The timing and isotopic character of regional hydrothermal alteration and associated epigenetic mineralization in the western sector of the Kaapvaal Craton (South Africa)

The Kaapvaal Craton of South Africa comprises an Archaean core of ≈3.5 Ga lithospheric and crustal rocks surrounded by younger accreted terrains of ≈3.0–2.7 and ≈2.1–1.9 Ga. The craton is covered by relatively undeformed 3.0–2.4 Ga supracrustal rocks, which show the effects of thermal and hydrothermal interaction. Part of this activity is manifested by a large number of epigenetic Pb–Zn (±Ag, Au, Cu, F) deposits in the cover rocks of the Kaapvaal Craton. These include small volcanic and breccia hosted deposits in mafic and felsic volcanic rocks of the 2.7 Ga Ventersdorp Supergroup and the Mississippi Valley-type (MVT) deposits in the carbonates of the Transvaal Supergroup.MVT mineralization at the Pering (and other Zn–Pb deposits) is hosted in fracture-generated N–S breccia bodies in the Paleoproterozoic carbonate succession of the western Kaapvaal Craton. The fluids carrying the metals were focused in vertical bodies within the fracture zones (FZ), the metals and the sulphur being carried together and precipitated in organic-rich sectors of the basin. Two small Pb–Zn deposits within mafic rocks of the Ventersdorp Supergroup, stratigraphically below the basin-hosted MVTs on the southwestern part of the Kaapvaal Craton have secondary chlorite which is extremely Rb-rich, associated with the mineralization. This chlorite and the associated altered basaltic host rocks give a Rb–Sr date of ≈1.98 Ga, and the associated galena Pb isotope data plot on the same array as those of other Pb–Zn deposits, the radiogenic intercept giving a date of ≈2.0 Ga. We interpret these data to indicate a craton-wide epigenetic fluid-infiltration event, which exploited the Maquassie Quartz Porphyry (MQP) as the aquifer and metal source.Sr isotopic results for the ore-zone gangue minerals show highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios (>0.710) which support earlier models that the origin of radiogenic Sr isotopic composition in the calcite cements is the felsic tuffs (MQP) of the Ventersdorp Supergroup occurring at deeper levels within the basin. Relationships between δ¹⁸O and δ¹³C performed on carbonate cements within the aquifers are complex: the range in δ¹³C for some of the cements represents a mixture from two sources and with a progression from heavy carbon in the host to somewhat lighter carbon in the cements. Similarly, the lighter δ¹⁸O values have a narrow range indicative of rapid exchanges between hydrous fluid and rock.     

Numerical study of three-dimensional shelf circulation on the Scotian Shelf using a shelf circulation model

A numerical shelf circulation model was developed for the Scotian Shelf, using a nested-grid setup consisting of a three-dimensional baroclinic inner model embedded inside a two-dimensional barotropic outer model. The shelf circulation model is based on the Princeton Ocean Model and driven by three-hourly atmospheric forcing provided by a numerical weather forecast model and by tidal forcing specified at the inner model's open boundaries based on pre-calculated tidal harmonic constants. The outer model simulates the depth-mean circulation forced by wind and atmospheric pressure fields over the northwest Atlantic Ocean with a horizontal resolution of 1/12°. The inner model simulates the three-dimensional circulation over the Gulf of St. Lawrence, the Scotian Shelf, and the adjacent slope with a horizontal resolution of 1/16°. The performance of the shelf circulation model is assessed by comparing model results with oceanographic observations made along the Atlantic coast of Nova Scotia and in the vicinity of Sable Island (on the Scotian Shelf) during two periods: October 2000–March 2001 and April–June 2002. Analysis of model results on Sable Island Bank indicates that tidal currents account for as much as ∼80% of the total variance of near-bottom currents, and currents driven by local winds account for ∼30% of the variance of the non-tidal near-bottom currents. Shelf waves generated remotely by winds and propagating into the region also play an important role in the near-bottom circulation on the bank.     

Inter‐annual variability in the effects of riparian woodland on micro‐climate,energy exchanges and water temperature of an upland Scottish stream

The influence of riparian woodland on stream temperature, micro‐climate and energy exchange was investigated over seven calendar years. Continuous data were collected from two reaches of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) with contrasting land use characteristics: (1) semi‐natural riparian forest and (2) open moorland. In the moorland reach, wind speed and energy fluxes (especially net radiation, latent heat and sensible heat) varied considerably between years because of variable riparian micro‐climate coupled strongly to prevailing meteorological conditions. In the forested reach, riparian vegetation sheltered the stream from meteorological conditions that produced a moderated micro‐climate and thus energy exchange conditions, which were relatively stable between years. Net energy gains (losses) in spring and summer (autumn and winter) were typically greater in the moorland than the forest. However, when particularly high latent heat loss or low net radiation gain occurred in the moorland, net energy gain (loss) was less than that in the forest during the spring and summer (autumn and winter) months. Spring and summer water temperature was typically cooler in the forest and characterised by less inter‐annual variability due to reduced, more inter‐annually stable energy gain in the forested reach. The effect of riparian vegetation on autumn and winter water temperature dynamics was less clear because of the confounding effects of reach‐scale inflows of thermally stable groundwater in the moorland reach, which strongly influenced the local heat budget. These findings provide new insights as to the hydrometeorological conditions under which semi‐natural riparian forest may be effective in mitigating river thermal variability, notably peaks, under present and future climates. © 2014 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

Prey and plastic ingestion of Pacific Northern Fulmars (Fulmarus glacialis rogersii) from Monterey Bay,California

Marine plastic pollution affects seabirds, including Pacific Northern Fulmars (Fulmarus glacialis rodgersii), that feed at the surface and mistake plastic for prey or incidentally ingest it. Direct and indirect health issues can result, including satiety and possibly leading to inefficient foraging. Our objective was to examine fulmar body condition, identify cephalopod diet to species, enumerate and weigh ingested plastic, and determine if prey number and size were correlated with ingested plastics in beach-cast fulmars wintering in Monterey Bay California (2003, n = 178: 2007, n = 185). Fulmars consumed mostly Gonatus pyros, G. onyx, and G. californiensis of similar size for both years. We found a significant negative correlation between pectoral muscle index and average size of cephalopod beaks per stomach; a significant increase in plastic categories between 2003 and 2007; and no significant correlation between number and mass of plastic compared with number and size of prey for either year.     

