bluuebunny/crossref_metadata_2025_split huggingface dataset:
The history of sediment flux to Atchafalaya Bay, Louisiana
Authors: McManus John 
Published: April 2008

Abstract

The Atchafalaya became a free-flowing river system in the mid-nineteenth century. It progressively increased its diversion of waters from the Mississippi until, by the mid-twentieth century, it threatened to capture the total flow of the main river. The installation of control structures has, temporarily at least, prevented total diversion, limiting the flows to 30% of the Mississippi; water discharge and sediment transport into the Atchafalaya have led to deposition throughout the basin. A major system of lakes and bayous has become largely filled with deltaic sands and layered silts, so that the waters currently drain almost directly into the Atchafalaya Bay. The natural outlet and one artificially created outlet each carry waters from the system. As a result of major flooding in 1973, sediment was flushed from the Atchafalaya Basin and began the subaerial emergence of deltas, which had been slowly building below the bay water surface since the 1950s. The two deltas have provided, parallel growth one largely in natural form, and the other substantially modified by the dredging needed to maintain a navigable routeway to Morgan City and the interior of the basin. The sub-aerial deltas initially grew rapidly, a process apparently slowing, but the sediment transported to the depositional areas has not diminished, rather the area of deposition within the receptor basin is increasing, so that apparent growth inevitably slows. By 1994 the two deltas occupied an area of more than 153 km
2
above the −0.6 m measuring datum, created in approximately 30 years. In each case the sediments show upward passage from fine clays to laminated silts and fine sands, within a succession rarely more that 3 m in thickness, as defined by the original 1890 bathymetry of the Bay. Within the Bay itself resuspension of deposited fine sediment due to wave activity and water set-up and set-down in response to variations in the directions of coastal winds creates plumes of material that travel through the basin and may leave to form suspended sediment plumes on the inner parts of the continental shelf. A total of 20–25 such plumes annually drift initially east of the Bay outlet but later move west towards the chenier coasts of western Louisiana and Texas.
Late Quaternary turbidite input into the east Mediterranean basin: new radiocarbon constraints on climate and sea-level control
Authors: Reeder Michael S. , Stow Dorrik A. V. , Rothwell R. Guy 
Published: April 2008

Abstract

The Late Pleistocene-Holocene (0–30 ka
bp
) allochthonous sedimentation in the Herodotus Basin of the eastern Mediterranean has been controlled, in part, by a combination of regional climatic change and eustatic sea-level fluctuation. A new series of radiocarbon dates, made on planktonic foraminifers and pteropod shells taken from the pelagic and hemipelagic intervals between individual turbidite units, has given bracketing dates for each major turbidity current event that deposited sand and mud on the Herodotus Basin plain. Two partly independent cycles are evident. Climate-induced cycles have lead to an alternation of periods of turbidites sourced from the Nile delta-fan system with those from the North African shelf and Anatolian rise. These correlate with pluvial and inter-pluvial climatic periods recognized in the Nile hinterland. Sea-level cycles have tended to focus turbidite emplacement, from whatever source, at periods of sea-level fall within the latest Wisconsin and sea-level rise from the Wisconsin-Holocene period. In addition to the Herodotus Basin Megaturbidite (HBM) described previously, six other beds with volumes in excess of 25 km
3
and wide lateral extent across the basin can be termed megaturbidites. There is no simple sea-level or climate control on the timing of these events, so we must conclude that triggering and emplacement of megaturbidites is independent and variable.
From graphical display to dynamic model: mathematical geology in the Earth sciences in the nineteenth and twentieth centuries
Authors: Howarth Richard J. 
Published: April 2008

Abstract
Graphical displays were used early in geophysics and crystallography, mineralogy, petrology and structural geology by the early 1800s, but nineteenth-century geology obstinately remained mainly descriptive. Charles Lyell’s quantitative classification of the Tertiary Sub-Era in 1828 was a notable exception. Nevertheless, by 1920 the quantitative approach had become established. W. C. Krumbein, who introduced the computer into geology in 1958, encouraged use of probabilistic sampling and process-response models. Early work focused on databases, statistical data analysis and display. By the 1970s, stochastic simulation, deterministic modelling and spatial ‘geostatistics’ (pioneered by Matheron and his co-workers), were of growing importance. The introduction of the personal computer and the graphical user interface in the 1980s brought well-proven quantitative methods out of the research environment onto the workbench and into the field. Since the mid-1980s, the analysis, display and modelling of behaviour in three dimensions, underpinned by spatial statistics, computational fluid-flow, visualization technology, etc., has proved of economic benefit to mining, petroleum geology and hydrogeology. Other, computationally intensive, methods likely to be of importance in the Earth sciences are the application of ‘robust’ statistical methods, increasing use of Bayesian methods in uncertainty (risk) estimation (as a result of a renewed interest in statistical intervals and forecasting), and computational mineralogy.
Norman Levi Bowen (1887–1956) and igneous rock diversity
Authors: Young Davis A. 
Published: April 2008

