Composition of sediments from the Arabian Sea

Thirty seven deep-sea sediment cores from the Arabian Sea were studied geochemically (49 major and trace elements) for four time slices during the Holocene and the last glacial, and in one high sedimentation rate core (century scale resolution) to detect tracers of past variations in the intensity of the atmospheric monsoon circulation and its hydrographic expression in the ocean surface. This geochemical multi-tracer approach, coupled with additional information on the grain size composition of the clastic fraction, the bulk carbonate and biogenic opal contents makes it possible to characterize the sedimentological regime in detail. Sediments characterized by a specific elemental composition (enrichment) originated from the following sources: river suspensions from the Tapti and Narbada, draining the Indian Deccan traps (Ti, Sr); Indus sediments and dust from Rajasthan and Pakistan (Rb, Cs); dust from Iran and the Persian Gulf (Al, Cr); dust from central Arabia (Mg); dust from East Africa and the Red Sea (Zr/Hf, Ti/Al). Corg, Cd, Zn, Ba, Pb, U, and the HREE are associated with the intensity of upwelling in the western Arabian Sea, but only those patterns that are consistently reproduced by all of these elements can be directly linked with the intensity of the southwest monsoon. Relying on information from a single element can be misleading, as each element is affected by various other processes than upwelling intensity and nutrient content of surface water alone. The application of the geochemical multi-tracer approach indicates that the intensity of the southwest monsoon was low during the LGM, declined to a minimum from 15,000-13,000 14C year BP, intensified slightly at the end of this interval, was almost stable during the Bölling, Alleröd and the Younger Dryas, but then intensified in two abrupt successions at the end of the Younger Dryas (9900 14C year BP) and especially in a second event during the early Holocene (8800 14C year BP). Dust discharge by northwesterly winds from Arabia exhibited a similar evolution, but followed an opposite course: high during the LGM with two primary sources-the central Arabian desert and the dry Persian Gulf region. Dust discharge from both regions reached a pronounced maximum at 15,000-13,000 14C year. At the end of this interval, however, the dust plumes from the Persian Gulf area ceased dramatically, whereas dust discharge from central Arabia decreased only slightly. Dust discharge from East Africa and the Red Sea increased synchronously with the two major events of southwest monsoon intensification as recorded in the nutrient content of surface waters. In addition to the tracers of past dust flux and surface water nutrient content, the geochemical multi-tracer approach provides information on the history of deep sea ventilation (Mo, S), which was much lower during the last glacial maximum than during the Holocene. The multi-tracer approach-i.e. a few sedimentological parameters plus a set of geochemical tracers widely available from various multi-element analysis techniques-is a highly applicable technique for studying the complex sedimentation patterns of an ocean basin, and, specifically in the case of the Arabian Sea, can even reveal the seasonal structure of climate change.

Lithology, micropaleontology and isotope record of sediments from the Nordic Seas

On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ~26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic delta18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ~13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic delta13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ~8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to today's steep east-west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.

(Table 1) Mineral composition of sand- and silt-sized fractions of bottom sediments from the Romanche Trench

A study was made of mineral composition of sand- and silt-sized fractions of recent clastic (riftogenic) sediments and solidified deposits collected from the bottom of the Romanche Trench during the first voyage of R/V Akademik Kurchatov. Similarity between mineral compositions of sediments and bedrocks (ultrabasites, gabbroids, diabases) was established. This similarity is a basis for considering the mineral complex of the deposits that have been derived from the bedrocks of the trench slopes, and have formed due to their submarine denudation accompanied by tectonic crushing. The same mineral composition was found in pieces of older consolidated deposits; this suggests that conditions of sedimentation similar to those at recent times have existed for a long time in the Romanche Trench.

(Table 1) Iron and manganese speciations in surface layer bottom sediments of the Pacific Ocean

Distribution of iron and manganese speciations in ocean sediments of a section from the coast of Japan to the open Pacific Ocean is under consideration. Determinations of total iron, as well as of reactive iron contents and of total manganese, as well as of Mn4+ contents have been done. Significant increase of total Fe content in sediments from the coast to the pelagic zone occurs without noticeable increase in reactive Fe content. Presence of layers of volcanic and terrigenous coarse clastic material in clayey sediments results to sharp change in iron content. Manganese content increases from near coastal to pelagic sediments more than 10 times; oxidation degree of sediments also increases. There are three types of bottom sediments different by contents of iron and manganese forms: reduced, oxidized (red clay), and transitional. Content of total Fe is almost does not change with depth in sediments, content of reactive Fe increases in reduced sediments, and decreases in oxidized ones. Manganese content in red clay mass increases several times.

