**WHITE SANDS GYPSUM DUNES: A TERRESTRIAL ANALOG TO NORTH POLAR DUNES ON
MARS?** A. Szynkiewicz^1^, L.M. Pratt^1^, M. Glamoclija^2^, C.H.
Moore^1^, Esther Singer^1^ and D. Bustos^3^, (^1^Indiana University
Department of Geological Sciences, 1001 East 10^th^ Street, Bloomington,
IN 47405-1405, <aszynkie@indiana.edu>; ^2^Geophysical Laboratory,
Carnegie Institution of Washington, 5251 Broad Branch Road NW
Washington, DC 20015; ^3^White Sands National Monument, P.O. Box 1086,
Holloman Air Force Base, NM 88330).

**Introduction:** Many dunes on Earth are indicators of an arid climate
where strong aeolian processes cause deposition of well-sorted, fine,
sandy material. Although dune *deposition* is primarily associated with
arid conditions, the deposition of the source sediments is usually
related to water-associated processes.

The White Sands gypsum dunes (WSgd) are the largest known aeolian
sulfate-rich deposits on Earth and are an excellent natural laboratory
for studying the origins of sulfate dunes. In particular, this
communication focuses on the interaction between hydrology and climate
that lead to the deposition of the source sediment for the dunes and
controls the current dune field morphology.

The WSgd may be a terrestrial analog to the gypsum-rich dunes of Olimpia
Undae (OUgd) detected in the north polar region of Mars. The OUgd appear
to have undergone complex chemical and physical evolution, possibly
involving hydrothermal heating and mobilization of sulfides by fluids,
followed by aeolian weathering \[1,2,3,4\].

The source area for gypsum in the OUgd has not yet been determined but
the OUgd are near the north polar cap, a plausible Martian water-source.
Also, the proximity of the OUgd to the outwash channels of the Chasma
Boreal \[2,4\] suggests that the gypsum supply might have been
controlled by water-involved processes similar to those on Earth.

We have used stable isotope methods at WSgd to determine the source of
the original dune sediments, document the involvement of water in their
deposition, and to trace the connection of these sediments to the
current dune field. We also present examples of aeolian sedimentary
features, recognizable on aerial photographs, that show the influence of
geochemical processes on the character of the surface morphology as seen
"from the air". This study of the WSgd region, at multiple scales, may
help in the understanding and recognition of similar processes operating
in the OUgd and possibly elsewhere on the surface of Mars.

**Geological setting:** The WSgd are located in south central New
Mexico, in the closed-drainage Tularosa Basin, a graben formed by
extensional tectonics in the Rio Grande Rift. The basin is bordered on
the east and west by mountain ranges containing Precambrian and
Paleozoic rocks. Both the ranges and Tularosa Basin show evidence of
tectonic processes within the last 20 ka. The pluvial Lake Otero existed
in the northern part of the basin during the late-Pleistocene/Holocene
and left gypsiferous evaporative deposits \[5\]. Regional deflation
events have been dated at 7 ka and 4 ka BP \[5\].

**Methods:** We sampled a 7 m thick sequence of Holocene age lakebeds
from a fault scarp in the southern part of the White Sands National
Monument in order to determine the sulfur isotope composition of this
probable sediment source for the dunes. Also, gypsum sand was sampled
and the sulfur isotope composition determined for samples from 8
transects across the northern, central and southern part of the dune
field inside the Monument. .

**Results and discussion:** The current groundwater system at White
Sands is dominated by SO~4~ ions originated from the dissolution, by
meteoric water, of Middle Permian strata with δ^34^S values from 10.8 to
12.3 ‰ (all values reported vs. VCDT). However, the range of δ^34^S
values, from the bottom (11.3 ‰) to the top (13.8 ‰), of the lakebed
sequence indicates that an SO~4~ ion source with higher δ^34^S must have
been involved. This source is the Lower Permian strata that has higher
δ^34^S values (12.5 to 14.4 ‰). Because, in the basin center, the Lower
Permian strata are overlain by the Middle Permian approximately 1000 m
beneath the surface, the positive excursion of δ^34^S values suggests
the influx, in the past, of water from deeper parts of the basin into
the playa. The change in sulfate source can be tied to climate evolution
toward more arid conditions and possibly to tectonic activity that might
have increased the release of deeper-seated groundwater through faults
extending into the playa surface.

