DOI: 10.1016/j.palaeo.2016.12.016
论文题名: Biostratigraphy and carbon and nitrogen geochemistry of the SPICE event in Cambrian low-grade metamorphic black shale, Southern Norway
作者: Hammer Ø. ; Svensen H.H.
刊名: Palaeogeography, Palaeoclimatology, Palaeoecology
ISSN: 0031-0182
出版年: 2017
卷: 468 起始页码: 216
结束页码: 227
语种: 英语
英文关键词: Alum shale
; Furongian
; HHXRF
; Oceanic anoxic event
; Rock-Eval
; Trilobites
英文摘要: We present new chemo- and biostratigraphical data from 90 m of Alum Shale Formation stratigraphy from the middle Cambrian (Series 3) to the lowermost Ordovician (Tremadocian) at Krekling, southern Norway. The alum shale was deposited during a transgression covering parts of the Baltic craton, and contains up to 13.5 wt.% total organic carbon (TOC) increasing up section through the Furongian. Pyrolysis confirms that the alum shale is affected by low grade metamorphism, either from a nearby pluton or during deep burial, where the remaining organic matter has lost most of the hydrocarbon productivity (S2 = 0–0.9 mgHc/g rock) compared to equivalent formations in Sweden (TOC = 8–22 wt.%; S2 = 20–80). Still, the carbon isotopic trend (δ13Corg) shows that the positive carbon isotope excursion known as the SPICE event (Furongian) is preserved and that the carbon isotopes did not fractionate significantly while undergoing metamorphism. The δ13Corg increases steadily from the base of the section, from − 30.8‰ to a maximum of − 27.3‰, likely due to increasing atmospheric oxygen concentrations and cyanobacteria productivity. The heaviest values are recorded in an interval with abundant pyrrhotite nodules, suggesting that the organic productivity led to euxinic conditions, as shown from several other SPICE-sections worldwide. The evolution of bulk rock nitrogen isotopes (δ15N) shows positive correlations with δ13Corg and TOC, with an increase from − 2 to + 1.5‰. Before the onset of the SPICE, negative δ15N values indicate that atmospheric nitrogen was fixed in the shallow ocean. During the transition from low to high TOC shale at about 18 m in the section, both the nitrogen content and the δ15N increase, but the δ15N is not high enough to imply water mass nitrogen reduction (denitrification) as a dominant process. Instead we suggest that the Krekling section was affected by sediment denitrification during SPICE, a process resulting in a ca. 1.5‰ post-depositional increase in δ15N. At the same time, in the upper parts of the water column, atmospheric nitrogen was still fixed by cyanobacteria. We conclude that the Cambrian ocean transgressed the Baltic craton and inhibited deep water upwelling, leaving geochemical signals characteristic of other greenhouse climates spanning the Phanerozoic. © 2016 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/67968
Appears in Collections: 过去全球变化的重建
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作者单位: Natural History Museum, University of Oslo, Pb. 1172, Oslo, Norway; Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Pb. 1028, Oslo, Norway
Recommended Citation:
Hammer Ø.,Svensen H.H.. Biostratigraphy and carbon and nitrogen geochemistry of the SPICE event in Cambrian low-grade metamorphic black shale, Southern Norway[J]. Palaeogeography, Palaeoclimatology, Palaeoecology,2017-01-01,468