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PAGE S 
24
Reconstruc ting past rainfall fluc tuations in

sedimentary archives, such as sand dunes,
are mediated by a number of factors, of
which changing moisture availability is just
one. Former river courses, and former lakes
fed by rivers, often record changes imported
from a climatic zone out side their dryland

are restricted to limestone terranes and tend
to be discontinuous records in drylands.
Isotopic records within hyrax middens have
growing potential, but are also spatially
restricted to areas with rock-ledge habi-

capable of recording changes in moisture
availability is extremely valuable for dryland
Quaternary environmental reconstruction.
Chemical tracers within sand dune pore
moisture offer a novel additional proxy for
paleomoisture availability. When and where
this mois ture moves downward, fluc tuations
in its chemical signature with depth can be
utilized to establish a record of changes to
moisture availability through time, based
on the transport of a signal built up in the

timescale of the preserved hydrostratigra-
phy depends on the rate of moisture move-


range of timescales are briefly explored to
demonstrate the utility of this approach.
Basis of tracer technique
Pore moisture in sand dunes above the
water table is described as the unsaturated

 
be reconstructed, based upon variations in

chloride signal is established in the near-sur-
face rec ycling zone, and this inherited signal
is then transmitted vertically in the moisture
that is infiltrating through the sediment, so
that a depth-based profile of samples repre-


precipitation contains solutes, including
-
tration of this input c an be established
empirically, and varies between regions as
 
input gets modified in the near-sur face
zone by evapotranspiration, which removes
water and enriches the chloride concentra-

of recycling, moisture moves downwards,
and movement is closest to homogenous
in sand-rich sediments containing some
finer-fraction sediment (e.g. Gehrels et al.

of chloride in pore-moisture at sequential
depths are a record of a climatically driven
process in the zone of rec ycling before

record can be read as low(high) chloride
concentrations recording high(low) moisture
availability. A certain degree of smoothing of
the record will have occurred in the rec y-
cling zone, including smoothing variations
between individual rainfall events.
Records of land-use changes
and precipitation

quantify increases in moisture flux through
sediments as a result of land-use change. In
southwestern Australia, clearance of native
eucalyptus vegetation increased recharge
 under intact vegeta-

under pas ture and cereal cropped land re-
-
strated clearly in the hydrostratigraphy by a
vertical displacement of a single subsurface
chloride peak in cleared sites compared to
intac t sites.

 -
west India, multi-decadal length hydro-
stratigraphies demonstrate a recharge rate

 in a non-irri-



as proxies for precipitation has also been

Pore moisture within desert sand dunes provides a novel archive for paleomoisture availability. Hydrostratigraphies
are produced from variations in chemical tracers in a vertical profile. The applicability has been demonstrated in
drylands in five continents and three examples are given here.
Squeezing a rainfall record out
of desert sand dunes

SCIENCE HIGHLIGHTS: OPEN SECTION
Figure 1:
p is concentration of chloride in precipitation and Cd is concentration of

PAGE S 
25

correlates well with both a local precipitation
-


Records of climatic change in central A sia
Arguably the most significant development
for dryland Quaternary environmental
reconstruction using this approach comes
from the decadal-scale resolution, mul-
ti-millennial length hydrostratigraphies from





independent paleohydrological proxies for
northern China have been made, which high-
light wet ter inter vals in this region around


wetter intervals are suppor ted by above-av-
erage values for precipitation reconstructed

 

phases of above-average ice-core accumu-


flood-drought index (based on his torical

However, the overall correspondence is not
perfect, with some larger incursions in the
-
drostratigraphies, suggesting some regional
complexity in paleohydrology in this region.
Reconstructing long-term
dryland aridification
Initially driven by a motivation to assess
-

waste without contaminating groundwater,
more than a dozen hydrostratigraphies re-





hydrostratigraphy at the Nevada nuclear
test core than contains an earlier c ycle from
higher recharge to greater aridity during


Conclusions and outlook
-
vironments is a novel and valuable archive,
providing a direct paleomoisture proxy,
where the enrichment of chloride ac ts as a
tracer for the balance between precipita-

sediments required for this approach cover
a portion of drylands where it is extremely
challenging to reconstruc t hydrological
variations over the Quaternary, owing to
poor preservation of biological material
and a scarcity of water-lain sediments and
speleothems.
Further development of modeling ap-
proaches that incorporate transient fluxes of
water and tracer input concentrations (Ginn


 
of the sediment texture at a site-by-site basis
and avoid sediments that experience a very
heterogeneous ver tical flow of water.
-



infiltration, hydrostratigraphies can record
low-resolution trends in humidit y-aridity
over deglacial timescales, a decadal-resolu-
tion paleomoisture proxy for the last two mil-
lennia or quantify changes in moisture flux in
sediment resulting from anthropogenic land
use change.

 
 
 
 

 abi.stone@manchester.ac.uk

  
  
  
  
 

 
  

 
  
  
  

  
 
 
SCIENCE HIGHLIGHTS: OPEN SECTION
Figure 2:(A)
(B)
(C)(D)
(E)
(F)
(G)


of the supporting proxy archive record wetter than average conditions.
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