全新世冲绳海槽北部表层海水温度和初级生产力对黑潮变迁的响应

在冲绳海槽北部B-3GC孔高分辨率地层年代框架的基础上,利用多种分子生物标志物和生源组分指标,重建了该地区全新世以来表层海水温度和生产力的变化历史.研究发现,U37K-表层海水温度和生产力的变化指示了全新世黑潮及对马暖流分支的变迁.10.6～7.3 ka BP期间,冲绳海槽北部主要受东海陆架冷水团影响,陆源物质输入量增加,初级生产力比较高,表层海水温度较低.7.3～4.6 ka BP期间,黑潮暖流对该区的影响迅速增强,表层海水快速增温,陆源物质输入减少,但是各种生产力指标显示出上升趋势,与该区上升流的出现和迅速发育有关.4.6～2.7 ka BP期间,即全新世普林虫低值事件(PME)期间,表层海水温度明显降低了0.8～1.4℃,陆源物质输入增加,初级生产力和各种生源组分的含量也达到较高水平,说明此时黑潮强度减弱,或者黑潮主流轴向太平洋方向发生偏移,该区再次受到陆架冲淡水的影响.2.7 ka BP以来,虽然黑潮暖流再次影响到该海区,但初级生产力和生源物质输出量继续大幅增高,可能与该地区上升流的持续发育有关.     

巽他海峡BAR9442岩心30 kaBP以来的沉积记录与古季风活动

为重建巽他海峡区古季风活动历史,通过对BAR9442岩心氧同位素、AMS14C测年、陆源碎屑标准偏差区间粒径分布,以及上升流种Globigerina bulloides等变化特点分析,并结合前人对该岩心粘土矿物的研究,获得如下认识:末次冰期30~17 kaBP,海平面下降,陆源粉砂物质输入增加; 南半球夏季日照率高,然而受冰期热带辐聚带位置影响,西北季风受阻,东南信风持续吹过研究海域形成类似东南季风的效应,气候冷干; 受印度东北季风影响,伊利石含量最高,上升流活动不发育, 古生产力降低.17~8 kaBP,海平面迅速上升,出现较强的表层穿越流活动,粘土物质成为沉积物的主要组分; 此时东南与西北季风都有增强,说明当时研究区气候有着更强的季节性.全新世8 kaBP以来,全球海平面上升使穿越流活动加强,沉积物粘土组分含量最大,西北季风带来温暖湿润的气候条件.      

Fluxes of material in the Arabian Sea and Bay of Bengal — Sediment trap studies

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.     

Glacial–interglacial productivity and environmental changes in Lake Biwa,Japan: A sediment core study of organic carbon,chlorins and biomarkers

Temporal changes in paleoproductivity of Lake Biwa (Japan) over the past 32 kyr have been studied by analyzing bulk organic carbon and photosynthetic pigments (chlorins) in the BIW95-5 core. Primary productivity was estimated on the assumption of C/N_org values of 8 for autochthonous organic matter (OM) and 25 for allochthonous OM and using an equation developed for the marine environment. The estimate indicates that primary productivity ranges from 50 to 90 g C m^?2 yr^?1 in the Holocene, while it is ～60 g C m^?2 yr^?1 on average in the last glacial. Pheophytin a and pheophorbide a are the major chlorins. A downcore profile of chlorin concentration normalized to autochthonous organic carbon (OC) shows a decreasing trend. Chlorin productivity was corrected by removal of the effect of post-burial chlorin degradation. The temporal profile of chlorin productivity thereby obtained resembles that from autochthonous OC.The difference in primary productivity between the Holocene and the glacial for the lake is markedly smaller than that for Lake Baikal situated in the boreal zone. This difference between the two lakes is probably caused by the difference in their climatic conditions, such as temperature and precipitation. Precipitation at Lake Biwa is relatively large during the glacial and the Holocene because of the continuous influence of the East Asian monsoon. Lake Baikal precipitation is generally small as a result of control by the continental (Siberia) climate regime. In addition, a significant difference in productivity between the glacial and the Holocene for Lake Baikal may be essentially controlled by the hydrodynamic systems in the lake.Lake Biwa terrigenous OM input events occurred at least five times over the period 11–32 kyr BP, suggesting enhanced monsoon activity. Molecular examination of the layer with a large input of terrigenous OM during the Younger Dryas indicates that concentrations of terrigenous biomarkers such as n-C₂₇–C₃₁ alkanes, lignin phenols, cutin acids, ω-hydroxy acids and C₂₉ sterols are high, suggesting that soil OM with peat-like material entered the lake as a result of flooding. An enhanced sedimentation rate in the last 3000 years might have been partially caused by agricultural activity around the lake.     

