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Response of bedrock channel width to tectonic forcing: Insights from a numerical model, theoretical considerations, and comparison with field data
No full textThe morphology of bedrock river channels is controlled by climatic and tectonic conditions and substrate properties. Knowledge of tectonic controls remains scarce. This is partly due to slow tectonic rates and long response times of natural channels and partly due to the difficulty in isolating and constraining tectonic forcing conditions in the field. To study the effect of tectonic forcing on channel geometry, we have developed a numerical model of the cross-sectional evolution of a detachment-limited channel. Its predictions are matched by an analytical model based on the assumption of the minimization of potential energy expenditure. Using these models, we illustrate how local tectonics can alter the observed width-discharge scaling and discuss published field data in light of our findings. Except for one case, the models fail to correctly describe field observations of well-constrained cases. This implies that the shear stress/stream-power family of models is too simple to describe the behavior of natural channels. Additional complexities such as sediment effects and discharge variability exert a strong control on channel morphology and need to be taken into account in the modeling of channel dynamics and steady state
Surface and deep-water hydrography on Gardar Drift (Iceland Basin) during the last interglacial period
No full textChanges in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric [delta]13C gradient during the penultimate glaciation was similar to the last glaciation with high- [delta]13C Glacial North Atlantic Intermediate water above ~ 2000 m, and low- [delta]13C water derived from the Southern Ocean below. During Termination II, low-[delta]13C water was present throughout the water column with minimum values at intermediate depths (~ 1500-2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic [delta]18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low- [delta]13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ~ 124 ka. These changes may have affected Atlantic Meridional Overturning Circulation
Heavy mineral constraints on Paleocene sand transport routes in the Faroe-Shetland Basin
No full textAn integrated sandstone provenance study involving heavy mineral analysis, major element
geochemistry of garnet, trace element geochemistry of rutile, detrital zircon age dating and
palynofloral analysis has been undertaken on Paleocene (T10-T38) sandstones in hydrocarbon
exploration wells in the Faroe-Shetland Basin. This study has led to the identification of four
siliciclastic sand types, each with a different provenance, together with volcaniclastic detritus. Sand
type I was mainly derived from basement lithologies (Lewisian and/or Moine/Dalradian) on the
Orkney-Shetland Platform, either directly or indirectly through recycling. Sand type II was derived
principally through recycling of the Triassic on the Orkney-Shetland Platform. Sand type III, which
is commonly found in association with volcaniclastic material, is interpreted as having an origin on
the western margin of the basin. Sand type IV represents recycled Carboniferous (Namurian-
Westphalian) sandstones on the Orkney-Shetland Platform. Although the westerly-derived sand
type (III) has a zircon age spectrum dominated by the Archaean, the pattern contrasts with that
found in Archaean-derived Paleocene sandstones from the Kangerlussuaq region of East Greenland.
Derivation from more local Archaean basement is therefore inferred: possibilities include the crust
below the Faroe Islands, the Blosseville Kyst of East Greenland and the Wyville-Thomson Ridge.
Palynofloral analysis indicates that sand type III is invariably found in association with the
‘Greenland flora’, providing further support for input from the west. Most of the terrigenous sand
was supplied from the Orkney-Shetland Platform, but western input is seen locally at T10, T35-T36
and T38 levels
Mesozoic marine tetrapod diversity: mass extinctions and temporal heterogeneity in geological megabiases affecting vertebrates
No full textThe fossil record is our only direct means for evaluating shifts in biodiversity through Earth’s history. However, analyses of fossil marine invertebrates have demonstrated that geological megabiases profoundly influence fossil preservation and discovery, obscuring true diversity signals. Comparable studies of vertebrate palaeodiversity patterns remain in their infancy. A new species-level dataset of Mesozoic marine tetrapod occurrences was compared with a proxy for temporal variation in the volume and facies diversity of fossiliferous rock (number of marine fossiliferous formations: FMF). A strong correlation between taxic diversity and FMF is present during the Cretaceous. Weak or no correlation of Jurassic data suggests a qualitatively different sampling regime resulting from five apparent peaks in Triassic–Jurassic diversity. These correspond to a small number of European formations that have been the subject of intensive collecting, and represent ‘Lagerstätten effects’. Consideration of sampling biases allows re-evaluation of proposed mass extinction events. Marine tetrapod diversity declined during the Carnian or Norian. However, the proposed end-Triassic extinction event cannot be recognized with confidence. Some evidence supports an extinction event near the Jurassic/Cretaceous boundary, but the proposed end-Cenomanian extinction is probably an artefact of poor sampling. Marine tetrapod diversity underwent a long-term decline prior to the Cretaceous–Palaeogene extinction
Tentaculate fossils from the Cambrian of Canada (British Columbia) and China (Yunnan) interpreted as primitive deuterostomes
No full textMolecular and morphological evidence unite the hemichordates and echinoderms as the Ambulacraria, but their earliest history remains almost entirely conjectural. This is on account of the morphological disparity of the ambulacrarians and a paucity of obvious stem-groups. We describe here a new taxon Herpetogaster collinsi gen. et sp. nov. from the Burgess Shale (Middle Cambrian) Lagerstätte. This soft-bodied vermiform animal has a pair of elongate dendritic oral tentacles, a flexible stolon with an attachment disc, and a re-curved trunk with at least 13 segments that is directed dextrally. A differentiated but un-looped gut is enclosed in a sac suspended by mesenteries. It consists of a short pharynx, a conspicuous lenticular stomach, followed by a narrow intestine sub-equal in length. This new taxon, together with the Lower Cambrian Phlogites and more intriguingly the hitherto enigmatic discoidal eldoniids (Cambrian-Devonian), form a distinctive clade (herein the cambroernids). Although one hypothesis of their relationships would look to the lophotrochozoans (specifically the entoprocts), we suggest that the evidence is more consistent with their being primitive deuterostomes, with specific comparisons being made to the pterobranch hemichordates and pre-radial echinoderms. On this basis some of the earliest ambulacrarians are interpreted as soft-bodied animals with a muscular stalk, and possessing prominent tentacles
