Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.     

Using local knowledge to improve understanding of groundwater supplies in parts of arid South Australia

Developing a good working knowledge of water in the landscape and the connectivity between surface and groundwater is germane to a resilient sustainable water supply to sustain human activities and a healthy environment, particularly in the drier parts of Australia. This paper presents a case study in the semi-arid Flinders Ranges of South Australia in which eliciting local knowledge from landholders was a key element in mapping surface and groundwater and investigating the connectivity between water resources. Local landholders provided a vital source of information to augment the limited scientific data initially available on groundwater resources. Groundwater recharge in the region comes from local rainfall onto individual unconnected groundwater catchments. In most of the region water supply seems to have been maintained over the longer term but there are a number of sites where former active springs are now dry or much reduced in size and water supplies from bores have substantially changed in quantity and quality. The involvement of landholders in the investigation of water resources encouraged them to consider the resource context of their stock watering points and incorporate more science into their understanding of the water resources that they rely on. This stimulated interest in investigating more about local water resources and changes in water management practices.     

Evaluation of water‐use policies for baseflow recovery during droughts in an agricultural intensive karst watershed: Case study of the lower Apalachicola–Chattahoochee–Flint River Basin,southeastern United States

The lower Apalachicola–Chattahoochee–Flint River Basin in the Southeast United States represents a major agricultural area underlain by the highly productive karstic Upper Floridan aquifer (UFA). During El Niño Southern Oscillation‐induced droughts, intense groundwater withdrawal for irrigation lowers streamflow in the Flint River due to its hydraulic connectivity with the UFA and threatens the habitat of the federally listed and endangered aquatic biota. This study assessed the compounding hydrologic effects of increased irrigation pumping during drought years (2010–2012) on stream–aquifer water exchange (stream–aquifer flux) between the Flint River and UFA using the United States Geological Survey modular finite element groundwater flow model. Principal component and K‐means clustering analyses were used to identify critical stream reaches and tributaries that are adversely affected by irrigation pumping. Additionally, the effectiveness of possible water restriction scenarios on stream–aquifer flux was also analysed. Moreover, a cost–benefit analysis of acreage buyout procedure was conducted for various water restriction scenarios. Results indicate that increased groundwater withdrawal in Water Year 2011 decreased baseflow in the lower Apalachicola–Chattahoochee–Flint River Basin, particularly, in Spring Creek, where irrigation pumping during April, June, and July changed the creek condition from a gaining to losing stream. Results from sensitivity analysis and simulated water restrictions suggest that reducing pumping in selected sensitive areas is more effective in streamflow recovery (approximately 78%) than is reducing irrigation intensity by a prescribed percentage of current pumping rates, such as 15% or 30%, throughout the basin. Moreover, analysis of acreage buyout indicates that restrictions on irrigation withdrawal can have significant impacts on stream–aquifer flux in the Basin, especially in critical watersheds such as Spring and Ichawaynochaway Creeks. The proposed procedure for ranking of stream reaches (sensitivity analysis) in this study can be replicated in other study areas/models. This study provides useful information to policymakers for devising alternate irrigation water withdrawal policies during droughts for maintaining flow levels in the study area.     

Littoral cladoceran community reassembly following the cessation of disturbance

The process of recovery from environmental impact provides insights into factors that drive community assembly. This study investigated community reassembly following perturbation, using littoral cladoceran microfossils. Whole-lake experimental manipulations of acids and nutrients in Lake 302N (L302N; 1972–1997) and acids in Lake 223 (L223; 1976–1994) at the Experimental Lakes Area (ELA; Ontario, Canada), provided an opportunity to use the sediment record to reconstruct the trajectory of recovery. Comparison was made to unmanipulated Lake 377 (L377) to estimate changes in regional baseline conditions. Recovery was evaluated at both the species and community level, using univariate and multivariate metrics. Up to 14 years after cessation of acid and nutrient additions, and following chemical recovery to pH > 6.5, both L302N and L223 littoral cladoceran communities failed to recover to their pre-manipulation states. Multivariate metrics demonstrated hysteresis along the recovery trajectory, and the movement to alternative states in littoral cladoceran communities in both lakes. The most recent littoral cladoceran community structure, reconstructed from the paleorecord, is driven by the persistence of species from the acidification period, suggesting biological resistance. Recovery states differed between L302N and L223, although their initial, pre-manipulation littoral cladoceran community structures were similar, reflecting multiple, independent recovery trajectories. Independent recovery trajectories are at odds with previous findings, which suggest that dispersal from an egg bank should support deterministic recovery trajectories. We conclude that littoral cladoceran microfossils can be used effectively to determine the community state with recovery and compare it to the pre-manipulation condition, particularly with the use of multivariate metrics.     

Coccolithophores as palaeoecological indicators for shifts of the ITCZ in the Cariaco Basin during the late Quaternary

Coccoliths were studied from the ODP Hole 1002C and core PL07‐39PC in the Cariaco Basin. Increases in Emiliania huxleyi are synchronous with decreases of Gephyrocapsa oceanica and vice versa. A new index (GEX) based on the relative abundances of these two taxa is proposed, and correlates with various other proxies. It is shown that GEX can serve as upwelling proxy. This confirms that the Intertropical Convergence Zone shifted north during the Bølling/Allerød, south during the Younger Dryas and back north during the Preboreal. The upwelling proxy shows few discrepancies with the terrigenous record. Copyright © 2008 John Wiley & Sons, Ltd.     

Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab₉₁Or₁An₈), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab_2.5Or_97.4An_0.1) associated with very pure porous albite (Ab_97.0Or_1.6An_1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 10⁴–10⁵ year after emplacement, but there are no microtextural constraints on the age of UPLF formation.