Microfabric analysis of Mn-carbonate laminae deposition and Mn-sulfide formation in the Gotland Deep, Baltic Sea

The manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy-based techniques allows identification of small-scale sedimentary and geochemical features associated with Mn-carbonate laminae, which has significant implications for models of Mn-carbonate formation. Varves occurring in the Littorina sequence contain up to four laminae that may be placed in a seasonal cycle, and kutnahorite laminae occur within varves only as a winter-early spring deposit. This kutnahorite laminae seasonality is in agreement with the seasonal distribution of major Baltic inflow events recorded in historical records, and a direct causal link between inflows and kutnahorite deposition is implied. Benthic foraminifera tests are found to be heavily encrusted in kutnahorite, implying that benthic recolonization during oxidation events occurs concurrently with kutnahorite formation. The relatively common occurrence of small (50 to 100 μm) hexagonal γ-Mn-sulfide pseudomorphs, associated with 13% of kutnahorite laminae studied, is reported in Gotland Deep sediments for the first time. Although Mn-sulfide crystals are not usually preserved in the sediment, the discovery of Mn-sulfide pseudomorphs suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied.     

Early Neoproterozoic evolution of Southeast Pakistan: evidence from geochemistry,geochronology, and isotopic composition of the Nagarparkar Igneous Complex

ABSTRACT

Rocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean ²⁰⁶Pb/²³⁸U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon ε_Hf^(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants.     

A method for determination of the isotopic composition of authigenic uranium in Baikal bottom sediments

A simple, reliable, and high-performance method has been proposed for direct determination of the isotopic composition of authigenic uranium in silica lacustrine sediments. The method is based on studying the kinetics of the selective extraction of authigenic uranium from sediments with weak solutions of ammonium hydrocarbonate followed by the ICP-MS analysis of the nuclides. To estimate the contamination of authigenic uranium by terrigenous one, the contents of ²³²Th and some other clastogenic elements in the extracts were measured simultaneously. The selectivity of extraction of authigenic uranium from the sediments treated with a 1% NH₄HCO₃ solution appeared to be no worse than 99%. The method was used to analyze the isotopic composition of authigenic uranium at several key horizons of a core dated before. The measurements directly prove that the ²³⁴U/²³⁸U values in Baikal water varied depending on climate, which contradicts the previous statements. The measured ²³⁴U/²³⁸U ratios in paleo-Baikal water match the values reconstructed from isotopic data for total uranium in the sediments on the supposition that the U/Th ratio is constant in the terrigenous part of the sediment. Direct experimental determination of total and authigenic nuclides in sediments enhances the potentiality of the method for absolute ²³⁴U-²³⁰Th dating of carbonate-barren lacustrine sediments, including those from Lake Baikal, within the intervals corresponding to the periods of glaciation, where the sediments contain a large fraction of terrigenous component. Given the fractions of terrigenous and authigenic uranium are accurately determined, we have an opportunity to study the variability of the sources of terrigenous matter and to refine the previous model for reconstructing the climate humidity in East Siberia.     

Instability of seawater pH in the South China Sea during the mid-late Holocene: Evidence from boron isotopic composition of corals

We used positive thermal ionization mass spectrometry (PTIMS) to generate high precision δ¹¹B records in Porites corals of the mid-late Holocene from the South China Sea (SCS). The δ¹¹B values of the Holocene corals vary significantly, ranging from 22.2‰ to 25.5‰. The paleo-pH records of the SCS, reconstructed from the δ¹¹B data, were not stable as previously thought but show a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values. The latter is likely caused by the large amount of anthropogenic CO₂ emissions since the Industrial Revolution but variations of atmospheric pCO₂ cannot explain the pH change of the SCS before the Industrial Revolution. We suggest that variations of monsoon intensity during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene. Results of this study indicate that the impact of anthropogenic atmospheric CO₂ emissions may have reversed the natural pH trend in the SCS since the mid-Holocene. Such ocean pH records in the current interglacial period can help us better understand the physical and biological controls on ocean pH and possibly predict the long-term impact of climate change on future ocean acidification.     

