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1.
Solid phase P speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the Great Barrier Reef (GBR) lagoon. This region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore, seawards of which biogenic carbonate sediment predominates. Phosphorus speciation results show large variations along this transect. Organic P and authigenic (apatite) P are the major chemical forms of phosphorus in the central GBR continental shelf sediments. Post-depositional reorganization of P was also observed, converting organic P and iron bound P (Fe-P) to authigenic (apatite) P. Phosphorus burial rate was estimated from measurements of total P concentration and excess 210Pb sediment mass accumulation rates. Burial efficiency varies significantly over the shelf. Inshore areas showed significant P remobilization from sediments to the water column (up to ∼50%). The mid and the outer shelf showed little evidence for remobilization (except for coral reef platform sediments), with more of the sediment P being in the less reactive authigenic apatite phases. An appreciable fraction of this non-labile authigenic apatite phase was identified as fish bone. P sources and sinks over the central part of the GBR shelf were quantified using a mass balance approach. This showed that Coral Sea shelf edge upwelling events are essential to satisfy the large P nutrient demand of the whole GBR lagoon. P inputs due to upwelling events were greater than those contributed by local rivers over an average year.  相似文献   

2.
Salinity variations in restricted basins like the Baltic Sea can alter their vulnerability to hypoxia (i.e., bottom water oxygen concentrations <2 mg/l) and can affect the burial of phosphorus (P), a key nutrient for marine organisms. We combine porewater and solid-phase geochemistry, micro-analysis of sieved sediments (including XRD and synchrotron-based XAS), and foraminiferal δ18O and δ13C analyses to reconstruct the bottom water salinity, redox conditions, and P burial in the Ångermanälven estuary, Bothnian Sea. Our sediment records were retrieved during the Integrated Ocean Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition 347 in 2013. We demonstrate that bottom waters in the Ångermanälven estuary became anoxic upon the intrusion of seawater in the early Holocene, like in the central Bothnian Sea. The subsequent refreshening and reoxygenation, which was caused by gradual isostatic uplift, promoted P burial in the sediment in the form of Mn-rich vivianite. Vivianite authigenesis in the surface sediments of the more isolated part of the estuary ultimately ceased, likely due to continued refreshening and an associated decline in productivity and P supply to the sediment. The observed shifts in environmental conditions also created conditions for post-depositional formation of authigenic vivianite, and possibly apatite formation, at ～8 m composite depth. These salinity-related changes in redox conditions and P burial are highly relevant in light of current climate change. The results specifically highlight that increased freshwater input linked to global warming may enhance coastal P retention, thereby contributing to oligotrophication in both coastal and adjacent open waters.  相似文献   

3.
In this study, redox-dependent phosphorus (P) recycling and burial at 6 sites in the Baltic Sea is investigated using a combination of porewater and sediment analyses and sediment age dating (210Pb and 137Cs). We focus on sites in the Kattegat, Danish Straits and Baltic Proper where present-day bottom water redox conditions range from fully oxygenated and seasonally hypoxic to almost permanently anoxic and sulfidic. Strong surface enrichments of Fe-oxide bound P are observed at oxic and seasonally hypoxic sites but not in the anoxic basins. Reductive dissolution of Fe-oxides and release of the associated P supports higher sediment-water exchange of PO4 at hypoxic sites (up to ∼800 μmol P m−2 d−1) than in the anoxic basins. This confirms that Fe-bound P in surface sediments in the Baltic acts as a major internal source of P during seasonal hypoxia, as suggested previously from water column studies. Most burial of P takes place as organic P. We find no evidence for significant authigenic Ca-P formation or biogenic Ca-P burial. The lack of major inorganic P burial sinks makes the Baltic Sea very sensitive to the feedback loop between increased hypoxia, enhanced regeneration of P and increased primary productivity. Historical records of bottom water oxygen at two sites (Bornholm, Northern Gotland) show a decline over the past century and are accompanied by a rise in values for typical sediment proxies for anoxia (total sulfur, molybdenum and organic C/P ratios). While sediment reactive P concentrations in anoxic basins are equal to or higher than at oxic sites, burial rates of P at hypoxic and anoxic sites are up to 20 times lower because of lower sedimentation rates. Nevertheless, burial of reactive P in both hypoxic and anoxic areas is significant because of their large surface area and should be accounted for in budgets and models for the Baltic Sea.  相似文献   

