Evaluating spatiotemporal variation in water chemistry of the upper Colorado River using longitudinal profiling

Stream water chemistry is traditionally measured as variation over time at fixed sites, with sparse sites providing a crude understanding of spatial heterogeneity. An alternative Lagrangian reference frame measures changes with respect to both space and time as water travels through a network. Here, we collected sensor-based measurements of water chemistry at high spatial resolution along nearly 500 km of the Upper Colorado River. Our objective was to understand sources of spatiotemporal heterogeneity across different solutes and determine whether longitudinal change manifests as smooth gradients as suggested by the River Continuum Concept (RCC) or as abrupt changes as suggested by the Serial Discontinuity Concept (SDC). Our results demonstrate that Lagrangian sampling integrates spatiotemporal variation, and profiles reflect processes that vary in both space and time and over different scales. Over each day of sampling, water temperature (T) and dissolved oxygen (DO) varied strongly in response to diel solar cycles, with most of the variation driven by sampling time rather than sampling location. Equilibration of T and DO with the atmosphere limited small scale spatial heterogeneity, with variation at the entire profile scale driven by regional climate gradients. As such, T and DO profiles more closely approximated the smooth gradients of the RCC (though including temporal sampling artefacts). Conversely, variation in specific conductance and nitrate (NO₃-N) was largely driven by spatial patterns of lateral inflows such as tributaries and groundwater. This resulted in discrete shifts in the profiles at or downstream of discontinuities, appearing as the profiles expected with the SDC. The concatenation of spatiotemporal variation that produces observed Lagrangian profiles presents interpretive challenges but also augments our understanding of where, how, and critically why water chemistry changes in time and space as it moves through river networks.     

Enhanced CO2 trapping in water ice via atmospheric deposition with relevance to Mars

It has been suggested that inclusions of CO₂ or CO₂ clathrate hydrates may comprise a portion of the polar deposits on Mars. Here we present results from an experimental study in which CO₂ molecules were trapped in water ice deposited from CO₂/H₂O atmospheres at temperatures relevant for the polar regions of Mars. Fourier-Transform Infrared spectroscopy was used to monitor the phase of the condensed ice, and temperature programmed desorption was used to quantify the ratio of species in the generated ice films. Our results show that when H₂O ice is deposited at 140-165 K, CO₂ is trapped in large quantities, greater than expected based on lower temperature studies in amorphous ice. The trapping occurs at pressures well below the condensation point for pure CO₂ ice, and therefore this mechanism may allow for CO₂ deposition at the poles during warmer periods. The amount of trapped CO₂ varied from 3% to 16% by mass at 160 K, depending on the substrate studied. Substrates studied were a tetrahydrofuran (C₄H₈O) base clathrate and Fe-montmorillonite clay, an analog for Mars soil. Experimental evidence indicates that the ice structures are likely CO₂ clathrate hydrates. These results have implications for the CO₂ content, overall composition, and density of the polar deposits on Mars.     

Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere

Recent observations suggest methane in the martian atmosphere is variable on short spatial and temporal scales. However, to explain the variability by loss reactions requires production rates much larger than expected. Here, we report results of laboratory studies of methane adsorption onto JSC-Mars-1, a martian soil simulant, and suggest that this process could explain the observations. Uptake coefficient (γ) values were measured as a function of temperature using a high-vacuum Knudsen cell able to simulate martian temperature and pressure conditions. Values of γ were measured from 115 to 135 K, and the data were extrapolated to higher temperatures with more relevance to Mars. Adsorptive uptake was found to increase at lower temperatures and larger methane partial pressures. Although only sub-monolayer methane surface coverage is likely to exist under martian conditions, a very large mineral surface area is available for adsorption as atmospheric methane can diffuse meters into the regolith. As a result, significant methane may be temporarily lost to the regolith on a seasonal time scale. As this weak adsorption is fully reversible, methane will be re-released into the atmosphere when surface and subsurface temperatures rise and so no net loss of methane occurs. Heterogeneous interaction of methane with martian soil grains is the only process proposed thus far which contains both rapid methane loss and rapid methane production mechanisms and is thus fully consistent with the reported variability of methane on Mars.     

