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1.
CaCO 3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial–interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic δ 13C values of −23‰ indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0–140 ka) and 17 through 19 (660–790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic δ 13C values to ca. −29‰ in portions of isotope stage 2 suggest that the local concentration of dissolved CO 2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments. 相似文献
2.
Pore water chemistry (total dissolved CO 2, NH 4, NO 3, NO 2, PO 4, Si(OH) 4, Ca, Mg, Fe, Mn, SO 4, H 2S and F, and titration alkalinity), solid phase chemistry (C org, P org, C TOT, N TOT, F, Si OPAL and S II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C org), very high porosity ( > 0.96 ml cm −3) and very low dry bulk density (< 0.1 g cm −3). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μ M, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation. The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites. 相似文献
3.
Organic carbon (OC) and nitrogen (N) contents and δ 13C and δ 15N values in total organic matter (OM) were measured in sub-surface sediments (0–30 cm sub-bottom) from 21 cores raised from the Laurentian Channel of the Gulf of St. Lawrence and the Labrador Sea, to document OM fluxes and storage along the eastern Canadian margin. Storage rates as high as 2.5 g m −2 yr −1 for OC and 0.2 g m −2 yr −1 for N are observed in the Laurentian Channel, suggesting that the shelf plays a significant role in terms of OM storage (from 1 to 2% of the primary production). Based on the isotopic composition of the essentially marine OM of the Labrador Sea (δ 13C/V-PDB=−21.9±0.4‰; δ 15N/AIR=7.6±0.6‰; n=12), there is no isotopic evidence for a significant relative input of terrestrial OM along the Laurentian Channel (δ 13C/V-PDB=−21.9±0.4‰; δ 15N/AIR=8.0±0.9‰; n=10), either due to high relative fluxes of marine OM and/or to the trapping of continental OM in the estuary and upstream. High storage rates of OM are also observed on the continental rise of the Labrador Sea (as high as 1.1 g C m −2 yr −1 and 0.09 g N m −2 yr −1). They contrast with one order of magnitude lower rates on the slope, due to low sedimentation rates (SR) and sediment winnowing by the Western Boundary Undercurrent (WBUC). Reduced early diagenetic alteration of OM is observed, particularly in the Laurentian Channel. It results in discrete (i) losses of OC and N, (ii) shifts in C/N ratios, suggesting preferential removal of N-bearing OM also highlighted by losses in total hydrolysable amino acids (HAA). In the Labrador Sea slope records, due to low SR, OM concentration changes linked to long term temporal variations may superimpose on these diagenetic trends, and some influence of the WBUC is noticeable. 相似文献
4.
The shore-zone sediments between Jeddah and Yanbu, west coast of Saudi Arabia, are composed mostly of skeletal carbonate sands. The nearshore sediments containing benthic foraminifera, algal fragments and molluscs are multimodal, the mean grain size varying between 0.76 and 2.35 ø. The beach sediments, except samples dominated by cerithid gastropods in some localities, are relatively finer than the nearshore sediments. Although the beaches to a great extent comprise sand-sized material, fine lime muds and coarse clastic gravels occur in certain areas. The dune sediments comprising mostly algal grains and ooids are very fine with 50 percent of the material in the 0.25–0.18 mm size grade. Except a general northward decrease in mean grain size, regional trends in the textural parameters of the sediments between Jeddah and Yanbu are not quite apparent. Lateral variations in the textural characteristics suggest a landward migration of the sediments in the shore zone under the influence of northerly and northwesterly winds. The carbon and oxygen (δ 13C + 4.80 to 4.84‰ PDB) (δ 13O − 0.04 to + 0.53‰ PDB) isotopic ratios of the lime muds occurring in certain shallow margins in the shore-zone, which are much higher than those of the green algae, indicate that the fine carbonates are at least in part inorganic in origin. Like the shallow-water carbonates in tropical seas, aragonite and high Mg-calcite are the dominant carbonate minerals in the shore-zone sediments. There is a landward increase in aragonite contents caused by the landward migration of fine material from the nearshore. The dominant clay mineral in the nearshore sediments is kaolinite with subordinate swelling chlorite and little illite. Kaolinite is contributed by the coastal regions under the sub-tropical humid climate. Swelling chlorite is considered to have been formed in the nearshore by mechanical mixture of chlorite and montmorillonite derived from the metamorphic and igneous terrains of the Tertiary mountains bordering the coastal plain. 相似文献
5.
