Geological structure and kinematics of normal faults in the Otway Basin,Australia, based on quantitative analysis of 3‐D seismic reflection data

The Otway Basin in the south of Victoria, Australia underwent three phases of deformation during breakup of the southern Australian margin. We assess the geometry and kinematics of faulting in the basin by analysing a 3‐D reflection seismic volume. Eight stratigraphic horizons and 24 SW‐dipping normal faults as well as subordinate antithetic faults were interpreted. This resulted in a high‐resolution geological 3‐D model (ca. 8 km × 7 km × 4 km depth) that we present as a supplementary 3‐D PDF (Data S1). We identified hard‐ and soft‐linking fault connections over the entire area, such as antithetic faults and relay ramps, respectively. Most major faults were continuously active from Early to Late Cretaceous, with two faults in the northern part of the study area active until at least the Oligocene. Allan maps of faults show tectonic activity continuously waned over this time period. Isopach maps of stratigraphic volumes quantify the amount of syn‐sedimentary movement that is characteristic of passive margins, such as the Otway Basin. We show that the faults possess strong corrugations (with amplitudes above the seismic resolution), which we illustrated by novel techniques, such as cylindricity and curvature. We argue that the corrugations are produced by sutures between sub‐vertical fault segments and this morphology was maintained during fault growth. Thus, they can be used to indicate the kinematics vector of the fault movement. This evidences, together with left‐stepping relay ramps, that 40% of the faults had a small component (up to 25°) of dextral oblique slip as well as normal (dip‐slip) movement.     

Characterizing Atmospheric Aerosols off the Atlantic Canadian Coast During C-FOG

Boundary-Layer Meteorology - Marine aerosols play an important role in the Earth’s climate, but their effects remain highly uncertain due to a poor understanding of their sources, properties,...     

A Lagrangian Cloud Model for the Study of Marine Fog

A large-eddy simulation model is coupled with a Lagrangian cloud model to study marine fog. In this model, aerosols and droplets are treated from a Lagrangian frame of reference, in contrast to the traditional bulk and bin microphysical models. Droplet growth via condensation is governed by Köhler theory and environmental conditions local to the droplet. Coupling to the vapour and temperature fields of the flow ensures mass, momentum, and energy conservation between the air and droplet phases. Based on the recent C-FOG field campaign, a simulation is performed which highlights the benefits and potential of this type of model. By initializing the simulation with the measured aerosol size distribution and making assumptions about the chemical composition of the multiple peaks, the simulations provide a clear explanation for the observed bimodal droplet distribution during C-FOG: high supersaturation levels cause condensational growth of nearly all coarse-mode aerosols (presumed to be composed of marine salt), as well as a large number of accumulation model aerosols (presumed to be of continental origin with a lower hygroscopicity). The largest peak in the resulting droplet distribution is created from coarse-mode aerosols with high hygroscopicity, while the secondary peak is only possible due to the limited impact of the largest peak on saturation levels inside the fog. Thus, for the simulated levels of supersaturation, it is the limited number of coarse-mode aerosols which is responsible for the emergence of a larger second peak.

     

The onset of the Messinian salinity crisis in the deep Eastern Mediterranean basin

Astronomical tuning of the Messinian pre‐salt succession in the Levant Basin allows for the first time the reconstruction of a detailed chronology of the Messinian salinity crisis (MSC) events in deep setting and their correlation with marginal records that supports the CIESM ( 2008 ) 3‐stage model. Our main conclusions are (1) MSC events were synchronous across marginal and deep basins, (2) MSC onset in deep basins occurred at 5.97 Ma, (3) only foraminifera‐barren, evaporite‐free shales accumulated in deep settings between 5.97 and 5.60 Ma, (4) deep evaporites (anhydrite and halite) deposition started later, at 5.60 Ma and (5) new and published ⁸⁷Sr/⁸⁶Sr data indicate that during all stages, evaporites precipitated from the same water body in all the Mediterranean sub‐basins. The wide synchrony of events and ⁸⁷Sr/⁸⁶Sr homogeneity implies inter‐sub‐basin connection during the whole MSC and is not compatible with large sea‐level fall and desiccation of the Mediterranean.     

