B14B-01 INVITED
Is the Low-Temperature Isotope Geochemistry of Mg Driven by Biology?
In the last decade, the development of a new field of isotope geochemistry sometime referred as non traditional stable isotope geochemistry together with a renewed interest for the origin of life has raised one of the key question in biogeochemistry: how can we fingerprint life beyond the conventional use of C-based molecules? A general idea has been to recognise that biomediated chemical reactions are rarely at thermodynamic equilibrium and strongly affected by kinetics with an associated isotopic fractionation much greater than the isotopic fractionation characteristic of the equilibrium reaction. This approach led to the false idea that stable isotope of heavy elements (mass greater than 40) can only be fractionated by biological reaction and divalent cations and among them Magnesium (Mg) are no exception. Differences in the relative abundances of Mg three stable isotopes, 24Mg (78.99%), 25Mg (10.00%), and 26Mg (11.01%), could be expected because of the large relative mass differences between 26Mg and 24Mg. However, Mg-bonds are dominantly ionic (a weak bond) and while kinetic processes affect all type of physico-chemical reactions, the scale of the isotopic effect of thermodynamic equilibrium is strongly related to the strength of chemical bonds. This view is supported by the lack of Mg isotopic fractionation during magmatic differentiation (Teng et al., doi:10.1016/j.epsl.2007.06.004) and the very small isotopic differences (less than 0.1‰ in δ26Mg) among igneous rock, when their analysis are immune of analytical artefact (Tipper et al., ). Therefore, the range of almost 5‰ for aqueous solution and associated precipitated material (e.g. Young and Galy, Tipper et al., doi:10.1016/j.epsl.2006.07.037) could be interpreted as widespread kinetic isotopic fractionation by Mg mobilisation at the surface of the Earth. Indeed, large isotopic fractionations are generated by biomineralisations and the largest is associated with the precipitation of low-Mg calcite by foraminifer. However, the incorporation of Mg into chlorophyll is characterised by a small (-0.5‰) isotopic enrichment factor (Black et al., ). In addition, the isotopic enrichment factor between solute and rocks is not systematically different between the arid Tibetan Plateau and the tropical Lesser Himalaya, suggesting that vegetation may only recycle a small amount of Mg in these catchments. (Tipper et al., doi:10.1016/j.gca.2007.11.029). Therefore large isotopic variations found in dissolved Mg cannot be accounted for by biologically mediated processes. Moreover, minute differences in the partitioning of 26Mg/24Mg and 26Mg/24Mg ratios (Young and Galy,) has suggested that an isotopic enrichment factor of - 2.7‰ is characteristic of the precipitation of low-Mg calcite at temperature around 20°C at thermodynamic equilibrium. Indeed, the mass-dependent fractionation laws that describe the partitioning of three or more isotopes are different for kinetic and equilibrium reactions. The 3-isotope systematics of Mg is likely to be the only avenue to pursue for distinguishing bio-mediated processes of Mg.
B14B-02
Mg Isotope Fractionation Between E. coli and Growth Medium
Magnesium is a major element in both microbial cells and minerals, immune to redox conditions and atmospheric interactions. In organic cells, Mg can be associated with membranes, with cytoplasm (either as an isolated ion or bound to proteins). Its isotope composition can be used to constrain the contribution of organic material to carbonate fluxes and the overall cycle of this element in the exogenous environment [1, 2]. Cells of DH5α E. coli strain were grown in Luria Broth medium and the Mg isotope fractionation between the cells and their growth medium determined after calcination in Pt crucibles, chemical purification by cation exchange chemistry in HCl medium [3] and isotopic analysis on a Nu HR MC-ICPMS. The yield is better than 96%. The Mg contents of 2.19 ± 0.08 mg per g DW in cells and 0.117 ± 0.001 mg per g DW in Luria Broth medium are consistent with literature data [4]. About half of the Mg initially present in the LB medium is taken up by the growing cells. At high cellular concentrations (OD600 = 3.5), cells are enriched in 26Mg by 0.97 ± 0.14 ‰ with respect to the culture medium. Although E. coli may not be a good proxy for oceanic plankton, such a substantial fractionation of Mg isotopes suggests that incorporation of even a few percent organic matter into oceanic oozes depletes oceanic Mg in its heavy isotopes and therefore accounts for the isotopic difference between riverine and marine Mg. [1] Drever, The Sea 5 (1974) 337-357 [2] Tipper et al., EPSL 250 (2006) 241-253 [3] Chang et al., JAAS 18 (2003) 296-301 [4] Outten et al., Science 292 (2001), 2488-2492
B14B-03
Calcium Isotope Geochemistry: Research Horizons and Nanoscale Fractionation Processes
