CARBON- AND NITROGEN ISOTOPE STRATIGRAPHY OF PHOSPHORITE- AND SEDEX-RELATED ROCKS OF THE CARBONIFEROUS-PERMIAN LISBURNE GROUP, NORTHERN ALASKA
In ramp facies, micritic carbonate above and below the phosphorite has δ18O of 25‰ and δ13C of 1‰, similar to micrite in correlative marine carbonates elsewhere in North America. Within the phosphorite unit, lime mudstones have δ18O of 25‰, but low δ13C (–5‰). A shale and spiculite unit at the top of the Lisburne shows δ18O of ~25‰ and δ13C decreasing upward from 1 to –4‰.
Organic C in micrite beds is isotopically lighter within the phosphorite unit (–31.0±0.3‰) than in the units above and below (–28.6±0.9‰). In the shale and spiculite unit, OM δ13C increases upward from –30 to –22‰. The carbonate-OM fractionation drops below 28‰ in the phosphorite unit and in the upper shale and spiculite unit. Corg/Ntotal of lime mudstone interbeds in the phosphorite unit are 3 to 25. The lower values suggest that N may be present in silicates as well as OM. Total N has δ15N values of 9–12‰.
At Anarraaq, the uppermost Kuna Fm. has isotopically light OM (–30.2±0.03 vs. –29.4±0.2‰ lower in the section) and shows an increase in δ15N (from 6 to 10‰) suggestive of denitrification. These attributes support correlation to the phosphorite unit of the Lisburne ramp facies. Overall, the isotopic data provide evidence that the phosphate, barite and sulfide deposits formed in association with a water mass that had experienced net oxidation of organic C and denitrification of nitrate. The data could be interpreted to reflect lower CO2 in surface water during this time, which would imply cooling, but could also reflect the methane release event that has been inferred previously from barite isotopes.