Petrography and provenance of Upper Cretaceous – Palaeogene sandstones in the foreland basin system of Central Nepal

Sedimentary deposits of the Cretaceous to Miocene Tansen Group of Lesser Himalayan association in central Nepal record passive-margin sedimentation of the Indian Continent with direct deposition onto eroded Precambrian rocks (Sisne Formation onto Kaligandaki Supergroup rocks), succeeded by the appearance of orogenic detritus as the Indian continent collided with Asia on a N-dipping subduction zone. Rock samples from two field traverses were examined petrographically and through detrital zircon U–Pb dating, one traverse being across the Tansen Group and another across the Higher and Tethyan Himalaya (TH). The Tansen Group depositional ages are well known through fossil assemblages. We examined samples from three units of the Tansen Group (Amile, Bhainskati, and Dumri Formations). The Sedimentary petrographic data and Qt F L and Qm F Lt plots indicate their ‘Quartzose recycled’ nature and classify Tansen sedimentary rocks as ‘recycled orogenic’, suggesting Indian cratonic and Lower Lesser Himalayan (LLH) sediments as the likely source of sediments for the Amile Formation (Am), the TH and the Upper Lesser Himalaya (ULH) as the source for the Bhainskati Formation (Bk), and both the Tethyan and Higher Himalaya (HH) as the major sources for the Dumri Formation (Dm). The Cretaceous–Palaeocene pre-collisional Am is dominated by a broad detrital zircon U–Pb ~1830 Ma age peak with neither Palaeozoic nor Neoproterozoic zircons grains, but hosts a significant proportion (23%) of syndepositional Cretaceous zircons (121–105 Ma) would be contributions from the LLH volcanosedimentary arc, Gangdese batholith (including the Xigaze forearc). The other formations of the Tansen Group are more similar to Tethyan units than to Higher Himalaya Crystalline (HHC). From the analysed samples, there is a lack of distinctive evidence or HH detritus in the Tansen basin. Furthermore, the presence of ~23±1 Ma zircons from the HH unit suggests that they could not have been exposed until the earliest Miocene time.     

Negotiating conflict resolution mechanisms for transboundary water treaties: A transaction cost approach

Conflict resolution mechanisms are recognised as indispensable constituting elements of transboundary water agreements. Earlier studies confirm, however, that such mechanisms are either incomplete, unsophisticated or absent from treaties. We argue that transaction costs, which occur during treaty negotiation, may constitute a barrier to the adoption of conflict resolution mechanisms in water treaties. Transaction costs are never equal and depend largely on the context in which negotiations take place. A content analysis of the treaties in the Transboundary Freshwater Dispute Database demonstrates that the adoption of conflict resolution mechanisms to transboundary water treaties is not random, but may be affected by external factors that influence the transaction cost of negotiating such mechanisms. Water scarcity and a history of cooperative hydro-relations are factors that coincide with the presence of conflict resolution mechanisms in treaties and are therefore considered to lower transaction costs. External resource dependency is believed to stir transaction costs to such an extent that it prevents the adoption of mature mechanisms. The same goes for political freedom, political heterogeneity and the presence of colonial signatories as these factors correlate with mechanism absence and the adoption of a low number of conflict resolution mechanisms per treaty. The effect of hydrological variability resulted insignificant, indicating that variability remains largely ignored by negotiators of transboundary water treaties. Our research further shows that if a mechanism for conflict resolution contains one element of maturity (e.g. institutionalisation or an activation procedure based on a unilateral rule) other elements of maturity are also likely to be present, providing negotiators with an incentive to negotiate conflict resolution mechanisms with at least one mature characteristic. Finally, the adoption of a high number of mechanisms coincides with the presence of institutionalised forms for conflict resolution, supporting the incentive of negotiating more than two mechanisms per treaty.     

Weathering, dust, and biocycling effects on soil silicon isotope ratios

Silicon isotope ratios (δ³⁰Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ³⁰Si values from a climate gradient of Hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ³⁰Si. In upper (<1 m deep) soil horizons along the climate gradient, Si depletion correlates with decreases of residual δ³⁰Si values in low rainfall soils and increases in high rainfall soils. Strong positive correlation between soil δ³⁰Si and dust-derived quartz and mica content show that both trends are largely controlled by the abundance of these weathering-resistant minerals. The data also lend support to the idea that fractionation of Si isotopes in secondary phases is controlled by partitioning of silicon between dissolved and precipitated products during the initial weathering of primary basalt. Secondary mineral δ³⁰Si values from lower (>1 m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential ²⁸Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ³⁰Si values in shallow (<1 m deep) soil horizons showed evidence of biocycling induced Si redistribution and substantially lower δ³⁰Si values than predicted. Low δ³⁰Si values in shallow soil horizons compared to predictions can be attributed to repeated fractionation as secondary minerals undergo additional cycles of dissolution and precipitation. Primary mineral weathering, secondary mineral weathering, dust accumulation, and biocycling are major processes in terrestrial Si cycling and these results demonstrate that each can be traced by δ³⁰Si values interpreted in conjunction with mineralogy and measures of Si depletion.     

