Carbon and oxygen isotope characteristics of foraminiferan from northern South China Sea sediments and their significance to late Quaternary hydrate decomposition

Carbon and oxygen isotope and dating analyses of foraminiferan in sediment cores collected from three different areas of the northern slope of the South China Sea were conducted,in order to examine the records of the gas hydrate decomposition events since the late Quaternary under the conditions of methane seepage.The results show that:1) the δ13C values of the benthic foraminiferan Uvigerina spp.(size range of 0.25-0.35 mm) are from-0.212% to-0.021% and the δ18O values of the planktonic foraminiferan Globigerinoides ruber(size range of 0.25-0.35 mm) are from-0.311% to-0.060%;2) three cores(ZD2,ZD3 and ZS5) from the bottom of a hole are aged for 11 814,26 616 and 64 090 a corresponding to the early oxygen isotope stage(MIS) I,III and IV final period,respectively;3) a negative-skewed layer of carbon isotope corresponds to that of MIS II(cold period),whose degree of negative bias is-0.2‰;and 4) the δ13C compositions of foraminiferans are similar to those of the Blake Ridge and the Gulf of Mexico sediments of the late Quaternary.According to the analysis,the reasons for these results are that the studied area is a typical area of methane seep environment in the area during MIS II due to the global sea-level fall and sea pressure decrease.Gas hydrate is decomposed and released,and a large number of light carbon isotopes of methane are released into the ocean,dissolved to inorganic carbon(DIC) pool and recorded in the foraminiferan shells.A pyrite layer developed in the negative bias layers of the foraminiferans confirms that the δ13C of foraminiferans is more affected by methane and less by the reduction of marine productivity and early diagenesis.The use of foraminiferan δ13C could accurately determine late Quaternary hydrate release events and provide evidence for both reconstructing the geological history of methane release events and exploring natural gas hydrate.

The stable isotope geochemical characteristics of dissolved inorganic carbon in northern South China Sea

The isotopic composition of dissolved inorganic carbon (DIC) in marine and estuarine environments has been studied in some detail because of its role in determining the isotopic composition of inorganic/organic matter and its applications to the study of various natural processes. The δ13CDIC values of sea water are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. In this paper, based on the description of the stable isotope geochemical characteristics of dissolved inorganic carbon in northern South China Sea, the following conclusions were drawn: 1) the δ13CDIC values of natural waters are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. And the low seawater δ13CDIC values of northern South China Sea are accompanied by a low level of sea primary productivity in this region; 2) according to the parameter relationship (δ13CDIC-1.1 PO4) between δ13CDIC and PO4, it is indicated that northern South China Sea is the source of atmospheric CO2; and 3) nitrogen (N) and phosphorus (P) are the elements which are the basis for phytoplankton growth. Based on the analysis of seawater δ13CDIC values and PO4 and NO3+NO2 concentrations, our data show that N is a limiting nutrient in northern South China Sea.

Stratal Carbonate Content Inversion Using Seismic Data and Its Applications to the Northern South China Sea

On the basis of the relationship between the carbonate content and the stratal velocity and density, an exercise has been attempted using an artificial neural network on high-resolution seismic data for inversion of carbonate content with limited well measarements as a control. The method was applied to the slope area of the northern South China Sea near ODP Sites 1146 and 1148, and the results are satisfaetory. Before inversion calculation, a stepwise regression method was applied to obtain six properties related most closely to the carbonate content variations among the various properties on the seismic profiles across or near the wells. These include the average frequency, the integrated absolute amplitude, the dominant frequency, the reflection time, the derivative instantaneous amplitude, and the instantaneous frequency. The results, with carbonate content errors of mostly ±5 % relative to those measured from sediment samples, show a relatively accurate picture of carbonate distribution along the slope profile. This method pioneers a new quantitative model to acquire carbonate content variations directly from high-resolution seismic data. It will provide a new approach toward obtaining substitutive high-resolution sediment data for earth system studies related to basin evolution, especially in discussing the coupling between regional sedimentation and climate change.

