珠江三角洲晚第四纪红树林的演化及其环境意义
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摘要
广州番禺东涌剖面样品的AMS(磁谱仪)年龄及红树林孢粉分析结果表明,珠江三角洲存在晚更新世和全新世的红树林,红树林是海侵的标志之一.按照检出红树属花粉的9个剖面的分布以及5处埋藏红树林腐木的分布,可以描绘出全新世海侵的边界.根据东涌剖面的沉积旋回、样品年龄、红树林花粉含量曲线、半咸水-淡水硅藻及热性-温性硅藻含量曲线的综合分析,可将沉积环境的变迁分为4个阶段.第一阶段和第三阶段的海侵有利于红树林的发展,第二阶段不利于红树林的发展,第四阶段的晚全新世,红树林的生长频繁,显示人为因素的影响.
The depositional environment of the Pearl River Delta in the Late Quaternary and the spatio-temporal change in the climate are discussed on the basis of serial sampling and the results of sporo-pollen analysis of mangroves and accelerator mass spectrometry dating of samples.The profile of Dongchong in Panyu,Guangzhou is taken as a representative and associated with correlative profiles.The research results show that there were mangroves during the Late Pleistocene and Holocene in the Pearl River Delta.The mangroves are an indicator of transgression.The boundary of Holocene transgression can be traced according to the distribution of nine profiles for which the content of Rhizophora sporo-pollen is observed and five locations of buried rotted mangrove wood.Changes in the depositional environment can be divided into four stages on the basis of comprehensive analysis of the sedimentary cycle,dating of samples and the three curves of the contents of mangrove sporo-pollen,brackish fresh water diatom and hot warm species diatom in the Dongchong profile.The first stage was the Last Subinterglacial Period of MIS3,before 25 000 aB.P.,and the lithology was a coarse bottom grain layer of the sedimentary cycle with depth from 24.8 to 29.8 m.The mangroves flourished,with the increase in mangrove abundance peaking at 6.5%.The brackish fresh water diatom content was only 20.0%,while a higher hot warm species diatom content indicated a warmer climate.This stage of transgression was favorable for the development of mangroves.The second stage was the final phase of the Last Glacial Period and Deglacial Period,from 25 000 to 11 000 aB.P.The lithology was a granule layer with depth from 16.0 to 24.8 m,and the mangrove sporo-pollen content decreased to 2.0%.No brackish fresh water diatom was detected,and the hot warm species diatom content decreased from 26.1% to 12.2%,and the climate became colder.This environment of the regression was unfavorable for mangroves.The third stage was the Early Holocene and Mid-Holocene,from 11 000 to 2 500 aB.P.,and the lithology was a sedimentary cycle of the bottom with depth from 9.1 to 16.0 m,which predicated the environment of the transgression.The mangroves again transitioned from a state of decline to a state of flourishing,with the sporo-pollen content increasing to 3.7%.The brackish fresh water diatom content decreased from 48.1% to 13.0%,while the hot warm species diatom content increased to 36.8%.This stage with warmer climate favored the growth of mangroves.The fourth stage was the Late Holocene,after 2 500 aB.P.,and the lithology was piebald clay of the sedimentary cycle of the top of the middle part with mantle rock,with the mantle rock depth ranging from 7.4 to 9.1 m,indicating a contaminated terrestrial environment,and the top was a sedimentary cycle.The proportion of brackish fresh water diatom was as high as about 57.9%,and later decreased sharply to 19.2%,while the hot warm species diatom content changed rapidly from 1.8% to 25.1%.The content of mangrove sporo-pollen increased to 7.3%,which was the highest level in the profile,and the content then quickly became very unstable.During the fourth stage of the Late Holocene,the mangrove growth varied frequently,and the continual alteration of the contaminative environment indicates the effects of artificial factors.
