长江口盐沼滩面演化的沉积和地球化学特征

Sedimentological and geochemical characteristics for evolution of salt marsh in the Changjiang River Estuary in China

选择长江口盐沼典型高程部位(高潮滩上部、中潮滩下部、光滩)柱状样,采用小间隔(1 cm)取样,通过粒度、颗粒有机碳(POC)含量与碳稳定同位素组成(δ13C)分析,研究盐沼滩面发育的沉积和地球化学特征,结果表明,盐沼沉积物的粒度参数、POC含量与δ13C值随深度变化明显;在盐沼典型高程部位沉积物的颗粒组成与地球化学特征差异显著,有明显的滩面趋势,这与滩面的规律性演化密切相关.柱样有限深度内有明显的相对淤积期与冲刷作用相对较强时期的交替,表明长江口盐沼滩面发育过程并不是线性的.盐沼沉积物黏粒含量对POC含量和δ13C值的垂向变化及滩面趋势影响显著,表明流域来水来沙明显制约河口盐沼的发育.对于沉积动力环境复杂、冲淤过程频繁、厚度为毫米级的砂质纹层和泥质纹层发育的河口盐沼,在沉积和地球化学研究中采用小间隔取样的效果明显,有助于揭示盐沼发育过程的更多信息,为年际滩面冲淤演变研究提供新途径.

Sedimentological and geochemical characteristics for the evolution of salt marshes in the Changjiang River Estuary （CRE） were studied based on analyses of grain sizes, contents of particulate organic carbon （POC） and compositions of stable carbon isotopes （δ^13C） for core samples in a small depth interval （1 cm） from three locations with typical elevations （the landward part of high tidal flat, the seaward part of middle tidal flat and bare flat） at the eastern Chongming beach salt marsh in the CRE. Results indicated that grain-size coefficients, POC contents and δ^13C values showed obvious fluctuations with depth, respectively. There existed predominant differences in grain size and geochemical characteristics between the three typical locations of the salt marsh, and clear tendencies from the high tidal flat to the bare flat in grain size and geochemical characteristics, respectively. This was correlated closely to the regular evolution of the tidal flat. Fluctuations between periods with weak erosion and periods with relative intense erosion were discerned from the limited depth of the cores from high tidal flat and middle tidal flat, suggesting that the evolution processes of the salt marsh were nonlinear in the CRE. Clay contents exerted great influences on both flat tendencies and vertical changes of POC contents and δ^13C values, respectively. This indicated that evolution of the salt marsh in the CRE was constrained obviously by discharges of water and suspended matter from fluvial catchments of the Changjiang River. Sampling in small depth interval （41 cm） was then effective in sedimentological and geochemical studies on the salt marshes with complex depositional dynamics, frequent accretion and erosion, and that muddy laminae and silty laminae in millimeter thickness developed well in strata. Evolution processes of salt marshes could be studied in detail by this sampling strategy, which provided a new measure for studies on annual development of salt marshes.