Late Quaternary stratigraphy and paleo-oceanography of the Grand Banks continental margin, eastern Canada

Quaternary sediments cored on the continental slope off the Grand Banks and on nearby seamounts and abyssal hills have been correlated back to at least isotopic stage 6. using lithologic and hioslratigraphic markers and warm-cold cyclicity in microfossil assemblages. The sequence is dated using limited oxygen isotope and ^l4C data. The oldest continental slope cores penetrate to a glacial stade with an extrapolated age of 50. 0(H) B. P. The oldest scamount cores probably penetrate to isotopic stage 13. Watcrmass distribution over the area results from interaction of the Labrador Current with locally generated shelf water, and to a lesser extent, the Gulf Stream. Planktic foraminiferal assemblages and sedimentological evidence suggest that arctic Labrador Current core water extends onto the eastern Grand Banks only during full interglacials. During intcrstadials, this arctic core water is absent, probably because its source-was blocked by ice. Cold water during stadials probably results mostly from local cooling on the continental shelf.     

Isotopic compositions of tropical East African flora and their potential as source indicators of organic matter in coastal marine sediments

The C and N stable isotope compositions of some flora of East Africa from coastal Tanzania and Amboseli National Park (Kenya) are used to assess if they can be used as a terrestrial end member during the estimation of terrestrial fraction in coastal marine sediments. The results of C isotope composition of various tree leaves, which average −29.3 ± 1.4%, indicate that these tropical higher land plant species follow a Calvin-Benson or non-Kranz (C₃) type of metabolism. The results for grass species, which average −13.2 ± 2.4%, indicate that most of them follow a Hatch-Slack or Kranz (C₄) type of metabolism. However, some of the succulent plants from the Amboseli National Park have δ¹³C values that average −14.7%, an indication that they follow a CAM (Crassulacean Acid Metabolism) type of metabolism. The N isotope values are relatively higher than expected for the terrestrial organic material. The average δ¹⁵N values for both tree and grass samples are higher than 5% and fall within the range normally considered to be marine. The high enrichment in ¹⁵N may be related to the environmental conditions in which plants thrive. Plants growing in sandy, dry and overgrazed environments are expected to be enriched in ¹⁵N owing to full utilisation of all available N species, regardless of their isotopic compositions. Other processes which may cause an enrichment in ¹⁵N include adsorption by various types of clay minerals, supply of ¹⁵N-enriched nitrate through sea-spray, and local denitrification, especially in swampy and lake margins where the input of organic matter may be higher than the rate of decomposition.The stable isotopic composition of organic C and N for surficial organic matter for the coastal marine sediments averages −17.0 ± 0.9% and 5.4 ± 1.1%, respectively. These values indicate a substantial contribution of C₄ plants and sea grasses. However, contribution of C₄ relative to that of sea grasses can not be evaluated owing to the fact that there is no significant difference in the isotopic compositions between the two groups.In the savannah environment, where a contribution from the C₄ types of plants might be substantial, the δ¹³C value for a terrestrial end member needs to be established prior to evaluation of the terrestrially derived organic matter in the marine environment. Owing to a significant contribution of sea grasses to the total organic matter preserved in coastal marine sediments, the stable isotopes of organic C seem to have a limited applicability as source indicators in the East African coastal waters. Furthermore, the results indicate that N stable isotopes seem to have a limited applicability as source indicators in coastal waters of East Africa. However, more work needs to be conducted to determine the terrestrial and sea grass end member values for the coastal areas.     

