Mercury, carbon, and nitrogen characteristics of Permafrost From 40,000 years old to modern age in Interior Alaska

Permafrost soils contain twice as much carbon as earth’s atmosphere and almost twice as much mercury as is stored in the total of all other soils, the ocean, and the atmosphere (Schuster et al., 2018). Significant nitrogen stocks have also been identified in permafrost (Strauss et al., 2022). The CRREL Permafrost Tunnel near Fairbanks, Alaska provides access to 500 meters of 40,000 year syngenetic permafrost. This is predominantly ice and carbon rich loess with ice wedges, segregated ice, and some large thermokarst cave ice features that are attributed to sudden permafrost thaw. The walls and ceiling of the Tunnel provide ready access to these varied permafrost soil and ice types. Above the Tunnel modern syngenetic permafrost is accessible via trenching and coring.

We surveyed the entire surface area of the walls and ceiling of the Tunnel with light distance and ranging (LiDAR). Return intensity values from different laser wavelengths allowed us to quantify exposed ice features versus ice cemented silt. Full LiDAR coverage of the entire tunnel interior after all artificial, floor, and extreme values were removed totaled 10,753,495 individual points. From this, we calculate 90.5 % of the surface area of the Tunnel walls and ceiling are represented by ice cemented silt and gravel and the remaining 9.5 % are ice features. Of the ice, 89.9 % are ice wedges and 10.1 % is thermokarst cave ice.

Based on these measurements, we collected roughly 80 SIPRE cores of frozen silt and ice features in the Tunnel and from modern permafrost above the Tunnel. Thawed soil and water ice were analyzed for major ions, trace metals, stable water isotopes, and carbon and nitrogen.

Major ion and trace metal concentrations are higher in ice wedges than replacement thermokarst cave ice. Mercury concentrations in ice cemented silt (n=28; 42.9 ng/g +/- 11.0), ice wedges (n=32; 54.0 ng/g +/- 16.3), thermokarst cave ice (n=17; 32.5 ng/g +/- 20.0), and the active layer above the Tunnel (n=5; 47.4 ng/g +/- 10.8) are similar to the 43 ng/g soil reported by Schuster et al. (2018). Carbon and nitrogen concentrations are greater in replacement thermokarst cave ice than in ice wedges and this is indicative of their formation during high intensity summer erosion events. Ice wedge total dissolved nitrogen (n= 30; 5.4 +/- 4.0 mg/L) and dissolved organic carbon (n= 30; 22.7 +/- 6.6 mg/L) values are greater than total dissolved nitrogen (n= 12; 2.9 +/- 4.3 mg/L) and dissolved organic carbon (n= 12; 17.4 +/- 21.0 mg/L) in thermokarst cave ice. Our values for yedoma permafrost inside the Permafrost Tunnel (n=28; 1.9 +/-0.6 kgN/cubic meter) and active layer above the Tunnel (n=5; 1.4 +/-0.5 kgN/cubic meter) are similar to the values of 0.9 kgN/cubic meter for yedoma and 1.6 kgN/cubic meter for active layer soils presented in Strauss et al. (2022).

Schuster PF, et a. Permafrost stores a globally significant amount of mercury. Geophysical Research Letters. 2018 Feb 16;45(3):1463-71.

Strauss J, et al. A globally relevant stock of soil nitrogen in the Yedoma permafrost domain. Nature Communications. 2022 Oct 14;13(1).