**GYPSUM DUNES FROM WHITE SANDS NATIONAL MONUMENT -- POTENTIAL
TERRESTRIAL ANALOG TO NORTH POLAR DUNES ON MARS.** A. Szynkiewicz^1^,
L.M. Pratt^1^, M. Glamoclija^2^, 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:** On Earth, gypsum dunes are rare and restricted to
semi-arid areas where older sedimentary sulfate-rich rocks undergo
dissolution by meteoric waters on/near the surface. Further, new
evaporitic deposits are formed from solute transport by surface runoff,
meteoric and/or hydrothermal groundwaters. Overall, gypsum dunes
appearance is closely connected with climate changes toward increasing
aridity after last global glaciaciation on Earth. The White Sands gypsum
dunes (WSgd) are the largest known on Earth. Their geological setting,
however, completely differs compared to the gypsum-rich dunes of Olimpia
Undae (OUgd) on Mars. In spite of this, we found few similar
morphological features within WSgd like those found on OUgd that might
provide some insights regarding the climate history of polar processes
on Mars.

The OUgd in the North Polar region of Mars reveal complex chemical and
physical evolution \[1,2,3,4\]. Gyspum was found in the presence of
dunes only, and its content decreases from the eastern edge of the main
polar north dune sea toward west following the main wind direction
\[1,2\]. Dunes contain gypsum grains of a few tens micrometers and
milimiters in size showing its elevated content within the dunes crest
\[1\]. The other component of dunes are mafic minerals and oxides. The
source area for gypsum has not been found yet. The spatial distribution
of gypsum suggests that region of meltwater channels could have
contained evaporites precipitated from water originated from the Chasma
Boreal melting events \[2,4\]. The regional geological and morphological
comparison suggests that gypsum deposits may result from circulation of
hydrothermal groundwater associated with magmatism at Alba Patera and
later eolian activity excavated this deposits \[3\]. On the contrary,
\[1\] proposed that elevated gypsum content may result from in-situ
alteration of sulfide and high-calcium-pyroxene-bearing dunes during
melting of the layered polar deposits. This hypothesis has a major
problem with "missing sulfide" as roughly 30 times more sulfide could be
needed than what likely was detected within the OUgd \[1\].

**Geological settings:** WSgd are located in southern central New
Mexico, in the closed-drainage basin of Tularosa formed by extensional
tectonism of the Rio Grande Rift. The pluvial Lake Otero existed in the
northern part of Tularosa Basin during the late-Pleistocene and left the
evaporative sequences with laminated clays and silts, laminated
gypsiferous marls, limestones, and massive silts containing large gypsum
crystals; today, these sediments are exposed in gullies and incised by
at least 20 playa lakes \[5\]. The aeolian processes are driven mainly
by hydrological changes in groundwater table \[5\], but vegetation and
surface cementation primarily acts to limit deflation as well \[6\].

**Results and discussion:** Three aspects of WSgd evolution relating to
climate variation will be discussed in comparison to OUgd: i) gypsum
sources, ii) groundwater discharge into interdunes areas, and iii)
dessication of dunes.

[Gypsum sources]{.underline} -- the sulfur isotopic analysis on gypsum
from dunes (mean δ^34^S, 13.15‰), lake beds (δ^34^S from 11.33 to
14.27‰), domes on the playa surface (mean δ^34^S, 13.07 ‰) and SO~4~^2-^
dissolved in local groundwater system (mean δ^34^S: 11.26‰ and 13.27‰)
showed that SO~4~^2-^ mostlikely originates from dissolution of two
different sequences of Permian evaporites located on and beneath the
surface. Contribution of each source seems to relate to a given climate
condition, wetter or drier, that governed varied inputs of sulfate from
shallow or deeper circulating groundwater system, respectively.
Additionaly, the alignment of some domes implies that local tectonism
could have released a deeper water through faults into the playa surface
in the past.