Interest in studies of calcium isotope variations in nature continues to increase. Investigations span human biology, plants and soils, oceanography and paleoclimate, early solar system processes, aqueous geochemistry, and silicate liquid structure. Variations in the 44Ca/40Ca ratio are generally small, about 5 ‰, but gradual small improvements in analytical capability now yield 0.05 to 0.1 ‰ resolution. The field is still plagued by a lack of universal standards for isotope ratios and data representation, but these are secondary issues. Traditional isotopic systems have been based in equilibrium thermodynamics, which can explain the magnitude and sign of observed mass-dependent fractionation behavior. For Ca isotopes this is not the case. There is still no reliable way to estimate the equilibrium free energy associated with isotopic exchange between most phases of interest. Experiments are difficult to interpret because it is almost impossible to precipitate minerals from aqueous solution at equilibrium at low temperature. Some studies suggest that, for example, there is no equilibrium isotopic fractionation between calcite and dissolved aqueous Ca. There is good evidence that most Ca isotopic fractionation is caused by kinetic effects. The details of the controlling processes are still missing, and without this mechanistic understanding it is difficult to fully understand the implications of natural isotopic variations. Recent work on dissolved Ca, calcite, and sulfates in both laboratory and natural settings is shedding light on where the fractionation may arise. There is emerging evidence for mass dependent fractionation associated with aqueous diffusion, but probably the primary source of the effects is in the details of precipitation of minerals from solution. This makes the fractionation potentially dependent on a number of factors, including solution composition and mineral growth rate. The next challenge is to develop appropriate experimental tests and combine them with micro- and nano-scale characterization, and to capture the critical processes in mathematical models. Some of the largest fractionation effects have been observed for silicate liquids, where both chemical and thermal diffusion generate large isotopic variations. Intake and transport of Ca in plants is also associated with substantial fractionation. Continuing work is beginning to place the fractionation into the context of global Ca cycles.
B14B-04
Behaviour of Sr, Ca, Nd and Li Isotopes During Granite Weathering: the Margeride Massif, France
The Massif Central region of France contains numerous mineral water springs with salinities up to 6 g/L. These high salinities develop due to water-rock interaction processes accompanying weathering of granitic rocks such as the Margeride massif, a 5 km-deep laccolith having an age of 323 ± 12 Ma and consisting mainly of granitoid and gneiss. In order to better constrain weathering processes, we have determined the Sr, Nd, Ca and Li-isotope compositions of the Margeride granite, weathered granite (arene) and saprolite, sediment and soil overlying the granite, and groundwater samples (e.g., mineral water springs) associated with the massif. 87Sr/86Sr ratios increase in the order apatite-plagioclase-K-feldspar-arene- sediments and soils-biotite, and are well correlated with Rb/Sr ratios. Mineral waters have 87Sr/86Sr ratios similar to that of plagioclase, but have higher Rb/Sr ratios. 44Ca/40Ca ratios of plagioclase and apatite are similar to that of the whole rock, while those of K-feldspar and biotite are significantly less. 44Ca/40Ca ratios of arene, soil and sediment are similar to or less than that of K- feldspar, reflecting complete loss of Ca from plagioclase and apatite during weathering. In contrast, 44Ca/40Ca ratios for the mineral waters are similar to or substantially greater than that of plagioclase, reflecting extensive calcium carbonate precipitation during ascent of the waters along the rock fracture network. 44Ca/40Ca ratios of the waters are as much as 3.5 per mil greater than that of seawater, and thus contain the heaviest Ca yet reported for terrestrial materials. 7Li/6Li ratios differ by a few per mil among the granite minerals; of the weathering products, arene and soils have the least 7Li/6Li ratios, while river bank sediment and arene surface sediment have progressively greater ratios. 7Li/6Li ratios of the mineral waters have the greatest values, reflecting preferential retention of 6Li in the weathering products. 143Nd/144Nd ratios are nearly invariant for the solid phase samples, but highly variable in the mineral waters. In order to characterize the isotopic signature of water interacting with the granite (IRf, or 'isotope ratio of the fluid'), a dissolution model was applied based on the hypothesis that dissolution of plagioclase, K-feldspar and apatite control IRf for Sr and Nd (Negrel, 2006). IRf was calculated for Sr and Nd isotopes after equilibration with minerals in the Margeride granite, and compared to values measured in the mineral waters in order to better define groundwater circulation processes in the laccolith. There is poor agreement between the Sr and Nd isotopic compositions predicted by the weathering model and those observed in the mineral waters. Thus, deep circulation involving interaction with another deep-seated rock type lying below the granite must be considered. Ca and Li isotopes proved most useful for determining the extent of secondary mineral formation during granite weathering and ascent of the mineral waters. Negrel Ph (2006) Water-granite interaction: clues from strontium, neodymium and rare earth elements in saprolite, sediments, soils, surface and mineralized waters. Applied geochemistry, 21, 1432-1454.
