加拿大不列颠哥伦比亚省Ursula Creek二叠纪-三叠纪边界的古环境和元素地球化学,以及与北美西部其他一些深水二叠纪-三叠纪界线陆架/斜坡剖面的比较

IF 1.9 3区 地球科学 Q1 GEOLOGY
Michael E. Brookfield, Alan G. Stebbins, Jeremy C. Williams, Robyn E. Hannigan
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引用次数: 0

摘要

Ursula Creek的二叠纪-三叠纪边界沉积物堆积在大陆架盆地中,或加拿大西部被动边缘的陆坡上,古纬度为30°N,沿潘塔拉西洋东部边缘。该地区位于西风带和东北信风带交界处的寒冷偏北洋流中,后者导致夏季沿海上升流。从最上层的二叠纪灰色放射虫燧石和灰色页岩到最下层的三叠纪灰色和黑色页岩以及细粒白云岩的转变是典型的深水潘他拉质沉积物。古地理位置和古环境与现在的加拿大西北太平洋边缘相当。Ursula Creek剖面揭示了长兴期海底氧值的逐渐下降,随后在长兴期晚期和整个早三叠世,真新世条件持续发展;这一转变与硅质海绵动物群的消失和多种放射虫种群的丧失相吻合。大部分碎屑沉积物是由北美洲西部沙漠的夏季东北信风提供的,尽管不同数量的碎屑沉积物可能是作为当代火山喷发的尘埃穿过潘特拉西海。相对古生产力变化表明,二叠纪-三叠纪边界的生产力没有一致的变化,产生的结果与东南部类似的Opal Creek剖面的结果相当。Ni/Co、Cu/Zn、U/Al和Th/U的比值表明,所有剖面的氧化还原条件都是可变的,但在二叠纪-三叠纪边界,有氧条件有向失氧状态转变的趋势。在这里和其他地方,边界上缺乏一致的元素地球化学变化,伴随着显著的同位素变化,这表明大气和海洋化学,而不是物源和海平面变化等物理变化,驱动了二叠纪-三叠纪的环境变化和物种灭绝。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Palaeoenvironments and elemental geochemistry across the Permian–Triassic boundary at Ursula Creek, British Columbia, Canada, and a comparison with some other deep-water Permian–Triassic boundary shelf/slope sections in western North America

Palaeoenvironments and elemental geochemistry across the Permian–Triassic boundary at Ursula Creek, British Columbia, Canada, and a comparison with some other deep-water Permian–Triassic boundary shelf/slope sections in western North America

The Permian–Triassic boundary sediments at Ursula Creek accumulated in a continental shelf basin, or on the continental slope of the western Canadian passive margin, at 30°N palaeolatitude along the eastern Panthalassic Ocean margin. The area lay within the cold northerly ocean currents at the junction of westerly and north-easterly trade wind belts, the latter causing summer coastal upwelling. The shift from uppermost Permian grey radiolarian cherts and grey shales to lowermost Triassic grey and black shales and fine-grained dolomites is typical of deep-water Panthalassic sediments. The palaeogeographical situation and palaeoenvironments are comparable to those of the present Canadian north-western Pacific margin. The Ursula Creek section reveals the progressive decline of seafloor oxygen values in the Changhsingian Stage), followed by the persistent development of euxinic conditions in the latest Changhsingian and throughout the Early Triassic; a transition that coincides with the disappearance of a siliceous sponge fauna and the loss of diverse radiolarian populations. Much of the detrital sediment was supplied by summer north-east Trade winds from the deserts of western North America, although variable amounts may have come across the Panthalassic Ocean as dust from contemporary volcanic eruptions. Relative palaeoproductivity changes show no consistent change in productivity across the Permian–Triassic boundary producing results that are comparable with those from the similar Opal Creek section to the south-east. The Ni/Co, Cu/Zn, U/Al and Th/U ratios indicate variable redox conditions in all sections, but with a tendency for oxic conditions to change to dysoxic across the Permian–Triassic boundary. The lack of consistent element geochemical changes across the boundary accompanied by significant isotopic changes, here and elsewhere, suggests that atmospheric and oceanic chemistry rather than physical changes, like provenance and sea-level changes, drove Permian–Triassic environmental changes and extinctions.

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