Wood export prediction at the watershed scale

Wood export from a watershed is a function of peak annual discharge, but one hydrologic relationship alone does not fully explain observed variability. Consideration of physical processes that influence the amount of wood available for transport is needed. However, wood recruitment, storage, mobilization, breakage, and transport rates and processes remain difficult to quantify. A theoretical wood transport equation focused on variations in discharge was the motivation for investigation into watershed‐specific wood export rates. Herein, multiplicative coefficients categorized by water year type are developed, paired with the equation, and validated to provide a new method for prediction of wood export at the watershed scale. The coefficients are defined as representing a broad suite of watershed processes that encompass spatio‐temporally variable scales. Two complementary datasets from the 1097 km² mountainous North Yuba River, California watershed were used. Wood surveys above New Bullards Bar Reservoir yielded a wood availability estimate of 250 000–300 000 m³ along the channel network. Annual wood export into the reservoir was field‐surveyed in 2010, 2012 and 2013, and estimated in seven years via remotely sensed images over the 30 year study period of water years 1985–2014. Empirical, watershed‐scale wood export rates ranged from 0.3–5.6%. Comparison of predicted quantities using the new DVWP (discharge variations modified by watershed processes) wood export equation to observed wood export quantities resulted in an aggregate error rate of ±10%. When individual wood export quantities were compared, predicted to observed varied by 0.5–3.0 times. Total wood export of 59 000–71 000 m³ was estimated over the 30 year period, yielding a rate of 1.8 to 2.2 m³/year/km². Wood export predictive capabilities at the watershed scale may help water resource and regulatory agencies plan for wood transfers to augment downstream ecosystems. Copyright © 2017 John Wiley & Sons, Ltd.     

Chemical remobilization of contaminant metals within floodplain sediments in an incising river system: implications for dating and chemostratigraphy

Metals such as Pb, Zn, Cd and Cu from historical mining activity have been used as stratigraphic markers for dating and provenancing vertically accreted, fine-grained floodplain overbank deposits. This study presents evidence for chemical remobilization of these metals within overbank sediments in the Tyne basin, UK. The evidence includes: breakdown of metal-bearing minerals (sulphides, carbonates, iron and manganese oxyhydroxides); shifts of chemical fractions within zones of relatively low pH towards more soluble and reactive phases; and accumulation of secondary iron and manganese oxyhydroxides at levels related to fluctuating water-table levels or to the breakdown of organic matter. All of this suggests that fine, centimetre-scale, chemostratigraphy using metal concentrations and ratios is unlikely to provide reliable data in river systems that have experienced, or are experiencing, major changes in water-table levels, or pedogenesis. Coarse tens of centimetre- to metre-scale, chemostratigraphy, when applied with caution, may still provide a means of delineating contaminated units. © 1998 John Wiley & Sons, Ltd.     

Conditions of ore mineral formation in certain Zambian Copperbelt deposits with special reference to the role of cobalt

Studies of the compositions of coexisting sulphide assemblages from certain Zambian Copperbelt deposits and of their textural relations and host rock environments have been undertaken by routine microscopy and using electron microprobe analysis. Special attention has been paid to sulphide assemblages containing cobalt. Using experimental data on the systems Cu-Co-S, Cu-Fe-Co-S and on sulphidation equilibria, together with the available information on equilibria involving gangue minerals and some new calculations, an attempt has been made to define the chemical conditions of ore formation and/or re-equilibration in several deposits (Baluba, Chibuluma West and Chibuluma, Chambeshi S.E.). If a re-equilibration temperature of 300°C is assumed the range of aS₂ prevailing during final formation of the assemblages was of the order 10^–7 – 10^–9 atm, more rarely dropping to 10^–11 – 10^–12 atm. If magnetite is present, and this is considered very unlikely in most cases, aO₂ may have reached as high as 10^–35 atm but it is generally likely to have been around 10^–50 atm or even lower. Values of aCO₂ are assumed to have been of the order of 10^–0.5 atm. Detailed studies of drill hole samples from the Chambeshi S.E. deposit suggest an important link between the petrology of the host rocks and the sulphide mineral chemistry. In particular, the control exerted on sulphide composition by aS₂ may have been related to the availability of sulphate in the form of diagenetic anhydrite or in sulphate-rich interstitial waters. The distinctive distribution of cobalt in the ores appears related to the distribution of amphibolite bodies and in turn to rift fault systems in the Basement which allowed upward movement of fluids enriched in magmatically derived cobalt.     