Abstract
By the beginning of the twentieth century, differentiation had emerged as the leading theory to explain the chemical and mineralogical diversity of igneous rocks. Soret diffusion, liquid immiscibility, compositional stratification of magma by gravity (Gouy-Chaperon effect), volatile transport, crystal settling and other processes had been advocated as mechanisms of differentiation, but no consensus was achieved regarding a dominant mechanism.
During a career spent primarily at the Geophysical Laboratory, Washington DC, Norman Levi Bowen (1887–1956) initiated a new approach to petrology. On the basis of experimental studies of rock-forming silicates and on physicochemical principles, Bowen argued against the importance of Soret diffusion, liquid immiscibility, volatile transport and assimilation as major causes of diversity. He formulated a comprehensive theory of differentiation that emphasized the role of separation of crystals from liquid. He reasoned that rocks of the subalkaline igneous rock series, including granite, have been derived from parental basalt by crystal separation, e.g. settling, filter-pressing or armouring of crystals.
Apart from its scientific merits, Bowen’s achievement rested on personal, institutional and technical factors that included his determination to dedicate virtually his entire career to solution of the problem of igneous rock diversity; his affiliation with the Geophysical Laboratory; the prior development of the quenching method and the calibration of the temperature scale to very high temperatures; and the influence of Arthur L. Day.
Plate tectonics, terranes and continental geology
Authors: Le Grand Homer E. 
Published: April 2008

Abstract

The ‘modern revolution’ in the Earth sciences is associated with the emergence of plate tectonics in the late 1960s. The assumption that the crust of the Earth was composed of a small number of rigid, non-deformable, mobile plates enabled a quantitative, kinematic description of current geological processes and reconstructions of past plate interactions. The simple model of plate theory
c.
1970, for example its depiction of a subduction zone, has since undergone considerable refinement. However, some geologists, especially those concerned with questions of continental tectonics, contend that plate theory in its current form is of limited value in addressing questions of continental tectonics, and prefer to employ the concept of allochthonous terranes in characterizing, describing and interpreting regional geology. These geologists may understandably take the view that plate tectonics is a kinematic grand generalization but thus far not particularly useful in making sense of the rocks at the local level.
From terrestrial magnetism to geomagnetism: disciplinary transformation in the twentieth century
Authors: Good Gregory A. 
Published: April 2008

Abstract
In 1900, researchers interested in Earth’s magnetism generally proclaimed all facets of magnetic phenomena to be within their purview. Most researchers in this field referred to themselves as ‘magneticians’ first and physicists or geologists second. After World War II, specialization increased. A number of distinct research areas appeared over several decades: the geodynamo theory and the study of the core-mantle boundary; palaeomagnetism and its growing connection to geology; the production of induced fields in Earth’s crust; and, among others, the electromagnetic phenomena of the upper atmosphere and near space. The former unity dissolved and the field fragmented. One result of fragmentation has been a loss of memory and a consequent misinterpretation of an important part of the history of geoscience. This paper relates the challenges of recovering a history obscured by later events.
Diversification and biogeography of bivalves during the Ordovician Period
Authors: Cope John C. W. 
Published: April 2008

Abstract
Bivalves have a wide distribution in the Lower and to a lesser extent Middle Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian. Recent discoveries have significantly increased our knowledge of Lower Ordovician bivalve faunas and their explosive radiation from the Early Ordovician apparently coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician, bivalves had begun to escape the confines of Gondwana and marked latitudinal differences in the composition of the faunas became apparent, with pteriomorphians showing clear preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes; surprisingly, nuculoids were both most diverse in terms of species and most abundant as a percentage of individuals within the bivalve populations at low latitudes. It was in the Late Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and Baltica, leading to a second major diversification particularly within the pteriomorphian bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-infaunal bivalves involving extinction of many genera, including the only Ordovician boring bivalves.
A brief history of the Indus River
Authors: Clift Peter D. 
Published: April 2008

