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1.
Late-Quaternary sections (about 1 Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter supply to the easternmost equatorial Atlantic Ocean. Variations in quantity and quality of the riverine organic matter reflect the interaction between the paleoclimatic development within the continental catchment area and the paleoceanographic conditions in the Congo river plume. To characterize the delivery of organic matter from terrigenous and marine sources, we used elemental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemistry, and organic petrology. High-amplitude fluctuations occurring about every 15-25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as would be anticipated in front of a major river. Fluctuations in Tmax from Rock-Eval pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine production offshore of the Congo. Organic petrological observations confirm the existence of abundant terrigenous plant tissues, both non-oxidized (vitrinite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterland, and implies more arid climatic conditions. Results from ratios of specific phenolic lignin components suggest that terrigenous organic matter in Late-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specific lignin phenols and the bulk '13Corg signature fosters the conclusion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbon isotopic signatures alone.  相似文献   

2.
Megafauna biomass and feeding guilds were studied on the NW Iberian upwelling Continental Margin in order to determine the presence of enriched zones pointing to enhanced particle input. We compare these findings with similar data obtained from a transect across the Celtic Continental Margin that represents a regime without coastal upwelling. Additionally sediment concentrations of phytopigments (chlorophyll-a, phaeophorbides) representing recent inputs of algal production and of nucleic acids (DNA, RNA) are used as proxies for microbial biomass, to assess if there was a relation between these parameters and the megafauna distribution. The sediment on the upper slope (<1600 m) of the Iberian Margin was found to be inhabited by filter-feeding megafauna (26–73% of total invertebrate density, and 1–35% of biomass), and contained relatively low levels of phytopigments (3–6 ng/cm3 chlorophyll-a) and nucleic acids (12–16 μg−1 DNA, 1.5–3.5 μg−1 RNA). In contrast, on the upper slope of the Celtic Margin the dominant component of the megafauna were deposit-feeders (57–92% of total invertebrate density, and 23–90% of biomass) and the sediments contained higher concentrations of phytopigments and nucleic acid. These observations, supplemented by video records revealing the presence of current ripples on the Iberian upper slope, show that these upper slope regions are non-depositional, high energy environments. Conditions at the lower slope and the abyssal plain on the Iberian transect were more quiescent with large deposit-feeding holothurians dominating the megafauna (72–94% of invertebrate biomass), and with relatively high sediment concentrations of phytopigments (7–9 ng/cm3 chlorophyll-a, 157–170 ng/cm3 phaeophorbides) and nucleic acids (21–38 μg−1 DNA, 2.4–5.5 μg−1 RNA). On the basis of our data we argue that the benthic food for the deepest stations on the Iberian transect does not consist of shelf derived organic matter. More likely, fast sinking offshore blooms, possibly associated with filaments of upwelling water, form the major contribution to the annual food supply of the deep living megafauna.  相似文献   

3.
We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola–Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being ‘basaltic’ in composition and the other similar to an ‘average shale’. The basaltic end-member comprises approximately 10–15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.  相似文献   

4.
Sedimentary, isotopic and bulk geochemical proxies measured in sediment samples of five gravity cores collected in the distal part of the Ogooue turbidite system (around 4000 m-depth) were used to develop a conceptual model to describe the accumulation of terrigenous organic matter (OM) during the last 200,000 yrs BP in the eastern part of the Gulf of Guinea. This model takes into account the influence of the different depositional processes (turbiditic vs hemipelagic sedimentation), geomorphological features and sea-level variations.Total organic carbon (TOC) and the stable organic carbon isotopes of the OM (δ13C) variability follow the highstand/lowstand (interglacial/glacial) cyclicity with a very low accumulation rate of terrigenous OM during periods of high sea-level and higher accumulation rate during period of low sea-level. A sea-level of 80–120 m below present day seems to favor the transfer of terrigenous sediments to the deep offshore environment through the turbidite system and thanks to the connection of the canyons heads with the river system presently located at the shelf edge at −120 m water depth.In this system, terrigenous OM matter delivered by the river accumulate in the sediments via two main processes. Indeed, a part of the terrigenous OM settles in combination with the finest particles forming hemipelagites, while another part, formed of very well preserved land plant debris, is transported and deposited far offshore with turbidity currents. The proportion of terrigenous OM accumulated due to turbidity currents is important as it can represent more than 70% of the carbon accumulated during sea-level lowstand. Moreover, terrigenous OM seems to preferentially accumulate in the levees and the lobes of the system notably due to the higher frequency of organic-rich turbidites.This study demonstrates that gravity flows, influenced by the sea-level variations, can significantly affect the terrigenous OM budget of the deep offshore Atlantic margins and that channel-levee complexes as well as turbidite lobes can be regarded as good sink for terrestrial organic carbon. These processes should be taken into consideration in the context of source rocks exploration but also for the estimation of the general carbon accumulation in ocean sediment.  相似文献   

