Chemical Alteration of Soils on Earth as a Function of Precipitation: Insights Into Weathering Processes Relevant to Mars

Details Chemical Alteration of Soils on Earth as a Function of Precipitation: Insights Into Weathering Processes Relevant to Mars Chemical Alteration of Soils on Earth as a Function of Precipitation: Insights Into Weathering Processes Relevant to Mars

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p23b..06a&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #P23B-06

Mathematics

Logic

1045 Low-Temperature Geochemistry, 0330 Geochemical Cycles, 0400 Biogeosciences

Scientific paper

Soils lie at the interface of the atmosphere and lithosphere, and the rates of chemical and physical processes that form them hinge on the availability of water. Here we quantify the effect of these processes on soil volume and mass in different rainfall regimes. We then use the results of this synthesis to compare with the growing chemical dataset for soils on Mars in order to identify moisture regimes on Earth that may provide crude analogues for past Martian weathering conditions. In this synthesis, the rates of elemental gains/losses, and corresponding volumetric changes, were compared for soils in nine soil chronosequences (sequences of soils of differing ages) - sequences formed in climates ranging from ~1 to ~4500 mm mean annual precipitation (MAP). Total elemental chemistry of soils and parent materials were determined via XRF, ICP-MS, and/or ICP-OES, and the absolute elemental gains or losses (and volume changes) were determined by normalizing data to an immobile index element. For the chronosequences examined, the initial stages of soil formation (103^ to 104^ yr), regardless of climate, generally show volumetric expansion due to (1) reduction in bulk density by biological/physical turbation, (2) addition of organic matter, (3) accumulation of water during clay mineral synthesis, and/or (4) accumulation of atmospheric salts and dust. Despite large differences in parent materials (basalt, sandstone, granitic alluvium), there was a systematic relationship between long-term (105^ to 106^ yr) volumetric change and rainfall, with an approximate cross-over point between net expansion (and accumulation of atmospheric solutes and dust) and net collapse (net losses of Si, Al, and alkaline earths and alkali metals) between approximately 20 and 100 mm MAP. Recently published geochemical data of soils at Gusev Crater (Gellert et al. 2004. Science 305:829), when normalized to Ti, show apparent net losses of Si and Al that range between 5 and 50% of values relative to adjacent rocks. However the chemical impact of globally distributed dust on Mars greatly affects the interpretation of these apparent elemental losses. From the available soil data, no Earth-based soil geochemical signature perfectly matches the reported Martian data, though arid soils in the Atacama Desert and elsewhere exhibit certain similarities (losses of Si, Al and gains of S). For both Earth and Mars, an understanding of the chemical signature of atmospherically derived elements is critical for calculating accurate measures of chemical weathering in soils.

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