Giving Credit where Credit is Due. A Practical Method to Distinguish between Human and Natural Factors in Carbon Accounting

Net carbon emissions from the biosphere differ from fossil-fuel based emissions in that: (i) a large proportion of biospheric carbon exchange is not under direct human control; (ii) land-use decisions often have only a small short-term effect on net emissions, but a large long-term effect; and (iii) biospheric carbon exchange is potentially reversible. Because of these differences, carbon accounting approaches also need to be different for fossil-fuel and biosphere-based emissions. Recognising that, the international negotiators at COP 7 adopted a range of guiding principles for accounting for biospheric carbon exchange, including: that accounting excludes removals resulting from (a) elevated carbon dioxide concentrations above pre-industrial level; (b) indirect nitrogen deposition; and (c) the dynamic effects of age structure resulting from activities and practices before the reference year. In this paper, we highlight some of the challenges in biospheric carbon accounting for Canada, the U.S.A, New Zealand and Australia, four nations for which biospheric net carbon exchange is large relative to fossil-fuel based emissions. We discuss an accounting scheme that is based on assessing changes in average carbon stocks due to changes in land use. That scheme is tailored to the special needs of biospheric carbon management and is consistent with the accounting principles adopted at COP 7. The paper shows how the accounting scheme would resolve many of the biospheric carbon accounting anomalies identified for the four nations we studied.     

The relative contributions of alpine and subalpine ecosystems to the water balance of a mountainous,headwater catchment

Climate change is affecting the hydrology of high‐elevation mountain ecosystems, with implications for ecosystem functioning and water availability to downstream populations. We directly and continuously measured precipitation and evapotranspiration (ET) from both subalpine forest and alpine tundra portions of a single catchment, as well as discharge fluxes at the catchment outlet, to quantify the water balance of a mountainous, headwater catchment in Colorado, USA. Between 2008 and 2012, the water balance closure averaged 90% annually, and the catchment ET was the largest water output at 66% of precipitation. Alpine ET was greatest during the winter, in part because of sublimation from blowing snow, which contributed from 27% to 48% of the alpine, and 6% to 9% of the catchment water balance, respectively. The subalpine ET peaked in summer. Alpine areas generated the majority of the catchment discharge, despite covering only 31% of the catchment area. Although the average annual alpine runoff efficiency (discharge/precipitation; 40%) was greater than the subalpine runoff efficiency (19%), the subalpine runoff efficiency was more sensitive to changes in precipitation. Inter‐annual analysis of the evaporative and dryness indices revealed persistent moisture limitations at the catchment scale, although the alpine alternated between energy‐limited and water‐limited states in wet and dry years. Each ecosystem generally over‐generated discharge relative to that expected from a Budyko‐type model. The alpine and catchment water yields were relatively unaffected by annual meteorological variability, but this interpretation was dependent on the method used to quantify potential ET. Our results indicate that correctly accounting for dissimilar hydrological cycling above and below alpine treeline is critical to quantify the water balance of high‐elevation mountain catchments over periods of meteorological variability. Copyright © 2015 John Wiley & Sons, Ltd.     

Multiwavelength observations of serendipitous Chandra X-ray sources in the field of A 2390

We present optical spectra and near-infrared imaging of a sample of 31 serendipitous X-ray sources detected in the field of Chandra observations of the A 2390 cluster of galaxies. The sources have  0.5–7 keV  fluxes of  (0.6–8)×10^-14 erg cm^-2 s^-1  and lie around the break in the  2–10 keV  source counts. They are therefore typical of sources dominating the X-ray Background in that band. 12 of the 15 targets for which we have optical spectra show emission lines at a range of line luminosities, and half of these show broad lines. These active galaxies and quasars have soft X-ray spectra. Including photometric redshifts and published spectra, we have redshifts for 17 of the sources, ranging from   z ∼0.2  up to   z ∼3  , with a peak between   z =1–2  . 10 of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic  2–10 keV  luminosities of  (0.2–3)×10⁴⁵ erg s^-1  , and absorbing columns of   N _H>10²³ cm^-2  . Both of these sources were detected in the mid-infrared by ISOCAM on the Infrared Space Observatory , which when combined with radiative transfer modelling leads to the prediction that the bulk of the reprocessed flux emerges at ∼100 μm.     

Physical explanation for the displacement-length relationship of faults using a post-yield fracture mechanics model

A plane strain model for a fault is presented that takes into account the inelastic deformation involved in fault growth. The model requires that the stresses at the tip of the fault never exceed the shear strength of the surrounding rock. This is achieved by taking into account a zone, around the perimeter of the fault surface, where the fault is not well developed, and in which sliding involves frictional work in excess of that required for sliding on the fully developed fault. The displacement profiles predicted by the fault model taper out gradually towards the tip of the fault and compare well with observed displacement profiles on faults. Using this model it is found that both (1) the shape of the displacement profile, and (2) the ratio of maximum displacement to fault length are a function of the shear strength of the rock in which the fault forms. For the case of a fault loaded by a constant remote stress, the displacement is linearly related to the length of the fault and the constant of proportionality depends on the shear strength of the surrounding rock normalized by its shear modulus. Using data from faults in different tectonic regions and rock types, the in situ strength of intact rock surrounding a fault is calculated to be on the order of 100 MPa (or a few kilobars). These estimates exceed, by perhaps a factor of 10, the strength of a well developed fault and thus provide an upper bound for the shear strength of the crust. It is also shown that the work required to propagate a fault scales with fault length. This result can explain the observation that the fracture energy calculated for earthquake ruptures and natural faults are several orders of magnitude greater than that for fractures in laboratory experiments.     