Abstract

The Indus River system is one of the largest rivers on the Asian continent, but unlike the Ganges-Brahmaputra system, the drainage of the Indus is dominated by the western Tibetan Plateau, Karakoram and tectonic units of the Indus Suture Zone, rather than the High Himalaya. The location of the river system relative to the Indus Suture Zone explains the deep exhumation north of that line in the Karakoram, compared with the modest erosion seen further east in Tibet. The modern Indus cuts Paleogene fluvial sedimentary rocks of the Indus Group located along the Indus Suture Zone in Ladakh, northern India. After the final marine incursion within the Indus Group in the early Eocene (<54.6 Ma), palaeo-current indicators changed from a north-south flow to an axial, westward pattern, synchronous with a marked change in sediment provenance involving erosion of South Tibet. The Indus probably was initiated by early Tibetan uplift following the India-Asia collision. The river has remained stationary in the suture since Early Eocene time, cutting down through its earlier deposits as they were deformed by northward folding and thrusting associated with the Zanskar backthrust at
c.
20 Ma. The Indus appears to have been located close to its present position within the foreland basin since at least Mid-Miocene time (
c.
18 Ma), and to have migrated only
c.
100 km east since Early Eocene time. In the Arabian Sea Paleogene fan sedimentation was significant since at least Mid-Eocene time (
c.
45 Ma). Sediment flux to the mid fan and shelf increased during Mid-Miocene time (after 16 Ma) and can be correlated with uplift of the Murray Ridge preventing sediment flow into the Gulf of Oman, tectonic uplift and erosion in the Karakoram and western Lhasa Block, and an enhanced monsoon triggered by that same uplift. Sedimentation rates fell during Late Miocene to Recent time. The Indus represents 18% of the total Neogene sediment in the basins that surround Asia, much more than all the basins of Indochina and East Asia combined (
c.
11%). Unlike the rivers of East Asia, which have strongly interacted as a result of eastward propagating deformation in that area, the Indus has remained uninterrupted and represents the oldest known river in the Himalayan region.
Monsoon-driven export fluxes and early diagenesis of particulate nitrogen and its δ
15
N across the Somalia margin
Authors: Brummer G. J. A. , Kloosterhuis H. T. , Helder W. 
Published: April 2008

Abstract

Settling nitrogen fluxes intercepted by sediment traps on the mid-slope and in the deep basin off Somalia show a consistent annual range of 3.4 ± 0.2‰ in their stable isotope composition. Seasonal minima in δ
15
N of 3.7‰ are associated with the moderate N fluxes derived from coastally upwelled water, which is rapidly carried offshore along eddy margins passing over the mooring sites during the SW monsoon (June–September). Coastal upwelling, offshore transport and deep wind mixing cease at the end of the SW monsoon, leading to enhanced utilization of the up to 20 μM of NO
3
−
in the photic layer, maxima in the N export flux, and an increasing δ
15
N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the δ
15
N increases to over 7‰. Cyanobacterial N
2
fixation probably diminishes the δ
15
N by 0.4–1.6‰ during the autumn intermonsoon (November–December) when settling N fluxes are lowest. Nutrient utilization remains high during the NE monsoon (January–March), when nutrient entrainment by deep wind mixing results in enhanced N export with maxima in δ
15
N of up to 7.4‰. Annual N fluxes have virtually the same δ
15
N of 6.0‰ in all traps despite considerable differences in both N flux and δ
15
N between the traps during the year and at different depths. In comparison with the annual δ
15
N of 6.0‰ arriving on the sea floor, core-top sediments are enriched by +0.6‰ on the upper slope (at 487 m) increasing to +2.9‰ in the deep basin (at 4040 m), whereas the N sediment burial efficiency declines from about 17% to 3%. It appears that the extent of oxic decomposition at the sediment-water interface is the most likely cause of such isotope enrichment. Similar positive gradients in δ
15
N with bottom depth have been reported from other continental margin transects and are generally attributed to increased nutrient utilization in the photic ocean with distance offshore. As for Somalia, nitrogen isotope fractionation as a result of oxic decomposition on the bottom rather than nutrient utilization at the ocean surface may account for the observed increase of sedimentary δ
15
N down continental margins in general.
Formation of varve-like laminae off Pakistan: decoding 5 years of sedimentation
Authors: Lückge A. , Reinhardt L. , Andruleit H. , Doose-Rolinski H. , von Rad U. , Schulz H. , Treppke U. 
Published: April 2008

Abstract
We studied Holocene sediments from the northeastern Arabian Sea near Pakistan, which were obtained from the same location in 1993 and 1998, to determine the composition and origin of laminated sediments for this 5 year time interval. Methods included geochemical, sedimentological and palaeontological analyses. We then compared our results with meteorological records, and satellite and sediment trap data. We suggest that short-term (few days) heavy rainfall periods in the hinterland and at the coast lead finally to flood events causing the deposition of light-coloured layers as event deposits on the continental slope. These layers are characterized by low percentages of biogenic compounds (i.e. organic matter, coccoliths and diatoms) and interpreted to have been deposited mainly during the winter season, when heavy rainfall can be expected. The thickness of the light layers seems to be related to the intensity of precipitation during a single flood event. In the 1997–1998 El Niño year, which was characterized by the strongest anomalies for the last 20 years, the thickest layer was deposited. The dark layers accumulate over the remaining larger part of the year and are characterized by an elevated input of biogenic material (marine organic matter, skeletal opal, foraminifera and coccoliths).
Discovery of the Toba Ash (
c.
70 ka) in a high-resolution core recovering millennial monsoonal variability off Pakistan
Authors: Von Rad Ulrich , Burgath Klaus-Peter , Pervaz Muhammad , Schulz Hartmut 
Published: April 2008