5.
Detailed sedimentological, geochemical and isotopic analyses were carried out on sediment samples from ODP Site 720A on the Indus Fan, Arabian Sea. High values of calcium carbonate associated with low values of Al and Ti from 0 to 375 ka, and low values of calcium carbonate along with high values of Al and Ti from 375 to 525 ka represent two distinct sedimentary sequences. The sediments deposited from 525 to 375 ka correspond to a turbidite sequence, characterized by a high terrigenous input of coarse-grained sediments composed mostly of sand and silt. The sediments deposited from 375 ka to the present day comprise a pelagic sequence, consisting of pelagic material and clay. The major turbidity flow between 375 and 525 ka resulted in the greatest development of the Indus Fan during the late Quaternary. Most of the active channels were buried by 375 ka, followed by deposition of mainly pelagic sequences since then. Enrichment of an Indus-derived Himalayan clay mineral assemblage (illite and chlorite) in both the turbiditic and pelagic sequences reveals that the source and supply of clay minerals to the Indus Fan were the same during pre- and post-turbidite deposition. At ODP Site 720A, Al, Ti and terrigenous material do not show any systematic changes with respect to glacial and interglacial periods, suggesting that sea-level changes are not directly responsible for the terrigenous material supply to this site. Rather, a major switch in distributary channels away from the western margin of the Indus Fan is suggested.  相似文献   

6.
Integrated studies on the hydrochemistry and water column rates of microbial processes in the eastern sector of the Black Sea along a standard 100-miles transect off Gelendzhik from the coast to the central part of the sea at water depths of 100–2170 m show that a series of warm winters and the absence of intense convective winter mixing resulted in a relatively low content of suspended particulate matter (SPM), particulate organic carbon (POC), and nutrients in the water column in March 2009. The relatively high SPM concentrations and the presence of isotopically light POC at the offshore station are indicative of the supply of terrigenous material from land and low contributions of phytoplanktonic organic matter to the composition of SPM. This may explain the low rates of biogeochemical processes in the water column near the coast. The surface layer at deep-water stations is dominated by isotopically heavy phytoplanktonic organic matter. This suggests that the supply of terrigenous material from land was insufficient in offshore deep-water areas. Therefore, warm winters and insufficient nutrient supply do not prevent photosynthesis in the photic layer of the deep-water zone, which generates organic substrates for heterotrophic aquatic communities. The results of isotopic analysis of POC, measurements of the rates biogeochemical processes, and the hydrochemical characteristics of the water column can be used to determine the nature and seasonal variability of the POC composition.  相似文献   

7.
通过对东北太平洋胡安·德富卡隆起西翼沉积柱样中的碳酸盐含量和浮游有孔虫氧同位素测试以及浮游有孔虫丰度、溶解指数和粗组分的统计,发现在晚第四纪约65 ka以来碳酸盐含量变化幅度大(0 4%~77 2%),相差非常悬殊,但其变化未呈现出冰期时溶解作用减弱、间冰期时溶解作用强烈这样明显的旋回性。粗组分分析结果显示,地层中有明显的浊流沉积发育。另外,碳酸盐含量和浮游有孔虫丰度分析结果揭露了研究区的 CCD在3 500 m左右。研究认为,在约65 ka来水深浅于3 500 m的区域碳酸盐含量的无规律性变化应与浊流沉积影响有直接的关系,而水深深于3 500 m的区域碳酸盐含量主要受控于深海碳酸盐的溶解作用。  相似文献   

