V53B-01 13:40h
Iron Isotopes in the Amazon River System: Weathering and Transport Signatures
Fractionation of iron isotopes will be an effective tool to investigate and quantify the environmental geochemistry of iron. Initial studies of stable iron isotopes show measurable fractionation in both field samples and laboratory studies spanning over 4$\permil$ in the 56/54 ratio. In this study, trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples were measured for their iron isotopic composition using a GV Instruments IsoProbe Multi-collector ICPMS. This system uses a hexapole collision cell to reduce molecular interferences and improve transmission. A plankton tow collected in a low salinity Amazon River plume in the open ocean had a $\delta$$^{56}$Fe value of -0.34$\permil$ relative to igneous rocks and a Fe:C ratio of $\sim$ 600 $\mu$mol/mol. It was inferred from the high Fe:C ratio that a majority of the Fe collected in the plankton tow was extracellular Fe and that the $\delta$$^{56}$Fe might reflect the composition of particles and Fe attached to the surface of the plankton. In order to investigate the source of Fe to the Amazon plume water, samples were collected from the Amazon River and region including filtered river water, suspended sediment, and a shelf porewater. River water-seawater mixing experiments were also performed to assess whether Fe flocculation in estuaries affects isotopic composition of the dissolved flux to the ocean. The overall Fe isotopic variation observed in the Amazon River system was 1.5$\permil$. The Fe from the dissolved and suspended loads of two main channel river sites was isotopically similar ($\sim$ -0.2 to -0.45$\permil$). The most depleted sample was the Amazon shelf porewater (-1.4$\permil$). The isotopically heaviest sample collected was the dissolved Fe from an organic rich tributary, the Negro River, in the Amazon River system (+0.16$\permil$). Although the Negro River dissolved phase was isotopically heavy relative to igneous rock, its suspended sediment Fe was isotopically light (-1$\permil$). The signature of the Negro was not observed downstream near the mouth. The variability in Fe isotopic composition from different types of river tributaries draining distinct weathering terrains suggests that Fe isotopes may reflect the degree or type of weathering or overall balance of Fe in a drainage basin. Based on river water-seawater mixing experiments, the $\delta$$^{56}$Fe signal of the Amazon River may be modified in the estuary when $>$90% of the Fe flocculates upon mixing with ocean water. In the river water-seawater mixing experiments, the Fe that flocculated was isotopically heavy compared with the riverine dissolved Fe by 0.19 $\pm$ 0.12 $\permil$ (2$\sigma$). This observation suggests that the dissolved Fe that is transported to the ocean is isotopically lighter ($\sim$ -1$\permil$ or lighter) than the river endmember. However, neither the proposed isotopically light Fe from the modified riverine input nor from shelf porewater matches the Amazon plume plankton tow iron isotopic composition. Processes in the euphotic zone (biological cycling/export, scavenging) may modify the riverine or shelf Fe input to the ocean by preferentially removing isotopically light Fe. The above studies of Fe isotope fractionation demonstrate that aqueous and biological samples in the environment have a measurable range in iron isotopic composition, and that these signals might be useful in tracking Fe pathways.
V53B-02 13:55h
Silicon Isotopes in River Waters: A First Look into Variability in the Continents
The isotopic fractionation during biological uptake of silicon has been studied as a proxy for silicic acid utilization in the marine environment (De La Rocha, et al 1997, Varela et al. 2004). These studies have shown that the Si isotope distribution in the ocean has been strongly affected by marine productivity. To what extent continental weathering also contributes to variations in the Si isotope composition of the ocean is unknown, because the inputs into the oceans have not been reliably quantified. We have measured the Si isotope compositions of river waters draining different lithologies over a seasonal cycle in order to constrain continental variability and contributions to the marine Si isotope budget. For our study we sampled four different rivers draining different catchment lithologies in Switzerland in March, May, August and October 2004. The average annual runoff of these rivers ranges from 10m$^{3}$/s to 110m$^{3}$/s, and the concentration of dissolved Si shows strong seasonal variation. Si isotopes were analysed using the high-resolution capacity of the Nu1700 (Nu Instruments, UK) multi-collector ICP-MS at ETH Zurich. Si isotope data are reported as $\delta ^{30}$Si and $\delta ^{29}$Si relative to the NIST NBS28 standard. The Si isotope composition of the river samples can be reproduced with two different chemical preparation methods with an average reproducibility of 0.15$\permil$ (2 $\sigma$). Preliminary results demonstrate that the Si isotope compositions of the rivers undergo seasonal variations and reveal clear differences between lithologically distinct catchments. The difference in $\delta ^{30}$Si between the rivers studied so far is up to 0.4$\pm$ 0.1$\permil$ and seasonal variations can be as high as 0.3$\pm$ 0.1$\permil$. Reference: De La Rocha, et al. 1997, G.C.A. 61 p5051-5056, Varela, et al. 2004, G.B.C. 18 GB1047.