(Table 2) Terrigenous, authigenic and biogenic composition of sediments at DSDP Hole 93-603B

Pelagic sedimentation during the Early Cretaceous at Site 603 produced alternations of laminated marly limestone and bioturbated limestone-a facies typical of the "Blake-Bahama Formation" of the western Atlantic. This limestone is a nannofossil micrite, rich in calcified radiolarians, with variable amounts of organic matter, pyritized radiolarian tests, fish debris, and micaceous silt. The laminated marly limestone layers are enriched in organic matter when compared with the intervals of bioturbated limestone. The organic carbon is predominantly terrestrial plant debris; where the organic-carbon content is in excess of 1%, there is also a significant marine-derived component. Laminations can result either from bands of alternately enriched and depleted opaque material and clay, or from bands of elongate lenses (microflasers) of micrite, which could be plastically deformed pellets or diagenetic features. The alternating intervals of laminated and bioturbated structures may have resulted from combined changes in surface productivity, in the influx of terrigenous organic matter, and in the intensity of bottom circulation, which led to episodic oxygen depletion in the bottom water and sediments. Variations in the relative proportions of laminated clay-rich and bioturbated lime-rich limestone and in the development of cycles between these structures make it possible to subdivide the Lower Cretaceous pelagic facies into several subunits which appear to be regional in extent. Bioturbated limestone is dominant in the Berriasian, laminated marly limestone in the Valanginian and Barremian-lower Aptian, and well-developed alternations between these end members in the Hauterivian. The Hauterivian to lower Aptian sediments contain abundant terrigenous clastic turbidites associated with a submarine fan complex. These changes in the general characteristics of the pelagic sediment component of the Blake-Bahama Formation at Site 603 are synchronous with those in the Blake-Bahama Basin (Sites 534 and 391) to the south. Carbonate sedimentation ended in the early Aptian, probably because of a regional shoaling of the carbonate compensation depth.

Compositional data analysis of Holocene sediments from the West Bengal Sundarbans, India: Geochemical proxies for grain-size variability in a delta environment

This paper is part of a special issue of Applied Geochemistry focusing on reliable applications of compositional multivariate statistical methods. This study outlines the application of compositional data analysis (CoDa) to calibration of geochemical data and multivariate statistical modelling of geochemistry and grain-size data from a set of Holocene sedimentary cores from the Ganges-Brahmaputra (G-B) delta. Over the last two decades, understanding near-continuous records of sedimentary sequences has required the use of core-scanning X-ray fluorescence (XRF) spectrometry, for both terrestrial and marine sedimentary sequences. Initial XRF data are generally unusable in ‘raw-format’, requiring data processing in order to remove instrument bias, as well as informed sequence interpretation. The applicability of these conventional calibration equations to core-scanning XRF data are further limited by the constraints posed by unknown measurement geometry and specimen homogeneity, as well as matrix effects. Log-ratio based calibration schemes have been developed and applied to clastic sedimentary sequences focusing mainly on energy dispersive-XRF (ED-XRF) core-scanning. This study has applied high resolution core-scanning XRF to Holocene sedimentary sequences from the tidal-dominated Indian Sundarbans, (Ganges-Brahmaputra delta plain). The Log-Ratio Calibration Equation (LRCE) was applied to a sub-set of core-scan and conventional ED-XRF data to quantify elemental composition. This provides a robust calibration scheme using reduced major axis regression of log-ratio transformed geochemical data. Through partial least squares (PLS) modelling of geochemical and grain-size data, it is possible to derive robust proxy information for the Sundarbans depositional environment. The application of these techniques to Holocene sedimentary data offers an improved methodological framework for unravelling Holocene sedimentation patterns.

Influence of smectite suspension structure on sheet orientation in dry sediments : XRD and AFM applications

The structure-building phenomena within clay aggregates are governed by forces acting between clay particles. Measurements of such forces are important to understand in order to manipulate the aggregate structure for applications such as dewatering of mineral processing tailings. A parallel particle orientation is required when conducting XRD investigation on the oriented samples and conduct force measurements acting between basal planes of clay mineral platelets using at. force microscopy (AFM). To investigate how smectite clay platelets were oriented on silicon wafer substrate when dried from suspension range of methods like SEM, XRD and AFM were employed. From these investigations, we conclude that high clay concns. and larger particle diams. (up to 5 μm) in suspension result in random orientation of platelets in the substrate. The best possible laminar orientation in the clay dry film, represented in the XRD 0 0 1/0 2 0 intensity ratio of 47 was obtained by drying thin layers from 0.02 wt.% clay suspensions of the natural pH. Conducted AFM investigations show that smectite studied in water based electrolytes show very long-range repulsive forces lower in strength than electrostatic forces from double-layer repulsion. It was suggested that these forces may have structural nature. Smectite surface layers rehydrate in water environment forms surface gel with spongy and cellular texture which cushion approaching AFM probe. This structural effect can be measured in distances larger than 1000 nm from substrate surface and when probe penetrate this gel layer, structural linkages are forming between substrate and clay covered probe. These linkages prevent subsequently smooth detachments of AFM probe on way back when retrieval. This effect of tearing new formed structure apart involves larger adhesion-like forces measured in retrieval. It is also suggested that these effect may be enhanced by the nano-clay particles interaction.