8.
楚科奇海盆M04柱晚更新世以来沉积古环境记录   总被引：1，自引：0，他引：1  
对"中国第五次北极科考"采自楚科奇海盆的M04柱进行粒度、冰筏碎屑、黏土矿物、岩心XRF扫描、沉积物颜色分析,初步建立了楚科奇海盆晚更新世MIS4期以来的沉积地层框架。MIS4期以来,楚科奇海盆M04柱沉积物粒度和黏土矿物组成具有明显的冰期/间冰期变化特征,冰期沉积物粒度分布以双峰态为主,由洋流搬运和海冰搬运沉积组分组成,伊利石含量高、高岭石含量低;间冰期沉积物具有三峰态粒度分布特征,由海冰搬运、洋流搬运和冰山搬运沉积组分组成,伊利石含量低、高岭石含量高。通过M04柱黏土矿物组合类型与北冰洋边缘海盆的表层沉积物黏土矿物组合类型对比表明,晚更新世以来楚科奇海盆沉积环境发生显著变化:温暖的间冰期受波弗特涡流驱动,波弗特海为研究区的物源输入提供了主要贡献;寒冷的冰期表层环流呈反向输运,细颗粒物源碎屑以东西伯利亚海的输入为主。  相似文献   

9.
To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The CORG:NTOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ13CORG and δ15NTOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (CORG:NTOT ratio) and isotopic (δ13CORG and δ15NTOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.  相似文献   

10.
冲绳海槽陆源碎屑峡谷通道搬运与海底扇沉积   总被引：20，自引：3，他引：20  
应用“向阳红16号”1992年地质调查和“向阳红9号”1995年地球物理调查的实际资料，并参照80年代以来有关研究成果，对冲绳海槽沉积物类型、陆源组分的堆积形式、沉积速率、物质通量以及沉积环境状况等进行了研究，结果表明，冲强海槽陆源碎屑主要集中在海底峡谷口外，形成海底扇沉积，海底扇以其与峡谷伴生而地势和缘、陆源组分含量高、沉积通量大、沉积物楔入体复合叠置为标志，揭示出海底峡谷在陆源碎屑向海槽输送过程中的通道作用；提出陆架潮流与海底峡谷内波、内潮汐的联合作用是陆源碎屑经峡谷通道向海槽持续搬运的主要动力因素，而黑潮摆动及其涡旋分支对峡谷上游沉积物的供给具有积极作用。  相似文献   

11.
High-resolution sedimentological and micropaleontological studies of several deep-sea cores retrieved from the levees of the Celtic and Armorican turbidite systems (Bay of Biscay — North Atlantic Ocean) allow the detection of the major oscillations of the British–Irish Ice Sheet (BIIS) and ‘Fleuve Manche’ palaeoriver discharges over the last 30,000 years, which were mainly triggered by climate changes.Between 30 and 20 cal ka, the turbiditic activity on the Celtic–Armorican margin was weak, contrasting with previous stratigraphic models which predicted a substantial increase of sediment supply during low sea-level stands. This low turbidite deposit frequency was most likely the result of a weak activity of the ‘Fleuve Manche’ palaeoriver and/or of a reduced seaward transfer of sediments from the shelf to the margin. However, two episodes of turbiditic activity increase were detected in the Celtic–Armorican margin, during Heinrich events (HE) 3 and 2. This strengthening of the turbiditic activity was triggered by the meltwater releases from European ice sheets and glaciers favouring the seaward transfer of subglacial material, at least via ‘Fleuve Manche’ palaeoriver.At around 20 cal ka, a significant increase of turbidite deposit frequency occurred as a response to the onset of the last deglaciation. The retreat of the European ice sheets and glaciers induced a substantial increase of the ‘Fleuve Manche’ palaeoriver discharges and seaward transfer of continentally-derived material into the Armorican turbidite system. The intensification of the turbiditic activity on the Celtic system was directly sustained by the widespread transport of subglacial sediments from the British–Irish Ice Sheet (BIIS) to the Celtic Sea via the Irish Sea Basin. A sudden reduction of turbiditic activity in the Armorican system, between ca. 19 and 18.3 cal ka, could have been triggered by the first well known abrupt sea-level rise (‘meltwater pulse’, at around 19 cal ka) favouring the trapping of sediment in the ‘Fleuve Manche’ palaeoriver valleys and the decrease of the seaward transfer of continentally-derived material.The maximum of turbiditic activity strengthening in the Celtic–Armorican margin, between ca. 18.3 and 17 cal ka, was induced by the decay of European ice sheets and glaciers producing the most extreme episode of the ‘Fleuve Manche’ palaeoriver runoff and a great seaward transfer of subglacial material into the Bay of Biscay. Between ca. 17.5 and 16 cal ka, the turbiditic activity significantly decreased in both Celtic and Armorican turbidite systems in response to a global re-advance of glaciers and ice sheets in Europe. The last episode of ice sheet retreat, between ca. 16 and 14 cal ka, is well expressed in the Celtic system by a new increase of the turbiditic activity. The major episode of sea-level rise at around 14 cal ka (‘Meltwater Pulse 1A’), precluding the seaward transfer of sediments, induced the end of turbiditic activity in both the Celtic and the Armorican system.Although two main phases of global sea-level rise seem to have had an effect on the Celtic–Armorican margin, this work proposes the BIIS retreat and associated riverine discharges as the main trigger mechanisms of the turbiditic activity in this region during the last 30,000 years.  相似文献   