4.
南海具有典型边缘海特征,其北部陆坡由于具有高沉积速率的地层剖面,可以用于研究亚轨道尺度气候变化。本研究以南海北部陆坡下部MD12 3432柱状样(19°16.88′N,116°14.52′E,水深2 125 m)为主要研究材料,分析其粗组分百分含量、底栖有孔虫丰度和特定环境指示属种的百分含量与丰度,并结合相关研究结果,运用多指标综合分析,重建了南海北部陆坡MIS 11期(距今约40万年)以来的表层初级生产力和底层水溶解氧含量的演化情况。与之前研究不同,本研究发现MIS11期以来南海北部的古生产力在温暖的间冰期高、在寒冷的冰期低,且主要受控于东亚夏季风影响下低纬地区降水强度的变化。在MIS 7期和MIS 9期可能有较高溶解氧含量的底层水进入南海北部,导致在高生产力的暖期中,喜氧的低生产力指示种Cibicidoides wuellerstorfi丰度增加。  相似文献   

5.
陈芳  陆红锋  刘坚  庄畅  吴聪  曹珺  周洋  刘广虎 《地球科学》2016,41(10):1619-1629
南海地区天然气水合物资源丰富,针对其分解方式的研究有助于资源的开采.对南海东北部天然气水合物钻探区GMGS08站位岩心沉积物开展沉积学、地球化学分析研究.结果表明:该站位自上而下分布11层含自生碳酸盐岩和双壳碎屑层(其中6层呈粥状沉积)以及2层自生碳酸盐岩灰岩层;各层自生碳酸盐岩除一个样品δ13C值稍高(-38.85×10-3)外,其他的δ13C值介于-41.36×10-3~-56.74×10-3,均低于-40.00×10-3,δ18O值介于2.94×10-3~5.37×10-3,明显偏重,表明其为天然气水合物分解的产物,形成于微生物对甲烷的缺氧氧化作用,甲烷主要源自生物成因;各层自生碳酸盐岩层中的有机质碳同位素负偏明显,最低达-82.44×10-3,可能与微生物活动有关;根据自生碳酸盐岩的分布推断该站位至少发生过6次天然气水合物分解释放,每期次自生碳酸盐岩的差异说明其甲烷通量强弱不同.   相似文献   

6.
Sediments of 13 piston cores from opposite continental slopes of the South China Sea, off southern China and Sabah (northern Borneo), were analyzed by sedimentological methods and dated by oxygen isotope stratigraphy. Sediments mostly consist of hemipelagic clay with 20% carbonate off Sabah and 40% off China. We calculated terrigenous and carbonate accumulation rates for up to 11 time-slices from the Holocene to oxygen-isotope stage 6. Terrigenous accumulation rates generally increase with water depth and reach a maximum at the middle slope off Sabah and at the lower continental slope off China. During glacial and interglacial times this distribution pattern did not markedly change, despite an increase of accumulation rates for glacial periods by a factor of 2 to 5 compared to interglacial periods. Rates are negatively correlated with positions of sea level, which controls the partition of fluviatile terrigenous material for deposition on shelf, slope, and abyssal plain. Carbonate accumulation rates are higher off China by a factor of 2 compared to Sabah, probably indicating higher calcareous plankton productivity.  相似文献   

7.
南海西南海域表层沉积物中微量元素Ba的地球化学特征   总被引：1，自引：1，他引：0  
分析南海西南海域表层沉积物中微量元素Ba的地球化学特征,并探讨生物成因Ba的分布及其与表层海水生产力的关系。南海西南海域表层沉积物Ba总量的变化范围为398~1 270 μg/g,平均值为851 μg/g,从上陆坡到下陆坡,沉积物中的Ba含量逐渐增加;微量元素Ba主要赋存于陆源碎屑相和生物成因相中,且明显受到沉积环境水深和陆源物质输入的影响。生物成因Ba含量的变化范围为30.6~938 μg/g,陆坡区和深海区平均值分别为495 μg/g、349 μg/g,占沉积物Ba总量的54％和51％,总体上,与沉积物Ba总量具有相对一致的分布特征。尽管研究区表层沉积物中Al、Ti成分为典型的陆源碎屑组分,但是,利用大陆上地壳Ba/Al比值和陆坡区沉积物回归分析获得的Ba/Al比值进行生物成因Ba的计算,过高地估算了沉积物中陆源Ba的含量;而采用页岩Ba/Ti比值来估算陆坡区表层沉积物中的生物成因Ba含量显得相对可靠。在深海区,利用经验的Ba/Al或Ba/Ti比值均不能获得有效的生物成因Ba值。因此,在获取沉积物中生物成因钡含量时,需结合各海区的特点选取合适的参数来扣除陆源Ba的含量。  相似文献   

