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!