12.
Distributions and sources of total organic carbon （TOC）in seabed sediments and their implications for hydrodynamics are analyzed, in the turbidity maximum of the Changjiang Estuary. Ecology ecoenvironmental effects of estuary water on the continuously increasing terrigenous organic carbon from the Changjiang River are also explored through variations of organic carbon content and water quality indicators. Results show that, hydrodynamics exert important influences on distributions of organic carbon in the tur- bidity maximum of Changjiang Estuary. For their redistribution effect of terrigenous organic carbon within the moving layer in the whole region, variations from land to sea are not indicated by surficial and vertical average values of TOC and total nitrogen （TN） contents in core sediment, as well as organic stable carbon isotopes in surface sediments. However, on the long-time scale, the trend of terrigenous organic carbon decreasing from land to sea is still displayed by variations of stable carbon isotopic average values becoming heavier from land to sea. Previous studies have shown that high content of Chl a cannot appear in the Changjiang Estuary in despite of adequate nourishment supply, because photosynthesis of phytoplankton is constrained by high suspended sediment concentration（SSC）. However, an area with a high content of Chl a occurs, which may be caused by resuspended benthic algae with bottom fine grain-size sediments. Tremendous pressures are imposed on the environment of Changjiang Estuary, because of uhrophication trends and special hydrodynamics. Phytoplankton bloom area tends to extend from the outer sea to the mouth of Changjiang River.  相似文献   

13.
The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the ‘Fleuve Manche’ palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.  相似文献   

14.
1Introduction TheBeringSea,locatedinthesub-arcticNorth Pacific,playsanimportantroleininfluencingtheevo- lutionaryprocessoftheglobalclimaticsystembecause itsseasonalseaiceisformedinrelativelowerlatitudes (Takahashi,1999).ItisalsoasinkofatmosphericCO2, whichisoriginatedfromtheeffectivebiologicalpump inthissea.Particulatefluxdatameasuredinthesea overthelast10aindicatethattheorganic/inorganic carbonratiowasalwaysgreaterthan1,whichexplains thattheBeingSeaoccupiesasignificantpositionin theproces…  相似文献   