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments from the southern Black Sea margin

The organic matter in the Late Cretaceous and Tertiary sediments from the southern Black Sea margin is assigned to the terrestrial-marine/terrestrial range of organic facies. Within this range, the stratigraphic section yields different organic facies types in response to different accumulation and preservation controlling processes. During the Late Companian-Maastrichtian, organic material from the shelf and slope was re-deposited in the deeper oxic parts of the basin. Rapid transport and sedimentation resulted in a higher degree of preservation of lipid-rich, terrestrial components (sporinite, cutinite, resinite) in comparison to the autochthonous sediments. The increase in organic carbon with increasing silt/clay content together with low carbon concentrations in the allochthonous sediments suggest that the accumulation of organic matter in the source areas was controlled by terrigenous influx and that the accumulation conditions were not favorable.In the Eocene (fore-arc basin), the higher content of marine organic matter can be explained by progressive shallowing of the environment and by reduced oxygen content in the bottom waters (reduced bioturbation).In the Miocene and Pliocene (back-arc basin), the organic fraction of the sediments from the basin margin is purely terrestrial and consists mostly of inertinite and reworked terrigenous liptinite indicating oxidative conditions. The dominance of inertodetrinite in the Miocene and of semifusinite in the Pliocene point to a change in the source area or to a higher energy transport or deposition conditions for the Miocene marginal sediments. In the basin interior, the higher content of marine organic matter is due to an oxygen deficiency or anoxic conditions in the bottom waters. Mineral associations indicate complete sulfate reduction and consequent methanogenesis. This is also implied in the hydrocarbon distributions. Periodic oxic conditions lead to a decrease in the marine liptinitic component. In the basin interior, however, the terrigenous fraction is still dominant, implying a continuous influx from the basin margins.The Late Cretaceous to Pliocene sediments are thermally immature (Rm<0.5%, T_max<435 °C).

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Zusammenfassung Die organische Substanz in den Sedimenten der Oberkreide und des Tertiärs der südlichen Schwarzmeerregion ist dem terrestrischen bis marin-terrestrischen Bereich organischer Fazies zuzuordnen. Innerhalb dieses Bereiches weisen die stratigraphischen Abschnitte unterschiedliche organische Faziestypen auf, die auf unterschiedliche, die Akkumulation und den Erhaltungsgrad der organischen Substanz kontrollierende Prozesse zurückzuführen sind.Während des Obercampan-Maastrichtiums und des Paläozäns (fore-arc Becken) wurde organisches Material des Schelf/-hanges in den tieferen oxischen Beckenbereichen resedimentiert. Die rasche Zufuhr und Ablagerung führte zu einem gegenüber den autochthonen Sedimenten höheren Erhaltungsgrad an lipidreichen, terrestrischen Komponenten (Sporinit, Cutinit, Resinit). Die Zunahme an organischem Kohlenstoff mit steigendem Silt-/Tonanteil bei insgesamt niedrigen Kohlenstoffkonzentrationen in den Resedimenten läßt vermuten, daß die Akkumulation organischer Substanz in den Liefergebieten durch terrigene Zufuhr bestimmt wurde und die Akkumulationsbedingungen ungünstig waren.Für das Eozän ist ein erhöhter Eintrag an marinem organischem Material zu verzeichnen, der mit der zunehmenden Verflachung des Ablagerungsraumes (fore-arc Becken) und einer Reduzierung im Sauerstoffgehalt des Bodenwassers (abnehmende Bioturbation) erklärt wird.Für das Miozän und Pliozän (back-arc Becken) ist die organische Fraktion der Ablagerungen des Beckenrandes rein terrestrisch und besteht zum größten Teil aus Inertinit und wieder aufgearbeitetem terrigenem Liptinit, die oxidative Verhältnisse anzeigen. Die Dominanz von Inertodetrinit im Miozän und Semifusinit im Pliozän indiziert eine Änderung im Liefergebiet oder ein höheres Energieniveau beim Transport bzw. im Ablagerungsraum der miozänen Randsedimente. Im Beckeninneren ist ein erhöhter Anteil an mariner organischer Substanz festzustellen, der auf Sauerstoffverarmung oder anoxische Verhältnisse im Bodenwasser zurückgeführt wird. Die Mineralassoziationen in den Sedimenten weisen auf vollständige Sulfatreduktion und nachfolgende Methanogenese hin, die sich auch mit den Kohlenwasserstoffverteilungen nachvollziehen läßt. Periodisch oxische Bedingungen führen zu einer Reduzierung der marin-liptinitischen Komponente. Im Beckeninneren dominiert jedoch auch die terrigene Fraktion (Huminit/Vitrinit, Inertinit), was auf kontinuierliche Zufuhr vom Beckenrand schließen läßt.Die Sedimente der Oberkreide bis Pliozän sind thermisch unreif (Rm<0.5%, T_max<435 °C).