Recent sediments and separated phosphate pellets ( 125–500 μm in diameter) from the Peru shelf have been analyzed for uranium decay-series isotopes and 14C in order to determine age relationships and mineralization rates. Uranium-series ages of pellets separated from one box core are significantly higher than AMS radiocarbon ages determined for the same pellets. These differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed. Postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages. Sediment radiocarbon and 210Pb sediment results, as well as some trends in the uranium-series data, suggest that high concentrations of phosphate pellets have accumulated in some Peru shelf sediments without extensive reworking. Individual pellets apparently form very quickly, on time scales of a few years. Estimated authigenic uptake rates of phosphorus into pellets ranges from 0.5 to 9.40 μmol-P cm− 2 yr− 1, somewhat higher than rates measured for nodules from the same area. This is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits. 相似文献
6.
The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems. Resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that corresponds to 1001 to 946 ka. Organic δ 13C values systematically increase from − 26‰ to − 21‰ and δ 15N values systematically decrease from 4‰ to < 0‰ as organic carbon mass accumulation rates increase in the sapropel layers. The increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total marine productivity. The precessional minima with which sapropels coincide were times of wetter climate that stratified the surface Mediterranean Sea, increased delivery of soil-derived phosphorus, and evidently amplified microbial primary production. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode that suggest that sapropel deposition was a climate-sensitive surface-driven phenomenon that was not accompanied by basin-wide stagnation. 相似文献
7.
Carbon and oxygen isotope ratios of 59 samples of Mesozoic ophicalcites from the north-central Apennines were measured. Strontium content and isotope ratios were only determined for selected samples. The data obtained (−4.1 ≤ δ 13C ≤ + 3.0; 16.5 ≤ δ 18O ≤ 30.1; 0.7076 ≤ 87Sr/86Sr ≤ 0.7086; 60 ≤ Sr ppm ≤ 1140) suggest that the carbonate cement of the Apennines ophicalcites had a marine origin. The 18O/16O values of most of the carbonates studied, however, indicate that the rocks have interacted with hydrothermal fluids, dominantly represented by sea water. In particular, the rocks from the northern section of the Apennines apparently reequilibrated their δ 18O content at higher temperature than those from the central zone. Strontium isotope ratios, close to the Cretaceous—Cenozoic ocean water values, are well in agreement with such a kind of sea water/ophicalcites interaction, even if the carbonate cement formed earlier during the Jurassic period. 相似文献
8.
New petrographic and major element geochemical data from modern Peru margin upper slope-outer shelf phosphorites are presented, which provide insight into their origin and paragenetic relationship with other authigenic minerals (glauconite, pyrite and dolomite) occurring in organic-rich sediments. Glauconites are precipitated relatively early following the partial reduction of ferric iron and, following this process, phosphate, pyrite, and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate. As in many ancient economic phosphorite deposits, the phosphatic facies here consist of nodules, crusts, coatings and strata composed of phosphatic pelletal grains (ooids, structureless grains, intraclasts, clumps and biogenic grains) in association with organic-rich biosiliceous sediments. All are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment-water interface. Important factors that influence which morphology will tend to develop include the amount of available pore space, the presence of suitable nucleation sites, the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions. Bacterial mediation may play an important, but as yet unspecified role in the precipitation process. Textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion-free Na-F-Mg-CO 3-substituted pore-water precipitates whereas ooids are inclusion-rich pore-water precipitates poor in lattice-substituted components. Variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore-water carbonate ion concentrations and thus relative degrees of organic-matter degradation. Phosphate and dolomite are intimately mixed, yet mineralogically distinct phases in phosphatized dolomicrites. Depth-stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate. Below depths of a few centimeters, excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite. Periodic sediment reorganization (bioturbation, current winnowing and erosion, mass wasting, etc.) plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment, as well as mixing ordered mineral parageneses into complicated sequences. 相似文献
9.
