Facies effects on the behaviour of Nd and Sr isotope systems in turbidite mudrocks during diagenesis

A detailed Sm/Nd, epsilon Nd and Rb/Sr profile through a 30-cm thick section of Silurian (Llandovery) interbedded turbiditic and hemipelagic mudrocks from the central Wales Basin shows well-marked chemical and isotopic trends. The variations reflect an interplay of depositional mode and diagenetic fractionation. Sm and Nd values are substantially higher and Sm/Nd ratios tend to be lower in the organic-rich hemipelagite layers due to diagenetic concentration in the hemipelagites. There is a corresponding depletion in the turbidite mudstones. Epsilon Nd values range from −0·8 to −7·1 and this is attributed to diagenetic modification of Sm/Nd ratios causing scatter in back-calculated epsilon values. Rubidium–strontium ratios in this succession fall within a narrow range, due to homogenization during diagenesis. By contrast, data from a hemipelagite-dominated (condensed) succession near the northern margin of the Welsh Basin show a lower range of epsilon Nd values and a higher scatter of Rb/Sr values, consistent with less fluid throughput during burial compaction and hence less diagenetic redistribution in these rocks. These patterns demonstrate the sensitivity of mudrock trace element and isotope compositions to both small-scale sedimentary structure and large-scale basin architecture.     

Evolution of the Nile deep‐sea turbidite system during the Late Quaternary: influence of climate change on fan sedimentation

This paper presents an overview of the evolution of the Nile deep‐sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep‐sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large‐scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep‐sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea‐level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood‐derived deposits occurred across the shallower part of the submarine delta during sea‐level rise between ca 14·8 and 5 ka. The most recent deep‐sea channel–lobe system was very active during this period of rising sea‐level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large‐scale slope failure. The Nile deep‐sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep‐sea turbidite system may be more active during periods of rising and high sea‐level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea‐level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large‐scale sedimentary system responded to monsoon‐driven climate change and sea‐level change in a system‐wide and contemporaneous manner.     

The Purple Saxifrage, Saxifraga oppositifolia, in Svalbard: two taxa or one?

Several studies have demonstrated high levels of genetic (DNA), ecophysiological, ecological, and morphological variation within the species Purple Saxifrage, Saxifraga oppositifolia , in Svalbard. It has recently been proposed that S. oppositifolia is represented by two conspicuously different subspecies in this archipelago: ssp. reptans , a late-flowering, prostrate ecotype of snow-protected, damp habitats, and ssp. pulvinata , an early-flowering, cushion-like ecotype of dry, wind-exposed heaths and ridges. It has also been suggested that the subspecies may be differentiated at the tetraploid and diploid levels, respectively, which would promote reproductive isolation. These hypotheses are tested by examining variation in morphology, ecology, and pollen size and stainability in 150 plants of S. oppositifolia growing in 50 vegetation samples at four sites in the Kongsfjorden area. Although analyses of the various data sets demonstrated the large variation within the species, the material could not be separated into distinct groups. The morphological variation was continuous along local ecological gradients. The pollen grains were fully stainable and the pollen diameter data showed a unimodal distribution, suggesting that the plants analysed represent only one ploidal level. These results reject a hypothesis that the morphologically intermediate plants are hybrids between two taxa at different ploidal levels. Thus, the conspicuous variation in S. oppositifolia in Svalbard probably results from local, in situ ecoclinal differentiation. Although this variation clearly is without taxonomic significance, it is important in the broader context of arctic conservation biology and the potential impact of global warming on arctic vegetation.     

Rock varnish in the Sonoran Desert: microbiologically mediated accumulation of manganiferous sediments

Rock varnish occurs in virtually all environments, most commonly in arid and semi-arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite-like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe-clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe-bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod-shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod-shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore-forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein-bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High-resolution mass spectrometry revealed non-hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro-organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.     