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Résumé La matière organique contenue dans les sédiments du Crétacé supérieur et du Tertiaire de la partie sud de la Mer Noire est à rapporter au domaine de facies organique terrestre à marin-terrestre. La série stratigraphique présente, dans les limites de ce domaine, divers types de facies organiques qui traduisent les divers processus qui régissent l'accumulation et la préservation.Au cours du Campanian supérieur-Maastrichtien, des matériaux organiques provenant du shelf et du talus continental ont été redéposés dans les parties oxygénées plus profondes du bassin. La rapidité du transport et de la sédimentation a entraîné la préservation de composants terrestres riches en lipides (sporonite, cutinite, résinite), dans une mesure plus élevée que dans les sédiments autochtones. L'augmentation de la teneur en carbone organique corrélative à celle de la fraction fine (boue et silt), de même que la faible concentration en carbone des sédiments allochtones indique que, dans la région-source, l'accumulation de matières organiques était régie par un afflux terrigène et que les conditions d'accumulation n'étaient pas favorables.A l'Eocène (bassin d'avant-arc) le contenu en matière organique marine est plus élevé, ce qui s'explique par la diminution progressive de la profondeur et par la réduction de la teneur en oxygène des eaux du fond (bioturbation réduite).Au Miocène et au Pliocène (bassin d'arrière-arc), la fraction organique des sédiments de la bordure du bassin est purement terrestre et consiste principalement en inertinite et en liptinite terrigène remaniée, ce qui indique des conditions oxydantes. La prédominance d'inertodétrinite au Miocène et de semifusinite au Pliocène indique soit une source différente, soit un transport ou un dépôt dans les conditions de plus haute énergie des sédiments miocènes marginaux. Vers l'intérieur du bassin, le contenu plus élevé en matière organique marine est dû à une déficience en oxygène ou à des conditions anoxiques dans les eaux de fond. Les associations minérales indiquent une réduction complète des sulfates et en conséquence une méthanogenèse, ce qui ressort également de la distribution des hydrocarbures. Des conditions oxydantes périodiques provoquent une diminution du composant liptinitique marin. Dans l'intérieur du bassin, toutefois, la fraction terrigène reste dominante, ce qui implique un afflux continu depuis les marges du bassin.Les sédiments d'âge crétacé supérieur à pliocène sont thermiquement immatures (Rm<0,5%; Tmax<435 °C).

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, , - . , , . - , . , ( , , ), . , , . , (force-arc Basin) ( ). (back-arc Basin) , . , , . , , . , . . ( , ), . : (Rm<0,5 %; T_max<435° ).

     

Molecular characterization of dissolved organic matter in pore water of continental shelf sediments

Dissolved organic matter (DOM) in sediment pore water is a complex molecular mixture reflecting various sources and biogeochemical processes. In order to constrain those sources and processes, molecular variations of pore water DOM in surface sediments from the NW Iberian shelf were analyzed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and compared to river and marine water column DOM. Weighted average molecular element ratios of oxygen to carbon ((O/C)_wa) and hydrogen to carbon ((H/C)_wa) provided general information about DOM sources. DOM in local rivers was more oxygenated ((O/C)_wa 0.52) and contained less hydrogen ((H/C)_wa 1.15) than marine pore water DOM (mean (O/C)_wa 0.50, mean (H/C)_wa 1.26). The relative abundance of specific compound groups, such as highly oxygenated aromatic compounds or nitrogen-bearing compounds with low H/C ratios, correspond to a high concentration of lignin phenols (160 μg/g sediment dry weight) and a high TOC/TN ratio (13.3) in the sedimentary organic matter and were therefore assigned to terrestrial sources. The lower degree of unsaturation and a higher relative abundance of nitrogen-bearing compounds in the pore water DOM reflected microbial activity within the sediment. One sampling site on the shelf with a high sediment accumulation, and a humic-rich river sample showed a wide range of sulfur compounds in the DOM, accompanied by a higher abundance of lipid biomarkers for sulfate-reducing bacteria, probably indicating early diagenetic sulfurization of organic matter.     

