渤海湾西北岸埋藏牡蛎礁礁顶上下沉积物中硅藻对“礁泥转换”古沉积环境的重建

渤海湾西北岸大吴庄牡蛎礁是近年来研究程度最高的礁体,天津空港物流中心牡蛎礁则是近年来新发现的礁体。该两处礁体上部的牡蛎个体内部充填的泥、个体之间的泥以及礁体上覆沉积泥层中的硅藻研究表明,礁顶上下（即礁体的上部-顶部和上覆沉积泥层的下部）均富含潮间带环境指示种的Auliscus caelatus,表明礁体发育后期和上覆沉积泥层的初期,均为潮间带环境。尽管如此,从礁顶向上覆沉积泥层转换方式在两处礁体却不相同:（1）大吴庄:Auliscus caelatus种向上逐渐减少,暗示着大吴庄剖面由礁体上部的潮间带中上部、直至上覆沉积泥层下部的潮间带上部、进一步向上覆沉积泥层上部的潮上带转化的趋势。（2）空港:牡蛎礁上覆沉积泥层出现了比例达6%~23%的河口咸水-半咸水种Achnanches brevipes,而下伏礁体内则未见该种,暗示此处泥层受河口作用显著影响。此种不同揭示了礁体停止发育的原因有所不同:在5 520cal BP时,大吴庄礁顶进入潮间带中上部,礁体经常脱离海水而发育停止;在约4 000 cal BP时,空港礁体所在位置因河流作用突然增强,在时间上看很可能是黄河改道在天津入海,古黄河携带大量泥砂入海,掩盖了潮间带的牡蛎礁而导致牡蛎礁发育停止。     

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)。     

渤海湾西北岸全新世牡蛎礁研究概述

环太平洋西岸,特别是渤海湾西北岸沿海平原全新世地层中广泛分布着由Crassostrea gigas牡蛎形成的礁体.总结了渤海湾西北岸30余年来众多研究者的主要成果.论述了C.gigas的定名沿革、生态习性、礁体特征、礁体与海面的关系.测定了C.gigas壳体的18O和13C同位素组成,发现了壳体生物碳酸钙的年际变化规律,结合可靠的14C数据,建立了正常建礁与个体分泌壳体之间的线性函数关系(约1 cm/a)、水平夹层的平均历时(约200a/层)等定量参数,进而提出"礁体历时假说",并将该地区时间跨度约7750～950 calBP的化石牡蛎礁划分为8道礁群,论证了礁体内部水平夹层和礁体之上泥质"转换层"可能记录的百年尺度的"缓变型"环境变化.     

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...     

稳定同位素贝壳年轮指示的牡蛎礁体水平夹层成因

渤海湾西岸众多的埋藏牡蛎礁体内,除正常的牡蛎壳体堆积层外,还广泛地存在一定数量的水平夹层.水平夹层的存在,可能指示了礁体的生长环境发生了变化.文章对组成礁体的正常建礁层和水平夹层中的牡蛎壳体,进行了贝壳年轮和稳定同位素分析研究,对比了水平夹层和正常建礁层中的壳体生长速率和壳体记录的稳定同位素变化范围与幅度,恢复了壳体记录的礁体生长环境,探讨了壳体同位素记录揭示的礁体水平夹层成因.结果表明,相对于相邻正常建礁层中的壳体,水平夹层中的壳体记录的季节性氧同位素变化幅度较小,这主要与壳体生长时期的季节性淡水输入变化较小或(和)年平均气温较低有关.水平夹层中的壳体记录的低温季节相对较多的淡水输入量和高温季节相对较少的淡水输入量,或(和)由于年平均气温较低而导致的壳体在一年中的生长周期缩短,是造成礁体中水平夹层形成的主要驱动因素.     

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。     

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。     

天津滨海新区中全新世以来地形地貌演化

根据最新获得的多项天津滨海新区海岸带地质调查资料,分别利用海区的浅地层剖面成果反演海底古地形、利用海区沉积钻孔了解沉积物时代、利用陆区的贝壳堤,埋藏牡蛎礁,古墓葬和埋藏砂体分布等,研究各时期古岸线的位置与形态,并与海区的沉积环境和地形地貌进行对比,对天津滨海新区中全新世以来的地形地貌演化进程及演化机制概括性介绍.自中全...     

天津地质调查中心积极支持国家海洋博物馆、七里海牡蛎礁博物馆的筹建工作

正隶属于国家海洋局、正在筹建的国家海洋博物馆,拟在其一楼正厅展出具有地方特色,记录了近7 000年来渤海湾西北岸海陆变迁的牡蛎礁。天津地质调查中心海岸带与第四纪地质研究所根据多年来的研究积累,主动提供获得礁体大样的合适地点(照片1),保障了牡蛎展品前期布展工作的顺利完成。海岸带与第四纪地质研究所还参加了天津市宁河区正在筹建的"七里海牡蛎礁博物馆"野外选址与钻探勘察工作,配合其他单位调查了近1万平方米范围内的48个12 m全取心孔的牡蛎礁分布,统计了各孔礁体顶、底板的深度,绘制了礁体的三维分布图     

A Model for Understanding the Effects of Sediment Dynamics on Oyster Reef Development

