B11C-0371
Mg Isotope Evolution During Water-Rock Interaction in a Carbonate Aquifer
To better understand how Mg isotopes behave during weathering and aqueous transport, we used a Nu Plasma MC-ICP-MS to measure δ26Mg values (relative to DSM-3) in water samples along a 236 km flow path in the Madison aquifer of South Dakota, a confined carbonate aquifer recharging in the igneous Black Hills. We also analyzed local granite and dolomite samples to characterize the Mg isotope composition of source rocks constituting the recharge zone and aquifer, respectively. Repeated analyses of Mg standard solutions yielded external precisions (2σ) better than 0.1 permil for δ26Mg(CAM-1, - 2.584±0.071, n=13; UIMg-1, -2.217±0.087, n=9.). The Madison aquifer provides a unique opportunity to quantify Mg isotope effects during water-rock interaction because (1) fluids and rock have chemically equilibrated over a much longer timescale (up to ~15 kyr) than can be simulated in laboratory experiments and (2) previous studies have determined the rates and mass-balances of de- dolomitization and other geochemical reactions controlling solute evolution along the flow path. Reactions important for changing the concentration and isotope composition of Mg include dolomite dissolution, Mg-for- Na ion exchange, calcite precipitation, and isotope exchange. δ26Mg values within the recharge region (0-17 km along flow path) vary between -1.08 and -1.63 permil, and then remain essentially constant at -1.408±0.010 permil(1σ, 5 samples) from 17 to 189 km. A final sample at 236 km shows an increase to -1.09 permil. Either mixing between different recharge waters or rapid isotope exchange between infiltrating waters and dolomite could control δ26Mg variability between 0 and 17 km. Likewise, reactive transport modeling suggests that preferential uptake of 24Mg during Mg-for-Na ion exchange might cause an increase in δ26Mg between 189 and 236 km. However, unchanging δ26Mg values observed throughout most of the aquifer clearly demonstrate that Mg isotopes are not fractionated during reactive transport. This suggests that Mg isotopes can conservatively trace weathering inputs and groundwater flow in dolomite-rich aquifers.
B11C-0372
Tracing mineral weathering reactions in the critical zone using Mg, Ca, and Sr isotopes, Luquillo Mountains, Puerto Rico
Mineral weathering in the critical zone directly impacts the availability of many important soil nutrients. As part of the USGS Water Energy and Biogeochemical Budgets (WEBB) program and the Critical Zone Exploration Network, we are investigating mineral nutrient distributions and fluxes in depth profiles (to 16 m) at five sites in the Bisley 1 catchment in the Luquillo Mountains of Puerto Rico. The Bisley 1 catchment contains a thick regolith developed on marine bedded, andesitic, volcaniclastic bedrock. Pore waters were sampled as a function of depth from nested suction water samplers. Pore water chemistry was analyzed and compared to total chemistry of solid samples taken from augered cores. Mg, Ca and Sr isotope ratios were measured of the pore waters at the Institut de Physique du Globe de Paris (Mg) and at the USGS in Menlo Park, CA (Ca, Sr). The Mg isotope ratios increase with increasing depth from δ26Mg = -0.772 at the surface to - 0.267 at depth, relative to the DSM3 standard. Sr isotope ratios vary from 0.70922 to 0.71016 87Sr/86Sr, with no discernible depth trend. The regolith is highly weathered and is depleted in primary minerals (except quartz) with respect to bedrock. Volumetric strain, calculated with respect to quartz, indicates approximately 25% volume collapse occurred relative to the original volume of the bedrock. Plagioclase, chlorite, pyroxene, and amphibole weather at the bedrock-regolith interface. The regolith contains quartz, kaolinite, other clays, and iron and manganese oxides. Increasing solid and pore water Mg concentrations and δ26Mg with depth likely indicate a two step weathering process wherein high-Mg chlorite dissolves at the bedrock-regolith interface and forms Mg-containing secondary clays and oxides, which then dissolve within the regolith profile.