V53B-03 14:10h
Si Isotopic Signatures of Diatoms in the Spring Southern Ocean
Marine Si isotopic signatures have been shown to be of great relevance in quantifying the diatoms nutrient utilization efficiency, a key factor for many studies related to oceanic carbon sequestration (De La Rocha et al., 1998; Brzezinski et al., 2002). In order to better constrain and apply this new tool, spring diatoms and seawater have been sampled at five stations distributed in different biogeochemical provinces of the Southern Ocean's Australian sector: Polar Front Zone (PFZ), Antarctic Zone (AZ), Sea Ice Zone (SIZ). Total ($>$0.45um), medium-sized (20$<$ $<$70um), and large diatoms ($>$70um) have been sampled at 2-4 depths in the upper 150m. Biogenic silica (BSi) was first digested in a hot diluted NaOH solution. After purification, silicon isotopic compositions of diatoms and seawater were then measured by MC-ICP-MS, in dry plasma mode using external Mg doping, with an overall repeatability of 0.08 p.mil (Cardinal et al., 2003). Results are expressed as \delta$^{29}$Si relatively to NBS28 standard. The isotopic composition of diatoms is generally homogeneous in the mixed layer and does not seem to exhibit an isotopic fractionation linked to a size effect. Diatoms \delta$^{29}$Si are systematically lighter than the ambient seawater signature, reflecting their preferential uptake of light isotopes (De La Rocha et al., 1997). We observe a trend of lighter isotopic signatures southward, both in diatoms and seawater samples, but the BSi isotopic gradient is much steeper with a diatoms \delta$^{29}$Si signature as low as -0.26 p.mil in the southernmost SIZ station, which strongly contrasts with the +0.65 p.mil signature measured on PFZ diatoms. Such latitudinal variation of diatom isotopic signature is well in accordance with the one observed in summer by Varela et al. (2004). In contrast, our spring samples display a difference between the ambient seawater and diatom isotopic signatures that strongly increases southward: it goes from 0.45 in the PFZ up to 1.08 p. mil in the SIZ. This points toward different time scales, likely to be recorded in the mixed layer between diatoms and seawater as a function of the zonal physical and ecological characteristics. The observed latitudinal trends will be discussed in terms of seasonal nutrient utilization and sources, sea ice influence, diatom assemblages and proxy validation.
V53B-04 14:25h
Zinc isotope variations in natural and cultured marine phytoplankton.
Zn is an essential micronutrient for marine phytoplankton. Zn distribution in the ocean is largely controlled by biological uptake, being drawn down from several nanomolar concentrations in the deep ocean to picomolar concentrations in the surface ocean. Zn isotopes may record this biological activity based on the preferential uptake of lighter Zn isotopes by phytoplankton. Marechal et. al. (2000) attribute a seasonal cycle in the Zn isotope composition of sediment trap material and global variations in the Zn isotope composition of manganese nodules to this biological fractionation. To better understand the processes controlling the distribution of Zn isotopes in the ocean, the isotopic composition of phytoplankton was investigated. In-situ plankton were collected by trace metal clean plankton tows from both the Atlantic and Pacific Oceans. In a region close to a continental source of Zn, approximately 300 km off the coast of Brazil (7.4 S, 31.4 W), the isotope composition of the plankton tow material closely resembles that of continental material. The isotopic composition of plankton tows from more remote locations in the open ocean may better record the effects of biological cycling. Additional samples from the Atlantic, as well as samples from the central Pacific near Hawaii and the North Pacific are being processed. This data may be used to better understand how biological processes control the global distribution of Zn isotopes in the oceans.
