Geochemical stratigraphy of the lavas of the Solund-Stavfjord Ophiolite Complex,W. Norway,and magma-chamber dynamics

Bulletin of Volcanology - The stratigraphy and geochemistry of the uppermost 200–300&;nbsp;m of the metabasalt sequence of the Solund-Stavfjord Ophiolite Complex of western Norway has been...     

Mobilisation and transport of dissolved organic carbon and iron in peat catchments—Insights from the Lehstenbach stream in Germany using generalised additive models

During the last decades, increasing exports of both dissolved organic carbon (DOC) and iron were observed from peat catchments in North America and Europe with potential consequences for water quality of streamwater and carbon storages of soils. As mobilisation and transport processes of DOC and iron in peat catchments are only partly understood, the purpose of this study was to elucidate these processes in an intensively monitored and studied system. Specifically, it was hypothesised that dissimilatory iron reduction in riparian peatland soils mobilises DOC initially adsorbed to iron minerals. During stormflow conditions, both DOC and iron will be transported into the stream network. Ferrous iron may be reoxidised at redox interfaces on its way to the stream, and subsequently, ferric iron could be transported together with DOC as complexes. To test these hypotheses, generalised additive models (GAMs) were applied to 14 years of weekly time series of discharge and concentrations of selected solutes measured in a German headwater stream called Lehstenbach. This stream drains a 4.19‐km² forested mountain catchment; one third of which is covered by riparian peatland soils. We interpreted results of different types of GAM in the way that (a) iron reduction drove the mobilisation of DOC from peatland soils and that (b) both iron and DOC were transported as complexes after their joint mobilisation to and within the steam. It was speculated that low nitrate availability in the uppermost wetland soil layer, particularly during the growing season, promoted iron reduction and thus the mobilisation of DOC. However, the influence of nitrate on the DOC mobilisation remains relatively uncertain. This influence could be further investigated using methods similar to the GAM analysis conducted here for other catchments with long‐term data as well as detailed measurements of the relevant species in riparian wetland soils and the adjacent stream network.     

Tectonic control of bioalteration in modern and ancient oceanic crust as evidenced by carbon isotopes

Abstract We review the carbon‐isotope data for finely disseminated carbonates from bioaltered, glassy pillow rims of basaltic lava flows from in situ slow‐ and intermediate‐spreading oceanic crust of the central Atlantic Ocean (CAO) and the Costa Rica Rift (CRR). The δ¹³C values of the bioaltered glassy samples from the CAO show a large range, between ?17 and +3‰ (Vienna Peedee belemnite standard), whereas those from the CRR define a much narrower range, between ?17‰ and ?7‰. This variation can be interpreted as the product of different microbial metabolisms during microbial alteration of the glass. In the present study, the generally low δ¹³C values (less than ?7‰) are attributed to carbonate precipitated from microbially produced CO₂ during oxidation of organic matter. Positive δ¹³C values >0‰ likely result from lithotrophic utilization of CO₂ by methanogenic Archaea that produce CH₄ from H₂ and CO₂. High production of H₂ at the slow‐spreading CAO crust may be a consequence of fault‐bounded, high‐level serpentinized peridotites near or on the sea floor, in contrast to the CRR crust, which exhibits a layer‐cake pseudostratigraphy with much less faulting and supposedly less H₂ production. A comparison of the δ¹³C data from glassy pillow margins in two ophiolites interpreted to have formed at different spreading rates supports this interpretation. The Jurassic Mirdita ophiolite complex in Albania shows a structural architecture similar to that of the slow‐spreading CAO crust, with a similar range in δ¹³C values of biogenic carbonates. The Late Ordvician Solund–Stavfjord ophiolite complex in western Norway exhibits structural and geochemical evidence for evolution at an intermediate‐spreading mid‐ocean ridge and displays δ¹³C signatures in biogenic carbonates similar to those of the CRR. Based on the results of this comparative study, it is tentatively concluded that the spreading rate‐dependent tectonic evolution of oceanic lithosphere has a significant control on the evolution of microbial life and hence on the δ¹³C biosignatures preserved in disseminated biogenic carbonates in glassy, bioaltered lavas.     

Heat generation versus depth of crystallization for Norwegian monzonitic rocks

Thorium, U and K analyses by γ-ray spectrometry of monzonitic intrusives from Lofoten-Vesterålen (ca. 1800 m.y.), Bjerkreim-Sogndal (ca. 1000 m.y.) and the Oslo region (ca. 275 m.y.) are used to calculate values of radioactive heat generation. A strong negative correlation between heat generation and depth of crystallization is demonstrated, indicating that the chemical zoning of the continental crust with respect to Th and U is of a rather regional and regular character, and was probably firmly established at least 1800 m.y. ago. The K contents, in contrast to Th and U, show an increase with depths of crystallization for these rocks. This may be explained by considerations of the pressure dependence of the partition coefficients for these elements between minerals and magma, and the importance of a fluid phase as a transport medium for the incompatible elements Th and U. A positive correlation between K content and age of intrusion is discussed on a tentative basis.     

A heat pulse technique for the determination of small‐scale flow directions and flow velocities in the streambed of sand‐bed streams

The hyporheic zone (HZ) has the capability to eliminate and attenuate nutrients and contaminants in riverine systems. Biogeochemical reactions and the potential elimination of contaminants are strongly controlled by the flow paths and dynamics in the HZ. Nevertheless, an easily applicable method for the field determination of flow patterns in the HZ is still lacking. Therefore, a heat pulse technique, which traces the movement of a short heat pulse in the upper part of the HZ and other sand beds, was developed. Five rods are vertically driven into the sediment of the streambed; one rod with a heater as point source located in about 10‐cm sediment depth and four rods with four temperature sensors in 3 cm distance, arranged concentrically with 7 cm diameter around the heating rod. Subsequently, a heat pulse is applied and the resulting breakthrough curves are indicative of flow velocities and flow directions in the streambed. A rough data analysis procedure is also suggested. In addition, laboratory experiments were performed to test the heat pulse technique. These experiments were validated based on coupled numerical modelling of flow and heat transport. First field tests of the method prove that the method is easily applicable under field conditions. These first field tests showed highly complex flow patterns with flow velocities from 1·8 to 4·9 cm min^?1 and flow directions from parallel to surface flow to opposite to surface flow. This suggests the need for a robust method to quantify hyporheic flow patterns in situ. Copyright © 2011 John Wiley & Sons, Ltd.