Geotimes
Highlights
Geochemistry
Neil C. Sturchio

Sometimes geochemists can learn something new from looking closely at something old, or something big from looking at something small. New tools for measuring geochemical compositions and structures of geological materials on microscopic scales are yielding exciting advances in our understanding of the Earth.

Good examples are petrographic, geochemical, and isotopic studies of detrital zircons in metamorphosed sediments from the Yilgarn Craton of Western Australia (Wilde and others, Nature, v. 409, p. 175-178; Mojszis and others, Nature, v. 409, p. 178-181). These studies revealed not only the oldest ages of any known minerals yet found in terrestrial rocks -- 4.3 to 4.4 billion years old -- but also evidence for widespread liquid water at the Earth's surface.

Another example is the insight gained into Phanerozoic seawater chemistry from examination of the compositions of individual primary fluid inclusions from marine halite deposits (Lowenstein and others, Science, 294, 1084-1086). The observed variations in fluid inclusion compositions with time are correlated with seafloor spreading rates, volcanic events, global sea level, and the mineralogies of marine limestones and evaporites.

Scientific exploration of the sea floor led to the discovery of a new type of submarine hydrothermal system, 15 kilometers off the axis of the Mid-Atlantic Ridge at 30 degrees north latitude near its intersection with the Atlantis Fracture Zone (Kelley et al., Nature, v. 412, p.145). This field of hydrothermal vents, dubbed the "Lost City," is on 1.5 milion-year-old crust. In discharging alkaline vent fluids at relatively low temperatures of 40 to 75 degrees Celsius that form carbonate chimneys up to 60 meters tall, its chemistry is distinct from all other known submarine hydrothermal systems. The chimney pictured here stretches as high as 30 feet. The white, sinuous spine is freshly deposited carbonate material. Photo courtesy of the University of Washington/Woods Hole Oceanographic Institution

Speaking of seawater, scientific exploration of the sea floor led to the discovery of a new type of submarine hydrothermal system, 15 kilometers off the axis of the Mid-Atlantic Ridge at 30 degrees north latitude near its intersection with the Atlantis Fracture Zone (Kelley and others, Nature, 412, 145-149). This field of hydrothermal vents, dubbed the "Lost City", is on 1.5 million year old crust. . In discharging alkaline vent fluids at relatively low temperatures of 40 to 75 degrees Celsius that form carbonate chimneys up to 60 meters tall, its chemistry is distinct from all other known submarine hydrothermal systems. Researchers think its characteristic fluid chemistry and heat content result from exothermic serpentinization reactions, whereby the mineral olivine reacts with seawater to form the mineral serpentine. The vent system supports dense microbial communities including anaerobic thermophiles.

Not only do these seafloor hydrothermal systems support dense microbial communities, so do mineral surfaces in laboratory beakers. As we noted in our geochemistry highlights review last year (Geotimes, July 2001), synchrotron X-ray methods are making a big difference in our understanding of small things, and biofilms on mineral surfaces are no exception. In a study comparing lead adsorption on polished alumina substrates in the presence and absence of B. cepacia biofilms, a group at the Stanford Synchrotron Radiation Laboratory used the long-period X-ray standing wave method. They found that the biofilm did not reduce the chemical activity of the alumina substrate to lead adsorption. The biofilm adsorbed significant amount of lead at solution concentrations exceeding one micromolar (Templeton and others, Proc. Natl. Acad. Sci., v. 98, p. 11897-11902). This indicates that although the presence of biofilms at mineral surfaces may alter the geochemical behavior of the surfaces, it does not altogether mask their intrinsic behavior as commonly speculated.

Another fundamental study, by a group using synchrotron X-ray reflectivity methods at the Advanced Photon Source, discovered static water density oscillations at the interface between muscovite and water and attributed these to the presence of two types of adsorbed water: one, possibly hydronium, substituting for the potassium ion in the ditrigonal sites; and the other forming a hydrogen-bonded network extending to at least three water molecules out from the basal oxygen plane of the tetrahedral sheet at the muscovite surface (Cheng and others, Phys. Rev. Letters, v. 87, p. 156103-1-156103-4).

Adsorption of ions and molecules (especially organic molecules) at mineral surfaces continues to be investigated intensively using new tools because microscopic, or "nanoscopic", processes at mineral-fluid interfaces are believed to control many macroscopic geochemical phenomena. For example, a study of organic carbon in black shale from the Cretaceous Western Interior Seaway of North America showed that 85% of the organic matter is correlated with mineral surface area, suggesting that adsorption of organic compounds to clay mineral surfaces controls the preservation and burial of organic carbon (Kennedy and others, Science, v. 295, p. 657-660). Macromolecular organic matter in black shales provides sustenance for microorganisms cultured from weathering profiles, as revealed by radiocarbon analysis of their membrane lipids (Petsch and others, Science, v. 292, p. 1127-1131). Another interesting discovery about natural organic matter is the widespread formation of semivolatile chlorinated organic compounds from inorganic chloride in soils during humification of plant material that was revealed by X-ray absorption spectroscopy (Myneni, Science, v. 295, p. 1039-1041).

And finally, I'll touch on a few highlights of the noteworthy progress being made in the realm of stable and radiogenic isotope geochemistry. Alan Matthews and others, as well as Clark Johnson and others, measured isotope fractionation between ferrous and ferric iron species in aqueous solutions, with both groups finding heavy isotope enrichments in ferric iron exceeding one per mil per mass unit (Mathews and others, Earth Planet Sci. Letters, v. 192, 81-92; Johnson and others, Earth Planet. Sci. Letters, v. 195, p. 141-153), consistent with theoretical predictions (Schauble and others, Geochim. Cosmochim. Acta, v. 65, p. 2487-2497). These data will facilitate interpretation of iron isotope data for natural systems (e. g., Sharma and others, Earth Planet. Sci. Letters, v. 194, p. 39-51). A kinetic isotope effect of 3.4 per mil accompanying microbial reduction of dissolved hexavalent chromium to trivalent chromium was measured in laboratory culture experiments and natural waters, an important finding for applications in the environmental geochemistry of toxic heavy metals (Ellis and others, Science, v. 295, p. 2060-2062). Jane Barling and others first measurements of the isotopic composition in natural molybdenum from marine sediments and seawater were reported to be on the order of several per mil (Earth Planet. Sci. Letters, v. 193, p. 447-457). A revised decay constant for lutetium-176 may have significant implications for studies of early Earth history (Scherer and others, Science, v. 293, p. 683-687).

Overall, geochemical research in the twenty-first century has gotten off to a running start. Interdisciplinary collaborations and advances in measurement techniques are opening rich opportunities for new discoveries and new insights into old problems.

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Sturchio is a professor in the Department of Earth and Environmental Sciences at the University of Illinois at Chicago, and is editor of the Geochemical News. E-mail.

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