Geochemistry of continental margin sediments from northwest Africa

The geochemistry of the non-carbonate fraction of sediments from the continental shelf and slope off northwest Africa has been investigated using R-mode factor analysis. The geochemistry is governed by a small number of factors controlled quantiatively by glauconite, phosphorite, quartz, clay minerals and secondary iron oxides. The distributions of the factors and of their controls are governed by mineral provenance and by depositional processes. Clay mineral factors predominate in fine grained sediments. Phosphorite, glauconite and oxide factors and quartz predominate in sands. Provenance controls the relative abundances of the minerals of the sand fraction all of which, except a proportion of the glauconite which is clearly recent, are relict and detrital. The recent glauconite is concentrated in poorly oxidised sediments off the southern Sahara. Provenance differences are detected between Saharan and Moroccan clays, and also exist between the clays of north and south Morocco.     

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.     

Manganese and copper fluxes from continental margin sediments

Total dissolvable Cu and Mn have been measured in seawaters collected from the continental shelf of the eastern Bering Sea. Copper concentrations of < 3 nmole kg⁻¹ were measured over the shelf break but concentrations increased to >4 nmole kg⁻¹ inshore of a hydrographie front over the 100 m isobath. Manganese concentrations also were low over the shelf break, <10 nmole kg⁻¹, and increased systematically to concentrations >10 nmole kg⁻¹ inshore of the hydrographic front. Depth distributions of Mn at all continental shelf stations showed gradients into the sediments, with concentrations typically >20 nmole kg⁻¹ in a bottom layer extending about 30 m off the bottom. Benthic Cu and Mn fluxes are indicated by cross-shelf pore water profiles that show interfacial concentrations more than an order of magnitude greater than in bottom water. These data and the results of a model of metal transport across the shelf suggest that Cu and Mn fluxes, estimated at 2 and 18 nmole cm⁻²y⁻¹, respectively, from continental shelf sediments may be one “source” of these metals to the deep sea.     

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.     

Pore water fluoride in Peru continental margin sediments: Uptake from seawater

Fluoride analyses display downward decreasing pore water gradients in Peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters, presumably by incorporation into carbonate fluorapatite. The profiles can be modeled as first-order F-removal with rate constants of ~3 yr^?1 and asymptotic F-concentrations deep in the cores of 35–45 μM, almost one-half the seawater value. The integrated flux of fluoride from seawater into organic-rich shelf sediments in coastal-upwelling zones (phosphatic muds) yields a contemporaneous global F-burial of 0.54 × 10¹⁰ mol-F yr^?1, about one-fifth the burial in other sinks (mostly carbonates and opal). The associated burial flux of phosphorus in shelf phosphorites is about 1.6 × 10¹⁰ mol-P yr^?1, comparable to P-burial in the deep sea with organic matter (~1.4 × 10¹⁰ mol yr^?1) and biogenic carbonates (~1.4 × 10¹⁰ mol yr^?1). Thus phosphorite formation on the Peru shelf is a significant contemporaneous process.     

The continental margin in Iceland — A snapshot derived from combined GPS networks

The tectonical setting in Iceland is quite complex due to the interaction of the Iceland hot spot and the Mid Atlantic Ridge. While in the north of the island one active spreading zone exists, the divergent motion in the centre and the south is distributed over at least two volcanic rift zones. The spreading rate increases linearly along the Western Volcanic Zone from north to south up to 8 mm/yr at the Hengill triple junction. On the contrary, the spreading rate of the parallel Eastern Volcanic Zone decreases from 16 mm/yr down to 6 mm/yr at the island's southern coast. The Hreppar microplate between the two predominant rift zones has an independent motion, which is distinct from that of the Eurasian and North American plates. A new detected feature is the spreading activity around the Hofsjökull volcanic zone located in the centre of Iceland with a significant rate of 6 mm/yr. During this investigation the coordinate sets of nearly 20 years of GPS data acquisition on Iceland were combined to get a velocity field for the surface of Iceland. This velocity field is based on a linear kinematic model with the consideration of local non-linear effects like volcano up-doming and displacements due to major earthquakes.     

