珠江三角洲晚第四纪环境演变的微体生物记录

珠江三角洲是主要由西江、北江和东江在珠江口湾内堆积而成的复合三角洲,自晚更新世开始形成。微体生物是珠江河口三角洲晚第四纪沉积记录中的重要组分,是重建第四纪环境变化极为有效的指标。本文样品取自PD孔岩心,钻孔位于珠江三角洲中部(113°28′35″E, 22°53′41″N),孔深16．7 m。代表约25 100年以来的沉积。PD孔中见有孔虫6属10种,其中以Ammonia beccani为优势种。根据地层分布特征,可识别3个有孔虫层位。介形类共7属9种,丰度较低,以Sinocytheridea impressa和Neomonoceratina delicata为代表,在5个层位相对富集。根据微体生物分布特征,并结合有机碳、氮以及磁化率数据,PD孔晚第四纪沉积可划分为6个沉积环境阶段。A．三角洲前缘阶段(-16．7～-13．5 m;25 100～24 000 aB．P．):岩性为粗砂、砂至砂质粉砂。主要含Ammonia属的4个种(A．beccarii、A．sp．、A．tepida、A．annectens),丰度均较低,其中Ammonia beccarii为优势种。不见适应于正常浅海的Elphidium属。介形类以滨岸类型Sinocytheridea impressa为主。出现少量双壳类。代表河口附近水深10 m以内软泥底质、受河流影响大、水体动荡的半咸水环境。有机碳含量和磁化率较低,同样指示低海平面时期的沉积。B．河口湾阶段(-13．5～-10．6 m;24 000～22 400 aB．P．):岩性为粘土质粉砂至粘土。前一阶段出现的Ammonia 4个种的丰度比较高,且出现浅海底栖分子Elphidium advenum和E．hispidulum,以及浮游有孔虫分子Globigerinoides sp．。介形类出现Bicornucythere leizhouensis、B．bisanensis、Alocopocythere goujoni、Pistocythereis bradyformis等适应水深较大的分子,而滨岸浅水分子较少。有机碳含量和磁化率相对升高。代表海平面相对较高时期的沉积。C．三角洲平原阶段(-10．6～-9．2 m;22 400～21 700 aB．P．):底部为一薄层花斑状风化粘土,表明曾短期露出水面。向上变为富含有机质的深灰色粉沙质粘土。不含微体生物和双壳类,反映不稳定的环境条件。由于海平面降低,磁化率表现为持续低值。有机碳和C/N比值很高,表明有丰富的陆源有机质输入,海洋影响较小。D．陆地风化阶段(-9．2～-7．4 m;21 700～3 700 aB．P．):岩性为1．6 m厚的杂色粘土,不含任何生物化石,有机碳含量极低,是长期强烈风化作用的产物,对应于末次冰盛期。E．潮坪沉积阶段(-7．4～-2．5 m;3 700～2 600 aB．P．):互层状深灰色粉沙质粘土、灰色粉沙和细沙,含广盐性有孔虫Ammonia beccarii和Arenoparrella asiatica,以及少量介形类,如Sinocytheridea impressa。典型的半咸水双壳类Potamocorbula amurensis较为丰富,另外还见藤壶和虫管碎片,指示盐度频繁波动的潮间带环境。F．上三角洲平原阶段(2 600 aB．P．以来):未见任何有孔虫和双壳类,顶部见少量陆相介形类Cypria yanchengensis,磁化率变化频繁,C/N比值较高,表明陆相影响不断增强,且环境条件不稳定。研究区晚第四纪古环境的演化是由海平面升降变化控制的。上述沉积环境变化序列表现出两次海进一海退旋回,分别对应阶段A至阶段D和阶段E至阶段F,相应于晚更新世和全新世的两次海平面上升事件。

LATE QUATERNARY MICROFAUNAS AND PALEOENVIRONMENTAL CHANGES RECORDED IN CORE SEDIMENTS OF THE PEARL RIVER DELTA, SOUTH CHINA

Late Quaternary core sediments from the middle area of the Pearl River Delta can be lithologically divided into six units. Unit 1,2, and 5 contain foraminifers and ostracods in different abundances. Faunal analysis suggests that they were deposited in brackish to marine environments. Unit 3 does not contain any fossils except plant debris, indicating a fluvial plain environment. Unit 4, characterized by a layer of mottled clay that is widely distributed in the Pearl River Delta area, is completely barren of fossils, representing an interval of continental weathering. Unit 6 bears only non-marine ostracods and was deposited on a fluvial plain. Combining foraminiferal and ostracod data with geochemical and susceptibility records allows to establish the following environmental stages in this area; A. Delta front stage (25 100～24 000 aB. P. ) ; B. Estuary stage (24 000～22 400 aB. P. ) ; C. Fluvial plain stage (22 400～21 700 aB. P. ) ; D. Continental weathering stage (21 700 ～ 3 700 aB. P. ) ; E. Tidal flat stage ( 3 700～2 600 aB. P. ) , and F. Upper fluvial plain stage (since 2 600 aB. P. ). In the Late Quaternary sedimentary succession two transgression-regression cycles can be recognized in the study area, corresponding to stages A to D and E to F, respectively. The palaeoenvironmental changes are believed to have been controlled by fluctuations of sea level.