8.
Utilizing a sequential extraction technique this study provides the first quantitative analysis on the abundance of sedimentary phosphorus and its partitioning between chemically distinguishable phases in sediments of the Bering Sea, the Chukchi Sea and the Mackenzie River Delta in the western Arctic Ocean. Total sedimentary phosphorus (TSP) was fractionated into five operationally defined phases: (1) adsorbed inorganic and exchangeable organic phosphorus, (2) Fe-bound inorganic phosphorus, (3) authigenic carbonate fluorapatite, biogenic apatite and calcium carbonate-bound inorganic and organic phosphorus, (4) detrital apatite, and (5) refractory organic phosphorus. TSP concentrations in surface sediments increased from the Chukchi Sea (18 μmol g−1 of dried sediments) to the Bering Sea (22 μmol g−1) and to the Mackenzie River Delta (29 μmol g−1). Among the five pools, detrital apatite phosphorus of igneous or metamorphic origin represents the largest fraction (~43%) of TSP. The second largest pool is the authigenic carbonate fluorapatite, biogenic apatite as well as CaCO3 associated phosphorus (~24% of TSP), followed by the Fe-bound inorganic phosphorus, representing ~20% of TSP. The refractory organic P accounts for ~10% of TSP and the readily exchangeable adsorbed P accounts for only 3.5% of TSP. Inorganic phosphorus dominates all of phosphorus pools, accounting for an average of 87% of the TSP. Relatively high sedimentary organic carbon and total nitrogen contents and low δ13C values in the Mackenzie River Delta together with the dominance of detrital apatite in the TSP demonstrate the importance of riverine inputs in governing the abundance and speciation of sedimentary phosphorus in the Arctic coastal sediments.  相似文献   

9.
In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production and transport, however, differ in both the regions. In the Arabian Sea, average annual fluxes are over 50gm-2y-1 in the western Arabian Sea and less than 27gm-2 y-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm-2y-1) and are least in the southern part of the Bay (37gm-2y-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers. Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents.  相似文献   

10.
Recent sedimentary records from the Arabian Sea   总被引：1，自引：0，他引：1  
An attempt is made to understand the redox conditions that prevailed in the north eastern continental margins of the Arabian Sea and in the nearby deep water regions during the past few centuries using short undisturbed sediment cores. The geochronology is accomplished using210Pb excess method and the proxy indicators chosen for productivity and associated redox changes are CaCO3, organic matter (OM), Mn and U along with major elements Fe and Al. Such changes in principle are related to high productivity in the overlying waters which in turn depend on monsoonal intensity that causes upwelling responsible for increase in productivity. Alongwith the published data on gravity cores from the same region, our measurements suggest the following: At ∼ 300 m water depth, south of 21°N, the sediment-water interface at depths of ∼ 300 m had been anoxic during the time span represented by the presently studied cores for approximately ∼ 700y as evidenced by low Mn/Al (< 0.7 × 10−2) and high U/Al (> 10−4) weight ratios. In some adjacent deeper regions, however, the environment turned oxic around ∼ 200 y BP. Whereas both Mn and Ra were lost to the overlying waters in the anoxic regions (depth ∼340m), the Mn that diffused from deeper sections appears to have mineralized at the sediment-water-interface. Studies of this type on long undisturbed cores from the margins of the Arabian Sea and the Bay of Bengal, involving several proxies and geochronology by more than one method are needed to understand short term environmental (and monsoonal intensity) changes of the recent past with high resolution.  相似文献   