Paleoclimate of Southwestern China for the Past 50,000 yr Inferred from Lake Sediment Records

Long sediment cores (12.5 and 13.5 m) from two lakes in Yunnan Province were used to infer the paleoclimate of southwest China over the past 50,000 yr. During the Holocene and marine isotope stage (MIS 3), bio-induced carbonate precipitation and organic matter (OM) production was high, suggesting warm temperatures and high primary productivity. In contrast, sediment inorganic carbon (IC) and organic carbon (OC) concentrations were low in last glacial deposits from 38,000 to 12,000 cal yr B.P., indicating cool temperatures and low productivity. The 50,000-yr record has alternating peaks of carbonate and coarse-grain (>38 μm) quartz that reflect warm, moist interglacial or interstadial conditions alternating with cold, dry glacial or stadial conditions, respectively. Spectral analysis of the carbonate and quartz signals reveals power concentrated at periods of 7200 and 8900 cal yr, respectively, that may reflect a nonlinear climate response to precessional forcing at a time of reduced eccentricity modulation (McIntyre and Molfino, 1996). Oxygen isotope values of calcite from Yunnan lake cores indicate the summer monsoon was weak during the last glaciation from 50,000 to 12,000 cal yr B.P. The summer monsoon intensified between 12,000 and 8000 cal yr B.P., but weakened gradually in response to insolation forcing during the mid-to-late Holocene. Our results support the Overpeck et al. (1996) model that posits a weak summer monsoon during the last glaciation that responded nonlinearly to insolation forcing when its intensity was affected by Eurasian snow cover and ice-sheet extent. The summer monsoon intensified and responded linearly to seasonal insolation forcing in the Holocene when ice volume diminished.     

Millennial-scale dynamics of the winter cold tongue in the southern South China Sea over the past 26 ka and the East Asian winter monsoon

Millennial-scale variations of the East Asian winter monsoon (EAWM) remain elusive due to sparse and controversial reconstructions. By compiling a variety of alkenone-based sea surface temperature (SST) estimates, we find that the west-east SST gradient in the southern South China Sea (SCS) well documents the temporal dynamics of the winter “cold tongue” off the southern Vietnam and by inference, variations in the EAWM intensity over the past 26 ka. Our results reveal that the winter “cold tongue” SSTs were significantly colder during Heinrich event 1 and the Younger Dryas event, resulting in an increased west-east SST gradient in the southern SCS due to a strengthened EAWM. Within dating uncertainties, an intensified EAWM during cold stadials was coeval with the shutdown or a reduction in strength of the Atlantic meridional overturning circulation (AMOC), exhibiting a strong linkage between the AMOC and the EAWM system. The west-east SST gradient also indicates an enhanced EAWM during the early Holocene, which may be induced by postglacial ice-sheet dynamics and a strong seasonal contrast in solar insolation. Our findings suggest that the EAWM was probably modulated by a complex interplay between the AMOC, solar insolation and ice-sheet dynamics on sub-orbital time scales.     

