The magneto-boussinesq approximation by scale analysis

Abstract

The Magneto-Boussinesq approximation is derived as a set of leading order equations in an asymptotic expansion. An analogous set of equations is derived for the case when the Alfvén speed is comparable to the sound speed.     

Oxygen and carbon isotope stratigraphy of the Palaeogene and Cretaceous limestones in the Bottaccione Gorge and the Contessa Highway sections, Umbria, Italy

We have made oxygen and carbon isotope measurements on limestone samples of Albian to Eocene age recovered from the Bottaccione Gorge section in the Umbrian Apennines, central Italy. Additional measurements have been made on topmost Maastrichtian and Palaeocene sediments from the adjacent Contessa Highway section. Our data from the Bottaccione Gorge show ¹³C maxima at the Cenomanian/Turonian boundary and during the Palaeocene, as well as a pronounced ¹³C minimum 7 metres above the clay layer at the K/T boundary. Our data from the Contessa Highway show a ¹³C minimum 9 metres above the K/T boundary clay. These minima are probably diagenetic artifacts. In the Bottaccione Gorge there is evidence for a hiatus in the early Middle Eocene. The Palaeocene of the Bottaccione Gorge appears to have been affected by slumping or faulting (in addition to the diagenetic overprinting) giving a spurious δ¹³C recovery after the K/T boundary, the Contessa Highway section appears to be more complete. The Coniacian-Maastrictian is characterized by relatively stable ¹³C values. Our data indicate an increase in δ¹⁸O (and therefore possible cooling) across the Cretaceous/Tertiary boundary in both the Bottaccione and Contessa Highway sections.     

Oceanographic and climatic implications of the Palaeocene carbon isotope maximum

We have compared detailed planktonic and benthonic foraminiferal carbon and oxygen isotope records from the Palaeocene and early Eocene successions at DSDP Site 577 (Shatsky Rise, North Pacific), a composite section derived from DSDP Leg 74 sites (Walvis Ridge, South Atlantic) and a composite section from ODP Leg 113 sites (Maud Rise, Weddell Sea). The δ¹³C records of Palaeocene and early Eocene Foraminifera at Site 577 and the Leg 74 sites show that an increase in δ¹³C values in surface waters at 64 Ma (end of Zone P1) resulted in increased vertical carbon isotope gradients (δ¹³C) between surface and deeper dwelling planktonic foraminifera, and between surface-dwelling planktonics and benthonic foraminifera which became progressively steeper until the iniddle Late Palaeocene (Zone P4). This steepening also occurs in the latest Palaeocene of the composite Leg 113 section and can be explained by an increase in surface ocean productivity. This increase in productivity probably resulted in an expansion of the oxygen minimum zone (OMZ). Benthonic δ¹³C values increased during the late Palaeocene in Site 577 and the composite Leg 74 section, suggesting that the Palaeocene carbon isotope maximum was composed of both within-ocean reservoir (increased surface water productivity) and between-reservoir (organic carbon burial) ftactionation effects. The benthonic δ¹³C increase lags the surface ocean δ¹³C increase in the early Palaeocene (63–64 Ma) suggesting that surface water productivity increase probably led an increase in the burial rate of organic carbon relative to carbonate sedimentation. Moreover, inter-site δ¹³C comparisons suggest that the locus of deep to intermediate water formation for the majority of the Palaeocene and the earliest Eocene was more likely to have been in the high southern latitudes than in the lower latitudes. Oxygen isotope data show a decline in deeper water temperatures in the early and early late Palaeocene, followed by a temperature increase in the late Palaeocene and across the PalaeoceneEocene boundary. We speculate that these changes in deeper water temperatures were related to the flux of CO₂ between the oceans and the atmosphere through a mechanism operating at the high southern latitudes.     

Recognition and implication of tectonic loading-induced reheating in the northern Variscan front (Belgium and northern France), based on an illite Kübler index and oxygen isotope study

Illite Kübler index (KI) and oxygen isotope (brachiopods and micrites) investigations have been performed on more than 300 Frasnian limestones sampled in one borehole and numerous outcrops in the Dinant Synclinorium (Belgium, northern France) of the northern Variscan front. The illite Kübler index and ¹⁸O data of a 3-km-thick, tectonically repeated Frasnian series from the Focant borehole are compared with their surrounding surface correspondents and document in-situ reheating induced by Variscan tectonic loading, which post-dated sedimentary burial alteration. The boundary between these two thermal processes (sedimentary burial and tectonic loading) on the Focant profile corresponds to an important location where the heat induced by the tectonic loading was equivalent to that Frasnian strata suffered during maximum sedimentary burial. Mainly based on this knowledge and on a former conodont colour alteration index study, the thickness of the eroded thrust sheet in the Focant area is estimated to be around 3,000 m. Oxygen isotopic exchange in these Frasnian closed carbonate systems, occurring under highest-grade diagenesis and anchimetamorphism, records two events. Brachiopods present a quite different and more homogeneous pattern, due to their higher resistance to heat alteration. These thermal events caused both ¹⁸O records to become increasingly lighter than the presumed original seawater signature. The comparison between KI and ¹⁸O profiles indicates that illite KI analysis is more appropriate than ¹⁸O in highlighting the temperature variations in the burial metamorphism at the periphery of orogenic belts.     

Structural style and stratigraphic architecture of fault propagation folding in extensional settings: a seismic example from the Smørbukk area, Halten Terrace, Mid-Norway

A number of recent papers have stressed the importance of lateral and vertical fault propagation on sediment geometries in active rift settings. However, the majority of these studies have been based on outcrop data. This contribution addresses the evolution of a single, major normal fault and its interaction with adjacent active faults using high-resolution 3D seismic data from the Smørbukk and Smørbukk South hydrocarbon fields, Halten Terrace, Mid-Norway. The major fault dividing the two fields, the Trestakk–Smørbukk fault, evolves from a southern segment with a well-defined set of rift wedges in its hangingwall to a northern segment where the fault tip is buried and a fault-tip fold is developed. Isochore maps of three Jurassic intervals illustrate a south to north evolution where, initially, Early Jurassic fault activity is limited to the southern part of the study area. Middle to Upper Jurassic intervals display a northwards migration in activity and linkage with two other major faults in the study area. This northwards migration had a profound effect on sediment geometries and depocentres in an area where previously only Late Jurassic rift activity has been recognized.     

Stable isotopic calibration of the earliest skeletal fossil assemblages in eastern Siberia (Precambrian-Cambrian boundary)

Sections from the eastern margin of the Siberian Platform provide important reference sections for stable isotopic correlation of the newly defined Precambrian–Cambrian boundary level since they contain some of the earliest pre-Tommotian shelly fossil assemblages known, and are amenable to δ¹³C stratigraphy. Stable isotopes are examined from the Dvortsy and Ulakhan–Sulugur sections (River Aldan), where sparse pre-Tommotian assemblages occur, and from the Uchur River region, where the putative Anabarites trisulcatus and Purella antiqua Zones of the Nemakit–Daldynian Stage are well-developed. Diagenetic resetting of δ¹⁸O and δ¹³C must be taken into account when comparing such sections. Simple corrections allow for stable isotopic correlation of basal Nemakit–Daldynian and Tommotian strata across eastern Siberia.