Comparisons of surface Chl a and primary productivity along three transects of the southern South China Sea, northern Java Sea and eastern Indian Ocean in April 2011

Results are presented about the changes in chlorophyll a density, carbon fixation and nutrient levels in the surfacewaters of three transects of the southern South China Sea （SCS）, northern Java Sea （JS） and eastern Indian Ocean （IO） duringApril 5-16 of 2011. The in situ Chl a concentration and carbon fixation showed decreasing trends from high to low latitudealong the three transects, while the photosynthetic rate of phytoplankton estimated from 14C incorporation displayed no simplevariation with latitude. Chl a concentration and carbon fixation in the IO water was lower than that in the JS water. Highersalinity and lower contents of dissolved inorganic nitrogen （DIN） and silicate （SiO3^2-） characterized the IO water as comparedto the SCS or JS water, and the PO4^3- content was lower in the IO water than in the SCS or JS water in most cases. Our resultsalso indicate the importance of DIN and SiO3^2- concentrations for the geographical changes in phytoplankton biomass andprimary productivity among the three regions.

Discovery of seep carbonate nodules as new evidence for gas venting on the northern continental slope of South China Sea

Seep carbonate nodules were firstlycollected from the southwestern Dongsha area onnorthern continental slope of South China Sea formineralogical and geochemical studies. The resultsof X-ray diffraction show that carbonate nodules arecomposed of Fe-rich domolite,siderite and a few cal-cite and aragonite, as well as a small quantity ofnon-carbonate minerals such as goethite, quartz andclay minerals. Fe-rich dolomite and siderite charac-terized by euhedral microcrystalline structure weredirectly precipitated at cold seeps. The δ 13C valuesfor Fe-rich dolomite and siderite in the bulk nodulesvary from ?18.24‰ to ?36.07‰, and the δ 18O valuesrange from 0.42 to 2.76‰. Their moderate depleted13C reflects that carbon origin is possibly thermoge- netic gas or mixed gas, which is evidence of gasventings in the seafloor. Moreover, massive wormtube fossils related to cold seeps were found on thesurfaces of carbonate nodules, and the conduitsand/or channels in semi-solidified nodules whichwere not filled by carbonate or sediment were alsoobserved. A preliminary conclusion is that activemicro gas venting with a conduit/channel diameterrange from 200 μm to 600 μm possibly exists inmodern seafloor of the carbonate nodule area.

Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δ44/40Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δ13C values of the carbonates of the southwestern Dongsha area varied between δ49.21‰ and δ16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δ18O values ranged from 2.25‰ to 3.72‰ VPDB. The δ13C value of the Baiyun Sag sample was 2.36‰ VPDB and the δ18O value was 0.44‰ VPDB. The δ13C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δ13C values suggesting a dominant methane carbon source for the light samples and mixtures of δ13C values for the heavier samples, with possibly an organic or seawater carbon source. The δ18O values indicated that there is enrichment in 18O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δ44/40Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation.

南海北部典型海域碳酸盐体系的分布特征

【目的】通过2022年春、夏季在油田海域开展现场观测,探讨碳酸盐体系的分布特征。【方法】对春、夏两季海水水温、盐度、pH值、溶解氧、总碱度、溶解无机碳的平面和垂直分布进行分析研究,并进行要素间相关性分析。【结果】调查海域水温和盐度水平分布均匀,变化较小;整个海域pH值介于8.1~8.2;总碱度和溶解无机碳水平分布没有明显统一规律。【结论】春、夏两季水温、pH值、溶解氧随深度增加而降低,盐度、总碱度、溶解无机碳随深度增加而升高;温度、有机物再矿化、生物作用可能是引起水体总碱度、溶解无机碳变化的主要因素。

