沉积成因Sr/Ba指示的福建潮控海湾全新世沉积环境及海平面意义

Holocene sedimentary environment and sea level significance of sedimentogenic Sr/Ba in Fujian tidal bays

福建沿海多发育潮控型海湾,潮滩平坦宽广,海湾水深较浅,有孔虫等微体生物较为匮乏,且其化石在地层中保存相对较差,致使沉积微相辨识存在一定难度,从而限制了深入理解该区全新世海平面变化过程及沉积环境响应。本研究拟利用不受保存环境限制的沉积成因Sr/Ba指标,探索其在福建宁德三沙湾地区沉积微相的辨识潜力,并结合AMS 14C年龄,探讨该区全新世以来的沉积环境演化及区域海平面变化。结果表明:①沉积成因Sr/Ba在三沙湾潮滩和海湾表层沉积物中平均值分别为9.06和20.43,具有显著差异。潮滩沉积物中的Sr/Ba明显高于长江等大河口地区(1~3),这是该区淡水输入量较少、潮滩盐度较高所致,这说明该指标辨识海陆过渡沉积相时需要考虑区域水文特点。②晚第四纪NDGK2钻孔沉积物Sr/Ba自下而上可分为3层(Ⅰ—Ⅲ):层Ⅰ为杂色硬黏土,Sr/Ba比值全孔最低(均值5.29),但也高于淡水环境,推测为前期海相沉积物的暴露改造;层Ⅱ—Ⅲ均为深灰色黏土,其中层Ⅱ均值为10.77,接近潮滩相;层Ⅲ均值为全孔最高(13.44),接近海湾相。结合AMS 14C年龄可知,NDGK2钻孔的河漫滩-潮滩-海湾沉积相演化受控于全新世海平面上升过程,潮滩相形成于约8.9 cal.kaBP,当时该区海平面大约在−21.0±2.5 m,最大海泛面出现于约8.2 cal.kaBP,之后出现了长达6 ka的低沉积速率时期,直至约2.2 cal.kaBP海湾内加速沉积。③该区15个钻孔全新统底部Sr/Ba均显现出明显的潮滩相特征,这为重建区域全新世相对海平面提供了可行性。

Tidal bays along the Fujian coast lack microorganisms(e.g.,foraminifera)and poor fossil preservation,resulting from wide tidal flats and shallow waters.This makes it difficult to rely on the fossils to understand the Holocene sea-level variation course and the sedimentary environment responses in this region.Therefore,this study aims to use the sedimentogenic Sr/Ba ratio index to identify the sedimentary facies in the Sansha Bay,Ningde,Fujian.In addition,AMS 14C dating was done to explore the sedimentary environment evolution and regional sea-level change since the Holocene.The findings revealed that:1)The mean value of sedimentogenic Sr/Ba ratio in the tidal flats sediments(9.06)is obviously lower than the surface sediments of Sansha Bay(20.43).However,the sedimentogenic Sr/Ba ratio in tidal flat sediments is significantly higher than those of large estuaries such as the Yangtze River estuary(1~3)where lower freshwater input and higher salinity in the tidal flat,suggesting that the regional hydrological characteristics shall be considered when applying the ratio index for examining the sedimentary facies in sea-land transition zones.2)The sedimentogenic Sr/Ba ratio of Core NDGK2 sediment of the Late Quaternary could be divided into three layers(Ⅰ-Ⅲ).From the bottom,LayerⅠis variegated hard clay,has a Sr/Ba mean value of 5.29,which is still greater than typical freshwater environments,indicating exposure and alteration of older sediments.Layers Ⅱ and Ⅲ consist of dark gray clay,of which Layer Ⅱ has a mean value of 10.77,similar to the tidal flats facies,while Layer Ⅲ has the greatest mean value of the entire core for 13.44,indicating the bay facies.The AMS 14C dates indicate the evolution of the fluvial-tidal flat-bay sedimentary facies in Core NDGK2 was controlled by the Holocene sea-level rise.The tidal flat facies was formed at~8.9 cal.kaBP when the sea level was at 21.0±2.5 m.The maximum flooding surface appeared at~8.2 cal.kaBP,followed by a long period of low sedimentation rate for 6 ka,and the sedimentation rate in the bay was accelerated again at~2.2 cal.kaBP.3)The sedimentogenic Sr/Ba ratios in all 15 cores at Pleistocene-Holocene boundary in the region show obvious tidal flat facies characteristics,indicating the applicability of sedimentogenic Sr/Ba ratio for reconstructing the Holocene relative sealevel change in the region.