LA-ICP-MS analyses on coral growth increments reveal heavy winter rain in the Eastern Mediterranean at 9 Ma.
LA-ICP-MS analyses on coral growth increments reveal heavy winter rain in the Eastern Mediterranean at 9 Ma.
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DOI：
10.1016/j.palaeo.2008.11.015
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发表时间：
2009-03
期刊：
影响因子：
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通讯作者：
R. Mertz‐Kraus;T. Brachert;K. Jochum;M. Reuter;B. Stoll
中科院分区：
文献类型：
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作者：
R. Mertz‐Kraus;T. Brachert;K. Jochum;M. Reuter;B. Stoll
珊瑚骨骼中的沉积物颗粒反映了珊瑚生长期间周围水中悬浮物质的组成和数量的变化。它们可以用来识别风暴频率增加的时期和相关的淡水排放。托托尼期(中新世晚期)克里特岛(爱琴海，地中海东部)的Porite珊瑚在X射线照片中显示出明显的年密度带。δ18O组分的变异性反映了7年内年平均海表面温度为7摄氏度的年带状变化。细小的沉积物颗粒集中在骨架孔隙度与生长增量平行的层中。珊瑚骨骼的化学成分的变化可能是环境变化的结果。因此，使用激光烧蚀电感耦合等离子体质谱(LA-ICPMS)测量了垂直于生长增量的横截面上空间分辨率为~500微米的元素浓度的变化。采用互补X射线衍射、X射线荧光和扫描电子显微镜分析技术对沉积物颗粒进行了表征。除了反映SST季节性的Sr/Ca和U/Ca变化外，文石层与包覆沉积物颗粒(如高岭石、蒙脱石、石英)的文石层的交替也反映了常量和微量元素的系统变化。这种元素模式是季节性环境机制的结果，在这种机制下，沉积物颗粒周期性地增加对珊瑚礁环境的输入，导致高浓度的非晶格结合元素(例如，Al、Mn、Mg)。根据化学、矿物学或地质证据，可以排除非晶格结合元素(即与粘土矿物有关的元素)的潜在来源，如气载撒哈拉尘埃、尼罗河来源的沉积物悬浮物以及火山灰坠落。相反，归一化元素模式表明，粘土矿物是当地岩石(如蛇绿岩)的风化产物，目前蛇绿岩出露在山区的克里特岛上，并在晚中新世形成易受侵蚀的岛屿。非晶格结合元素的高浓度与冬季月的低SST相关。δ-18O与锶/钙季节性之间的海温估算差异表明，来自冬季强降水的淡水导致了δ-18O年振幅的降低，并将悬浮物输送到近岸礁体。因此，我们将非晶格结合元素浓度的季节性与地中海东部地区在中新世晚期的气候变化联系起来。我们的研究结果表明，地中海东部可能至少在中新世晚期~9 Ma出现过降水季节性较强、冬季降水事件较多的地中海型气候。
Sediment particles incorporated into coral skeletons reflect variation in composition and amount of suspended material in ambient water during coral growth. They can be used to identify periods of enhanced storm frequency and associated freshwater discharge. Tortonian (Late Miocene) Porites corals from Crete (Aegean Sea, Eastern Mediterranean) show pronounced annual density bands in X-ray photographs. δ18O compositional variability reflects the annual banding equivalent with a ~7 °C annual sea surface temperature (SST) cycle over a seven-year period. Fine sediment particles are concentrated in layers with skeletal porosity parallel to growth increments. Variations in the chemical composition of coral skeletons can result from changes in the environment. Therefore, variations in element concentrations with a spatial resolution of ~500 µm along a transect perpendicular to growth increments were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Complementary X-ray diffraction, X-ray fluorescence and scanning electron microscope analytical techniques were applied in order to characterize the sediment particles. Besides Sr/Ca and U/Ca variability indicating SST seasonality, the alternation of layers of pure aragonite and layers of aragonite coated with sediment particles (e.g., kaolinite, montmorillonite, quartz) is reflected in systematic variations in major and trace element concentrations. This element pattern results from a seasonal environmental mechanism where periodically enhanced input of sediment particles into the coral reef environment causes high concentrations of non-lattice bound elements (e.g., Al, Mn, Mg). Potential sources for the non-lattice bound elements (i.e., elements related to the clay minerals) such as airborne Saharan dust, Nile-derived sediment suspension as well as volcanic ash fall can be excluded on the basis of chemical, mineralogical or geological evidence. In contrast, normalized element patterns indicate that the clay minerals represent weathering products of local rocks, e.g., ophiolites, which at present crop out on Crete in mountainous areas, and formed islands exposed to erosion during the Late Miocene. High concentrations of the non-lattice bound elements correlate with low SSTs of the winter months. The discrepancy in SST estimations between δ18O and Sr/Ca seasonality suggests that fresh water originating from intense winter rain caused a reduction of the annual δ18O amplitudes and transported suspended material to the near-shore reef. Thus, we relate the seasonality in non-lattice bound element concentrations to climate variations in the Eastern Mediterranean region during the Late Miocene. Our results indicate that the present-day Mediterranean-type climate with a strong seasonality of precipitation and heavy winter rainfall events may have occurred at least temporarily during the Late Miocene at ~9 Ma in the Eastern Mediterranean.