Today, eolian processes are inhibited by relatively high groundwater
table, advanced dunes cementation and re-exposition of old lacustrine
clay sequence in the playa surface being a region of gypsum source for
WSgd in the past. The clay remains moist, most if not all of the time,
and contains varied in size (fine to coarse grained) secondary gypsum
crystals originated from groundwater currently discharging on the
playa's surface. This example shows that depending on the type of
deposited sediments on the playa surface in the past, the source region
may be subjected to exhaustion of gypsum. In analogy to OUgd, it may be
proposed that lack of source area for gypsum implies the limited
resources of gypsum sediments in the north polar region of Mars.

[Groundwater discharge into the interdune areas]{.underline} -- was
possible to observe during the winter 2006/2007. Last time, this process
was observed in WSgd around 60 years ago. A monsoon season of 2006
abundant in precipitation caused a rapid discharge of groundwater and

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| **ONLY FIGURE "C" WILL BE INCLUDED**                                  |
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| Fig. 1. A) New gypsum sediments formed after groundwater discharge    |
| into interdune areas during winter 2006/2007. Field of traverse dunes |
| in WSNM. Photo taken by first author from altitude about 1,500 feet   |
| on October 2007. A dark color of interdune areas relsted from higher  |
| moisture content; B) Examples of dunes with associated bright patches |
| which may be gypsum \[1\].                                            |
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flooding of interdune areas that led to formation of shallow ponds.
After the water went down, a thin layer of new gypsum-rich sediments
were deposited on the surface of interdune areas (Fig. 1A). Mostly, new
sediments consist of fine-grained gypsum re-worked by waves and its
further cementation by loss of water due to evaporation. The OUgd
inclose bright patches of material in low areas between dune crests (Fig
1B) that in the way of occurance resemble the new deposits observed in
WSgd. The bright patches are relatively low in gypsum compared to the
dunes themselves but grain-size effect resulted from limitation of the
orbiter camera may cover up the real content of gypsum in the interdune
areas of OUgd \[1\].

[Dessication of dunes]{.underline} -- were manifested by cross-cutting
cracks under an angle of 90º after hot summer of 2007 (Fig. 2A). One
system of cracks propagated parallel to the cross-bedding originated
from dunes migration. The second system of cracks did not follow any
sedimentary structures. Some of OUgd have an eroded rather than fresh
appearance indicating inactivity and possible induration (Fig. 2B). A
close similiratity of the way of cracks propagation between WSgd and
OUgd implies a possible important loss of water by OUgd in the past.
More advanced incision of

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  Fig. 2. A) cross-cutting cracks formed at dune       
  slopes after summer. Photo taken by first author in  
  the WSNM area of traverse dunes on October 2007; B)  
  examples of eroded and possibly indurated dunes      
  within Olympia Undae \[1\].                          
  ---------------------------------------------------- ----------------------------------------------------

OUgd may have resulted from further eolian erosion rather than surface
or groundwater activity.

**Conclusions:** By analogy to the origin and evolution of WSgd, the
reported chemical composition and advanced physical weathering of OUgd
on Mars \[1\] may indicate complex processes taken place under few
different climate conditions (episodes). We selected at least two wet
episodes and two dry episodes that could have been involved during
evolution of OUgd:

1)  wet condition -- deposition of sulfate-rich sediments in the region
    of meltwater channels mostlikely from precipitation of evaporites
    from water originated from the Chasma Boreal melting events;

2)  dry condition -- formation of gypsum-rich dunes by eolian deflation
    of sediments deposited earlier in the region of melting channels;

3)  wet condition -- cementation of dunes by capillary fringe,
    groundwater discharge or/and another sources of surface water;

4)  dry condition - desiccation of dunes and formation of cross-cutting
    cracks; further erosion of dunes through existing cracks mostlikely
    by wind.

**Acknowledgments:** This study was supported by the NASA Astrobiology
Institute team award directed by L.M. Pratt. Sincerest thanks are due to
Parker Bradley and Kevin Dunshee from a Division of Heron Aerospace
Corp. in Alamogordo for organizing the flight and permission from the
White Sands Missile Range for flying over the dune field of White Sands;
to Esther Singer for help in preparation of lake sediments for sulfur
isotopic analysis.

**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 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*.