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  • 1
    Publication Date: 2019-07-13
    Description: Ancient lake deposits and valley networks on Mars provide strong evidence that its surface was once modified by liquid water, but the extent of that modification is still debated. Ancient lacustrine deposits in Milna Crater provide insight into the timescale and fluid volume required to construct fluvially derived sedimentary deposits near the Noachian-Hesperian boundary. Placing the lacustrine deposits their regional context in Paran Valles provides a quantitative measurement of the intermittency of large, water-mediated sediment transport events in that region.
    Keywords: Astrophysics
    Type: MSFC-E-DAA-TN44742 , International Conference on Early Mars: Geologic, Hydrologic, and Climatic Evolution and the Implications for Life; 2-6 Oct. 2017; Flaggstaff, AZ; United States
    Format: application/pdf
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  • 2
    Publication Date: 2016-06-24
    Description: Fill terraces along rivers represent the legacy of aggradation periods that are most commonly attributed to climate change. In the North Fork of the San Gabriel River, an arid bedrock landscape in the San Gabriel Mountains, California, a series of prominent fill terraces was previously related to climate-change–induced pulses of hillslope sediment supply that temporarily and repeatedly overwhelmed river transport capacity during the Quaternary. Based on field observations, digital topographic analysis, and dating of Quaternary deposits, we suggest instead that valley aggradation was spatially confined to the North Fork San Gabriel Canyon and was a consequence of the sudden supply of unconsolidated material to upstream reaches by one of the largest known landslides in the San Gabriel Mountains. New 10 Be-derived surface exposure ages from the landslide deposits, previously assumed to be early to middle Pleistocene in age, indicate at least three Holocene events at ca. 8–9 ka, ca. 4–5 ka, and ca. 0.5–1 ka. The oldest and presumably most extensive landslide predates the valley aggradation period, which is constrained by existing 14 C ages and new luminescence ages to ca. 7–8 ka. The spatial distribution, morphology, and sedimentology of the river terraces are consistent with deposition from far-traveling debris flows that originated within, and mined, the landslide deposits. Valley aggradation in the North Fork San Gabriel Canyon therefore resulted from locally enhanced sediment supply that temporarily overwhelmed river transport capacity, but the lack of similar deposits in other parts of the San Gabriel Mountains argues against a regional climatic signal. Our study highlights the potential for valley aggradation by debris flows in arid bedrock landscapes downstream of landslides that occupy headwater areas.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 3
    Publication Date: 2016-05-22
    Description: River deltas worldwide are currently under threat of drowning and destruction by sea-level rise, subsidence, and oceanic storms, highlighting the need to quantify their growth processes. Deltas are built through construction of sediment lobes, and emerging theories suggest that the size of delta lobes scales with backwater hydrodynamics, but these ideas are difficult to test on natural deltas that evolve slowly. We show results of the first laboratory delta built through successive deposition of lobes that maintain a constant size. We show that the characteristic size of delta lobes emerges because of a preferential avulsion node—the location where the river course periodically and abruptly shifts—that remains fixed spatially relative to the prograding shoreline. The preferential avulsion node in our experiments is a consequence of multiple river floods and Froude-subcritical flows that produce persistent nonuniform flows and a peak in net channel deposition within the backwater zone of the coastal river. In contrast, experimental deltas without multiple floods produce flows with uniform velocities and delta lobes that lack a characteristic size. Results have broad applications to sustainable management of deltas and for decoding their stratigraphic record on Earth and Mars.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
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  • 4
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-11
    Description: Earth’s most severe climate changes occurred during global-scale "snowball Earth" glaciations, which profoundly altered the planet’s atmosphere, oceans, and biosphere. Extreme rates of glacioeustatic sea level rise are predicted by the snowball Earth hypothesis, but supporting geologic evidence has been lacking. We use paleohydraulic analysis of wave ripples and tidal laminae in the Elatina Formation, Australia—deposited after the Marinoan glaciation ~635 million years ago—to show that water depths of 9 to 16 meters remained nearly constant for ~100 years throughout 27 meters of sediment accumulation. This accumulation rate was too great to have been accommodated by subsidence and instead indicates an extraordinarily rapid rate of sea level rise (0.2 to 0.27 meters per year). Our results substantiate a fundamental prediction of snowball Earth models of rapid deglaciation during the early transition to a supergreenhouse climate.
