Transit Times and Rapid Chemical Equilibrium Explain Chemostasis in Glacial Meltwater Streams in the McMurdo Dry Valleys, Antarctica

Abstract

Fluid transit time is understood to be an important control on the shape of concentration‐discharge (C‐q) relationships, yet empirical evidence supporting this linkage is limited. We investigated C‐q relationships for weathering‐derived solutes across seven Antarctic glacial meltwater streams. We hypothesized that (H1) solute fluxes in McMurdo Dry Valley streams are reaction limited so that C‐q relationships are characterized by dilution and that (H2) transit time explains between‐stream variability in the degree of C‐q dilution. Results show that C‐q relationships are chemostatic because solute equilibrium times are shorter than stream corridor fluid transit times. Between‐stream variability in the efficiency of solute production is positively correlated with transit time, suggesting that transit time is an important control on the solute export regime. These results provide empirical evidence for the controls on weathering‐derived C‐q relationships and have important implications for Antarctic ecosystems and solute export regimes of watersheds worldwide.

Concentration versus streamflow trends of major ions and tritium in headwater streams as indicators of changing water stores

“…Variations in solute concentrations with streamflow (discharge) in rivers and streams contain vital information on residence times, processes of solute generation, biogeochemical processes, and sources of stream water; thus, documenting the controls on solute geochemistry is vital to understanding catchment functioning (Ameli et al, ; Bieroza, Heathwaite, Bechmann, Kyllmar, & Jordan, ; Bouchez et al, ; Clow & Mast, ; Creed et al, ; Diamond & Cohen, ; Duncan, Band, & Groffman, ; Godsey, Kirchner, & Clow, ; Guan et al, ; Hunsaker & Johnson, ; Ibarra, Moon, Caves, Chamberlain, & Maher, ; Kim, Dietrich, Thurnhoffer, Bishop, & Fung, ; Koger, Newman, & Goering, ; Uhlenbrook & Hoeg, ; Wlostowski, Gooseff, McKnight, & Lyons, ; Zhang, Harman, & Ball, ). There are several potential sources of the water in streams and rivers that may have significantly different geochemistry (Cartwright, Gilfedder, & Hofmann, ; Cook, ; Gonzales, Nonner, Heijkers, & Uhlenbrook, ; Nathan & McMahon, ; Yu & Schwartz, ).…”

“…(Bieroza et al, ; Clow & Mast, ; Diamond & Cohen, ; Godsey et al, ; Koger et al, ; Musolff, Schmidt, Selle, & Fleckenstein, ; Wlostowski et al, ). Equation is a power law relationship where the exponent b is the slope of the log C versus log Q trend and a is a constant.…”

“…This overcomes the problem that solute concentrations in rainfall can be spatially and temporally variable (Hrachowitz, Fovet, Ruiz, & Savenije, ; Kirchner, Tetzlaff, & Soulsby, ) and are thus difficult to account for in detail. The concentrations of major ions such as Cl, Na, Ca, and K do decrease with increasing streamflow in many catchments; however, values of b are commonly significantly greater than −1 (Clow & Mast, ; Diamond & Cohen, ; Godsey et al, ; Musolff et al, ; Wlostowski et al, ).…”

“…If the geochemistry of these water stores is more similar to baseflow than surface runoff, there may also be little change in stream geochemistry. Increased weathering may occur as the catchment wets up due to the reactive surface area increasing, potentially increasing the flux of weathering‐derived solutes at high streamflows (Ameli et al, ; Godsey et al, ; Wlostowski et al, ).…”

“…Where flushing of weathering products by recent rainfall has occurred, the transit times of the water at high streamflows may be very short. Mean transit times (MTTs) may be calculated from reaction rates or modelling (Ameli et al, ; Wlostowski et al, ). Alternatively, MTTs of the water from within the catchment that contributes to streamflow may be estimated using radioactive tracers such as tritium ( 3 H) (Morgenstern, Stewart, & Stenger, ; Stewart, Morgenstern, McDonnell, & Pfister, ) or from time series of Cl concentrations and/or stable isotope ratios (Kirchner et al, ; McGuire & McDonnell, ).…”