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Photic-zone euxinia and anoxic events in a Middle-Late Devonian shelfal sea of Panthalassan continental margin, NW Canada: Changing paradigm of Devonian ocean and sea level fluctuations
Global and Planetary Change ( IF 3.9 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.gloplacha.2020.103153
P. Kabanov , C. Jiang

Abstract The latest Eifelian – Frasnian strata of the Mackenzie Valley, NW Canada, provide an excellent archive of paleoceanographic signals imprinted in oxic and anoxic facies deposited in close proximity. Fondoformic black-shale strata preserve fingerprints of four global anoxic events (Kacak, Frasnes, Middlesex, and Rhinestreet), which receives confirmation with δ13Corg data. The discovery of 2,3,6- and 3,4,5-trimethyl aryl isoprenoids (biomarkers of green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of photic-zone euxinia as a common state of oceanographically open and semi-restricted shelfal basins of the Middle Devonian – Early Mississippian; a condition impossible under present-day vigorous thermohaline circulation, but consistent with models of greenhouse ocean depicting drastic slowdown in watermass turnover, reversals of deep ocean circulation, greatly expanded oxygen minimum zones, and profoundly changed nutrient flows. The rocks under study were deposited in an oceanographically open basin with fluctuating chemocline as attested by unstable presence of gammacerane in GCMS spectra and co-occurrence of signatures of water-column euxinia and pyritized hyalosponge spicules indicating episodes of weak bottom oxygenation. Shallow-water carbonate banks in the same basin show signatures of reduced hydrodynamic activity and do not record sea level changes in excess of several meters, which lines up with the shortage of evidence for high-amplitude base-level fluctuations in coeval strata worldwide. This supports discarding changes in sea level as the principal control over Devonian anoxic events and instead suggests pulsatory expansions of thick, semi-continuous oxygen minimum zones of the greenhouse ocean. Sea level fluctuations could still be involved as non-glacial (thermal and aquifer?) eustatic transgressions of a very modest amplitude.

中文翻译:

加拿大西北部 Panthalassan 大陆边缘中-晚泥盆世陆架海的光区euxinia 和缺氧事件:泥盆纪海洋和海平面波动的变化范式

摘要 加拿大西北部 Mackenzie 山谷最新的 Eifelian – Frasnian 地层提供了一个极好的古海洋学信号档案,这些信号印记在附近沉积的好氧和缺氧相中。Fondoformic 黑色页岩地层保留了四个全球缺氧事件(Kacak、Frasnes、Middlesex 和 Rhinestreet)的指纹,这些事件得到了 δ13Corg 数据的确认。2,3,6- 和 3,4,5-三甲基芳基异戊二烯(绿色硫细菌的生物标志物)在缺氧事件水平和之间的发现有助于越来越多的证据表明光区euxinia是海洋学上的一种常见状态中泥盆纪—密西西比纪早期的开放和半限制陆架盆地;在当今剧烈的温盐环流下不可能出现的条件,但与描绘水团周转急剧放缓、深海环流逆转、最低含氧区大大扩大以及养分流动发生深刻变化的温室海洋模型一致。研究中的岩石沉积在海洋学开放的盆地中,具有波动的趋化层,GCMS 光谱中伽玛蜡烷的不稳定存在证明了这一点,并且水柱euxinia 和黄铁矿化的透明海绵针状体的特征的共同出现表明了弱底部氧化的事件。同一盆地中的浅水碳酸盐岩滩显示出水动力活动减少的特征,并且没有记录超过几米的海平面变化,这与世界范围内同时代地层高幅度基线波动的证据不足相吻合。这支持将海平面变化视为泥盆纪缺氧事件的主要控制因素,而是建议温室海洋中厚的、半连续的最低氧区的脉动扩张。海平面波动仍可能作为幅度非常适中的非冰川(热和含水层?)超海侵而参与。
更新日期:2020-05-01
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