Assessment of deterministic PMF modelling approaches

Probable maximum flood (PMF) event estimation has challenged the scientific community for many years. Although the concept of the PMF is often applied, there is no consensus on the methods that should be applied to estimate it. In PMF estimation, the spatio-temporal representation of the probable maximum precipitation (PMP) as well as the choice of modelling approach is often not theoretically founded. Moreover, it is not clear how these choices influence PMF estimation itself. In this study, combinations of three different spatio-temporal PMP representations and three different modelling approaches are applied to determine the PMF of a mesoscale basin keeping the antecedent catchment conditions and the total precipitation amount constant. The nine resulting PMF estimations are used to evaluate each combination of methods. The results show that basic methods allow for a rough estimation of the PMF. In cases where a physically plausible and reliable estimation is required, sophisticated PMP and PMF estimation approaches are recommended.

EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR A. Viglione     

The Sensitivity of the Phanerozoic Inorganic Carbon System to the Onset of Pelagic Sedimentation

The onset of pelagic sedimentation attending the radiation of pelagic calcifiers during the Mesozoic was an important divide in Earth history, shifting the locus of significant carbonate sedimentation from the shallow shelf environments of the Paleozoic to the deep sea. This shift would have impacted the CO₂ cycle, given that decarbonation of subducted pelagic carbonate is an important return flux of CO₂ to the atmosphere. Coupled with the fact that the mean residence time of continental platform and basin sedimentary carbonate exceeds that of the oceanic crust, it thus becomes unclear whether carbon cycling would have operated on a substantially different footing prior to the pelagic transition. Here, we examine this uncertainty with sensitivity analyses of the timing of this transition using a coupled model of the Phanerozoic atmosphere, ocean, and shallow lithosphere. For purposes of comparison, we establish an age of 250 Ma (i.e., after the Permo-Triassic extinctions) as the earliest opportunity for deposition of extensive biogenic pelagic carbonate on the deep seafloor, an age that predates known occurrences of pelagic calcifiers (and intact seafloor). Although an approximate boundary, we do show that attempts to shift this datum either significantly earlier or later in time produce model results that are inconsistent with observed trends in the mass–age distribution of the rock record and with accepted trends in seawater composition as constrained by proxy data. Significantly, we also conclude that regardless of the timing of the onset of biogenic pelagic carbonate sedimentation, a carbon sink involving seawater-derived dissolved inorganic carbon played a critical role in carbon cycling, particularly in the Paleozoic. This CaCO₃ sink may have been wholly abiogenic, involving calcium derived either directly from seawater (thus manifest as a direct seafloor deposit), or alternatively from basalt–seawater reactions (represented by precipitation of CaCO₃ in veins and fissures within the basalt). Despite the uncertainty in the source and magnitude of this abiogenic CaCO₃ flux, it is likely a basic and permanent feature of global carbon cycling. Subduction of this CaCO₃ would have acted as a basic return circuit for atmospheric CO₂ even in the absence of biogenically derived pelagic carbonate sedimentation. Lastly, model calculations of the ratio of dissolved calcium to carbonate ion (Ca²⁺/CO₃ ^2?) show this quantity underwent significant secular evolution over the Phanerozoic. As there is increasing recognition of this ratio’s role in CaCO₃ growth and dissolution reactions, this evolution, together with progressive increases in nutrient availability and saturation state, may have created a tipping point ultimately conducive to the appearance of pelagic calcifiers in the Mesozoic.     

PCB contamination from sampling equipment and packaging

Work reported in this paper suggests that there are cases of contamination of soil and water samples by polychlorinated biphenyls (PCB) from paper and plastic packaging. Soil samples, which have been stored in paper bags for more than 20 years, share a similar congener distribution as the bags. Analyses showed a predominance of light congeners. PCB-analyses of water also indicate that water samples could be contaminated by plastic packaging. All analytical results of solid material packaging, as well as soil stored in the packaging, show a high relative amount of light weight PCB congeners. The paper bags that were analysed are made of strong paper and very popular among geochemists because of their watertight quality. These paper bags were manufactured more than 20 years ago. The plastic packaging that was analysed was produced in 1997 and 2008. The analyses of plastic and paper show that the raw material that has been used in the production at different times contains a wide concentration range of PCB. Re-sealable plastic bags, which contained the highest levels of PCB of the plastic material, are used by researchers world-wide as sampling bags for soils and sediments. This paper raises an important issue that packaging may potentially contaminate the samples that they hold.     

Biological metal accumulation in the red beds (Guettioua Member) of the Bathonian (Dogger) of Msemrir,Central High Atlas,Morocco

A fluviatile sequence is described from the Dogger (Bathonian) of Msemrir (Central High Atlas, Morocco). The base comprises laminites with fine clouds of galena and some zinc minerals, while the rest of the profile consists of point bars with conglomerates. Due to their limitation on lower energy levels, the sulphide minerals are interpreted as biological metal accumulations of bacterial metabolic processes.

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Zusammenfassung Aus dem Dogger (Bathonium) vom Msemrir (Zentraler Hoher Atlas, Marokko) wird ein vorwiegend fluviatiles Profil beschrieben. An der Basis besteht es aus laminierten Serien, in denen feinverteilter Bleiglanz vorkommt. Am Top des Profils finden sich vor allem große Rinnenkörper mit Konglomeraten und vereinzelten Knochenresten. Die sulfidischen Vererzungen in den niedrig-energetischen Bereichen werden als biogenetische Anreicherungen, hervorgerufen durch bakterielle Umsetzungen, interpretiert.

