Thermal Dynamics of the Active Layer Along a Hydrologic Gradient Bordering Lakes in the McMurdo Dry Valleys, Antarctica

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1 Thermal Dynamics of the Active Layer Along a Hydrologic Gradient Bordering s in the McMurdo Dry Valleys, Antarctica Michael N. Gooseff Department of Civil & Environmental Engineering, Pennsylvania State University, University Park, PA J. E. Barrett Department of Biological Sciences, Virginia Polytechnic Institute, Blacksburg, VA Scott Ikard Geology & Geological Engineering Department, Colorado School of Mines, Golden, CO Melissa Northcott Geology & Geological Engineering Department, Colorado School of Mines, Golden, CO Cristina Vesbach Department of Biology, University of New Mexico, Albuquerque, NM Lydia Zeglin Department of Biology, University of New Mexico, Albuquerque, NM Abstract With little precipitation (<10 cm water equivalent annually as snow), soils of the McMurdo Dry Valleys (MDV) have limited water available to support hydrological or biogeochemical processes. Active layer depths across most of this landscape are <1 m. Saturated sediments are obvious in wetted margins on the shorelines of lakes, extending for up to ~10 m into a zone where typically arid MDV soils prevail. We propose that wetted margins of MDV lakes will differ from arid soils across the rest of the landscape in their active layer depth and temperature regimes because of the consistent presence of water within these wetted margins. We have monitored temperatures along a wetted margin of s Fryxell, Bonney, and Joyce. During the austral summer, we found that drier soils promoted shallower thaw depths and that, at the same depths, wet soils generally had lower temperatures and smaller diurnal fluctuations than dry soils. Keywords: active layer; Antarctica; McMurdo Dry Valleys; temperature time series. Introduction Soil temperature dictates physiological constraints on biological activity (Kirshbaum 1994) and is especially important in the McMurdo Dry Valleys (MDV) of Antarctica (Fig. 1) where temperature is a primary control over biogeochemical cycling and biotic communities (Doran et al. 2002b, Parsons et al. 2004, Aislabie et al. 2006). Similarly, soil moisture is a limiting factor in Antarctic soil ecosystems where low temperatures severely limit the availability of liquid water (Kennedy 1993). Most soil studies in the MDV have emphasized the top 10 cm of the soil profile, within the active layer that is generally 60 cm thick (Campbell et al. 1998). There has been extensive research on active layer thermal dynamics in the Arctic, with emphasis on numerical modeling of these dynamics (Hinkel 1997), and their response to changing climate (Hinkel et al. 2001). Arctic active layer soils exhibit seasonal changes in surface energy balances comparable to those in the MDV, though the magnitudes and types of energy exchange differ greatly due to the presence of extensive vegetation in the Arctic, as well as substantial spring snowmelt infiltration and rain infiltration during the summer. There have been several studies of permafrost and active layer processes in the Victoria Land (Bockheim et al. 2007, Guglielmin 2006), but little attention has been paid to active layer thermal dynamics on floors of the MDV, particularly with respect to dependence upon soil moisture. Bockheim and Tarnocai (1998) note that most of the surficial permafrost in the MDV is dry permafrost, which has very low water content (<5%), and Pringle et al. (2003) have documented thermal properties of MDV permafrost and active layer from two fairly high elevation sites in the region. The shores of MDV water bodies are continually proximal to liquid water during the austral summer, and wick water from the lake and stream edges to the dry mineral soil. These wetted margins are visually apparent, and represent a hydrologic gradient in soil moisture from saturated at the water body to dry conditions at distal locations. Around lakes, Gooseff et al. (2007) found that the dimensions of these wetted margins vary as a function of both shore slope and depth of the active layer. We propose that, because these wetted margins represent an obvious gradient of soil moisture, the active layer thermal dynamics across these wetted margins should vary, with general soil moisture condition, due to the differences in thermal conductivity and heat capacity of the soil matrix with changes in soil moisture status. 529

