硅藻Auliscus caelatus对渤海湾西北岸大吴庄埋藏牡蛎礁形成环境的复原

通过对渤海湾西北岸大吴庄牡蛎礁18个层位牡蛎壳内泥质填充物样品的硅藻分析,首次发现在渤海湾埋藏牡蛎礁中富含代表潮间带环境的Auliscus caelatus种硅藻化石,体积分数高达15%～40%,显示整个牡蛎礁形成于潮间带环境。根据潮间带标志种Auliscus caelatus,结合生活在海湾内侧河口处环境的Cyclotella stylorum和沿岸分布的内湾指标群种Thalassionema nitzschioides 3种硅藻比例在垂向上的变化,将牡蛎礁分为潮间带下部(Ⅰ带)、潮间带中上部(Ⅱ带)、潮间带中下部(Ⅲ带)和潮间带中上部(Ⅳ带)4个硅藻带。牡蛎礁上半部(Ⅱ带和Ⅲ带)中的4条水平夹层中的硅藻,与其下伏相邻正常建礁层的硅藻对比,均显示出Auliscus caelatus体积分数显著偏低、咸水种硅藻的比例偏高以及硅藻种属类型偏多等特征,推测出各水平层的海相性和开放性均有强于下伏相邻正常层的倾向。水平夹层的形成原因:一是牡蛎礁发育后期,礁体所处环境发生显著变化,从距河口相对较远的潮间带下部(不含水平层的Ⅰ带)环境转为距河口相对较近的潮间带中上部(Ⅱ带—Ⅳ带)环境,较浅的海水环境使得牡蛎礁易受外界环境的影响;二是来自海洋方向的风暴潮或较强的波浪,将牡蛎礁向海一侧的潮间带泥沙冲到礁顶,导致牡蛎倒伏死亡后被泥沙掩埋。     

天津空港牡蛎礁:中全新世环境恶化与新构造控礁作用

新发现的天津空港埋藏牡蛎礁体和下伏、上覆泥层的沉积学、AMS14C年代学研究显示,厚2.3m的礁体分为3部分,起讫年龄是6170～3980cal BP。5900～4100cal BP的中部,是受全新世冷事件4环境恶化控制的缓慢发育期。礁体的结束,则与冷事件3在时间上大致吻合。根据新的年龄数据,将该礁体的归属重新厘定为Ⅲ-2礁群。探讨礁体主要发育区与新构造活动的关系,表明全新世牡蛎礁平原主要位于海河-宝坻-工部-蓟运河断裂圈闭的北西-南东向区域内。这一认识,进一步从实证角度证实了牡蛎礁形成于渤海湾"湾中之湾"的推测。     

渤海湾4.2ka事件初步研究

相距3~20km的岭头、罾口河、俵口和空港物流中心4处埋藏牡蛎礁体的年代学研究证实,开始发育的时间不同,但同时在4.2~4.0ka cal BP时结束建礁,即其灭亡与"4.2ka事件"在时间上重合。岭头、罾口河剖面O稳定同位素和罾口河剖面孢粉数据进一步表明,4.5~4.3ka cal BP时研究区气候出现变冷变干的趋势,4.3~4.2ka cal BP逐渐好转,推测这一气候波动变化过程可能是对"4.2ka事件"的响应。     

渤海湾全新世牡蛎礁:时空分布和海面变化标志点

以他人和本研究小组对渤海湾西北岸牡蛎礁平原、毗邻浅海区的全新世埋藏牡蛎礁和现生牡蛎礁的研究为基础,重新考订了与礁体有关的数据的沉积学意义及年龄,进一步厘定了牡蛎礁的形成时间、礁体顶板和底板的高度.在原有的9个礁群的基础上,增加了目前发现的最老的早全新世晚期礁群;在沿海低地新发现的滨海湖、空港牡蛎礁,暂作为北淮淀礁群的亚...     