The Precambrian—Cambrian boundary: A symposium

Contributions to a symposium of the IUGS Subcommission on Cambrian Stratigraphy at the International Geological Congress in Montreal in 1972 are the basis of this paper which is intended to give guidance to further work of what is now a Working Group on the Precambrian—Cambrian boundary of the International Geological Correlation Programme (UNESCO/IUGS). The proposed principles for the definition of the boundary are based on those accepted by the Commission on Stratigraphy, particularly on stratigraphic classification and on defining the Silurian—Devonian boundary. These matters are discussed by the authors in general terms and by W.B. Harland in terms of procedure. They are illustrated by examples of stratigraphic sequences across the boundary in Siberia given by A.Yu. Rozanov and discussed critically by V.E. Savitsky. New relevant discoveries of Early Cambrian faunas and their stratigraphic setting are reviewed by A. Boudda et al. P.E. Cloud's contribution, published elsewhere, and work in Australia are reviewed by the authors.The conclusions, based not only on the symposium material but also on current opinion, are that no single sequence which could serve as a reference standard for the Precambrian—Cambrian boundary has yet been identified but that many significant ones have been described which will be further examined. While the general limits for the choice of a boundary definition should lie between the horizons with fossils of Ediacaran type and those with diversified trilobite assemblages, a number of “Working Reference Points” exist in that interval which are correlatable horizons of faunal change. These cannot be defined simply as horizons of first appearance.     

Investigating the surface process response to fault interaction and linkage using a numerical modelling approach

In order to better understand the evolution of rift‐related topography and sedimentation, we present the results of a numerical modelling study in which elevation changes generated by extensional fault propagation, interaction and linkage are used to drive a landscape evolution model. Drainage network development, landsliding and sediment accumulation in response to faulting are calculated using CASCADE, a numerical model developed by Braun and Sambridge, and the results are compared with field examples. We first show theoretically how the ‘fluvial length scale’, L_f, in the fluvial incision algorithm can be related to the erodibility of the substrate and can be varied to mimic a range of river behaviour between detachment‐limited (DL) and transport‐limited (TL) end‐member models for river incision. We also present new hydraulic geometry data from an extensional setting which show that channel width does not scale with drainage area where a channel incises through an area of active footwall uplift. We include this information in the coupled model, initially for a single value of L_f, and use it to demonstrate how fault interaction controls the location of the main drainage divide and thus the size of the footwall catchments that develop along an evolving basin‐bounding normal fault. We show how erosion by landsliding and fluvial incision varies as the footwall area grows and quantify the volume, source area, and timing of sediment input to the hanging‐wall basin through time. We also demonstrate how fault growth imposes a geometrical control on the scaling of river discharge with downstream distance within the footwall catchments, thus influencing the incision rate of rivers that drain into the hanging‐wall basin. Whether these rivers continue to flow into the basin after the basin‐bounding fault becomes fully linked strongly depends on the value of L_f. We show that such rivers are more likely to maintain their course if they are close to the TL end member (small L_f); as a river becomes progressively more under supplied, i.e. the DL end member (large L_f), it is more likely to be deflected or dammed by the growing fault. These model results are compared quantitatively with real drainage networks from mainland Greece, the Italian Apennines and eastern California. Finally, we infer the calibre of sediments entering the hanging‐wall basin by integrating measurements of erosion rate across the growing footwall with the variation in surface processes in space and time. Combining this information with the observed structural control of sediment entry points into individual hanging‐wall depocentres we develop a greater understanding of facies changes associated with the rift‐initiation to rift‐climax transition previously recognised in syn‐rift stratigraphy.     

Surface and subsurface water contributions to streamflow from a mesoscale watershed in complex mountain terrain

An understanding of surface and subsurface water contributions to streamflow is essential for accurate predictions of water supply from mountain watersheds that often serve as water towers for downstream communities. As such, this study used the end‐member mixing analysis technique to investigate source water contributions and hydrologic flow paths of the 264 km² Boulder Creek Watershed, which drains the Colorado Front Range, USA. Four conservative hydrochemical tracers were used to describe this watershed as a 3 end‐member system, and tracer concentration reconstruction suggested that the application of end‐member mixing analysis was robust. On average from 2009 to 2011, snowmelt and rainwater from the subalpine zone and groundwater sampled from the upper montane zone contributed 54%, 22%, and 24% of the annual streamflow, respectively. These values demonstrate increased rainwater and decreased snow water contributions to streamflow relative to area‐weighted mean values derived from previous work at the headwater scale. Young water (2.3 ± 0.8 months) fractions of streamflow decreased from 18–22% in the alpine catchment to 8–10% in the lower elevation catchments and the watershed outlet with implications for subsurface storage and hydrological connectivity. These results contribute to a process‐based understanding of the seasonal source water composition of a mesoscale watershed that can be used to extrapolate headwater streamflow generation predictions to larger spatial scales.     

Redshift distribution of the submillimeter extragalactic background light

The submillimeter (submm) extragalactic background light (EBL) traces the integrated star formation history throughout the cosmic time. Deep blank-field 850 μm and 1.4 GHz surveys and optical follow-up have been only able to determine the redshift of ∼20% of the submm EBL. The majority (80%) of the submm EBL is still below the confusion and sensitivity limits of current submm and radio instruments. We break through these limits with stacking analyses on our deep 850 μm image in the GOODS-N and find that the submm EBL mostly comes from galaxies at redshifts around 1.0. This redshift is much lower than the redshift of z=2–3 previously implied from radio identified submm sources. This result significantly decreases the number of high redshift galaxies that may be seen by ALMA.