Geochemical expressions of anoxic conditions in Nordåsvannet,a land-locked fjord in western Norway

The Nordåsvannet fjord in western Norway is a modern semi-enclosed basin suitable for studying sedimentary cycles as they occur under anoxic bottom conditions. It is characterized by strongly anoxic conditions in the water column and bottom sediments. Diagenetic pyrite formation occurs in the sediments, and syngenetic pyrite is formed in the lower water column. Organic matter burial in the fjord exceeds that of other environments with normal marine or upwelling conditions. This is due to the better preservation of organic matter. Organic matter composition appears to have changed over time with higher fractions of terrigenous organic matter being present in the most recent sediments. This may be a result of increased input of terrigenous organic matter, possibly due to sewage supply to the fjord over the last decades. Organic C and CaCO₃ contents of the sediments do not appear to reflect a productivity signal. Calcium carbonate content is influenced by chemogenic calcite formation. Biogenic opal content appears to reflect a productivity signal, but different degrees of dissolution may obscure its clear recognition.     

Sea-Surface Temperatures and the History of Monsoon Upwelling in the Northwest Arabian Sea during the Last 500,000 Years

Arabian Sea sediments record changes in the upwelling system off Arabia, which is driven by the monsoon circulation system over the NW Indian Ocean. In accordance with climate models, and differing from other large upwelling areas of the tropical ocean, a 500,000-yr record of productivity at ODP Site 723 shows consistently stronger upwelling during interglaciations than during glaciations. Sea-surface temperatures (SSTs) reconstructed from the alkenone unsaturation index (U^K′₃₇) are high (up to 27°C) during interglaciations and low (22-24°C) during glaciations, indicating a glacial-interglacial temperature change of >3°C in spite of the dampening effect of enhanced or weakened upwelling. The increased productivity is attributed to stronger monsoon winds during interglacial times relative to glacial times, whereas the difference in SSTs must be unrelated to upwelling and to the summer monsoon intensity. The winter (NE) monsoon was more effective in cooling the Arabian Sea during glaciations then it is now.     

Variability of East Asian Winter Monsoon in Quaternary Climatic Extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.     

Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)

Phosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO₄²⁻ concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (∼900 to 2800 vs. ∼70 to 120 μmol m⁻² yr⁻¹). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO₄²⁻ from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO₄²⁻ from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (∼7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO₄²⁻ concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation.     

Paläoklima und paläoozeanische Verhältnisse im SW-Pazifik während der letzten 6 Millionen Jahre (DSDP - Site 594, Chatham Rücken, östlich Neuseeland)

Detailed sedimentological investigations were performed on sediments from DSDP-Site 594 (Chatham Rise, east of New Zealand) in order to reconstruct the evolution of paleoclimate and paleoceanographic conditions in the Southwest Pacific during the last 6 million years. The results can be summarized as follows:

1. High accumulation rates of biogenic opal and carbonate and the dominance of smectites in the clay fraction suggest increased oceanic productivity and an equable dominantly humid climate during the late Miocene.
2. During Pliocene times, decreasing contents of smectites and increasing feldspar/quartz ratios point to an aridification in the source area of the terrigenous sediments, culmunating near 2.5 Ma. At that time, accumulation rates of terrigenous components distinctly increased probably caused by increased sediment supply due to intensified atmospheric and oceanic circulation, lowered sea level, and decreased vegetation cover.
3. A hiatus (1.45 to 0.73 Ma) suggests intensified intermediate-water circulation.
4. Major glacial/interglacial cycles characterize the upper 0.73 Ma. During glacial times, oceanic productivity and terrigenous sediment supply was distinctly increased because of intensified atmospheric and oceanic circulations and lowered sea level, whereas during interglacials productivity and terrigenous sediment supply were reduced.
5. An increased content of amphibols in the sediments of Site 594 indicates increased volcanic activities during the last 4.25 Ma.