渤海湾西北岸埋藏牡蛎礁体中的 壳体形态与沉积环境*

渤海湾西北岸沿岸平原分布的众多埋藏牡蛎礁体中的壳体形态和泥沙沉积物特征是不尽相同的。不同礁体中的壳体形态有明显的区别,有些礁体中的壳体细窄,有些礁体中的壳体较宽厚; 与此相对应,具有不同壳体形态的礁体中的沉积物颗粒和粘土含量也有明显的差别。文章分别对组成大吴庄和岭头两个礁体剖面的牡蛎壳体和泥质沉积物进行测量和分析,结果表明大吴庄礁体中壳体的重量随高度增长的速率大于岭头礁体中的壳体,而两个礁体中壳体的壳重随体积增长的速率大致相同; 组成礁体的壳体形态与礁体的沉积环境有关,沉积物较细、粘土含量高时发育细窄的壳体,沉积物较粗、粘土含量低时发育宽厚的壳体。渤海湾西北岸地区埋藏牡蛎礁体中的壳体本身记录了其生长时的环境信息,可以通过分析礁体中的壳体形态来恢复礁体建礁过程中的沉积环境。     

渤海湾西北岸大吴庄牡蛎礁测年与古环境变化

对位于渤海湾牡蛎礁平原东部的大吴庄牡蛎礁一个完整剖面的系统^14C年代测定与校正表明,该礁体的持续时间约为1600年(7200～5600calBP)。以壳体生长纹层氧、碳同位素获得的礁体平均建造速率1cm／yr来计算礁体的持续时间,结果仅为^14C计年历时的三分之一。^14C测年与壳体同位素计年结果的对比表明,礁体的每一水平层平均占据了一定的时间(约200年),这一时间段是不适宜礁体正常建造的“缓变型地质环境恶化期”。礁体正常建礁层及水平夹层的时空分布与地区性气候变化曲线对比结果表明,水平夹层与正常建礁层的转换与地区性气温波动相对应。水平夹层代表的“缓变型地质环境恶化期”,对应着地区性气温偏冷时期。     

天津宁河俵口-罾口河牡蛎礁体时空分布

为天津市宁河区在俵口-罾口河间拟建的"七里海牡蛎礁博物馆"进行了详细的全取心钻探调查和高程测量等前期勘查工作。结果表明:1)在面积约80 000 m~2、地表起伏仅0.7 m的调查区内,全部36个钻孔均钻遇并穿透牡蛎礁体。礁体顶板和底板的平均埋深分别是3.1 m和8.1 m(相当于现代黄海85高程-2.737 m和-7.684 m),礁体平均厚度约5 m。调查区东部,尤其是东南部礁体顶板埋深浅、底板埋深大,礁体较厚。已有的年代学证据显示,俵口牡蛎礁体发育于全新世早期,约4 000 cal BP时结束建礁;罾口河礁体发育于7 120 cal BP,约4 210 cal BP时结束建礁。两处礁体的终结均因为被潮间带泥质沉积物掩埋。根据俵口-罾口河礁体的时空展布,将此处的大范围礁体归属于渤海湾西北岸牡蛎礁平原第VI礁群。2)全新世以来,该地区相继经历了海侵和海退过程,可辨识出因海水进退与深度变化造成的潮上带盐沼→潮间带→潮下带浅海至潮间带中下部→潮间带中上部→潮上带沼泽-盐沼环境的完整的沉积相演化过程。在这一过程的中期,发育了牡蛎礁体。     