Basin analysis and economic geology of the Northern Mount Isa Basin

The Mount Isa Basin is a new concept used to describe the area of Palaeo- to Mesoproterozoic rocks south of the Murphy Inlier and inappropriately described presently as the Mount Isa Inlier. The new basin concept presented in this thesis allows for the characterisation of basin-wide structural deformation, correlation of mineralisation with particular lithostratigraphic and seismic stratigraphic packages, and the recognition of areas with petroleum exploration potential. The northern depositional margin of the Mount Isa Basin is the metamorphic, intrusive and volcanic complex here referred to as the Murphy Inlier (not the "Murphy Tectonic Ridge"). The eastern, southern and western boundaries of the basin are obscured by younger basins (Carpentaria, Eromanga and Georgina Basins). The Murphy Inlier rocks comprise the seismic basement to the Mount Isa Basin sequence. Evidence for the continuity of the Mount Isa Basin with the McArthur Basin to the northwest and the Willyama Block (Basin) at Broken Hill to the south is presented. These areas combined with several other areas of similar age are believed to have comprised the Carpentarian Superbasin (new term). The application of seismic exploration within Authority to Prospect (ATP) 423P at the northern margin of the basin was critical to the recognition and definition of the Mount Isa Basin. The Mount Isa Basin is structurally analogous to the Palaeozoic Arkoma Basin of Illinois and Arkansas in southern USA but, as with all basins it contains unique characteristics, a function of its individual development history. The Mount Isa Basin evolved in a manner similar to many well described, Phanerozoic plate tectonic driven basins. A full Wilson Cycle is recognised and a plate tectonic model proposed. The northern Mount Isa Basin is defined as the Proterozoic basin area northwest of the Mount Gordon Fault. Deposition in the northern Mount Isa Basin began with a rift sequence of volcaniclastic sediments followed by a passive margin drift phase comprising mostly carbonate rocks. Following the rift and drift phases, major north-south compression produced east-west thrusting in the south of the basin inverting the older sequences. This compression produced an asymmetric epi- or intra-cratonic clastic dominated peripheral foreland basin provenanced in the south and thinning markedly to a stable platform area (the Murphy Inlier) in the north. The fmal major deformation comprised east-west compression producing north-south aligned faults that are particularly prominent at Mount Isa. Potential field studies of the northern Mount Isa Basin, principally using magnetic data (and to a lesser extent gravity data, satellite images and aerial photographs) exhibit remarkable correlation with the reflection seismic data. The potential field data contributed significantly to the unravelling of the northern Mount Isa Basin architecture and deformation. Structurally, the Mount Isa Basin consists of three distinct regions. From the north to the south they are the Bowthorn Block, the Riversleigh Fold Zone and the Cloncurry Orogen (new names). The Bowthom Block, which is located between the Elizabeth Creek Thrust Zone and the Murphy Inlier, consists of an asymmetric wedge of volcanic, carbonate and clastic rocks. It ranges from over 10 000 m stratigraphic thickness in the south to less than 2000 min the north. The Bowthorn Block is relatively undeformed: however, it contains a series of reverse faults trending east-west that are interpreted from seismic data to be down-to-the-north normal faults that have been reactivated as thrusts. The Riversleigh Fold Zone is a folded and faulted region south of the Bowthorn Block, comprising much of the area formerly referred to as the Lawn Hill Platform. The Cloncurry Orogen consists of the area and sequences equivalent to the former Mount Isa Orogen. The name Cloncurry Orogen clearly distinguishes this area from the wider concept of the Mount Isa Basin. The South Nicholson Group and its probable correlatives, the Pilpah Sandstone and Quamby Conglomerate, comprise a later phase of now largely eroded deposits within the Mount Isa Basin. The name South Nicholson Basin is now outmoded as this terminology only applied to the South Nicholson Group unlike the original broader definition in Brown et al. (1968). Cored slimhole stratigraphic and mineral wells drilled by Amoco, Esso, Elf Aquitaine and Carpentaria Exploration prior to 1986, penetrated much of the stratigraphy and intersected both minor oil and gas shows plus excellent potential source rocks. The raw data were reinterpreted and augmented with seismic stratigraphy and source rock data from resampled mineral and petroleum stratigraphic exploration wells for this study. Since 1986, Comalco Aluminium Limited, as operator of a joint venture with Monument Resources Australia Limited and Bridge Oil Limited, recorded approximately 1000 km of reflection seismic data within the basin and drilled one conventional stratigraphic petroleum well, Beamesbrook-1. This work was the first reflection seismic and first conventional petroleum test of the northern Mount Isa Basin. When incorporated into the newly developed foreland basin and maturity models, a grass roots petroleum exploration play was recognised and this led to the present thesis. The Mount Isa Basin was seen to contain excellent source rocks coupled with potential reservoirs and all of the other essential aspects of a conventional petroleum exploration play. This play, although high risk, was commensurate with the enormous and totally untested petroleum potential of the basin. The basin was assessed for hydrocarbons in 1992 with three conventional exploration wells, Desert Creek-1, Argyle Creek-1 and Egilabria-1. These wells also tested and confrrmed the proposed basin model. No commercially viable oil or gas was encountered although evidence of its former existence was found. In addition to the petroleum exploration, indeed as a consequence of it, the association of the extensive base metal and other mineralisation in the Mount Isa Basin with hydrocarbons could not be overlooked. A comprehensive analysis of the available data suggests a link between the migration and possible generation or destruction of hydrocarbons and metal bearing fluids. Consequently, base metal exploration based on hydrocarbon exploration concepts is probably. the most effective technique in such basins. The metal-hydrocarbon-sedimentary basin-plate tectonic association (analogous to Phanerozoic models) is a compelling outcome of this work on the Palaeo- to Mesoproterozoic Mount lsa Basin. Petroleum within the Bowthom Block was apparently destroyed by hot brines that produced many ore deposits elsewhere in the basin.