     

Organic Matter Accumulation in the Upper Permian Dalong Formation from the Lower Yangtze Region, South China

The Late Permian was marked by a series of important geological events and widespread organic-rich black shale depositions, acting as important unconventional hydrocarbon source rocks. However, the mechanism of organic matter (OM) enrichment throughout this period is still controversial. Based on geochemical data, the marine redox conditions, paleogeographic and hydrographic environment, primary productivity, volcanism, and terrigenous input during the Late Permian in the Lower Yangtze region have been studied from the Putaoling section, Chaohu, to provide new insights into OM accumulation. Five Phases are distinguished based on the TOC and environmental variations. In Phase I, anoxic conditions driven by water restriction enhanced OM preservation. In Phase II, euxinic and cycling hydrological environments were the two most substantial controlling factors for the massive OM deposition. During Phase III, intensified terrestrial input potentially diluted the OM in sediment and the presence of oxygen in bottom water weakened the preservation condition. Phase IV was characterized by a relatively higher abundance of mercury (Hg) and TOC (peak at 16.98 wt%), indicating that enhanced volcanism potentially stimulated higher productivity and a euxinic environment. In Phase V, extremely lean OM was preserved as a result of terrestrial dilutions and decreasing primary productivity. Phases I, II and IV are characterized as the most prominent OM-rich zones due to the effective interactions of the controlling factors, namely paleogeographic, hydrographic environment, volcanism, and redox conditions.     

Major changes in East Asian climate in the mid-Pliocene: Triggered by the uplift of the Tibetan Plateau or global cooling?

Since the mid-Pliocene, East Asian climates have experienced significant changes. One view suggests that significant uplift of the Tibetan Plateau during this period could have been responsible for these dramatic changes in the strength of the East Asian monsoon and for Asian interior aridification, while some other authors attribute these changes to the ongoing global cooling and rapid growth of the Arctic ice-sheet. Up to the present, which factor dominates the major changes of East Asian climate in the mid-Pliocene is still a contentious issue. This study presents an analysis of several climate proxies including grain-size, (CaO^* + Na₂O + MgO)/TiO₂ ratio, Na/Ka ratio and dust accumulation rates of the Xifeng Red Clay sequence in the eastern Chinese Loess Plateau and the Xihe Pliocene loess-soil sequence in West Qinling. They reveal that aridity in the continental interior and winter monsoon circulation both intensified, whereas the East Asian summer monsoon showed a weakening rather than intensifying trend since the mid-Pliocene. These changes are also supported by the other multi-proxy records from various regions in East Asia. Previous numerical modeling studies have demonstrated that uplift of the Tibetan Plateau would have simultaneously enhanced continental-scale summer and winter monsoon strength as well as central Asian aridity. The mid-Pliocene climate changes in East Asia are therefore unlikely to be a response to Plateau uplift. On the contrary, our recent modeling results give support to the view that ongoing cooling could have intensified both the aridity of the interior and the strength of the winter monsoon, but weakened the summer monsoon in East Asia.     

Geochemical Record of the Cenomanian–Turonian Anoxic Event in Tunisia: Is it Correlative and Isochronous to the Biotic Signal?

In Tunisia, five Bahloul spaced sections, Bargou, Jerisa, Guern Halfaya, Kherij and Gafsa were analyzed for biostratigraphy (foraminifera and radiolarians) and major and trace elements. This high-resolution biostratigraphic and chemostratigraphic integrated analyses for the Late Cenomanian–Early Turonian Bahloul Formation provide new insight into the palaeoceanographic evolution of the southern Tethyan margin. Relative low abundance of related terrigenous Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba, Cu, and Ni (organic matter related trace elements) suggests that the Bahloul, deposited during a relatively short period (0.5 Ma), was of relatively elevated primary productivity and minimal detrital input. While higher D* values concurrent with lower Ti/Al ratios are interpreted as caused by enhanced fluvial material contribution, due to more humid climate during the OAE-2. Enhanced humidity triggered probably fluvial influxes, resulted in a sluggish water circulation and consequent anoxic/euxinic conditions favoring the preservation of organic matter at the bottom. Enrichments in redox-sensitive trace metals U, V, and Mo in the Bahloul Formation deposits and redox indices, such as V/(V+Ni), U/Th, V/Cr, and Ni/Co, indicate that oxygen-restricted conditions prevailed during the Late Cenomanian to earliest Turonian times and correlate well with relative abundances of some foraminiferal and radiolarian paleo-environmental relevant indicators. High Baxs values and Uauth may indicate anoxic conditions at least at the water–sediment interface during the Bahloul Formation deposition and provide information about low to moderate sulfate-reduction reactions.     