Twenty kilogrammes of crusts and slabs of indurated carbonate sediment, usually referred to as hardgrounds, were dredged along the eastern steep wall of the Bannock Basin during the 1984 cruise of R.V. Bannock. The crusts range in thickness from one to a few centimetres and the fragments of these crusts are irregular in shape. Their surface is always uneven and their colour ranges from white to brownish dark grey. Some slabs are impregnated along one side by ferromanganese sesquioxides, and borings occur in several samples. Serpulid tubes have been observed in one instance. The borings and serpulids suggest formation of the hardgrounds at or close to the sediment/water interface and exposure at the seafloor. The degree of lithification is generally different on the inferred upper and lower sides of the slabs. An upward increase of lithification across the slabs is reflected by mineralogy, ultrastructure and stable isotope composition of the carbonate. X-ray diffraction analyses indicate high-magnesian calcite as the predominant carbonate with minor amounts of low-magnesian calcite and dolomite. Occasionally, large gypsum crystals are attached to the hardgrounds and sometimes smaller ones are dispersed through the carbonate matrix. An increase in diagenesis is reflected by the passage from friable, nodular nannofossil chalk to nannofossil limestone and hard xenotopic calcite micrite. Overgrowth of coccoliths and internal cementation of the tests of planktonic foraminifera by high-Mg calcite increase from chalk to limestone. In the hard, fully cemented micrites, coccoliths can no longer be recognised in the xenotopic fabric. Pteropods occur as dissolution moulds with aragonite preserved as only tiny relics. Carbon and oxygen isotope analyses were performed on different samples. The progressive lithification to chalk and limestone is marked by a shift in the δ18O values from +1.2‰ to +5.4‰ (PDB). This change indicates that precipitation of high-Mg calcite and possibly also recrystallisation of the original biogenic carbonate took place within cold and hypersaline brines which were enriched in 18O. The oxygen isotope data suggest that lithification and gypsum precipitation occurred under identical conditions. The carbon isotope data show progressive diagenetic change from values near +1‰ to values of +3‰. This change may reflect a contribution of methanogenetic CO2 to the hypersaline brine. 相似文献
10.
The stable isotope stratigraphy of Planulina wüllerstorfi in three box cores from Ontong Java Plateau in the western equatorial Pacific shows that there existed a bathyal thermocline near the depth of 2000 m during glacial time. Waters below the boundary were colder by about 1°C, those above were about the same as now. The deep waters were slightly enriched and the upper waters depleted in nutrients, compared with present values. The differences in δ 13C to the present (after correcting for the global change) are of the order of 0.1‰ more positive (above 2000 m) and 0.1‰ more negative (below), corresponding to changes in phosphate of the order of 0.1 μmol/kg (less phosphate above, and more below the boundary). 相似文献
11.
冷水珊瑚也称为深水珊瑚, 在生物多样性、生态资源和科研价值等方面具有重要意义。文章对采自南海北部冷泉区的冷水珊瑚骨骼碎屑进行测定, 鉴定出冷水珊瑚2个种( Crispatotrochus sp.1和 Crispatotrochus sp.2), 以及4个属[ Balanophyllia ( Balanophyllia)、 Balanophyllia ( Eupsammia)、 Lochmaeotrochus和 Enallopsammia]。测定的冷水珊瑚的 δ13C为-7.36‰~-1.15‰, δ18O为-1.38‰~3.67‰, 与全球冷水珊瑚碳氧同位素组成相似, 但明显不同于南海暖水珊瑚、冷泉碳酸盐岩及低温热液成因碳酸盐岩的碳氧同位素组成。 相似文献
12.