我国季风边缘区湖泊沉积记录的全新世亚洲夏季风衰退事件

亚洲夏季风是全球季风系统的重要组成部分,亚洲夏季风的变化对其控制区域自然生态系统的多样性和生态平衡,以及社会经济发展有重要的影响。本文选择位于现代亚洲夏季风边缘区对季风变化响应敏感的湖泊达连海为研究对象,基于陆生植物残体和全有机质的AMS ¹⁴C定年建立了钻孔顶部24.6 m沉积物的年代框架,利用粒度指标重建了全新世研究区水文变化过程以及亚洲夏季风衰退事件序列。结果显示,沉积物中存在数层砂层,代表了湖泊低水位时期,进而指示了亚洲夏季风衰退事件。这些事件处在11.6~11.3 cal.ka B.P.、10.4~9.5 cal.ka B.P.、6.4~6.0 cal.ka B.P.、4.6~4.4 cal.ka B.P.、3.7~3.4 cal.ka B.P.、3.1~2.9 cal.ka B.P.以及2.0~0.9 cal.ka B.P.,可以发现中晚全新世以来亚洲夏季风衰退事件发生的频率显著增加。进一步与北半球高纬地区与低纬地区的气候突变事件记录对比显示,全新世百年-千年时间尺度上亚洲夏季风强度的变化与低纬ENSO活动存在密切的联系。

     

Chemistry of miocene plume tholeiites from northwest Iceland

The earliest (Miocene) plateau basalts of northwest Iceland form an olivine tholeiite series with elevated contents of Ti, K, P, Rb, Ba and Sr. They are closely similar to ‘plume’ tholeiites of the Faeroes (Paleocene-Eocene) and the Reykjanes Peninsula, Iceland (Recent) and confirm the Miocene renaissance in northeast Atlantic plume activity previously suggested on geophysical grounds.It is argued that the elevated contents of Ti, K, etc., are due to the ascent in a plume column of high pressure alkalic magmas and their re-equilibration to low pressure olivine tholeiites largely by additional melting at 10–20 km depth and 1150–1300°C.The 1800 m northwest Iceland sequence lacks stratigraphic variation suggesting random extraction from an extensive melt region with a nearly stable range of P-T-chemical conditions.     

Temporal changes in Holocene δ18O records from the northwest and central North Iceland Shelf

The region of north Iceland is highly sensitive climatically owing to its location with respect to atmospheric and oceanographic fronts. In this study we present total carbonate and δ¹⁸O records of benthic and planktic Foraminifera from nine sediment cores from the North Iceland Shelf. The results of this work indicate that the deglaciation of the Vestfirdir Peninsula was completed by 10 200 cal. yr BP. The 8200 cal. yr BP cold event is present only as a minor isotopic event, and seems not to have had much of a cooling effect on the bottom waters of the northwest Iceland shelf. The Holocene maximum warmth, attributed to a stronger North Icelandic Irminger Current, occurred between ca. 7800 and 6200 cal. yr BP. Over the past 4500 cal. yr BP a general cooling trend has occurred on the North Iceland Shelf, and superimposed on this overall cooling trend are a number of oscillations between periods when relatively warmer and cooler waters occupied the shelf. Relatively cooler waters were present at 4200–4000 cal. yr BP, 3200–2900 cal. yr BP, 2500–2350 cal. yr BP and 600–200 cal. yr BP, whereas relatively warmer waters were present on the shelf at 3750–3450 cal. yr BP, 2800–2600 cal. yr BP and 1700–1000 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.     

The present day formation of apatite in Mexican continental margin sediments

Results of pore water and sediment analyses from the western Mexican continental margin strongly suggest the present day formation of apatite. The interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments. Apatite is identified within this maximum via X-ray diffraction but is elsewhere undetectable in the core. Radioisotopic thorium, uranium, and radium data support the conclusion that this deposit is modern. The present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value.     