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Résumé Nous présentons et décrivons une séquence fluviatile du Bathonien de Msemrir (Haut Atlas Central, Maroc). La base est constituée de laminites dans lesquelles se trouve de la galène finement répartie. La couche suivante comporte de grands chenaux contenant des conglomérats et, ça et là, des débris osseux et des restes de végétaux. Les minéralisations sulfurées dans les zones de basse énergie sont interprétées comme des accumulations biogénétiques dues à une activité bactérienne.

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Reconstructing South African cities: Contemporary strategies and processes in the urban core

In the 1990s, South Africa and its cities have entered an unprecedented era of change. This transition has been characterised by a burgeoning of civil society and negotiation processes around the key policy and development challenges facing the country. Within this context, the beginnings of a new post-apartheid urban reconstruction era has been marked by the dual challenges of neutralising the damaging anomalies created in cities by apartheid urban engineering, and of shaping efficient, functional and well-managed cities for the future. The core areas or central city zones of the major metropolitan complexes — comprising inner city residential components and central business district economic sectors — are now emerging as a priority area in urban reconstruction. This paper briefly outlines the evolution of the South African central city within the context of overall metropolitan morphology, before considering several of the key issues and processes currently shaping inner city and CBD zones.     

Field comparison of passive sampling and biological approaches for measuring exposure to PAH and alkylphenols from offshore produced water discharges

Polycyclic aromatic hydrocarbons (PAH) and alkylphenols (AP) that are present in routine discharges of produced water (PW) from the offshore industry continue to cause concern. The suitability of biological methods and chemical based passive samplers to determine exposure to these compounds was tested by deploying them around an oil installation and at reference locations in the North Sea. PAH and AP were analysed either as parent compounds in passive samplers and mussel tissue or as metabolites in fish bile. Generally the pattern of exposure relative to proximity to the discharge was represented by mussels, SPMDs and fish for PAH. Fish and SPMDs showed good correlation for PAH accumulations, whereas some differences were apparent between mussels and SPMDs. POCIS was the only technique tested that could accurately measure the most abundant AP in PW. The advantages of biologically independent measures of exposure for inclusion in discharge monitoring studies are outlined.     

Development and Intercomparison of Condensed Isoprene Oxidation Mechanisms for Global Atmospheric Modeling

A new condensed isoprene oxidation mechanism forglobal atmospheric modeling (MIM) was derived from ahighly detailed master chemical mechanism (MCM). In abox model intercomparison covering a wide range ofboundary layer conditions the MIM was compared withthe MCM and with five other condensed mechanisms, someof which have already been used in global modelingstudies of nonmethane hydrocarbon chemistry. Theresults of MCM and MIM were generally in goodagreement, but the other tested mechanisms exhibitedsubstantial differences relative to the MCM as well asrelative to each other. Different formation yields,reactivities and degradation pathways of organicnitrates formed in the course of isoprene oxidationwere identified as a major reason for the deviations.The relevance of the box model results for chemistrytransport models is discussed, and the need for avalidated reference mechanism and for an improvedrepresentation of isoprene chemistry in global modelsis pointed out.     

The petrogenesis of leucogranitic dykes intruding the northern Semail ophiolite, United Arab Emirates: field relationships, geochemistry and Sr/Nd isotope systematics

The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10–100 m scale metaluminous to peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites, which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite Moho. Compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a _H2O and pressures of 3 to 5 kbar. The LILE (Sr, Rb and Ba) covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments. Initial ⁸⁷Sr/⁸⁶Sr ratios of analysed dykes vary between 0.710 and 0.706 at initial ɛ_Nd values of between −6.3 and −0.5. Cogenetic units of a composite sill from Ra's Dadnah yield a Sm-Nd isochron age of 98.8 ± 9.5 Ma (MSWD = 1.18). Geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill (Haybi complex) sediments, preserved as greenschist (Asimah area) to granulite facies (Bani Hamid area) ophiolitic metamorphic sole terranes. The Sr-Nd isotope systematics suggest that hybrid granitic melts were derived from pre-magmatic mixing of two contrasting subduction zone sources. Received: 17 December 1998 / Accepted: 19 July 1999     

A 40 ka record of temperature and permafrost conditions in northwestern Europe from noble gases in the Ledo‐Paniselian Aquifer (Belgium)

The Ledo‐Paniselian Aquifer in Belgium offers unique opportunities to study periglacial groundwater recharge during the Last Glacial Maximum (LGM), as it was located close to the southern boundary of the ice sheets at that time. Groundwater residence times determined by ¹⁴C and ⁴He reveal a sequence of Holocene and Pleistocene groundwaters and a gap between about 14 and 21 ka, indicating permafrost conditions which inhibited groundwater recharge. In this paper, a dataset of noble gas measurements is used to study the climatic evolution of the region. The derived recharge temperatures indicate that soil temperatures in the periods just before and after the recharge gap were only slightly above freezing, supporting the hypothesis that permafrost caused the recharge gap. The inferred glacial cooling of 9.5°C is the largest found so far by the noble gas method. Yet, compared to other palaeoclimate reconstructions for the region, recharge temperatures deduced from noble gases for the cold periods tend to be rather high. Most likely, this is due to soil temperatures being several degrees higher than air temperatures during periods with extended snow cover. Thus the noble‐gas‐derived glacial cooling of 9.5°C is only a lower limit of the maximum cooling during the LGM. Some samples younger than the recharge gap are affected by degassing, possibly related to gas production during recharge in part of the recharge area, especially during times of melting permafrost. The findings of this study, such as the occurrence of a recharge gap and degassing related to permafrost and its melting, are significant for groundwater dynamics and geochemistry in periglacial areas. Copyright © 2010 John Wiley & Sons, Ltd.