A Lagrangian model for wind- and wave-induced near-surface currents

A theory is outlined for time-dependent currents induced near the sea surface in deep water, away from coastal boundaries, by a variable wind stress and deep-water wave field. It is based on the theory of Weber (1983) which uses a second-order perturbation expansion of the Navier-Stokes equations in Lagrangian coordinates and includes the Coriolis effect. It uses an eddy viscosity formulation for both wave dissipation and momentum transfer within the current field: the eddy viscosity ν may be allowed to vary with depth. The wind stress may be time-varying and the wave field may vary in both space and time.For the case of a constant ν, the results agree with those of Ursell (1950), Hasselmann (1970) and Pollard (1970) in the limit ν→0, and the steady-state results agree with those of Weber. For a particular case of depth-varying ν, results (obtained from numerical simulations) are in better general agreement with observations of wind-induced surface drift than when a constant ν is used.An outline is given of the application of the theory to the case of a random sea state. There are good prospects for using output data from numerical wave prediction models to drive the equations of this near-surface current model.     

Recruitment patterns of Serpula vermicularis L. (Polychaeta, Serpulidae) in Loch Creran, Scotland

This study aimed to contribute to conservation management of reefs of Serpula vermicularis by increasing understanding of the factors influencing larval settlement. The study was carried out in Loch Creran, which supports the most extensive known development of S. vermicularis reefs in the world. Settlement plates were deployed to examine the influence of season, depth, reef density, substrate type and orientation. Monthly deployment of plates revealed settlement of S. vermicularis to occur predominantly from mid-June to mid-October, peaking in late August to early September. Settlement of Pomatoceros spp. peaked much earlier, in late May to early June. Deployment of plates at different depths revealed a marked reduction in S. vermicularis settlement intensity between 6 and 12 m. As this corresponds with the deeper limit of the peripheral fringe of serpulid reefs in the loch, it is suggested that this limit is imposed by a depth-correlated settlement response, rather than reduction in available substrata. Comparisons of various substrata showed a preference by S. vermicularis larvae for a slate over a scallop substrate and no evidence of enhanced recruitment to occupied or unoccupied tubes of S. vermicularis, suggesting that gregarious attraction is unlikely to be a factor causing reef formation. Settlement onto the upper side of a horizontal scallop substrate was found to be insignificant in comparison with the underside or a vertically orientated scallop. Evidence for the role of light in controlling the depth and substrate-orientation preferences of S. vermicularis larvae is discussed. Based on the results of this study, recommendations are made regarding remediation of areas suffering reef damage.     

Carbon stable isotopes in estuarine sediments and their utility as migration markers for nursery studies in the Firth of Forth and Forth Estuary, Scotland

The stable carbon isotope ratios (δ¹³C) of the organic fraction of intertidal sediments in the Forth Estuary and the Firth of Forth, Scotland, were measured to determine if terrestrially derived carbon was present in the estuarine sediments. It was hypothesised that differences in the inputs from marine vs. terrestrial sources to the organic carbon of estuarine and marine sediments, as well as differences in ambient seawater stable oxygen isotope (δ¹⁸O) ratios between the estuary and the Outer Firth, would allow the use of these two stable isotopes as habitat markers for juvenile plaice (Pleuronectes platessa), to allow determination of nursery habitats. Muddy and sandy sediments from the estuary and sandy sediments from the Outer Firth were sampled and δ¹³C measured. Juvenile plaice were caught at two estuarine sites and at two Outer Firth sites and otoliths were removed for δ¹³C and δ¹⁸O analysis. The sandy sediments in the estuary showed a strong gradient of δ¹³C enrichment with distance down the estuary, while the muddy sediments showed a much shallower gradient. δ¹³C and δ¹⁸O measured in the carbonate of juvenile plaice otoliths showed no clear difference between otoliths of fish caught at one of the estuarine sites and at the two Outer Firth sites. However, the isotope ratios of both carbon and oxygen in plaice otoliths from the other estuarine site showed the expected trend of depletion in the heavier isotopes. While the measurements recorded here did not conclusively distinguish between otoliths from juveniles caught in the estuarine site and those caught in the other three sites, they show that stable isotopes have potential to distinguish between estuarine habitats with terrestrial carbon inputs, and coastal marine habitats with predominantly marine carbon inputs.     

Can turbidity caused by Corophium volutator (Pallas) activity be used to assess sediment toxicity rapidly?

