Unsaturated zone hydrostratigraphies: A novel archive of past climates in dryland continental regions

Natural chemical tracers contained in moisture within unsaturated zone (USZ) sediments have very significant potential as a novel archive of past climate and palaeoenvironmental conditions in dryland environments. However, this potential has yet to be fully realised. The tracer signature is established in the near-surface zone, with evapotranspiration processes enriching the concentration inherited from input from atmospheric deposition (predominantly rainfall). This pore-moisture tracer signature then moves vertically through the USZ sediments toward the water table, producing a hydrostratigraphy that, with increasing depth, records changes to the moisture balance at the surface through time. The small fluxes of water through the USZ in dryland climates means that this hydrostratigraphy approach is particularly valuable in these environments for providing climate records longer than the instrumental data period. This paper sets out the current state of the art in the use of the USZ as an archive. We explain the nature of the USZ, the basis of the tracer technique and also the field and sampling methodologies employed. Examples of application worldwide by broad geographic region are also evaluated. This shows that the USZ hydrostratigraphy approach can be used across three key timescales: (i) decadal to multi-decadal resolution records covering hundreds of years that provide information about recent climate fluctuations and patterns of land-use change; (ii) multi-decadal through to millennial scale length records which may record wetter and drier events down to decadal scale resolution and (iii) Last Interglacial through to the Holocene records of lower resolution that indicate broad shifts over multi-millennial timescales. We also explore the questions and challenges surrounding the depositional flux of tracer inputs and the extent to which infiltration is non-uniform, before setting the agenda for their future potential use alongside related proxies for palaeohydrology.