Mangrove forest degradation indicated by mangrove-derived organic matter in the Qinzhou Bay, Guangxi, China, and its response to the Asian monsoon during the Holocene climatic optimum
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摘要: 在全球气候变暖背景下,了解红树林生态系统对亚洲季风的响应可以通过重塑近岸沉积物记录的红树林演化过程实现。本文以广西钦州湾一钻孔岩心的一段时间跨度为5.6 ~ 7.7 cal. ka BP(相当于全新世大暖期)的沉积物为研究对象,在沉积物总有机碳、总氮含量和有机碳稳定碳同位素组成(δ13Corg)分析基础上,利用总有机碳与总氮含量之比(C/N)和δ13Corg, 采用有机质来源三端元混合模型,定量计算出沉积物混合源有机质中红树林源、陆源和海洋生物源有机质贡献比例,并以红树林源有机质贡献为红树林演化的示踪参数。研究表明,在全新世大暖期,沉积物中的红树林源有机质贡献相对较大,但却在~7.3、~6.9和~6.2 cal. ka BP呈现出3次显著的降低,指示了广西钦州湾红树林呈发生了3次明显衰替。3次红树林衰退事件恰好对应于亚洲夏季风的减弱(冬季风增强)事件,由此表明,即便在利于红树林繁盛的全新世大暖期,极端气候事件的发生(如受亚洲季风驱动的冷干事件)也会导致红树林的衰退。Abstract: The response of mangrove ecosystems to the Asian monsoon in the future global warming can be understood by reconstructing the development of mangrove forests during the Holocene climatic optimum (HCO), using proxies preserved in coastal sediments. The total organic matter in sediments of a segmented core, with calibrated age ranges between 5.6 and 7.7 cal. ka BP and corresponding to the HCO, from the Qinzhou Bay in Guangxi, China, is quantitatively partitioned into three end-members according to their sources: mangrove-derived, terrigenous, and marine phytoplanktonic, using a three-end-member model depicted by organic carbon isotope (δ13Corg) and the molar ratio of total organic carbon to total nitrogen (C/N). The percentage of mangrove-derived organic matter (MOM) contribution is used as a proxy for mangrove development. Three visible drops in MOM contribution occurred at ca. 7.3, ca. 6.9, and ca. 6.2 cal. ka BP, respectively, are recognized against a relatively stable and higher MOM contribution level, indicating that three distinct mangrove forest degradations occurred in the Qinzhou Bay during the HCO. The three mangrove forest degradations approximately correspond to the time of the strengthened/weakened Asian winter/summer monsoon. This indicates that even during a period favorable for the mangrove development, such as the HCO, climatic extremes, such as cold and dry events driven by the strengthened/weakened Asian winter/summer monsoon, can trigger the degradation of mangrove forests.
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