The standard toxicity test organism, Corophium volutator, exhibits a behavioural response to contaminated sediments that causes increased turbidity of overlying water. We quantify the effects of this response to an estuarine sediment spiked with copper and hydrocarbon contaminated sediments from an oil installation in the North Sea. Turbidity measured 24 h after the start of a toxicity test shows a strong relationship with contaminant concentrations and with mortality after 10 days. Turbidity measurements can therefore give a rapid indication of sediment toxicity, permitting a reduction in storage time of sediments to be used in dilution series and toxicity identification evaluation (TIE) tests, reducing the likelihood of contaminants degrading prior to testing.     

Deposition and diagenesis of the lacustrine-fluvial Cangfanggou Group (uppermost Permian to Lower Triassic), southern Junggar Basin,NW China: a contribution from sequence stratigraphy

The Junggar Basin in NW China contains lacustrine hydrocarbon source rocks which are among the highest quality of hydrocarbon potential in the world. Oil reservoirs in the basin are very substantial: target reservoirs span Carboniferous to Tertiary strata and include Permo-Triassic lacustrine and fluvial sandstones. The Junggar Basin was a foreland basin during the late Permian to Cenozoic, possibly with strike-slip tectonics at the southern margin during Mesozoic time. The Cangfanggou Group, as one of the major reservoirs, is well-exposed in the eastern part of the southern Junggar Basin. A measured outcrop section and a number of borehole logs coupled with resistivity logs were used to attempt sequence stratigraphic analysis. Detailed sedimentological studies on the outcrops and borehole cores have demonstrated that the Cangfanggou Group is characterized by alternating lacustrine and fluvial deposits. Four depositional sequences have been recognized. For each sequence, the basal boundary is marked by erosional truncation of fluvial channel conglomeratic sandstones in sharp contact with underlying lacustrine or floodplain mudstones. The top of each lowstand systems tract is normally overlain by the transition to lacustrine or maximum flooding surface. The transgressive systems tract is normally not identifiable at the basin margin, but was developed in the basinward area and characterized by interbedded fining-upward distal fluvial and shallow lacustrine deposits. The highstand systems tract at the basin margin is characterized by very thick floodplain mudstones or shallow lacustrine deposits, and by typical coarsening-upward parasequences of shallow lacustrine deposits in more basinward areas. Sediment input to the basin was controlled by tectonics and climate. Depositional sequences were probably controlled by fluctuating change of lake level: this was in turn controlled by climate (runoff), modified by tectonics in specific areas.The sandstones studied are exclusively volcanic litharenites. Diagenetic studies suggest that the calcite cementation, pore-filling clay minerals and zeolites occluded substantial porosity in the sandstones examined because they are compositionally immature. However, notable secondary porosity in varying proportions is present in the sandstones of the Cangfanggou Group, resulting from the dissolution of unstable detrital grains. The lowstand fluvial/distal fluvial sandstones recorded the highest average porosity and highest permeability, in which some primary porosity may remain because early formed clay coatings inhibited further compaction. The combination of residual primary porosity and significant amount of secondary porosity in the sandstones of the Cangfanggou Group may constitute moderate to good reservoirs. In contrast, the lacustrine fine-grained sandstones is characterized by clay authigenesis and zeolitization, in which the porosity was obliterated by the zeolites and extensive illitization; the lowstand fluvial channel sandstones in the basin margin areas are characterized by extensive calcite cementation which greatly reduced the porosity and permeability.This is the fifth paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.     

Evidence for Neotethys rooted within the Vardar suture zone from the Voras Massif, northernmost Greece