2 530 Ninth International Conference on Permafrost Bonney Joyce Fryxell Asgard Mtns. y alle or V l Tay kr Ku ih ills Figure 1. Map of Taylor Valley, Antarctica and s Joyce, Bonney and Fryxell. Table 1. Distances to thermocouple strings from the edge of the lake, in meters (XX), and depth of deepest thermocouple, in meters (YY), in the format XX,YY. All thermocouples are spaced by 10 cm vertically on a string. The number of thermocouples at a location is determined by the depth of penetration possible when deployed, with a maximum of 0.5 m depth. String I II III IV V Lk. Fryxell 0.0, , , , , 0.33 Study Plot Lk. Bonney 0.0, , , , , 0.50 String V String IV String III N Ferrar Gl. Taylor Gl. A Saturation 78 S 162 E String II String I permafrost B Lk. Joyce 0.0, , , , , 0.17 Site Description and Methods Site description The MDV are located on the western edge of the Ross Sea, at approximately 78 S 162 E. The climate is cold, with annual mean temperatures of -20 C, and dry, with <10 cm precipitation (all as snow) annually (Doran et al. 2002a). The water balance of closed-basin lakes on the valley floors are maintained by incoming glacial meltwater stream flow and losses due to annual ablation of perennial ice covers, and evaporation of open water moats that form along the lake shores during the austral summer. Thus, lake shore soils and sediments are immediately adjacent to liquid water for approximately 2.5 months annually. Due to the dry nature of the MDV and the presence of continuous permafrost, these shoreline sediments wick lake water several meters inland from the shoreline. Methods In January 2005, we deployed thermocouple strings with 10 cm vertical spacing along transects across wetted margins of the north shore of Joyce, the south shore of the east lobe of Bonney, and the north shore of Fryxell (Fig. 1). Five thermocouple strings were deployed at each lake-side location (Fig. 2A): String I at the water edge, String II 20 cm from the shoreline, String III at a point bisecting the wetted margin, String IV just before the outside edge of the wetted margin, and String V outside of the wetted margin (Fig. 2B, Table 1). Thus we collected active layer temperature data across three hydrologic gradients, from saturated conditions at String I to dry conditions at Figure 2. (A) General layout of thermocouples along transects, across wetted margins, (B) wetted margin around Fryxell, extending ~10 m from shoreline. String V. Thermocouple strings were built in a lab prior to deployment and then inserted into the ground using a thin steel rod attached to the end of the thermocouple string. The rod was subsequently removed after being inserted to the point of refusal. At each transect, thermocouple data was collected by a Campbell Scientific CR-10XT datalogger via an AM-25T multiplexer. In addition, a thermistor was also deployed which collected air temperature data near the soil surface, in a shaded location. During the and field seasons, we were able to visit the dataloggers intermittently. Records are not continuous for the entire time of deployment due to datalogger failures. At Joyce, data was collected on a 4 h interval from Jan. 2005, from 12 Jul. to 15 Dec. 2005, and on a 15 min interval from 27 Dec to 04 Jan At Fryxell, data was collected on a 15 min interval from Jan. 2005, on a 4 h from 02 Jul. to 05 Dec. 2005, on a 1 h interval from 07 Jan. to 01 Feb. 2006, and on a 4 h interval from 01 Feb. to 30 Jan At Bonney, data was collected on a 15 min interval from Jan. 2005, and on a 4 h interval from Jan. 2006, and from 14 Dec to 02 Feb We analyzed the collected subsurface temperature data by calculating 1) total degree-days of thaw (DDT) for each thermocouple record and 2) the daily amplitudes of temperature at every site, for days on which complete data collection was available (539 d at Fryxell, 81 d at

3 Go o s e f f e t a l. 531 Bonney, and 166 d at Joyce). We then computed frequency-duration curves (FDCs) using these data to illustrate comparisons among the thermocouple strings at each site, along the hydrologic gradients. Temperature FDCs that plot to the right on such graphs indicate a greater proportion of large diurnal cycles compared to those that plot toward the left. Our expectation is that thermocouple records in the drier shore sediments (i.e., Strings IV and V) will have greater diurnal variation than strings in more saturated conditions (Strings I-III). Because each thermocouple string is not deployed to a common depth, we cannot compare all FDCs on the same plot. Thus, for a more fair comparison, we have only