Geochemical mobility of arsenic in the surficial waters from Argentina

The presence of arsenic (As) in surface water constitutes an important environmental risk, where mobility and adsorption processes are responsible for its behavior in the sediment–water interface. Therefore, the assessment of adsorption, mobility and water availability of arsenic in freshwater sediments, with agricultural, livestock and urban soil uses was performed. Arsenic concentrations in sediments ranged from 5.4 to 15.9 mg kg^?1 (total) and 2.8 to 6.5 mg kg^?1 (labile), and those of iron and manganese were 11,563–23,500 and 140.6–662.1 mg kg^?1, respectively. The As levels in water were significantly lower than those of sediments. Results would suggest that As co-precipitation and adsorption on Fe oxides are probably the major route of immobilization, determining its low lability. Manganese did not present an outstanding contribution to the retention, and cation-exchange capacity, pH and organic matter of sediments did not show an influence on the mobility of As.     

Oceanic circulation as an element in palaeogeographical reconstructions: the Arenig (early Ordovician) as an example

A new conceptual palaeo-oceanographical model is outlined in this paper. The model differs from previous models by using shifts in Hadley circulation caused by orbital variations, and the rotation rate of the Earth, to locate the position of the planetary oceanic low-and high-pressure systems, around which the planetary ocean surface currents flow. Adapting the model to the Arenig (early Ordovician) the temperate low pressure zones were found to be located at 50° latitude and the subtropical high pressure zones at 25° latitude.
Traditionally, most Palaeozoic palaeogeographical recon-structions are reconstructed using palaeomagnetic data supplemented with data from climate-sensitive lithofacies and palaeo-biogeographical distributions. However, as a new approach in palaeogeographical reconstructions, the con-ceptual palaeo-oceanographical model is combined with palaeobiogeographical data for the Arenig series, comple-menting the palaeomagnetic data, and resulting in a new, refined palaeogeographical reconstruction.     

An analytical solution for rapidly predicting post-fire peak streamflow for small watersheds in southern California

Following wildfires, the probability of flooding and debris flows increase, posing risks to human lives, downstream communities, infrastructure, and ecosystems. In southern California (USA), the Rowe, Countryman, and Storey (RCS) 1949 methodology is an empirical method that is used to rapidly estimate post-fire peak streamflow. We re-evaluated the accuracy of RCS for 33 watersheds under current conditions. Pre-fire peak streamflow prediction performance was low, where the average R² was 0.29 and average RMSE was 1.10 cms/km² for the 2- and 10-year recurrence interval events, respectively. Post-fire, RCS performance was also low, with an average R² of 0.26 and RMSE of 15.77 cms/km² for the 2- and 10-year events. We demonstrated that RCS overgeneralizes watershed processes and does not adequately represent the spatial and temporal variability in systems affected by wildfire and extreme weather events and often underpredicted peak streamflow without sediment bulking factors. A novel application of machine learning was used to identify critical watershed characteristics including local physiography, land cover, geology, slope, aspect, rainfall intensity, and soil burn severity, resulting in two random forest models with 45 and five parameters (RF-45 and RF-5, respectively) to predict post-fire peak streamflow. RF-45 and RF-5 performed better than the RCS method; however, they demonstrated the importance and reliance on data availability. The important parameters identified by the machine learning techniques were used to create a three-dimensional polynomial function to calculate post-fire peak streamflow in small catchments in southern California during the first year after fire (R² = 0.82; RMSE = 6.59 cms/km²) which can be used as an interim tool by post-fire risk assessment teams. We conclude that a significant increase in data collection of high temporal and spatial resolution rainfall intensity, streamflow, and sediment loading in channels will help to guide future model development to quantify post-fire flood risk.     

Continuous versus event‐based sampling: how many samples are required for deriving general hydrological understanding on Ecuador's páramo region?

