Records of oceanic germanium:silicon (Ge:Si) preserved in diatoms show temporal variations that reflect changes in the marine Si cycle over time. These variations may be due to (1) the relative rates of Ge removal in opal and non-opal phases, (2) the ratio of weathering and/or hydrothermal Ge inputs, and (3) the internal cycling of Si in the ocean and Ge/Si fractionation by diatoms. Germanium isotopes may help constrain the influences of these factors. A scientist for the University of Southern California plans to analyze seawater from the Atlantic and Pacific Ocean, river water from selected watersheds, and sediment pore waters from iron-rich reducing sediments in the California Margin and Gulf of Mexico to address the following questions: (1) Does Ge isotope variability exist within and between ocean basins?; (2) Is there Ge isotope fractionation during low-temperature weathering?; and (3) Are Ge isotopes fractionated during sediment diagenesis? Overall, this study will determine if and where significant isotopic differences exist in the sources and sinks for inorganic Ge in the oceans and should document the utility of this tracer for unraveling the complexities of the marine Si cycle and its temporal variation. In addition, the researcher plans to analyze the isotopic composition of methyl germanium in the oceans to evaluate if its isotopic composition provides clues about its biogeochemical cycling.
As regards broader impacts, one graduate and one undergraduate student would be trained and supported as part of this project.
Motivation for Study Germanium (Ge) is an element with a behavior that tracks that of silicon (Si) in the ocean water column, but its sources and sinks have some behaviors that are distinct from those of Si. These differences have acted to cause the Ge/Si ratio in the ocean to vary in the past, but it is difficult to decipher whether variations in sources or sinks of these elements have played the major role in regulating this ratio. The variations are related to glacial/interglacial cycles, and one implication of the past variations is that they may reflect evidence that global weathering rates depend on climate in an unexpected manner. Ultimately, the weathering of silicate minerals (the major source of Si) acts to regulate the concentration of atmospheric CO2, so assessing the variation of past weathering rates is important for understanding the global carbon cycle. Isotopic Composition of Germanium Added to the Ocean Ge has 4 stable isotopes whose relative abundance can be measured with a multi-collector mass spectrometer. If natural processes act to affect the relative abundance of these isotopes in the sources or sinks of Ge, the isotopic ratio of Ge in seawater may prove to be a useful tracer to evaluate the present and past magnitude of sources and sinks of Ge, and its cousin Si. The two major sources of Ge are river and hydrothermal inputs. As part of this project, we have developed methods to measure the isotopic composition of the small concentrations of Ge that are found in river water and seawater. We have analyzed water from several rivers that drain regions dominated by different types of rock. Results indicate that rivers have some variability, but are enriched in the heavy isotope, relative to the germanium in the bulk silicate earth. This enrichment is evidence that isotope fractionation occurs during weathering and this enrichment is transferred to seawater. Previous observations indicate that Ge in hydrothermal waters is relatively depleted in the heavy isotope (Siebert et al, 2011) compared to rivers. We have analyzed seawater from the Atlantic, the Pacific and the Gulf of Mexico, and our results show that seawater is well mixed, with an isotopic composition that lies between these two primary Ge sources. Others have shown that little isotope fractionation occurs during diatom uptake of Ge (Mantoura et al., 2005, Mantoura, 2006), so that the isotopic composition of Ge preserved in buried diatoms should offer a record of the past composition of the oceans. Isotopic Composition of Germanium Removed from the Ocean to the Sediments The overall balance for the isotopic composition of seawater depends on the extent of fractionation caused byboth inputs and removal processes. While the burial of diatom remains does not seem to fractionate the isotopes from seawater values, about half the Ge is removed from the ocean by incorporation into non-opal phases, probably iron-rich clays. In a study to evaluate Ge cycling in the Gulf of Mexico, we measured one sample of the pore water of the coastal sediments that indicated little fractionation seems to occur during Ge burial. We also made several analyses of Ge that is remineralized in sediments, and escapes removal by both the opal and non-opal phases, but these suggest that the heavy isotope is preferentially removed in the non-opal sink. These conflicting results complicate the story, but may reflect spatial variability in the strength of the non-opal sink. Further measurements are planned to define the role of sedimentary processes on Ge isotopes. Ge Cycling in the Coastal Gulf of Mexico As part of our work in the Gulf of Mexico, we constructed a germanium budget for the coastal waters near the region influenced by the Mississippi river plume. Results showed that as diatoms grow, they discriminate against Ge, relative to Si. Sediments in this region also have the right chemistry to sequester Ge into a non-opal phase, likely an iron-rich clay. Broader Impacts Discoveries made during this project indicate that the isotopes of germanium offer insights into the cycling of Ge and Si, although the story is not yet fully revealed. Analyses of Ge isotopes in buried opal may offer some insight into past variations, or lack of past variations, in silicate weathering, an important aspect in regulating the carbon cycle. During the life of this project, two graduate students and one undergraduate student received training in methods of chemical analysis of silicon, and the isotopic analysis of germanium. They also received training in data compilation and interpretation. References Mantoura, S., A. Galy, C.L. DeLaRocha, H. Elderfield, and A. Shemesh (2005), A Germanium Isotope Record From Diatom Opal, Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abstract PP33D-07. Mantoura, S.C. (2006) Development and Application of Opal Based Paleoceanographic Proxies, Unpublished Ph.D thesis, 218pp., University of Cambridge, England.