Glaciation and deglaciation of the Libyan Desert: The Late Ordovician record
No full textDetailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW-SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed
The osteology of Magnosaurus nethercombensis (Dinosauria, Theropoda) from the Bajocian (Middle Jurassic) of the United Kingdom and a re-examination of the oldest records of tetanurans
No full textMagnosaurus nethercombensis from the Lower Bajocian (Middle Jurassic) of Dorset, UK is a valid species, possessing a
single autapomor phy: the presence of anteroposteriorly elongate foramina, inclined anterodorsally and located ventrally on the lateral surface of the dentary. It is the oldest known definite tetanuran dinosaur and shows two tetanuran features: a reduced ischial peduncle of the ilium and the presence of a marked femoral extensor groove. Other records of putative early tetanurans are reviewed: ‘Zanclodon cambrensis’ from the Rhaetian of Wales; remains from the Norian–Hettangian of Switzerland;
Shuvosaurus and Protoavis from the Norian of Texas; Eshanosaurus from the Hettangian of China; theropod remains
originally included in the syntype series of Scelidosaurus from the Hettangian–Sinemurian of England; a fragmentary
skeleton from the Sinemurian of Italy; Cryolophosaurus from the Sinemurian–Pliensbachian of Antarctica; the partial
skeleton of a small theropod from the Toarcian of Morocco; and the lost syntype material of ‘Streptospondylus cuvieri’ from the Toarcian of Whitby. None of these records can be confidently considered to be the earliest tetanuran record. An early Middle Jurassic age for the earliest-known tetanuran is more consistent with a restricted content of Ceratosauria, comprising Ceratosaurus, Elaphrosaurus, and abelisauroids, than with a wider content including coelophysoids, due to reduction in the length of phylogenetic ghost lineages
Mississippian Paleocean Chemistry from Biotic and Abiotic Carbonate, Muleshoe Mound, Lake Valley Formation, New Mexico, U.S.A.--Discussion
No full textHasiuk and Lohmann (2008) present a geochemical study of cement and crinoid remains from the much studied Mississippian Muleshoe mud mound, New Mexico, U.S.A. They deduce Mississippian paleocean chemistry from data on altered cement and crinoids that are assumed to have recrystallized in sea water and retained their original marine chemistry. It is surprising in this context that they present data for just one brachiopod, as brachiopods are accepted as one of the most reliable marine proxies! They do not discuss the interpretation of Muleshoe brachiopod data made by Wu and Chafetz (2002). The latter authors differentiate intermound from mound brachiopods by their {delta}13C values; higher mound values are thought to be due to seepage of 13C-enriched waters produced by methanogenic bacteria. The Muleshoe mound is interpreted by Stanton et al. (2000) as being deposited in water ~ 200 m deep in the dysoxic oxygen-minimum zone (OMZ). Consequently, can Muleshoe carbonate influenced by methane seepage and deposited in the OMZ be regarded as an appropriate choice of material for an open marine proxy
Variation in size of living articulated brachiopods with latitude and depth
No full textGeographical variations in animal characters are one of the main subjects for study in macroecology. Variation with latitude has received special interest. Articulated brachiopods are possibly the commonest macrofossil with large variations in size of taxa through the fossil record. Here, we investigate trends in size of the 3 main orders of articulated brachiopod with latitude and depth. Data were insufficient to identify patterns in Thecideida (a micromorph taxon only recorded from low latitudes). Rhynchonellida had no clear trends in size with latitude or depth. Terebratulida exhibited hemispheric differences in size relations, with increasing length of species towards the pole in the south and no significant trend in the north. Tropical species were small (<20 mm length between 10°N and 10°S), and the largest species were found between 30° and 60° latitude in both hemispheres. There were no articulated brachiopods recorded from the high arctic, and support for a continuous trend in size with latitude was small or absent. In Terebratulida, there was a significant decrease in species length with depth of 1.7 mm per 100 m depth increase. These trends could be explained by competition for space and reduced availability of habitat with progressive depth beyond the continental shelf
Are palaeoscolecids ancestral ecdysozoans?
No full textThe reconstruction of ancestors is a central aim of comparative anatomy and evolutionary developmental biology, not least in attempts to understand the relationship between developmental and organismal evolution. Inferences based on living taxa can and should be tested against the fossil record, which provides an independent and direct view onto historical character combinations. Here, we consider the nature of the last common ancestor of living ecdysozoans through a detailed analysis of palaeoscolecids, an early and extinct group of introvert-bearing worms that have been proposed to be ancestral ecdysozoans. In a review of palaeoscolecid anatomy, including newly resolved details of the internal and external cuticle structure, we identify specific characters shared with various living nematoid and scalidophoran worms, but not with panarthropods. Considered within a formal cladistic context, these characters provide most overall support for a stem-priapulid affinity, meaning that palaeoscolecids are far-removed from the ecdysozoan ancestor. We conclude that previous interpretations in which palaeoscolecids occupy a deeper position in the ecdysozoan tree lack particular morphological support and rely instead on a paucity of preserved characters. This bears out a more general point that fossil taxa may appear plesiomorphic merely because they preserve only plesiomorphies, rather than the mélange of primitive and derived characters anticipated of organisms properly allocated to a position deep within animal phylogeny