Seasonal controls on DOC dynamics in nested upland catchments in NE Scotland

Spatial and temporal variability of hydrological responses affecting surface water dissolved organic carbon (DOC) concentrations are important for determining upscaling patterns of DOC export within larger catchments. Annual and intra‐annual variations in DOC concentrations and fluxes were assessed over 2 years at 12 sites (3·40–1837 km²) within the River Dee basin in NE Scotland. Mean annual DOC fluxes, primarily correlated with catchment soil coverage, ranged from 3·41 to 9·48 g m^?2 yr^?1. Periods of seasonal (summer–autumn and winter–spring) DOC concentrations (production) were delineated and related to discharge. Although antecedent temperature mainly determined the timing of switchover between periods of high DOC in the summer‐autumn and low DOC in winter‐spring, inter‐annual variability of export within the same season was largely dependent on its associated water flux. DOC fluxes ranged from 1·39 to 4·80 g m^?2 season^?1 during summer–autumn and 1·43 to 4·15 g m^?2 season^?1 in winter–spring.Relationships between DOC areal fluxes and catchment scale indicated that mainstem fluxes reflect the averaging of highly heterogeneous inputs from contrasting headwater catchments, leading to convergent DOC fluxes at catchment sizes of ca 100 km². However, during summer–autumn periods, in contrast to winter–spring, longitudinal mainstem DOC fluxes continue to decrease, most likely because of increasing biological processes. This highlights the importance of considering seasonal as well as annual changes in DOC fluxes with catchment scale. This study increases our understanding of the temporal variability of DOC upscaling patterns reflecting cumulative changes across different catchment scales and aids modelling of carbon budgets at different stages of riverine systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Wet removal of highly soluble gases

Partition, not kinetics, ultimately determines the concentration of highly soluble gases in cloud droplets. Partition equations are formulated and applied to idealized air-mass thunderclouds and precipitating stratus. Contribution to aqueous concentrations from sub-cloud scavenging of highly soluble gases is estimated at between 10 and 20% under relatively unpolluted conditions. Data indicate that evaporation can produce enhancements in concentration of as much as a factor of 3. The calculations give large-scale mean coefficients of wet removal of highly soluble gases of about 2.8×10^-6 s^-1 (4-day residence time) for air-mass thunderclouds and precipitating stratus. Removal is so effective that the mean scale heights of these gases should be decreased to 2 km or less. The criterion of high solubility in this paper is that K ^H (Henry's Law coefficient) > 10⁵ mol l^-1 atm^-1. Gases that are effectively highly soluble include HCl, HNO₃, H₂SO₄, H₂O₂, NH₃ in acid droplets, SO₂ in oxidizing droplets (and probably some light amines and sulfonic acids), but not SO₂ in the absence of oxidants, nor HCHO. A variation of removal coefficient and scale height with solubility is presented. A comparison of atmospheric NH₃ concentrations deduced from rain NH₄ ⁺ and measured directly gives reasonable agreement.     

The representer method for two-phase flow in porous media

The representer method is applied to a one-dimensional two-phase flow model in porous media; capillary pressure and gravity are neglected. The Euler–Lagrange equations must be linearized, and one such linearization is presented here. The representer method is applied to the linear system iteratively until convergence, though a rigorous proof of convergence is out of reach. The linearization chosen is easy to calculate but does not converge for certain weights; however, a simple damping restores convergence at the cost of extra iterations. Numerical experiments are performed that illustrate the method, and quick comparison to the ensemble Kalman smoother is made. This research was supported by NSF grant EIA-0121523.     

Late Holocene coastal sand movements in the Outer Hebrides, N.W. Scotland

Lithostratigraphical and biostratigraphical investigation of coastal marshes along the Atlantic coast of the Outer Hebrides from Lewis in the north to Barra in the south discloses inland-tapering sand units within marshland areas. The inland extent of each sand unit has been radiometrically dated and the units have been collectively interpreted as a proxy for past coastal storminess. The data appear to indicate that for the study sites investigated, the majority of the sand units were produced during episodes of climate deterioration both prior to and after the well-known period of Medieval warmth (MWP). Many were produced after ca. AD 1400. It is argued that the episodes of sand blow indicated by the deposits may reflect periods of increased cyclogenesis in the Atlantic associated with increased sea ice cover and an increase in the thermal gradient across the North Atlantic region.     

