In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Samples of a core (52 m) of ablating Antarctic ice were analysed for ¹⁴CO and ¹⁴CO₂ by accelerator mass spectrometry. The data were compared with a ¹⁴C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ ¹⁴C at depths below 10 m, which we attribute to ¹⁴C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 ¹⁴C ka BP.     

Petrogenesis of a Stromatic Migmatite (Nelaug, Southern Norway)

The stromatic migmatites of Nelaug (Tvedestrand area, SouthernNorway) are investigated in detail. They show well developedlayers of leucosomes, mesosomes and melanosomes. It is establishedthat the mesosomes and leucosomes of these migmatites are differentfrom each other texturally, mineralogically, and chemically.Also combinations of leucosome plus adjacent melanosome portionsare chemically different from those of the mesosomes. Theseobservations do not agree with the findings of Mehnert (1971)and do not fit into his genetic model. The mesosome layers and the leucosome + melanosome combinationsare taken to represent the chemical compositions of the countryrock, a metagraywacke with relicts of primary rhythmic layering(Touret, 1965). The mineralogical composition of the layersvaries from granitic to tonalitic. Relict textures indicatethat the leucosome portions were initially occupied by layersof granitic composition relatively rich in K-feldspar, whereasthe mesosomes are the representatives of those metagraywackelayers which were relatively rich in plagioclase. An almostisochemical transformation of a paragneiss into the investigatedstromatic migmatite is established. Melting experiments performed at PH₂O= 5 Kb yielded solidustemperatures of 640±7 °C for all layers. The Composition of plagioclases present in the different layersis explained by isochemical partial melting and in situ crystallization.The chemical, mineralogical, and textural findings support themodel of almost isochemical transformation already establishedfor the Arvika migmatites (Johannes & Gupta, 1982).     

Growth and submarine fossilization of algal cup reefs,Bermuda*

Small charges of explosives were used to section cup-shaped reefs that occur on the margins of the Bermuda Platform. Study of these artificial outcrops, up to 10 m high, and the samples collected from them show how the reef-building community is rapidly converted to well-lithified reef rock in the marine development. The reefs, known locally as boilers and breakers, occur along the wave-swept south shore of the Bermuda Islands and around the northeast and northwest margins of the Platform. They are cup-shaped, up to 30 m in long dimensions, and rise up to the sea surface as much as 12 m above the surrounding sea floor. The reefs are built by an intergrowth of encrusting organisms, principally crustose coralline algae, an encrusting hydrozoan, Millepora sp., and an attached gastropod, Dendropoma irregulare. The growth framework of these algal cup reefs has extensive voids: large and intermediate-sized growth framework and shelter pores; borings of bivalves and sponges; and both intra- and inter-particle pores. A variety of vagile and sessile organisms (coelobites) inhabit these pores: an encrusting Foraminifera, Homotrema rubrum, is the most abundant attached coelobite; the tests of a variety of benthic Foraminifera and ostracods are common: branched coralline algae, barnacles, bivalves, ahermatypic corals, bryozoans, and burrowing crustaceans occur in varying abundance. Beginning millimetres below the living surface, internal sediments accumulate in the extensive voids. Coarse-grained skeletal sand derived from the surface of the reefs is characteristic of the larger voids; lime mud with the tests of planktonic Foraminifera and planktonic algae occurs generally in the smaller voids. Most specimens from the interior of the reefs show multiple generations of internal sediment that vary in grain size, composition, and colour. The sand-sized sediments are pumped into the voids by the frequent and intense wave action; the lime mud settles out in the smaller, less agitated pores. Cementation of internal sediments and surrounding growth frame begins centimetres below the living surface; it is so pervasive that marble-hard reef rock is developed within 1/2 m or less. The cement is principally high-magnesium calcite of micrite size, and subordinately acicular aragonite, but there are locally wide variations in crystal size and morphology. The occurrence of the cement within the reefs well below sea level, the isotope ratios of the cement crystals, the mineralogy, and the age inferred from radiocarbon age determinations of the growth frame all indicate that the cement is submarine and deposited from water of oceanic composition. The algal cup reefs of Bermuda demonstrate the reef-building ability of a community of encrusting organisms that form only crusts in the intertidal zone of the Mediterranean and Northern Brazil. The cup reefs of the northern margins of the Bermuda Platform are true reefs, not merely veneers covering eroded blocks of Pleistocene limestone. In their composition, location, and early diagenesis, the cup reefs closely resemble the algal or lithothamnion ridge of Pacific atolls. Synsedimentary cementation of internal sediments and growth frame makes a major contribution to the rigidity of these ocean-facing reefs and atoll rims. The assemblage of features that characterize the submarine fossilization of the cup reefs is widespread elsewhere in the modern seas: the floors of the Persian Gulf and parts of the Mediterranean; the margins of Pacific atolls; and the reefs off the north coast of Jamaica. This fossilization is characterized by reiterated generations of coelobites, internal sediments, and synsedimentary cements that can in time replace a major part of the original growth framework. Major variations in the sequence of these generations from pore to pore is the signature of this kind of fossilization. The same features of fossilization are described from reefs in the Devonian, Permian, and Triassic.     

