Dynamic, structured heterogeneity of water isotopes inside hillslopes

Abstract

Use of the stable isotopes of water (dD, d 18 O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (328) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo-meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…This "two-water-world" hypothesis (McDonnell, 2014) is controversial (Berry et al, 2017;Sprenger et al, 2016) and could be at odds with the existence of subsurface reservoirs such as layers of saprolite and fractured, partly weathered immobile material that hold water that is accessed by trees (Oshun et al, 2016). For example, in seasonally dry climates, trees may derive a significant portion of their moisture from immobile weathered material well below the soil (Zwieniecki and Newton, 1996;Graham et al, 2010;Nie et al, 2012).…”

“…There have been a number of recent papers building upon the early work in Graham Allisons' laboratory exploring water isotope fractionation in subsurface pools (Allison et al, 1983). This new work investigates methodologies of extraction, isotope fractionation during water uptake by plants, and interpretation of isotope data (Oerter et al, 2014;Orlowski et al, 2016a, b;Oshun et al, 2016;Zhao et al, 2016;Gaj et al, 2017;Johnson et al, 2017;Vargas et al, 2017). These papers provide new insights at the same time that they add to the ongoing controversy about what explains water isotope variation in the many possible subsurface pools, highlighting the need for research.…”

“…These observations point out that there is a generally uncharacterized heterogeneity of water resources, nutrients, and fluxes in the CZ related to trees (Johnson and Lehmann, 2006;Oshun et al, 2016;Bowling et al, 2017). These findings are now forcing researchers to develop new ways to investigate the parts of the CZ that trees access.…”

“…These findings are now forcing researchers to develop new ways to investigate the parts of the CZ that trees access. In turn, this is driving a new re-calculation of the types, sizes, and residence times of water inventories that are available to plants in catchments (Oshun et al, 2016) and how water use is changing with atmospheric carbon content (Keenan et al, 2013). We also know that nearly all tree species host mycorrhizal fungi in symbiotic association with their roots (Read, 1997).…”

Isotopic hydrograph separation in two small mountain catchments during multiple events

“…Although hydrologists became more aware of the limitations (e.g. Genereux and Hooper, 1998;Burns, 2002, Oshun et al, 2016 and the technique does not contribute anymore to the development of ideas on runoff formation (Klaus and McDonnell, 2013), it may still be useful in improving the understanding of runoff generation in individual case studies conducted in various climatic and physiographic conditions. High-frequency sampling of rainfall and runoff and P A P E R A C C E P T E D .…”