H52B-01
Distinguishing Between Lithogenic and Biogenic Processes in a Soil Chronosequence
It is increasingly recognized that many geochemical processes observed in soils result from a combination of chemical weathering and biological cycling. This relationship is particularly important in understanding the distribution of inorganic macronutrients which are products of chemical weathering and are used extensively in the biosphere. Approaches to differentiating between these processes are being investigated in a soil chronosequence (65 to 260 kyrs) developed on silicic sediments contained on marine terraces near Santa Cruz California. An example of what we term the lithogenic/biogenic fractionation occurs soil pore waters in which specific cation ratios such as Ca/Na, Mg/Na and Sr/Na are spatially delineated and are dependent on the extent of mineral reactions and the degree plant uptake. These fractionation are further discussed in terms of other major elements as well as Ge/Si ratios and the isotopes of Ca, Sr and Si. Results of this study indicate that important macronutrients, including K and Ca, are tightly cycled to considerable depths in the profiles which weathering-dominated elements such as Na and Si produce net exports.
H52B-02
Microbial community evolution across a granitic chronosequence, Santa Cruz, California
Ongoing research at a marine terrace chronosequence in Santa Cruz, CA includes quantification of biogeochemical and physical processes to better understand the evolution of soil development and the soil microbial community. The chronosequence, formed on granite-derived marine sediments, is located in a Mediterranean climate zone and is dominated by grassland vegetation. Two features of the chronosequence relevant to the soil microbial community are the progressive depletion of primary minerals--containing nutrients such as Ca, K, and P--with soil age, and the increase of the aboveground plant biomass from terrace 1 (65 ka) to terrace 2 (90 ka) with a subsequent decrease for terraces 3 (137 ka) and 5 (226 ka). This study investigated soil microbial community size and composition in both the surface soils (0-0.07 m) and in subsurface depth profiles (down to at least 2 meters) across the chronosequence. Trends in the surface concentrations of carbon and microbial biomass carbon (MBC) were similar to the aboveground plant biomass with an increase from terrace 1 to terrace 2 and then a slight decrease in terraces 3 and 5. Soil microbial community composition trends, measured by phospholipid fatty acids (PLFA), with soil age were somewhat different. Concentrations of the 18:29,12 lipid, a lipid derived from fungi as well as fine plant roots, declined with soil age. As expected from previously published studies of soil microbial communities with depth, concentrations of carbon, MBC, and PLFAs declined with increasing depth in the subsurface. Preliminary PLFA data indicated two main microbial community changes with depth. First, the 18:29,12 concentration decreased as a fraction of the total PLFA concentration, and second, branched chain lipids commonly associated with gram-positive bacteria increased as a fraction of the total PFLA concentration. The variation of soil carbon and the soil microbial community as a function of depth appeared to change with soil age. Shallow subsurface (0.2-1 m) carbon, MBC, and PLFA concentrations declined more rapidly relative to surface concentrations in the older terraces (3 and 5) than in terrace 2. Interestingly, the decline in the 18:29,12 lipid fraction was more pronounced in the younger terrace 2 soil than in the older terrace 3 and 5 soils. Results from this first study of soil microbiology across a long-term (declining ecosystem) grassland chronosequence differed from similar studies on forested long-term chronosequences. In particular, the 18:29,12 lipid declined as a fraction of the total PLFA pool in the surface soil with soil age, rather than increasing as was the case for the forested chronosequences. Additionally, this was the first study of subsurface soil microbiology across a long-term chronosequence. PLFA results indicated that variations in the soil microbial community with age were more significant in the subsurface than at the surface. These changes in the soil microbial community were likely connected to changes in the plant biomass. Plant biomass changes were, in turn, probably driven by the increased depletion of primary minerals and nutrients such as Ca or P with soil age across the chronosequence. At the Santa Cruz chronosequence, changes in soil chemistry due to chemical weathering appear to be driving changes in the ecology of the soil microbial community.
