CARBON- AND NITROGEN ISOTOPE STRATIGRAPHY OF PHOSPHORITE- AND SEDEX-RELATED ROCKS OF THE CARBONIFEROUS-PERMIAN LISBURNE GROUP, NORTHERN ALASKA

The Lisburne Group, a succession of mostly Carboniferous carbonates, is exposed in the Brooks Range (BR) of northern Alaska and underlies the North Slope. In ramp facies of the central and western BR, the upper Lisburne contains phosphorites with interbedded black shale and lime mudstone. Deeper water facies in the western BR host large sedex Zn-Pb deposits at Red Dog and Anarraaq, and massive barite. We measured isotopes (δ¹³C, δ¹⁸O, δ¹⁵N) in carbonate and sedimentary organic matter (OM) through ramp sections at Skimo and Confusion Creeks, and in OM in the deeper-water Kuna Fm. at Anarraaq.

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. C_org/N_total 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.