Ordination of sediments from the Cape Hatteras continental margin

Ordination is a multivariate technique developed by plant ecologists which has proven effective in the interpretation of paleoenvironments. It allows gradational relationships among samples to be depicted in contrast to other quantitative techniques which classify samples into discrete groups. In this study, ordination is used to interpret textural data for 62 bottom samples taken from the Cape Hatteras, North Carolina, continental margin. The ordination suggests the existence of six sedimentary facies that are similar to those obtained by cluster analysis using a dendrograph display. The facies represented are: littoral sands and outer-shelf shelly sands; inner-shelf sands; outer-shelf sands and silts; outer-shelf slope silts; and two highly biogenic, deep-water silt and clayey silt facies with similar characteristics. The facies are related in a qualitative manner to the environmental processes operating off Cape Hatteras. Chicago Bridge Technical Paper No. 5139.     

Record of a Mid-Pleistocene depositional anomaly in West Antarctic continental margin sediments: an indicator for ice-sheet collapse?

Modern global warming is likely to cause future melting of Earth's polar ice sheets that may result in dramatic sea-level rise. A possible collapse of the West Antarctic Ice Sheet (WAIS) alone, which is considered highly vulnerable as it is mainly based below sea level, may raise global sea level by up to 5–6 m. Despite the importance of the WAIS for changes in global sea level, its response to the glacial–interglacial cycles of the Quaternary is poorly constrained. Moreover, the geological evidence for the disintegration of the WAIS at some time within the last ca. 750 kyr, possibly during Marine Isotope Stage (MIS) 11 (424–374 ka), is ambiguous. Here we present physical properties, palaeomagnetic, geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the West Antarctic continental margin in the Amundsen Sea and spans more than the last 1 Myr. Within the sedimentary sequence, proxies for biological productivity (such as biogenic opal and the barium/aluminum ratio) and the supply of lithogenic detritus from the West Antarctic hinterland (such as ice-rafted debris and clay minerals) exhibit cyclic fluctuations in accordance with the glacial–interglacial cycles of the Quaternary. A prominent depositional anomaly spans MIS 15–MIS 13 (621–478 ka). The proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single, prolonged interglacial period. Even though no proxy suggests environmental conditions much different from today, we conclude that, if the WAIS collapsed during the last 800 kyr, then MIS 15–MIS 13 was the most likely time period. Apparently, the duration rather than the strength of interglacial conditions was the crucial factor for the WAIS drawdown. A comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout MIS 15–MIS 13. Some of these anomalies are observed in the pelagic Southern Ocean and the South Atlantic and might originate in major ice-sheet drawdown in Antarctica, but further research is required to test this hypothesis.     

Late Proterozoic to Early Palaeozoic supracrustal succession of Sierra Leone: an aulacogen at the western margin of the West African Craton

Three parallel zones of Upper Proterozoic to Lower Palaeozoic sedimentary-volcanic cover rocks have been recognized in western Sierra Leone. The Kasila Group m the SW ist metamorphosed to the granulite facies and grades into the Marampa Schist Group, and this in turn grades into the unmetamorphosed Rokel River Group in the NE. Prior to this investigation, these rocks were thought to belong to different ages; the most highly metamorphosed was thought to be the oldest and the unmetamorphosed, the youngest. This view is unrealistic since field mapping has failed to reveal any sharp contacts between them and all available age data indicate their involvement in the Pan-African (ca. 550 Ma) Orogeny A model is proposed involving an intracontinental graben (aulacogen) extending from western Sierra Leone into the adjacent territories developed about 1000 Ma ago. During the Pan-African Orogeny a portion of the aulacogen fill overlying the remobilized basement was subjected to a regional metamorphism and deformation. The grade of metamorphism and the degree of deformation decreased steadily from the centre of the Pan-African domain to the contact with the West African craton. The unmetamorphosed and mildly deformed to undeformed equivalent rests on the craton.