11.
冰期时的中国海——研究现状与问题   总被引：19，自引：3，他引：19  
汪品先 《第四纪研究》1990,10(2):111-124
沉积柱状样分析表明:末次冰期最盛时,东海缩剩冲绳海槽,南海变为袋状海湾,大河在陆坡入海,使南海北部陆坡与冲绳海槽海水浊度大增,沉积中碳酸盐含量被陆源物稀释,使剖面中CaCo3旋回与大西洋相似、与太平洋相反。因极锋南移,亲潮可能影响冲绳海槽,太平洋温水可能进入巴士海峡,南海表层冬、夏双向海流变为环流,加上沿岸冲淡水影响,使南海北部陆坡冬季表层水温比间冰期低6—9℃,季节差达8—10℃,远超过太平洋同纬度区的数值。  相似文献   

12.
Two gravity sediment cores (GH99‐1239 and GH99‐1246) obtained from the north‐eastern Japan Basin in the East Sea/Japan Sea were analyzed for the orbital‐ and millennial‐scale paleoceanographic changes. Chronostratigraphically, core GH99‐1239 represents a continuous sedimentary record since 32 ka, based on correlation of distinct lithological markers (i.e. dark layer or TL layer) with those in core GH98‐1232 collected nearby. For core GH99‐1246, the age model is constructed through correlation of lightness (L*) values and tephra (Aso‐4 and Toya) layers with those in the well‐dated Oki Ridge core (MD01‐2407), indicating about 134 ka of sedimentation since the latest Marine Isotope Stage (MIS) 6. New geochemical data from both cores corroborate orbital‐scale paleoceanographic variation, such that surface‐water productivity, represented by biogenic opal and total organic carbon (TOC) contents, increased during MIS 1 and MIS 5; CaCO3 contents do not show such distinct glacial–interglacial cycles, but were influenced by dissolution and preservation rather than foraminiferal production. During the glacial periods when sea ice was prevalent, surface‐water productivity was low, and bottom‐water conditions became anoxic, as indicated by high total sulfur (TS) contents and high Mo concentrations. The geochemical data further document millennial‐scale paleoceanographic variability, corresponding to a series of thin TL layers in response to Dansgaard–Oeschger cycles but irrespective of the glacial or interglacial periods. In particular, thin TL layers formed during MIS 3 are characterized by less TOC (about 1%) and TS (about 0.4%) contents and lower Mo (about 5 p.p.m.) concentration, whereas those during MIS 4 and MIS 5 exhibit more TOC (up to 4%) and TS (up to 5%) contents and higher Mo (up to 120 p.p.m.) concentration. Such a discrepancy is attributed to different degree of surface‐water productivity and of bottom‐water oxygenation, which is closely related to the sea level position and extent of ventilation. Flux of the East China Sea Coastal Water controlled by millennial‐scale paleoclimatic events is the most critical factor in deciding the properties of TL layers in the north‐eastern Japan Basin. Our results strongly confirm that TL layers in the Japan Basin also validate the unique feature of basin‐wide paleoceanographic signals in the East Sea/Japan Sea. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
中国近海天然气水合物的研究进展   总被引：40，自引：1，他引：40  
南海、东海具有形成天然气水合物的良好动力学环境和丰富的烃类气体来源。根据卫星对海面增温异常的观测、底水气体地球化学、标志矿物和流体组成的研究表明 ,南海、东海海底存在强烈的烃流体活动和排气作用 ;南海南北陆坡区海底气体主要由CH4组成 ,前者多为微生物成因气 ,后者多为热解气 ;冲绳海槽热液沉积区的气体 ,主要为CO2 (86 % ) ,其次为CH4、H2 、H2 S(14 % ) ,分别来自岩浆流体及陆源有机质的降解 ,也属热解成因气。地震地球物理的研究主要集中于南海东北部、北部、南部陆坡区和冲绳海槽中南部 ,测线长度还很有限 ,虽然都有BSR标志的发现 ,但质量比较好 ,研究程度也比较高的还只有南海东北部主动陆缘和北部被动陆缘的一些海域。通过沉积物烃含量和热释光的研究 ,南海、东海共获得 6个地球化学异常区。在综合对比物化探、地热等项资料的基础上初步认为 :笔架南 (Ⅰ )、台西南—东沙 (Ⅱ )异常区是寻找水合物的最佳远景区 ;琼东南—西沙海槽(Ⅲ )、中建南—中业北 (Ⅳ )和冲绳海槽南部异常区是寻找水合物和常规油气藏的有利地区 ,但Ⅳ区更有利于寻找油气 ,其余 2区更有利于寻找水合物 ;南沙海域的研究程度总体上比较低 ,但在其中的南沙海槽 ,物化探异常标志均优 ,甲烷含量较其它地区高 2个数量级  相似文献   

