کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
5783190 1637947 2017 37 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Transient deep-water oxygenation in the early Cambrian Nanhua Basin, South China
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
Transient deep-water oxygenation in the early Cambrian Nanhua Basin, South China
چکیده انگلیسی
Many late Neoproterozoic to early Cambrian fossils of multicellular eukaryotes, including those of benthic animals, are found preserved under anoxic and even euxinic bottom-water conditions, which is contradictory to the consensus that oxygen is essential to eukaryotes. To investigate this conundrum, we conducted an integrated study of iron speciation, redox-sensitive trace elements, and Mo isotopes (δ98Mo) on the black shale interval of the lower Cambrian Hetang Formation (∼535-521 Ma) at Lantian, South China, in which benthic sponge fossils are abundant in the lower member (LM) but absent in the upper member (UM). Iron speciation data point to uniformly anoxic-ferruginous conditions in the LM and euxinic conditions in the UM, whereas the trace-element and δ98Mo data show greater secular variation in redox conditions. The LM shows higher mean trace element concentrations (Mo: 108 ppm, U: 36 ppm, V: 791 ppm) and lower and more variable δ98Mo (+0.13 to +1.76‰) relative to the UM (Mo: 45 ppm, U: 18 ppm, V: 265 ppm, δ98Mo: +1.59 to +1.67‰), and ratios of redox-sensitive trace element concentrations to total organic carbon are significantly more variable and higher on average in the LM relative to the UM. The appearance of sponge fossils and lower δ98Mo values correlate strongly with gray (i.e., lighter-colored) layers in the LM. These patterns can best be interpreted as recording mainly euxinic conditions throughout deposition of the study units, with more intense background euxinia in the LM relative to the UM, but also with frequent transient oxygenation events in the LM that do not appear in the UM. The transient oxygenation events of the LM led to the initial colonization of the deep Nanhua Basin by sponges, and the termination of these events in the UM caused sponge faunas to disappear until a general rise in O2 levels later in the Cambrian permitted their return to deeper-water habitats. Our study also illustrates that multiple geochemical and paleobiological proxies exhibit different responses in 'poikiloredox' environments (i.e., characterized by small-scale spatial and high-frequency temporal variations), which can lead to apparent contradictions between metazoan fossil occurrences and their inferred watermass redox conditions.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Geochimica et Cosmochimica Acta - Volume 210, 1 August 2017, Pages 42-58
نویسندگان
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