Late Quaternary changes in surface productivity and oxygen minimum zone (OMZ) in the northwestern Arabian Sea: Micropaleontologic and sedimentary record at ODP site 728A

Changes in the abundance of selected planktic foraminiferal species and some sedimentological parameters at ODP site 728A were examined to understand the fluctuations in the surface productivity and deep sea oxygenation in the NW Arabian Sea during last ∼540 kyr. The increased relative abundances of high fertility taxa, i.e., Globigerinita glutinata and Globigerina bulloides mainly during interglacial intervals indicate intense upwelling. Strong SW summer monsoon probably increased the upwelling in the western Arabian Sea during interglacial intervals and caused high surface productivities due to the lateral transport of eutrophic waters. Most of the glacial periods (i.e., MIS 2, 4, 6, 8 and 12) are characterized by higher relative abundances of Neogloboquadrina pachyderma and Neogloboquadrina dutertrei associated with Globigerinoides ruber. The more stratified condition and deep mixed layer due to increased NE winter monsoon are mainly responsible for the higher relative abundances of N. pachyderma during glacial periods. Some of the glacial intervals (i.e., MIS 6 and 8) are also characterized by pteropod spikes reflecting deepening of aragonite compensation depth (ACD) and relatively less intense oxygen minimum zone (OMZ) in this region due to deep sea mixing and thermocline ventilation, and relatively less intense surface productivity during winter monsoon. The interglacial periods are largely devoid of pteropod shells indicating more aragonite dissolution due to increased intensity of OMZ in the northwestern Arabian Sea.     

Enhanced Benthic Nutrient Flux During Monsoon Periods in a Coastal Lake Formed by Tideland Reclamation

Sediment and water quality were investigated in an artificial coastal lake (Saemangeum Lake, Korea) that was formed by constructing a 33-km long sea-dyke offshore from the mouths of two adjacent rivers, which discharge into the Yellow Sea. Sediment showed drastic increases in fine fraction (silt and clay) after the dyke construction. TN, TP, and OC contents in the sediment showed the similar spatial variation to that of fine fraction. A mixing model indicated benthic fluxes of nutrients such as PO₄, NH₄, and SiO₂, which were considerably elevated during the summer monsoon season. It is revealed that this phenomenon was associated with the development of strong salinity stratification, elevated water temperature, and increased groundwater discharge. However, a change in the sedimentation environment due to dyke construction is suggested as the primary reason for the enhanced benthic fluxes.     

Biomarker seasonality study in Lake Van, Turkey

The endorheic Lake Van in eastern Anatolia (Turkey) is the world’s largest soda lake and it is an important site in paleoclimate studies to understand past continental conditions in western Asia. In order to gain further insights into the biomarker signatures in Lake Van’s sediments we have analyzed particulate material in sediment traps deployed between August 2006 and July 2007. The biomarkers used were long chain alkenones (LCAs C₃₇-C₃₉, haptophyte lipids), isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs, Archaea membrane lipids) and pigments (chlorins and fucoxanthin). The biomarker fluxes indicate a strong seasonality in export primary productivity and the phytoplankton community structure. The highest total mass and organic carbon fluxes were found in summer, coupled to strong stratification while the lowest mass fluxes occurred in winter at the time of water column mixing. With increasing temperatures in early spring, phytoplankton export productivity grew, coupled with an increase of total mass flux and organic carbon, which might be associated to enhanced nutrient input from snowmelt runoff. The percentage of C_37:4 shows some correspondence with observed seasonal changes in Lake Van’s stratification structure. We also evaluated the potential applicability of molecular temperature proxies derived from Archaea and haptophyte lipids. The use of the TEX₈₆ proxy was precluded by low GDGT abundances. Estimated LCA temperatures were consistent with temperatures in the photic zone but no seasonality changes were observed despite the wide annual temperature range measured at Lake Van.     