Ship board testing of a deoxygenation ballast water treatment

A ship board trial of a deoxygenation method for treating ballast water was carried out during a voyage from Southampton (United Kingdom) to Manzanillo (Panama). A nutrient solution added to two ballast tanks encouraged bacterial growth, resulting in a gradual change to an anoxic environment. Samples were taken from two treated tanks and two untreated tanks to assess changes in the abundance and viability of zooplankton, phytoplankton and bacteria. The work was carried out before the International Maritime Organization (IMO) standard was agreed so only a broad indication of whether the results achieved the standard was given. For the zooplankton, the standard would have been achieved within 5 or 7 days but the phytoplankton results were inconclusive. The biological efficacy was the result of the combination of several factors, including the treatment, pump damage and an increase in the water temperature during the voyage.     

The evolution of coastal barrier systems: a case study of the Middle-Late Pleistocene Wilderness barriers,South Africa

Barrier systems contain lengthy, but complex, records of long-term environmental fluctuations. The Wilderness embayment, South Africa, contains a system of shore-parallel barriers reaching up to 200 m above modern sea level. This study reports the results of chronological, topographical (both on- and off-shore), sedimentological and micromorphological analyses within the Wilderness embayment. Sixty-one new luminescence ages from sixteen sites in unconsolidated dunes and three separate barriers are presented which, when combined with previously published luminescence ages from the area, provide a high-resolution chronological framework for the emplacement and evolution of the barrier system. The preserved barriers have been constructed within at least the last two glacial–interglacial cycles with notable phases between 241–221 ka, 159–143 ka, 130–120 ka, 92–87 ka and post 6 ka. Multiple phases of barrier construction occurred during sea-level highstands, with sediment deposition on each individual barrier occurring over at least two interglacials. Holocene evolution of the system sheds light on earlier events, with dune preservation occurring only during early regression from the Mid-Holocene highstand. Tectonic stability at Wilderness allowed glacio-eustatically formed shorelines to occupy similar positions on multiple occasions. This, in conjunction with a relatively humid climate and a well-vegetated landscape, enabled deflated sediment from beaches to form dunes which stacked upon each other to form an extensive and complex vertical accretionary sequence. Repeated erosion and recycling of pre-existing barriers as well as barrier construction on what is currently the off-shore platform during still-stands in sea-level regressional cycles, when sea levels dropped below ca ?50 m from the present day, has added to the complexity of the preserved terrestrial barrier record. The Wilderness barrier system contrasts with barriers developed elsewhere in the world where higher rates of crustal uplift have allowed preservation of a more complete and more widely spaced palaeorecord. This research also shows the utility of integrating off-shore topography as revealed by bathymetry, with terrestrial topographic data for the better understanding of the evolution of palaeo-coastlines and the preserved dune record found on present-day coastal plains. Local variation in the topography of the continental shelf at Wilderness has generated spatial and temporal complexity within the sedimentary records of individual barriers as well as having a significant influence on preservation.     

Bidirectional stream–groundwater flow in response to ephemeral and intermittent streamflow and groundwater seasonality

It is often assumed that the net groundwater flow direction is towards the channel in headwater streams in humid climates, with magnitudes dependent on flow state. However, studies that characterize stream–groundwater interactions in ephemeral and intermittent streams in humid landscapes remain sparse. Here, we examined seasonally driven stream–groundwater interactions in response to temporary streamflow on the basis of field observations of streamflow and groundwater on an adjacent hillslope. The direction of hydraulic head gradients between the stream and groundwater shifted seasonally. The stream gained water (head gradients were towards the stream) when storage state was high. During this period, streamflow was persistent. The stream lost water to the groundwater system (head gradients were away from the stream) when storage state was low. During this period, streamflow only occurred in response to precipitation events, and head gradients remained predominantly away from the stream during events. This suggested that mechanisms other than deep groundwater contributions produced run‐off when storage was low, such as surface and perched subsurface flowpaths above the water table. Analysis of the annual water balance for the study period showed that the residual between precipitation inputs and streamflow and evapotranspiration outputs, which were attributed to the loss of water to the deeper, regional groundwater system, was similar in magnitude to streamflow. This, coupled with results that showed bidirectionality in stream–groundwater head gradients, indicated that headwaters composed of temporary (e.g., ephemeral and intermittent) streams can be important focal areas for regional groundwater recharge, and both contribute to and receive water, solutes, and materials from the groundwater system.     

