古盐度指标与渤海湾盆地古近系海侵事件初探

Paleosalinity Proxies and Marine Incursions into the Paleogene Bohai Bay Basin Lake System,Northeastern China

盐度是水体最基本的属性之一,古盐度是古环境、古气候和古生物研究中常用的指标。对来自不同海域的现代水体盐度和沉积物地球化学数据进行整理,发现硼/镓(B/Ga),锶/钡(Sr/Ba)和硫/总有机碳(S/TOC)等比值与水体盐度具有显著的相关性,可有做作为示踪水体盐度的指标。这些化学元素的双变量图显示,每个指标的不同阈值指向了不同盐度的沉积环境:1)B/Ga<3指示淡水相,3~6指示半咸水相,>6指示海水相;2)Sr/Ba<0.2指示淡水相,0.2~0.5指示半咸水相,>0.5指示海水相;3)S/TOC<0.1指示淡水相,>0.1指示半咸水和海水相。基于上述盐度指标,我们对渤海湾盆地沾化凹陷沙河街组沙三下亚段的古盐度和沉积环境进行了重建,并结合古生物、矿物等信息对渤海湾盆地古近纪发生的海侵事件进行了梳理,揭示了四次主要海侵事件。

Salinity is a fundamental property of watermasses that is useful in paleoenvironmental and paleoecological studies,yet the theory of application of geochemical proxies to paleosalinity reconstruction is underdeveloped.Here,we explore the use of three elemental ratios for paleosalinity reconstruction:boron/gallium(B/Ga),strontium/barium(Sr/Ba),and sulfur/total organic carbon(S/TOC)ratios.A large set of modern aqueous and sedimentary chemical da-ta was compiled representing a range of salinity facies(i.e.,freshwater,brackish,marine)in order to test the rela-tionships of these proxies to ambient watermass salinity and to determine their viability for paleosalinity analysis.Sedi-ment data were limited to fine-grained siliciclastic units(muds/shales/mudstones)without significant carbonate con-tent,in which the elements of interest were mainly acquired through adsorption of dissolved species to forge a connec-tion between elemental proxy values and watermass chemistry.In modern systems,watermass salinity is correlated with these proxies,yielding r of+0.99 and+0.76 for aqueous and sediment B/Ga,+0.66 and+0.54 for aqueous and sediment Sr/Ba,+0.98 for aqueous sulfate,and+0.66 for sedimentary S/TOC.These relationships establish the basis for using these elemental ratios as paleosalinity proxies.Elemental crossplots permitted estimation of approximate sa-linity thresholds for each proxy:(1)B/Ga is<3 in fresh water,3-6 in brackish water,and>6 in marine facies;(2)Sr/Ba is<0.2 in fresh water,0.2-0.5 in brackish water,and>0.5 in marine facies;and(3)S/TOC is<0.1 in fresh water and>0.1 in brackish water and marine facies.In order to address the marine influence on sediment accu-mulation and to identify variations in paleosalinity during deposition of the Mid-Eocene lower Es3(Es3 L)member of the Shahejie Formation,this study analyzed multiple paleosalinity proxies(B/Ga,Sr/Ba and S/TOC)in fine-grained siliciclastic(argillaceous)sediments of the Luo69 drillcore from the Dongying Depression.All three proxies yielded similar paleosalinity interpretations,with peak salinity and marine influence(i.e.,maximum B/Ga,Sr/Ba and S/TOC values)occurring at the base of the Es3 L member,and progressively declining salinities,representing a shift to-ward brackish conditions,upsection over an interval of 185.5 m representing~2.2 Myr of sedimentation.This paleosa-linity record suggests gradual long-term variations that were probably due to changes in tectonically controlled gate-ways linking the Bohai Bay Basin to the Yellow Sea.In addition to analysis of the Mid-Eocene Es3 L member,pub-lished data on the stratigraphic distribution of marine fossils,glauconite,and elemental paleosalinity proxies are ap-plied to evaluate the history of marine incursions into the Bohai Bay lake system throughout the mid-to late Paleogene(~50-24 Ma).This analysis revealed four intervals of probable major marine incursions dating to the Early Eocene,the Middle Eocene,the Early Oligocene,and the Late Oligocene.