渤海湾西北岸几道牡蛎礁

渤海湾西北岸划分出了第Ⅰ道—第Ⅵ道牡蛎礁的平面分布(图1);对牡蛎礁壳体的几何形态特征进行了描述(图2),并将牡蛎礁壳体进行了剖面切割(图3);对牡蛎礁壳体进行了X光衍射与激光拉曼(活—古)矿物晶型的对比分析(表1,图4);对俵口村BKP20-2剖面(-2.84m)牡蛎礁一个体右壳:在双目镜下对其内部结构与矿物组分进行了分析(图5);通过机械挖掘剖面及钻探获得:牡蛎礁体的空间堆积厚度与赋存的海拔高度及其各剖面垂直方向上的14C年代数据;并获得牡蛎礁体堆积的内部结构特征(图6);以及牡蛎礁体水平夹层与CaCO3含量变化(图7),其中水平夹层CaCO3含量往往偏高,初步分析可能与气候偏冷有关。对牡蛎礁壳体内充填物及壳外沉积物进行了粒度分析(表3),粒度组分及概率累积曲线等(图9)为分析牡蛎礁的生态环境提供了佐证:即牡蛎礁基本以潮下带生活环境为主。以俵口BKP20剖面为例分析了牡蛎礁体堆积物从埋深-2.14～-6.90m的生物组合特征(表4)。以上资料为获得牡蛎礁堆积体的生长-发育-消亡等演化模式提供了依据(图10)。     

渤海湾西部浅海区活牡蛎礁调查结果及资源保护建议

采用旁侧声呐、浅地层剖面仪和单波束水深测量多种物探仪器联机,同步调查天津大神堂浅海区的水下活牡蛎礁资源,为合理保护活牡蛎礁资源提供科学依据。调查结果表明,该区活牡蛎资源由3个分散礁体组成,总面积仅约3km2,礁体最大厚度1.2m,平均厚度0.6m;调查区海底地形平坦,水深介于3.4~5.6m之间,因受人类活动严重破坏,仅在有礁体发育的局部范围内有高程约1m的起伏波动。为了生态资源的完整,必须对现存的活牡蛎礁体进行保护和恢复。     

渤海湾泥质海岸带近现代地质环境变化研究(Ⅱ):成果与讨论

渤海湾泥质海岸带国土资源大调查以其特有的方法、手段和要求,获得了一批带有地质调查特点的、关于全新世地层与近现代地质环境变化的阶段性成果,主要包括:1)该地区海岸带全新世地层具有4种沉积类型:(A)远离海岸的传统的"三分"型;(B)海相层与陆(海陆过渡)相层的多次交替;(C)末次冰期后期未接受沉积,全新统半咸水的海陆过渡相直接覆于历时约10～20ka的沉积间断面上,向上,进一步发育为海相层;(D)沿海低洼地区在末次盛冰期后的晚更新世晚期即已进入海侵阶段,晚更新世晚期与全新世早期的海陆过渡相沉积是连续的,向上继续发育海相层.2)从以海水影响为主的开放潮坪(及浅海区)环境转变为(陆源影响逐渐加强的)湖-盐沼环境,是研究区中晚全新世地质环境转变的重要特征,建立了表征这两种不同环境的非正式地层单位("灰色单元"与"黄色单元").3)牡蛎礁与贝壳堤的对比方案,以牡蛎礁对应潮坪的贝类繁盛期,牡蛎礁中的水平再搬运夹层和/或上覆泥质盖层(往往形成"岭地")对应贝壳堤成堤.4)大比例尺区调地质点(浅孔)揭示了中晚全新世(近现代)地质环境变化,编绘出约4500calB.P.以来小于1ka时段的二维、假三维环境系列图.一个实例是Ⅱ贝壳堤向陆侧的洼地甚至早于该堤的形成,推测洼地边缘的NNW向的埋藏线性凸起可能受同一方向的新构造活动控制.5)渤海湾沿海平原线性砂坝(障壁岛)-湖/盐沼成因假说.作为堆积贝壳堤正地形的补偿,堤前因"簸选"贝壳导致的潮坪负地形,同样可发育湖沉积.6)210pb/137Cs测年研究提供了该地区现代沉积速率的初步结果(＜0.1cm/a或＞3cm/a不等),据此将海岸带划分为不同的沉积类型区.     