Two cores from the Bannock Basin in the eastern Mediterranean have been analysed for δ 180 and the δ 13C in planktonic foraminifera. One core (02-PC) was extracted from the anoxic brine, the other (08-GC) from a plateau east of the brine. The absence of sapropelic muds in Core 08-GC from the two Holsteinian euxinic cycles, together with the presence of isotopic spikes, suggests that the plateau has risen during the last ca. 200,000 years by ca. 2.5 mm/yr. The δ180 amplitude of Globigerinoides ruber in eastern Mediterranean cores is only ca. 0.5%. larger than for this species in North Atlantic cores. This suggests that the surface oxygen isotopic composition of both bodies of water followed each other fairly closely during the Late Pleistocene, except during the stagnant phases. 相似文献
13.
潮滩是陆海交互影响的重要区域,在陆海物质交换的过程中扮演着重要的角色.象山港位于浙江宁波,是典型的半封闭式港湾,受人类活动影响显著.本文取其重要支港西沪港为研究区,自其潮上带至潮间带采集表层沉积样品,进行总有机碳(Total Organic Carbon,TOC)、总氮(Total Nitrogen,TN)和稳定碳同位... 相似文献
14.
The stable carbon isotope composition of particulate organic carbon (POC) from plankton, sediment trap material and surface sediments from the Atlantic sector of the Southern Ocean were determined. Despite low and constant water temperatures, large variations in the δ13C values of plankton were measured. 13C enrichments of up to 10‰ coincided with a change in the diatom assemblage and a two-fold increase in primary production. Increased CO 2 consumption as a result of rapid carbon fixation may result in diffusion limitation reducing the magnitude of the isotope fractionation. The δ13C values of plankton from sea-ice cores display a relationship with the chlorophyll a content. High ‘ice-algae’ biomass, in combination with a limited exchange with the surrounding seawater, results in values of about − 18 to − 20‰. It is assumed that these values are related to a reduced CO 2 availability in the sea-ice system. In comparison with plankton, sinking krill faeces sampled by traps can be enriched by 2–5‰ in 13C (e.g. central Bransfield Strait). In contrast, the transport of particles in other faeces, diatom aggregates or chains results in minor isotope changes (e.g. Drake Passage, Powell Basin, NW Weddell Sea). A comparison between the δ13C values of sinking matter and those of surface sediments reveals that 13C enrichments of up to 3–4‰ may occur at the sediment-water boundary layer. These isotopic changes are attributed to high benthic respiration rates. 相似文献
15.
Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. 238U concentrations and 238U/ 232Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, 238U and 238U/ 232Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of 230Th and 232Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm −2 yr −1; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink. 相似文献
16.
利用2003年大洋DY105-12,14航次在中太平洋海山获取的KXD28富钴结壳样品,采用气相色谱(GC)内标法对该结壳样品的分层样进行了可溶有机质(氯仿沥青“A”)及其族组成(饱和烃、芳烃、非烃)、正构烷烃、类异戊二烯烃的定量分析,并结合总有机碳(TOC)及有机碳同位素(δ 13C)分析,探讨了结壳样品有机质来源及富钴结壳组成与生长环境之间的关系。结果表明:(1)富钴结壳中有机质生物母源主要为海洋生物和菌藻类,并混有少量陆源物质;有机碳同位素同样也指示了海洋水生生物碳同位素特征;(2)KXD28结壳从底层到最外层有机碳同位素发生了很大变化,结壳中有机碳同位素组成变化与结壳生长过程中的海洋环境气候相符合:富钴结壳生长前期δ 13C组成由正变负,对应全球气候变冷,南极底层流向太平洋挺进;而富钴结壳生长后期δ 13C逐渐偏正,与全球气候变暖和南极底层流萎缩相关联。 相似文献
17.
Distinctive light–dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO 3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light–dark cycles. Opal concentrations are independent of both TOC and CaCO 3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light–dark cycles. δ 13C org and δ 15N tot values do not vary across light–dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable δ 15N tot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles. 相似文献
18.