Sources of organic carbon in the Portuguese continental shelf sediments during the Holocene period

Organic C (OC) and total N (TN) concentrations, and stable isotope ratios (δ¹³C) in muddy deposit sediments of the Northern and Southern Portuguese continental shelf were used to identify sources of fine-sized organic matter (<63 μm) during the Holocene period. Sedimentary columns off the Guadiana (core CRIDA 05), Tagus (core MD 992332) and Douro (core KSGX 57) estuaries are characterised by elemental and isotopic values that reflect distinct sources of organic matter (OC/TN and δ¹³C ranging, respectively, from 8.5 to 21 and from −22.4‰ to −27‰). Intense supplies to the Guadiana continental shelf of fine terrigenous particles during the Younger-Dryas Event are closely linked with higher OC/TN values and lower δ¹³C ratios. During the postglacial transgression phase, an increasing contribution of marine supplies (up to 80%) occurred. Higher δ¹³C (up to −22.4‰) values and low OC/TN ratios (down to 8.5) are found as the sea level approaches the current one. The Upper Holocene records emphasize the return to enhanced terrestrial supplies except for the Little Climatic Optimum between the 11th and 15th centuries AD. This climatic event is especially obvious in the three cores as a return to marine production and a decrease in terrestrial sediment supply to the continental shelf. The return to a cooling event, the Little Ice Age, between the 15th and 19th centuries AD, is mirrored by decreased terrigenous supplies in core KSGX 57. Gradually increasing sedimentation in estuaries, as well as formation of coastal dune fields, have been hypothesized on the basis of increasing δ¹³C and decreasing OC, TN and OC/TN values.     

Structure,mechanical properties and evolution of the lithosphere below the northwest continental margin of India

International Journal of Earth Sciences - The continental breakup history at the northwest continental margin of India remained conjectural due to lack of clearly discernable magnetic anomaly...     

Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245-0 ka) from the Niger and Congo deep-sea fans, however, reveal that C_org/N_tot ratios and isotopic signatures of bulk organic matter (δ¹³C_org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C_org/N_tot and δ¹³C_org (up to −17‰) were driven by generally nitrogen-poor but ¹³C-enriched terrigenous plant debris and SOM from C₄/C₃ vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C₃/C₄ vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C_org/N_tot and δ¹³C_org (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and ¹²C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C_org/N_tot and δ¹³C_org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO₂.     

Modern and Holocene foraminifera and sediments on the continental shelf off Troms, North Norway

Based on studies of gravity cores from two transverse troughs on the shelf and earlier investigations, the surface sediments are divided into three main facies: bouldery and pebbly sand on the banks and the shelf break; sand on the flanks and outer parts of the troughs and sandy mud in the inner parts of the troughs. Besides a depth control, the distribution must have been influenced by relatively rapidly moving bottom currents in the outer parts of the troughs. The distribution and composition of the modern benthic foraminiferal fauna (e.g. C. lobatulus/T. angulosa in the outer reaches and C. obtusalBolivina spp. in the inner reaches) is mainly controlled by the bottom current regime and sediments. The planktic fauna dominated by N. pachyderma (R) correlates well with the winter surface temperatures. The stratigraphi-cal analysis shows that the 10,000–9,600 years B.P. period experienced high rates of deposition probably due to meltwater runoff from the continental ice sheet. At ca. 9,700 B.P. a minimum in the production of N. pachyderma (R) indicates a temporary cooling of the surface water. During the 9,600–7,800 B.P. period the rate of deposition was reduced. At the end of this period the foraminiferal fauna changed towards one like the modern fauna, reflecting improving ecological conditions. At ca. 7,800 B.P. the sediments became coarser due to reduced input of detrital sediments and an increased production of sand-sized biogenic material. Since then the shelf environment has been fairly stable up to the present time.     

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

全球典型被动陆缘盆地油气成藏特征

以被动陆缘油气富集理论为指导,通过研究南大西洋和印度洋被动陆缘盆地油气地质条件,分析油气差异富集因素。研究结果表明:被动陆缘盆地主要发育3套含油气系统,其中印度洋地区主要发育克拉通陆内裂谷期含油气系统,裂陷期含油气系统,漂移期含油气系统。南大西洋地区主要发育裂陷期含油气系统,漂移早期含油气系统和漂移晚期含油气系统。根据不同油气成藏特征,总结为10类成藏模式,不同成藏模式发育背景和成藏主控因素各异。为被动陆缘盆地不同构造沉积演化阶段油气勘探方向提供研究基础,对相关或类似盆地勘探研究提供借鉴。     

Limestone lithofacies from the South African continental margin

Carbonate sedimentation has played an important part in the Cainozoic history of the South African offshore areas. The Agulhas Bank sector of the continental shelf and upper slope is covered by a patchy veneer of relict and Recent sediment. Under this thin cover of sediment lies a succession of Cainozoic carbonate rocks very different from those exposed onshore. The Agulhas Bank rocks have been divided into the following lithofacies: (1) lagoonal-quartzose lime wackestones; (2) pelagicforaminiferal lime packstones; and (3) infralittoral-skeletal lime packstones. The rocks within each lithofacies are not necessarily time-equivalents; indeed any one lithofacies may have a considerable time range.     

