Monitoring climate variability over the past 116 years in coral oxygen isotopes from Ningaloo Reef, Western Australia

 116-year record of coral skeletal δ¹⁸O is presented from a colony of Porites lutea from Ningaloo Reef, western Australia. Interannual variability of sea-surface temperatures (SST) inferred from skeletal δ¹⁸O is dominated by a 9.5-year period, and may constitute a characteristic signal of the Leeuwin Current. On long-terms coral skeletal δ¹⁸O indicates a near-continuous increase of SST at Ningaloo Reef over one century. The skeletal δ¹⁸O time series was checked for the presence of seasonal cooling events resulting from major volcanic eruptions. An ∼1  °C cooling is evident following the eruption of Pinatubo in 1991, which reproduces the results of previous investigations. However, only weak or no signals can be related to the eruptions of Krakatau (1883) and Agung (1963). Received: 9 November 1998 / Accepted: 12 April 1999     

Assessment of the relative importance of major sediment‐transport mechanisms in the central Great Barrier Reef lagoon

The delivery, flux and fate of terrigenous sediment entering the Great Barrier Reef lagoon has been a focus of recent studies and represents an ongoing environmental concern. Wave‐induced bed stress is the most significant mechanism of sediment resuspension in the Great Barrier Reef, and field data and mathematical modelling indicates that the combined effects of short‐period wind waves, longer period swell waves, and tidal and wind‐driven currents can often exceed the critical bed stress for resuspension. Suspended‐sediment concentrations at 20 m water depth indicate resuspension seldom occurs on the middle shelf under normal wave conditions. Non‐cyclonic turbidity events are generally confined to the inner shelf. The wave climate in the southern sector of the central Great Barrier Reef lagoon is the most erosive, and resuspension of outer shelf sediments was hindcast for recorded cyclones. Wind‐driven, longshore currents are fundamental to the northward movement of sediment, and the annual northward mass flux from embayments undergoing resuspension in the Burdekin region is estimated to be one order of magnitude larger than the mass of sediment introduced by a moderate flood plume. Strong onshore winds are estimated to generate significant three‐dimensional bottom return currents on approximately 30–70 days per year, forming a potentially significant offshore‐directed sediment flux during high suspended‐sediment concentration events on the inner shelf.     

Platinum-Group Elements in Sulphide Minerals, Platinum-Group Minerals, and Whole-Rocks of the Merensky Reef (Bushveld Complex, South Africa): Implications for the Formation of the Reef

The concentrations of platinum-group elements (PGE), Co, Re,Au and Ag have been determined in the base-metal sulphide (BMS)of a section of the Merensky Reef. In addition we performeddetailed image analysis of the platinum-group minerals (PGM).The aims of the study were to establish: (1) whether the BMSare the principal host of these elements; (2) whether individualelements preferentially partition into a specific BMS; (3) whetherthe concentration of the elements varies with stratigraphy orlithology; (4) what is the proportion of PGE hosted by PGM;(5) whether the PGM and the PGE found in BMS could account forthe complete PGE budget of the whole-rocks. In all lithologies,most of the PGE (65 up to 85%) are hosted by PGM (essentiallyPt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride).Lesser amounts of PGE occur in solid solution within the BMS.In most cases, the PGM occur at the contact between the BMSand silicates or oxides, or are included within the BMS. Pentlanditeis the principal BMS host of all of the PGE, except Pt, andcontains up to 600 ppm combined PGE. It is preferentially enrichedin Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, butexcludes both Pt and Pd. Chalcopyrite contains very little ofthe PGE, but does concentrate Ag and Cd. Platinum and Au donot partition into any of the BMS. Instead, they occur in theform of PGM and electrum. In the chromitite layers the whole-rockconcentrations of all the PGE except Pd are enriched by a factorof five relative to S, Ni, Cu and Au. This enrichment couldbe attributed to BMS in these layers being richer in PGE thanthe BMS in the silicate layers. However, the PGE content inthe BMS varies only slightly as a function of the stratigraphy.The BMS in the chromitites contain twice as much PGE as theBMS in the silicate rocks, but this is not sufficient to explainthe strong enrichment of PGE in the chromitites. In the lightof our results, we propose that the collection of the PGE occurredin two steps in the chromitites: some PGM formed before sulphidesaturation during chromitite layer formation. The remainingPGE were collected by an immiscible sulphide liquid that percolateddownward until it encountered the chromitite layers. In thesilicate rocks, PGE were collected by only the sulphide liquid. KEY WORDS: Merensky Reef; Rustenburg Platinum Mine; sulphide; platinum-group elements; image analysis; laser ablation ICP-MS     

新疆巴楚地区中上奥陶统生物礁群落古生态学

新疆巴楚东部地区良里塔格山出露有中奥陶统达瑞威尔阶一间房组和上奥陶统中部凯迪阶良里塔格组的生物礁。一间房组礁的厚度为数米到十多米,以棘屑滩为礁基,主要造礁生物为托盘类,形成典型的骨架岩,托盘类中央腔形成的原生孔隙发育,基质为灰泥,纹层状、皮壳状的藻类缠结包裹托盘类生长,可定为皮壳状藻托盘类密群落。良里塔格组的礁体主要由分枝状绿藻形成骨架岩,基质为灰泥,原有的原生或次生孔隙现被亮晶方解石充填;该组下部一些直径约1 m的小型礁由丰富的分枝状绿藻和少量乳孔藻构成,为分枝状绿藻密群落。良里塔格组上部较大的礁体（直径10~30 m）由分枝状绿藻和一种未知的球状生物（可能也是绿藻）构成,可称为“球状藻”分枝状绿藻密群落。     

