COMPOSITIONS OF ORGANIC CARBON AND NITROGEN AND CARBON ISOTOPE OF POSTGLACIAL SEDIMENTS IN THE YANGTZE RIVER DELTA AND THE PALEOENVIRONMENT IMPLICATION
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摘要: 长江三角洲的CM和HM孔揭示了冰后期以来的河床相、河漫滩-河口湾相、浅海相和三角洲相的沉积环境。钻孔的有机碳、总氮和有机碳同位素δ13C以及碳酸盐组成在不同沉积相中变化规律不一致,基本上落在现代长江主要支流和干流的沉积有机质组成范围内,而偏离河口地区的表层沉积有机质组成,反映长江流域的高等植被是三角洲冰后期沉积有机质的主要来源,而水生生物有机质的贡献较少。冰后期早期河床相和晚期三角洲相的沉积有机质组成变化要大于中间的河漫滩、河口湾到浅海相沉积。冰后期沉积有机质和碳酸盐组成基本上可以反映冰后期长江流域的古气候变化特征,主要的古气候波动变化与孢粉研究结果相似;但全新世大暖期和古洪水等气候变化事件在沉积有机质组成上的反映不显著。显然,河口三角洲地区由于陆海相互作用强烈,沉积地层不连续,且沉积有机质来源比较复杂和沉积环境变化大,运用全岩有机元素和有机碳同位素以及CaCO3组成指示古环境有相当的难度和多解性,需要寻找更敏感与可靠的古气候与古环境的替代指标。Abstract: Postglacial deposition in the Yangtze River delta primarily consists of riverbed, flood plain as well as estuarine, shallow marine and delta facies. Compositions of organic elements, organic carbon isotope and calcium carbonate show systematic variations in core CM and HM sediments, suggesting paleoclimatic variability in the Yangtze drainage basin including the delta during the postglacial period. The analytical results correspond well with the pollen data but there are also some fluctuations. Organic matter deposited in the fluvial and delta facies mainly came from terrestrial C3 plant, and intense human activity during last 2 000 years might have changed the soil weathering and vegetation types in the drainage basin. Because of intense land-sea interaction and widespread discontinuity in the postglacial sedimentary strata of the Yangtze River delta it is difficult to reliably decipher the palaeoenvironmental changes in the estuarine and delta area just through the study of bulk compositions of organic elements, carbon isotope and CaCO3. Therefore, seeking more sensitive and reliable proxies for palaeoenvironmental changes is urgently needed.
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Key words:
- paleoenvironment /
- organic elements /
- sediment /
- Holocene /
- the Yangtze River
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