北部湾北部上更新统的沉积特征及其记录的古环境

黄向青; 梁开; 夏真; 张顺枝; 林进清; 马胜中; 霍振海

摘要

Under the tropical and sub-tropical climatic background,northern Beibu Gulf is an interaction zone between cold and warm air masses and is the gulf closest to Indian Ocean in China resulting frequent climatic fluctuations.By comparison with neighboring areas especially northern areas' rich data and research results,the study on palaeo-climatic fluctuations since Late Pleistocene in northern Beibu Gulf and their characteristics have almost not carried out yet.The seismic profile interpretation and geological core sediments analysis have revealed that there were buried river channels during Quaternary under seafloor outside Qinzhou Bay Mouth,which formed during sea transgression in Holocene and had become the transition zone from submarine slope to submarine plain.There are main run offs now Fangcheng River,Maoling River,Qin River,Dafeng River and Nanliu River from west to east along coastline of Qinzhou Bay and its neighboring areas.This study obtained whole-length 80.05 m core sediments (core ZK9) from Quaternary strata to the upper surface of weathered crust through drill equipment in position of 21°24′20″N,108°33′57″E located at northern part of a sunken area outside the Qinzhou Bay Mouth by seismic interpretation.Based on 1 m interval,13 sediments samples of Upper Pleistocene from 25.1 m to 9.7 m(103～ 11.5 ka) and 4 samples for age dating were analyzed and identified in laboratories in fundamental sedimentology of granularity,micro-geochemical elements,detrital minerals and micro-palaeontology and age measurement.The age dating results were 103± 10 ka (25.1 m),66±7 ka(20.8 m),62±6 ka(20.2 m)by optical stimulated luminescence method(OSL)and 11598±51 cal.a B.P.(10.2 m)by 14C method respectively and the age frame was then established through linear interpolation.The study results showed that there were tropical and sub-tropical sporopollen species of Microlepia,Polypodiaceae,Castanopsis and so in this Upper Pleistocene core sediments,the dominant diatom species was Cyclotella striata and there wasn't any foraminifera found.The sediments composition was sand,silt and clay with pebbles in some depths,and the detrital minerals included quartz,feldspar,pyrite,ilmenite,limonite and so on with dominant quartz averaged 89.02％ in concentration.The concentration of micro-geochemical elements ranged from 9.64 ～ 246.86 μg/g within which Ba reached the highest,and there were significant correlation between Co,Ni,Sr,Zn,Zr,Ga,Ba beside Cu and Cr which were more sensitive to oxidation-reduction condition.The study also revealed that the sedimentary environment was of fluvial/riverine facie with the characteristics of time or depth fluctuations and then divided into two primary stages:the Last Interglacial and the Last Glacial and further four climatic stages periodically.The Stage One 103～60 ka(25.1 ～ 19.6 m)was the Last Interglacial with fluvial facie and warm event,and Stage Two was divide into three sub-stages:Sub-stage One 60～46 ka(19.6～ 17.1 m) Early Ice Order with generally-low(hydrodynamic)energy,46～19 ka(17.1 ～ 11.5 m)Interglacial Order with generally-high energy and 19～ 11.5 ka(11.5 ～ 9.7 m)Late Ice Order with generally-low energy.The above four climatic stages could be basically corresponded to palaeo-climates of same times in neighboring regions of Yun-Gui Plateau and South of Changjiang River.During above Interglacial Order of the Last Glacial (17.1 ～ 11.5 m),semi-saline diatom species Cyclotella striata appeared in some depths indicating that the river systems possibly had ever connected into northern continent shelf of South China Sea.There were possibly a millennial-scale warm event in 20.9 m depth(age 67 ka under age frame and a little more earlier than 20.8 m of laboratory dating result 66 ka)with sediments granularity parameters mean grain size Mz 4.29 φ,sorting σ1 4.68 φ,skewness SkI almost 0 and kurtosis KG 1.58,concentration of pyrite 12.8025％,concentration of micro-geochemical elements 14.2～650 μg/g,Σ (micro-geochemical elements) 1342.4 μg/g as well as organic matter 3.72％,all of which reached their maximum of this whole Upper Pleistoncene core sediments except Cu and Cr.There were also sharp growth percentages in silt composition 119.46％ and granularity parameters-94.70％～ 43.78％,and limonite and pyrite exceeding 1000％,micro-geochemical elements 374.72％ in maximum,and organic matter 853.85％ except Cr indicating humid and warm climatic background,strong weathering and leaching and also significant chemical reduction condition.By making concentration comparison with ZK4 and ZK5 sediments core of the Pleistocene strata in neighboring area respectively:the concentration of above warm event was 10429.8 times of ZK5 in pyrite (pyrite concentration is 0 in ZK4 and limonite is 0 in both ZK4 and ZK5),over 1.0 and maximum 28.8 and 17.6 except Cu and Cr in micro-geochemical elements,Σ (micro-geochemical elements)was both 2.8,organic matter 16.5 and 18.9 respectively.This event might be an example of frequent fluctuation thereafter globally 72 ka Cold Event and corresponded in age to Dansgaard-Oeschger Warm Events (D-O fluctuation)labeled 19 in GISP 2 ice core meaning that the occurring extent of D-O fluctuation had extended to further southwestern part in monsoon region of China by comparison with D-O labeled 20 ～ 18 of stalagmite records found in Hulu cave in Nanjing,Sanbao cave in Hubei and Xinya cave in Chongqing.As a climatically sensitive zone and geologically sunken developed areas,the relationship of northern Beibu Gulf with global climatic change needs much more attention.%晚更新世100 ka以来作为重要气候阶段对后续环境演化影响深远.北部湾位于东亚低纬度季风区,为冷暖气团影响交汇带,是我国最靠近印度洋的大型海湾,沿海地势崎岖,山洪暴发,需要加强对该阶段气候波动及演化研究.在北部湾北部钦州湾口外近浅海钻取了海底岩芯,岩芯直达第四系地层下伏风化壳且进尺为80.05 m,文章对上更新统25.1～9.7 m岩芯段沉积物进行了粒度、微量元素、碎屑矿物、微体古生物测试鉴定以及14C与光释光测年.对数据的研究结果显示,该岩芯段孢粉为热带-亚热带山地植被,所处沉积环境为河流相,沉积要素波动变化并具有阶段性特征,物源来自周边陆区风化侵蚀.划分出4个岩芯深度段及相应的4个气候阶段,并对测年数据应用线性插值方法得到气候阶段为:末次间冰期(103～ 60 ka)水动力低能-高能环境、末次冰期早冰阶(60～46 ka)低能环境、末次冰期间冰阶(46～19 ka)低能-高能环境、末次冰期晚冰阶(19～11.4 ka)低能环境,均与相邻云贵高原和中国长江以南地区同期区域气候具有可比性.除此之外,还辨识出67 ka前后出现了一次暖事件,具有颗粒变细,有机质、微量元素、黄铁矿和褐铁矿等含量出现异常峰值并且相对增幅极高的特征,表明环境湿热和微生物还原活跃等.该暖事件在时间上与Dansgaard-Oeschger旋回(D-O旋回)序号19暖事件最为接近,序号18暖事件则次之,说明在我国季风区的湖北三宝洞、重庆新崖洞、南京葫芦洞石笋研究已发现的对应D-O旋回暖事件序号20～18时期,再往西南沿岸同样发生了与D-O旋回可对应的暖事件.

北部湾北部上更新统的沉积特征及其记录的古环境

摘要

著录项

相似文献

相关主题

期刊订阅