B14B-05
Carbonate Accumulation and Dissolution Events and the Seawater Calcium Isotope Record
Calcium carbonate sedimentation in the ocean represents the largest calcium and carbon sink in the combined atmosphere, biosphere, and ocean system, thus connecting the global carbon and calcium cycles. Determining fluctuations in calcium carbonate sedimentation over climate transitions provides important information on how the coupled calcium-carbon biogeochemical system behaves and reveals feedbacks between processes that control it. The calcium isotope records of two periods of extreme change in the global calcite compensation depth (CCD) and climate over the Cenozoic, the Eocene-Oligocene Transition (EOT) and the Paleocene-Eocene Thermal Maximum (PETM), will be examined at high resolution using marine (pelagic) barite. Large fluctuations in the CCD may be reflected in seawater Ca concentrations and Ca isotopic composition resulting from short term imbalances in marine Ca sources and sinks. A simple isotopic mass balance model is constructed to compare predicted and observed isotopic fluctuations. The permanent deepening of the CCD during the EOT which coincided with a transition from a 'greenhouse' to an 'icehouse' world is not accompanied by a long-term increase in seawater Ca concentration. Instead, our results suggest that the seawater Ca concentration might have decreased at this time due to an increase in calcium carbonate sedimentation associated with the deepening of the CCD. Preliminary results from the PETM will also be presented. These results present the first high resolution Ca isotope measurements over the EOT and PETM.
B14B-06
Strontium-86 Labeling Experiments Show Spatially Heterogeneous Skeletal Formation in the Scleractinian Coral Porites porites
We present first results of a long-term effort to label calcium carbonates formed by marine organisms with stable isotopes to obtain information about the dynamics of the biomineralization processes. The skeleton of the scleractinian coral Porites porites was labeled three times with enhanced concentrations of 86Sr. The distribution of 86Sr in the skeleton can be imaged with the NanoSIMS ion microprobe with a spatial resolution of ca. 200 nm and combined with images of the skeletal ultra-structure. Importantly, the distribution of the 86Sr label in the P. porites skeleton was found to be strongly heterogeneous. This is inconsistent with the existence of a continuous Extracellular Calcifying Fluid (ECF) reservoir at the surface of the growing skeleton, which is implicit in most geochemical models for coral biomineralization. These new experimental capabilities promise a much more detailed view of skeletal growth dynamics for a wide range of marine organisms that biomineralize carbonate structures.
