Diffusion of helium isotopes in silicate glasses and minerals : implications for petrogenesis and geochronology

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1989Helium mobility in geologic materials is a fundamental constraint on the petrogenetic origins of helium isotopic variability and on the application of radiogenic and cosmogenic helium geochronology. 3He and 4He volume diffusivities determined at 25-600°C in basaltic glasses by incremental-heating and powder storage experiments (using a diffusion model incorporating grain size and shape information to obtain high precision) are three to four orders of magnitude greater than for common cations. Diffusion in tholeiitic glass can be described by an Arrhenius relation with activation energy = 16.85±.13 Kcal/mole and log Do = -2.37±.06, although low temperature data are better described by a distribution of activation energies model . The best estimate for D at 0°C in tholeiitic glass is 5±2 x 10-16 cm2/s, an order of magnitude higher than the results of Kurz and Jenkins (1981) but lower than suggested by Jambon, Weber and Begemann (1985). Measurements in an alkali basalt show that helium diffusion is composition dependent (Ea = 14.4±.5 Kcal/mole; log Do = 3.24±.2), and roughly five times faster than in tholeiites at seafloor temperatures. The corresponding timescales for 50% helium loss or exchange with seawater (1 cm spheres) are about one million years for mid-ocean-ridge- basalts, and about 100,000 years in seamount alkali basalts. Radiogenic 4He diffusion has a higher activation energy (27±2 Kcal/mole; log Do = +2.4±1.0) than inherited (magmatic) helium, suggesting very low mobility (D = 3xl0-19 cm2/s at 0°C; factor of 5 uncertainty) and that U+Th/4He geochronology of fresh seafloor basalt glasses is unlikely to be hampered by helium loss. Measured isotopic diffusivity ratios, D3He/D4He, are not composition dependent, average 1.08±.02, and vary slightly with temperature, consistent with an activation energy difference of 60±20 cal/mole. This result differs from the inverse-square-root of mass prediction of 1.15, and may be explained by quantization of helium vibrational energies. These results suggest preferential loss of 3He will be minimal at low temperature (D3He/D4He = 1.02± .03 at 0°C). Therefore, alteration of magmatic 3He/4He ratios in basaltic glasses on the seafloor will occur only by helium exchange with seawater, and be important only for samples with low helium contents (<10-8 ccSTP/g), such as those found in island arc environments. Extrapolating the glass results to magmatic temperatures yields diffusivities similar to melt values, and suggests D3He/D4He approaches 1.15 at these and higher temperatures. Helium diffusivities in olivine and pyroxene at magmatic and mantle temperatures (900-1400°C) are higher than for cations, (E = 100±5 Kcal/ mole, log Do = +5.1±.7; and 70±10 Kcal/mole, log Do = +2.1±1.2, respectively), but are still too low to transport or homogenize helium in the mantle or even in magma chambers. However, diffusion equilibrates melts and mantle minerals within decades, and interaction with wall-rocks may be enhanced for helium in comparison to other isotopic tracers because of its greater mobility. Rapid exchange of helium within xenoliths and with their host magmas set limits on origin depths and transport times for xenoliths which exhibit helium isotopic disequilibrium between minerals, or between the magma and the xenolith. Phenocrysts equilibrate helium too rapidly to exhibit zoned isotopic compositions, and are likely to retain magmatic helium quantitatively in rapidly cooled volcanic extrusives. The 100-fold higher He diffusivity in pyroxene than olivine at 1000°C allows diffusive loss effects to be evaluated in more slowly cooled rocks, when cogenetic minerals can be measured. Diffusivities of cosmic- ray produced 3He in surface exposed rocks are several orders of magnitude higher than for inherited helium. However, activation energies for olivine and quartz, 25±4 Kcal/mole (log Do = 3.7±.8) and 25.2±.9 Kcal/mole (log Do = +.2±.4) respectively, still suggest low diffusivities at surface temperatures of approximately 10-22 and 10-20 cm2/s. Equations for simultaneous helium production and diffusive loss allow model ages for surface exposure to be corrected for helium loss, and demonstrate that cosmogenic 3He geochronology will not be limited by helium loss for timescales of approximately 1 million years in quartz and 10 million years or more in olivine. The measurements also suggest that radiogenic 4He produced by U and Th decay may be a useful dating method in quartz. Application of the diffusion measurements demonstrates that part of the wide range of 3He/4He ratios (.01 to 9 Ra) of a suite of dredged basalts and andesites from the Woodlark Basin, (western Pacific) reflects post-eruptive helium addition, from seawater in glasses with low He contents and from U and Th decay in mafic mineral separates. In unaltered samples, 3He/4He ratios for tholeiites from the Woodlark Spreading Center are 8-9 Ra, similar to mid-ocean-ridges, but distinctly different than the ratio of 6.9±.2 Ra observed in Kavachi submarine volcano basaltic andesites. Helium isotopic systematics in cogenetic pyroxenes and olivines from these samples demonstrate that this is a magmatic signature, and not the result of preferential 3He loss by diffusion. Coupled Sr and He isotopic systematics in these and other samples from the region suggest the sub-arc mantle has been enriched in radiogenic helium supplied by subducted Pacific lithosphere.Thanks to the Chemistry Department, and the WHOI Education Office for providing financial support and a nice place to work. Parts of this research was funded by NSF grants OCEBS-16082, EAR86-l06ll, and OCE87-16970