Tephrochronology of marine sediments around the Island of Montserrat, Lesser Antilles volcanic arc

The recent history of the Soufrière Hills Volcano, Montserrat, Lesser Antilles volcanic arc, is reconstructed using data obtained from recently drilled submarine cores.Tephra layers in these cores preserve a record of the volcanic history of Montserrat back to ~250 ka on the basis of micropaleontology and stable isotope stratigraphy. Stratigraphic relationships identified in the cores collected in 2002 and 2005 document the fate of both pyroclastic flows entering the ocean to the east of Montserrat and carbonate-rich turbidites sourced from the carbonate platformssurrounding the islands of the Lesser Antilles. Using oxygen isotope stratigraphy, micropalaeontological analysis and Carbon-14 dating, it can be shown that three significant volcanic events, including the on-going eruption, have occurred over the last 12 ka. Preceding this was a time of volcanic quiescence, with three carbonate-rich turbidite events being documented in many of the cores. Our data suggest that these events occurred during Marine Isotope Stage 2, following the Last Glacial Maximum (LGM) and onset of post-glacial sea level rise.

Distribution of organic-rich sediments through the Phanerozoic

It is commonly assumed that rates of accumulation of organic-rich strata have varied through geologic time with some periods that were particularly favorable for accumulation of petroleum source rocks or coals. A rigorous analysis of the validity of such an assumption requires consideration of the basic fact that although sedimentary rocks have been lost through geologic time to erosion and metamorphism. Consequently, their present-day global abundance decreases with their geologic age. Measurements of the global abundance of coal-bearing strata suggest that conditions for coal accumulation were exceptionally favorable during the late Carboniferous. Strata of this age constitute 21% of the world's coal-bearing strata. Global rates of coal accumulation appear to have been relatively constant since the end of the Carboniferous, with the exception of the Triassic which contains only 1.75% of the world's coal-bearing strata. Estimation of the global amount of discovered oil by age of the source rock show that 58% of the world's oil has been sourced from Cretaceous or younger strata and 99% from Silurian or younger strata. Although most geologic periods were favourable for oil source-rock accumulation the mid-Permian to mid-Jurassic appears to have been particularly unfavourable accounting for less than 2% of the world's oil. Estimation of the global amount of discovered natural gas by age of the source rock show that 48% of the world's oil has been sourced from Cretaceous or younger strata and 99% from Silurian or younger strata. The Silurian and Late Carboniferous were particularly favourable for gas source-rock accumulation respectively accounting for 12.9% and 6.9% of the world's gas. By contrast, Permian and Triassic source rocks account for only 1.7% of the world's natural gas. Rather than invoking global climatic or oceanic events to explain the relative abundance of organic rich sediments through time, examination of the data suggests the more critical control is tectonic. The majority of coals are associated with foreland basins and the majority of oil-prone source rocks are associated with rifting. The relative abundance of these types of basin through time determines the abundance and location of coals and petroleum source rocks.