plotted FDCs for thermocouple strings that have common or very similar depths (within 3 cm of each other). From s Fryxell and Bonney, we plot only Strings III, IV and V. From Joyce, we plot only Strings II, III, and IV. Results and Discussion Temperatures in the subsurface vary throughout the year from -50 C to just above 10 C. At Fryxell, the time series data indicate a tight coupling of the temperature records near the shore (Strings I and II) and increasing vertical temperature gradients at more distal locations (Strings III, IV, and V) (Fig. 3). Similar patterns are evident from data collected at Bonney (Fig. 4) and Joyce (Fig. 5). It is worth noting that none of these locations appear to go through an extensive periods of zero-curtain condition during thawing (Figs. 3 5), as has been observed in arctic active layers (Hinkel et al. 2001). The only freeze-up data available is from Fryxell, which does indicate a few days of zero-curtain condition close to the shorelines (Strings I and II) at Fryxell (Fig. 3). The vertical temperature gradients (Figs. 3 5) are not perfectly comparable as the thermocouples are deployed at different absolute depths along each thermocouple string. Despite these differences, these gradients are approximately comparable for Strings I-IV at Fryxell, as the penetration depths are 50 cm, 50 cm, 45 cm, and 45 cm, respectively. At Bonney, Strings III, IV, and V are all deployed to the same depths, and it is evident that String IV appears to have the greatest diurnal fluctuation in temperature, at the 10 cm depth (Fig. 4). At Joyce, String IV is offset by 3 cm from Strings II and III (Fig. 5). To compare integrated heating among locations along the soil moisture gradients, the DDT values are presented in Table 2. As expected, there is a pattern of diminishing degree-days above freezing with increasing depth. Similar to the analyses of temperature time series, these data are not ideally comparable among thermocouple strings within a site because of different absolute depths. The vertical pattern exceptions are String II and String V at Fryxell, and String I at Bonney. These unexpected results are likely due to one of three explanations: (1) surface evaporation which may drive cooling in the upper layers of the soil, (2) longitudinal movement of cool water from Figure 3. shore soil temperatures from thermocouple strings I through V at Fryxell.

4 532 Ninth International Conference on Permafrost Table 2. Degree-days of thaw (DDT, C d) for entire temperature records of thermocouples and thermistors. The table is organized by thermocouple string and position along the string with 1 = shallowest and 5 = deepest. Thermocouple String TC# I II III IV V Fryxell Air Bonney Air Joyce Air the shoreline outward toward the dry soils, or (3) change in surface conditions. String I at Bonney was found to be inundated at the surface in Jan. 2006, due to rising lake levels. Thus the temperature signal at this location becomes more representative of benthic interaction with a water column than a soil exposed to atmosphere. The longitudinal movement of water is a possibility at String II at Fryxell, given that the water at the lake shore (String I) is generally cooler within the same ~depths (positions 4 and 5). The comparisons of temperature dynamics are also informed by the frequency analysis of diurnal temperature amplitudes (Fig. 6). At the Fryxell transect, the diurnal amplitudes of temperatures increase for near-surface thermocouples from String III to IV to V, with similarities between III and IV, and much more variable overall at V (red curves, Fig. 6A). The same pattern exists at the thermocouples in the 2 nd position (green curves, Fig. 6A), but changes at the 3 rd thermocouple position, with Strings III and V similar in their FDC curves, and that of IV being generally less variable (blue curves, Fig. 6A). This is unexpected, and may be due to (1) the fact that these mid-transect locations are buffered at depth because of greater soil moisture or (2) lateral movement of water (i.e., along a direction that is parallel to the shoreline), which was not investigated here. At Bonney, String IV is more variable at thermocouple positions 1 and 2, than Strings III and V (red and green curves, Fig. 6B). Similar to the patterns observed at Fryxell, this sequence changes at position 3, and String IV becomes generally less variable than Strings III and V (blue curves, Fig. 6B). This Figure 4. shore soil temperatures from thermocouple strings I through V at Bonney.