B14B-07
New Perspectives on the Marine Strontium Isotope Record of the last 27 Ma from delta 88/86Sr and 87Sr/86Sr Systematics Using the TIMS Sr-Double-Spike Technique
Applying a recently developed Sr-double spike the Sr-isotope fractionation for both 88Sr/86Sr and
87Sr/86Sr ratios in water and carbonates can now precisely be determined with the TIMS-
technique. Repeated measurements of standard materials (NIST SRM 987, IAPSO seawater standard, JCp-1
carbonate standard) showed that δ 88Sr/86Sr (reported as δ
88/86Sr=((88Sr/86Sr)sample/(88Sr/86Sr)NBS987)-1)*1000) and 87Sr/86Sr ratios
can be determined with an external reproducibility in the order of better than 20 ppm and about 6 ppm,
respectively, for NIST SRM 987, IAPSO and JCp-1, respectively. Using the double spike TIMS technique the
statistical error for determination of δ 88/86Sr is reaching a whole procedure reproducibility of about
15 ppm. All standard measurements are in general accord with previous observations (1,2) indicating that
δ 88/86Srseawater and 87Sr/86Srseawater is about 0.39±0.02 ‰ and
0.709285(6) , (n=12), respectively, and that the marine carbonates being isotopically lighter than seawater
(JCp-1, δ 88Sr/86Sr: 0.20±0.02; 87Sr/86Sr : 0.709219(6); n=3). Note, that also
87Sr/86Sr carbonate ratios is isotopically about 70 ppm lighter than seawater as expected from
isotope fractionation during the precipitation of calcium carbonates (1).
For a first application of our double spike on 13 bulk carbonate samples of aragonitic composition,
representing marine shallow water mollusks (0 to 100m water depth), it has been shown that their bio-
stratigraphic ages are in accordance with the classical Strontium isotope stratigraphy (SIS) after McArthur
and Howarth (2004). The samples span an age range from the the Late Oligocene to Pleistocene (about 27
Ma). The δ 88Sr/86Sr-record shows variations in the order of about 0.2 ‰. The
87Sr/86Srseawater record tends to show higher values compared to the classical normalized
87Sr/86Sr record. The offset is not constant rather correlated to the 88Sr/86Sr ratios.
Preliminary interpretation take seawater temperature variations and varying Sr supply from isotopically
distinctively different sources into account.
References:
(1) Fietzke J. and Eisenhauer A. (2006) Determination of temperature-dependent stable strontium isotope
(88Sr/86Sr) fractionation via bracketing standard MC-ICP-MS. Geochemistry, Geophysics, Geosystems 7,
doi:10.1029/2006GC001243.
(2) Halicz L., Segal I., Fruchter N., Stein M. and Lazar B. (2008) Strontium stable isotopes fractionate in the
soil environments? Earth Planet Sci Lett, 272, 406-411.
(3) McArthur J.M. and Howarth R.J. (2004) Strontium isotope stratigraphy. In: A Geologic Time Scale, eds.
Felix M. Gradstein, James G. Ogg, and Alan G. Smith. Cambridge University Press, 96-105.
B14B-08
High-Precision Measurement of Radiogenic and Stable Sr Isotopes; Applications to the Marine Sr Record
Strontium isotope systematics in terrestrial materials consists of stable 88Sr/86Sr and 84Sr/86Sr ratios, and the radiogenic 87Sr/86Sr ratio. Growing interest in natural mass dependent 88Sr/86Sr variations indicate that terrestrial samples vary by ~1000 ppm, and so we have developed a double-spike TIMS technique that allows us to determine 87Sr/86Sr and 88Sr/86Sr ratios to be better than 10 ppm and 84Sr/86Sr to 70 ppm external precision respectively. Variations in seawater 87Sr/86Sr ratios over time reflect changes in the flux and composition of material delivered to the oceans. Such records cannot provide a unique interpretation of the changing inputs to the oceans because it is not possible to resolve source inputs from weathering fluxes, in part because any natural variations in 88Sr/86Sr are lost during correction for instrumental mass fractionation. However, our new double-spike data allows us to utilise combined radiogenic (87Sr/86Sr) and stable (δ88Sr) isotopes and potentially resolve these two important parameters. Measurements of the composition of the key input and output fluxes to the oceans allows us to construct a model for the evolution of seawater Sr. Source variations are dominated by shifts in 87Sr/86Sr whereas silicate weathering tends to drive riverine inputs to heavy δ88Sr values. In contrast to previous modelling of Sr isotopes in seawater, we can also model the effects of the carbonate output flux on the seawater composition. This output flux is isotopically light and mass-dependently related to seawater. Our initial data utilise foraminifera to reconstruct the recent (2.5 Ma) 87Sr/86Sr and δ88Sr seawater record and track changing input fluxes.