中国南海北部枯竭气田CO_(2)封存潜力展望:以崖城13-1气田和东方1-1气田为例

碳捕集与封存(Carbon Capture and Storage,CCS)技术作为缓解全球气候变暖、减少CO_(2)排放的有效路径之一,其潜力评估至关重要。目前CCS技术主要包括枯竭油气藏封存技术、CO_(2)强化石油(天然气)开采封存技术、CO_(2)驱替煤层气封存技术以及咸水层CO_(2)封存技术。枯竭的油气藏具有良好的圈闭构造,地质条件十分适合CO_(2)封存。南海北部油气勘探开发历经半个世纪,已建成荔湾气区、东方气区、乐东气区、崖城气区和陵水中央峡谷带气区等五大气区,“南海万亿方大气区”建设稳步推进。同时,部分气田的开发生产进入了末期(枯竭期),且该类气田CO_(2)含量较高,在对该类气田地质特征描述的基础上,依据已知的天然气产出量,对其CO_(2)的有效封存量进行了计算。根据初步的定量计算结果,优选的崖城13-1气田和东方1-1气田的CO_(2)有效封存量分别为1.2×10^(8) t和2.3×10^(8) t,显示了十分可观的CO_(2)封存潜力,研究结果为海上碳封存目标优选提供技术支持。

南海北部神狐海区的自生碳酸盐岩烟囱——海底富烃流体活动的记录

大陆边缘海的流体喷流活动或水合物分解都会导致自生碳酸盐岩的形成。南海北部神狐海区出现的自生碳酸盐岩主要为烟囱状,以铁白云石、文石、方解石碳酸盐矿物为主;稳定同位素研究显示,烟囱的δ13CPDB值在-40.18‰^-38.69‰、δ18OPDB值在3.75‰~4.31‰之间变化,显示了导源于甲烷厌氧氧化作用的特征,是海底富含甲烷的流体活动的最终产物。持续或间断的流体喷流活动,使神狐海区碳酸盐岩烟囱发生单阶段或多阶段沉淀。

南海北部及珠江口细菌生产力研究

2004年2～3月和2004年8～9月两个航次中对珠江口及南海北部海域的异养浮游细菌生物量、生产力及其调控机制进行了观测研究．结果表明，营养物质的供应对调查区域真光层水体内的细菌生物量和生产力起着主要控制作用，从而导致冬季航次珠江口-陆架-外海调查断面表层细菌生物量和生产力呈现沿盐度梯度向外海逐渐降低的特征．就南海北部调查区域而言，冬季真光层异养细菌生物量（C）平均为（712±290）mg／m^2，夏季平均为（937±397）mg／m^2；真光层细菌生产力（C）冬季平均为（65．1±42．8）mg／（m^2·d），夏季平均（52．5±28．6）mg／（m^2·d）．本调查中，南海北部海区IBP和IPP比值范围是4％～96％，平均为26％，IBP和IPP比值与初级生产力呈负相关，其分布特征与新生产力，比的分布趋势相反，显示了异养细菌在真光层物质循环过程中所发挥的作用在南海近岸富营养海域和外海寡营养海域之间的差异．

南海北部陆坡九龙甲烷礁冷泉碳酸盐岩沉积岩石学特征

本文对南海北部陆坡九龙甲烷礁采集的两个冷泉碳酸盐岩样品TVG-1和TVG-11进行了矿物学、岩石学和碳氧同位素分析研究。X-射线衍射分析(XRD)结果表明,自生矿物主要为文石、白云石和高镁方解石等,碎屑矿物含量较少。TVG-11中存在大量的文石,而TVG-1只保留了已转化为方解石的文石假象,据此可以推断TVG-1的形成要早于TVG-11。扫描电镜(SEM)显微结构特征显示,自生矿物主要是通过细菌的厌氧氧化作用,直接从冷泉中沉淀出来的。碳、氧同位素测试结果显示,碳酸盐岩具有强13C亏损和一定程度的18O富集的特征,TVG-1和TVG-11的δ13C值分别为-46.22‰和-52.88‰,均低于-40‰,表明碳源于微生物作用的甲烷厌氧氧化,是指示该海域存在冷泉的重要证据;TVG-1和TVG-11的δ18O值分别为3.19‰和4.07‰,存在一定程度的18O富集。碳氧同位素和矿物的显微结构特征都说明九龙甲烷礁碳酸盐岩是烃类渗漏微生物作用而形成的自生碳酸盐岩,可能是天然气水合物分解引起的沉积,显示该区存在水合物的可能性很大。

