduction in vegetation cover occurs through any disturbance process, there is potential for aeolian transport and dust emission. We find that as mean gap size between vegetation elements scaled by vegetation height increases, total horizontal aeolian sediment flux increases and explains 58% of the variation in total horizontal aeolian sediment flux. We also test a probabilistic model of wind erosion based on gap size between vegetation elements scaled by vegetation height (the Okin model), which predicts measured total horizontal aeolian sediment flux more closely than another commonly used model based on the average plant area observed in profile (Raupach model). The threshold shear velocity of bare soil appears to increase as gap size between vegetation elements scaled by vegetation height increases, reflecting either surface armoring or reduced interaction between the groundwater capillary zone and surface sediments. This work advances understanding of the importance of measuring gap size between vegetation elements scaled by vegetation height for empirically estimating Q and for structuring process-based models of desert wind erosion in groundwater-dependent vegetation.
Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers
(2017-10-02) Shobe, Charles M.; Hancock, Gregory S.; Eppes, Martha C.; Small, Eric E.
Erosion processes in bedrock-floored rivers shape channel cross-sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross-sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross-sections. We hypothesize that average cross-section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from similar to 0.1m/kyr to >0.8m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross-sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross-sections on one channel. All cross-sections showed significant (p
On the Cutting Edge: Teaching Help for Geoscience Faculty
(2010-01-01) Manduca, Cathryn A.; Fox, Sean P.; Iverson, Ellen R.; Macdonald, R. Heather
Constraining the timescales of sediment sequestration associated with large woody debris using cosmogenic Be-7
(2010-01-01) Fisher, G. B.; Fisher, G. B.; Magilligan, F. J.; Kaste, J. M.
The beneficial ecogeomorphic functions associated with large woody debris (LWD) in fluvial environments are well documented and include positive sediment impacts such as channel margin sequestration, increased substrate heterogeneity, and decreased channel embeddedness, as well as numerous secondary benefits such as nutrient retention and increased habitat heterogeneity. Despite an extensive literature documenting such positive sediment attributes of LWD in forested channels, a quantitative analysis of in-channel sediment storage times associated with channel obstructions has traditionally been difficult to assess. In this study along a 9 km stretch of the Ducktrap River in coastal Maine we present a novel application of fallout cosmogenic Be-7 (t(1/2) = 53 days) coupled with a constant initial activity (CIA) sediment aging model to quantitatively assess transitional bed load storage times in bars associated with in-channel obstructions (LWD and boulders). We find that reach-scale variability in unit stream power and LWD frequency affect sediment storage times, with transport-limited reaches providing longer-term sediment sequestration (generally > 100 days) associated with in-channel obstructions than supply limited ones (<100 >days). Estimates of sediment baraccumulation rates also varied between reaches from 0.2 g cm(-2) d(-1) in the supply limited reach to 0.7 g cm(-2) d(-1) in the transport-limited reach. Last, greater frequency of sites, increased sediment volumes and storage times, and naturally viable recruitment mechanisms for LWD in forested channels document its superior ecogeomorphic function when compared to boulders in this study, even in the Ducktrap river, where twentieth century logging has greatly reduced the size, frequency, and geomorphic efficacy of in-channel wood. This study has implications for channel restoration efforts and documents a novel application of 7Be and CIA methodology to constraining transitional bed load storage times in the fluvial environment.
Spatial and vertical distribution of mercury in upland forest soils across the northeastern United States
(2013-01-01) Richardson, Justin B; Friedland, Andrew J.; Engerbretson, Teresa J; Kaste, James M.
Assessing current Hg pools in forest soils of the northeastern U.S. is important for monitoring changes in Hg cycling. The forest floor, upper and lower mineral horizons were sampled at 17 long-term upland forest sites across the northeastern U.S. in 2011. Forest floor Hg concentration was similar across the study region (274 +/- 13 mu g kg(-1)) while Hg amount at northern sites (39 +/- g ha(-1)) was significantly greater than at western sites (11 +/- 4 g ha(-1)). Forest floor Hg was correlated with soil organic matter, soil pH, latitude and mean annual precipitation and these variables explained approximately 70% of the variability when multiple regressed. Mercury concentration and amount in the lower mineral soil was correlated with Fe, soil organic matter and latitude, corresponding with Bs horizons of Spodosols (Podzols). Our analysis shows the importance of regional and soil properties on Hg accumulation in forest soils. (C) 2013 Elsevier Ltd. All rights reserved.
Glacier response to North Atlantic climate variability during the Holocene
(2015-01-01) Balascio, N. L.; D'Andrea, W. J.; Balascio, N. L.; Bradley, R. S.
Small glaciers and ice caps respond rapidly to climate variations, and records of their past extent provide information on the natural envelope of past climate variability. Millennial-scale trends in Holocene glacier size are well documented and correspond with changes in Northern Hemisphere summer insolation. However, there is only sparse and fragmentary evidence for higher-frequency variations in glacier size because in many Northern Hemisphere regions glacier advances of the past few hundred years were the most extensive and destroyed the geomorphic evidence of ice growth and retreat during the past several thousand years. Thus, most glacier records have been of limited use for investigating centennial-scale climate forcing and feedback mechanisms. Here we report a continuous record of glacier activity for the last 9.5 ka from southeast Greenland derived from high-resolution measurements on a proglacial lake sediment sequence. Physical and geochemical parameters show that the glaciers responded to previously documented Northern Hemisphere climatic excursions, including the '8.2 ka' cooling event, the Holocene Thermal Maximum, Neoglacial cooling, and 20th century warming. In addition, the sediments indicate centennial-scale oscillations in glacier size during the late Holocene. Beginning at 4.1 ka, a series of abrupt glacier advances occurred, each lasting similar to 100 years and followed by a period of retreat, that were superimposed on a gradual trend toward larger glacier size. Thus, while declining summer insolation caused long-term cooling and glacier expansion during the late Holocene, climate system dynamics resulted in repeated episodes of glacier expansion and retreat on multi-decadal to centennial timescales. These episodes coincided with ice rafting events in the North Atlantic Ocean and periods of regional ice cap expansion, which confirms their regional significance and indicates that considerable glacier activity on these timescales is a normal feature of the cryosphere. The data provide a longer-term perspective on the rate of 20th century glacier retreat and indicate that recent anthropogenic-driven warming has already impacted the regional cryosphere in a manner outside the natural range of Holocene variability.

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