    Keywords: Geochemistry, Geophysics
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 5
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    Geological Society of America (GSA)
    In: Geology
    Publication Date: 2017-02-23
    Description: Water flowing over sand in fluvial and marine settings often results in the formation of current ripples. Found in modern and ancient deposits on Earth and Mars, ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records. Despite decades of observations of current ripples, there is no universal scaling relation to predict their size or to distinguish them from dunes. Here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows. Results show that ripples are larger with more viscous fluids, coarser grains, smaller bed shear stresses, and smaller specific gravity of sediment. The scaling relation also segregates ripples from dunes, highlighting a narrow regime of transitional bedforms that have morphologic properties and sediment transport conditions that overlap with both ripples and dunes. Our analysis shows that previous absolute size–based definitions of ripples and dunes only hold for certain conditions, such as water flows transporting siliciclastic grains on Earth. The new theory allows estimates of ripple sizes in foreign fluids and on other planets, including meter-scale ripples in methane flows on Titan or in viscous brines on Mars.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 6
    Publication Date: 2011-11-23
    Description: Large bedrock landslides have been shown to modulate rates and processes of river activity by forming dams, forcing upstream aggradation of water and sediment, and generating catastrophic outburst floods. Less apparent is the effect of large landslide dams on river ecosystems and marine sedimentation. Combining analyses of 1-m resolution topographic data (acquired via airborne laser mapping) and field investigation, we present evidence for a large, landslide-dammed paleolake along the Eel River, CA. The landslide mass initiated from a high-relief, resistant outcrop which failed catastrophically, blocking the Eel River with an approximately 130-m-tall dam. Support for the resulting 55-km-long, 1.3-km3 lake includes subtle shorelines cut into bounding terrain, deltas, and lacustrine sediments radiocarbon dated to 22.5 ka. The landslide provides an explanation for the recent genetic divergence of local anadromous (ocean-run) steelhead trout (Oncorhynchus mykiss) by blocking their migration route and causing gene flow between summer run and winter run reproductive ecotypes. Further, the dam arrested the prodigious flux of sediment down the Eel River; this cessation is recorded in marine sedimentary deposits as a 10-fold reduction in deposition rates of Eel-derived sediment and constitutes a rare example of a terrestrial event transmitted through the dispersal system and recorded offshore.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 7
    Publication Date: 2015-02-04
    Description: Detailed chemical abundances for five stars in two Galactic globular clusters, NGC 5466 and NGC 5024, are presented from high-resolution optical (from the Hobby-Eberley Telescope) and infrared spectra (from the SDSS-III APOGEE survey). We find [Fe/H] = –1.97 ± 0.13 dex for NGC 5466, and [Fe/H] = –2.06 ± 0.13 dex for NGC 5024, and the typical abundance pattern for globular clusters for the remaining elements, e.g. both show evidence for mixing in their light element abundance ratios (C, N), and asymptotic giant branch contributions in their heavy element abundances (Y, Ba, and Eu). These clusters were selected to examine chemical trends that may correlate them with the Sgr dwarf galaxy remnant, but at these low metallicities no obvious differences from the Galactic abundance pattern are found. Regardless, we compare our results from the optical and infrared analyses to find that oxygen and silicon abundances determined from the infrared spectral lines are in better agreement with the other α-element ratios and with smaller random errors.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 8
    Publication Date: 2016-10-08
    Description: Deciphering erosion rates over geologic time is fundamental for understanding the interplay between climate, tectonic, and erosional processes. Existing techniques integrate erosion over different time scales, and direct comparison of such rates is routinely done in earth science. On the basis of a global compilation, we show that erosion rate estimates in glaciated landscapes may be affected by a systematic averaging bias that produces higher estimated erosion rates toward the present, which do not reflect straightforward changes in erosion rates through time. This trend can result from a heavy-tailed distribution of erosional hiatuses (that is, time periods where no or relatively slow erosion occurs). We argue that such a distribution can result from the intermittency of erosional processes in glaciated landscapes that are tightly coupled to climate variability from decadal to millennial time scales. In contrast, we find no evidence for a time scale bias in spatially averaged erosion rates of landscapes dominated by river incision. We discuss the implications of our findings in the context of the proposed coupling between climate and tectonics, and interpreting erosion rate estimates with different averaging time scales through geologic time.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
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  • 9
    Publication Date: 2014-02-28
    Description: Sediment transport in mountain channels controls the evolution of mountainous terrain in response to climate and tectonics and presents major hazards to life and infrastructure worldwide. Despite its importance, we lack data on when sediment moves in steep channels and whether movement occurs by rivers or debris flows. We address this knowledge gap using laboratory experiments on initial sediment motion that cross the river to debris-flow sediment-transport transition. Results show that initial sediment motion by river processes requires heightened dimensionless bed shear stress (or critical Shields stress) with increasing channel-bed slope by as much as fivefold the conventional criterion established for lowland rivers. Beyond a threshold slope of ~22°, the channel bed fails, initiating a debris flow prior to any fluvial transport, and the critical Shields stress within the debris-flow regime decreases with increasing channel-bed slope. Combining theories for both fluvial and debris-flow incipient transport results in a new phase space for sediment stability, with implications for predicting fluvial sediment transport rates, mitigating debris-flow hazards, and modeling channel form and landscape evolution.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 10
    Publication Date: 2014-06-28
    Description: Upstream knickpoint propagation is an important mechanism for channel incision, and it communicates changes in climate, sea level, and tectonics throughout a landscape. Few studies have directly measured the long-term rate of knickpoint retreat, however, and the mechanisms for knickpoint initiation are debated. Here, we use cosmogenic 3 He exposure dating to document the retreat rate of a waterfall in Ka’ula’ula Valley, Kaua‘i, Hawai‘i, an often-used site for knickpoint-erosion modeling. Cosmogenic exposure ages of abandoned surfaces are oldest near the coast (120 ka) and systematically decrease with upstream distance toward the waterfall (〈10 ka), suggesting that the waterfall migrated nearly 4 km over the past 120 k.y. at an average rate of 33 mm/yr. Upstream of the knickpoint, cosmogenic nuclide concentrations in the channel are approximately uniform and indicate steady-state vertical erosion at a rate of ~0.03 mm/yr. Field observations and topographic analysis suggest that waterfall retreat is dominated by block toppling, with sediment transport below the waterfall actively occurring by debris flows. Knickpoint initiation was previously attributed to a submarine landslide ca. 4 Ma; however, our dating results, bathymetric analysis, and landscape-evolution modeling support knickpoint generation by wave-induced sea-cliff erosion during the last interglacial sea-level highstand ca. 120–130 ka. We illustrate that knickpoint generation during sea-level highstands, as opposed to the typical case of sea-level fall, is an important relief-generating mechanism on stable or subsiding steep coasts, and likely drives transient pulses of significant sediment flux.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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