Salt Marsh and Fringing Oyster Reef Transgression in a Shallow Temperate Estuary: Implications for Restoration,Conservation and Blue Carbon

The importance of intertidal estuarine habitats, like salt marsh and oyster reef, has been well established, as has their ubiquitous loss along our coasts with resultant forfeiture of the ecosystem services they provide. Furthering our understanding of how these habitats are evolving in the face of anthropogenic and climate driven changes will help improve management strategies. Previous work has shown that the growth and productivity of both oyster reefs and salt marshes are strongly linked to elevation in the intertidal zone (duration of aerial exposure). We build on that research by examining the growth of marsh-fringing oyster reefs at yearly to decadal time scales and examine movement of the boundary between oyster reef and salt marsh at decadal to centennial time scales. We show that the growth of marsh-fringing reefs is strongly associated to the duration of aerial exposure, with little growth occurring below mean low water and above mean sea level. Marsh-shoreline movement, in the presence or absence of fringing oyster reefs, was reconstructed using transects of sediment cores. Carbonaceous marsh sediments sampled below the modern fringing oyster reefs indicate that marsh shorelines within Back Sound, North Carolina are predominantly in a state of transgression (landward retreat), and modern oyster-reef locations were previously occupied by salt marsh within the past two centuries. Cores fronting transgressive marsh shorelines absent fringing reefs sampled thinner and less extensive carbonaceous marsh sediment than at sites with fringing reefs. This indicates that fringing reefs are preserving carbonaceous marsh sediment from total erosion as they transgress and colonize the exposed marsh shoreline making marsh sediments more resistant to erosion. The amount of marsh sediment preservation underneath the reef scales with the reef’s relief, as reefs with the greatest relief were level with the marsh platform, preserving a maximum amount of carbonaceous sediments during transgression by buffering the marsh from erosional processes. Thus, fringing oyster reefs not only have the capacity to shelter shorelines but, if located at the ideal tidal elevation, they also keep up with accelerating sea-level rise and cap carbonaceous sediments, protecting them from erosion, as reefs develop along the marsh.     

海南岛排浦珊瑚礁区的现代沉积体系及其演化过程

海南岛排浦礁区由珊瑚岸礁和堤礁及其间水域组成。因堤礁的障壁作用和丰富的陆源物质供应,研究区内形成清水和浑水两类沉积环境,产出清水碳酸盐和浑水碳酸盐两列沉积体系,并形成礁源沉积、陆源沉积和混合沉积三类沉积物。文中详细论述了各类沉积的特征,讨论了沉积体系的演化过程:全新世早期是单一的陆源碎屑沉积体系,全新世中期海侵,气温转暖,形成早期排浦岸礁与大铲堤礁的雏型,全新世晚期堤礁进入成熟阶段,其障壁作用加强,最终形成清水与浑水两种沉积环境和两列沉积体系。     

渤海湾全新世贝壳堤与牡蛎礁:古气候与海面变化

讨论了早全新世晚期以来渤海湾西岸贝壳堤平原成堤、西北岸牡蛎礁平原(及毗邻浅海区)建礁过程的同时性特征和气候变化与这一特征之间的关系,以及堤、礁记录的相对海面变化。堤、礁大致同时发育,可分为6期,依次分别被寒冷事件5、4、3、2和1分隔。堤底板前、后缘高差大致对应3m的大潮差、礁顶板大致对应海平面。据二者分别重建了南部贝壳堤平原、北部牡蛎礁平原的相对海面变化:南部自7ka cal BP以来基本与现代海平面等高,小的波动在±1m之间,压实固结作用抵消了中全新世的相对高海面;北部中全新世的相对海平面位置因新构造和固结压实双重下沉作用的影响,已位于现代海平面2~3m以下。     