塔里木盆地下古生界岩溶型储层类型及特征

塔里木盆地下古生界岩溶型储层可划分为同生岩溶、风化壳岩溶和埋藏岩溶等三种类型。同生岩溶型储层主要为礁滩体受三级或四级相对海平面下降导致的短暂暴露和淡水淋溶的产物。风化壳岩溶型储层包括层间岩溶和潜山岩溶两个亚类:层间岩溶为碳酸盐岩中—短期抬升剥蚀和淡水淋溶的产物,地形起伏平缓,与上覆碳酸盐岩呈平行不整合接触;潜山岩溶为碳酸盐岩长期抬升剥蚀和淡水淋溶的产物,地形起伏强烈,与上覆石炭系、侏罗系—白垩系碎屑岩呈角度不整合接触,包括石灰岩潜山和白云岩潜山。埋藏岩溶型储层又可分为原源埋藏岩溶和异源埋藏岩溶两个亚类,前者为有机酸、盆地热卤水、TSR成因的硫化氢等成岩流体作用的产物,后者为深源热液作用的产物。认为塔里木盆地岩溶型储层是多期次岩溶作用叠加改造的结果。     

Modeling the Merensky Reef,Bushveld Complex,Republic of South Africa

The Merensky pegmatoid (normal reef) in the western Bushveld Complex is commonly characterized as a pyroxene-rich pegmatoidal unit with a base that is enriched in chromite and platinum-group element-bearing sulfides overlying a leuconorite footwall. Models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile-induced remelting. The consequences of the latter interpretation are investigated using the numerical modeling program IRIDIUM, which links diffusive and advective mass and heat transport with a phase equilibration routine based on the MELTS program. The initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite, both initially at 1,190°C and 2 kbar. 2 wt% of a volatile fluid composed of 75 mol% H₂O, 20 mol% CO₂ and 5 mol% H₂S is then added to the lower 20 cm of the pyroxenite. The system is then allowed to evolve under conditions of chemical diffusion in the liquid. The addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite. However, chemical diffusion across the leuconorite–pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite, preferential re-precipitation of sulfide and chromite and concentration of the PGE at this boundary. These results mimic actual mineral and compositional profiles across the Merensky pegmatoid and illustrate that long-term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage.     

Accretion patterns in Holocene tropical coral reefs: do massive coral reefs in deeper water with slowly growing corals accrete faster than shallower branched coral reefs with rapidly growing corals?

It is a widely held concept that tropical coral reefs in shallower water with branched acroporid corals should accrete faster than those in deeper water dominated by massive corals. Results from a study of Holocene development of the largest Atlantic reef system, including paleo-waterdepth data, challenge these concepts. In Belize barrier and atoll reefs, reef accretion-rates range from 0.46 to 7.50 m/kyr, and average 3.03 m/kyr, as measured along 33 dated reef sections. Interestingly, accretion-rates increase with increasing paleo-waterdepth, and sections dominated by massive corals accumulated even slightly faster than those with branched acroporids. Published data from some other reef locations reveal no significant trends when plotting reef accretion-rate versus paleo-waterdepth, also indicating that the above-mentioned concepts should be questioned. Massive corals apparently are more resistant and accrete in lower disturbance conditions in slightly deeper water (5–10 m) and higher accomodation (space available for sediment deposition) as compared to shallow water (0–5 m) branched acroporids, which repeatedly get broken and leveled out during tropical cyclones.     

Assessing the potential for satellite image monitoring of seagrass thermal dynamics: for inter- and shallow sub-tidal seagrasses in the inshore Great Barrier Reef World Heritage Area,Australia

Seagrass meadows are at increasing risk of thermal stress and recent work has shown that water temperature around seagrass meadows could be used as an indicator for seagrass condition. Satellite thermal data have not been linked to the thermal properties of seagrass meadows. This work assessed the covariation between 20 in situ average daily temperature logger measurement sites in tropical seagrass meadows and satellite derived daytime SST (sea surface temperature) from the daytime MODIS and Landsat sensors along the Great Barrier Reef coast in Australia. Statistically significant (R²?=?0.787–0.939) positive covariations were found between in situ seagrass logger temperatures and MODIS SST temperature and Landsat sensor temperatures at all sites along the reef. The MODIS SST were consistently higher than in situ temperature at the majority of the sites, possibly due to the sensor’s larger pixel size and location offset from field sites. Landsat thermal data were lower than field-measured SST, due to differences in measurement scales and times. When refined significantly and tested over larger areas, this approach could be used to monitor seagrass health over large (10⁶?km²) areas in a similar manner to using satellite SST for predicting thermal stress for corals.     

The influence of pearl oyster farming on reef fish abundance and diversity in Ahe,French Polynesia

Many cultured pearl farms are located in areas of the Pacific that have thriving, highly diverse fish communities but the impacts of farming on these communities are poorly understood. We studied the effects of pearl oyster farming on shore fish abundance and diversity in the lagoon of Ahe, French Polynesia by adapting roving diver census methods to the coral reef bommies of the lagoon and compared 16 sites with high pearl farming impact to others with no direct impact. Pearl farming has a slightly positive effect on reef fish abundance (N) and no significant impact on fish diversity (H) or community composition. This is important when considering the ecological sustainability of pearl farming in French Polynesia and suggests that a potential synergy between pearl farms and marine conservation should be further explored.