The isotopic composition of oxygen and carbon in the subfossil mollusc shells of the Baltic Sea as an indicator of palaeosalinity

The carbon (δ¹³ C) and oxygen (δ¹⁸O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ¹⁸O values of water show a good correlation with the salinity of the Baltic. This correlation served as a basis for reconstructing palaeosalinity and for stratifying the marine sediments according to the δ¹⁸O values of the carbonate skeletons of subfossil shells. The δ¹³C values in shells are mainly determined by the isotopic composition of land-originating bicarbonate, especially in the carbonate skeleton of Lymnaea balthica , which inhabits the immediate coastal zone. According to the δ¹⁸O data, salinity in the investigated area (the coastal area of W and NW Estonia) was highest (about 9–11%) during the Littorina stage. The Limnae a stage had, in general, a salinity similar to the contemporary one, but during some phases possibly exceeding it by 2–3%.     

Early Holocene history of the southwestern Baltic Sea: the Ancylus Lake stage

One of the most discussed stages in the history of the Baltic Sea is the Ancylus Lake phase. This paper presents detailed information from the Darss Sill threshold area as well as the adjacent basins, i.e. the Mecklenburg Bay and Arkona Basin located in the southwesternmost Baltic. The threshold area was transgressed at the Baltic Ice Lake maximum phase and during the following regression about 10.3 ka BP a river valley was incised in the Darss Sill to a level of 23-24 m below present sea level (b.s.l.). Preboreal sediments in the study area show lowstand basin deposition in the Arkona Basin and the existence of a local lake in Mecklenburg Bay. The lowstand system is followed by the Ancylus Lake transgression that reached a maximum level of 19 m b.s.l. Thus, at the maximum level the water depth was about 5 m over the threshold, and the shore level fall during the Ancylus Lake regression must be in the same range. The Darss Sill area is the key area for drainage of the Ancylus Lake, and if the previously suggested regression of 8-10 m in southeastern Sweden is to be achieved, isostatic rebound must also play a role. The existence of the so-called Dana River in the Darss Sill area cannot be supported by our investigations. We observed no signs of progressive erosion of the Darss Sill area in the Early Holocene, and there are no prograding systems in Mecklenburg Bay that can be related to the Ancylus Lake regression. On the contrary, local lakes developed in Mecklenburg Bay and in the Darss Sill threshold area. In the Darss Sill area, marl was deposited in a lake in the valley that developed after the final drainage of the Baltic Ice Lake. Studies of diatoms and macrofossils, combined with seismic interpretation and radiocarbon dating, provide detailed information about the chronology and the relative shore level of these lake phases as well as about environmental conditions in the lakes.     

Darss Sill as a biological border in the fossil record of the Baltic Sea: evidence from diatoms

Biostratigraphical and palaeoecological analyses of cores along a transect from Femer Belt to the Arkona Basin reveal that North Sea waters began to enter the western Baltic Sea between 8600 and 8400 calibrated years BP. Studies of diatoms indicate that Mecklenburg Bay was characterised by slightly brackish-water conditions between 8400 and 8000 cal. years BP. At around 8000 cal. years BP increasing salinity is indicated by a strong dominance of the diatoms Paralia sulcata and Dimeregramma minor. Some centuries later another diatom assemblage appeared and became dominant in Mecklenburg Bay. This assemblage includes Hyalinella lateripunctata and Pravifusus hyalinus species typical of shallow water areas along the Atlantic coast today. At this time the first marine molluscs made their appearance. The oldest shell of a marine mollusc found in our material is dated to 7600 cal. years BP. The associated assemblage that includes adult specimens of the gastropod Aporrhais pespelicani indicates higher salinities than today.During the Littorina Sea stage a marine diatom flora with P. sulcata, Catenula adhaerens and D. minor crossed the Darss Sill and became widely distributed in the Arkona Basin, Pomeranian Bay and the Baltic Sea proper. In contrast, taxa indicative of the Hyalinella lateripunctata/P. hyalinus assemblage are only found west of the Darss Sill in Femer Belt and Mecklenburg Bay. Apparently, the Darss Sill threshold has been acting as an important salinity border from around 7800 cal. years BP until today.     

