Terrestrial Silicon Isotope Geochemistry

Introduction

When released by weathering, Si is soluble in water but also takes part in a number of inorganic and biologically mediated reactions that lead to secondary precipitates. Knowledge of the Si-isotopic fractionations that take place as Si moves from primary minerals to secondary minerals, biotic products and river water will provide greater specificity to our understanding of the global Si-cycle. On continents, a limited set of measurements suggest that Si-isotopes will help in quantifying the contribution of terrestrial ecosystems to the global biogeochemical Si-cycle. This is of immense importance as we do not understand why dissolved Si in rivers and oceans has such a fundamentally different d³⁰Si composition (generally positive) than igneous and metamorphic rocks (generally negative). In order to produce a ‘positive ocean’ from ‘negative continents’, a major fractionation must take place during the transformation from crystalline Si in rocks to dissolved riverine and marine Si, which also must lead to a residual product with even more negative d³⁰Si values than the parent rocks.

Goal

The main goal of our research is to characterize Si-isotopic fractionation during the partitioning of primary mineral Si into soil clay minerals, phytoliths, and leaching water. In order to monitor and determine the magnitude and direction of possible Si-isotopic fractionations, we synthesized clay minerals and phytoliths under controlled conditions. Allophane, a ubiquitous non-crystalline soil mineral, was synthesized, and 54% of the initial Si was incorporated into the solid. Opaline phytoliths were synthesized by growing corn and wheat plants in growthchambers from hydroponic solutions providing a constant supply of Si.

Experimental Results

The experimental results demonstrate that the mechanism involved in clay mineral and opal formation do fractionate Si-isotopes. Furthermore, the fractionation is in the direction required for explaining the observed isotopic difference between Si fixed in rocks and dissolved Si in waters. They also allow us to hypothesize that in terrestrial weathering systems, we should see opposite d³⁰Si-trends of soil minerals (negative) and soil waters (positive). Those trends should become more pronounced with increasing weathering intensity and soil age. We are testing this hypothesis by sampling minerals, opal phytoliths and stream water from different age landscapes, and hence different weathering intensities, from the Hawaiian Islands.

For more information on this project please contact Karen Ziegler: kziegler@geog.ucsb.edu

Back to Projects