泛巽他盆山体系北部湾古泥沼旋回与古生态效应

Palaeo-peat Cycles in Beibu Gulf of Pan-Sunda and Their Palaeo-Ecological Significance Under Plateau-Mountain-Basin Complex Backgrounds

南海西北部北部湾在地质环境方面与狭义巽他陆架相似而成为一体构成泛巽他,同属青藏高原及其挤出阶梯地貌盆山体系。然而,关于其指示环境变化和碳循环的古泥沼分布及其气候-构造驱动机制、作用与联系等缺乏关注,其研究滞后于上述热点巽他陆架相关进展。为加强对这些内容的认识,本文的研究在北部湾北部华南陆缘沉降带进行了80.05 m进尺第四系全岩心钻取,立足于沉积物微体古生物、粒度、碎屑矿物组分、微量元素含量等基础沉积环境指标的实验测试、鉴定以及测年数据,并结合前人相关资料和结果进行综合分析。结果表明,岩心底部年龄为中更新世后期171.0 ka,孢粉化石呈热带和亚热带植被面貌,主要有栲属、栎属等以及鳞盖蕨属、水龙骨科等,其含量变化显示出3个主要气候变化阶段,与倒数第二冰盛期、末次间冰期、末次冰期以及冰消期等相联系,从56.0 ka始见有半咸水种硅藻化石条纹小环藻、柱状小环藻等,有孔虫化石出现于中全新世6.0 ka接受海进。总体上以砂为主,但局部粉砂和黏土(泥)含量较高,最高依次可达65.78%、59.71%,元素含量为2.6×10^(-6)~347×10^(-6),碎屑矿物稳定出现石英、长石、风化矿物,以前者占优,平均含量88.57%,余下矿物相间出现,具有陆源性,各要素随气候阶段而变化。存在5个泥沼旋回,沉积环境有所差异,在冰期和间冰期均有分布,出现炭化腐木以及邻区炭化层。青藏高原隆起导致的区域强烈地形落差在中—晚更新世业已构建,盆山体系成为海洋暖湿气流与地形作用的水热有效利用地带,北部湾冰期出露地表古水系广布,森林和草地植被共存,有助于泥沼的维护发展。气候波动性是泥沼的本质驱动因素,除了间冰期之外,冰期的强烈暖阶振荡亦为其成因;泛巽他北部湾在冰期同样具有碳库意义,具有转移与弥补中高纬度生产力衰减之作用。

Beibu Gulf is located in the summer monsoonal zone along the southeastern margin of Euro-Asia continent as a coastal basin of Titetan Plateau,Hengduan Mountains and their sketch,which is also a broadest shelf along northern South China Sea(SCS).It constitutes an integrity with Sunda shelf called Pan-Sunda in the aspects of geomorphoric and geological environment showing an ecological connection and similarity on a larger scale.The Plateau-Mountain-Basin complex is characteristized by a semi-closed,long,and multiple-fractal-dimensional coast line but receiving less attention on palaeo-peat development in Beidu Gulf.We collected a complete Quaternary sediment core ZK9 with a length of 80.05 m.Subsequently,we performed lab analysis,dating,and identification of micro-paleontological,granularity,geochemical micro-elements,and detrital minerals for 92 samples.With combination of the data analysis and shallow seismic profiles collection,the results show that the ZK9 bottom age was 171.0 ka.There was tropical and subtropical monsoonal vegetation of main Castanopsis sp.,Quercus sp.,Microlepia sp.,Polypodiaceae and Cibotium barometzetc etc appeared from~165 ka in a variation style and primarily fell into three evolution stage 165-110 ka,110-10.5 ka,and Holocene relating to PGM,last inter-glacial,glacial and deglacial,and basically corresponded to change stages of granularity,geochemical-micro elements,and detrital minerals below.Sparse diatom Cyclotella striata and C.stylorum displayed from 56.0 ka and diatom species became abundant during Holocene,and warm shallow-water type of foraminifera appeared during the SCS transgression from middle-Holocene(6.0 ka).Main composition is comprised of more sand,but the gray and silt both became abundant in some depth or ages with maximum 65.78%,59.71%,respectively,and generally bad sorting.There were relatively richer detrital minerals with dominant quartz of an average 88.57%,and others such as feldspar,and weathering minerals including mica and hematite.Geochemical micro-elements concentration ranged from 2.6×10^(-6)to 347×10^(-6),with an average organic matter of 1.07%.A series of P1-P5 peat cycles showed millennium scales driven by strong warming oscillation during glacial/interglacial periods,one of which was polar-coupled oscillation D-O/AIM a11 revealing a highly unstable southward shift of westlies,and it developed different sedimentary environments under fluvial/riverine and brackish facies but also shared some common characteristics in micro-element behaviour,organic matter growth,fine sediments and reduction pyrite and so on.There was also highly carbonizated wood in upper core and black-charcoaled sub-strata in neighboring core during some peat cycles exhibiting a carbon reservoir feature like Sunda shelf.Peat cycles indicated that the neo-tectonic uplift movement caused a significant vertical terrain differences and patterns in the Plateau-Maintain-Basin complex during mid-late Pleistocene.Therefore,it acted as an intensive water production,utilization,maintaining and gradient transport area under warm and humid Indian and East-Asian summer monsoonal field through the climatic-tectonic coupled effect.Beibu Gulf was ever a closed basin before opening to the sea thereafter it became a coastal plain in Late Pleistocene,and the reconstruction for its lowland surface distributed woods and grass and widely distributed palaeo-river system when being exposed during glacial stages.In summary,Beibu Gulf was sensitive to global and regional climatic changes,and this exposed coastal basin might have also played a role in southward transfer and compensation for high-middle latitude shrink of palaeo-productivity and-ecological function.The river systems incised to the continent slope to transport biogenic matters into sea basin could be an important mechanism to enhance marine CO_2 biological pump especially from the viewpoint of whole low-latitude monsoonal Pan-Sunda during glacial stages except the way of aerial dust input at high latitude,meaning Pan-Sunda fedbacked regional even global changes.