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Zusammenfassung Aus Sierra Leone werden drei Untereinheiten von oberproterozoischen bis unterpaläozoischen vulkano-sedimentären Schichten beschrieben, die nebeneinander liegen und ineinander übergehen.Die Kasila-Gruppe im SW, die bis zur Granulitfazies metamorphisiert ist, geht in die Marampa-Schiefer-Gruppe über, die wiederum in die unmetamorphisierte Rokel-River-Gruppe im NE überleitet. Vor dieser Untersuchung wurden diesen Gesteinen unterschiedliche Alter zugeordnet; die am stärksten metamorphisierten wurden als älteste, die unmetamorphisierten als jüngste Gesteine angesehen. Diese Betrachtung ist unrealistisch, da Geländekartierungen keine scharfen Kontakte erkennen lassen und alle erhältlichen Altersdaten die Einbeziehung in die Panafrikanische Orogenese (ca. 550 Ma) anzeigen. Es wird daher angenommen, da\ sich vor ungefähr 1000 Ma ein intrakontinentaler Graben (Aulakogen) entwickelte, der vom westlichen Sierra Leone bis in die benachbarten Territorien reichte. Während der Panafrikanischen Orogenese wurde ein Teil der dem remobilisierten Basement auflagernden Aulakogenfüllung einer Regionalmetamorphose und Deformation unterworfen. Die Stärke der Metamorphose und der Grad der Deformation nimmt vom Zentrum der panafrikanischen Region bis zum Kontakt mit dem westafrikanischen Kraton hin gleichmä\ig ab. Auf dem Kraton selbst liegen die nicht metamorphisierten, schwach- bis undeformierten Schichten (Rokel-River Gruppe).

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Résumé On a reconnu, en Sierra Leone occidentale, trois zones parallèles de roches volcano-sédimentaires, d'âge protérozoÏque supérieur à paléozoÏque inférieur. Le groupe de Kasila, dans la région sud-ouest, est métamorphisé dans le faciès des granulites; il passe progressivement au groupe des Schistes de Marampa, qui passe lui-mÊme au groupe non métamorphique de Rokel River, dans la région nord-est. Jusqu'ici, ces roches ont été considérées comme d'âges différents, les plus métamorphiques étant les plus vieilles, et les non-métamorphiques les plus jeunes. Ce point de vue s'est avéré incorrect; en effet, la cartographie n'a pas révélé de contacts nets entre ces roches et, par ailleurs, les données disponibles relatives à leur age relient leur évolution à l'oro genèse pan-africaine (ca 550 Ma). Le modèle proposé fait appel à un graben intra-continental (aulacogène) qui s'étend de la Sierra Leone occidentale aux territoires contigus et s'est développé, il y a 1000 Ma environ; lors de l'orogenèse pan-africaine, la partie du remplissage de l'aulacogène qui recouvre le socle remobilisé a été soumise à une déformation et à un métamorphisme régionaux. Le degré du métamorphisme ainsi que l'intensité de la déformation décroissent régulièrement du centre du domaine pan-africain jusqu'au contact avec le craton ouest-africain. Sur le craton lui-mÊme reposent les couches équivalentes non métamorphiques, peu ou pas déformées, du groupe de Rokel-River.

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- - -, . - , , , — -. ; , — . , .. , - .. 550 . , 1.0 / lakogen /, - . - , . - - . , , — .

