Monday, October 28, 2013
Despite the cold (and overly ambitious) days of fieldwork in Barrow, the consensus among LANL’s hydrology and geochemistry research team is that our 2nd Synoptic Sampling Campaign in September 2013 was a huge success! Our field crew consisted of a highly motivated and hard-working team from LANL including: Lily Cohen (post-bac, and research superwoman); Garrett Altmann (post-masters, and GIS/remote sensing guru); Marvin Gard (staff, and world class innovator/inventor); and Heather Throckmorton (postdoctoral researcher, and isotope geochemist and enthusiast).
The goal of our research is to assess isotopic and geochemical tracers to infer hydrologic processes, and vertical and lateral movement of carbon across the broader landscape. Our most recent sampling campaign in September 2013 has been an extension of a previous campaign in July 2013, during which we established 17 sites across the broader landscape (both inside and outside the BEO) to collect water samples for analyses. Our sites were initially selected by looking at satellite images of the BEO and surrounding region to identify main drainage channels and outlets from different aged drained thaw lake basins (DTLBs), as locations that integrate hydrologic signals over larger areas. (see below map)
This September we revisited the same 17 site-locations as in July (please refer to map) in order to identify seasonal changes/variability, with the thawing of frozen soils throughout the summer, and corresponding deepening of the frost table.
During our most recent trip this September, we noticed several clues supporting seasonal differences in hydrology relative to July conditions. Our first clue, as expected, was the significant deepening of the active layer as we measured the depth to the frost table at all of our sites. This presumably has significant implications for subsurface hydrologic flow, as deeper mineral rich soil horizons become hydraulically conductive.
Seasonal melt correlating hydrologic processes with geomorphologic features was also really exemplified by a dramatic thermokarst feature that we encountered on our walk in to our site 11 (see map). We previously observed this feature in July as well, but the advanced thawing/melt over the season has promoted slumping on the ridge, and resulted in the interesting feature below.
Additionally, although surface waters can often appear stagnant in the BEO and surrounding region, at a couple of our sites in July we had observed very gentle lateral surface flow and drainage. During our recent trip in September, we noticed visibly greater lateral surface (and subsurface) flow at several of our sites relative to what we observed earlier in the season.
Another notable difference we observed from July to September was an increase in lateral extension of surface water in ponds and drainages at several of our sites. In the vast and hummocky tundra landscape, lateral expansion of surface water would typically be difficult to notice from visible inspection only, with such few distinct reference points in the landscape. However, when we sampled in July, we stationed along the edges of ponded or draining surface waters, marking these locations with bamboo for our return trip in September. During our recent September trip, upon revisiting these locations, we estimated that in many cases the water extended a few meters farther than in July (laterally); and in one larger drainage channel (site 8 on attached map), we estimated lateral expansion of at least 25 meters.
In addition to seasonal variation in hydrology (and significantly colder weather in September!), we’ve noticed some very exciting spatial variation across the broader landscape highlighted by differences in a variety of features across our sites, including: lateral surface hydrologic flow rates; hydrologic conductivity (i.e., the rate we can extract water); depth to frost table; soil properties; and basic field parameter measurements relating to groundwater redox and geochemistry.
In particular, we were excited and encouraged by the outcome of Iron Reduction in Sediment (IRIS) probes that we installed in July, as an indicator of below-ground oxidation–reduction conditions. Dissolution of the ferrihydrite coat on probes showed interesting patterns that varied across sites, which we will quantify with image analyses to better understand redox microsites and variation across the broader landscape (photo below on left). At some locations, IRIS probes developed black speckles of iron sulfide, providing additional insight into below ground biogeochemistry (photo below on right). To our knowledge, this is the first time this technology has been deployed in this type of environment, and we feel that IRIS probes seem promising for improving our understanding of spatial and temporal variation in redox across a range of scales.
We are looking forward to pursuing analyses on water and soil samples back at LANL to better understand how spatial and seasonal variations in hydrology may correspond to and influence biogeochemical processes and C transport. For example, do we see different sources of water spatially and seasonally? Different sources of carbon across the landscape, as organic rich soils thaw throughout the summer and leach or become transported? How do these different hydrologic conditions affect biogeochemistry and redox? Microbial processes? C transport? These are only a few of the questions we are excited to address. So far so good-as our research group, as well as soil and water samples, have all made it back to LANL intact!