As a consequence of the remote location of the Andean páramo, knowledge on their hydrologic functioning is limited; notwithstanding, these alpine tundra ecosystems act as water towers for a large fraction of the society. Given the harsh environmental conditions in this region, year‐round monitoring is cumbersome, and it would be beneficial if the monitoring needed for the understanding of the rainfall–runoff response could be limited in time. To identify the hydrological response and the effect of temporal monitoring, a nested (n = 7) hydrological monitoring network was set up in the Zhurucay catchment (7.53 km²), south Ecuador. The research questions were as follows: (1) Can event sampling provide similar information in comparison with continuous monitoring, and (2) if so, how many events are needed to achieve a similar degree of information? A subset of 34 rainfall–runoff events was compared with monthly values derived from a continuous monitoring scheme from December 2010 to November 2013. Land cover and physiographic characteristics were correlated with 11 hydrological indices. Results show that despite some distinct differences between event and continuous sampling, both data sets reveal similar information; more in particular, the monitoring of a single event in the rainy season provides the same information as continuous monitoring, while during the dry season, ten events ought to be monitored. Copyright © 2016 John Wiley & Sons, Ltd.     

Timing and pace of ice-sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

Understanding the pace and drivers of marine-based ice-sheet retreat relies upon the integration of numerical ice-sheet models with observations from contemporary polar ice sheets and well-constrained palaeo-glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine-geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice-marginal landforms, to reconstruct the timing and rate of ice-sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine- to a terrestrially-based ice-sheet margin. Seafloor bathymetric data in the form of moraines and grounding-zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid-shelf where it formed a large grounding-zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice-free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice-sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio-isostatically induced, high relative sea level. Net rates of ice-sheet retreat across the shelf were slow (62–19 m a⁻¹) and reduced (8 m a⁻¹) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine-based sectors of ice sheets.     

Laterally constrained inversion of helicopter-borne frequency-domain electromagnetic data

Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost.For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data.The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment.If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter.     

The first high-amplitude δ Scuti star in an eclipsing binary system

Consider radar ranging of a distant galaxy in a Friedman–Lemaître cosmological model. In this model the comoving coordinate of the galaxy is constant; hence, the equations of null geodesics for photons travelling to the distant galaxy and back imply Here, τ_e, τ_r and τ_o are, respectively, the times of emission, reflection and observation of the reflected photons, and a (τ) is the scalefactor. Since the Universe is expanding, a (τ) is a monotonically increasing function, so the return traveltime, τ_o−τ_r, must be greater than the forward traveltime, τ_r−τ_e. Clearly, space expands, and on their way back, the photons must travel a longer distance! This paper explains why this argument for the Expansion of Space (EoS) is wrong. We argue that, unlike the expansion of the cosmic substratum, the EoS is unobservable. We therefore propose to apply to it – just like to the ether – Ockham's razor.     

Glacial and periglacial floodplain sediments regulate hydrologic transfer of reactive iron to a high arctic fjord

The transport of reactive iron (i.e. colloidal and dissolved) by a glacier‐fed stream system draining a high relief periglacial landscape in the high Arctic archipelago of Svalbard is described. A negative, non‐linear relationship between discharge and iron concentration is found, indicative of increased iron acquisition along baseflow pathways. Because the glaciers are cold‐based and there are no intra‐ or sub‐permafrost groundwater springs, baseflow is principally supplied by the active layer and the colluvial and alluvial sediments in the lower valley. Collectively, these environments increase the flux of iron in the stream by 40% over a floodplain length of just 8 km, resulting in 6 kg Fe km^?2a^?1 of reactive iron export for a 20% glacierized watershed. We show that pyrite oxidation in shallow‐groundwater flowpaths of the floodplain is the most important source of reactive iron, although it is far less influential in the upper parts of the catchment where other sources are significant (including ironstone and secondary oxide coatings). Microbial catalysis of the pyrite oxidation occurs in the floodplain, enabling rapid, hyporheic water exchange to enhance the iron fluxes at high discharge and cause the non‐linear relationship between discharge and reactive iron concentrations. Furthermore, because the pyrite oxidation is tightly coupled to carbonate and silicate mineral weathering, other nutrients such as base cations and silica are also released to the stream system. Our work therefore shows that high Arctic floodplains should be regarded as critically important regulators of terrestrial nutrient fluxes to coastal ecosystems from glacial and periglacial sources. Copyright © 2015 John Wiley & Sons, Ltd.