Saturday, 15 July 2006

Genesis of the Hyperarid Soils of the Atacama Desert: Analogue for Mars?.

Michael S. Howell1, Brenda Buck2, Jason A. Rech3, Amy Brock2, and Joel Prellwitz3. (1) Univ of Nevada, Las Vegas, Dept Geoscience, 4505 Maryland Parkway Box 4010, Las Vegas, NV 89154, (2) Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Parkway Box 4010, Las Vegas, NV 89154, (3) Miami Univ, 114 Shideler Hall, Oxford, OH 45056

The sulfate-salt-rich soils found in the Atacama Desert are arguably the oldest and driest soils found on Earth (Rech et al., 2003; Bao et al., 2004). Previous research indicates that hyperaridity has been sustained in this regionsince at least the mid-Miocene (Alpers and Brimhall, 1988). The combination of sulfate minerals and hyperaridity suggest these soils may be a possible analogue for surfaces on Mars. This study focuses on soil profiles from the Baquedano Nitrate District, approximately 80 km NE of Antofagasta, Chile. This area was heavily mined in the late 19th and early 20th century, and new mining operations have recently opened exploration pits throughout the valley. Since these soils are so heavily cemented, profiles were chosen from mined outcrops and exploration pits. Soil profiles on two west facing medial alluvial fans and one east sloping fluvial stream terrace were described according to standard USDA techniques outlined in Schoenberger et al. 2002 and sampled using a diamond-blade saw to obtain fresh samples. Surface vesicular A horizons containing soluble salts (Avz) range from 1 to 8 cm thick, are pink (7.5YR 7/3) or light brown (7.5YR 6/3) and are slightly to moderately effervescent. The underlying Byz horizon (8-17 cm thick) is slightly to noneffervescent and contains illuvial gypsum and other soluble salts as Stage I snowballs, Stage II nodules, and incipient Stage III cementation in the center of coarse columnar peds (e.g. Buck and Van Hoesen, 2002). Color of this horizon is highly variable depending upon salt concentration, but varies from white (2.5YR 8/1) to brown (7.5YR 5/4). An underlying Bz horizon (4-7 cm thick) characterized by loose unconsolidated white (2.5YR 8/1), noneffervescent, crystalline thenardite (stage III) occurs on both alluvial fan sites but is not present on the fluvial terrace site. In contrast, the pinkish gray (7.5YR 7/2) fluvial terrace Byz2 horizon is 80 cm thick and contains similar characteristics as the overlying Byz1 horizon. At all three sites, an abrupt, wavy boundary at the base of the Bz or Byz2 horizon separates the underlying massive, soluble salt-cemented stage III Bzm horizons. These horizons vary slightly between sites but are characterized by poorly sorted coarse sand and gravel clasts suspended in a matrix of strongly cemented illuvial (stage III) soluble salts. Patterned ground with vertical cracks extending into the soils was observed and described at all three sites. Both small (<10 cm diameter) and large (2 m diameter) polygonal patterned surface features, defined by either small (1 to 5 cm width) or large (10 to 20 cm width) vertical cracks filled with silt, sand, and clasts (< 10 cm length), are present. All vertical cracks narrow with depth and the majority of the narrower cracks terminate at a depth of 15 to 25 cm. Larger vertical cracks, however, propagate well into Bzm horizons. The deepest vertical crack observed extended to the limits of the profile exposure at a depth of 318 cm. Previous research described these features as sand dikes and desiccation polygons (Ericksen, 1981). Our observations indicate that these features are not the result of desiccation processes alone, but are formed via a complicated set of processes termed salt heave. Salt heave includes (1) precipitation & dissolution; (2) hydration & dehydration; and (3) differential thermal expansion & contraction of salt minerals. In the Atacama, these processes occurring over extended lengths of time have resulted in a complicated distribution of patterned ground with vertical fissures extending deep into the underlying soil. The fissures exhibit a topographic low at the surface that concentrates runoff from rare precipitation events flushing the most soluble salts to greater depths. Eolian dust and surface clasts are also concentrated along these fissures creating the surficial expression of the patterned ground. These processes create soils with highly heterogeneous characteristics at different scales across these geomorphic surfaces. Ongoing research includes determining the salt mineralogy of these soils, micromorphological relationships within fissures, and obtaining cosmogenic dates of the geomorphic surfaces adjacent to study sites. These results, along with the similarities with patterned ground photos and sulphate minerals on Mars (Vaniman et al., 2004), may provide the best known analogue to pedogenic processes in Martian soils on Earth.

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