Late pleistocene and holocene sedimentation in the central and eastern Persian Gulf

The sandfraction of the sediment was analysed in five cores, taken from 65 m water depth in the central and eastern part of the Persian Gulf. The holocene marls are underlayn by aragonite muds, which are probably 10-11,000 years old. 1. The cores could be subdivided into coarse grained and fine grained layers. Sorting is demonstrated by the following criteria: With increasing median values of the sandfraction - the fine grained fraction decreases within each core; - the median of each biogenic component, benthonic as well as planktonic, increases; - the median of the relict sediment, which in core 1179 was carried upward into the marl by bioturbation, increases; - the percentages of pelecypods, gastropods, decapods and serpulid worms in the sandfraction increase, the percentages of foraminifera and ostracods decrease; - the ratios of pelecypods to foraminifera and of decapods to ostracods increase; - the ratios of benthonic molluscs to planktonic molluscs (pteropods) and of benthonic foraminifera to planktonic foraminifera increase (except in core 1056 and 1179); - the ratio of planktonic molluscs (pteropods) to planktonic foraminifera increases; - the globigerinas without orbulinas increase, the orbulinas decrease in core 1056. Different settling velocities of these biogenic particles help in better understanding the results : the settling velocities, hence the equivalent hydrodynamic diameters, of orbulinas are smaller than those of other globigerinas, those of planktonic foraminifera are smaller than those of planktonic molluscs, those of planktonic molluscs are smaller than those of benthonic molluscs, those of pelecypods are smaller than those of gastropods. Bioturbation could not entirely distroy this "grain-size-stratification". Sorting has been stronger in the coarse layers than in the finer ones. As a cause variations in the supply of terrigenous material at constant strength of tidal currents is suggested. When much terrigenous material is supplied (large contents of fine grained fraction) the sedimentation rates are high: the respective sediment surface is soon covered and removed from the influence of tidal currents. When, however, the supply of terrigenous material is small, more sandy material is taken away in all locations within the influence of terrigenous supply. Thus the biogenic particles in the sediment do not only reflect the organic production, but also the influence of currents. 2. There is no parameter present in all cores that is independently variable from grain size and can be used for stratigraphic correlation. The two cores from the Strait of Hormus were correlated by their sequences of coarse and fine grained layers. 3. The sedimentation rates of terrigenous material, of total planktonic and benthonic organisms and of molluscs, foraminifera, echinoids and ophiuroids are shown in table 1 (total sediment 6.3-75.5 cm/1000 yr, biogenic carbonate 1.9-3.6 cm/1000 yr). The sedimentation rates of benthonic organisms are nearly the same in the cores of the Strait of Hormus, whereas near the Central Swell they are smaller. In the upper parts of the two cores of the Strait of Hormus sedimentation rates are higher than in the deeper parts, where higher median values point to stronger reworking. 4. The sequence of coarse and fine grained intervals in the two cores of the Hormus Strait, attributed to variations in climate, as well as the increase of terrigenous supply from the deeper to the upper parts of the cores, agrees with the descriptions in the literature of the post Pleistocene climate as becoming more humid. The rise of sea level is sedimentologically not measurable in the marly sediments - except perhaps for the higher content of echinoids in the lower part of core 1056. These may be attributed to the influence of a migrating wave-base. 5. The late Pleistocene aragonite mud is very fine grained (> 50%< 2 p) and poor in fossils (0.5-1.8%) biogenic particles of