The depositional environment of the Pearl River Delta in the Late Quaternary and the spatio-temporal change in the climate are discussed on the basis of serial sampling and the results of sporo-pollen analysis of mangroves and accelerator mass spectrometry dating of samples.The profile of Dongchong in Panyu,Guangzhou is taken as a representative and associated with correlative profiles.The research results show that there were mangroves during the Late Pleistocene and Holocene in the Pearl River Delta.The mangroves are an indicator of transgression.The boundary of Holocene transgression can be traced according to the distribution of nine profiles for which the content of Rhizophora sporo-pollen is observed and five locations of buried rotted mangrove wood.Changes in the depositional environment can be divided into four stages on the basis of comprehensive analysis of the sedimentary cycle,dating of samples and the three curves of the contents of mangrove sporo-pollen,brackish fresh water diatom and hot warm species diatom in the Dongchong profile.The first stage was the Last Subinterglacial Period of MIS3,before 25 000 aB.P.,and the lithology was a coarse bottom grain layer of the sedimentary cycle with depth from 24.8 to 29.8 m.The mangroves flourished,with the increase in mangrove abundance peaking at 6.5%.The brackish fresh water diatom content was only 20.0%,while a higher hot warm species diatom content indicated a warmer climate.This stage of transgression was favorable for the development of mangroves.The second stage was the final phase of the Last Glacial Period and Deglacial Period,from 25 000 to 11 000 aB.P.The lithology was a granule layer with depth from 16.0 to 24.8 m,and the mangrove sporo-pollen content decreased to 2.0%.No brackish fresh water diatom was detected,and the hot warm species diatom content decreased from 26.1% to 12.2%,and the climate became colder.This environment of the regression was unfavorable for mangroves.The third stage was the Early Holocene and Mid-Holocene,from 11 000 to 2 500 aB.P.,and the lithology was a sedimentary cycle of the bottom with depth from 9.1 to 16.0 m,which predicated the environment of the transgression.The mangroves again transitioned from a state of decline to a state of flourishing,with the sporo-pollen content increasing to 3.7%.The brackish fresh water diatom content decreased from 48.1% to 13.0%,while the hot warm species diatom content increased to 36.8%.This stage with warmer climate favored the growth of mangroves.The fourth stage was the Late Holocene,after 2 500 aB.P.,and the lithology was piebald clay of the sedimentary cycle of the top of the middle part with mantle rock,with the mantle rock depth ranging from 7.4 to 9.1 m,indicating a contaminated terrestrial environment,and the top was a sedimentary cycle.The proportion of brackish fresh water diatom was as high as about 57.9%,and later decreased sharply to 19.2%,while the hot warm species diatom content changed rapidly from 1.8% to 25.1%.The content of mangrove sporo-pollen increased to 7.3%,which was the highest level in the profile,and the content then quickly became very unstable.During the fourth stage of the Late Holocene,the mangrove growth varied frequently,and the continual alteration of the contaminative environment indicates the effects of artificial factors.
引文
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[2]李平日,方国祥,黄光庆.珠江三角洲全新世环境演变[J].第四纪研究,1991,2(6):130-139.
[3]杨小强,李华梅,余素华.从SX97孔分析深圳沿海近3万年来海平面及气候变迁[J].地球化学,2003,32(2):146-154.
[4]黄镇国,李平日,张仲英,等.深圳地貌[M].广州:广东科技出版社,1983:117-118.
[5]蓝先洪.珠江三角洲第四纪沉积特征[J].沉积学报,1996,14(2):155-161.
[6]雷作淇,郑卓.黄茅海晚第四纪孢粉组合与环境变迁[J].热带海洋,1990,9(4):24-27.
[7]严维枢,范时清,吴作基,等.香港晚第四纪古环境和沉积作用[J].华南地震,1988,8(3):39-51.
[8]黄镇国,李平日,张仲英,等.珠江三角洲形成发育演变[M].广州:科学普及出版社广州分社,1982.
[9]黄镇国,张伟强,陈俊鸿.珠江三角洲生物埋葬群与环境变迁[J].地理学报,1995,50(4):310-322.
[10]黄镇国,张伟强,许世友,等.中国日本全新世环境演变对比研究[M].广州:广东科技出版社,2002:181-188.
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