Formation of Box Canyon, Idaho, by megaflood: implications for seepage erosion on Earth and Mars

Amphitheater- headed canyons have been used as diagnostic indicators of erosion by groundwater seepage, which has important implications for landscape evolution on Earth and astrobiology on Mars. Of perhaps any canyon studied, Box Canyon, Idaho, most strongly meets the proposed morphologic criteria for groundwater sapping because it is incised into a basaltic plain with no drainage network upstream, and approximately 10 cubic meters per second of seepage emanates from its vertical headwall. However, sediment transport constraints, ^4He and ^14C dates, plunge pools, and scoured rock indicate that a megaflood (greater than 220 cubic meters per second) carved the canyon about 45,000 years ago. These results add to a growing recognition of Quaternary catastrophic flooding in the American northwest, and may imply that similar features on Mars also formed by floods rather than seepage erosion

Sequential Separation of Uranium, Hafnium and Neodymium from Natural Waters Concentrated by Iron Coprecipitation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/1/ggr12049-sup-0002-TableS1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/2/ggr12049-sup-0003-TableS2.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/3/ggr12049-sup-0008-TableS7.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/4/ggr12049-sup-0001-FigS1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/5/ggr12049.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/6/ggr12049-sup-0004-TableS3.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/7/ggr12049-sup-0005-TableS4.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/8/ggr12049-sup-0006-TableS5.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111909/9/ggr12049-sup-0007-TableS6.pd

Noble gas signatures in Greenland : tracing glacial meltwater sources

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 9311–9318, doi:10.1002/2015GL065778.This study represents the first comprehensive noble gas study in glacial meltwater from the Greenland Ice Sheet. It shows that most samples are in disequilibrium with surface collection conditions. A preliminary Ne and Xe analysis suggests that about half of the samples equilibrated at a temperature of ~0°C and altitudes between 1000 m and 2000 m, with a few samples pointing to lower equilibration altitudes and temperatures between 2°C and 5°C. Two samples suggest an origin as melted ice and complete lack of equilibration with surface conditions. A helium component analysis suggests that this glacial meltwater was isolated from the atmosphere prior to the 1950s, with most samples yielding residence times ≤ 420 years. Most samples represent a mixture between a dominant atmospheric component originating as precipitation and basal meltwater or groundwater, which has accumulated crustal 4He over time.University of Michigan; Packard Foundation; Department of Earth and Environmental Sciences Turner fellowship2016-05-0

Resistant calcification responses of Arctica islandica clams under ocean acidification conditions

Ocean acidification (OA) directly impacts marine calcifying organisms including ecologically and commercially important shellfish species such as Arctica islandica (A. islandica). To test whether documented growth resilience of A. islandica to OA is a general response across ages and populations or a function of adaptation to local habitat, we cultured juvenile and adult clams collected from an environment with little pH variation under four pH levels (7.5, 7.7, 7.9, and 8.1) for three months and integrated our understanding with relevant literature. The average shell growth over the experiment among all (69) individuals was 57 ± 55 μm, and there were no statistically significant differences in growth among pH treatments, including the control treatment, despite the general growth rate differences between juveniles and adults. Our results show that A. islandica can maintain its shell growth even in aragonite undersaturated (Ω < 1) conditions (0.65 and 0.83 for pH 7.5 and 7.7 treatments, respectively), supporting the hypothesis that resistance to OA conditions is likely a generalized response across populations. Although the present results show A. islandica can maintain their shell growth under short-term OA, long-term impacts of OA on A. islandica shell growth and other physical parameters including shell density and microstructure are still needed to better assess the sustainability of A. islandica in a more acidified future and to provide guidance on managing this important shellfish stock.publishedVersio

Microbial communities of the Lemon Creek Glacier show subtle structural variation yet stable phylogenetic composition over space and time

Glaciers are geologically important yet transient ecosystems that support diverse, biogeochemically significant microbial communities. During the melt season glaciers undergo dramatic physical, geochemical and biological changes that exert great influence on downstream biogeochemical cycles. Thus, we sought to understand the temporal melt-season dynamics of microbial communities and associated geochemistry at the terminus of Lemon Creek Glacier (LCG) in coastal southern Alaska. Due to late season snowfall, sampling of LCG occurred in three interconnected areas: proglacial Lake Thomas, the lower glacial outflow stream and the glacier’s terminus. LCG associated microbial communities were phylogenetically diverse and varied by sampling location. However, Betaproteobacteria, Alphaproteobacteria and Bacteroidetes dominated communities at all sampling locations. Strict anaerobic groups such as methanogens, SR1, and OP11 were also recovered from glacier outflows, indicating anoxic conditions in at least some portions of the LCG subglacial environment. Microbial community structure was significantly correlated with sampling location and sodium concentrations. Microbial communities sampled from terminus outflow waters exhibited day-to-day fluctuation in taxonomy and phylogenetic similarity. However, these communities were not significantly different from randomly constructed communities from all three sites. These results indicate that glacial outflows share a large proportion of phylogenetic overlap with downstream environments and that the observed significant shifts in community structure are driven by changes in relative abundance of different taxa, and not complete restructuring of communities. We conclude that LCG glacial discharge hosts a diverse and relatively stable microbiome that shifts at fine taxonomic scales in response to geochemistry and likely water residence time

An open source Bayesian Monte Carlo isotope mixing model with applications in Earth surface processes

The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multicomponent mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems, we expand our model to two additional case studies: deriving melt sources from δ18O, δD, and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ɛNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil data sets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems.Key Points:Open source BMC model determines source contributions in Earth surface systemsEffectively applied to stable and radiogenic isotope systems in various settingsModel able to encompass end‐member uncertainties and multiple isotopic systemsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/1/ggge20708.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/2/ggge20708-sup-0001-2014GC005683-ts01.pd

A high‐throughput system for boron microsublimation and isotope analysis by total evaporation thermal ionization mass spectrometry

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98802/1/rcm6619.pd

Radiogenic isotopic compositions of low concentration dust and aerosol from the GISP2 ice core

We have produced a record of radiogenic isotope compositions of particulate and soluble dust fractions from the GISP2 ice core, from basal ice to the surface. For each respective dust fraction we measured Sr and Nd isotope values to constrain the provenance of mineral dust and sea salt aerosol transported to the Greenland Ice Sheet. We have produced high-precision measurements of εNd values on sub-ng quantities of Nd in ice core samples via TIMS with 1013 ohm resistors. The range of measured εNd values of soluble material (− 12.9 ± 2.6 to − 7.3 ± 0.9) is interpreted to represent sea salt aerosol based on Sr isotope compositions that resemble seawater values (0.711007 ± 0.000030 to 0.709416 ± 0.000072). Therefore, the εNd potentially provides important insights to the atmospheric circulation patterns at high latitudes at least through MIS 5. We compare εNd values of surface seawater from ocean basins and deltas at high latitudes to alternating relatively radiogenic and unradiogenic εNd at GISP2. We suggest these changes represent transitions between a climate system dominated by enhanced and persistent Arctic Oscillation positive anomalies (as determined from sea level and ocean bottom pressure measurements), and a state representative of modern atmospheric circulation, which typically exhibits either negative anomalies or none altogether

Hf-W chronology of the eucrite parent body

The Hf-182-W-182 systematics of 12 whole-rock eucrites, including basaltic and cumulate samples, have been investigated. The Hf-W isotope systematics of both basaltic and cumulate eucrites are consistent with derivation from a single mantle source characterized by a strongly suprachondritic Hf/W ratio (Hf-180/W-184 of similar to 19). The elevated Hf/W of this mantle source was established by core formation within similar to 1 Ma after CAI formation or, alternatively, represents that of the residual melt of a magma ocean from which the eucrites ultimately formed. In the latter case the time of core formation is more uncertain and core formation may have occurred slightly later than similar to 1 Ma. The investigated basaltic eucrites fall into three distinct age groups with Hf-W ages of similar to 4 Ma (Stannern), similar to 11 Ma (Bereba, Bouvante) and similar to 22 Ma (Camel Donga, Juvinas) after CAI formation and provide evidence for a protracted history of magmatism and crustal metamorphism on the eucrite parent body, lasting for at least similar to 20 Ma. Evidence for even later activity is provided by the cumulate eucrites, which exhibit only small if any variations in W-182/W-184 in spite of variable Hf-180/W-184, indicating cooling below the Hf-W closure temperature when Hf-182 was nearly extinct. On the basis of three cumulate eucrites, a Hf-W age of 38 +/- 21Ma is inferred, but even younger ages would also be consistent with the Hf-W data. (C) 2015 Elsevier Ltd. All rights reserved