南海东北部天然气水合物成藏演化地质过程

南海东北部天然气水合物赋存类型多样,包括块状、瘤状、薄层状、脉状及分散状等五种形式,独自出现或相互共生成藏,并伴生碳酸盐岩,且分布在水深600~900m、海底及其以下的200m地层之内。其中前四种水合物常形成于浅断裂隙、沉积物层间薄弱带、海底表面等局部地区,粒径大于正常沉积物孔隙,水合物肉眼可见,故又称可视水合物;分散状水合物充填于微米—纳米级孔隙的沉积物中,肉眼难以分辨,故这类水合物为非可视水合物。据钻井揭示,以海底之下90m处地层为界,研究区发育上、下两套水合物矿层,上部水合物矿层以可视水合物为主,下部水合物矿层为非可视水合物。其中上部水合物矿层发育上、下两个水合物层段,在各水合物层段的顶部均发育自生碳酸盐岩,反映为地史时期与水合物藏演化相伴生的古冷泉。综合分析认为,天然气水合物赋存形式及水合物藏演化受控于多种因素作用,包括岩性、粒度、岩相(古河道、扇体)等沉积作用,新构造活动及断层作用,流体及气体运移,以及影响水合物结晶成核生长的热流、盐度、时间等。其中可视水合物主要受构造因素控制,多为活跃流体携带气体以渗漏形式局部聚集形成;而分散状水合物则多受沉积因素影响,气体以扩散形式运移至适宜温压域的沉积层内形成水合物。上新世以来的地质事件是水合物形成分解的诱发因素,冰期、间冰期变化与水合物形成分解有一定耦合关系。水合物形成时代应早于上覆的自生碳酸盐岩,晚于其赋存的母岩沉积层,研究区上部水合物矿层的上段形成于晚更新世,下段形成于中更新中期—晚更新世,下部水合物矿层形成中更新世早期—中更新世中期;水合物矿藏共经历了3次水合物藏发育期和2次水合物藏破坏期。伴随水合物演化,发育两期自生碳酸盐岩,海底自生碳酸盐岩形成于更新世末期,上部水合物矿层之下段的顶部碳酸盐岩形成于中更新世早期。多层水合物构成的复式水合物矿藏系统,是扩散、渗漏驱动的二元成藏动力学过程综合作用的结果。

南海北部深海甲烷冷泉自生碳酸盐岩显微结构特征与流体活动关系初探

对2002年"海洋四号"在南海北部深水冷泉活动区水深约3000m海底获得的1块自生碳酸盐岩展开了岩石学、地球化学和显微结构的分析研究。结果表明,该HD-83站位岩石样品为有多孔不规则块体,属于与扩散流有关的冷泉自生碳酸盐岩。其自生碳酸盐矿物成分为镁方解石或文石,含量达全岩的17%。全岩碳同位素值δ13C为-61.403‰,氧同位素值δ18O为3.5606‰,属于甲烷冷泉喷口成因自生碳酸盐岩,甲烷来源以微生物成因为主。该岩石显微结构有泡孔状、充填通道、不规则裂隙、虫孔边缘加厚4种主要类型。通过对比,推知这些显微结构可能与甲烷分解排气释放或流体逃逸通道有关。本研究注意到在岩石的裂隙或孔道中菌丝体普遍存在,可能属于菌席的组成份子,依靠通道内流体进行化能合成。