     

Late Quaternary sediments and stratigraphy on the continental shelf off Troms and west Finnmark, northern Norway

Lithologic, paleontologic, and chronostratigraphic investigation of 13 gravity cores indicates the following environmental evolution: a high- (mid-) arctic period with a slight influx of ice-rafted debris occurred during the early middle Weichselian followed by a mid- (high-) arctic environment with a high influx of iceberg-rafted debris during the remainder of the middle Weichselian. The continental ice sheet probably did not extend beyond the inner shelf during middle Weichselian and a minimum relative sea level was ca. −120 m. A low-arctic environment occurred during (parts of) the late Weichselian with an initial winnowing of the sediments. The Norwegian Current entered the area during this substage. A high- (mid-) boreal environment occurred during the Holocene with high winnowing activity in the early Holocene. Winnowing is still very active on the shallower banks in contrast to the deeper banks where it has ceased. Relatively high percentages of carbonate in the form of biogenic skeletal remains occur in the Holocene sediments.     

Flooding of the continental shelves as a contributor to deglacial CH4 rise

The transition from the last glacial and beginning of Bølling–Allerød and Pre‐Boreal periods in particular is marked by rapid increases in atmospheric methane (CH₄) concentrations. The CH₄ concentrations reached during these intervals, ～650–750 ppb, is twice that at the last glacial maximum and is not exceeded until the onset of industrialization at the end of the Holocene. Periods of rapid sea‐level rise as the Last Glacial Maximum ice sheets retreated and associated with ‘melt‐water pulses’ appear to coincide with the onset of elevated concentrations of CH₄, suggestive of a potential causative link. Here we identify and outline a mechanism involving the flooding of the continental shelves that were exposed and vegetated during the glacial sea‐level low stand and that can help account for some of these observations. Specifically, we hypothesize that waterlogging (and later, flooding) of large tracts of forest and savanna in the Tropics and Subtropics during the deglacial transition and early Holocene would have resulted in rapid anaerobic decomposition of standing biomass and emission of methane to the atmosphere. This novel mechanism, akin to the consequences of filling new hydroelectric reservoirs, provides a mechanistic explanation for the apparent synchronicity between rate of sea‐level rise and occurrence of elevated concentrations of ice core CH₄. However, shelf flooding and the creation of transient wetlands are unlikely to explain more than ～60 ppb of the increase in atmospheric CH₄ during the deglacial transition, requiring additional mechanisms to explain the bulk of the glacial to interglacial increase. Similarly, this mechanism has the potential also to play some role in the rapid changes in atmospheric methane associated with the Dansgaard–Oeschger cycles. Copyright © 2012 John Wiley & Sons, Ltd.     

佳木斯地块东缘晚古生代大陆边缘构造演化

佳木斯地块位于中国东北微陆块群的最东缘,其东缘地区晚古生代的岩浆和沉积演变进程为欧亚大陆东缘由被动陆缘向活动陆缘构造环境的转化提供了关键证据。年代学和地球化学研究表明,佳木斯地块东缘中泥盆世黑台组砂岩,形成于被动陆缘的构造环境,黑台组上覆的老秃顶子组流纹岩也形成于被动陆缘的构造环境;晚石炭世珍子山组砂岩,形成于活动陆缘的构造环境;早二叠世的二龙山组安山岩以及相邻地区早二叠世的其它火成岩形成于活动陆缘的构造环境。同时,佳木斯地块东缘泥盆-二叠纪的沉积地层也呈现出由浅海相到陆相地层转化的特征。因此,佳木斯地块东缘由被动陆缘向活动陆缘的转化应该发生在中泥盆世到晚石炭世,而该构造环境的转化也为晚古生代时期蒙古-鄂霍茨克洋向欧亚大陆之下俯冲过程的研究提供了关键信息。

     

Holocene denudation of the northwest sector of Iceland as determined from accumulation of sediments on the continental margin