Holocene relative sea‐level changes in Harris,Outer Hebrides,Scotland, UK

Evidence for relative sea‐level changes during the middle and late Holocene is examined from two locations on the Atlantic coast of Harris, Outer Hebrides, Scotland, using morphological mapping and survey, stratigraphical, grain size and diatom analysis, and radiocarbon dating. The earliest event identified is a marine flood, which occurred after 7982–8348 cal. a (7370 ± 80 ¹⁴C a) BP, when the sea crossed a threshold lying at ?0.08 m Ordnance Datum Newlyn (OD) (?2.17 m mean high water springs (MHWS)) before withdrawing. This could have been due to a storm or to the Holocene Storegga Slide tsunami. By 6407–6122 cal. a (5500 ± 60 ¹⁴C a) BP, relative sea levels had begun to fall from a sandflat surface with an indicated MHWS level of between 0.08 and ?1.96 m (?2.01 to ?4.05 m). This fall reached between ?0.30 and ?2.35 m (?2.39 to ?4.44 m) after 5841–5050 cal. a (4760 ± 130 ¹⁴C a) BP, but was succeeded by a relative sea‐level rise which reached between 0.54 and ?1.57 m (?1.55 to ?3.66 m) by 5450–4861 cal. a (4500 ± 100 ¹⁴C a) BP. This rise continued, possibly with an interruption, until a second sandflat surface was reached between 2.34 and ?0.26 m (0.25 to ?2.35 m) between 2952–3375 cal. a (3000 ± 80 ¹⁴C a) and 1948–2325 cal. a (2130 ± 70 ¹⁴C a) BP, before present levels were reached. The regressive episode from the earliest sandflat is correlated with the abandonment of the Main Postglacial Shoreline. It is maintained that the fluctuations in relative sea level recorded can be correlated with similar events elsewhere on the periphery of the glacio‐isostatic centre and may therefore reflect secular changes in nearshore sea surface levels. Despite published evidence from trim lines of differential ice sheet loading across the area, no evidence of variations in uplift between the locations concerned could be found. Copyright © 2009 John Wiley & Sons, Ltd.     

Godunov Mixed Methods on Triangular Grids for Advection–Dispersion Equations

A time-splitting approach for advection–dispersion equations is considered. The dispersive and advective fluxes are split into two separate partial differential equations (PDEs), one containing the dispersive term and the other one the advective term. On triangular elements a triangle-based high resolution Finite Volume (FV) scheme for advection is combined with a Mixed Hybrid Finite Element (MHFE) technique to solve dispersion. This approach introduces an error proportional to the time step and the overall scheme is only first order accurate if special care is not taken in the definition of the numerical flux approximation for advection. By incorporating the diffusive effects into the definition of this numerical flux, near second order accuracy (up to a logh factor) can be proved theoretically and validated by numerical experiments in both one- and two-dimensional cases.     

Oxidation states of the upper mantle recorded by megacryst ilmenite in kimberlite and type A and B spinel lherzolites

The intrinsic oxygen fugacities of homogeneous, inclusion-free, megacryst ilmenites from the Frank Smith, Excelsior, Sekameng and Mukorob kimberlite pipes in southern Africa, and the alnöitic breccia in the Solomon Islands have been determined. Similar measurements have been made of the type A and B spinel peridotites from San Carlos in Arizona. The type A peridotites are characterised by oxygen fugacities close to the iron-wüstite buffer, similar to those of equivalent peridotite specimens from other continental and island arc environments. In strong contrast, the type B peridotites and all of the ilmenite megacrysts range between the oxygen fugacities defined by the nickelnickel oxide and fayalite-magnetite-quartz buffers. A close relationship between type B peridotites, oxidized metasomatizing fluids in the upper mantle and oxidized, silicaundersaturated magma types is suggested. It is unlikely that a solid elemental carbon phase can be an equilibrium crystallization product of kimberlite magmas if the ilmenite megacrysts represent the redox state of kimberlite melts. The ultimate source of the oxidizing fluids and the development of such a wide dispersion (>4 orders of magnitude) in oxygen fugacities of the upper mantle is not clear, but may involve recycled lithosphere, fluids from the lower mantle or result from the relatively rapid diffusion of H₂, compared with other potential volatile species, in the mantle.     

Garnet peridotite xenoliths in South African kimberlite pipes and their petrogenesis

Whole rock analyses are presented for nine garnet peridotite, two garnet pyroxenite, one garnet free peridotite and two highly altered garnet peridotite xenoliths from South African kimberlite pipes. Garnets from eight of these samples have also been chemically analysed, while olivine compositions in the twelve fresh samples have been estimated by X-ray diffraction. The chemistry and mineralogy of these specimens are compared with the results of previous studies, and the controversies concerning the petrogenesis of these xenoliths are discussed in detail.