Permafrost soil cores were collected on the North Slope of Alaska during
the ice-free summer months of June-August 2018 near the Toolik Field
Station. Soil cores were collected from within the permafrost layer (at
85 cm below the surface) of Imnavait Creek wet sedge tundra and Toolik
Lake tussock tundra soils. In June 2022, soil was sampled from a
thermokarst failure on the shore of Lake LTER 395 on the North Slope of
Alaska, where an abrupt collapse of thawing soil exposed deeper
permafrost soil. Soil was sampled from the permafrost layer exposed in
the headwall of the thermokarst failure (\> 80 cm below the surface)
using MilliQ-rinsed pickaxes. The permafrost and thermokarst soil
samples were collected as previously described in detail (Bowen et al.,
2020). All soils were stored in freezers at the Toolik Field Station
until overnight shipment to Woods Hole Oceanographic Institution (WHOI).
DOC was leached from the permafrost and thermokarst soils as previously
described (Bowen et al., 2020). Briefly, frozen soil and MilliQ water
were mixed in 5-gallon, MilliQ-rinsed HDPE buckets and allowed to leach
in the dark for up to 48 hours at 4 °C. Both the soil-to-water ratio of
soil leachates and the leaching time were adjusted to achieve a final
concentration of ~1500 μM DOC in the leachates, as estimated from the
absorbance of chromophoric dissolved organic matter at 305 nm
(*a*<sub>305</sub>). All leachates were passed through MilliQ-rinsed 60
μm mesh screens to remove the largest particulates and then through
MilliQ-rinsed 5 μm high-capacity Whatman cartridge filters. Subsamples
of each leachate for light exposure experiments were then filtered
through 0.2 μm high-capacity cartridge filters. Soil leachates were
stored at 4 °C until further use, and chromophoric dissolved organic
matter (CDOM) was measured for each permafrost leachate as previously
described (Cory et al., 2014)
Wavelength-dependent, daily water column rates of absorption of
ultraviolet (UV) and visible sunlight (*Q<sub>aλ</sub>*; mol photon
m<sup>-2</sup> d<sup>-1</sup>) by chromophoric dissolved organic matter
(CDOM) in each soil leachate were quantified as:
*Q*<sub>*aλ*</sub> (*mol* *photons* *m*<sup>−2</sup>*d*<sup>−1</sup>) = *Q*<sub>*dso* − *λ*</sub>(1−*e*<sup>−*K*<sub>*dλ*</sub>*z*</sup>)
where *Q<sub>dso-λ</sub>* is the photon flux directly below the water
surface, derived from direct measurements of UVB, UVA, and PAR fluxes of
sunlight at the Toolik Field Station that are apportioned to a clear-sky
solar spectrum and corrected for atmospheric scattering;
*K*<sub>*d*λ</sub> is the attenuation coefficient of the water column;
and *z* is the water column depth (Cory et al. 2014). *Q<sub>aλ</sub>*
was calculated for each permafrost soil leachate using an attenuation
coefficient derived from the absorption spectrum for CDOM in the soil
leachate, the daily incoming photon flux spectrum (mol photon
m<sup>-2</sup> d<sup>-1</sup> nm<sup>-1</sup>) measured on 21 June 2018
at the Toolik Field Station on the North Slope of Alaska, and the
average water column depth (0.5 m) of Imnavait Creek, a small headwater
creek draining tundra soils near the Toolik Lake Field Station.
References:
Bowen, J. C., Ward, C. P., Kling, G. W., & Cory, R. M. (2020). Arctic
amplification of global warming strengthened by sunlight oxidation of
permafrost carbon to CO<sub>2</sub>. *Geophysical Research Letters,
47*(12), 0–3. https://doi.org/10.1029/2020GL087085
Cory, R. M., Ward, C. P., Crump, B. C., & Kling, G. W. (2014). Sunlight
controls water column processing of carbon in arctic fresh waters.
*Science, 345*(6199), 925–928. https://doi.org/10.1126/science.1253119
Rieb, E. C., Polik, C. A., Ward, C. P., Kling, G. W., & Cory, R. M.
Controls on the respiration of ancient permafrost carbon in sunlit
arctic surface waters. *In review.*