Radiocarbon-dated marine cores, measurements of sediment density and seismic surveys were used to estimate the sediment and mass accumulation rates (m/kyr and kg/m²/kyr) in the troughs from the southwest to north-central Iceland shelf (i.e. northwest sector of Iceland). The 3.5-kHz seismic survey showed varying thicknesses of acoustically transparent sediment in the troughs, whereas the inter-trough banks were largely devoid of sediment. The survey showed a pervasive reflector 1 to ≥60 m below the sea floor, which turned out to be Saksunarvatn tephra, dated at 10 180±60 cal. yr BP. The 3.5-kHz analogue data were digitized at 1-min intervals and provided 1645 estimates of maximum sediment thickness and 979 estimates of sediment accumulation over the last 10 200 cal. yr BP. Maximum sediment accumulation occurred in the mid-troughs and not, as expected, in the fjords. The median sediment accumulation rate (SAR) based on the core data was 0.23 m/kyr, but was 0.77 m/kyr based on the seismic data: the difference is attributed to coring limitations. Based on the volume of offshore sediment and the contributing terrestrial drainage area, the Holocene denudation of northern Iceland (c. 50 000 km²) is calculated to have been between 0.02 to 0.05 m/kyr, substantially lower that the 1-3 m/kyr derived from the suspended sediment load of rivers from southern Iceland but in agreement with the rate of accumulation of Holocene glacial lacustrine sediments in central Iceland.     

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km². Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/10³ y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales’ largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/10³ y, with the upper slope accumulating a lower 0.23 m/10³ y since transgression. Transgression produced a diachronous (14–10 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic‐carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.     

Structure and evolution of an obliquely sheared continental margin: Rio Muni, West Africa

Oblique-shear margins are divergent continental terrains whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni margin, equatorial West Africa, where the ca. 70-km-wide Ascension Fracture Zone (AFZ) exhibits oblique–slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole lithosphere failure and continental breakup (ca. 117 Ma). Oblique extension is recorded also by strike–slip and oblique–slip fault geometry within the AFZ, and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of continental margin kinematics. At transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and oblique-shear margins may experience several tens of millennia between breakup and drift. Noncoeval breakup and drift have important consequences for the fit of the equatorial South American and African margins because, in reconstructing the configuration of conjugate continental margins at the time of their breakup, it cannot be assumed that highly segmented margins like the South Atlantic will match each other at their ocean–continent boundaries (OCBs). Well known ‘misfits’ in reconstructions of South Atlantic continental margins may be accounted for by differential timing of breakup and drifting between oblique-shear margins and their adjacent rifted segments.     

The high-supply,current-dominated continental margin of southeastern South America during the late Quaternary

The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-¹⁴C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.     

东北地区晚古生代海相地层属于大陆边缘沉积的确定

东北地区晚古生代海相地层围绕佳—蒙地块呈环带状分布。岩屑长石砂岩等非稳定型沉积发育,岩性横向变化较大,岩相不稳定,火山岩发育等说明其属于活动型沉积。从佳—蒙地块的核心到边缘缝合带可见有古陆—类陆表海—陆棚海—深海的较为明显岩相带变化。从环带状分布、活动型沉积和构造岩相带有规律分布等方面确定东北地区晚古生代海相地层属于佳—蒙地块的大陆边缘沉积。     

Late Quaternary relative sea-level changes and the role of glaciation upon continental shelves

Sea levels of the past 20 kyr are largely determined by the response of the Earth to the last ice age. Consequently, sea-level indicators are an important source of information about the interaction between cryosphere and hydrosphere and the solid Earth. Based on the material presented at a recent European Science Foundation conference, the present paper pin-points some of the topics currently under discussion with respect to sea-level evidence found on continental shelves. These topics include possible effects of erosion and changes in palaeotidal ranges on indicators of former relative sea levels as well as the problems involved in the determination of palaeo-water depth in addition to former sea levels. More evidence is being gathered for substantial small-scale patterns in the sea-level changes at or nearby to the former ice margins. These patterns are not reproduced by the available geophysical models, which reconcile on first-order level only the large-scale pattern.     

