Paleoenvironmental changes across the Carnian/Norian boundary in the Black Bear Ridge section, British Columbia, Canada
Paleoenvironmental changes across the Carnian/Norian boundary in the Black Bear Ridge section, British Columbia, Canada
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DOI：
10.1016/j.palaeo.2015.10.008
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
T. Onoue;J. Zonneveld;M. Orchard;Misa Yamashita;K. Yamashita;Honami Sato;Soichiro Kusaka
中科院分区：
文献类型：
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作者：
T. Onoue;J. Zonneveld;M. Orchard;Misa Yamashita;K. Yamashita;Honami Sato;Soichiro Kusaka
加拿大不列颠哥伦比亚省东北部的黑熊岭剖面由上卡尼统至下诺里统(上三叠统)陆缘地层连续出露。该剖面已被建议作为卡尼安/诺里安界线(CNB)的候选全球层型剖面和点(GSSP)。为了评价该剖面记录的晚卡尼期至诺里纪早期的环境变化，我们研究了该剖面中87Sr/86Sr、δ~(13)C和δ~(18)O值的地层变化，以及氧化还原敏感元素(V、Ni和Cr)的值。研究部分位于不列颠哥伦比亚省东北部威利斯顿湖的北岸。黑熊山脊剖面沉积于北美克拉通被动西缘的远端斜坡环境中，横跨北冰洋的地层表现出δ13C值正向漂移，氧化还原指数V/(V1+VNi)和V/Cr值相应增加。δ~(13)C值和氧化还原指数的同步增加表明，海洋有机碳的埋藏率随着水体中缺氧条件的发展而增加。西特提斯西部上三叠统段(如西西里岛的Pizzo Mondello段)碳酸盐岩的δ13C值也有增加的报道，这表明在CNB段内缺氧条件的发展是广泛的，影响到潘塔拉桑洋和特提斯海。这项研究的地球化学数据以及对黑熊岭剖面牙形刺生物地层学的研究表明，海洋缺氧条件的开始可能是北冰洋动物群更替事件的原因。这一缺氧事件的原因尚不清楚，但87Sr/86Sr和δ13C同位素数据在很大程度上排除了该事件是由甲烷水合物的解离和与大规模火山活动有关的排气触发的可能性。
The Black Bear Ridge section in northeastern British Columbia, Canada, consists of a continuously exposed sequence of Upper Carnian through Lower Norian (Upper Triassic) continental margin strata. The section has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Carnian/Norian boundary (CNB). In order to assess Late Carnian to Early Norian environmental changes recorded in the section, we examined stratigraphic variations in87Sr/86Sr, δ13C, and δ18O values, and also values of redox sensitive elements (V, Ni and Cr), in the CNB interval. The study section is located along the north shore of Williston Lake, northeastern British Columbia. The Black Bear Ridge section was deposited in a distal ramp environment on the passive western margin of the North American craton.The strata across the CNB display a positive shift in δ13C values and a corresponding increase in the redox indices V/(V + Ni) and V/Cr. The synchronous increase in δ13C values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column. An increase in δ13C values in carbonate rocks across the CNB has also been reported from Upper Triassic sections in the western Tethys (e.g., in the Pizzo Mondello section, Sicily), which suggests that the development of anoxic conditions within the CNB interval was widespread, affecting both the Panthalassan Ocean and Tethyan Sea. The geochemical data from this study, as well as from research into conodont biostratigraphy in the Black Bear Ridge section, show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the CNB. The cause of this anoxic event is unknown, but the87Sr/86Sr and δ13C isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large-scale volcanic activity.