HiRISE views enigmatic deposits in the Sirenum Fossae region of Mars

HiRISE images together with other recent orbital data from Mars define new characteristics of enigmatic Hesperian-aged deposits in Sirenum Fossae that are mostly 100-200 m thick, drape kilometers of relief, and often display generally low relief surfaces. New characteristics of the deposits, previously mapped as the “Electris deposits,” include local detection of meter-scale beds that show truncating relationships, a generally light-toned nature, and a variably blocky, weakly indurated appearance. Boulders shed by erosion of the deposits are readily broken down and contribute little to talus. Thermal inertia values for the deposits are ∼200 J m⁻² K⁻¹ s^−1/2 and they may incorporate hydrated minerals derived from weathering of basalt. The deposits do not contain anomalous amounts of water or water ice. Deflation may dominate degradation of the deposits over time and points to an inventory of fine-grained sediment. Together with constraints imposed by the regional setting on formation processes, these newly resolved characteristics are most consistent with an eolian origin as a loess-like deposit comprised of redistributed and somewhat altered volcanic ash. Constituent sediments may be derived from airfall ash deposits in the Tharsis region. An origin directly related to airfall ash or similar volcanic materials is less probable and emplacement by alluvial/fluvial, impact, lacustrine, or relict polar processes is even less likely.     

Landscape and climatic changes during the end of the Late Prehistory in the Amblés Valley (Ávila, central Spain), from 1200 to 400 cal BC

Correlations between ecological and cultural changes occurred during a short period between the end of Subboreal and the beginning of Subatlantic in the Amblés Valley (Ávila, central Spain) are analyzed, taking into account palaeopalynological and archaeological data. Plant dynamics from pollen analyses, both from archaeological sites and peat bogs, have been interpreted in relation to human settlements and the transformation of economic practices. These provided a comprehensive hypothesis on human/climate interactions at the beginning of the 1st millennium cal BC. There was an ecological crisis in the region that lasted for a century (ca. 850–760 cal BC). This was especially sharp due to both the geographical constraints of the area and its previous agrarian history. This event implied a sudden and abrupt climatic change from xeric and warm conditions (Subboreal) to more humid and cooler ones (Subatlantic). Environmental stress derived from climatic crisis could be an important factor in the explanation of the historical process, whose main consequences were both the origin of the Iron Age peasant villages and the end of the ‘dehesa’ type landscape.     

Three monthly coral Sr/Ca records from the Chagos Archipelago covering the period of 1950–1995 A.D.: reproducibility and implications for quantitative reconstructions of sea surface temperature variations

In order to assess the fidelity of coral Sr/Ca for quantitative reconstructions of sea surface temperature variations, we have generated three monthly Sr/Ca time series from Porites corals from the lagoon of Peros Banhos (71°E, 5°S, Chagos Archipelago). We find that all three coral Sr/Ca time series are well correlated with instrumental records of sea surface temperature (SST) and air temperature. However, the intrinsic variance of the single-core Sr/Ca time series differs from core to core, limiting their use for quantitative estimates of past temperature variations. Averaging the single-core data improves the correlation with instrumental temperature (r > 0.7) and allows accurate estimates of interannual temperature variations (~0.35°C or better). All Sr/Ca time series indicate a shift towards warmer temperatures in the mid-1970s, which coincides with the most recent regime shift in the Pacific Ocean. However, the magnitude of the warming inferred from coral Sr/Ca differs from core to core and ranges from 0.26 to 0.75°C. The composite Sr/Ca record from Peros Banhos clearly captures the major climatic signals in the Indo-Pacific Ocean, i.e. the El Niño–southern oscillation and the Pacific decadal oscillation. Moreover, composite Sr/Ca is highly correlated with tropical mean temperatures (r = 0.7), suggesting that coral Sr/Ca time series from the tropical Indian Ocean will contribute to multi-proxy reconstructions of tropical mean temperatures.     

Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming

We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.     

Comparison of risk from pyroclastic density current hazards to critical infrastructure in Mammoth Lakes,California, USA,from a new Inyo craters rhyolite dike eruption versus a dacitic dome eruption on Mammoth Mountain

Renewed volcanic activity near Mammoth Lakes, California, in the form of dome-collapse pyroclastic density currents (PDCs) from either a new eruption at Mammoth Mountain or the Inyo craters would pose a significant hazard to critical infrastructure there. This paper compares the risk from PDC impact hazards upon selected critical infrastructure from: (1) a 100 m tall dacite dome on Mammoth Mountain and (2) three 200 m tall rhyolite domes at the southern end of the Inyo craters. For each scenario, maximum estimated dynamic pressure and velocity from two PDC volumes (10⁶ and 10⁷ m³) are modeled with the EXPLORIS PDC software (Toyos et al. Nat Hazards 41(1):99–112, 2007). Risk to critical infrastructure from Mammoth Mountain PDCs would be much greater than the Inyo PDCs because of both location and the greater kinetic energy of the Mammoth PDC material, providing comparative insight to planners should a real eruption at one location or the other be forthcoming.     