南海台西南区碳酸盐岩矿物学和稳定同位素组成特征——天然气水合物存在的主要证据之一

南海台西南区是中国南海中天然气水合物赋存的最有利场所。研究表明,该区的碳酸盐岩主要以结壳、烟囱的形式出现,结壳的裂隙或孔洞中常常充填有淡黄-白色的文石晶体。碳酸盐岩中自生碳酸盐矿物主要为文石、高镁方解石,少量白云石、铁白云石和菱铁矿。扫描电子显微镜(SEM)分析表明,文石主要呈针状、长柱状、放射束状,高镁方解石呈颗粒状。碳酸盐岩的碳同位素δ13C值主要在-56·878‰~-32·829‰PDB之间,大多数小于-40‰PDB,显示了生物甲烷成因碳源的特征;氧同位素δ18O值在2·1875‰~5·045‰PDB之间,主要在4‰PDB以上,这种较重的氧同位素比值表明,天然气水合物分解产生的富18O水体可能是碳酸盐岩沉淀的流体源。矿物学和碳氧稳定同位素研究表明,南海台西南区的碳酸盐岩为细菌性甲烷成因碳酸盐岩,可能与天然气水合物有关,显示了该区水合物存在的可能性很大。     

Origin of authigenic carbonates in sediment from the deep Bering Sea

Forty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high-magnesium calcite and protodolomite, less commonly siderite, rhodo-chrosite, low-magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty-five per cent of carbonate beds occurred below 400 m sub-bottom depth and 70% in sediment older than 4 m.y. δ¹³C values averaged -17.20^0/00 PDB and δ¹⁸O ranged from 18.59 to 34–11^0/00SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO₂ produced in concert with CH₄ during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3–5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627–1057 m sections at temperatures of 7–85°C. No apparent calcite precursor of biogenic origin was found for these authigenic carbonates.     

Dissolved Al in sediments and waters of the East China Sea: Implications for authigenic mineral formation

Numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans. In this study we show, using dissolved Al distributions in sediments and waters from the nearshore regions of the East China Sea, that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater. Statistical analyses of dissolved Al-Si-H⁺ relations in surface sediments indicate that the minerals forming in East China Sea sediments low in dissolved Fe are dioctahedral chlorites with an average composition EX_0.91Mg_0.77Al_5.0Si_2.7O₁₀(OH)₈ (where EX = exchangeable + 1 cation). This composition is also consistent with dissolved Al and Si measurements as a function of salinity in turbid overlying waters. Results suggest a dissolution—reprecipitation mechanism for clay mineral reconstitution. This mechanism can help to explain why different authigenic clays are found in different areas of the oceans. In the East China Sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments. Thus, the formation of these minerals has a relatively small impact upon dissolved Si distributions. Clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to pH, as the composition of minerals formed should be a direct function of the H⁺ activity in the surrounding environment.     

Carbon isotopic composition of fossil leaves from the Early Cretaceous sediments of western India

Stable carbon isotope analysis of fossil leaves from the Bhuj Formation, western India was carried out to infer the prevailing environmental conditions. Compression fossil leaves such as Pachypteris indica, Otozamite kachchhensis, Brachyphyllum royii and Dictyozamites sp. were recovered from three sedimentary successions of the Bhuj Formation, Early Cretaceous in age. A chronology was established based on faunal assemblage and palyno-stratigraphy and further constrained by carbon isotope stratigraphy. The three sampling sites were the Karawadi river bank near Dharesi; the Chawad river bank near Mathal; and the Pur river section near Trambau village in Gujarat. The Dharesi sample was also analyzed to investigate intra-leaf δ ¹³C variability. The mean δ ¹³C of the leaf was − 24.6 ± 0.4‰ which implied negligible systematic change along the leaf axis. The Mathal sample was fragmented in nature and showed considerable variation in carbon isotopic composition. The Trambau sample considered to be the oldest, dating to the middle of Aptian (ca. 116 Ma), shows the most depleted value in δ ¹³C among all of them. The overall δ ¹³C trend ranging from mid Aptian (ca. 116 Ma) to early Albian (ca. 110 Ma) shows a progressive increase in δ ¹³C from −26.8 to −20.5‰. Based on these measurements the carbon isotopic composition of atmospheric carbon dioxide of the Aptian–Albian period is estimated to be between −7.4 and −1.7‰. The ratio of the partial pressure of carbon dioxide in leaf to that of the ambient atmosphere calculated based on a model is estimated to be similar to that of the modern plants. This indicates that the Early-Cretaceous plants adapted to the prevailing high carbon dioxide regime by increasing their photosynthetic uptake.     

