基于沉积成因地化元素指标的闽北海湾晚更新世海侵地层辨识及其意义*

福建沿岸晚更新世地层中微体古生物化石较为缺乏,极大限制了对该区晚第四纪海侵过程的深入研究。针对这一问题,本研究以闽北2个海湾内的第四纪钻孔为研究对象,通过有孔虫化石以及沉积成因的地球化学元素(Sr/Ba、Mn/Fe)分析,结合三沙湾表层沉积物相应指标,探讨沉积成因的地球化学元素指示海侵地层的可行性。结果显示,2个海湾钻孔的晚第四纪地层中均发育3套疑似海相深灰色黏土层,其中开放海湾霞浦湾钻孔底部黏土层中缺失有孔虫,半封闭海湾三沙湾钻孔中有孔虫仅出现于顶部黏土层中。这3层黏土层的Sr/ Ba值远大于1,和现代潮滩—河口湾体系相近,且出现的海相沟鞭藻和自生黄铁矿为典型的海相地层特征。基于已有的OSL和AMS¹⁴C年龄框架,这3层海侵层分别对应于MIS5、MIS3和MIS1期,Sr/Ba和Mn/Fe均值从MIS5到MIS1期不断增大,结合现代潮滩-河口湾体系指标,表明从MIS5到MIS1期海侵强度的增加。由此可见,沉积成因地球化学元素指标可以有效辨识闽北海湾晚更新世海侵地层,这将有助于进一步了解该区晚第四纪海侵过程及其控制因素。

Identification and significance of the Late Pleistocene transgressive strata in the bays of northern Fujian Province based on geochemical element indicators of sedimentary origin

The lack of micropaleontological records in the Late Pleistocene strata along the Fujian coasts has greatly restricted the further understanding on the regional land-sea interaction. This study aims to identify the transgressive strata of the Late Pleistocene in the two bays of northern Fujian,by using geochemical elements of sedimentary origin. Based on the analysis of the foraminifera and surficial sediments of Sansha Bay,the Sr/ Ba and Mn/ Fe are used as proxies to identify the possibility of transgression in the study area. The results suggest that there are three sets of dark grey layers in the two late Quaternary cores might be formed in a marine environment. Foraminifera are missed at the bottom of drilling cores in the open Xiapu Bay,while foraminifera are only observed in the upper drilling cores of semi-closed Sansha Bay. However,the sedimentary Sr/ Ba ratios in the three clay layers are much higher than 1 and similar to the modern tidal flat and bay system,indicating typical marine origins. This is also supported by the occurrence of marine dinoflagellates and abundant authigenic pyrites in these layers. Based on the existing chronology of the core(NDGK2),the three transgressive layers are possibly corresponding to the high-stands of the MIS5,MIS3 and MIS1,respectively. The mean ratios of Sr/ Ba and Mn/ Fe show an increase trend from MIS5 to MIS1,indicating marine influence is enhanced compared with the ratios of modern ti dal flat-bay system. This study shows geochemical elements of sedimentary origin can be used as an effective proxy to identify the Late Pleistocene transgression strata along the Fujian coast,which will help to further understand the process and controlling factors of the late Quaternary transgression in this area.