14.
Increasing eutrophication and seasonal anoxia in bottom water in the Changjiang Estuary and its adjacent waters has progressed in recent decades, caused by elevated anthropogenic N and P input. Sedimentary biogenic elements were investigated to determine whether the biogenic proxies could be used in paleoenvironmental studies in an energetic estuary, as well as to reconstruct the histories of environmental changes in the East China Sea (ECS). Two 210Pb-dated cores from the coastal and offshore waters were analyzed for organic C (TOC) and its stable isotope (δ13C), total N (TN), biogenic Si (BSi), total P (TP) and P species. In coastal sediment, the variations of P species, especially Fe-P, Al-P and detrital apatite P (Det-P), reflected the dry–wet oscillations in the Changjiang River for the past century, which has influenced the sediment grain size and terrestrial material input. Much lower BSi content (0.756%) at 16–22 cm likely recorded the pronounced decrease in silicate flux in the Changjiang River and its lower flow in the late 1980s. In offshore sediment, higher concentrations of TOC, TN, BSi, Ex-P, Fe-P and Lea-OP indicated higher primary productivity in response to the strong winter monsoons during the 1960s–1980s, and their 20-a fluctuations were in agreement with the decadal variations of the winter monsoon. Low contents and little variations of Al-P and Det-P indicated the slight influence of the terrestrial sediment input in offshore waters. The influence of human activities on the environment in recent decades has also been recorded in coastal sediment. Grain-size normalized concentrations of TOC, TN, TP, Ex-P, Fe-P and Lea-OP increased by 24%, 23%, 15%, 13% and 51% in the upper 16 cm of coastal sediment, indicating elevated P and N load and primary productivity since the 1990s. Elevated TN/TP ratios and decreased BSi/TOC recorded the changed nutrient structure and the decrease in the proportion of the diatom to phytoplankton community. However, the sediment record indicated that the eutrophication might actually have started from the end of the 20th century rather than the reported middle of 20th century. In contrast, biogenic elements in offshore sediment did not reflect disturbance by human activities. This study revealed that multi-nutrient proxies in sediment in the ECS could indicate natural environmental changes including runoff and the winter monsoon over the past century, as well as the influence of human activities in recent decades. Phosphorus species with distinct origins and biogeochemical behaviors could effectively reflect different aspects of past environmental conditions.  相似文献   

15.
通过对南海北部的ODP 1148站岩芯600 mcd以上(约30 Ma以来)的沉积物中自生富集Mn、Cd和Mo等过渡金属元素的含量变化的研究,并结合相关的化学组成结果,探讨了岩芯内部氧化-还原条件的变化以及相关元素的活动特征,反演了相应沉积时期的环境演变.结果显示,岩芯387 mcd以上,自生Mn富集明显,代表氧化的环境;387～485 mcd之间,自生Cd含量明显富集,Mn含量显著降低,代表少氧的环境;485 mcd以下,Mn和Cd含量极低,自生Mo明显富集,代表缺氧的环境.随氧化-还原条件的变化,Mo存在明显的向下迁移并在缺氧界面的缺氧一方达到最大值的趋势,而Cd在少氧环境形成的固相态则可能在缺氧环境下不稳定,溶解态的Cd有向上迁移的趋势,并且在少氧/缺氧界面的少氧一方富集.这些过渡金属元素记录的氧化-还原条件的变化,反映出ODP 1148站所在海区的沉积环境变化:早期有较丰富的陆源输入,表层海水生产力较高,随着南海不断扩张以及全球海平面上升,该海区表层海水生产力逐渐降低.  相似文献   

