南海中央海盆晚中新世深海火山碎屑沉积的遗迹学特征及意义

Characteristics and Significance of Trace Fossils in Late Miocene Deep-Sea Volcaniclastic Sediments in the Central Basin of South China Sea

火山碎屑沉积是南海深海沉积的重要组成部分,因记录了火山活动影响下特殊的深海环境演变过程,具有重要的科研价值.为了解火山活动期和活动间歇期底栖环境的演变过程,对IODP349航次U1431站火山碎屑岩地层的生物遗迹记录开展了研究.研究发现,在中中新世-晚中新世(12.5～7.4 Ma,主要在晚中新世)期间遗迹化石在该套地层的粉砂岩和泥岩层中广泛发育,以Zoophycos、Thalassinoides、Chondrites和Planolites为主要类型.晚中新世研究区所在南海深海海盆经历了多次火山活动期和火山活动间歇期的交替演变,研究站位遗迹化石组合特征与现代南海中央海盆遗迹化石种属相似,指示了火山活动间歇期深海环境特征,沉积速率在5～15 cm/ka范围水体平静,同时有足够的营养物质供给,适合底栖生物的生存.通过分析遗迹化石组合与地层岩性特征之间的联系,研究发现遗迹化石不仅受限于底质岩性(主要发育在粉砂岩和泥岩中),还受控于沉积物的含氧量.研究推测海底火山活动为表层海水提供营养盐,促使浮游生物发育,浮游生物死亡后沉降至海底,增加了表层沉积物有机碳的供给,但消耗大量氧气,不利于底栖生物活动;反之,在火山活动间歇期,洋底表层沉积物有机碳含量降低,溶解氧消耗减少,有利于底栖生物生存.

Volcaniclastics is one of the important compositions of deep-sea sediments produced by volcanic eruptions. It is with great research values for studies on paleoenvironmental evolution in the deep-sea volcanic environment. The purpose of this work is to reveal the pattern of the Late Miocene benthic environment revolution in the central basin of the South China Sea (SCS) by analyzing the trace fossil assemblages in the volcaniclastic sequence drilled at site U1431 during the International Ocean Discovery Program (IODP) expedition leg 349. The study shows that trace fossils were developed well in siltstone and claystone during the Middle-Late Miocene (12.5–7.4 Ma, mainly in Late Miocene), and were dominated by Zoophycos,Thalassinoides,Chondrites, and Planolites.The volcanoes in the central basin of the SCS experienced repeated alternations of eruption and quietness during Late Miocene. The trace fossil assemblage in site U1431 implies that the basin is a quiet deep-sea environment with sedimentary rate varying between 5–15 cm/ka during volcanic activity quiet periods. By analyzing the associations between the trace fossil assemblages and the lithologic stratigraphic features, the study shows that the composition of the trace fossil assemblage is limited by substrate lithology and the dissolved oxygen concentration in the water masses at the sediment/water interface. The study also suggests that during eruptions, submarine volcanic activities can potentially provide nutrients for surface seawaters, and these nutrients can promote phytoplankton growth and subsequently increase the supply of organic carbon to the seafloor. The organic carbon on the sea floor then may consume the dissolved oxygen in the sediment pore water, making it unfavorable to the activities of the benthic organisms. In contrast, during volcanic eruption quiet periods, flux of organic carbon to the seafloor possibly reduced. Consumption of the dissolved oxygen in the pore water thus reduced, and the growth of the benthic organisms subsequently developed.