HR: 08:30h
AN: B11B-03    [PDF]
TI: Natural Variations of $\delta$$^{30}$Si Values During 4 Million Years of Progressive Basalt Weathering, Hawaii
AU: * Ziegler, K
EM: kziegler@geog.ucsb.edu
AF: Dept. of Geography, University of California, Santa Barbara, CA 93106 United States
AU: Chadwick, O A
AF: Dept. of Geography, University of California, Santa Barbara, CA 93106 United States
AU: Brzezinski, M A
AF: Dept. of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106 United States
AU: Kelly, E F
AF: Dept. of Soil and Crop Sciences, Colorado State University, Ft. Collins, CO 80523 United States
AB: Terrestrial silicate weathering rates and processes are important for studies of modern and paleo-climate. Recently, Si isotope geochemistry has shed light on Si cycling in marine systems, which is directly linked to the terrestrial Si cycle as nearly 80% of the Si is supplied by continental run-off water. Understanding the terrestrial Si-isotope geochemistry is crucial for a more comprehensive understanding of the global Si cycle. Weathering of primary minerals in the pedosphere releases Si that can be incorporated into secondary minerals or leached into rivers and oceans. These are active Earth surface processes that are dependent on local climate and regional tectonics. If we understand how these processes impact soil formation we can interpret present soil properties in the context of past controls on weathering systems. Few direct tracers of weathering status exist; we most commonly use Sr isotopes to track silicate weathering, but it would be far more useful to use Si isotopes. Here we present the results of the first systematic effort to understand the variations of natural abundances of Si isotopes along a soil development gradient. We find that Si isotopes in soils are a promising indicator of weathering status in terrestrial systems and the rivers that drain them. Isotopic data from rock, secondary soil minerals and soil water from 5 soil pits along a 4.1 Ma basaltic weathering chronosequence on Hawaii demonstrate large and systematic variations of $\delta$$^{30}$Si values. Unweathered basalt has a $\delta$$^{30}$Si value of -0.5$\permil$. Initial weathering leaches most of the Si from the top meter of soil, and converts the remaining Si into amorphous soil clay minerals. Bulk soil from $>$30 cm depth has $\delta$$^{30}$Si values 0.5$\permil$ more negative than fresh basalt. Soil water simultaneously evolves toward more positive $\delta$$^{30}$Si values, leading to a Si-isotopic difference between solid and aqueous Si of 2.1$\permil$. As weathering progresses, primary minerals are exhausted, and secondary amorphous minerals are transformed into crystalline phases. This second stage of mineral transformation produces even more negative $\delta$$^{30}$Si$_{bulk soil}$ signatures. Repeated transformation cycles cause the Si-isotopic ratios of both soil minerals and soil water to move in parallel towards more negative values. The oldest soil has a $\delta$$^{30}$Si composition 2.1$\permil$ more negative than that of fresh basalt, but still displays a 2.3$\permil$ solid-aqueous difference. The top 30 cm of older soils contain atmospheric dust that masks Si-isotopic weathering signatures. $\delta$$^{30}$Si values of these soils of around -0.5$\permil$ are more positive than those from deeper horizons. The top 0-5 cm of the soils show slightly lower $\delta$$^{30}$Si$_{bulk soil}$ values due to the presence of biogenic opal (phytoliths). Our data show that weathering and clay mineral and phytolith formation creates large natural Si-isotope variations, and confirm that secondary phases are a sink for negative Si isotopes. They also suggest that water from young, tectonically active areas will be of relatively high $\delta$$^{30}$Si values, whereas water from old, stable continents will have relatively low values. Such a distribution is substantiated by the few, globally distributed river data.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 4805 Biogeochemical cycles (1615)
DE: 4885 Weathering
SC: Biogeosciences [B]
MN: 2003 Fall Meeting