Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

Globally, dissolved inorganic carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here, we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low‐lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon, particulate organic carbon and DIC concentrations and stable isotope ratios of DIC (δ¹³C_DIC) at two time scales: seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser was used to measure diel DIC concentration and δ¹³C_DIC changes at a 15‐min temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55 and 3.82 mg/l and δ¹³C_DIC values ranging from ?19.7 ± 0.31‰ to ?17.1 ± 0.08‰. In contrast, at the seasonal scale, we observed wet season DIC variations predominantly from mixing processes and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/l) and δ¹³C_DIC values of river water (by 5.4‰) largely result from proportional increases in subsurface inflows from the savanna plains (C₄ vegetation) region relative to inflows from the rainforest (C₃ vegetation) highlands. The high DIC river load during the wet season resulted in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river, there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial carbon sink in the savannas of northern Australia during wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of a probabilistic approach for rock wedge failure

For rock slope engineering, uncertainty and variability are inherent in data collected on orientation and strength of discontinuities, yielding a range of results. Unfortunately, conventional deterministic analysis based on the factor of safety concept, requires a fixed representative value for each parameter without regard to the degree of uncertainty involved. Therefore, the deterministic analysis fails to properly represent uncertainty and variability, so common in engineering geology studies. To overcome this shortcoming, the probabilistic analysis method was proposed and used for more than a decade in rock slope stability analysis. However, most probabilistic analyses included a deterministic model as part of the analysis procedure causing subsequent problems, which went uncorrected. The objectives of this paper are to develop a solution for these difficulties in probabilistic analyses and to propose an appropriate simulation procedure for the probabilistic analysis of rock wedge failures. As part of the solution, probability of kinematic instability and probability of kinetic instability are evaluated separately to provide a proper, combined evaluation for failure probability. To evaluate the feasibility of this new probabilistic approach, the procedure is applied to a practical example, a major, highway rock cut in North Carolina, USA. Results of the probabilistic approach are compared to those of the deterministic analysis; findings are significantly different, indicating that the deterministic analysis does not depict rock slope variations, particularly where significant scatter in parameter data occurs.     

Investigation of vertical distribution and morphology of indigenous organic matter at Sleeping Bear site,Michigan

This study evaluates the nature and origin of particulate organic carbon and organic coatings on aquifer sands upgradient from a fuel spill site near the Sleeping Bear Dunes National Lakeshore in Michigan. The distribution of carbon was found to be highly complex due to the occurrence of high organic carbon horizons, bounded above and below by high carbonate sediments. The organic coatings on the sands were examined using white light and fluorescence microscopy and by scanning electron microscopy. Core samples were analyzed for organic and inorganic carbon, solution pH, humic/fulvic acid ratios, and insoluble organic matter content (that is, humin) as a function of depth from the ground surface. The organic geochemistry of the soil profile at this site was found to be significantly influenced by the carbonates producing a sharp boundary of precipitated organic matter. This boundary was followed by coatings of predominantly fulvic acid salts on mineral grains deeper in the soil column. The coatings extended into the aquifer. The existence of native organic films on sand grains is well documented in the soils literature. The study reported here was greatly aided by this information and provides the framework for future studies concerning the influence of carbon distribution, chemical identity, and morphology on contaminant fate and transport processes.     

Polychlorinated biphenyls in sewage sludge and sediments of the New York Bight

Sediments of the New York Bight were analysed for polychlorinated biphenyls (PCB) and found to be heavily contaminated by such substances. The highest concentrations are adjacent to an offshore sludge dump zone, implying that the primary source of PCBs is sewage sludge in the area under consideration. Widespread transport of PCBs out of the bight is not evident as the area of contamination is limited to areas of mud accumulation. The PCB profile in a core of these mud facies can be historically correlated to the commercial production of PCBs.     

Indication of a geomagnetic low-inclination excursion in supposed Middle Weichselian interstadial marine clay at Rubjerg,Denmark

Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ± 190?12,650 ± 180 B.P.), and a secular variation with an amplitude of 10–12° in the inclincation and a “period” roughly estimated at about 350–400 years was found in the Younger Yoldia Clay.Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 11° with α₉₅ = 3°. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”.     

论找矿勘查的风险性和降低风险的预测性勘查战略

找矿勘查的高风险性表现在找到可赢利矿床的成功率低和找矿发现的成本越来越高,这不仅影响了矿业经济的发展,也影响了公众对地球科学的认识。根据国际找矿勘查的一些统计资料和实例分析了造成找矿勘查高风险性的原因,提出降低风险的预测性勘查战略。区域地质背景、成矿作用的复杂性和勘查者所具有的知识、思维能力、技术手段的局限性等客观因素使找矿勘查的风险不可避免,但不少找矿勘查的失败是由勘查者在资料使用、观察、思维等方面的主观失误造成的。勘查者可以通过增加尝试的次数来获取成功,更可取的是通过提高矿床预测的准确度,以预测性勘查促找矿发现,进而降低找矿勘查的风险。     

Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

The Publishers wish to apologise for the incorrect representationof figure 10 which accompanied this paper. The correct figureis reproduced below.     

Transient heat flow in a stratified medium

The equation of heat conduction is solved for a horizontally stratified medium initially at constant temperature and subject to a step-function surface temperature change. The problem is solved by Laplace transformation and by applying a concept of further stratifications of the medium into unitary layers of constant ratio of thickness to the square root of thermal diffusivity. The solution takes the form of an infinite sum of complementary error functions, with coefficients given by recursion relations, is suitable for numerical applications and offers an attractive alternative to harmonic and quasi transient approaches in calculating the penetration of transient surface temperature variations into a layered medium. Use of the theorem of superposition yields a general expression for an arbitrary surface temperature function. The utility of the theory is illustrated by modelling examples of palaeoclimatically induced subsurface temperature and heat flow perturbations.     

Interpretation of vector magnetograph data including magneto-optic effects

In this paper, the presence of Faraday rotation in measurements of the orientation of a sunspot's transverse magnetic field is investigated. Using observations obtained with the Marshall Space Flight Center's (MSFC) vector magnetograph, the derived vector magnetic field of a simple, symmetric sunspot is used to calculate the degree of Faraday rotation in the azimuth of the transverse field as a function of wavelength from analytical expressions for the Stokes parameters. These results are then compared with the observed rotation of the field's azimuth which is derived from observations at different wavelengths within the Fei 5250 Å spectral line. From these comparisons, we find: the observed rotation of the azimuth is simulated to a reasonable degree by the theoretical formulations if the line-formation parameter η _o is varied over the sunspot; these variations in η _o are substantiated by the line-intensity data; for the MSFC system, Faraday rotation can be neglected for field strengths less than 1800 G and field inclinations greater than 45°; to minimize the effects of Faraday rotation in sunspot umbrae, MSFC magnetograph measurements must be made in the far wings of the Zeeman-sensitive spectral line.