Recent climatic trends and linkages to river discharge in Central Vietnam

In this study, change in rainfall, temperature and river discharge are analysed over the last three decades in Central Vietnam. Trends and rainfall indices are evaluated using non‐parametric tests at different temporal levels. To overcome the sparse locally available network, the high resolution APHRODITE gridded dataset is used in addition to the existing rain gauges. Finally, existing linkages between discharge changes and trends in rainfall and temperature are explored. Results are indicative of an intensification of rainfall (+15%/decade), with more extreme and longer events. A significant increase in winter rainfall and a decrease in consecutive dry days provides strong evidence for a lengthening wet season in Central Vietnam. In addition, trends based on APHRODITE suggest a strong orographic signal in winter and annual trends. These results underline the local variability in the impacts of climatic change at the global scale. Consequently, it is important that change detection investigations are conducted at the local scale. A very weak signal is detected in the trend of minimum temperature (+0.2°C/decade). River discharge trends show an increase in mean discharge (31 to 35%/decade) over the last decades. Between 54 and 74% of this increase is explained by the increase in precipitation. The maximum discharge also responds significantly to precipitation changes leading to a lengthened wet season and an increase in extreme rainfall events. Such trends can be linked with a likely increase in floods in Central Vietnam, which is important for future adaptation planning and management and flood preparedness in the region. Copyright © 2012 John Wiley & Sons, Ltd.     

Grain size,areal thickness distribution and controls on sedimentation of the 1991 Mount Pinatubo tephra layer in the South China Sea

The June 15, 1991 climactic eruption of Mt. Pinatubo produced an extensive, largely co-ignimbrite-derived airfall ash layer on Luzon Island and across the central South China Sea. The layer covers an area of ~4×10⁵ km² with a volume of 5.5 km³. Near the coast of Luzon, the deposit consists of two units: a normally graded basal ash bed, unimodal in grain size, and a finer-grained, internally structureless upper ash bed showing grain size bimodality. With increasing distance from the source, the coarse particle populations of the two units merge and migrate towards a near-constant fine population (~11 m); the distal region is covered by a fine-mode dominated, virtually ungraded single ash layer. The reversal of the winds from easterly directions at upper-tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse- and fine-mode components fell out from high-altitude eruption clouds. The high-velocity upper-level winds, however, would have transported fine-grained ash particles far beyond the South China Sea, which suggests that their settling was accelerated by aggregation. The boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling, coarse-grained particles. The particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper-level winds. At lower elevations, the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward, producing a thickness maximum in the medial region (at about 160 km from source). The strong displacement of fines, possibly aided by wind turbulence, led to a break in bulk tephra thinning rates close to the coastline. In the distal region, outside the influence of the typhoon, southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry. Within this region, at about 420 km from source, fallout of particle aggregates created a second thickness maximum. Comparison of the field data with previous experimental observations and tephra flux records in the deep sea (Wiesner et al. 1995; Carey 1997; McCool 2002) implies that the transport of ash in the water column was largely determined by vertical density currents. Differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface. Considering typical fall rates of convective plumes, modifications of the initial fallout position of the particles by the South China Sea current system are on the order of only a few kilometers. The results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions, distances, and fallout modes in the deep sea.Editorial responsibility: R. CioniAn erratum to this article can be found at     

Mineralization and deformation of the Malanjkhand terrane (2,490–2,440 Ma) along the southern margin of the Central Indian Tectonic Zone

The Malanjkhand Cu–Mo–Au deposit, located near the northwest margin of the Malanjkhand batholith (terrane), is a strategic and significant porphyry-style deposit that experienced a protracted 50 m.y. deformational history shortly after its formation at 2,490±8 Ma (Stein et al. 2004). In a recent study, Panigrahi et al. (2004) averaged U–Pb SHRIMP zircon data from a pooled set of samples from the Malanjkhand batholith to advocate a meaningless intermediate age of ~2,476 Ma for the Malanjkhand granitoid and its Cu–Mo–Au deposit. In the northwest part of the Malanjkhand batholith, Re–Os dating of occurrence-specific molybdenite captures not only the age of porphyry-style mineralization and associated magmatism, but also elucidates a complex deformational history that extends to ~2,450 Ma. In the central part of the Malanjkhand batholith, Re–Os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the Devgaon Mo prospect unequivocally shows that deformation ceased at this location no later than 2,470–2,465 Ma. The deformational history recorded at the Malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the Malanjkhand terrane with units in the poorly understood (proto) Central Indian Tectonic Zone (CITZ) along its southern margin, the Central Indian shear zone. The timing for this convergence is Late Archean–Early Paleoproterozoic.Comment on “Age of granitic activity associated with copper–molybdenum mineralization at Malanjkhand, Central India” by Panigrahi MK et al. (Mineralium Deposita 39:670–677)