南海珊瑚礁硼同位素组成及其环境意义

采用正热电离质谱方法测定了中国南海诸岛７０００ａ以来的珊瑚礁的硼同位素的硼含量。讨论了珊瑚礁中硼同位素 硼含量,Ｐ不管不顾忻代等参数的关系。结果表明,所测定的珊瑚礁硼同位素组成变化范围为２２．７￣２４．８‰,并且与珊瑚 硼含量呈正相关关系。根据硼同位素与海水ＰＨ值的关系计算出过去７０００ａ南海泊ＰＨ值变化８．１０￣８．４１。初步探讨了硼同位素组成与南海海平面变化的关系。     

成岩过程中碳酸盐-二氧化碳平衡体系的热力学模拟

碳酸盐矿物的溶解与沉淀在沉积岩成岩作用中具有非常重要的意义,在与之有关的地层流体中,CO_2组分受到了广泛关注.本文选取CaCO_3(方解石)-H_2O-CO_2与CaMg(CO_3)2(白云石)-H_2O-CO_2体系作为研究对象,利用热力学方法在假设体系达到平衡的前提下对100～7000m深度范围的温度压力条件下的CaCO_3-H_2O-CO_2与CaMg(CO_3)2-H_2O-CO_2体系中各组分的平衡浓度([H~+]、[Ca~(2+)]、[Mg~(2+)]……)进行了数值模拟,并对不同埋藏深度的温度压力下方解石、白云石的溶解与沉淀,流体对CO_2的吸收与释放,pH变化等相关地质过程进行了讨论.研究表明:随埋深增加,体系的pH值越来越趋于酸性,但碳酸盐并没有持续溶解,而是呈先溶解后沉淀的趋势.对比CaCO_3-H_2O-CO_2体系中碳酸盐的溶解量和沉淀量,可将小于700m的深度范围定义为碳酸盐溶解带,700～2500m的深度范围定义为补偿带,大于2500m的深度范围定义为沉淀带.对比两个体系的固体相体积变化发现,含CO_2的溶蚀性流体虽然不能在白云岩地层中溶出较灰岩地层更多的阳离子,但会产生更高的孔隙度.     

Geological structure and composition of the Curonian Spit (Baltic Sea)

The paper presents new data on geology of the Curonian Spit based on results of the study of data on the engineering-geological transect compiled during the construction of gas pipe in the Russian sector of the spit. The data show that, along with eolian sand, peat deposits recovered in both its Russian and Lithuanian sectors make up a significant portion of the spit (data on the Lithuanian sector are adopted from the available literature). Based on new data, existing concepts on geology, paleogeography, geoecology, and stability of the Curonian Spit are discussed and refined.     

Factors influencing methane-derived authigenic carbonate formation at cold seep from southwestern Dongsha area in the northern South China Sea

This paper carried out mineral and geochemical studies on a profile through a diagenetic methane-derived authigenic carbonate sample that was collected from southwestern Dongsha area of the northern South China Sea. Five samples locating in the cross-sectional middle mainly consist of dolomite and quartz, and two samples close to the surface have a small amount of Mg-calcite. The δ¹³C values of the samples vary between ?30.59 and ?0.30 % VPDB, with δ¹⁸O values ranging from 3.07 to 3.59 % VPDB, δ^44/40Ca values ranging from 1.35 to 1.47 % SRM915a, indicating a contribution of methane to the carbon pool where the precipitation of authigenic carbonates occurred. Based on the isotope values alone, it can not be distinguished if the carbon source is thermogenic gas or a mixture of biogenic methane and marine dissolved inorganic carbon. The δ¹⁸O values are in general consistent with dolomite precipitation from a fluid similar to present seawater. The observed small variation might be related to the oxygen isotope composition of seep fluid. The relative small range in calcium isotope values suggests that relatively constant growth conditions and precipitation from seawater. The central part of the carbonate nodule formed under the strong influence of methane seepage, and the external part is less influenced by methane, either due to reduced methane flux to the surface or caused by erosional exhumation of the carbonate nodule from greater depth to the sediment surface.     