V53B-05 14:40h
Controls on the Transition Metal Isotopic Composition of Seawater: Diatom Culture Experiments
Many transition metals are essential micronutrients for marine phytoplankton. As a result the expectation is that biological processes play an important, perhaps a dominant, role in their marine isotope geochemistry. These observations raise the prospect of using isotope records to trace transition metal micronutrient usage in the past oceans, an issue that is of importance to the efficiency of the biological pump and atmospheric carbon dioxide. As such, the characterisation of trace metal isotopic fractionations associated with marine primary productivity are an important scientific goal. Here we report fractionations associated with Fe, Cu and Zn sequestration by diatoms, one of the main primary producers in the oceans. Axenic unialgal cultures of {\it Thalassiosira weissflogii} and {\it Thalassiosira pseudonana} were established in artificial seawater + F/2 medium at 18$\deg$C on a 16:8 light:dark cycle. The cultures were filtered to separate diatom material from residual media and analysed for Zn, Cu and Fe concentrations and isotope composition using techniques described elsewhere$^{1,2}$. Aliquots of the starting medium were also measured for each batch of cultures. The diatom organic material shows small, but consistent and resolvable, positive fractionations (0.1-0.3 per mil) for Fe, Cu and Zn relative to the starting medium. In the case of all three metals, but particularly for Zn (70-95% depending on experiment size), the diatoms had sequestered a large proportion of the available metal, suggesting that the fractionation factor for metal usage by the diatoms is much greater than 1.0001 to 1.0003. Time-series experiments are under way to determine the exact magnitude of the fractionation factor. The mass-balance is supported by the fact that the residual medium is around -0.4 per mil for Zn. The fact that diatoms incorporate trace metals that are isotopically heavier than the nutrient pool is a surprising result, the expectation having been that, as with carbon, the biological usage of trace metals would result in kinetic fractionations$^{3}$. The positive fractionations necessitate an equilibrium process and, perhaps, active extra-cellular sequestration of trace metals. The second broader implication is that given the proposed role of diatoms in controlling the extreme depletion of Zn in open ocean surface waters, particularly in the Pacific where surface waters are have up to a factor of 250 less Zn than deep waters$^{4}$, the depletion of the light isotope in surface waters and its enrichment in deep waters are predicted to be extreme. Zn, and other trace metal, isotopes may have an important role in recording this process in the past oceans. $^{1}$ C. Archer and D. Vance, 2004, J. Anal. Atom. Spectr. 19, 656-665. $^{2}$ J. Bermin, et al., 2004, this volume. $^{3}$ Pichat, S et al., 2003, Earth Planet. Sci. Lett. 210, 167-178. $^{4}$ Lohan, M.C. et al., 2002, Deep-Sea Res. II 49, 5793-5808.
V53B-06 14:55h
Thallium Isotope Constraints on Hydrothermal Water Fluxes at Mid-Ocean Ridge Axes and Flanks
The hydrothermal circulation that occurs at mid-ocean ridge axes and flanks has profound effects on the chemical budgets of the oceans but our understanding of the relevant fluxes is incomplete. Here, we use new thallium (Tl) isotope and concentration data for hydrothermal fluids and rocks from ODP Hole 504B to obtain independent estimates of the high- and low-temperature (T) hydrothermal water fluxes at spreading axes and ridge flanks. Seawater is characterized by relatively uniform Tl isotope compositions and concentrations of $\epsilon^{205}Tl$ = -6 and 65 pmol/kg, respectively ($\epsilon^{205}Tl$ represents the deviation of the $^{205}Tl/^{203}Tl$ ratio of a sample from the standard in parts per 10,000). In contrast, high-T hydrothermal fluids from ridges axes display $\epsilon^{205}Tl = -2\pm1$, indistinguishable from unaltered mantle rocks. The correlation of Tl and Cl abundances indicates an average Tl content of 10-25 nmol/kg for high-T endmember fluids. The low-T alteration of the upper volcanic zone of ODP Hole 504B is associated with Tl-uptake from seawater. The isotope fractionation that occurs during the uptake generates Tl-rich rocks that have $\epsilon^{205}Tl$ as low as -16. The sheeted dike complex displays low Tl contents due to