珠江三角洲MIS 5间冰期海平面波动新证据

NEW EVIDENCE FOR SEA LEVEL CHANGES DURING THE MARINE ISOTOPE STAGE 5 IN THE PEARL RIVER DELTA

珠江三角洲第四系广泛发育沉积间断,致使该区域的古环境重建仍然存在诸多疑问,特别是对全新统以下的沉积旋回发育时代及其反映的海平面变化历史等争议尚大。近年从珠江三角洲中部平原钻取的一个长50多米的岩芯中,首次在第四纪地层中见到6个明显的红色沉积层,综合利用沉积学、年代学及区域对比分析后认为,该套地层包含了MIS 6以来的沉积,6个红色沉积层中,除最顶部红色层为末次冰期(MIS 4~2)形成的典型花斑粘土层外,其余5个红色层均应为MIS 5间冰期海平面阶段性降低的沉积产物,反映在MIS 5海侵过程中海平面有过多次显著波动。地球化学和矿物学分析结果进一步指示,自下而上6个红色沉积层经受过的化学风化作用强度增大,指示MIS 5以来研究区相对水面波动降低的幅度和(或)持续时间从早期到晚期呈增加趋势。

The Pearl River Delta(PRD)is formed by interaction of land and sea at the month of Pearl River system.Researches on the developing history of the PRD have been conducted since about a hundred years ago,and many important results have been gradually achieved.Nevertheless,due to the complex basement topography,the widespread sedimentary discontinuities in the PRD,and the difficulty of obtaining accurate ages for sedimentary sequences in coast areas,many questions in the PRD's paleoenvironment reconstruction still remain uncertain,with the environmental evolution Pre-Holocene being particularly controversial.This study presents new findings concerning the Marine Isotope Stage 5 in the PRD region,China based on the study of a 58.5 m borehole(22°43'13.98"N,113°23'44.63"E).The core stratum contains seven terrestrial units(T1,T2a^T2e and T3)and seven marine units(M1,and M2a^M2f).Six radiocarbon dates obtained from the upper part of the core and OSL dating obtained from the adjacent cores of the PRD as well as regional comparison are used to help define the chronology of stratigraphic units.Mineralogy and geochemical analysis were used to reconstruct the paleoenvironment.The results show that before the area was inundated by the last interglacial sea level rise,an older terrestrial unit of sand and gravel(T3)was laid down in the receiving basin.During the Marine Isotope Stage 5(MIS 5),stratigraphic units including five terrestrial units of red silt and clay(T2a^T2e)and six marine units of grey silt and clay(M2a^M2f)were developed,with red sediment layers corresponding to relatively lower sea level and weathering period,and the grey layers corresponding to relatively higher sea level and marine sedimentary period,indicating that sea level had fluctuated obviously during MIS 5 in the PRD region.The MIS 5 stratum was subsequently subaerially exposed when sea-level deeply regressed during the last glacial period(MIS 4~2),causing the uppermost section to be strongly weathered(T1).Since the early Holocene,rapid postglacial rise in sea level initiated a new phase of sedimentation(M1).