H52B-03
Application of a quantitative pedogenic energy model to predict critical zone structure and function
Energy-based pedogenic models present a framework for quantitatively linking pedogenesis and mineral weathering to soil system energy through-flow. This study presents refinement and application of the Quantitative Pedogenic Energy Model (QPEM) at global, watershed, and pedon spatial scales. The QPEM model framework is based an open-system principles and uses rates of effective energy and mass transfer (EEMT; kJ m-2 yr-1) to predict soil development and pedogenic environments. Study objectives included: (i) derivation of a global equation for estimating EEMT; (ii) testing the QPEM framework at the various spatial scales using a combination of global soil datasets and site specific pedon data; (iii) incorporation of EEMT into a soil mass balance model of silicate weathering; and (iv) development of quantitative transfer functions between pedogenic indices and EEMT. We derived a 2-D Gaussian expression for estimating EEMT from mean annual temperature (MAT) and mean annual precipitation (MAP) (R2=0.96) using a global climate dataset. Global EEMT patterns demonstrated distinct latitudinal variation in total EEMT and the percent of EEMT derived from biologic and climatic sources. Pedon data demonstrated significant linear and non-linear functions between EEMT and a variety of pedogenic indices including pedon depth, clay content, subsurface chemical index of alteration, and the ratio of free Fe- oxides to total Fe. Watershed scale stream Si-flux data demonstrated a significant linear relationship to EEMT (r2=0.82), whereas depletion of soil Si relative to bedrock demonstrated a significant exponential rise to maximum function (r2=0.76) with increasing EEMT. At the watershed scale, modeled pedon depth and landscape denudation data indicated a feedback between EEMT, denudation and rates of silicate weathering. Furthermore, at all of the observed scales, significant differences in EEMT were observed amongst soil classes according to U.S. Soil Taxonomy. The presented data analysis indicates the potential for using rates of effective energy and mass transfer in an energy-balance approach to characterize critical zone structure and function.
H52B-05
Chemical Weathering Rate and Climate: Measurements in the Idaho Batholith
Many biogeochemical and Earth surface processes depend critically on chemical weathering. Chemical weathering supplies nutrients to soils, accelerates physical erosion by weakening bedrock, and buffers Earth's climate over geologic time by modulating atmospheric carbon dioxide concentrations. Measurements of chemical weathering rates (and of their dependence on external factors such as climate) are thus important for understanding the biogeochemical evolution of the soil-bedrock system and the geomorphological evolution of the Earth's surface. Chemical weathering fluxes can be measured in mountainous terrain using cosmogenic radionuclide measurements of long-term denudation rates, coupled with measurements of the rock-to-soil enrichment of chemically inert tracers. Here we use this method to measure chemical weathering fluxes at a series of sites along a 1500-meter altitudinal transect on Pilot Peak in the granitic Idaho Batholith. Concentrations of cosmogenic 10Be in soil-borne quartz indicate that total denudation rates range from 61±5 to 201±17 t km-2 yr-1 among the sites on this elevation transect. In rock and soil samples from the same sites, concentrations of zirconium --- an element we assume is chemically immobile --- are slightly enriched in soil relative to parent rock, implying that chemical weathering fluxes account for 0-20% of the total denudation rate. When combined, these 10Be and zirconium data imply that chemical weathering fluxes at these sites range from -5±8 to 30±4 t km-2 yr-1, and are fastest at the highest elevations where it is coldest -- contrary to the expectation that chemical weathering rates should decrease as temperature decreases. This suggests that variations in factors other than temperature are the dominant controls on chemical weathering rates at Pilot Peak.