Evolution of an active continental margin as exemplified by the Alpine history of the Caucasus

A plate-tectonics model of the Alpine evolution of the Caucasus is suggested. According to the model, in the Jurassic-Neocomian the Caucasian territory comprised the shelf of the East European platform, the marginal sea of the Great Caucasus, the Pontian-Transcaucasian island arc, the Anatolian-Minor Caucasian oceanic basin (Tethys) and the Iranian-Turkish microcontinent. Along the northern margin of the oceanic basin a convergent plate juncture extended. Part of the Caucasus, situated north of this plate boundary, represented the West Pacific-type active margin of the East European platform. In the Middle Cretaceous the Iranian-Turkish microcontinent collided with the Pontian-Transcaucasian island arc and as a result the Transcaucasian-Minor Asian continental block originated. In the central part of the latter an extensive Paleogene andesitic belt formed, with the Black Sea-Adjara-Trialetian and Talysh-South Caspian basaltic rift troughs on its rear (northern) side (incipient Black Sea and South Caspian basins). Major plate boundary shifted south, into the Zagros-Taurus basin, though the Anatolian-Minor Caucasian suture zone remained mobile in the Upper Cretaceous and Paleogene. From the Oligocene, under conditions of ongoing convergence of the Eurasian and Afro-Arabian continental blocks, the present-day intracontinental mountainous foldbelt has developed.     

Phosphate regeneration from sediments of the Peru continental margin by dissolution of fish debris

Generally, oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes. Stoichiometric modelling of such reactions takes into consideration varying proportions of ‘decomposable’ organically bound P to account for the ratios among nutrients in depth-concentration profiles of near-surface sediments. New results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off Peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P.Dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations. Phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10% of the nutrient pool in the waters of the Peru undercurrent. Partitioning of P among the two sources reveals further that fish debris phosphate is about four times more important than organically bound P in nutrient generation from sediments of the Peru continental margin. Not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area.     

南祁连盆地木里冻土区第四纪沉积物中可溶有机质的来源及其与天然气水合物的关系

在南祁连盆地木里坳陷钻遇中国陆域第一例天然气水合物实物样品。通过对木里冻土区钻遇水合物区域（DK井区）第四纪沉积物及邻区湖相沉积物中可溶有机质开展有机地球化学分析,发现DK井区浅地表沉积物中明显存在热解成因的可溶有机质,而邻区老人湖湖相沉积物中可溶有机质以高等植物输入为主。与区内烃源岩对比,热解成因的可溶有机质主要来源于中侏罗世烃源岩,晚三叠世尕勒得寺组烃源岩也有一定的贡献。结合该区水合物气源研究成果,DK井区沉积物中可溶有机质与形成水合物的烃类气体可能属于同源,侏罗纪、三叠纪烃源岩热演化生成的烃类能够沿断裂、砂岩裂隙等通道向上运移或渗漏至浅地表,其中甲烷、重烃气等轻质组分能够在稳定带内与地层水结合形成天然气水合物。木里地区第四纪沉积物中可溶有机质的来源解析能够为天然气水合物的勘探提供新的视角。     

Late Quaternary cycles of mangrove development and decline on the north Australian continental shelf

Mangrove communities in the Australian tropics presently occur as narrow belts of vegetation in estuaries and on sheltered, muddy coasts. Palynological data from continental-shelf and deep-sea cores indicate a long-term cyclical component of mangrove development and decline at a regional scale, which can be linked to specific phases of late Quaternary sea-level change. Extensive mangrove development, relative to today, occurs during periods of marine transgression, whereas very diminished mangrove occurs during marine regressions and during rarer periods of relative sea-level stability. Episodes of flourishing mangrove cannot be linked to phases of humid climate, as has been suggested in studies elsewhere. Rather, the cycle of expansion and decline of mangrove communities on a grand scale is explained in terms of contrasting physiographic settings characteristic of continental-shelf coasts during transgressive and regressive phases, in particular by the existence, or lack, of well-developed tidal estuaries. Copyright © 1999 John Wiley & Sons, Ltd.