Implications of Fine-Scale Geochemical Depth Trends in the Active Layer of a Continuous Permafrost Landscape near Barrow, Alaska

Abstract:

As part of the US DOE, Office of Science, Next Generation Ecosystem Experiment-Arctic project, we have been using environmental tracers (naturally occurring stable isotopes and geochemical species) to understand hydrological and geochemical processes within polygonal ground in a continuous permafrost area in the Arctic coastal plain. The study site is characterized by a thin zone of active layer development (typically <50 cm). This condition makes it difficult to understand development of geochemical gradients between the near surface and the frost line because traditional sampling using pumping causes mixing which can obscure depth gradients. We have applied a passive approach by using a series of diffusion cells that are installed at different depths within the active zone. The cells are filled with deionized water and over time, they equilibrate with the adjacent active layer water chemistry (ions diffuse into the cell, but the water in the cell does not exchange). Using this approach we have collected a series of fine resolution depth profiles within saturated zones in the active layer. Results over the last three years often show well-developed and sometimes substantial geochemical gradients for multiple analytes. Such gradients imply minimal vertical mixing within the active zone. Reductions in permeability with depth and lack of strong hydrological gradients likely limit vertical mixing. We also noted that the strength of the depth gradients varies across the landscape reflecting differences related to microtopography and drainage conditions. These results suggest that there are likely to be substantial fine-scale depth variations in biogeochemical processes such as methane and carbon dioxide production. Hydrological models should also reflect limited mixing with depth.