珠江三角洲埋藏古森林地层年代与沉积特征

STRATIGRAPHIC CHRONOLOGY AND SEDIMENTARY CHARACTERISTICS FOR BURIED ANCIENT FORESTS IN THE PEARL RIVER DELTA

珠江三角洲四会市和高要市两地埋藏古森林在发育历史时期形成了一层或多层腐殖质层。其中,四会埋藏古森林腐殖质层发育起止年代大致为4218±91 cal.aB.P.～3291±24cal.aB.P.;高要埋藏古森林腐殖质层发育有3期,它们的年代大致为49l0±64～1966±42cal.aB.P.,但在4347±63～4017±35cal.aB.P.和3658±45～3539±19cal.aB.P.之间发育中断,形成两层灰白色粘土层,3期腐殖质层(从上至下)发育持续的时间分别为1500a,400a和500a,约1000±500a。在四会埋藏古森林腐殖质层中,有机碳含量在26.2%～48.9%之间变化,δ^(13)C值波动介于-29.8‰～-25.6‰之间,其中,粘土层与腐殖质层边界点的有机碳含量为26.2%,δ^(13)C值为-25.6‰,对应的年代为3291±24cal.aB.P.;在高要埋藏古森林腐殖质层中,有机碳含量在20.3%～64.0%之间变化,δ^(13)C值波动介于-30.9‰～-29.0‰之间,而在腐殖质层之间的粘土层的中心位置,有机碳含量从上至下分别为1.0%和8.8%,对应的δ^(13)C值分别为-28.2‰和-27.8‰,较相邻腐殖质层平均δ^(13)C值偏正约2.0‰至2.5‰。腐殖质层有机碳含量与δ^(13)C值显示,埋藏古森林腐殖质层形成于湿地环境,而粘土层中有机碳含量和δ^(13)C值与腐殖质层中的显著差异及粘土层的沉积特征则说明粘土层很可能形成旱地环境。沉积环境干湿变化的周期与腐殖质层持续的时间一致,大致为1000±500a,这种变化可能与中全新世以来气候在千百年尺度上的波动相关,而四会和高要两地古森林湿地发育的起止时间不一致则主要与两地的地理位置及地形不同相关。

Sihui and Gaoyao are both located in the Pearl River Delta. The buried ancient forests with an area about 0.02km^2 in Sihui is situated at Longfu（23 ～22＇359＂N, 112～42＇497＂E） , the center of Sihui. The buried ancient forest profile in Sihui is 4.8m high, consisting of four layers ： the yellow brown coarse aleuritic clay layer （ layer A,0 ～ 1.5m）, the grey fine aleuritic clay layer（ layer B, 1.5 -2.0m）, the brown humic layer（ layer C, 2.00 -4.41m） with the altitude of about 0m on the surface and the grey aleuritic clay layer（layer D,4.41 ～ 4. 80m）. The buried ancient forest with an area of about 0.03km^2 in Gaoyao is situated at B aizhu （22～ 53＇19＂N, 112～19＇66＂E）, a foreland basin at the southwest of Gaoyao. The sedimentary profile in Gaoyao is 5.6m in depth and the altitude is about -5m on the surface of the first humic layer. The surface layer is the grey fine aleuritie clay layer（A）about 75cm thick,where charcoal and plant detritus are found. Lots of stumps of about 40cm in height are found standing on the first humic layer. Three humic layers （ B, D, and F） are found in the Gaoyao profile, separated by two white grey clay layers（C and E）with a depth of about 27cm and 36em from up to down,respectively. 27 samples, including 23 sediment samples and four samples from the bark and core of two ancient trees from Sihui profile, as well as 27 samples consisting of 25 sediment samples and one charcoal and one plant detritus sample from Gaoyao profile, are collected. The AMS-＇4C dating method, organic carbon （OC）o～3C analysis with a precision of 0.02%o and OC content of sediments analysis are applied in all these samples. Results show that the humic layer of ancient forests in Sihui began to develop at about 4218 ±91cal. aB. P. and ended at around 3291±24cal. aB. P. In other locations from Gaoyao, the humic layer started earlier than 4910_± 64cal. aB. P. and terminated at around 1966±42eal. aB. P., however, the humic layer was interrupted twice during 4347±63cal. aB. P. to 4017±35cal. aB. P. ,and 3658±45cal. aB. P. to 3539±19cal. aB. P. when two white-grey clay layers were formed. Each humic layer formed either in Gaoyao or Sihui continued about 1000_±500a. Except for the site at 201cm the boundary to clay layer, OC contents varied between 33.4% and 48.9%, and the o～3C values fluctuated between -29.9%o and -29. 0%o in the humic layer of buried ancient forests in Sihui. While,in the humic layers of buried ancient forests in Gaoyao, OC contents varied between 20. 3% and 64.0%, and the ～3C values fluctuated between -30.9%o and -29.0%o,however,less than 1%o shift of bJ3C value was observed in each humic layer. The above evidences all suggest that the humic layers should be formed in a wetland environment which continued approximately 1000_±500a each time. The appearance of the clay layers represented the transition of the sedimentary environment. As showed in the upper clay layers in Sihui profile, the OC contents were shifted from 0. 8% to 7.8% with OC 8～3C values fluctuating between -29.0%o and -26.0%o,especially,at 201cm the boundary site to clay layer,a positive shift of ～3C about 4. 1%o is observed. As the same,the OC contents at the central sites of the clay layers C,E are 1.0% and 8.8% ,and the OC 813C values are -28.2%o and -27.8%o correspondingly, showing positive shifts about 2.0%o and 2.5%o from the mean OC 8＇3C values in the close humic layers in Gaoyao profile. Combined with the field observation,the significant differences in OC contents and OC ＆^13C values between clay layers and humic layers reflected a sedimentary environment transition from a wetland one to a dry land one during the time when the clay layers were formed. The transition of sedimentary environment was likely related with the climate fluctuation in millennium and centennial scale since Mid-Holocene. Locations and geography played an important role on the initial time and termination time of the wetland environment in Sihui and Gaoyao.