5 Go o s e f f e t a l. 533 Figure 6. Frequency-duration curves (FDCs) for daily amplitudes in soil temperatures at A) Fryxell, B) Bonney, and C) Joyce. Each curve represents the entire record for a single thermocouple. Strings with comparable depths of thermocouple deployments are presented for comparison. Strings are distinguished by symbols on the curves: nearest the lake has no symbol, the next one out has a cross symbol, and the furthest an open circle. Similar depths are indicated by pattern. Figure 5. shore subsurface temperatures from thermocouple strings I through V at Joyce. may indicate here too that String III is well buffered because of (1) enhanced soil moisture at depth at this position, (2) this mid-transect location is a transition point between the influence of convected heat from either longitudinal (i.e., away from the shoreline) flowing water from the shore to the dry soils, or (3) the influence of lateral flow of water not investigated here. At Joyce, there is a consistent pattern of String IV having greater diurnal variability, in

6 534 Ninth International Conference on Permafrost general, than String V at thermocouple positions 1, 2, and 3 (red, green, and blue lines, respectively, Fig. 6C). At the 4 th thermocouple position (next to deepest), String V is much more variable in daily temperature fluctuation than String III at Fryxell (black curves, Fig. 6A), and very similar for Strings III-V at Bonney (gray curves, Fig. 6B). At the deepest thermocouple location (positions 5) temperature amplitudes are similarly buffered at Strings III and IV at both Fryxell (Fig. 6A) and Bonney (Fig. 6B). These three sets of results generally support our expectation, that drier soils would be warmer and more variable than wetter soils. In particular, this notion is supported by the evident temperature time series magnitudes, which are greater at the distal thermocouple strings than at the near-shore strings, as well as by the general patterns of increased DDF at the distal locations compared to the nearshore locations. Finally, for the most part, locations that have less soil moisture are more variable in daily temperature amplitude than more saturated soils. Conclusions We found that, in general, soils with little soil moisture were warmer and more variable in temperature than wetter soils. This investigation focused on locations of soil moisture gradients near three lakes in the MDV, which may host variable microbial communities due to potential dependence upon habitat conditions, namely soil temperature and water content. These factors also influence biogeochemical processes across these hydrologic gradients. Acknowledgments The authors gratefully acknowledge Raytheon Polar Services Corp., Petroleum Helicopters, Inc., and UNAVCO for logistical and field support, and the McMurdo Long Term Ecological Research project. This research was funded by the National Science Foundation under collaborative research grants OPP , , and Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. References Aislabie, J.M., Chhour, K.L., Saul, D.J., Miyauchi, S., Ayton, J., Paetzold, R.F. & Balks, M.R Dominant bacteria in soils of Marble Point and Wright Valley, Victoria Land, Antarctica. Soil Biology & Biochemistry 38: Bockheim, J.G. & Tarnocai, C Nature, occurrence and origin of dry permafrost. Proceedings of the Seventh International Permafrost Conference, Bockheim, J.G., Campbell, I.B. & McLeod, M Permafrost distribution and active-layer depths in the McMurdo dry valleys, Antarctica. Permafrost and Periglacial Processes 18(3): Campbell, I.B., Claridge, G.G.C., Campbell, D.I. & Balks, M.R Permafrost properties in the McMurdo Sound-Dry Valley region of Antarctica. Proceedings of the Seventh International Permafrost Conference, Doran, P.T., McKay, C.P., Clow, G.D., Dana, G.L., Fountain, A.G., Nylen, T. & Lyons, W.B. 2002a. Valley floor climate observations from the McMurdo dry valleys, Antarctica, Journal of Geophysical Research 107(D24): 4772, doi: /2001jd Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. & Parsons, A.N. 2002b. Antarctic climate cooling and terrestrial ecosystem response. Nature 415: Gooseff, M.N., Barrett, J.E., Northcott, M.L., Bate, D.B., Hill, K., Zeglin, L., Bobb, M. & Takacs-Vesbach, C Controls on soil water dynamics in near-shore lake environments in an Antarctic polar desert. Vadose Zone Journal 6: Guglielmin, M Ground surface temperature (GST), active layer and permafrost monitoring in continental Antarctica. Permafrost and Periglacial Processes 17(2): Hinkel, K.M Estimating seasonal values of thermal diffusivity in thawed and frozen soils using temperature time series. Cold Regions Science and Technology 26(1): Hinkel, K.M., Paetzold, F., Nelson, F.E. & Bockheim, J.G Patterns of soil temperature and moisture in the active layer and upper permafrost at Barrow, Alaska: Global and Planetary Change 29(3 4): Kennedy, A.D Water as a limiting factor in the Antarctic terrestrial environment: A biogeographical synthesis. Arctic and Alpine Research 25(4): Kirschbaum, M.U.F The temperature-dependence of soil organic-matter decomposition, and the effect of global warming on soil organic-c storage. Soil Biology & Biochemistry 27: Parsons, A.N., Barrett, J.E., Wall, D.H., & Virginia, R.A Soil carbon dioxide flux from Antarctic Dry Valley soils. Ecosystems 7(3): Pringle, D.J., Dickinson, W.W., Trodahl, H.J. & Pyne A.R Depth and seasonal variations in the thermal properties of Antarctic Dry Valley permafrost from temperature time series analysis. Journal of Geophysical Research 108(B10): 2474, doe: /2002jb