Diffusion of helium isotopes in silicate glasses and minerals : implications for petrogenesis and geochronology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989.Includes bibliographical references (leaves 194-199).by Thomas W. Trull.Ph.D

Temporal changes in size distributions of the Southern Ocean diatom Fragilariopsis kerguelensis through high-throughput microscopy of sediment trap samples

Some aspects of the life cycle of the Southern Ocean diatom Fragilariopsis kerguelensis have been investigated previously, but many of its details have not been surveyed in nature. We investigated material from a two-year sediment trap time series by high-throughput imaging and image analysis, looking for morphometric signals of life cycle stages. Valve length distributions appeared close to unimodal but positively (right-) skewed. Size cohorts resulting from synchronized sexual reproduction events were not clearly distinguishable. Nevertheless, based on changes in valve length distributions, we found three general seasonal phases. These corresponded to periods of proliferation (with higher proportions of smaller cells during late spring/early summer), cessation of growth (relative loss of smaller cells during late summer/early autumn), and overwintering (little change in size distributions, with an increased proportion of large cells). We discuss possible causes of these signals, and their relevance to growth, sexual activity and adaption to environmental conditions, such as grazing pressures and the need for an overwintering strategy.This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the priority programme 1158 ‘Antarctic Research with comparative investigations in Arctic ice areas’ under grant nr. BE4316/4-1, KA1655/3-1; and by an outgoing scholarship, as well as travel expenses, granted by the Helmholtz Graduate School for Polar and Marine Research (POLMAR). Part of this work was supported by the Australian Government’s Australian Antarctic Science Grant Program under project number 4078, and Macquarie University (A. RigualHernández and L. Armand)

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009):1143-1167, doi:10.1016/j.dsr.2009.04.001.An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport in the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234Th:238U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites- even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of 234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 μm at ALOHA and >5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10’s to 100’s km at a resolution that is difficult to obtain with other methods.Funding for VERTIGO in the US was provided primarily by research grants from the US National Science Foundation Programs in Chemical and Biological Oceanography with additional support by the US Department of Energy (DAS). For TWT, support came from the Australian Cooperative Research Centres program

Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.info:eu-repo/semantics/publishedVersio

Limitation of Algal Growth by Iron Deficiency in the Australian Subantarctic Region

In March 1998 we measured iron in the upper water column and conducted iron- and nutrient-enrichment bottle-incubation experiments in the open-ocean Subantarctic region southwest of Tasmania, Australia. In the Subtropical Convergence Zone (∼42°S, 142°E), silicic acid concentrations were low (\u3c 1.5μM) in the upper water column, whereas pronounced vertical gradients in dissolved iron concentration (0.12-0.84 nM) were observed., presumably reflecting the interleaving of Subtropical and Subantarctic waters, and mineral aerosol input. Results of a bottle-incubation experiment performed at this location indicate that phytoplankton growth rates were limited by iron deficiency within the iron-poor layer of the euphotic zone. In the Subantarctic water mass (∼46.8°S, 142°E), low concentrations of dissolved iron (0.05-0.11nM) and silicic acid (\u3c 1μM) were measured throughout the upper water column, and our experimental results indicate that algal growth was limited by iron deficiency. These observations suggest that availability of dissolved iron is a primary factor limiting phytoplankton growth over much of the Subantarctic Southern Ocean in the late summer and autumn