The geochemistry of iron in Recent tidal-flat sediments of the Wash area,England: a mineralogical,Mössbauer,and magnetic study

Undisturbed core samples of Recent sediments from the Wash tidal flats, East Anglia, England, obtained using a Delft corer, were studied with special reference to the diagenesis and geochemical behaviour of iron. The Mössbauer effect in ⁵⁷Fe was used to monitor the distribution of Fe between different phases as a function of depth, together with the magnetic mineralogy and palaeomagnetic properties.The cores consist of, successively downwards: 0.36 m brown clay; 1.5 m finely laminated silts and fine sands, and 7.14 m homogeneous fine sands. The dominant minerals are quartz, feldspar, calcite and clay minerals, and chemical analysis for Al, Si, Mg, Mn, Ca, Fe, Na, K showed variations closely linked to lithological changes. Illite is the most abundant clay mineral (mean 48%), followed by mixed layer illite-montmorillonite and montmorillonite, kaolinite and chlorite. Chlorite is the major iron-bearing clay mineral and represents 4 to 10% of the <2 μm fraction throughout the core. Sulphide minerals are present throughout the core, including framboidal pyrite.Computer fit analysis of the Mössbauer spectra of best quality showed contributions from Fe²⁺ and Fe³⁺ in clay minerals (essentially chlorite), low-spin Fe²⁺ in pyrite, and magnetically ordered iron in greigite (Fe₃S₄). Systematic variations, as a function of sample depth, indicate a relative increase in the amount of Fe in pyrite at the expense of the clay minerals.Magnetite and titanium-bearing magnetite are the carriers of natural magnetic remanence in these sediments.The direction and intensity of natural remanence in the samples compare well with the known secular variation of the Earth's magnetic field derived from the historic-archaeomagnetic record and this enables the samples to be dated and sedimentation rates to be determined (1.5 mm yr^?1 for the upper 2 m and ~7.7 mm yr^?1 for the lower 7 m).     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.     

Impacts of aerosol chemical composition on microphysics and precipitation in deep convection

Aerosols affect precipitation by modifying cloud properties such as cloud droplet number concentration (CDNC). Aerosol effects on CDNC depend on aerosol properties such as number concentration, size spectrum, and chemical composition. This study focuses on the effects of aerosol chemical composition on CDNC and, thereby, precipitation in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. The MCE was observed during the 1997 department of energy's Atmospheric Radiation Measurement (ARM) summer experiment. Double-moment microphysics with explicit nucleation parameterization, able to take into account those three properties of aerosols, is used to investigate the effects of aerosol chemical composition on CDNC and precipitation. The effects of aerosol chemical compositions are investigated for both soluble and insoluble substances in aerosol particles. The effects of soluble substances are examined by varying mass fractions of two representative soluble components of aerosols in the continental air mass: sulfate and organics. The increase in organics with decreasing sulfate lowers critical supersaturation (S_c) and leads to higher CDNC. Higher CDNC results in smaller autoconversion of cloud liquid to rain. This provides more abundant cloud liquid as a source of evaporative cooling, leading to more intense downdrafts, low-level convergence, and updrafts. The resultant stronger updrafts produce more condensation and thus precipitation, as compared to the case of 100% sulfate aerosols. The conventional assumption of sulfate aerosol as a surrogate for the whole aerosol mass can be inapplicable for the case with the strong sources of organics. The less precipitation is simulated when an insoluble substance replaces organics as compared to when it replaces sulfate. When the effects of organics on the surface tension of droplet and solution term in the Köhler curve are deactivated by the insoluble substance, S_c is raised more than when the effects of sulfate on the solution term are deactivated by the insoluble substance. This leads to lower CDNC and, thus, larger autoconversion of cloud liquid to rain, providing less abundant cloud liquid as a source of evaporative cooling. The resultant less evaporative cooling produces less intense downdrafts, weaker low-level convergence, updrafts, condensation and, thereby, less precipitation in the case where organics is replaced by the insoluble substance than in the case where sulfate is replaced by the insoluble substance. The variation of precipitation caused by the change in the mass fraction between the soluble and insoluble substances is larger than that caused by the change in the mass fraction between the soluble substances.     

