Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles.We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed.An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.     

Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11

The Mid-Brunhes dissolution interval (MBDI) represents a period of global carbonate dissolution, lasting several hundred thousand years, centred around Marine Isotope Stage (MIS) 11. Here we report the effects of dissolution in ODP core 982, taken from 1134 m in the North Atlantic. Paradoxically, records of atmospheric CO₂ from Antarctic ice-cores reveal no long term trend over the last 400 kyr and suggest that CO₂ during MIS 11 was no higher than during the present interglacial. We suggest that a global increase in pelagic carbonate production during this period, possibly related to the proliferation of the Gephyrocapsa coccolithophore, could have altered marine carbonate chemistry in such a way as to drive increased dissolution under the constraints of steady state. An increase in the production of carbonate in surface waters would cause a drawdown of global carbonate saturation and increase dissolution at the seafloor. In order to reconcile the record of atmospheric CO₂ variability we suggest that an increase in the flux of organic matter from the surface to deep ocean, associated with either a net increase in primary production or the enhanced ballasting effect provided by an increased flux of CaCO₃, could have countered the effect of increased calcification on CO₂.     

Formation of 35S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord,Denmark) during short-term 35SO42− reduction measurements

Analyses of the specific products of ³⁵SO₄^2? reduction measurements were made in marine sediments in Denmark. We injected tracer quantities of ³⁵SO₄^2? into cores, incubated the cores, and assayed for ³⁵S-labelled acid volatile sulfides. Additionally, we assayed for ³⁵S-labelled elemental sulfur by extraction with CS₂, and for ³⁵S-labelled pyrite by reduction with chromium (II). We separately determined that elemental sulfur which formed in situ and that which formed by oxidation during the acid distillation of acid volatile sulfides.In subtidal sediments in Limfjorden, ³⁵S-labelled elemental sulfur and pyrite make up 14–32% of the reduced sulfur formed in short-term (0–48 hours) ³⁵SO₄^2? reduction experiments, at all depths studied (0–15 cm). Labelled elemental sulfur which formed in situ during the incubations at depths below 1 cm made up a fairly constant 5–11% of the total labelled reduced sulfur, from 0–1 cm, it made up 27%. An additional small amount (1–2% from 1–15 cm and 5% from 0–1 cm) of labelled elemental sulfur was formed during the acid-distillation step in our assay for labelled acid-volatile sulndes. Pyrite contained 4–13% of the total labelled reduced sulfur at all depths. Rates of sulfate reduction in Limfjorden were linear over the period 0–48 hours, and ³⁵S-pyrite made up a nearly constant percentage of the ³⁵S-labelled reduced sulfur formed over this time period.Estimates of sulfate reduction rates for Limfjorden which do not include elemental sulfur and pyrite as products are 19% too low. At Kysing Fjord, estimates of sulfate reduction which do not include elemental sulfur and pyrite are 24% to 32% too low. Thus, while previously published data on sulfate reduction in similar environments are probably low, they are not greatly in error.     

The characteristics of Pc 3 and Pc 4 geomagnetic micropulsations recorded at Sanae,Antarctica

Digital dynamic spectra of micropulsations recorded at SANAE (L ～ 4) show that Pc 3 pulsations have frequencies which decrease throughout the day. Both the onset frequency and the rate of decrease of frequency depend on the level of magnetic activity during the previous night. The variation of Pc 3 amplitudes and frequencies is explained in terms of the position of the plasmapause and the associated Pc 3 resonance region in the plasmatrough.For Pc 4 pulsations a constant frequency is observed on most days and it is not possible to infer the presence of a Pc 4 resonance region.     

Resistivity of partially saturated Triassic Sandstone

The variation in resistivity with saturation of poorly cemented Triassic Sandstone samples from a site in the English Midlands has been measured. The measurements were obtained using an adapted four‐electrode technique, which utilizes conductive gelling agents between electrodes, avoiding the need to sputter electrodes directly on to samples – a difficult process with such friable samples. The measurements provide important information regarding the way in which resistivity varies with saturation in the Triassic Sandstone. The resulting variation in the observed resistivity versus saturation curves indicates the presence of significant pore‐scale variation between samples. Measurements have also been conducted on fully saturated samples. These indicate significant variation in the matrix conductivity between samples. The results have important implications for field‐scale monitoring of the unsaturated zone.     

'Linked' debrites in sand-rich turbidite systems – origin and significance

Abstract The outer parts of a number of small Late Jurassic sandy deep‐water fans in the northern North Sea are dominated by the stacked deposits of co‐genetic sandy and muddy gravity flows. Sharp‐based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast‐rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a post‐depositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water‐rich sand bed just deposited by a co‐genetic concentrated gravity current. As such, they are referred to as ‘linked debrites’ to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low‐friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low‐concentration gravity currents should be more important here. In fact, evidence for transport by low‐concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast‐rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast‐charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand‐rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.     

Hybrid sediment gravity flow deposits – Classification, origin and significance

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.