南海北部大陆边缘盆地深水区储层类型及沉积演化

南海北部大陆边缘盆地深水区储层类型与构造、沉积演化有密切的关系。南海北部陆缘盆地深水区经历了裂陷期、裂坳过渡期、坳陷期3个沉积演化阶段,不同沉积演化阶段发育不同类型的储层。深水区的储层主要包括:始新统—下渐新统裂陷期的河流和扇三角洲相砂岩;上渐新统裂坳过渡期的(扇)三角洲—滨浅海相砂岩;裂后坳陷期中新统的滨海相砂岩、台地碳酸盐岩及深水浊积砂岩。其中,南海北部陆缘深水区广泛分布的斜坡扇、盆底扇、进积楔形体和峡谷水道等低位砂体具有巨大的勘探潜力。

南海北部陆缘深水区礁相碳酸盐岩的地球物理特征

利用地震、钻井及测井资料,通过地震剖面的精细解释对南海北部陆缘深水区中新世礁相碳酸盐岩分布进行研究,并采用地球物理反演及地震属性研究方法,分析已有钻井的礁相碳酸盐岩的储层物性。结果表明:储层物性良好的礁相碳酸盐岩具有较高渗透率、高孔隙度、高波阻抗及高P波速度,同时纵向及横向上具有不均一性;储集物性较差的礁相碳酸盐岩渗透率和孔隙度都比较低,而纵向及横向均一性较好,波阻抗也相对较高;琼东南盆地深水区礁相碳酸盐岩在储层物性上接近于流花11-1(LH11-1)储层,表现为高速度、强波阻抗以及横向上的不均一性,可以作为较好的储层。

东沙西南海域表层底栖有孔虫碳同位素对冷泉活动的指示

底栖有孔虫碳同位素负偏记录是冷泉活动的重要指标之一。对南海北部东沙西南海域4个含碳酸盐结核的表层沉积物样品中的6种底栖有孔虫进行了氧碳同位素分析,结果表明,表生属种Discanomalina semiungulata、Cibicides wullerstorfi、Cibicides pseudoungerianus、Cibicides lobatulus的碳同位素,相对同水深南海现代表生底栖有孔虫有明显的负偏记录,尤其是表生种D.semiungulata基本都偏负;内生种Lenticulina orbicularis也有部分负偏,而Uvigerina auberiana的碳同位素则明显偏正;几种底栖有孔虫的碳同位素波动幅度都明显较大,明显超出了南海最近两个冰期-间冰期旋回中碳同位素的变化幅度。其中生活在蠕虫管壁上的活体底栖有孔虫D.semi-ungulata和C.lobatulus的同位素都呈明显的负偏,认为该区底栖有孔虫碳同位素这种负偏和波动幅度大可能是由于冷泉活动造成的。活体底栖有孔虫碳同位素负偏表明该区可能存在有正在活动的冷泉,其中表生种D.semiungulata的大量出现可以作为南海冷泉活动的一个潜在指示标志。

2009年冬、夏季南海北部超微型浮游生物的分布特征及其环境相关分析

2009年2月(冬季)和8月(夏季)在南海北部海域(nSCS)采用流式细胞术对聚球藻、原绿球藻、超微型光合真核生物3类超微型光合浮游生物和异养浮游细菌的丰度和碳生物量的时空分布特征进行了研究,并分析了其与环境因子之间的关系。结果表明,夏季聚球藻和原绿球藻的平均丰度高于冬季,超微型光合真核生物和异养浮游细菌的丰度反之,为冬季高于夏季。聚球藻、超微型光合真核生物和异养浮游细菌在富营养的近岸陆架海域丰度较高,而原绿球藻高丰度则出现在陆坡开阔海域。在垂直分布上,聚球藻主要分布在跃层以上,跃层以下丰度迅速降低;原绿球藻高丰度主要出现在真光层底部;超微型光合真核生物在水层中的高值同样出现在真光层底部,且与Pico级份叶绿素a浓度分布一致;异养浮游细菌在水体中的分布与聚球藻类似。这些分布格局的差异,取决于环境条件的变化和4类超微型浮游生物生态生理适应性的差异。在超微型光合浮游生物群落中,各类群碳生物量的贡献因季节和海域类型的不同而发生变化:聚球藻在夏季近岸陆架区占超微型光合浮游生物总碳生物量的41%,原绿球藻在陆坡开阔海成为主要贡献者(50%),超微型光合真核生物碳生物量以冬季为高(在近岸陆架区占比68%)。冬、夏季异养浮游细菌碳生物量均高于超微型光合浮游生物碳生物量。

