Allochthonous Components Research Articles

The relation between productivity and temperature in the Pliocene North Atlantic at the onset of northern hemisphere glaciation: a palynological study

High resolution palynological analysis of DSDP Cores 607/607A shows for the interval between 2.8 and 2.2 Ma B.P. (which includes the onset of major northern hemisphere glaciations) a 41 ka cyclicity characterised by much higher palynomorph concentrations for the cooler intervals than for the warmer ones. Variation in dilution and concentration of palynomorphs can neither be explained by differential input of terrigenous clastics or carbonate, nor by differences in sedimentation rate, sediment density or selective preservation of palynomorphs. Subdivision of the palynomorph record in terms of autochthonous and allochthonous components, provides a way to detect changes in open ocean productivity and transport through time. It seems that a negative correlation between productivity and temperature in the latest Pleistocene open Atlantic had already been established before the major onset of northern hemisphere glaciations. A conceptual model is proposed to explain the observed lead of the palynological record relative to the isotope and carbonate records is proposed. This early response can result from changes in North Atlantic surface ocean circulation induced by changing atmospheric circulation.

Diopatrichnus odlingi n.isp. (annelid tube) and associated ichnofabrics in the White Limestone (M. Jurassic) of Oxfordshire: sedimentological and palaeoecological significance

The shell-lined tubes ( Diopatrichnus odlingi n.isp.) frequently associated with Epithyris in the highly bioturbated shelly micrites of the White Limestone (Bathonian) at Kirtlington (Oxfordshire) represent the reworked caps of a tube-building polychaete similar to those of the modern onuphid Diopatra . The associated facies includes two distinct ichnofabrics. Ichnofabric 1 represents complex tiering in a relatively quiet subtidal environment with extensive bioturbation due mainly to deep tier crustacean burrows ( Thalassinoides ), with Diopatrichnus as an allochthonous component. Ichnofabric 2 has a more diverse suite of trace fossils associated with partial preservation of primary cross-stratification, and a lower bioturbation grade possibly formed on a channel bar or shoal.

Lake Sediments from the Kashmir Himalayas: Inverted 14C Chronology and Its Implications

We have measured 14C, 210Pb and 137Cs profiles in two representative cores from Manasbal Lake, Kashmir, India. The sedimentation rate derived from 210Pb and 137Cs in the upper part of the core is in the range of 3.4 to 5.5 mm yr−1. In contrast, 14C ages show an inversion at depths >20 cm. These results are attributed to the erosion of the ubiquitous 10–20-m-thick loess mantle, based on the similarity of 14C ages of the inversion layer in the sediments and the paleosols present in the catchment area. Frequency-dependent mineral magnetic susceptibility (χfd), carbon to nitrogen ratios and pigment concentrations in the profile show a significant amount of allochthonous component in the lake deposits and support the conclusion that the 14C dates do not reflect the chronology of the in-situ lake sedimentation but episodic deposition of the surrounding loess. Thus, 14C serves as a useful tracer to understand source components of the sediments.

Lipids in sediments from Beibu Gulf South China Sea and their geochemical significance

Examination of solvent-extractable lipids in sediments from Beibu Gulf, South China Sea by capillary gas chromatography (GC) and capillary gas chromatography-mass spectrometry (GC-MS) resulted in the first identifications of β, γ-carotanes and their C19 to C31 homologs in the modern sediments from the China Seas. Distributional features of these (mainly triterpenoid and steroid hydrocarbons) biomarkers are discussed and the possibility of using these compounds as indicators of source of materials, diagenesis and depositional conditions is explored. It was found that the biomarkers in this region are mixtures of allochthonous (terrigenous and/or crude oil leak or pollution) and antochthonous (marine) organic components. The biomarders have proved to be of value for understanding sedimentation.

Sediment chemistry and diatom stratigraphy of two high arctic isolation lakes, Truelove Lowland, Devon Island, N.W.T., Canada

Three sediment cores from two lakes, Fish Lake and Phalarope Lake, in Truelove Lowland, Devon Island, N.W.T. were analyzed for diatoms and chemical composition. Multivariate statistical techniques using a range of chemical variables successfully isolated three sediment groupings in the cores. Allochthonous and autochthonous chemical components in the sediments have been used to reconstruct paleoenvironmental conditions. The two lakes began approximately 10600 years ago as shallow marine lagoons that were isolated from the sea as a result of glacio-isostatic rebound. Based on the presence of distinctive diatom assemblages, the three stratigraphic zones are identified as a basal marine zone, an intermediate and transitional brackish/marine zone and an upper freshwater zone. Following isolation from the sea, the lakes were flushed with freshwater produced by snow and ice melt. In Fish Lake, the period of transition from marine to freshwater, which began approximately 7000 years ago, lasted approximately 800 years. In Phalarope Lake, which was isolated from the sea approximately 5000 years ago, flushing by fresh water was completed only within the last 300 years. Fe, Cr, and Mo in the sediments are associated with the isolation phase when lake sedimentation is sensitive to the presence of brackish water and erosion within the lake catchments. In particular, the precipitation of Mo as MoS2 reflects the presence of hypolimnetic anoxia associated with lake isolation. During the early post-isolation phase the response of lake biota to an influx of nutrients is reflected in an increase in biological silica and organic carbon in the lake sediments. On the other hand, the generally low organic content of the sediments indicates that sedimentation in these lakes has been largely determined by variations in non-biogenic factors through time. During the mid Holocene the progressive stabilization of surface materials within the lake catchments is marked by decreasing Cr, As and Na in the sediments. At the same time, an increase in allochthonous Mn and Fe is attributed to progressive soil development. During the last 2500 years the catchments have experienced decreased erosion resulting in a decrease in both allochthonous clastic input and lake productivity.

Foraminiferal assemblages as flandrian palaeo-environmental indicators

The sediments deposited during the Flandrian Stage in Swansea Bay, South Wales, have been sampled by offshore gravity coring and drilling, and by drilling in the intertidal and supratidal zones. Sedimentological and microfaunal analyses have revealed that the late Devensian boulder-clay surface was progressively transgressed over the period 10,000–2,500 years B.P., and that a sequence of environments from the supralittoral marsh through high mudflats, low mudflats, littoral and sublittoral sandflats developed; each palaeoenvironment is characterised by its own, distinctive foraminiferal assemblage, with indigenous and allochthonous components.

The Moni Mélange, Cyprus: an olistostrome formed at a destructive plate margin

Detailed mapping has shown that the Moni Mélange of southern Cyprus consists of two separate allochthonous components: a variety of Triassic to Cretaceous sedimentary and volcanic rocks representative of an original continental margin sequence, and substantial sheets of serpentinite derived from Upper Cretaceous Troodos-type oceanic crust. All these allochthonous rocks were emplaced by sliding into an in situ host matrix of Upper Cretaceous bentonitic clays and radiolarian siltstones of deep water hemipelagic facies. The postulated geotectonic setting involves a phase of late Cretaceous northward subduction of Troodos-type oceanic crust beneath a continental margin now located in southern Turkey. Subduction culminated in the Maastrichtian in a major collision of oceanic crust with the adjacent continental margin. This triggered off major oceanward gravity sliding of continental margin rocks. The collision was also associated with deep tectonic slicing of Troodos oceanic crust with the result that sheets of partly serpentinized ultramafic rocks were emplaced by sliding into the host sediments of the mélange from the opposite direction to the other olistoliths. Soon afterwards there was a brief period of submarine erosion, then local tectonic movements ceased, suggesting an end to subduction in the area.