Influence of processes of the formation of authigenic clay minerals of terrigenous rocks on reservoir properties: Evidence from the rocks of the Vartov Arch

Filtration-capacity properties of terrigenous reservoir rocks significantly depend on catagenetic transformations of clay minerals. Results of our research revealed the authigenic nature and formation stages of kaolinite and chlorite. The volume and shape of pore space define the morphology of clay particles. Depending on the pore volume, kaolinite crystallizes either as thick tabular aggregates or as fan-shaped intergrowths. The formation of authigenic clays in pores of terrigenous reservoirs decreases filtration-capacity properties of rocks with low permeability and, conversely, increases filtration-capacity properties of rocks with high primary permeability and large pore volume.     

Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage

Deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques. Rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets. A variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating Amazon delta sediments indicates that both of these general assumptions are unjustified. Apparent lack of significant biogenic silica storage in deltaic environments, particularly in the tropics, may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica. Biogenic silica particles buried in suboxic Amazon delta deposits can be unaltered, partially dissolved, covered with aluminosilicate or metal-rich coatings, or completely reconstituted into authigenic K-Fe-rich aluminosilicate minerals. Pore water (K, Mg, F, Si) and solid-phase distributions, direct observations of particles, laboratory experiments, and depositional context indicate that authigenic clays form rapidly (<1 yr) in the seasonally reworked surface layer (∼ 0.5-2 m) of the delta topset and are disseminated during sediment remobilization. Fe, Al-oxide rich debris derived from the tropical drainage basin is an abundant reactant, and thus the supply of biogenic silica is a major control on the amount of clay formed.The mild 1% Na₂CO₃ alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0.1N HCl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution. Well-crystallized clays are not significantly affected by this modification nor is bulk Amazon River bed sediment. The two-step procedure indicates that ∼90% of the biogenic silica originally present in deposits is converted to clay or otherwise altered, raising the effective quantity of biogenic silica stored from ∼33 to ∼296 μmol Si g⁻¹ (∼1.8% SiO₂). Biogenic Si stored in the delta increases away from the river mouth, across shelf and along the dispersal system where primary production is highest. The K/Si ratio of labile authigenic material is ∼0.19 mol mol⁻¹, far higher than Amazon River suspended matter (∼0.07 mol mol⁻¹). Diagenetic models indicate formation rates in the mobile sediment layer of ∼2.8 μmol K g⁻¹ yr⁻¹ (∼16 μmol Si g⁻¹ yr⁻¹). Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial increases the deltaic storage of riverine Si to ∼22% of the Amazon River input. The rapid formation of aluminosilicates from biogenic SiO₂, seawater solutes, and remobilized Fe, Al-oxides represents a form of reverse weathering. Rapid reverse weathering reactions in tropical muds and deltaic deposits, the largest sediment depocenters on Earth, confirms the general importance of these processes in oceanic elemental cycles.     

Two-stage contamination during crustal assimilation: isotopic evidence from volcanic rocks in eastern Nevada

Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclaserich andesites and dacites have greater Sr and ⁸⁷Sr/⁸⁶Sr, but less Nd and lower _Nd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two bites. The separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of ⁸⁷Sr/ ⁸⁶Sr versus _Nd that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.     

从UB-2孔硅藻记录看末次冰期至早全新世日本海古海洋环境

日本海南部郁陵海盆的UB-2孔沉积物硅藻记录,反映了11000—28000aBP该区古海洋和古气候的变化。对比日本海和格陵兰冰芯(GRIP)δ18O数据,UB-2孔硅藻暖水种比率及淡水—海滨种含量变化清楚地记录了氧同位素3期(MIS3)、末次盛冰期(LGM)、波令—阿勒罗德暖期(B/A)以及新仙女木冷事件(YD)等古气候事件。淡水—海滨种硅藻含量在末次盛冰期阶段明显升高及暖水种数量的显著减少,表明此时日本海为低温、低盐的古海洋环境,这可归因于气候变冷、海平面下降造成的日本海当时相对封闭的海洋环境。自15200aBP起,Paraliasulcata含量逐渐上升,这可能与海平面上升引起的古环境变化有关,可以作为对马海峡开启的标志。全球气候变化引起的海平面变化是该区域古气候变化的主要控制因素。