南海北部夏季浮游细菌生长效率初步研究

2010年9月对南海北部海域细菌生长效率、细菌呼吸率、细菌生产力及相关生态、环境因子进行了调查研究。结果表明,细菌生产力(C)和细菌呼吸率(C)表层平均值分别为0.08±0.02mg·m 3·h 1和0.92±0.18mg·m 3·h 1,细菌生长效率在6.20%—9.84%范围波动。细菌生产力、细菌生长效率的空间分布呈现由近岸向外海逐渐递减趋势,而细菌呼吸率的分布特征相对不显著。细菌呼吸率和细菌需碳量明显高于初级生产力,表明南海北部海域整体呈异养性特征,初级生产过程不足以满足细菌生长代谢对碳的需求。尽管细菌生产力与细菌呼吸率相关性显著,但与两者相关的环境因子不完全一致,说明它们可能存在不同的调控机制。综合分析细菌生产力、细菌呼吸率和相关生态环境因子,结果表明,南海北部异养细菌二次生产和呼吸代谢2种不同的生态功能在微食物环以及整个生态系统的物质循环和能量流动中均发挥重要作用。

南海北部冷泉碳酸盐岩的矿物、岩石及地球化学研究进展

南海北部陆坡海底冷泉碳酸盐岩广泛发育,已从30个站位采集到冷泉碳酸盐岩。南海北部冷泉碳酸盐岩呈结壳状、结核状、烟囱状、角砾状、块状等,主要自生矿物有文石和方解石,部分站位样品有白云石和铁白云石及少量菱铁矿。冷泉碳酸盐岩具有较轻的碳同位素组成特征,西沙海槽海域、神狐海域、东沙西南海域、东沙东北海域和台西南海域碳酸盐岩的δ13CPDB分别为29.6‰—13.3‰(平均21.0‰),40.4‰—38.7‰(平均39.8‰),36.1‰—18.2‰(平均26.8‰),61.4‰—32.8‰(平均49.3‰)和57.6‰—35.7‰(平均48.3‰),显示了较大的变化范围,反映了复杂的碳源。冷泉碳酸盐岩的δ18OPDB为0.4‰—7.5‰,同样显示了较大的变化范围,反映可能与富集18O的地层水和/或天然气水合物分解水有关。神狐海域及东沙东北部九龙甲烷礁海域冷泉碳酸盐岩的稀土元素页岩标准化配分模式既具有Ce正异常,同时也有Ce负异常特征,说明冷泉碳酸盐岩形成过程中可能存在氧化还原条件的变化。南海北部陆坡不同海域,甚至同一站位的冷泉碳酸盐岩均表现出不同的矿物、同位素及元素组成特征,可能反映了冷泉渗漏流体和天然气水合物稳定状态的差异。

近3万年来南海北部碳酸盐旋回及古气候意义

选取2005年“海洋四号”在南海北部海域溶跃面之上获得的ZHS-176柱状样品，分析了其碳酸钙的含量，结果表明，南海北部海域近3万年来碳酸盐含量在间冰阶时较高，冰阶时降低，其旋回特征自下而上主要分为5个阶段：（Ⅰ）末次冰期以来，碳酸钙含量偏低；（Ⅱ）冰消期，碳酸钙含量升高，出现第一个高峰；（Ⅲ）早冰后期，碳酸钙含量明显降低，出现低谷；（Ⅳ）中冰后期，碳酸钙含量升高，出现第二个高峰；（V）晚冰后期，碳酸钙含量略微降低。ZHS-176柱碳酸盐含量变化总体表现出“大西洋型旋回”的特征，但在早冰后期，尤其是在8．5～7．5kaBP时出现了一次明显的低钙事件，可能与陆源物质输入增加以及深海溶解作用增强有关。