Rapid formation of marsh-edge cliffs, Jiangsu coast, China
Rapid formation of marsh-edge cliffs, Jiangsu coast, China
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中国江苏沿海沼泽边缘悬崖的快速形成
DOI：
10.1016/j.margeo.2017.02.001
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发表时间：
2017
期刊：
影响因子：
2.9
通讯作者：
Gao Shu
中科院分区：
文献类型：
--
作者：
Zhao Yangyang;Yu Qian;Wang D;an;Wang Ya Ping;Wang Yunwei;Gao Shu
互花米草的人工引入和快速扩张极大地改变了滩涂的自然演变，特别是江苏沿海的中国。滨海沼泽边缘(沼泽边缘悬崖)高程差的出现，表明潮滩经历了严重的区域侵蚀和由平缓坡面向阶梯状剖面的状态转变。然而，控制沼泽边缘悬崖形成和滩涂状态转换的机制仍不清楚，沼泽-悬崖-滩涂系统的未来演化也不明朗。本文以江苏盐城自然保护区为例，通过考察2008-2014年间的跨岸地形变化和2013-2014年间的沉积物特征、植被生物量、沼泽沉积速率、海流和海浪，首次展示了江苏盐城自然保护区沼泽边悬崖在不到5年的时间里快速形成的过程。利用一维跨岸模型估算了潮滩跨岸剖面变化时波浪和水流的相对优势，有助于解释沼泽边缘悬崖的形成。引入浮游植物后，沼泽地面积迅速增加。互花米草(约4.3-5.3亿厘米/年)加速了沼泽边缘向海的移动，缩小了裸露平原的宽度；同时，近海沉积物供应的减少引发了较低的裸露平原的下切(例如，2010年至2011年平均海平面最大约0.5百万米)，加强了波浪作用，向陆地方向侵蚀裸露的平原。这两个过程共同导致海浪(0.43-0.90亿N/m~2)高于海流(主要是0.40亿N/m~2)，这与数值模拟结果一致。因此，基床坡度从2008年的0.6‰增加到2014年的1‰，容易受到海浪的攻击。在2011-2013年间，下伏沉积物的植被-根系结合和沼泽边缘的波浪破碎最终促进了沼泽边缘悬崖的形成。据粗略估计，2013-2014年间，这些悬崖后退了2000万至1.1亿米。未来滩涂的演变伴随着悬崖的不断后移，新沼泽区的启动和向海扩展，以及新的沼泽边缘悬崖的形成，类似于滞后效应，因为向海推进和向后侵蚀所需的时间几乎相当于9-50年。潮滩演化的这种滞后效应可能会进一步改变当地生物群落结构和海岸有机碳流的途径。
The artificial introduction and rapid expansion ofSpartinaalternifloraLoisel (S. alterniflora) have greatly changed the natural evolution of tidal flats, especially along the Jiangsu coast, China. The occurrence of an elevation gap at the seaward marsh margin (marsh-edge cliffs) manifests that tidal flats have experienced severe regional erosion and the state transformation from the smooth slope to the stepped profile. However, the mechanisms controlling the formation of marsh-edge cliffs and the state transformation of tidal flats remain elusive while the future evolution of a marsh-cliff-flat system is unclear. Here we show the rapid formation process of marsh-edge cliffs within less than five years in Yancheng Nature Reserve, Jiangsu coast, China, as a fantastic example for the first time, via examining the cross-shore topographic changes during 2008–2014 and investigating sediment characteristics, vegetation biomass, marsh deposition rate, currents and waves during 2013–2014. A one-dimension cross-shore model was used to estimate the relative predominance of waves and currents in response to the variations of tidal flat cross-shore profile and help interpret the formation of marsh-edge cliffs. The fast accretion of the marsh area after the introduction ofS. alterniflora(approximately 4.3–5.3 cm/yr) accelerated the seaward shift of marsh margin, narrowing the width of bare flat; meanwhile, reduced sediment supply from offshore triggered the down-cutting of the lower bare flat (e.g., the maximum of about 0.5 m at the mean sea level from 2010 to 2011), strengthening wave actions to erode bare flat landward. These two processes jointly led to the predominance of waves (0.43–0.90 N/m2) over currents (mostly < 0.40 N/m2) on the upper bare flat, which is consistent with the numerical modeling outcomes. As a consequence, the bed slope increased from 0.6‰ in 2008 to 1.0‰ in 2014, being vulnerable to wave attacks. The vegetation-root bonding of underlying sediments and the wave breaking at the marsh margin finally facilitated the formation of marsh-edge cliffs during 2011–2013. The cliffs were roughly estimated to retreat 20–110 m during 2013–2014. The future evolution of tidal flat accompanied by the continuous backward shifting of cliffs, the initiation and seaward spread of new marsh area, and the formation of new marsh-edge cliffs, resemble the hysteresis effect, as seaward advance and backward erosion will take almost equivalent time of nearly 9–50 years. This hysteresis effect of tidal flat evolution might further alter the local biological community structure and the pathway of coastal organic carbon flows.