SID flare production and Mt. Wilson magnetic class: An alternative interpretation

The statistical results presented by Achong and Stahl (1984) may alternatively be interpreted as demonstrating a strong dependence of SID flare production on Mt. Wilson magnetic class of the parent sunspot group.     

Spatial patterns and environmental risks of ringnet fishing along the Kenyan coast

Ringnet fishing began in the early 20th century and is practised worldwide, mainly to target nearshore pelagic species. The method was introduced to Kenya’s coastal waters by migrant fishers from Tanzania. However, the impacts of this fishing gear remain poorly assessed. We assessed the spatial distribution of ringnet fishing effort and its possible effects on ecosystem components, such as coral reefs, marine megafauna and marine protected areas, on the south coast of Kenya. We tracked 89 ringnet fishing trips made from December 2015 to January 2016 and used spatial multicriteria analysis to determine hotspots of possible environmental risks. The results showed that habitat type and bathymetric profile influenced the spatial distribution of ringnet fishing effort. Mixed seagrass and coral habitats had the highest concentration of the effort. Most of the habitats in the study area were moderately exposed to the impacts of the ringnet fishery. The study identifies high-risk areas that require spatial measures to minimise possible environmental risks of the gear both to habitats and to endangered sea turtles.     

Environmental tracers as indicators of karst conduits in groundwater in South Dakota,USA

Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ¹⁸O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration.     

Impacts of alien ‘ecosystem engineers’ overwhelm interannual and seasonal shifts in rocky-shore community composition on Marcus Island,South Africa

The South African coastline has been invaded by numerous alien species. Rare pre-invasion (1980) and post-invasion datasets (2001 and 2012) exist for Marcus Island, a small land-tied island in Saldanha Bay, South Africa. These snapshot datasets of the island’s intertidal invertebrate community were complemented with monitoring across seasons, from 2014 to 2016. Invertebrate communities were compared among the summers of 1980, 2001, 2012, 2014, 2015 and 2016 to assess interannual differences, while invertebrates and algae were monitored quarter-annually to assess seasonal changes. In addition, the population dynamics of the alien mussel Mytilus galloprovincialis were monitored. Differences in invertebrate communities between consecutive summers were significant but much smaller than changes induced by the arrival of alien species. Invertebrate and seaweed communities differed among years and shore zones but not among seasons, whereas species diversity differed among years, seasons and shore zones, with zones having the strongest influence. The invasion by M. galloprovincialis, and ensuing spatial and temporal variability in its recruitment, emerged as the most important factor influencing community composition, overshadowing interannual and seasonal changes. This work highlights that the impacts of alien species can be distinguished from natural variability by combining long-term monitoring with surveys at finer temporal scales. This is an important step in extending our understanding of the impacts of marine alien species.     

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province

15011993

Abstract

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone.

Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico.

Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.     

The galloping chromosphere

Solar Physics - Oscillating velocity fields can be observed on Hα filtergrams as a shifting pattern of intensity fluctuations known as ‘the galloping chromosphere’. The...     

Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface

Changes in streamflow and water table elevation influence oxidation–reduction (redox) conditions near river–aquifer interfaces, with potentially important consequences for solute fluxes and biogeochemical reaction rates. Although continuous measurements of groundwater chemistry can be arduous, in situ sensors reveal chemistry dynamics across a wide range of timescales. We monitored redox potential in an aquifer adjacent to a tidal river and used spectral and wavelet analyses to link redox responses to hydrologic perturbations within the bed and banks. Storms perturb redox potential within both the bed and banks over timescales of days to weeks. Tides drive semidiurnal oscillations in redox potential within the streambed that are absent in the banks. Wavelet analysis shows that tidal redox oscillations in the bed are greatest during late summer (wavelet magnitude of 5.62 mV) when river stage fluctuations are on the order of 70 cm and microbial activity is relatively high. Tidal redox oscillations diminish during the winter (wavelet magnitude of 2.73 mV) when river stage fluctuations are smaller (on the order of 50 cm) and microbial activity is presumably low. Although traditional geochemical observations are often limited to summer baseflow conditions, in situ redox sensing provides continuous, high‐resolution chemical characterization of the subsurface, revealing transport and reaction processes across spatial and temporal scales in aquifers.     

Microstructure and compositional changes across biotite‐rich reaction selvedges around mafic schollen in a semipelitic diatexite migmatite

Biotite‐rich selvedges developed between mafic schollen and semipelitic diatexite in migmatites at Lac Kénogami in the Grenville Province of Quebec. Mineral equilibria modelling indicates that partial melting occurred in the mid‐crust (4.8–5.8 kbar) in the range 820–850°C. The field relations, petrography, mineral chemistry and whole‐rock composition of selvedges along with their adjacent mafic schollen and host migmatites are documented for the first time. The selvedges measured in the field are relatively uniform in width (~1 cm wide) irrespective of the shape or size of their mafic scholle. In thin section, the petrographic boundary between mafic scholle and selvedge is defined by the appearance of biotite and the boundary between selvedge and diatexite by the change in microstructure for biotite, garnet, plagioclase and quartz. Three subtypes of selvedges are identified according to mineral assemblage and microstructure. Subtype I have orthopyroxene but of different microstructure and Mg# to orthopyroxene in the mafic scholle; subtype II contain garnet with many mineral inclusions, especially of ilmenite, in contrast to garnet in the diatexite host which has few inclusions; subtype III lack orthopyroxene or garnet, but has abundant apatite. Profiles showing the change in plagioclase composition from the mafic schollen across the selvedge and into the diatexite show that each subtype of selvedge has a characteristic pattern. Four types of biotite are identified in the selvedges and host diatexite based on their microstructural characteristics. (a) Residual biotite forms small rounded red‐brown grains, mostly as inclusions in peritectic cordierite and garnet in diatexite; (b) selvedge biotite forms tabular subhedral grains with high respect ratio; (c) diatexite biotite forms tabular subhedral grains common in the matrix of the diatexite; and (d) retrograde biotite that partially replaces peritectic cordierite and garnet in the diatexite. The four groups of biotite are also discriminated by their major element (EMPA) and trace elements (LA‐Q‐ICP‐MS) compositions. Residual biotite is high in TiO₂ and low in Sc and S, whereas retrograde biotite has high Al₂O₃, but low Sc and Cr. Selvedge and diatexite biotite are generally very similar, but selvedge biotite has higher Sc and S contents. Whole‐rock compositional profiles across the selvedges constructed from micro‐XRF and LA‐Q‐ICP‐MS analyses show: (a) Al₂O₃, FeO, MgO and CaO all decrease from mafic scholle across the selvedge and into the diatexite; (b) Na₂O is lowest in the mafic scholle, rises through the selvedge and reaches its maximum about 20–30 mm into the diatexite host; (c) K₂O is lowest in the mafic scholle and reaches its highest value in the first half of the selvedge, then declines before reaching a higher, but intermediate value, about 20 mm into the diatexite. Of the trace elements, Cs and Rb show distributions very similar to K₂O.