Germanium and silicon isotope geochemistry in terrestrial and marine low-temperature environments

• Main Author: Baronas, Jokubas Jotautas
• Format: Dataset
• Language: English
• Published: University of Southern California Digital Library (USC.DL) 2017
• Subjects: Geological Sciences; germanium; silicon; isotope geochemistry; seawater; rivers; soils; global budget; weathering; marine sediment diagenesis; Greenland
• Online Access: https://dx.doi.org/10.25549/usctheses-c40-403765; https://digitallibrary.usc.edu/asset-management/2A3BF166SGA3

The conditions on the surface of planet Earth, including climate and its ability to support life, are controlled by a complex interaction of physical and chemical processes, taking place over a range of spatial and temporal scales. Due to the ubiquity of silicon (Si) in the solid Earth, a number of these processes involve the transformation and translocation of silica-containing compounds (i.e., the global Si cycle). The rates of the various chemical reactions and mass fluxes within the Si cycle are often difficult to assess directly, especially in the geological past. Therefore, geochemical tracers can help evaluate their importance in controlling the evolution of Earth's climate, landscape, and life. ❧ In this PhD dissertation, I investigate the use of germanium (Ge) and Si isotope composition (δ⁷⁴Ge and δ³⁰Si, respectively) in natural fluids and solids to trace various Earth surface processes, including silicate rock weathering and marine sediment authigenesis. Ge, like Si, is primarily supplied by the weathering of silicate rocks on continents, and consumed during biogenic silica production in the oceans, making it a reliable tracer of the Si cycle. In particular, different chapters of this dissertation are focused on establishing Ge isotope systematics in various different Earth surface environments, as there is very little prior data available. A second objective is to demonstrate how the combined use of δ⁷⁴Ge, δ³⁰Si, and Ge/Si signatures can yield insights otherwise unobtainable if either proxy was applied in isolation. ❧ Ge and Si are primarily supplied to the ocean via rivers and hydrothermal fluids, and removed via burial of biogenic silica and authigenic phases that precipitate within marine sediments. In Chapter 2, I presented the first δ⁷⁴Ge data from a number of analytically challenging low temperature fluid samples, including seawater and river water. The dissolved δ⁷⁴Ge composition of rivers and seawater was significantly heavier (2.0-5.6 ‰ and 3.2±0.4 ‰, respectively) than the source silicate rocks (0.4-0.8 ‰), indicating significant Ge isotope fractionation during low temperature processes. High temperature ridge-axis hydrothermal fluids exhibited δ⁷⁴Ge of 0.7-1.6 ‰ and are likely controlled by isotopic equilibration with hydrothermal quartz. Low temperature ridge-flank hydrothermal fluids had δ⁷⁴Ge of 2.9-4.1 ‰, consistent with isotopic fractionation during Ge adsorption or co-precipitation with Fe oxyhydroxides. A steady state isotopic mass budget for the ocean was used to predict that Ge sequestration during sediment authigenesis (i.e. the non-opal Ge sink) must involve a Δ⁷⁴Ge_{authigenic-seawater} fractionation factor of -0.6±1.8 ‰. ❧ In Chapter 3 I sought to directly observe and quantify the δ⁷⁴Ge fractionation associated with marine sediment authigenesis, using Ge/Si and δ³⁰Si data to provide additional constraints. The same set of sedimentary processes appeared to control dissolved Ge dynamics in all three studied continental margin sites (San Pedro Basin, Santa Monica Basin, and Gulf of Mexico continental shelf). First, biogenic Si (bSi) dissolution supplies dissolved Ge to the pore waters with δ⁷⁴Ge = ~3 ‰, identical to seawater composition. Second, reductive dissolution of Fe oxides (FeOx) in the subsurface sediments results in pore water δ⁷⁴Ge as low as 1.3-1.9 ‰, coinciding with a Ge/Si maximum of up to 3 µmol/mol. It is unclear whether the Fe oxides are of lithogenic or authigenic origin (i.e., supplied as settling detrital particles or previously formed in-situ. However, variations in the benthic dissolved Ge flux and its isotopic composition suggest sensitivity to redox conditions, indicating −1.2 ‰ difference in δ⁷⁴Ge between FeOx and bSi. Third, δ³⁰Si fractionation indicates that authigenic aluminosilicates precipitate throughout the sediments. The latter process results in significant dissolved Ge draw down (Ge/Si ratio decreases to 0.3 µmol/mol) without any significant Ge isotope fractionation (pore water δ⁷⁴Ge ~2 ‰). Overall, authigenically buried Ge is ~1 ‰ lighter than seawater, close to the value calculated in Chapter 2. ❧ Chapters 4-6 focus on Ge and Si isotope systematics during continental weathering. In Chapter 4, a detailed study of soils, streams, and ground water in the