After that time, gypsiferous sediments underwent extensive deflation
that led to the formation of the White Sands dune field with parabolic
and traverse dunes prevailing on the eastern and western sides,
respectively. To explain the evolution of the dune field, we used δ^34^S
values as a tracer of the gypsum source and compared them to the studied
lakebed sequence. The increase of δ^34^S values in gypsum sand from the
SW towards the NE of the dune field follows the prevailing wind
direction (Fig. 1) and suggests that the eastern parabolic dunes were
formed from deflation of sediments from the top of the lakebed sequence,
and the western traverse dunes were sourced by gypsum coming from older
lakebeds that were deflated during a later episode(s). Based on our
data, we propose that an approximately 4 m thick sequence of playa
sediments was removed from the source region to form the White

sands dunes and that the process was essentially unidirectional.

Today, in the White Sands area, aeolian processes are limited by a
relatively high groundwater table \[5\], advanced dune cementation
\[6\], and re-exposure of an old lacustrine, sulfate-poor, clay sequence
on the playa surface. This indicates that, if the sediment sequence in
the playa includes non-gypsiferous sediments, the source region may be
subject to exhaustion of gypsum. Similarly, the lack of an observed
(currently) source area for gypsum at the OUgd, suggests that there were
limited amounts of gypsiferous sediments in the north polar region of
Mars in the past, or that such gypsiferous sediments have been buried by
younger rocks.

Remote sensing is the dominant technique for investigating the Martian
surface. Studies of comparable terrestrial geological systems, as
analogs, should increase our understanding of geological processes and
related morphological features and so help in our interpretation of the
geological history of Mars. Aerial photographs of the White Sands area
reveal well-developed cross-bedding features related to aeolian
transport of sand particles and reveal a long-term record of migration
because of advanced cementation (Fig. 2). Large horizontal cross-bedding
features are best exposed in the interdune areas and less so in the
downwind slopes of dunes. These are stoss slopes of migrating dunes that
have been cemented by gypsum and are erosion resistant. In the central
parts of interdune areas stoss slopes are progressively buried by
wet/damp interdune deposits \[7\]. After groundwater
![](media/image1.png){width="2.808333333333333in"
height="2.1118055555555557in"}

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  ![](media/image2.jpeg){width="3.0819444444444444in"
  height="2.053472222222222in"}

  Fig. 2. An aerial photo, taken from an altitude of about 450 m in
  October of 2007 over the heart of the White Sands dunes and revealing a
  longer-term record of migration because of advanced cementation of
  stoss slopes.
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discharge into interdune areas during the 2006/2007 winter, new
interdune gypsum-rich deposits partly covered large cross-bedding
features. This process is best visible around interdune areas at the
bottom of downwind and slip slopes (Fig. 2).

**Conclusions:** The δ^34^S values in gypsum sand from the WSgd indicate
that the dune field was mainly sourced by gypsum precipitated from an
old deep groundwater system that discharged into the White Sands area .
WSgd formation appears to be linked to a specific time period
characterized by a distinct climatic and/or tectonic regime during the
Holocene.

The geographical position of OUgd on Mars indicates that a similar
sulfate-rich aeolian system might have formed under Martian polar
conditions. The spatial distribution of gypsum within OUgd may be
connected to melting events in the Martian north polar cap and the OUgd
may be indicators of water-involved processes on Mars similar to those
observed at WSgd.

**Acknowledgments:** This study was supported by the NASA Astrobiology
Institute team award directed by L.M. Pratt. Sincere thanks are due to
Parker Bradley and Kevin Dunshee from a Division of Heron Aerospace
Corp. in Alamogordo for obtaining high quality aerial photographs; and
to Dave Love for providing new information about the geology of the
White Sands area.

**References:** \[1\] Fishbaugh K.E. et al. (2007) *JGR, 112, E07002*.
\[2\] Fishbaugh K.E. et al. (2006) *LPSC XXXVII,* Abstract#1642. \[3\]
Tanaka (2006) *Fourth Mars Polar Science Conference,* Abstract#8024.
\[4\] Langevin et al. (2005) Science, 307,1584-1586. \[5\] Langford R.P.
(2003) *Quatern Internat, 104, 31-39*. \[6\] Schenk C.J. and Fryberger
S.G. (1988) *Sediment Geol 55, 109-120*. \[7\] Kocurek G. et al. (2007)
*Sediment Geol 197, 313-331.*