Segregating gas from melt: an experimental study of the Ostwald ripening of vapor bubbles in magmas

Diffusive coarsening (Ostwald ripening) of H₂O and H₂O-CO₂ bubbles in rhyolite and basaltic andesite melts was studied with elevated temperature–pressure experiments to investigate the rates and time spans over which vapor bubbles may enlarge and attain sufficient buoyancy to segregate in magmatic systems. Bubble growth and segregation are also considered in terms of classical steady-state and transient (non-steady-state) ripening theory. Experimental results are consistent with diffusive coarsening as the dominant mechanism of bubble growth. Ripening is faster in experiments saturated with pure H₂O than in those with a CO₂-rich mixed vapor probably due to faster diffusion of H₂O than CO₂ through the melt. None of the experimental series followed the time^1/3 increase in mean bubble radius and time⁻¹ decrease in bubble number density predicted by classical steady-state ripening theory. Instead, products are interpreted as resulting from transient regime ripening. Application of transient regime theory suggests that bubbly magmas may require from days to 100 years to reach steady-state ripening conditions. Experimental results, as well as theory for steady-state ripening of bubbles that are immobile or undergoing buoyant ascent, indicate that diffusive coarsening efficiently eliminates micron-sized bubbles and would produce mm-sized bubbles in 10²–10⁴ years in crustal magma bodies. Once bubbles attain mm-sizes, their calculated ascent rates are sufficient that they could transit multiple kilometers over hundreds to thousands of years through mafic and silicic melt, respectively. These results show that diffusive coarsening can facilitate transfer of volatiles through, and from, magmatic systems by creating bubbles sufficiently large for rapid ascent.     

Relative contribution of foliar and fine root pine litter to the molecular composition of soil organic matter after in situ degradation

The influence of litter quality on soil organic matter (SOM) stabilization rate and pathways remains unclear. We used ¹³C/¹⁵N labeled litter addition and Curie-point pyrolysis gas chromatography–mass spectrometry combustion-isotope ratio mass spectrometry (Py–GC–MS–C–IRMS) to explore the transformation of litter with different composition and decay rate (ponderosa pine needle vs. fine root) to SOM during 18 months in a temperate conifer forest mineral (A horizon) soil. Based on ¹³C Py–GC–MS–C–IRMS the initial litter and bulk soil had ∼1/3 of the total pyrolysis products identified in common. The majority was related either to carbohydrates or was non-specific in origin. In bulk soil, carbohydrates had similar levels of enrichment after needle input and fine root input, while the non-specific products were more enriched after needle input. In the humin SOM fraction (260 yr C turnover time) we found only carbohydrate and alkyl C-derived compounds and greater ¹³C enrichment in the “carbohydrate” pool after fine root decomposition. ¹⁵N Py–GC–MS–C–IRMS of humic substances showed that root litter contributed more than needle litter to the enrichment of specific protein markers during initial decomposition.We found little evidence for the selective preservation of plant compounds considered to be recalcitrant. Our findings suggest an indirect role for decomposing plant material composition, where microbial alteration of fine root litter seems to favor greater initial stabilization of microbially derived C and N in SOM fractions with long mean turnover times, such as humin, compared to needles with a faster decay rate.     

Options for monitoring and estimating historical carbon emissions from forest degradation in the context of REDD+

Measuring forest degradation and related forest carbon stock changes is more challenging than measuring deforestation since degradation implies changes in the structure of the forest and does not entail a change in land use, making it less easily detectable through remote sensing. Although we anticipate the use of the IPCC guidance under the United Framework Convention on Climate Change (UNFCCC), there is no one single method for monitoring forest degradation for the case of REDD+ policy. In this review paper we highlight that the choice depends upon a number of factors including the type of degradation, available historical data, capacities and resources, and the potentials and limitations of various measurement and monitoring approaches. Current degradation rates can be measured through field data (i.e. multi-date national forest inventories and permanent sample plot data, commercial forestry data sets, proxy data from domestic markets) and/or remote sensing data (i.e. direct mapping of canopy and forest structural changes or indirect mapping through modelling approaches), with the combination of techniques providing the best options. Developing countries frequently lack consistent historical field data for assessing past forest degradation, and so must rely more on remote sensing approaches mixed with current field assessments of carbon stock changes. Historical degradation estimates will have larger uncertainties as it will be difficult to determine their accuracy. However improving monitoring capacities for systematic forest degradation estimates today will help reduce uncertainties even for historical estimates.