tropical, supply-limited weathering environment of La Selva, Costa Rica was used to elucidate the competing controls of secondary mineral precipitation and vegetation uptake on δ⁷⁴Ge, δ³⁰Si, and Ge/Si signatures. The soils of La Selva lowlands are strongly weathered and composed almost exclusively of secondary minerals. The streams, however, reflect a mixture of lowland soil waters and interbasin groundwater, the latter representing volcanic rock weathering at higher elevations. Similar degrees of isotopic fractionation were observed during weathering of fresh volcanic rock by groundwater (Δ³⁰Si_{clay-fluid} = −1.2 ± 0.1 ‰ and Δ⁷⁴Ge_{clay-fluid} = −2.6 ± 0.2 ‰) and of chemically depleted lowland soils by rainwater (Δ³⁰Si_{clay-fluid} = −2.4 ± 0.6 ‰ and Δ⁷⁴Ge_{clay-fluid} = −3.0 ± 0.5 ‰). The observed Ge/Si and Ge and Si isotope signatures are best explained by the precipitation of secondary and tertiary clays, with vegetation playing a negligible role. The magnitude of fractionation observed in the fluids and the solids was shown to depend on the chemical mobility of each element. Due to the high degree of Ge retention in secondary products, weathering fluid δ⁷⁴Ge signatures were more strongly fractionated than δ³⁰Si. The opposite was true for the δ⁷⁴Ge and δ³⁰Si composition of the residual soils. ❧ Chapter 5 focuses on the other climatic extreme, investigating Ge/Si and δ⁷⁴Ge behavior during glacial weathering processes in West Greenland. The field study was coupled with a long-term river water and sediment incubation experiment to provide a direct observation of Ge/Si and δ⁷⁴Ge evolution with continued weathering. The dissolved Ge/Si ratios in periglacial streams, the Watson River, and its tributaries ranged from 0.9 to 2.2 µmol/mol, higher than most non-glacial rivers around the world, and likely reflecting preferential dissolution of Ge-rich biotite during subglacial weathering. Dissolved δ⁷⁴Ge of the Watson River was 0.86±0.24 ‰, only slightly heavier than the river suspended load (0.48±0.23 ‰), indicating limited precipitation of secondary weathering products during the short rock-water contact times associated with glacial weathering. Incubating unfiltered river water with its suspended sediment in the laboratory for 1.5‐2 years has resulted in the reduction of dissolved Ge/Si to ~0.5 µmol/mol, indicating significant Ge removal from solution with increased rock-water contact time, most likely due to adsorption to Fe oxyhydroxides. At the same time, dissolved δ⁷⁴Ge increased to 1.9‐2.2 ‰. The Δ⁷⁴Ge_{sec-diss} fractionation factor was calculated as −2.1±1.4 ‰, in good agreement with previously determined values for Ge adsorption onto Fe oxide particles and similar to the values determined for tropical weathering in Chapter 4. ❧ Chapter 6 presents an overview of δ⁷⁴Ge, δ³⁰Si, and Ge/Si data in a number of world's rivers, spanning different climatic and geomorphic regimes. Co-variation of all three proxy signatures confirmed that fluid composition is primarily determined by the precipitation of secondary weathering products. Ge/Si and δ³⁰Si signatures deviated from this trend only where biological uptake of Si is thought to have a major effect on the river chemistry. In contrast, δ⁷⁴Ge appeared unaffected by the limited uptake of Ge by diatoms or vegetation. Despite the broad similarity between the riverine δ⁷⁴Ge, δ³⁰Si, and δ⁷Li behavior, each proxy displayed a unique global relationship with silicate chemical weathering intensity. These differences likely stem from variable affinity of each element to different secondary weathering phases (Fe and Al oxides, various aluminosilicates). ❧ Finally, Chapter 7 presents and updated global Ge isotope budget, taking into account the revised global riverine δ⁷⁴Ge composition from Chapter 6. The knowledge of δ⁷⁴Ge dynamics gained from the preceding chapters is used to investigate the glacial-interglacial changes in Ge and Si cycles, using the paleorecord previously presented by Mantoura (2006). Since diatom δ⁷⁴Ge composition appears insensitive to vital effects, sedimentary diatom δ⁷⁴Ge paleorecords can be used to track secular changes in the seawater signature that result from the shifting balance between the different Ge sources and sinks. However, similar δ⁷⁴Ge composition of various input fluxes and limited isotopic fractionation during authigenic Ge removal from pore water (~−1 ‰ relative to seawater), coupled with short Ge residence time in the ocean, results in only small δ⁷⁴Ge_{SW} variations over glacial-interglacial cycles. More work is needed to confirm and explain the small ~0.5 ‰ positive excursion during glacial termination T2 at 130 ka. In summary, δ⁷⁴Ge paleorecords should be insensitive to local biological overprinting and could help identify large magnitude perturbations in the global Ge and Si cycles.