Biogeochemical Weathering of Iron-Silicate Minerals

Open Access
Buss, Heather L.
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
July 06, 2006
Committee Members:
  • Susan Louise Brantley, Committee Chair
  • Maryann Victoria Bruns, Committee Member
  • Lee Kump, Committee Member
  • Christopher Howard House, Committee Member
  • spheroidal weathering
  • biotite
  • hornblende
  • iron cycling
  • siderophores
  • etch pits
In the Rio Icacos watershed of Puerto Rico, which has one of the fastest documented chemical weathering rates of granitic rock in the world, the quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick interface of partially weathered rock layers called rindlets. Plagioclase dissolution and biotite oxidation are the first observable weathering reactions in this system although it remains unknown which of these reactions occurs first. Dissolution of plagioclase produces porosity and may retard dissolution of other silicate minerals within the rindlet zone. Biotite weathering occurs in two stages: by oxidation within the rindlet zone and by alteration to kaolinite within the saprolite. Within a 0.07 m zone near the rindlet-saprolite interface, we document the fastest rate of hornblende dissolution in the field ever reported: 6.0 x 10-13 mol m-2 s-1. The release of Fe(II) during hornblende dissolution supports the growth of iron-oxidizing bacteria at depth in the Rio Icacos saprolite. Fixation of CO2 by these bacteria supports growth of heterotrophic bacteria. A conceptual model is presented to quantify the effect of Fe(II) release during bedrock weathering on the microbial population at depth in the saprolite. By consuming Fe(II) and O2, iron-oxidizing bacteria may affect spheroidal fracturing, which is likely driven by diffusion of O2 into the bedrock and oxidation of Fe-silicate minerals. Microbial populations also affect the mechanisms and rates of mineral weathering via the production of organic moieties that interact with mineral surfaces. Spectroscopic and microscopic techniques were utilized to investigate Fe-silicate glass surfaces after incubation with siderophores, extracellular polysaccharides (EPS), and/or under colonies of bacteria. Siderophores enhanced release of Fe, Al, and Si. Release of these elements was greatest in the presence of both EPS and siderophores, but negligible in the presence of EPS alone. Siderophore-exposed surfaces contained widespread, small etch pits and greater surface roughness than bacteria-exposed surfaces, which contained fewer, localized, larger etch pits. This is the first documented case in which mineral surface pitting by siderophores has been observed. Small acidic molecules released from the EPS may have shuttled Fe from the surfaces to the siderophores, enhancing general dissolution.