Mutual replacement reactions in alkali feldspars II: trace element partitioning and geothermometry

Perthitic alkali feldspar primocrysts in layered syenites in the Klokken intrusion in South Greenland, underwent dissolution–reprecipitation reactions in a circulating post-magmatic aqueous fluid at ~450°C, and are to a large degree pseudomorphs. These ‘mutual replacement’ reactions provide a perfect natural experiment with which to study trace element partitioning between sodium and potassium feldspars growing simultaneously. The reactant ‘phase’ was a cryptoperthitic feldspar consisting of low albite and low microcline in a coherent sub-μm ‘braid’ intergrowth and the product phases were ‘strain-free’ incoherent subgrains of low albite and low microcline forming microporous patch perthites on scales up to 200 μm. The driving force for the reaction was reduction of coherency strain energy. The mechanisms of this process are described in Part I. Five mixed braid perthite–patch perthite crystals were analysed for major and trace elements using laser ablation-inductively coupled plasma mass spectrometry with a 19 μm beam diameter. This gave bulk analyses of the braid texture, which were in the range Ab_73–54Or_45–27An_4.3–0.8, but could resolve Ab- and Or-rich patches in patch perthite. The major element bulk compositions of the crystals were retained during the replacement reactions. Major components in patches plot on tielines in the Ab–Or–An ternary system that pass through or very close to the parent braid perthite composition and indicate local equilibrium on the scale of a few tens of mm. Many trace elements, including REE, were lost to the fluid during the deuteric reactions, but the effect is large only for Fe and Ti. Cs, Pb and Sr were added to some crystals. Plots of log distribution coefficient D for Rb, Ba, Pb, Eu²⁺, La and Ce between Or- and Ab-rich patches against ionic radius are straight lines, assuming eightfold coordination, and to a first approximation are independent of ionic charge. K also lies on these lines, and the smaller ions Na and Ca lie close to them. The best linear fits were obtained using ionic radii for ^[8]K and ^[8]Ca, but there is ambiguity as to whether ^[7]Na or ^[5]Na is most appropriate. The linear relationship shows that the listed trace elements are in the feldspar M-site rather than in inclusions. Tl is in M although an exact D could not be obtained. The very large Cs ion partitions strongly into the Or-rich phase but its D value appears to be less than predicted by extrapolation. The near-linearity arises because partitioning is occurring between two solids into sites which have similar Young’s moduli, so that the parabolas that normally represent trace element partitioning between crystals and liquids (which have negligible shear strength) approximately cancel out. Ga and Be are in T-sites, as well as some of the Fe and Ti present, although part is in oxide inclusions. The site of Sc is unclear, but if structural it is likely to be T. Partitioning on M-sites is a potential geothermometer but because the effective size of the irregular M-site is defined by its K and (Na + Ca) contents, which are controlled by ternary solvus relationships, its calibration is not independent of conventional two-feldspar geothermometers. Trace elements may however provide a useful means of confirming that feldspar pairs are in equilibrium, and of recognising feldspar intergrowths produced by non-isochemical replacement rather than exsolution. Two-feldspar geothermometry for the ternary phases in the low-albite microcline patch perthites gives temperatures above the stability range of microcline, markedly so if a correction is made for Si–Al ordering. This is probably because current geothermometers are too sensitive to low concentrations of An in ordered Or-rich feldspars. This interpretation is supported by two-feldspar assemblages growing at known temperatures in geothermal systems and sedimentary basins. This paper and the earlier Part I are dedicated in the memory of J. V. Smith and W. L. Brown, both of whom died in 2007, in acknowledgement of their unrivalled contributions to the study of the feldspar minerals over more than half a century. An erratum to this article can be found at     

Fractionation of silicon isotopes during biogenic silica dissolution

Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO₂, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO₂ suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO₂ production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor ε_DSi–BSi of −0.55‰, corresponding to a fractionation factor α_30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20 °C. Thus, the dissolution of bSiO₂ produces dissolved silicon with a δ³⁰Si value that is 0.55‰ more negative than its parent bSiO₂, an effect that must be accounted for when interpreting oceanic δ³⁰Si distributions. The δ³⁰Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ³⁰Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for 10–30% of the difference in diatom δ³⁰Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb³⁰SiO₂ of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage.