Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars

A Late Noachian-aged alluvial fan complex within Harris Crater in far western Terra Tyrrhena, Mars, is comprised of two well-defined source regions and associated discrete depositional lobes. Three fan units were recognized based on common morphological characteristics, thermal properties and spectral signatures. Although the entire fan complex has been subjected to extensive erosional degradation, the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred; the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow. This transition from fluvial to late-stage debris flow(s) suggests a decline in available water and/or change in sediment supply. The thermal inertia values obtained for all three fan surface units (mean values ranged from 318 to 344 J m⁻² K⁻¹ s^−1/2) are typical for coarse-grained and/or well-indurated materials on Mars, but subtle variations point to important distinctions. Variations in aeolian bedform coverage as well as the density of ridges (inferred inverted channels) and boulders contribute to these subtle fan thermophysical differences and likely reflect changes in the fan depositional mechanisms and variations in post-depositional modification histories. The majority of the alluvial fan surface has a spectral signature that is broadly similar to TES “Surface Type 2” (ST2), with some important exceptions at long wavelengths. However, a unique spectral component was identified in one of the fan units (unit 3), that likely reflects lithological differences from other fan materials. This spectral attribute of unit 3 matched locations within the western catchment providing confirmation of provenance and supporting the contention that sediment supply changed over time as the fan developed. Finally, we applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history. Using the computed eastern fan volume (32 km³), significant water, likely from precipitation, was involved in fan construction (>50 km³) and an extensive period of fan formation occurred over millennia or longer.     

Rotational modeling of Hyperion

Saturn’s moon, Hyperion, is subject to strongly-varying solid body torques from its primary and lacks a stable spin state resonant with its orbital frequency. In fact, its rotation is chaotic, with a Lyapunov timescale on the order of 100 days. In 2005, Cassini made three close passes of Hyperion at intervals of 40 and 67 days, when the moon was imaged extensively and the spin state could be measured. Curiously, the spin axis was observed at the same location within the body, within errors, during all three fly-bys—~ 30° from the long axis of the moon and rotating between 4.2 and 4.5 times faster than the synchronous rate. Our dynamical modeling predicts that the rotation axis should be precessing within the body, with a period of ~ 16 days. If the spin axis retains its orientation during all three fly-bys, then this puts a strong constraint on the in-body precessional period, and thus the moments of inertia. However, the location of the principal axes in our model are derived from the shape model of Hyperion, assuming a uniform composition. This may not be a valid assumption, as Hyperion has significant void space, as shown by its density of 544± 50  kg m⁻³ (Thomas et al. in Nature 448:50, 2007). This paper will examine both a rotation model with principal axes fixed by the shape model, and one with offsets from the shape model. We favor the latter interpretation, which produces a best-fit with principal axes offset of ~ 30° from the shape model, placing the A axis at the spin axis in 2005, but returns a lower reduced χ ² than the best-fit fixed-axes model.     

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249, 148-161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal.By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U-Th)/He thermochronometry. We show how (U-Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U-Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1-0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U-Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U-Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories.Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U-Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U-Th)/He dates are actually older than the corresponding fission-track dates.     

Streambank sediment loading rates at the watershed scale and the benefit of riparian protection

Streambank erosion is a pathway for sediment and nutrient loading to streams, but insufficient data exist on the magnitude of this source. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. The objective of this research was to quantify watershed‐scale streambank erosion and estimate the benefits of riparian protection. The research focused on Spavinaw Creek within the Eucha‐Spavinaw watershed in eastern Oklahoma, where composite streambanks consist of a small cohesive topsoil layer underlain by non‐cohesive gravel. Fine sediment erosion from 2003 to 2013 was derived using aerial photography and processed in ArcMap to quantify eroded area. ArcMap was also utilized in determining the bank retreat rate at various locations in relation to the riparian vegetation buffer width. Box and whisker plots clearly showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks, statistically significant based on non‐parametric t‐tests. The total soil mass eroded from 2003 to 2013 was estimated at 7.27 × 10⁷ kg yr.^?1, and the average bank retreat was 2.5 m yr.^?1. Many current erosion models assume that fluvial erosion is the dominant stream erosion process. Bank retreat was positively correlated with stream discharge and/or stream power, but with considerable variability, suggesting that mass wasting plays an important role in streambank erosion within this watershed. Finally, watershed monitoring programs commonly characterize erosion at only a few sites and may scale results to the entire watershed. Selection of random sites and scaling to the watershed scale greatly underestimated the actual erosion and loading rates. Copyright © 2016 John Wiley & Sons, Ltd.     

The structure and transformation of the nanomineral schwertmannite: a synthetic analog representative of field samples

The phase transformation of schwertmannite, an iron oxyhydroxide sulfate nanomineral synthesized at room temperature and at 75 °C using H₂O₂ to drive the precipitation of schwertmannite from ferrous sulfate (Regenspurg et al. in Geochim Cosmochim Acta 68:1185–1197, 2004), was studied using high-resolution transmission electron microscopy. The results of this study suggest that schwertmannite synthesized using this method should not be described as a single phase with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nanometers in diameter, within a characteristic ‘pin-cushion’-like amorphous matrix. The difference in synthesis temperature affected the density of the needles on the schwertmannite surface. The needles on the higher-temperature schwertmannite displayed a dendritic morphology, whereas the needles on the room-temperature schwertmannite were more closely packed. Visible lattice fringes in the schwertmannite samples are consistent with the powder X-ray diffraction (XRD) pattern taken on the bulk schwertmannite and also matched d-spacings for goethite, indicating a close structural relationship between schwertmannite and goethite. The incomplete transformation from schwertmannite to goethite over 24 h at 75 °C was tracked using XRD and TEM. TEM images suggest that the sample collected after 24 h consists of aggregates of goethite nanocrystals. Comparing the synthetic schwertmannite in this study to a study on schwertmannite produced at 85 °C, which used ferric sulfate, reveals that synthesis conditions can result in significant differences in needle crystal structure. The bulk powder XRD patterns for the schwertmannite produced using these two samples were indistinguishable from one another. Future studies using synthetic schwertmannite should account for these differences when determining schwertmannite’s structure, reactivity, and capacity to take up elements like arsenic. The schwertmannite synthesized by the Regenspurg et al. method produces a mineral that is consistent with the structure and morphology of natural schwertmannite observed in our previous study using XRD and TEM, making this an ideal synthetic method for laboratory-based mineralogical and geochemical studies that intend to be environmentally relevant.