Terrigenous sedimentation processes along the continental margin off NW Africa: implications from grain-size analysis of seabed sediments

The terrigenous fraction of seabed sediments recovered along the north‐west African continental margin illustrates spatial variability in grain size attributed to different transport mechanisms. Three subpopulations are determined from the grain‐size analyses (n = 78) of the carbonate‐free silt fraction applying an end‐member modelling algorithm (G. J. Weltje, 1997). The two coarsest end‐members are interpreted as representing aeolian dust, and the fine‐grained end‐member is related to fluvial supply. The end‐member model thus allows aeolian fallout to be distinguished from fluvial‐sourced mud in this area. The relative contributions of the end‐members show distinct regional variations that can be related to different transport processes and pathways. Understanding present‐day sediment dispersal and mixing is important for a better understanding of older sedimentary records and palaeoclimate reconstructions in the region.     

Geomorphic evidence for Holocene glacial advances and sea level fluctuations on eastern Vestfirðir, northwest Iceland

New geomorphic and chronological data of Holocene advances of the Drangajökull Ice Cap, located on eastern Vestfirðir, northwest Iceland, are presented. At least two glacial advances and two transgressions during the Holocene are interpreted from moraines and raised beach deposits, respectively. Geomorphic evidence is concentrated in the three valleys adjacent to the modern outlet glaciers of the Drangajökull Ice Cap: Kaldalónsjökull, Leirufjarðarjökull, and Reykjarfjarðarjökull. The valley surrounding Kaldalónsjökull contains a vegetated Holocene moraine with a minimum radiocarbon age of ∼2600 cal. yr BP, which provides geomorphic evidence for Neoglacial activity on eastern Vestfirðir. The second extensive Holocene glacial advance on eastern Vestfirðir occurred during the Little Ice Age, and moraines associated with this advance are present in all three outlet glacier valleys. The Neoglacial advance is the most extensive ice advance on eastern Vestfirðir. Raised beaches parallel to the coastlines of Ísafjarðardjúp and Jökulfirðir, at an elevation of approximately 5 m a.s.l., suggest a minor transgression at ∼3000 cal. yr BP based on radiocarbon ages of shells. A minor transgression of 0.3–0.5 m a.s.l. is associated with the timing of the Little Ice Age advance. Correlation of geomorphic events with sediment proxy records facilitates distinguishing local perturbations from regional North Atlantic climate signals. This study supports regional interpretations of climatic instability during the Holocene.     

Holocene temperature history of northern Iceland inferred from subfossil midges

The Holocene temperature history of Iceland is not well known, despite Iceland's climatically strategic location at the intersection of major surface currents in the high-latitude North Atlantic. Existing terrestrial records reveal spatially heterogeneous changes in Iceland's glacier extent, vegetation cover, and climate over the Holocene, but these records are temporally discontinuous and mostly qualitative. This paper presents the first quantitative estimates of temperatures throughout the entire Holocene on Iceland. Mean July temperatures are inferred based upon subfossil midge (Chironomidae) assemblages from three coastal lakes in northern Iceland. Midge data from each of the three lakes indicate broadly similar temperature trends, and suggest that the North Icelandic coast experienced relatively cool early Holocene summers and gradual warming throughout the Holocene until after 3 ka. This contrasts with many sites on Iceland and around the high-latitude Northern Hemisphere that experienced an early to mid-Holocene “thermal maximum” in response to enhanced summer insolation forcing. Our results suggest a heightened temperature gradient across Iceland in the early Holocene, with suppressed terrestrial temperatures along the northern coastal fringe, possibly as a result of sea surface conditions on the North Iceland shelf.