末次冰期以来南海北部物源及古环境变化的有机地球化学记录

文章在AMS～14C定年的基础上建立了南海北部MD05-2905钻孔（20°08.17′N,117°21.61′E）末次冰期以来有机地球化学记录的时间序列,利用其中的正构烷烃、甾醇和烯酮等有机分子标志物对南海北坡25ka以来沉积物物源、气候环境变化进行探讨。长链正构烷烃总含量和浮游植物标志物总含量表明25.0～14.5kaB.P.时段南海北部沉积物中陆源物质输入、海洋初级生产力浮游植物输入均较高,14.5～8.5kaB.P.时段迅速降低,全新世8.5kaB.P.之后稳定在较低水平。末次冰期海平面低,陆架出露面积大,陆源物质搬运至沉积地点的距离短,强盛的冬季风及其驱动的洋流有利于大量的陆源物质搬运至该沉积地点;14.5kaB.P.,对应于MWPIa时期,陆源物质输入迅速减少,海洋初级生产力的迅速降低,印证了此时海平面的快速上升。C31/C17物源参数表明陆源物质输入和海洋内生源输入在25.0～14.5kaB.P.时段和14.5～8.5kaB.P.时段分别占据了主要地位,这表明海平面变化对陆源物质输入影响可能更大;全新世8.5ka之后,陆源、海洋内生源输入交替占据主要地位,这可能与东亚季风增强有关。从有机分子标志物研究来看,海平面的变化对陆源物质输入及总海洋初级生产力可能均有重要控制作用,但海洋初级生产力中某些藻类如硅藻可能受控于不同的气候因素。C28甾醇含量指示硅藻在MWPIa和MWPIb时期大量增多,这可能是14.5～8.5kaB.P.海洋内生源输入相对较多的重要原因,同时,对11.0～8.5kaB.P.时段的碳酸钙低值事件可能有一定贡献,但其对海陆环境的响应还有待于进一步研究。     

Reply to D. F. Lascelles. Discussion on ‘Microplaty hematite—its varied nature and genesis’ by R. C. Morris and ‘Genesis modelling for the Hamersley BIF-hosted iron ores of Western Australia: a critical review’ by R. C. Morris and M. Kneeshaw

Abstract

The third (Es₃) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es₃ (Es₃^L) and upper Es₃ (Es₃^U) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es₃^L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es₃^U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es₃^L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es₃^U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es₃^U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es₃^L and Es₃^U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.     

南海北部1987～1988年颗粒物质和硅藻通量的季节性变化:季风气候与 El Nino的响应

南海北部1987年9月～1988年10月沉积物捕获器中颗粒物质和硅藻通量的季节性变化受到季风气候的控制.颗粒物质与硅藻在东北和西南季风盛行期增加,在季风转变期减少.浅层和深层的颗粒总通量、蛋白石通量、碳酸钙通量、蛋白石/颗粒总通量比值、碳酸钙/颗粒总通量比值、有机碳/磷的比值以及浅层硅藻Thalassionema nitzschioides、Coscinodiscus excentricus、Coscinodiscus nodulifer、Nitzschia marina和Rhizosolenia bergonii的通量在东北季风期间明显地增加了,这些变化可能与1987年～1988年发生的EINino事件相关.