SHRIMP U–Pb ages of diagenetic and hydrothermal xenotime from the Archaean Witwatersrand Supergroup of South Africa

SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime ²⁰⁷Pb/²⁰⁶Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.     

基于波动方程的广义屏叠前深度偏移

地震波传播算子的计算效率和精度是制约三维叠前深度偏移的关键因素. 广义屏传播算子(GSP, Generalized Screen Propagator)是一种在双域中实现的广角单程波传播算子. 这一方法略去了在非均匀体之间发生的交混回响,但它可以正确处理包括聚焦、衍射、折射和干涉在内的各种多次前向散射现象. 通过背景速度下的相移和扰动速度下的陡倾角校正,广义屏算子能够适应地层速度的强烈横向变化. 这种算子可以直接应用于炮集叠前偏移,通过将广义屏算子作用于双平方根方程,还可以获得一种高效率、高精度的炮检距域叠前深度偏移方法,用于二维共炮检距道集和三维共方位角道集的深度域成像. 本文首先简述了炮检距域广义屏传播算子的理论,进而讨论了共照射角成像(CAI, Common Angle Imaging)条件,由此给出各个不同照射角(炮检距射线参数)下的成像结果,进而得到共照射角像集. 由于照射角和炮检距的对应关系,共照射角像集又为偏移速度分析和AVO(振幅随炮检距变化)分析等提供了有力工具.     

In situ sampling reveals rapid uptake and depuration of polycyclic aromatic hydrocarbons by surf clams (Paphies subtriangulata) affected by the Rena oil spill

ABSTRACT

To investigate the uptake and depuration of polycyclic aromatic hydrocarbons associated with the Rena oil spill we sampled the surf clam Paphies subtriangulata at two open coast locations (6?km apart) just prior to oil coming ashore (7 October 2011), then at 1–3 week intervals for the next 4 months. Total polycyclic aromatic hydrocarbons (tPAH) increased at both sites from 1 to 96–124?µg?kg^?1 (wet weight) by 18 October before declining to low levels (<4?µg?kg^?1) by February 2012. Ongoing sampling throughout 2012–2014 included three additional sites to the north east (up to 30?km away) and a site 5?km to the south east revealing tPAH levels generally <10?µg?kg^?1 except in October 2013 where levels ranged between 39–45?µg?kg^?1 at all sites. A comparison of PAH component profiles with oil-contaminated beach sediment indicated that the high levels observed in surf clams between October–December 2011 were clearly associated with the Rena spill. However, the October 2013 peak had a PAH profile inconsistent with weathered Rena oil, suggesting an alternative source of contamination. Our results highlight the potential for P. subtriangulata as a PAH monitoring tool but recognise more study is needed to better quantify baseline levels and uptake and depuration dynamics.     

Back to the future: reflections and directions of South African marine bioinvasion research

Biological invasions continue to increase around the world, with impacts on many coastal marine systems. Here we review the South African marine invasion literature which, despite the field being relatively new, has grown to have significant presence in both the local and international arenas. Of the 79 papers reviewed, 70% focused on the establishment and spread of alien species, with modes of transport and introduction largely overlooked. An emphasis was also apparent towards field studies, in particular survey work, with few experimental studies. The overwhelming majority of papers focused on a single species, the Mediterranean mussel Mytilus galloprovincialis, reflecting the scale of this invasion and the tractable nature of rocky shores as study systems. With the exception of this one species, the impacts of marine alien species have rarely been quantified. We suggest that future research extends the taxonomic coverage of present work and develops a better understanding of the mechanisms of introduction, establishment and spread of marine alien species. Through an experimental approach, the drivers of altered ecological patterns and processes resulting from invasions should be addressed, providing insight into associated impacts. This approach will maintain the local applicability and international relevance of South African marine invasion research.     

Individual responsibility for the oceans? An evaluation of marine citizenship by UK marine practitioners

In recent times, the governance of the marine environment has evolved from being primarily top down and state directed to being more participatory and community based. This paper proposes that an extension of this trend would be the inculcation of a societal sense of marine citizenship to deliver the sustainable management and protection of the marine environment through enhanced individual involvement in policy development and implementation. The potential role of marine citizenship in UK marine governance was examined by surveying UK marine practitioner’s views on both the current and future role of marine citizenship. Observations from this study identified three key factors for discussion including conflicting opinions over the implementation of marine citizenship into policy; that marine-focused education and a sense of personal attachment are integral to the development of marine citizenship; and that as yet, the potential implications of an enhanced sense of marine citizenship are uncertain. This research highlights the recognition amongst marine practitioners that higher levels of citizen involvement in the management of the marine environment would greatly benefit the marine environment, with additional benefits possible through an increased sense of marine citizenship.     

SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.     