H52B-06
Processes Influencing Regolith Development on Bodmin Moor, UK
Regolith is disaggregated, transportable material above bedrock created by physical processes such as fracture generation, frost cracking, tree throw, and animal burrowing and chemical processes such as dissolution and mineral transformation. Models typically parameterize the rate of regolith generation as a function of its thickness, without specifying the physical and chemical processes involved. This study examines a non-glaciated, granitic hillslope on Bodmin Moor, southwest England, UK, to identify processes creating regolith. The site has a temperate climate with relatively high mean annual temperature (MAT,~10°C) and rainfall (1381- 2580mm). We dug five soil pits ~80cm deep along a 120 m transect to examine the saprolite-regolith interface and extract solid phase and soil pore water samples. We find no evidence of physical processes such as frost cracking or bioturbation creating regolith. The site is treeless with vegetation dominated by grass. Although the site experiences ground frost and has sufficient water content to support frost cracking, a model of freezing front penetration indicates that the saprolite-regolith boundary does not fall within the frost cracking window of -3 to -10°C. A decrease in MAT of ~3°C would lower the depth of frost penetration to the current saprolite-regolith boundary (80cm). We speculate that frost cracking may have operated during the periglacial conditions of the Quaternary and would have pre-conditioned the bedrock for chemical processes currently operating. We use soil pore water and solid phase chemistry to determine the chemical processes at work. Pore water concentrations of K (4.5 mg/L), Na (14.2 mg/L), SO42- (24.9 mg/L), Mg (1.7 mg/L), and Ca (1.7 mg/L) are highest in O-horizons and decrease as a function of distance from the surface, indicating that biological processes have a strong effect on geochemical cycling in the upper part of the soil. Soil water at greater depths within the regolith are likely under-saturated and able to transform saprolite into regolith via dissolution. A mass balance approach is used on the solid phase chemical data to quantify mass gains and losses. We calculate τ, the percent mass change of an element relative to parent material, using Zr as an immobile element. Na shows high mass losses (-0.84) regardless of landscape position. The τ values for K, Al, Mg, and Mn are dependent on landscape position with higher mass losses upslope (K -0.78, Al -0.67, Mg -0.35, and Mn -0.56) and lower mass losses or mass gains downslope (K -0.18, Al 0.24, Mg 0.43, and Mg 0.70). Quantitative mineralogy shows that mineral transformations occur across the regolith-saprolite boundary. Feldspar percentages decrease as a function of distance above the boundary while clay percentages increase. Chemical processes currently dominate the transformation of saprolite into disaggregated and transportable regolith at Bodmin Moor. We speculate that loss of feldspar and formation of clay weakens the material enough to render it susceptible to transport by soil creep. We cannot presently eliminate the possibility, however, that frost cracking was important in the evolution of the regolith in the Quaternary.
H52B-07 INVITED
Interplay between physical movements of soils and mineral grains and chemical weathering
Most soil biogeochemistry studies treat the soils and their inorganic and organic constituents as physically immobile. Those soil materials, however, are in perpetual motion due to the conversion of bedrock to soils, colluvial transport, and vertical mixing by various biophysical perturbations of the soils. Subsequently, a soil is continuously replaced by the materials from the neighboring soils and the underlying parent material, while its individual horizons are gradually mixed with the materials in the neighboring horizons. The movements of bulk soil materials are ultimately driven by moving individual mineral grains. While rarely appreciated, these physical movements of soil's mineral components operate in the presence of strong vertical and topographic gradients of the rates of mineral dissolution and leaching. The result is that the physical movement of soil constituents affects chemical weathering. The fluxes of soil materials (via physical movements and solute fluxes) in and out of a soil system defined by a researcher determine the time length that the materials reside in the system. The residence time, together with the system-specific rates of chemical weathering, determine the degree of weathering of the materials within the system. This presentation provides a new mathematical framework to consistently quantify the residence times of minerals, individual soil horizons, soil profiles, and an entire soil within a watershed boundary. Soil age, which is equivalent of the time length since the cessation of erosion or deposition on level grounds, becomes a special case of the residence time. The model is combined with empirical data to quantitatively illustrate the impacts that the physical motion of soil constituents have on the rates of chemical weathering. The data are drawn from ongoing field and laboratory studies focusing on the impact of river incision, colluvial flux, bioturbation, and agricultural tillage on the vertical and lateral variation of elemental composition within the soils.