total sediment. The sand fraction consists almost entirely of white clumps, c. 0.1 mm in diameter (1177), composed of aragonite needles and of detrital minerals with the same size (1201). The argonite mud was probably not formed in situ, because the water depth at time of formation was at most 35 m at least 12 m. The sorting of the sediment (predominance of the fine grained sand), the absence of larger biogenic components and of pellets, c. 0.2-0.5 mm in diameter, which are typical for Recent and Pleistocene locations of aragonite formation, as well as the sedimentological conditions near the sampling points, indicate rather a transport of aragonite mud from an area of formation in very shallow waters. Sorting as well as lenticular fabric in core 1201 point to sedimentation within the influence of currents. During alternating sedimentation - and reworking processes the aragonitic matrix was separated from the silt - and sand-sized minerals. The lenses grade into touches because of bioturbation. 6. In core 1056 D2 from Hormus Bay the percentages of organic carbon, total nitrogen and total carbonate were determined. With increasing amounts of smaller grain sizes the content of organic matter increases, whereas the amount of carbonate decreases. The amounts of organic carbon and of nitrogen decrease with increasing depth, probably due to early-diagenetic decomposition processes. Most of the total nitrogen is of organic origin, only about 10% may well be inorganically fixed as ammonium-nitrogen. In the upper part of the core the C/N-ratio increases with increasing depth. This may be connected with a stronger decomposition of nitrogen-containing organic compounds. The general decrease of the C/N-ratios in the lower part of the core may be explained by the relative increase of inorganically fixed ammonium-nitrogen with decreasing content of organic matter.研究对取自波斯湾（Persian Gulf）中部及东部海域、水深65米处的5根岩芯（core）中的沉积物（sediment）砂粒级组分（sandfraction）开展了分析。全新世（Holocene）泥灰岩（marls）之下伏有形成年代约为10000~11000年的文石泥（aragonite mud）。 1. 所有岩芯均可划分为粗粒层与细粒层。沉积物的分选性可通过以下判据体现： 随着砂粒级组分中位值升高 - 单根岩芯内的细粒级组分占比降低； - 各生物组分（biogenic component）（包括底栖（benthonic）与浮游（planktonic）生物组分）的中位值升高； - 残留沉积物（relict sediment）的中位值升高——在岩芯1179中，残留沉积物因生物扰动（bioturbation）被搬运至泥灰岩地层中； - 砂粒级组分中双壳类（pelecypods）、腹足类（gastropods）、十足类（decapods）及龙介虫（serpulid worms）的占比升高，而有孔虫（foraminifera）与介形类（ostracods）的占比降低； - 双壳类与有孔虫的比值、十足类与介形类的比值均升高； - 底栖软体动物与浮游软体动物（翼足类（pteropods））的比值、底栖有孔虫与浮游有孔虫的比值均升高（岩芯1056与1179除外）； - 浮游软体动物（翼足类）与浮游有孔虫的比值升高； - 岩芯1056中，不含圆球虫（orbulinas）的抱球虫（globigerinas）类占比升高，而圆球虫类占比降低。 这些生物颗粒的沉降速度差异有助于更好地解释上述结果：圆球虫类的沉降速度（即等效流体动力直径）小于其他抱球虫类；浮游有孔虫的沉降速度小于浮游软体动物；浮游软体动物的沉降速度小于底栖软体动物；双壳类的沉降速度小于腹足类。 生物扰动并未完全破坏这种“粒级分层”现象。粗粒层的分选性优于细粒层。推测其成因是：在潮流（tidal currents）强度恒定的条件下，陆源物质（terrigenous material）补给量发生变化。当陆源物质补给量大（细粒级组分占比高）时，沉积速率较高，相应的沉积物表面会被快速覆盖，脱离潮流的影响；反之，当陆源物质补给量较小时，陆源影响范围内的所有区域都会被带走更多的砂质物质。因此，沉积物中的生物颗粒不仅反映了有机生产力，还受到水动力条件的影响。 2. 所有岩芯中均不存在独立于粒级变化、可用于地层对比（stratigraphic correlation）的参数。来自霍尔木兹海峡（Strait of Hormus）的两根岩芯通过其粗、细粒层序列完成了地层对比。 3. 表1列出了陆源物质、总浮游与底栖生物、软体动物、有孔虫、海胆类以及蛇尾类的沉积速率（总沉积物沉积速率为6.3~75.5 cm/1000 yr，生物碳酸盐沉积速率为1.9~3.6 cm/1000 yr）。霍尔木兹海峡岩芯中的底栖生物沉积速率基本一致，而在中央隆起（Central Swell）附近的岩芯中，该速率更低。霍尔木兹海峡两根岩芯的上部沉积速率高于下部，而下部较高的粒级中位值表明其受到了更强的改造作用。 4. 霍尔木兹海峡两根岩芯中的粗、细粒间隔序列（归因于气候变化），以及从岩芯深部到浅部陆源补给量的升高趋势，均与文献中关于晚更新世（late Pleistocene）后气候逐渐变得湿润的描述相符。在泥灰岩沉积物中，海平面上升的沉积学信号难以识别——仅岩芯1056下部的海胆类占比升高可能属于例外，这或许可归因于迁移的浪基面（wave-base）影响。 5. 晚更新世文石泥颗粒极细（>50%的组分粒径<2 μm），且生物颗粒占总沉积物的比例极低（0.5%~1.8%）。砂粒级组分几乎完全由直径约0.1 mm的白色团块（岩芯1177）组成，这些团块由文石针及同尺寸的碎屑矿物构成（岩芯1201）。文石泥大概率并非原地形成，因为其形成时期的水深至多为35 m，至少为12 m。 沉积物的分选性（以细粒砂为主）、缺乏大型生物组分及直径约0.2~0.5 mm的球粒（该类组分是现代及更新世文石形成区域的典型特征），以及采样点附近的沉积学条件，均表明文石泥是从极浅水区的形成区域经搬运而来。 岩芯1201中的分选特征与透镜状构造（lenticular fabric）均表明其沉积作用受水动力条件控制。在沉积与改造的交替过程中，文石基质与粉砂级、砂级矿物发生了分离。受生物扰动作用影响，透镜状沉积体逐渐过渡为相互接触的形态。 6. 对取自霍尔木兹湾的岩芯1056 D2中的有机碳、总氮及总碳酸盐含量进行了测定。随着细粒级组分占比升高，有机质含量随之升高，而碳酸盐含量则降低。有机碳与总氮的含量随深度增加而降低，这可能与早期成岩作用（early-diagenetic）中的分解过程有关。 总氮中的绝大部分为有机来源，仅约10%可能以铵态氮（ammonium-nitrogen）的形式被无机固定。在岩芯上部，C/N比值（C/N-ratio）随深度增加而升高，这可能与含氮有机化合物的更强分解作用有关。而岩芯下部的C/N比值整体降低，这可通过有机质含量下降时无机固定的铵态氮占比相对升高来解释。