15.
The sources and distribution of organic matter (OM) in surface waters and sediments from Winyah Bay (South Carolina, USA) were investigated using a variety of analytical techniques, including elemental, stable isotope and organic biomarker analyses. Several locations along the estuary salinity gradient were sampled during four different periods of contrasting river discharge and tidal range. The dissolved organic carbon (DOC) concentrations of surface waters ranged from 7 mg l−1 in the lower bay stations closest to the ocean to 20 mg l−1 in the river and upper bay samples. There was a general linear relationship between DOC concentrations and salinity in three of the four sampling periods. In contrast, particulate organic carbon (POC) concentrations were significantly lower (0.1–3 mg l−1) and showed no relationship with salinity. The high molecular weight dissolved OM (HMW DOM) isolated from selected water samples collected along the bay displayed atomic carbon:nitrogen ratios ([C/N]a) and stable carbon isotopic compositions of organic carbon (δ13COC) that ranged from 10 to 30 and from −28 to −25‰, respectively. Combined, such compositions indicate that in most HMW DOM samples, the majority of the OM originates from terrigenous sources, with smaller contributions from riverine and estuarine phytoplankton. In contrast, the [C/N]a ratios of particulate OM (POM) samples varied significantly among the collection periods, ranging from low values of 5 to high values of >20. Overall, the trends in [C/N]a ratios indicated that algal sources of POM were most important during the early and late summer, whereas terrigenous sources dominated in the winter and early spring.In Winyah Bay bottom sediments, the concentrations of the mineral-associated OM were positively correlated with sediment surface area. The [C/N]a ratios and δ13COC compositions of the bulk sedimentary OM ranged from 5 to 45 and from −28 to −23‰, respectively. These compositions were consistent with predominant contributions of terrigenous sources and lesser (but significant) inputs of freshwater, estuarine and marine phytoplankton. The highest terrigenous contents were found in sediments from the river and upper bay sites, with smaller contributions to the lower parts of the estuary. The yields of lignin-derived CuO oxidation products from Winyah Bay sediments indicated that the terrigenous OM in these samples was composed of variable mixtures of relatively fresh vascular plant detritus and moderately altered soil OM. Based on the lignin phenol compositions, most of this material appeared to be derived from angiosperm and gymnosperm vascular plant sources similar to those found in the upland coastal forests in this region. A few samples displayed lignin compositions that suggested a more significant contribution from marsh C3 grasses. However, there was no evidence of inputs of Spartina alterniflora (a C4 grass) remains from the salt marshes that surround the lower sections of Winyah Bay.  相似文献   

16.
Recent exploration revealed the high potential for hydrocarbon in the deepwater sags, Pearl River Mouth Basin, northern South China Sea. This paper reports its Cenozoic sedimentary evolution through backstripping of high precision depth data of interpreted sequence boundaries. Local backstripping parameters were mapped based on well and geophysical data. Sensitivity analysis indicates that the reliability of decompaction results were largely improved by using the local porosity parameters and the lithological parameters that vary with grid nodes. Maps of sedimentation rates of 17 sequences from 65 Ma to the present were constructed, showing the spatial–temporal variation of the sedimentation rate. Three rapid depositional stages, 65–32, 29–23.3, 18.5–10.5 Ma, and three slow depositional stages, 32–29, 23.3–18.5, 10.5–0 Ma, were identified with abrupt changes of sedimentary patterns. The three rapid depositional stages were in accord with syn-rifting stage, the first post-rifting depositional stage, and the second post-rifting depositional stage, respectively. And the three slow depositional stages were in keeping with three tectonic events respectively. Several significant sedimentary discontinuities at 32, 23.3 and 10.5 Ma were observed and discussed. The comparison between the study area and the ODP Site 1148 at 32–23.3 Ma indicates that before ~29 Ma the ODP Site 1148 was at similar sedimentation regime as that in the Baiyun and Liwan sags, but significant diversity appeared after ~29 Ma, when a large quantity of terrigenous sediments was trapped by strong post-rifting subsidence in the Baiyun and Liwan sags and could not reach the lower slope areas. Study revealed that the most rapid accumulation from 18.5 to 17.5 Ma might be mainly owing to the large sediment supply during this strong monsoon period.  相似文献   

17.
The Cenozoic margins of the Norwegian-Greenland Sea offer ideal conditions for passive margin studies. A series of structural elements, first observed on these margins, led to the concept of volcanic passive margins. Questions still remain about the development of such features and the location of the boundary between oceanic and continental crust. Despite the thin sediment cover of the margins, seismic reflection data are not able to image the deeper structures due to the occurrence of igneous rocks at shallow depth.This paper presents a 320-km long profile perpendicular to the strike of the main structural units of the Lofoten Margin in Northern Norway. A geological model is proposed, based on observations made with ocean bottom seismographs, which recorded seismic refraction data and wide angle reflections, along with a seismic reflection profile covering the same area. Ray-tracing was used to calculate a geophysical model from the shelf area into the Lofoten basin. The structures typical of a volcanic passive margin were found, showing that the Lofoten Margin was influenced by increased volcanic activity during its evolution. The ocean/continent transition is located in a 30-km wide zone landwards of the Vøring Plateau escarpment.The whole margin is underlain by a possibly underplated, high velocity layer. Evidence for a pre-rift sediment basin landwards of the escarpment, overlain by basalt flows, was seen. These structural features, related to extensive volcanism on the Lofoten Margin, are not as distinct as further south along the Norwegian Margin. Viewed in the light of the hot-spot theory of White and McKenzie (1989) the Lofoten Margin can be interpreted as a transitional type between volcanic and non-volcanic passive margin.  相似文献   