16.
乌梁素海和岱海沉积磷形态分布的差异性研究   总被引：3，自引：0，他引：3  
湖泊沉积物中磷的含量及形态分布等地球化学信息可以用来反演湖泊流域的古环境及污染历史。本文以乌梁素海和岱海两个不同类型的湖泊为研究对象,对两个湖泊沉积柱芯中磷形态分布的差异性进行了研究。结果表明,乌梁素海和岱海沉积柱芯中沉积磷的主导形态均为ACa P、De P和 Or P,但乌梁素海表层沉积物和沉积柱芯中Al P和 ACa P的含量水平明显高于岱海,而Ex P、De P和 Or P的含量水平明显低于岱海。两个湖泊沉积磷形态分布的差异性能较好地揭示出湖泊流域的地理环境、化学风化程度以及湖泊富营养化类型的差异性,沉积柱芯中ACa P含量及其分布特征能很好的指示湖泊自身发展过程中的富营养化水平,而De P含量及其分布特征则能较好的揭示出整个湖泊流域的古环境古气候信息。  相似文献   

17.
《Geochimica et cosmochimica acta》1999,63(11-12):1723-1734
Factors controlling the burial of organic carbon (OC) in Late Quaternary sediments on the NW Mexican continental margin are assessed using a suite of box and piston cores strategically located on the shelf-slope rise with respect to the intense oxygen minimum in this region. An OC concentration maximum occurs on the mid-slope, below the core of an intense water-column O2 minimum, due to current winnowing on the outer shelf, the preferential accumulation of organic matter in fine-grained deposits, and the offshore decrease in the settling flux of organic detritus. The organic matter at all water depths is overwhelmingly marine. Hydrogen indices (HI) are higher on the slope (>300 mg HC/g TOC) than on the shelf (<300 mg HC/g TOC), where current winnowing has promoted organic matter degradation, but there is no difference in HI in slope sediments accumulating under well oxygenated and O2-deficient conditions. The degree of winnowing appears to be the primary factor affecting the preservational quality of organic matter deposited on this margin.Rates of accumulation of OC and opal are all higher in the interglacial intervals when compared with the glacial deposits over the last 140,000 yr. However, matrix-corrected HI values in the mid- and lower-slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. Thus, cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differential preservation. HI values in Late Quaternary sediments from several continental margins, including NW Mexico, and euxinic basins correspond to type II kerogen, irrespective of bottom water O2 concentrations. Therefore, the preservation of oil-prone kerogen in productive margin settings does not appear to be restricted to sediments deposited under conditions of low bottom water O2 concentrations as envisioned in models of petroleum source-rock deposition.  相似文献   

18.
Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.  相似文献   

19.
The spatio-temporal changes in sedimentation rate along the continental margin of the monsoon-dominated eastern Arabian Sea during the last 24 kyr have been estimated from a compilation of 58 radiocarbon dated cores, in order to understand the effect of glacial-interglacial variation on sedimentation. The sedimentation rate has been computed for four-time slices, viz. the last glacial maximum, glacial-interglacial transition, early Holocene and late Holocene. We report that the average sedimentation rate in the slope to the abyssal region of the entire eastern Arabian Sea, although higher during the Holocene as compared with that during the last glacial maximum and glacial-interglacial transition, does not significantly vary during all four-time intervals. The sedimentation rate during any particular time interval, however, varies from core to core, thus indicating zonal changes. We report four zones of relatively high sedimentation rate, viz. the northeastern Arabian Sea, the region off the Gulf of Khambhat, the region off Goa and Mangalore, and off the southern tip of India. We suggest that a complex interaction of land-ocean-atmospheric processes controlled sedimentation rate in the eastern Arabian Sea during the last 24 kyr in such a way that the average sedimentation rate does not vary significantly, even during highly contrasting climatic conditions.  相似文献   

20.
Surface sediment samples from the shelf and continental slope off southwest Africa and sediment cores from the deepest part of the Black Sea were analyzed for sterols. Because the organic matter in these anoxic sediments is relatively well-preserved, the input from source organisms in the water column is important in controlling sterol distribution patterns. The sterol distribution on the Namibian shelf is complex, probably because of the great spatial and temporal variability of biological productivity caused by seasonal upwelling and changes in oxygen concentration. The Black Sea, perhaps because of greater physical stability of the water column, has sterol distributions which can be explained by microbial activity or chemical processes acting on a constant input of organic carbon from surface production.  相似文献   

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