2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 15
Presentation Time: 9:00 AM-6:00 PM


JOHNSON, C.A., USGS, Box 25046, MS 963, Denver, CO 80225, DUMOULIN, Julie A., N/a, U.S. Geol Survey, 4210 University Dr, Anchorage, AK 99508, SLACK, John F., MS-954 National Ctr, US Geol Survey, 12201 Sunrise Valley Dr, Reston, VA 20192 and WHALEN, Michael T., Department of Geology and Geophysics, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, AK 99775, cjohnso@usgs.gov

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. 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.