18.
《Marine Geology》2001,172(3-4):205-223
A mathematical wave erosional model was used to study the effect of high sea levels during the penultimate (oxygen isotopic stage 7) and last interglacials (substage 5e), and in the late Holocene (stage 1), on the present morphology of wave-cut shore platforms. Sea level was considered to have been either the same as today or 2.25 m lower during the penultimate interglacial, and 2.25, 4.5 or 6.75 m higher than today during the last interglacial stage. The model suggested that inherited, gently sloping shore platforms in resistant rocks may be essentially protected today from erosion by high storm waves. The lowest platform gradients were in runs with mesotidal (3 m) range, and usually with low wave periods, low surf attenuation rates and weak rocks. Modern platform gradients increased with the difference in elevation between sea levels during successive interglacial stages. Shore platforms were widest in runs in which sea level was the same as today in the penultimate interglacial and 4.5 m higher than today during the last interglacial. Constant sea level, and high, last interglacial sea levels with considerable overlapping between the zones of high duration values between the mean neap high and low tidal levels in stages 7, 5e and 1, were conducive to the development of wide shore platforms, whereas sea levels lower than today's in stage 7 tended to produce narrower platforms. In general, higher sea levels during the last interglacial tended to produce higher cliff–platform junctions than constant sea level, unless the sea was lower than today during the penultimate interglacial stage. There was a lack of supratidal ledges in macrotidal (9 m) model runs with high initial gradients; this suggests that gently sloping, inherited shore platforms are essential for the subsequent development of supratidal, nonstructural ledges in high tidal environments during periods of higher sea level. Intertidal ledges developed in the upper portion of the modern intertidal zone, under a variety of tidal and sea level conditions. These ledges can develop independently of lithological or structural influences, and without any change in sea level. A single high sea level may also simultaneously produce two ledges at different supratidal elevations in mesotidal environments.  相似文献   

19.
In the eastern equatorial Atlantic Ocean, changes in the concentration of carbonate in Late Quaternary sediments resulted from reduced production of carbonate in surface waters and increased dilution with non-carbonate sediments during glacial maxima. During glacial stages, production of carbonate in surface water (measured as its accumulation rate in shallow, undissolved cores) decreased by one half. The glacial accumulation rate of non-carbonate components increased 1.5 to 4 times over Holocene values; the greatest increases occurred in the deepest cores.

Carbonate dissolution during stages 2, 3 and 4 increased the proportion of foraminiferal fragments and decreased the accumulation rate of susceptible species in the deep sites. In shallow sites, slightly increased dissolution can be detected during stage 3 while greatly increased dissolution occurred during stage 4. Bathymetric profiles of foraminiferal fragmentation and accumulation document a shoaling of the foraminiferal lysocline by 1000 m during glacial isotopic stages. We present a mass balance model of sediment accumulation for carbonate and insoluble components and from this model we estimate the rate of downslope transport and dissolution of carbonate at the Sierra Leone Rise. Our results show that during stage 4 the rate of carbonate loss to dissolution was greater than the rate observed today or during other interglacial stages. The calculated rates of dissolution for stages 2 and 3 are not significantly different from those calculated for stage 1.  相似文献   


20.
对CSH1岩心全样沉积物样品进行元素地球化学分析,揭示了过去88ka冲绳海槽北部沉积物成分、水动力条件及陆源碎屑物质源区风化历史。冲绳海槽北部碎屑沉积物母岩主要以长英质为主,在MIS 1期沉积物存在大量火山源物质。过去88ka,沉积物源区风化程度较弱,但是自冰消期以来有逐渐增强的趋势。沉积物Zr/Nb比值表明在MIS 1期和MIS 5.1期水动力较为强烈,这与黑潮增强的时间一致,可能是指示黑潮强度的一个指标。冲绳海槽北部陆坡沉积物陆源碎屑贡献在低海平面时期显著增加,而在MIS 1和MIS 5.1期生源贡献显著增加。冲绳海槽北部沉积物成分变化明显受到黑潮和入海径流的调节,实际上受海平面和东亚季风的制约。  相似文献   

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