Dinoflagellate cysts abundance and events of ODP Hole 189-1168A sediments

Palynomorphs were studied in samples from Ocean Drilling Program (ODP) Leg 189, Hole 1168A (slope of the western margin of Tasmania; 2463 m water depth). Besides organic-walled dinoflagellate cysts (dinocysts), broad categories of other palynomorphs were quantified in terms of relative abundance. In this contribution, we provide an overview of the early late Eocene-Quaternary dinocyst distribution and illustrate main trends in palynomorph distribution. Dinocyst species throughout Hole 1168A are largely cosmopolitan with important contributions of typical low-latitude taxa and virtual absence of endemic Antarctic taxa. Dinocyst stratigraphic distribution broadly matches that known from the Northern Hemisphere and equatorial regions, although significant differences are noted. Selected potentially biochronostratigraphically useful events are summarized. The distribution of dinocysts in the middle-upper Miocene interval is rather patchy, probably due to prolonged exposure to oxygen. An important general aspect in the dinocyst assemblages is the near absence of Antarctic endemic species and the apparent influence of relatively warm waters throughout the succession at Site 1168. General palynomorph distribution indicates continued deepening from an initial shallow, even restricted, marine setting from late Eocene-Quaternary times. A curious massive influx of small skolochorate acritarchs is recorded throughout the late early-early middle Miocene; the significance of this signal is not yet understood. A general long-term oligotrophic nature of the surface waters influencing Site 1168 is suggested from the low abundance of (proto) peridinioid, presumably heterotrophic, species. The overall dinocyst distribution pattern corresponds to the long-term existence of a Leeuwin-like current influencing the region, including Site 1168, confirming results of earlier studies on other microfossil groups. The occasional influence of colder surface water conditions is, however, also apparent, notably during the late Pliocene-Quaternary, indicating the potential of high-resolution dinocyst analysis for future paleoceanographic studies.本研究针对大洋钻探计划（Ocean Drilling Program, ODP）第189航次1168A钻孔（塔斯马尼亚西缘陆坡，水深2463米）的样品开展了孢粉型化石（palynomorphs）研究。除有机壳沟鞭藻囊孢（dinocysts）外，研究还对其他孢粉型化石的大类群进行了相对丰度定量分析。本文概述了晚始新世早期至第四纪的沟鞭藻囊孢分布特征，并阐明了孢粉型化石分布的主要趋势。 1168A钻孔中的沟鞭藻囊孢物种整体为广布型，包含大量典型低纬度类群，几乎未见南极特有类群。尽管存在显著差异，但沟鞭藻囊孢的地层分布大体与北半球及赤道地区的已知分布模式相符。本文总结了若干潜在具有生物地层学应用价值的关键事件。中中新统至上中新统层位的沟鞭藻囊孢分布较为零散，这可能与沉积物长期处于氧化环境有关。沟鞭藻囊孢组合的一个重要共性特征是几乎无南极特有种，且1168站位整个沉积序列均显示出相对暖水的显著影响。 整体孢粉型化石分布特征表明，自晚始新世至第四纪，该区域从初始的浅海甚至局限海环境持续发生海侵加深。在早中新世晚期至早中中新世期间，记录到一次大规模的小型具刺疑源类（skolochorate acritarchs）涌入事件，该信号的地质意义目前尚未明确。表层水体中（原）多甲藻目类群（推测为异养型物种）丰度较低，这暗示影响1168站位的表层水体长期处于贫营养状态。 整体沟鞭藻囊孢分布模式与影响该区域（包括1168站位）的长期类勒温海流（Leeuwin Current）的存在相吻合，这验证了此前针对其他微古生物类群的研究结果。不过，表层冷水环境的偶发影响也十分明显，尤其在晚上新世至第四纪时期，这表明高分辨率沟鞭藻囊孢分析可用于未来的古海洋学研究。