Relationships between lithology and diagenesis in a limeston—marl facies transition: an electron microprobe study

A 3 km long lateral facies transition from Oligo-Miocene skeletal grainstones into packstones and mudstones displays systematic variation in carbonate skeletal fragment alteration and secondary carbonate mineral composition. Within the skeletal grainstone end-member all aragonitic grains have dissolved, all Mg-calcite skeletal fragments have altered to calcite, and calcite has been precipitated as both void-filling spar and as syntaxial overgrowths on echinoid fragments. Within the transitional grainstone-packstone lithologies some skeletal fragments show preservation of their original mineralogy, while calcite, ferroan calcite spar and dolomite have precipitated in void spaces. Within the packstone-mudstone end-member of the transition, aragonitic grains have been preserved and Mg-calcite skeletal fragments have retained most of their magnesium. Thin, originally porous grainstone layers within the pack-stone-mudstones contain sideroplesite and ferroan calcite spar in void spaces, and iron has been incorporated in skeletal fragments which were originally Mg-calcite. In these grains iron has been incorporated at the expense of magnesium until the grains approach the composition of the surrounding ferroan calcite cement. The lateral variation in diagenetic alteration across this facies transition is a function of differing groundwater movement and composition, controlled ultimately by the composition, porosity and permeability of the host lithologies.     

A Barometer for Garnet Amphibolites and Garnet Granulites

new barometer based on the equilibrium: has been calibrated with experiments conducted in the piston-cylinderapparatus. Reversed equilibria have been obtained using well-calibrated2-54 cm NaCl furnace assemblies, Ag₈₀Pd₂₀capsules withf_O2 bufferedat or near iron-wustite. The equilibrium is located between10.6–10.8,12.0–12.2, 13.2–13.4 and 14.2–14.4kb, at 800, 900, 1000, and 1100?C, respectively. The barometer is applicable in both garnet-bearing amphibolitesand granulites. Its greatest potential is in garnet amphiboliteswherein multi-variant amphibole-bearing mineral assemblagesdo not define pressure and few, if any, well-calibrated barometersare available. Application of the garnet-rutile-ilmenite-plagioclase-quartzbarometer in amphibolite and granulite terranes yields geologicallyreasonable pressures that are in agreement with other well-calibratedbarometers in those terranes where comparisons can be made.     

Dolomite formation in the dynamic deep biosphere: results from the Peru Margin

Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values. The results are compared with the chemistry, and δ¹³C and ⁸⁷Sr/⁸⁶Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre‐dates the formation of framboidal pyrite. Most dolomite layers show ⁸⁷Sr/⁸⁶Sr‐ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ¹³C values of the dolomite are not in equilibrium with the δ¹³C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and ⁸⁷Sr/⁸⁶Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (<30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co‐occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot‐spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ¹³C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ¹³C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon‐rich sedimentary sequences can provide a useful window into the palaeo‐dynamics of the deep biosphere.     

Sedimentology and stratigraphy in the cave Hamnsundhelleren,western Norway

The stratigraphy in Hamnsundhelleren is as follows. A basal weathered rock bed of unknown age is followed by laminated clay deposited under stadial conditions and correlated with palaeomagnetism to the Laschamp excursion (43–47 000 yr BP). Angular blocks, bones and clay above this are ¹⁴C dated to the Ålesund Interstadial (28–38 000 yr BP). Another stadial laminated clay following the Ålesund Interstadial includes a palaeomagnetic excursion correlated with Lake Mungo (28 000 yr BP). The newly discovered Hamnsund Interstadial above this consists of frost-weathered clay and scattered angular blocks. It is ¹⁴C dated to 24 500 yr BP on bones mixed into the Ålesund Interstadial. The Hamnsund Interstadial is succeeded by another stadial laminated clay and then a Late-glacial–Holocene mixture of bones and blocks. In Hamnsundhelleren and other similar caves four successive phases of sedimentary environments for each ice-free–ice-covered cycle have been identified: (i) ice-free phase (deposition of bones and frost-weathered blocks); (ii) subaerial ice-dammed lake phase (sand or silt deposited in a lateral glacial lake); (iii) subglacial ice-dammed lake phase (cave closed by ice, deposition of till, debris flows and laminated clay); (d) ice-plugged phase (cave is plugged by frozen lake water and/or glacial ice, no deposition).