Pollen ages and depths in ODP Site 184-1144

A high-resolution pollen record (sampling interval averages 820 years) has been obtained from ODP Site 1144 (water depth 2037 m), northern South China Sea. The 504-m sequence (in composition length) covers the last 1.03 million years according to micropaleontological and isotopic stratigraphy. The pollen assemblages are characterized by high proportions of Pinus and herb pollen, and by their frequent alternations. Based on these alternations, 29 pollen zones have been recognized that are closely correlated to the Marine Oxygen Isotope Stages (MIS) 1-29. Pinus- dominant pollen zones correspond to interglacial periods with lighter delta18O values, while herb-marked ones relate to the heavier delta18O stages assigned to glacials. Judging from the pollen data, the exposed northern continental shelf of the South China Sea during the glacials was covered by grassland, and the extensive northern shelf has formed only since MIS 6 (ca. 150 ka), probably as a result of tectonic subsidence. Tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago. The summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance, and the result shows high values in general occurring at interglacials, with the maxima at MIS 15, 5e and 1. The relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon.研究团队从南海北部大洋钻探计划（Ocean Drilling Program, ODP）1144站位（水深2037米）获取了一套高分辨率孢粉记录，其采样间隔平均为820年。该504米厚的沉积序列经微古生物与同位素地层学分析，其沉积记录覆盖了过去103万年的地质时段。该序列的孢粉组合以高含量的松属（Pinus）花粉与草本花粉为典型特征，且二者呈现频繁的交替更迭。研究据此共识别出29个孢粉带，这些孢粉带与海洋氧同位素阶段（Marine Oxygen Isotope Stages, MIS）1至29高度吻合。以松属花粉为主的孢粉带对应于δ¹⁸O值偏轻的间冰期，而以草本花粉为标志的孢粉带则对应于δ¹⁸O值偏重的冰期阶段。根据孢粉数据推断，冰期时南海北部裸露的大陆架曾被草原植被覆盖；而广阔的北部陆架直至海洋氧同位素6阶段（MIS 6，约15万年前）才逐渐形成，其成因可能与构造沉降作用相关。乔木花粉通量可作为冬季风演化的代用指标，研究显示冬季风在60万年前开始增强。夏季风的演化则可通过总孢粉与孢子总量中蕨类孢子的占比来近似反映，结果显示蕨类孢子占比的高值普遍出现于间冰期，峰值分别出现在MIS 15、MIS 5e以及MIS 1阶段。60万年前之前，蕨类孢子占比相对偏高且变化幅度较小，这或许表明在冬季风增强之前，区域气候处于更为稳定的湿润状态。