Chemometric perspectives on plankton community responses to natural iron fertilisation over and downstream of the Kerguelen Plateau in the Southern Ocean

International audienceWe examined phytoplankton community responses to natural iron fertilisation at 32 sites over and downstream from the Kerguelen Plateau in the Southern Ocean during the austral spring bloom in October–November 2011. The community structure was estimated from chemical and isotopic measurements (particulate organic carbon – POC; 13C-POC; particulate nitrogen – PN; 15N-PN; and biogenic silica – BSi) on size-fractionated samples from surface waters (300, 210, 50, 20, 5, and 1 μm fractions). Higher values of 13C-POC (vs. co-located 13C values for dissolved inorganic carbon – DIC) were taken as indicative of faster growth rates and higher values of 15N-PN (vs. co-located 15N-NO3 source values) as indicative of greater nitrate use (rather than ammonium use, i.e. higher f ratios).Community responses varied in relation to both regional circulation and the advance of the bloom. Iron-fertilised waters over the plateau developed dominance by very large diatoms (50–210 μm) with high BSi / POC ratios, high growth rates, and significant ammonium recycling (lower f ratios) as biomass built up. In contrast, downstream polar frontal waters with a similar or higher iron supply were dominated by smaller diatoms (20–50 μm) and exhibited greater ammonium recycling. Stations in a deep-water bathymetrically trapped recirculation south of the polar front with lower iron levels showed the large-cell dominance observed on the plateau but much less biomass. Comparison of these communities to surface water nitrate (and silicate) depletions as a proxy for export shows that the low-biomass recirculation feature had exported similar amounts of nitrogen to the high-biomass blooms over the plateau and north of the polar front. This suggests that early spring trophodynamic and export responses differed between regions with persistent low levels vs. intermittent high levels of iron fertilisation

Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as εNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October–November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative εNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic εNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic εNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica

An assessment of the use of sediment traps for estimating upper ocean particle fluxes

Author Posting. © Sears Foundation for Marine Research, 2007. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 65 (2007): 345–416, doi: 10.1357/002224007781567621This review provides an assessment of sediment trap accuracy issues by gathering data to address trap hydrodynamics, the problem of zooplankton "swimmers," and the solubilization of material after collection. For each topic, the problem is identified, its magnitude and causes reviewed using selected examples, and an update on methods to correct for the potential bias or minimize the problem using new technologies is presented. To minimize hydrodynamic biases due to flow over the trap mouth, the use of neutrally buoyant sediment traps is encouraged. The influence of swimmers is best minimized using traps that limit zooplankton access to the sample collection chamber. New data on the impact of different swimmer removal protocols at the US time-series sites HOT and BATS are compared and shown to be important. Recent data on solubilization are compiled and assessed suggesting selective losses from sinking particles to the trap supernatant after collection, which may alter both fluxes and ratios of elements in long term and typically deeper trap deployments. Different methods are needed to assess shallow and short- term trap solubilization effects, but thus far new incubation experiments suggest these impacts to be small for most elements. A discussion of trap calibration methods reviews independent assessments of flux, including elemental budgets, particle abundance and flux modeling, and emphasizes the utility of U-Th radionuclide calibration methods.WG meetings and production of this report was partially supported by the U.S. National Science Foundation via grants to the SCOR. Individuals and science efforts discussed herein were supported by many national science programs, including the U.S. National Science Foundation, Swedish Research Council, the International Atomic Energy Agency through its support of the Marine Environmental Laboratory that also receives support from the Government of the Principality of Monaco, and the Australian Antarctic Science Program. K.B. was supported in part by a WHOI Ocean Life Institute Fellowship