Validation of Ground-Based Visible Measurements of Total Ozone by Comparison with Dobson and Brewer Spectrophotometers

Comparisons of total column ozone measurements from Dobson, Brewer and SAOZ instruments are presented for the period 1990 to 1995 at seven stations covering the mid- and the high northern latitudes, as well as the Antarctic region. The main purpose of these comparisons is to assess, by reference to the well established Dobson network, the accuracy of the zenith-sky visible spectroscopy for the measurement of total ozone. The strengths and present limitations of this latter technique are investigated. As a general result, the different instruments are found to agree within a few percent at all stations, the best agreement being obtained at mid-latitudes. On average, for the mid-latitudes, SAOZ O₃ measurements are approximately 2% higher than Dobson ones, with a scatter of about 5%. At higher latitudes, both scatter and systematic deviation tend to increase. In all cases, the relative differences between SAOZ and Dobson or Brewer column ozone are characterised by a significant seasonal signal, the amplitude of which increases from about 2.5% at mid-latitude to a maximum of 7.5% at Faraday, Antarctica. Although it introduces a significant contribution to the seasonality at high latitude, the temperature sensitivity of the O₃ absorption coefficients of the Dobson and Brewer instruments is shown to be too small to account for the observed SAOZ/Dobson differences. Except for Faraday, these differences can however be largely reduced if SAOZ AMFs are calculated with realistic climatological profiles of ozone, pressure and temperature. Other sources of uncertainties that might affect the comparison are investigated. Evidence is found that the differences in the air masses sampled by the SAOZ and the other instruments contribute significantly to the scatter, and the impact of the tropospheric clouds on SAOZ measurements is displayed.     

West Antarctic Ice Sheet collapse – the fall and rise of a paradigm

It is now almost 30 years since John Mercer (1978) first presented the idea that climate change could eventually cause a rapid deglaciation, or “collapse,” of a large part of the West Antarctic ice sheet (WAIS), raising world sea levels by 5 m and causing untold economic and social impacts. This idea, apparently simple and scientifically plausible, created a vision of the future, sufficiently alarming that it became a paradigm for a generation of researchers and provided an icon for the green movement. Through the 1990s, however, a lack of observational evidence for ongoing retreat in WAIS and improved understanding of the complex dynamics of ice streams meant that estimates of likelihood of collapse seemed to be diminishing. In the last few years, however, satellite studies over the relatively inaccessible Amundsen Sea sector of West Antarctica have shown clear evidence of ice sheet retreat showing all the features that might have been predicted for emergent collapse. These studies are re-invigorating the paradigm, albeit in a modified form, and debate about the future stability of WAIS. Since much of WAIS appears to be unchanging, it may, no longer be reasonable to suggest there is an imminent threat of a 5-m rise in sea level resulting from complete collapse of the West Antarctic ice sheet, but there is strong evidence that the Amundsen Sea embayment is changing rapidly. This area alone, contains the potential to raise sea level by around ~1.5 m, but more importantly it seems likely that it could, alter rapidly enough, to make a significant addition to the rate of sea-level rise over coming two centuries. Furthermore, a plausible connection between contemporary climate change and the fate of the ice sheet appears to be developing. The return of the paradigm presents a dilemma for policy-makers, and establishes a renewed set of priorities for the glaciological community. In particular, we must establish whether the hypothesized instability in WAIS is real, or simply an oversimplification resulting from inadequate understanding of the feedbacks that allow ice sheets to achieve equilibrium: and whether there is any likelihood that contemporary climate change could initiate collapse.     

Exploring high-end scenarios for local sea level rise to develop flood protection strategies for a low-lying delta—the Netherlands as an example

Sea level rise, especially combined with possible changes in storm surges and increased river discharge resulting from climate change, poses a major threat in low-lying river deltas. In this study we focus on a specific example of such a delta: the Netherlands. To evaluate whether the country’s flood protection strategy is capable of coping with future climate conditions, an assessment of low-probability/high-impact scenarios is conducted, focusing mainly on sea level rise. We develop a plausible high-end scenario of 0.55 to 1.15 m global mean sea level rise, and 0.40 to 1.05 m rise on the coast of the Netherlands by 2100 (excluding land subsidence), and more than three times these local values by 2200. Together with projections for changes in storm surge height and peak river discharge, these scenarios depict a complex, enhanced flood risk for the Dutch delta.