南极半岛西摩岛始新世拉揭塞塔组Teredolites遗迹化石研究

Characterization of the trace fossil Teredolites longissimus(Apectoichnus longissimus) from the Eocene La Meseta Formation,Seymour Island,Antarctic Peninsula

Teredolites遗迹化石是一类赋存于海相地层中的以木质基底为特征的钻孔遗迹组合,是由海相钻木类双壳动物(如船蛆、海笋等)寄生于漂浮或沉入海洋的树干中形成。Teredolites遗迹化石可指示浅海沉积环境,提供寄生木材的分类信息、古地理信息以及埋藏过程中的环境信息。本文研究了南极半岛西摩岛拉揭塞塔组顶部首次发现的一例Teredolites longissimus(Apectoichnus longissimus)遗迹化石,层位时代为晚始新世,根据木基底内保存的船蛆化石的发育特征和分布特征,确定本例为船蛆幼年群体对木基底的初期感染形成的钻孔遗迹。利用场发射扫描电镜(FESEM)和X射线能谱仪(EDS)对基底木化石和船蛆化石进行深入研究分析,将本例Teredolites遗迹化石的基底木化石定为裸子植物罗汉松科(Podocarpaceae)叶枝杉型木属未定种(Phyllocladoxylon sp.),证实了叶枝杉型木属在西摩岛上的分布延续至始新世晚期;船蛆化石内部赋存大量草莓状黄铁矿,其中粒径超过10μm的草莓状黄铁矿占比达39.2%,最大粒径达到44μm,粒径的统计和分析显示其形成于氧化水体沉积环境,证明始新世晚期南极半岛地区古海洋处于氧化海状态。

The ichnogenus Teredolites, produced by wood-boring marine bivalves(Teredinidae or Pholadidae), describes trace assemblage of club-shaped borings in log-ground in marine strata. Teredolites are taphonomically indicative of shallow marine environments, thus they can provide valuable information on the taxonomy and distribution of wood substrates as well as the environmental setting during the burial. This article investigates the trace fossil Teredolites longissimus(Apectoichnus longissimus) discovered in the upper part of the La Meseta Formation of Seymour Island, Antarctic Peninsula. The age of this horizon is constrained in the Late Eocene and the development and distribution features of teredinid body fossils preserved in the wood substrate indicate an incipient stage of infestation. Wood substrate and teredinid body fossils were investigated thoroughly using field emission scanning electron microscope(FESEM) and energy dispersive X-ray spectrometer(EDS). The xylic substrate was identified as Phyllocladoxylon sp. of family Podocarpaceae, which confirmed the existence of Phyllocladoxylon had lasted until the Late Eocene in Seymour Island. Framboidal pyrite was discovered in abundance inside teredinid body fossils, 39.2% of the framboids were above 10 μm in grain size, and the maximum grain size reached 44 μm;the size distribution analysis indicated they were formed under oxic condition, suggesting the paleo-ocean in the Antarctic Peninsula area was oxic at the end of the Eocene.