Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure,India

The Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta‐supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study is aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium‐osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the ¹⁸⁷Os/¹⁸⁸Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.     

Growth increment and stable isotope analysis of marine bivalves: Implications for the geoarchaeological record of El Niño

The 1982–1983 El Niño event afforded the opportunity to develop criteria for the recognition of ancient El Niños using mollusks from archaeological sites along coastal South America. A combination of growth increment and stable isotope analyses indicated that elevated sea surface temperatures during large scale El Niños leave a record decodable from the growth patterns of selected bivalve shells. The intertidal venerid Chione subrugosa displayed a pronounced break in the valve margin profile following the 1982-1983 event but provided an inconsistent stable isotope pattern. The subtidal carditid Trachycardium procerum, on the other hand, preserved a discernible and diagnostic growth interruption as well as an expected trend in stable isotope indicators of salinity and temperature change. We conclude that some of the major culturally disruptive El Niño events can be recognized in the geoarchaeological record by these techniques, especially if ancillary information, such as faunal distribution patterns, are also considered. Perhaps the most serious constraint upon application of this approach involves microstratigraphic resolution of shell midden deposits. Stratigraphic sampling of midden material should be accompanied, if possible, by sampling of proximal natural strata. The chances of discovery of major El Niño perturbations in the geoarchaeological record of shell middens is enhanced by the catastrophic nature of such events and by the indication that major El Niños have a high probability of being closely spaced in time.     

Chronology of the last deglaciation and Holocene environmental changes in the Sisimiut area,SW Greenland based on lacustrine records

The Sisimiut area was deglaciated in the early Holocene, c. 11 cal. ka BP. At that time the lowlands were inundated by the sea, but the isostatic rebound surpassed the global sea‐level rise, and the lowlands emerged from the sea. The pioneer vegetation in the area consisted of mosses and herbaceous plants. The oldest remains of woody plants (Empetrum nigrum) are dated to c. 10.3 cal. ka BP, and remains of Salix herbacea and Harrimanella hypnoides are found in slightly younger sediments. The maximum occurrence of statoblasts of the bryozoan Plumatella repens from c. 10 to 4.5 cal. ka BP probably reflects the Holocene thermal maximum, which is also indicated in geochemical proxies of the lake sediments. A maximum in organic matter accumulation in one of the three studied lakes c. 3 cal. ka BP can probably be ascribed to a late Holocene short‐duration temperature maximum or a period of increased aridity. Cenococcum geophilum sclerotia are common in the late Holocene, implying increased soil erosion during the Neoglaciation. A comparison with sediment and macrofossil records from inland shows similar Holocene trends and a similar immigration history. It also reveals that there has been a significant gradient throughout the Holocene, from an oceanic and stable climate at the outer coast to a more continental and unstable climate with warmer summers and drier conditions close to the margin of the Greenland ice sheet, where the buffer capacity of the sea is lower.     

Biomarkers in sedimentary sulfides of precambrian age

Samples of the Mt Isa formation (Australia, c, 1.5 Ga), and the Shungit formation (U.S.S.R., c. 2.3 Ga) were studied by organic geochemical means. All samples were freed of any low-molecular-weight solvent soluble organic material in order to insure the authenticity of the analysed material. The isolation of biochemical compounds entrapped in sulfides was the major goal of this work.Organic compounds that were entrapped during the early stages of sulfide formation may obviously survive extended periods of time and can be released by a mild hydrogenation of the sulfides. Preliminary investigations of the hydrocarbon fraction indicate n-alkanes, monomethylalkanes, cycloalkanes, and an unknown series of branched alkanes as major constituents. Their distribution patterns show great selectivity with respect to structures and chain lengths of individual compounds. Differences between the hydrogenation reaction product and the sample extract may arise from the release of a different kind of lipid material through dissolution of the sulfides.     

Steroid and triterpenoid distributions in bransfield strait sediments: Hydrothermally-enhanced diagenetic transformations

Recent sediments from Bransfield Strait, Antarctica have been analyzed for triterpenoid and steroid hydrocarbons, sterols and steroid ketones to consider the effects of hydrothermal fluids on the sedimentary organic matter. The steroid distributions in unaltered and altered sediments are controlled more by inputs from source organisms than by the effects of hydrothermal activity, which is suggested to be limited to low temperature alteration. Nevertheless, chemical reactions occurred in altered sediments and include dehydration of sterols to sterenes, isomerization of triterpenes and sterenes, rearrangement of sterenes to diasterenes and reductive processes leading to generation of phytane from phytol via phytenes.     

A formulation for diagnostic anomaly models

It is shown that the anomaly model governing the difference between two integrations of the equations of motion can be written in a form that is formally linear in the anomalies, by choosing the basic state to be the mean of the results of the two integrations.     

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province

15011993

Abstract

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone.

Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico.

Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.     

Climatic change has stabilized An Editorial

The National Center for Atmospheric Research is sponsored by the National Science Foundation.