The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.     

Vulnerability of water supply from the Oregon Cascades to changing climate: Linking science to users and policy

Despite improvements in understanding biophysical response to climate change, a better understanding of how such changes will affect societies is still needed. We evaluated effects of climate change on the coupled human-environmental system of the McKenzie River watershed in the Oregon Cascades in order to assess its vulnerability. Published empirical and modeling results indicate that climate change will alter both the timing and quantity of streamflow, but understanding how these changes will impact different water users is essential to facilitate adaptation to changing conditions. In order to better understand the vulnerability of four water use sectors to changing streamflow, we conducted a series of semi-structured interviews with representatives of each sector, in which we presented projected changes in streamflow and asked respondents to assess how changing water availability would impact their activities. In the McKenzie River watershed, there are distinct spatial and temporal patterns associated with sensitivity of water resources to climate change. This research illustrates that the implications of changing streamflow vary substantially among different water users, with vulnerabilities being determined in part by the spatial scale and timing of water use and the flexibility of those uses in time and space. Furthermore, institutions within some sectors were found to be better positioned to effectively respond to changes in water resources associated with climate change, while others have substantial barriers to the flexibility needed to manage for new conditions. A clearer understanding of these opportunities and constraints across water use sectors can provide a basis for improving response capacity and potentially reducing vulnerability to changing water resources in the region.     

Paleomagnetic and rock-magnetic survey of eocene dike swarms from the Tecalitlan area (Western Mexico): Tectonic implications

For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacán block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacán block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α₉₅ = 8.9°. The corresponding mean paleomagnetic pole position is P_lat = 75.7°, P_long = 166.6°, K = 31, A₉₅ = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacán Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.     

Historical channel narrowing along the Rio Grande near Albuquerque,New Mexico in response to peak discharge reductions and engineering: magnitude and uncertainty of change from air photo measurements

Over the last century, geomorphic processes along the Middle Rio Grande have been altered by flood control and bank stabilization projects, intensified land and water use, and climate change. In response to potential risks to infrastructure and ecological integrity, recent (1985–2008) adjustment was investigated and historic (1918–1985) changes in Rio Grande channel planform through the Albuquerque, New Mexico, area were reviewed, especially in relation to changes in annual peak discharge and river engineering measures. Using a GIS, channel characteristics were digitized from georeferenced photographs and analyzed with particular attention to quantifying potential measurement error and its propagation. Error associated with average channel widths and channel area ranged between 4 and 13%. For smaller polygons, e.g. islands, error was higher (11 to 40% for width and >200% for area) because width error is large relative to polygon width. Between 1918 and 1963, average channel widths decreased 8 m/yr, from 516 ± 67 m to 176 ± 7 m, mostly due to decreasing peak flows and the implementation of flood control and other engineering measures. From 1985 to 2008, widths decreased 0·7 m/yr, from 176 ± 23 m to 146 ± 5 m, accompanied by an increase in vegetated island area which largely coincided with low flow periods. Narrowing was concentrated at tributary inputs and in the upstream part of the reach, where bedload trapping by Cochiti Dam has caused degradation. Bank protection structures and dense vegetation limit bank erosion in the reach, but erosion is significant where expanding islands, incision, and increased meandering force water against banks. Copyright © 2010 John Wiley & Sons, Ltd.     

Bayesian hierarchical models for soil CO<Subscript>2</Subscript> flux and leak detection at geologic sequestration sites

Proper characterizations of background soil CO₂ respiration rates are critical for interpreting CO₂ leakage monitoring results at geologic sequestration sites. In this paper, a method is developed for determining temperature-dependent critical values of soil CO₂ flux for preliminary leak detection inference. The method is illustrated using surface CO₂ flux measurements obtained from the AmeriFlux network fit with alternative models for the soil CO₂ flux versus soil temperature relationship. The models are fit first to determine pooled parameter estimates across the sites, then using a Bayesian hierarchical method to obtain both global and site-specific parameter estimates. Model comparisons are made using the deviance information criterion (DIC), which considers both goodness of fit and model complexity. The hierarchical models consistently outperform the corresponding pooled models, demonstrating the need for site-specific data and estimates when determining relationships for background soil respiration. A hierarchical model that relates the square root of the CO₂ flux to a quadratic function of soil temperature is found to provide the best fit for the AmeriFlux sites among the models tested. This model also yields effective prediction intervals, consistent with the upper envelope of the flux data across the modeled sites and temperature ranges. Calculation of upper prediction intervals using the proposed method can provide a basis for setting critical values in CO₂ leak detection monitoring at sequestration sites.