Previous field studies have demonstrated that sedimentation is an important factor that can limit oyster reef growth and restoration success. High relief reefs are more productive and resilient than low relief reefs, in part, because increasing reef height reduces sedimentation and enhances oyster growth. In this study, we investigated the relationship between initial reef height and reef development using a simple model. The model contains three coupled differential equations that describe changes in oyster volume, shell volume, and sediment volume per unit area of reef with time. The model was used to investigate how parameters such as flow speed, sediment grain size, and food concentration affect reef survival and final reef height. Whether or not a reef survives depends primarily on the shape of the sediment concentration profile relative to the initial reef height. Over a long time period, three different steady-state reef heights are possible, depending on the environmental parameters and initial reef height: (1) If growth outpaces sedimentation, the reef achieves the maximum possible height, which is independent of sedimentation parameters; (2) if deposition outpaces growth and the shear stress does not exceed the critical shear stress, the reef is buried in sediment and dies; and (3) if deposition outpaces growth and the shear stress exceeds the critical shear stress, a reduced steady-state height is achieved that depends on both growth and sedimentation parameters. The model can be used to assess the ways in which measurable environmental parameters affect reef restoration success.     

渤海湾西北岸牡蛎礁体对区域性构造活动与水动型海面变化的响应

渤海湾地区广泛分布的古海面标志物,为建立该地区的相对海面变化曲线提供了良好的基础。过去20年来该地区已相继建立了一些相对海面变化曲线,但在讨论影响古海面标志物高程的因素时,未将构造活动与水动型海面变化及均衡作用对高程的贡献量加以区分。文章以该地区7个点的古海面标志物及相关层位的高程测量与年代学测定结果,经与地区性预测海面变化曲线进行对比后,定量讨论了该地区海面变化(包括绝对海面变化与冰川、水均衡作用)与构造活动对古海面标志物高程的贡献;并以壳体的稳定同位素、地层记录与古地震研究资料讨论了牡蛎礁体在建礁过程中所记录的构造下沉。     

渤海湾全新世贝壳堤和牡蛎礁的古环境

贝壳堤和牡蛎礁是渤海湾沿海全新世的两类重要地质现象。本文运用沉积学、年代学(ＡＭＳ１４Ｃ测年和古地磁)和生物碳酸钙氧、碳同位素测定等方法，研究了贝壳堤、牡蛎礁及相关泥质沉积物的形成和地层序列，考订了各类海面标志点的地质意义和１４Ｃ数据的可靠程度，校正了文献中的１４Ｃ数据，测定了区内全新世晚期软体动物壳体的δ１３Ｃ平均值，以及δ１８Ｏ水／盐度和δ１３Ｃ贝壳／Ｔ等基础数据，绘出了地区性相对海面变化的时间－深度图，研究了新构造活动对相对海面变化的影响。基于上述研究，进一步探讨了本区约４０００ｃａｌＢ．Ｐ．以来的环境变化及其原因。     

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.     

Sediment Suspension and Deposition Across Restored Oyster Reefs of Varying Orientation to Flow: Implications for Restoration

The eastern oyster, Crassostrea virginica, is a prominent ecosystem engineer, whose reefs exhibit strikingly consistent morphologies at multiple spatial scales throughout its North American range. These distinct morphologies are thought to form by interactions of nascent reef structures with hydrodynamics. We carried out two field studies to determine if historical reef configurations applied in a restoration context would improve reef persistence and restoration outcomes. We collected seabed and water column observations across constructed reefs of three orientations representative of those found historically throughout the oyster’s range: parallel or perpendicular to tidal currents or circular. Areas adjacent to reefs were sites of fine sediment trapping, with lower flow velocities, evidence of particle settling, and more fine sediments on the seabed relative to off-reef reference sites. The water column above the reef crest exhibited higher acoustic backscatter, higher flow velocities, and larger particles in suspension, consistent with local erosion of flocculated fine sediment from the reef crest. Perpendicular reefs produced conditions that were more conducive to reef persistence and improved oyster performance, including high flow velocities and enhanced resuspension of sediments from the reef, compared to parallel or circular reefs. Particle trapping in areas between reefs has the potential to inhibit reef growth between existing reef structures, providing support for hypotheses of landscape-scale reef pattern formation. Oyster reef restoration efforts can benefit from this improved understanding of biophysical interactions arising from reef orientation that contribute to sediment dynamics on constructed oyster reefs.     

Habitat partitioning between the xanthid crabsPanopeus herbstii andEurypanopeus depressus on intertidal oyster reefs (Crassostrea virginica) in southeastern North Carolina

The abundances of the xanthid crabsPanopeus herbstii andEurypanopeus depressus were examined relative to surface oyster shell cover, surface oyster cluster volume, subsurface shell content, substrate sand and silt composition, and oyster reef elevation. During August 1986 through July 1987, xanthid crabs were collected monthly from twelve 0.25 m²×15 cm deep quadrats, during low tide, from intertidal oyster reefs in Mill Creek, Pender County, North Carolina, USA, with respective quadrat details recorded. The abundance ofP. herbstii, and to a lesser degreeE. depressus, was positively correlated with surface shell cover. The abundance ofE. depressus, and to a lesser degreeP. herbstii, was positively correlated with surface cluster volume. The majority ofP. herbstii inhabited the subsurface stratum of the oyster reef, whereas the majority ofE. depressus inhabited the cluster stratum. Seasonality (i.e., temperature) appeared to influence the strata habitation of both species, with a higher incidence of cluster habitation during warmer months and a lower incidence during colder months. Crab abundance was not related to other factors examined, such as subsurface shell, substrate sand and silt composition, or elevation within the oyster reef. The analyses show thatP. herbstii andE. depressus have partitioned the intertidal oyster reef habitat, withE. depressus exploiting surface shell clusters andP. herbstii the subsurface stratum.     

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。