Three conflicting models are currently proposed for the location and tectonic setting of the Eurasian continental margin and adjacent Tethys ocean in the Balkan region during Mesozoic–Early Tertiary time. Model 1 places the Eurasian margin within the Rhodope zone relatively close to the Moesian platform. A Tethyan oceanic basin was located to the south bordering a large “Serbo-Pelagonian” microcontinent. Model 2 correlates an integral “Serbo-Pelagonian” continental unit with the Eurasian margin and locates the Tethys further southwest. Model 3 envisages the Pelagonian zone and the Serbo-Macedonian zone as conjugate continental units separated by a Tethyan ocean that was sutured in Early Tertiary time to create the Vardar zone of northern Greece and former Yugoslavia. These published alternatives are tested in this paper based on a study of the tectono-stratigraphy of a completely exposed transect located in the Voras Mountains of northernmost Greece. The outcrop extends across the Vardar zone, from the Pelagonian zone in the west to the Serbo-Macedonian zone in the east.Within the Voras Massif, six east-dipping imbricate thrust sheets are recognised. Of these, Units 1–4 correlate with the regional Pelagonian zone in the west (and related Almopias sub-zone). By contrast, Units 5–6 show a contrasting tectono-stratigraphy and correlate with the Paikon Massif and the Serbo-Macedonian zone to the east. These units form a stack of thrust sheets, with Unit 1 at the base and Unit 6 at the top. Unstacking these thrust sheets places ophiolitic units between the Pelagonian zone and the Serbo-Macedonian zone, as in Model 3. Additional implications are, first, that the Paikon Massif cannot be seen as a window of Pelagonian basement, as in Model 1, and, secondly, Jurassic andesitic volcanics of the Paikon Massif locally preserve a gneissose continental basement, ruling out a recently suggested origin as an intra-oceanic arc.We envisage that the Almopias (Vardar) ocean rifted in Triassic time, followed by seafloor spreading. The Almopias ocean was consumed beneath the Serbo-Macedonian margin in Jurassic time, generating subduction-related arc volcanism in the Paikon Massif and related units. Ophiolites were emplaced onto the Pelagonian margin in the west and covered by Late Jurassic (pre-Kimmeridgian) conglomerates. Other ophiolitic rocks formed within the Vardar zone (Ano Garefi ophiolite, Unit 4) in latest Jurassic–Early Cretaceous time and were not deformed until Early Tertiary time. The Vardar zone finally sutured in the Early Tertiary creating the present imbricate thrust structure of the Voras Mountains.     

Tectonic evolution of the Intra-Pontide suture zone in the Armutlu Peninsula, NW Turkey

The Armutlu Peninsula and adjacent areas in NW Turkey play a critical role in tectonic reconstructions of the southern margin of Eurasia in NW Turkey. This region includes an inferred Intra-Pontide oceanic basin that rifted from Eurasia in Early Mesozoic time and closed by Late Cretaceous time. The Armutlu Peninsula is divisible into two metamorphic units. The first, the Armutlu Metamorphics, comprises a ?Precambrian high-grade metamorphic basement, unconformably overlain by a ?Palaeozoic low-grade, mixed siliciclastic/carbonate/volcanogenic succession, including bimodal volcanics of inferred extensional origin, with a possibly inherited subduction signature. The second unit, the low-grade znik Metamorphics, is interpreted as a Triassic rift infilled with terrigenous, calcareous and volcanogenic lithologies, including basalts of within-plate type. The Triassic rift was unconformably overlain by a subsiding Jurassic–Late Cretaceous (Cenomanian) passive margin including siliciclastic/carbonate turbidites, radiolarian cherts and manganese deposits. The margin later collapsed to form a flexural foredeep associated with the emplacement of ophiolitic rocks in Turonian time. Geochemical evidence from meta-basalt blocks within ophiolite-derived melange suggests a supra-subduction zone origin for the ophiolite. The above major tectonic units of the Armutlu Peninsula were sealed by a Maastrichtian unconformity. Comparative evidence comes from the separate Almacık Flake further east.Considering alternatives, it is concluded that a Mesozoic Intra-Pontide oceanic basin separated Eurasia from a Sakarya microcontinent, with a wider Northern Neotethys to the south. Lateral displacement of exotic terranes along the south-Eurasian continental margin probably also played a role, e.g. during Late Cretaceous suturing, in addition to overthrusting.     

A pulsed extension model for the Neogene–Recent E–W-trending Alaşehir Graben and the NE–SW-trending Selendi and Gördes Basins, western Turkey

We have developed a significant body of new field-based evidence relating to the history of crustal extension in western Turkey. We establish that two of the NE–SW-trending basins in this region, the Gördes and Selendi Basins, whose sedimentary successions begin in the early Miocene, are unlikely to relate to late-stage Alpine compressional orogeny or to E–W extension of Tibetan-type grabens as previously suggested. We argue instead that these basins are the result of earlier (?) late Oligocene, low-angle normal faulting that created approximately N–S “scoop-shaped” depressions in which clastic to lacustine and later tuffaceous sediments accumulated during early–mid-Miocene time, separated by elongate structural highs. These basins were later cut by E–W-trending (?) Plio–Quaternary normal faults that post-date accumulation of the Neogene deposits. In addition, we interpret the Alaşehir (Gediz) Graben in terms of two phases of extension, an early phase lasting from the early Miocene to the (?) late Miocene and a young Plio–Quaternary phase that is still active. Taking into account our inferred earlier phase of regional extension, we thus propose a new three-phase “pulsed extension” model for western Turkey. We relate the first two phases to “roll-back” of the south Aegean subduction zone and the third phase to the westward “tectonic escape” of Anatolia.