本文测定了海南儋州湾南岸柱状沉积物的粒度、总有机质参数(TOC、C/N和 δ13C)和类脂生物标志物含量,并通过端元混合模型使用红树植物特征标志物蒲公英萜醇含量、长链正构烷烃含量和δ13C值半定量区分了海南儋州湾南岸沉积有机质来源,尤其是红树林的贡献.另外,通过估算沉积物的有机碳储量来评估儋州湾红树林区域的储碳能力.在... 相似文献
19.
The speciation of sedimentary sulfur (pyrite, acid volatile sulfides (AVS), S 0, H 2S, and sulfate) was analyzed in surface sediments recovered at different water depths from the northwestern margin of the Black Sea. Additionally, dissolved and dithionite-extractable iron were quantified, and the sulfur isotope ratios in pyrite were measured. Sulfur and iron cycling in surface sediments of the northwestern part of the Black Sea is largely influenced by (1) organic matter supply to the sediment, (2) availability of reactive iron compounds and (3) oxygen concentrations in the bottom waters. Biologically active, accumulating sediments just in front of the river deltas were characterized by high AVS contents and a fast depletion of sulfate concentration with depth, most likely due to high sulfate reduction rates (SRR). The δ34S values of pyrite in these sediments were relatively heavy (−8‰ to −21‰ vs. V-CDT). On the central shelf, where benthic mineralization rates are lower, re-oxidation processes may become more important and result in pyrite extremely depleted in δ34S (−39‰ to −46‰ vs. V-CDT). A high variability in δ34S values of pyrite in sediments from the shelf-edge (−6‰ to −46‰ vs. V-CDT) reflects characteristic fluctuations in the oxygen concentrations of bottom waters or varying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resulting in low AVS concentrations and light δ34S values. Anoxic conditions in the bottom waters overlying shelf-edge sediments or periods of high accumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyrite isotopic composition (−37‰ to −39‰ vs. V-CDT) of sediments in the permanently anoxic deep sea (1494 m water depth) reflect the formation of pyrite in the upper part of the sulfidic water column and the anoxic surface sediment. The present study demonstrates that pyrite, which is extremely depleted in 34S, can be found in the Black Sea surface sediments that are positioned both above and below the chemocline, despite differences in biogeochemical and microbial controlling factors. 相似文献
20.
An important discovery during Ocean Drilling Program Leg 175, when investigating the record of upwelling off Namibia, was the finding of a distinct Late Pliocene diatom maximum spanning the lower half of the Matuyama reversed polarity chron (MDM, Matuyama Diatom Maximum) and centered around 2.6–2.0 Ma. This maximum was observed at all sites off southwestern Africa between 20°S and 30°S, and is most strongly represented in sediments of Site 1084, off Lüderitz, Namibia. The MDM is characterized by high biogenic opal content, high numbers of diatom valves, and a diatom flora rich in Southern Ocean representatives (with Thalassiothrix antarctica forming diatom mats) as well as coastal upwelling components. Before MDM time, diatoms are rare until ca. 3.6 Ma. After the MDM, in the Pleistocene, the composition of the diatom flora points to increased importance of coastal upwelling toward the present, but is accompanied by a general decrease in opal and diatom deposition. Here we present a simple conceptual model as a first step in formalizing a possible forcing mechanism responsible for the record of opal deposition in the upwelling system off Namibia. The model takes into account Southern Ocean oceanography, and a link with deepwater circulation and deepwater nutrient chemistry which, in turn, are coupled to the evolution of North Atlantic Deep Water (NADW). The model proposes that between the MDM and the Mid-Pleistocene climate revolution, opal deposition off Namibia is not directly tied to glacial–interglacial fluctuations (as seen in the global δ 18O record), but that, instead, a strong deepwater link exists with increased NADW production (as seen in the deepwater δ 13C record) accounting for higher supply of silicate to the thermocline waters that feed the upwelling process. The opal record of Site 1084 shows affinity to eccentricity on the 400-kyr scale but not for the 100-kyr scale. This points toward long-term geologic processes for delivery of silica to the ocean. 相似文献
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