Abstract

A discrete Toba Ash layer in the northeastern Arabian Sea was detected near the base of a 20.2 m long piston core (289KL) recovered from the oxygen minimum zone off the Indus delta. In addition to the Toba Ash, we discovered two highly disseminated, vitreous, rhyolitic ‘ash layers’ in two annually laminated box cores: a ‘Younger Ash’ (about
ad
1885–1900), and an ‘Older Ash’ (about
ad
1815–1830). The glass shards were probably derived from eruptions of Indonesian volcanoes, although it was not possible to correlate these two ashes with well-known historical eruptions. We discuss source, transport and deposition of distal ash-fall layers in the Arabian Sea, which are derived from violent ultra-Plinian eruptions on the Indonesian volcanic archipelago, as well as their use for palaeoclimatic correlation. Core 289KL has a complete, high-resolution stratigraphic record of the past 75 ka with 21 interstadials (IS) or Dansgaard-Oeschger (D-O) cycles and equivalents of Heinrich events H1–H6. The high-frequency record of this core shows rapid climate oscillations with periods around 1.5 ka and can be tuned precisely to the δ
18
O record of a Bay of Bengal core and to the GISP-2 ice core from Greenland. The Toba event (70 ± 4 ka
bp
), which is well documented in the Arabian Sea and Bay of Bengal records at the end of IS-20, as well as in the Greenland ice, is an excellent stratigraphic marker horizon to validate this correlation. The apparent synchronous appearance of the various D-O oscillations and Heinrich events, which has been documented for many northern hemisphere localities, can be explained only by fairly rapid atmospheric circulation changes. Changes in the intensity of the Indian summer monsoon are tightly coupled with suborbital climate oscillations in the northern hemisphere via atmospheric moisture and heat circulation.
The seasonal and vertical distribution of living planktic foraminifera in the NW Arabian Sea
Authors: Peeters Frank J. C. , Brummer Geert-Jan A. 
Published: April 2008

Abstract

The NW Arabian Sea is characterized by a strong seasonal contrast in surface water hydrography. During the SW monsoon of 1992, we encountered strong coastal upwelling characterized by low sea surface temperatures (SST), high nutrient concentrations, a shallow thermocline and a near-surface chlorophyll maximum. By contrast, the hydrography during the NE monsoon of 1993 was characterized by a relatively warm nutrient-depleted surface mixed layer and a deep chlorophyll maximum. We show that the faunal composition, depth habitat and abundance of living planktic foraminifera respond to the hydrographic changes controlled by the seasonally reversing monsoon system. Total shell concentrations (> 125 μm) ranged from 4 to 332 individuals (ind.) m
−3
during upwelling and from 3 to 85 ind. m
−3
during the non-upwelling season. During upwelling, the fauna was dominated by
Globigerina bulloides
. During non-upwelling the fauna was characterized by relatively high concentrations of tropical symbiont-bearing species such as
Globigerinoides ruber, Globigerinoides sacculifer
and
Globigerinella siphonifera
, whereas concentrations of
Globigerina bulloides
were an order of magnitude lower. Factor analysis on 15 species yields an upwelling assemblage (UA), a tropical assemblage (TA) and a subsurface assemblage (SA). A fourth factor represents the distribution of the species
Globigerina falconensis
, which is mainly found in subsurface waters during the non-upwelling period (NE monsoon). A model is presented to calculate the base of the productive zone from the vertical shell concentration profile of a given species. The model is validated by comparing the range in calcification temperatures of
G. bulloides
, derived from its δ
18
O, with the
in situ
sea-water temperature range of the productive zone as predicted from the model. It appears that shell growth (calcite precipitation) is restricted to the productive zone as defined by this method. The average calcification temperature of
G. bulloides
corresponds to the point of maximum change in the shell concentration profile (i.e. the inflection point). For most shallow-dwelling species, the inflection point is found at or below the depth of the chlorophyll maximum, although above the main thermocline. This study indicates that the depth habitat and abundance of different species varies seasonally. Consequently, the abundance and stable isotope composition of specimens in the fossil record reflects a mixture of specimens that were produced at various depths during the different seasons.