Biostratigraphy and carbon and nitrogen geochemistry of the SPICE event in Cambrian low-grade metamorphic black shale, Southern Norway

Abstract

We present new chemo- and biostratigraphical data from 90m 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 13.5wt.% 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=0–0.9mgHc/g rock) compared to equivalent formations in Sweden (TOC=8–22wt.%; 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 18m 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.