渤海湾巨葛庄贝壳堤与下伏泥层有孔虫组合的海面变化意义

根据巨葛庄贝壳堤及下伏泥层的有孔虫研究,讨论了贝壳堤底板和下伏泥层各类沉积环境的海面意义,以此作为海面标志点重建的古海面误差范围为±0.225~±0.625m。结合该贝壳堤底板和下伏泥层顶板的年龄数据,重建了3900~3300cal BP时的海面高度为-2.955~-2.715m。     

Impact of environmental parameters on coral reef development and drowning: Forward modelling of the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310)

The sedimentological and chronological analysis of the last deglacial reef sequences of Tahiti (French Polynesia), drilled during the Integrated Ocean Drilling Program Expedition 310, provide a high‐resolution data set allowing a well‐constrained forward modelling study. This study represents the first attempt to model in three dimensions the coral reef development of Tahiti during the last deglacial sea‐level rise (23 000 to 6000 cal yr bp ) using the software dionisos developed by IFP Energies nouvelles. It allows the testing of the reconstructed last deglacial sea‐level curve and the different environmental parameters (for example, wave energy and sediment fluxes) that could have influenced the reef development. These last deglacial reef sequences form two prominent ridges occurring seaward of the living barrier reef that consist of successive submerged reefs. These reefs have been prone to drowning because the window of maximum carbonate production rate is inhibited by high water turbidity (sediment supply from a nearby river), shallow depth of wave action and substrate availability. These factors, combined with rapid sea‐level rise, have driven the growth of retrograding reef pinnacles. Local factors (substratum nature, sediment supply and wave energy) were the main processes that induced the drowning of the inner ridge, whereas interplay of local and global factors (acceleration of the sea‐level rise) was responsible for the drowning of the outer ridge. This particular acceleration of sea‐level rise of 16 m between 14·6 ka and 14 ka bp corresponds to meltwater pulse 1A.     

Control of Carbonate Sedimentation and Reef Growth in Llandovery Sequences on the Northwestern Margin of the Yangtze Platform, South China

The later Telychian (late Llandovery, Silurian) sea-level highstand was a suitable setting for global carbonate deposition and reef growth in epeiric seas. However, evidence from the northwest margin of Yangtze Platform indicates that small carbonate platforms developed in rapidly-subsiding small basins and were principally controlled by muddy clastic input. In particular, sediments of the Ningqiang Formation, late Telychian, usually more than 2000 m thick, are mostly shales, but eight major units of discontinuous (15 km maximum width) and relatively thin (120 m maximum thickness) reef-bearing carbonates, which developed when the sedimentation rate apparently lessened, occur within a relatively short time interval. This interval is between upper griestonensis to spiralis-grandis graptolite biozones, estimated as a c. 2 Ma duration. More than 30 small- to medium-scale patch reefs occur in several parts of the sequence, but only within the carbonate units. Shelly faunas common throughout the sequence reveal water depth to have been shallow during deposition of the Ningqiang Formation equivalent to BA2–3, which has a depth range from low intertidal to the base of the photic zone. BA3 is interpreted as being no more than 60 m deep (Boucot, 1975), which is above normal wave base, frequently affected by storms (Chen et al., 1996), and is regarded as optimum depth for high diversity of Silurian faunas (Boucot, 1975; Brett, 1991). Thus, the rate of sediment accumulation kept pace with basement subsidence, and was a substantial factor for limiting reef growth. Sharp contacts between carbonate units and shales indicate that carbonate units are constrained by frequent inputs of terrigenous debris, as the major cause for termination of carbonate deposition. Therefore, carbonate platforms, and reefs they contained, formed during times when sediment input to the basin lessened and ended when it increased; present evidence does not allow correlation to modeled dry episodes, and we interpret the control to be principally tectonic. Overall, sedimentation in the region was terminated by the end of Telychian time by tectonic uplift of the Yangtze Platform; the southwestward migration of palaeocoastline shows this progression. Sedimentation ceased until Middle Devonian time. Ludlow marine transgression has been recognized in the offshore area of Ningjiang Bay.