leaching of the rocks by high-T hydrothermal fluids. Taken together, these observations indicate that high-T vent fluids do not acquire significant Tl from the altered Tl-rich rocks of the volcanic section. With this constraint, the high-T axial water flux can be calculated, assuming that Tl is leached with an efficiency of $60-95%$ from 1.0-1.4 km of sheeted dikes and upper gabbros, which have a Tl concentration of $3\pm1$ ppb. These parameters yield a high-T water flux of 0.2-2.5 x $10^{13}$ kg/yr, equivalent to a heat flux of 0.1 to 1.2 TW. This result is in excellent agreement with other geochemical estimates of high-T water fluxes, e.g., those based on Li isotopes (Chan et al., 2002) or the Sr isotope profile of ODP Hole 504B (Teagle et al., 2003). If the total axial hydrothermal power output is about 2 TW, the geochemical data indicate that at least some heat loss at mid-ocean ridges is due to diffuse low-T fluid flow. The Tl data acquired for off-axis fluids and the volcanic zone rocks are in accord with significant ($30-80%$) losses of Tl from circulating seawater. If it is assumed that 15-35 ppb of Tl are added to the top 600 m of ocean crust, the low-T water flux of ridge flanks can be calculated as 1-5 x $10^{16}$ kg/yr, which is equivalent to an average fluid exit temperature of about $3-10\deg$C. This result is in accord with a recent Mg-based estimate (Mottl, 2003), which indicates that about $90-98%$ of the ridge flank power output occurs at cool sites with fluid temperatures of less than $20\deg$C.
V53B-07 15:10h
An estimate of the Germanium isotopic composition of the Ocean.
Ge is a trace element in seawater whose biogeochemistry is dominated by its Si-like behaviour. Its residence time is poorly constrained but could be close to the mixing time of the ocean. In addition, hydrothermal vents are enriched in Ge (relative to Si) and this excess has been witnessed in the water column. Moreover, Si isotopic variations have been reported in the ocean, related to the precipitation of biogenic opal, while the Si residence time is slightly higher than the Ge residence time. Therefore, variations in the isotopic composition of dissolved Ge in the ocean are expected provided that at least one of the major input or output of Ge has a different isotopic composition. Given the low Ge concentration (around 40 picomol/kg) and the state-of-the art analytical facilities, a direct measurement of the isotopic composition of the seawater is barely conceivable. The major input of Ge into the ocean are the rivers and the hydrothermal vents, while the removal of Ge occurs through the precipitation of biogenic opal and the early diagenesis of passive margins. The mechanism of the later is, however, not well established but could be related to the precipitation of Fe-oxyhydroxide. So the measurement of marine authigenic minerals, biogenic silica and the comparison with an estimate of the bulk silicate Earth (BSE) composition will give some constraints on the Germanium isotopic composition of the ocean. A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate, sulfide, and biogenic material. The analyses were performed using a continuous flow hydride generation system coupled to a Nu Instrument MC-ICPMS. Samples have been purified through anion and cation exchange resins to separate Ge from matrix elements and potential interferences. Deep sea clays have a similar isotopic composition that MORBs or granites, suggesting that isotopic composition of the dissolved Ge in rivers might not be very distinct from the BSE composition. On the other hand, modern deep-sea sponges have \delta$^{74}$Ge values clustered at 1.7\permil (standardised to BSE) while the \delta$^{74}$Ge values of modern diatoms are more variables and can be as high as 2.5\permil. Modern authigenic clays (glauconite) are also enriched in heavy isotopes by 2\permil. In addition, the Ge isotopic fractionation during Ge sorption on goethite has experimentally determined to be +1.5\permil. Therefore, we already have 3 reasons to believe that the Germanium isotopic composition of the ocean is enriched in heavy isotopes (relative to BSE). The exact amplitude of the isotopic enrichment factor will be determined on cultured diatoms and will be associated to the isotopic composition of diatoms from core-top from the Southern Ocean to give a first estimate of the isotopic difference between the ocean and the BSE.