Rates of Holocene isostatic uplift and relative sea-level lowering of the Baltic in SW Finland based on studies of isolation contacts

Southwestern Finland was covered by the Weichselian ice sheet and experienced rapid glacio-isostatic rebound after early Holocene deglaciation. The present mean overall apparent uplift rate is of the order of 4-5 mm/yr, but immediately after deglaciation the rate of crustal rebound was several times higher. Concurrently with land uplift, relative sea level in the Baltic basin during the past more than 8000 years was also strongly affected by the eustatic changes in sea level. There is ample evidence from earlier studies that during the early Litorina Sea stage on the southeastern coast of Finland around 7000 yr BP (7800 cal. yr BP), the rise in sea level exceeded the rate of land uplift, resulting in a short-lived transgression. Because of a higher rate of uplift, the transgression was even more short-lived or of negligible magnitude in the southwestern part of coastal Finland, but even in this latter case a slowing down in the rate of regression can still be detected. We used evidence from isolation basins to obtain a set of 71 ¹⁴C dates, and over 30 new sea-level index points. The age-elevation data, obtained from lakes in two different areas and located between c. 64 m and 1.5 m above present sea level, display a high degree of internal consistency. This suggests that the dates are reliable, even though most of them were based on bulk sediment samples. The two relative sea-level curves confirm the established model of relatively gradually decreasing rates of relative sea-level lowering since c. 6100 yr BP (7000 cal. yr BP) and clearly indicate that the more northerly of the two study areas experienced the higher rate of glacio-isostatic recovery. In the southerly study area, changes in diatom assemblages and lithostratigraphy suggest that during the early Litorina Sea stage (8300-7600 cal. yr BP) eustatic sea-level rise exceeded land uplift for hundreds of years. Evidence for this transgression was discovered in a lake with a basin threshold at an elevation of 41 m above sea level, which is markedly higher than any previously known site with evidence for the Litorina transgression in Finland. We also discuss evidence for subsequent short-term fluctuations superimposed on the main trends of relative sea-level changes.     

Moraine ridges formed from subglacial frozen-on sediment slabs and their differentiation from push moraines

Throughout the 1980s the annual cycle of ice-front activity along the stationary north margin of the ice-cap Myrdalsjökull, southern Iceland, produced a complex ridge, 4 m high, composed of imbricately stacked slabs of frozen, clast-paved lodgement till dipping up-glacier. Further observations in 1994 revealed that glaciofluvial processes and associated deposits may be involved in the final stage of ridge production depending on local climate and meltwater drainage pattern. It is concluded that at the margin of Myrdalsjökull the progressive stacking of subglacial frozen-on sediment slabs to form a moraine ridge is a fundamentally similar mechanism to that involved in the incremental double-layer model reported from Styggesdalsbreen, southern Norway. This study has also identified internal characteristics which are of potential use for distinguishing between moraine ridges formed by this mechanism and push moraines formed by proglacial thrusting.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.