Effects of electrolytes on attachment of aquatic bacteria to solid surfaces

The influence of electrolytes on the adhesion to surfaces by two freshwater bacteria (Pseudomonas fluorescens, Chromobacterium sp.), a marine bacterium (Pseudomonas sp. NCMB 2021), and eight estuarine isolates was investigated with interference reflection microscopy (IRM). IRM is a light microscope technique which allows evaluation of changes in bacterium-substratum separation distance within a range of 0 to ca. 100 nm by changes in image darkness and brightness. The addition of calcium chloride (5 mM) or lanthanum chloride (75 μM or 125 μM) to suspensions of the freshwater bacteria produced changes in the image consistent with a reduction in cellsurface separation distance. However, the addition of calcium chloride to suspensions of the estuarine or marine bacteria caused little or no change in image. As the addition of electrolytes would be expected to facilitate adhesion by reducing electrostatic repulsion between the two surfaces, the results suggest that electrostatic repulsion may be of little consequence to bacterial adhesion in saline environments.     

El Nino/Southern oscillation signals in the global tropical ocean

The monthly mean sea surface temperature data of 6 areas are used to study the El Nino/Southern Oscillation signals in the global tropical ocean. These areas are in the 5oN-5oS latitude zone at 1) eastern Pacific (110o-l40oW), 2) western Atlantic (30o-50oW), 3) eastern Atlantic (10oW-10oE), 4) western Indian Ocean (30o-50oE), 5) central Indian Ocean (70o-90oE) and 6) far western Pacific (120o-140oE), and the data cover the 120-month period of December 1968 to November 1978.A power spectrum analysts shows that the characteristic time of the El Nino/Southern Oscillation (about 3-4 years) appears not only in the eastern Pacific but also in other areas of the tropics except for the western Pa-cific, where the spectrum is of white noise. The amplitude of oscillation in the eastern Pacific is about 4 times larger than the others, making the El Nino/Southern Oscillation signal the strongest in this area. According to a cross-spectrum analysis, there is no time lag between the variation in the central Indian Ocean and that in the eastern Pacific. These two areas oscillate simultaneously and comprise the main feature of the El Nino/ Southern Oscillation. Other tropical areas are related with time lags, as shown by correlation and coherence calculations.It should be noted that the sea surface temperature in the eastern Pacific oscillates in phase with that in the Indian Ocean, while the pressure oscillations in these two areas are out of phase with each other, according to the Southern Oscillation definition. It is suggested that the Southern Oscillation cannot be explained simply by the sea surface temperature anomalies.Variations in the far western equatorial Pacific do not have the time scale of the El Nino/Southern Oscilla-tion, perhaps because it is a buffer zone between the monsoon system and the trade wind system.     

Elimination of hydraulic effects for cassiterite in a Malaysian Stream

Geochemical patterns for elements, such as Sn, W and Au, present in drainage sediments as resistate heavy minerals are often erratic and difficult to interpret. To investigate the source of these problems and develop methods of eliminating them we have compared the behavior of Sn, present as cassiterite, and associated pathfinder elements downstream from a small primary Sn deposit in Perak, Peninsular Malaysia.Dispersion trains for the pathfinder elements are characterized by smooth decay patterns and differences in concentrations between high- and low-energy environments, characterized by coarse-and medium-grained sands respectively, are not significant. In contrast, Sn (and magnetite) concentrations are extremely erratic with significantly higher concentrations in high- compared to low-energy environments. As a result the dispersion train for Sn exhibits no regular decay pattern away from its source. These findings suggest that the action of the stream is analogous to that of sluice box, with light minerals being winnowed away and cassiterite, together with magnetite, accumulating. For all but the finest sizes this process, which is most efficient in high-energy environments, causes considerable local variability in Sn content of the sediments. However, because the hydraulic behavior of cassiterite and magnetite is similar, but magnetite is not associated with the primary mineralization, the Sn/magnetite ratio can be used to eliminate Sn anomalies resulting from local variations in hydraulic conditions.The concept of hydraulic equivalence of cassiterite and magnetite was extended to examining the relationship between Sn and different size fractions of the light minerals that constitute the bulk of most sediments. Greatest contrast is obtained when the Sn content of the −270 mesh (−53 μm) fraction is re-expressed as its hydraulic equivalent concentration in −65 + 100 mesh (−212 + 150 μm) material.For exploration purposes it is concluded that: (1) providing cassiterite is present in the fine size fractions, sampling of this material will reduce hydraulic effects, thereby reducing data variability, and can also increase the length of the anomalous dispersion train; and (2) hydraulic effects can also be reduced by re-expressing Sn concentrations as ratios to magnetite (provided this is not associated with the primary Sn mineralization) or a hydraulically equivalent size fraction of the light minerals that constitute the bulk of the sediment. Similar principles probably apply to the interpretation of geochemical data for other elements dispersed in drainage sediments as heavy minerals; this warrants further investigation.