B11C-0373
Foliar nutrients and Sr isotopes along a climate gradient on Mt. Haleakala, Maui
On the northeast, windward side of Mt. Haleakala, Maui, a trade-wind inversion layer at 1900 m controls the uppermost extent of a tropical montane cloud forest. Treeline marks the boundary between the cool and moist cloud forest below and the drier and colder locations above. Within the cloud forest, Metrosideros polymorpha is the dominant canopy tree. Above treeline, the predominant species are a xerophytic shrub and a non-native grass; M. polymorpha exists only as isolated individuals within the drier, cooler shrub and grasslands. We sampled surface soils and vegetation of the three species along four elevational transects to observe how nutrient availabilities and sources change across treeline. Transects were comprised of locations within the upper extent of the cloud forest, at treeline, and in the shrub and grassland communities. Soils were analyzed for C, N, net N mineralization rates, and plant-available P. Foliar samples were analyzed for C, N, P, base cations, micronutrients, and the isotopic ratio of 87Sr/86Sr. Total soil N and net mineralization rates decrease with increasing elevation across treeline. N content of the grass and shrub reflect soil characteristics and also significantly decrease above treeline, although this trend is not observed in M. polymorpha. Preliminary isotopic results suggest for all three species a change in source of nutrients from predominately rock-derived in the upper extent of the cloud forest to partially precipitation- derived in the shrub and grasslands.
B11C-0374
Improvements in Precise and Accurate Isotope Ratio Determination via LA-MC-ICP-MS by Application of an Alternative Data Reduction Protocol
An alternative approach for the evaluation of isotope ratio data using LA-MC-ICP-MS will be presented. In contrast to previously applied methods it is based on the simultaneous responses of all analyte isotopes of interest and the relevant interferences without performing a conventional background correction. Significant improvements in precision and accuracy can be achieved when applying this new method and will be discussed based on the results of two first methodical applications: a) radiogenic and stable Sr isotopes in carbonates b) stable chlorine isotopes of pyrohydrolytic extracts. In carbonates an external reproducibility of the 87Sr/86Sr ratios of about 19 ppm (RSD) was achieved, an improvement of about a factor of 5. For recent and sub-recent marine carbonates a mean radiogenic strontium isotope ratio 87Sr/86Sr of 0.709170±0.000007 (2SE) was determined, which agrees well with the value of 0.7091741±0.0000024 (2SE) reported for modern sea water [1,2]. Stable chlorine isotope ratios were determined ablating pyrohydrolytic extracts with a reproducibility of about 0.05‰ (RSD). For basaltic reference material JB1a and JB2 chlorine isotope ratios were determined relative to SMOC (standard mean ocean chlorinity) δ37ClJB-1a = (-0.99±0.06) ‰ and δ37ClJB-1a = (-0.60±0.03) ‰ (SD), respectively, in accordance with published data [3]. The described strategies for data reduction are considered to be generally applicable for all isotope ratio measurements using LA-MC-ICP-MS. [1] J.M. McArthur, D. Rio, F. Massari, D. Castradori, T.R. Bailey, M. Thirlwall, S. Houghton, Palaeogeo. Palaeoclim. Palaeoeco., 2006, 242 (126), doi: 10.1016/j.palaeo.2006.06.004 [2] J. Fietzke, V. Liebetrau, D. Guenther, K. Guers, K. Hametner, K. Zumholz, T.H. Hansteen and A. Eisenhauer, J. Anal. At. Spectrom., 2008, 23, 955-961, doi:10.1039/B717706B [3] J. Fietzke, M. Frische, T.H. Hansteen and A. Eisenhauer, J. Anal. At. Spectrom., 2008, 23, 769-772, doi:10.1039/B718597A
B11C-0375
Design and Application of a Sr-84/Sr-87-Double Spike to Determine Natural Strontium Isotope Fractionation in Carbonates and Silicates
In order to precisely determine 88Sr/86Sr- and 87Sr/86Sr-isotope variations in natural
samples using TIMS-technique we developed a mixed 87Sr/84Sr-double spike from two solutions
enriched in 84Sr and 87Sr, respectively. After mixing the two solutions the Sr-spike ratios have
precisely been determined by calibration to the NBS 987 standard. For the determination of natural
88Sr/86Sr- and 87Sr/86Sr-isotope variations in carbonates and silicates two TIMS
measurements are required: an unspiked and a spiked run where the Sr-isotope ratios are arbitrarily
normalized to a fixed Sr isotope ratio (e.g. mean of the first block). For denormalization and data reduction we
adopted the algorithm for Ca isotope measurements (1) presented earlier by Heuser et al.(2003) modified
for Sr-isotope measurements. It was found that best results can be achieved if the
84Srspike/84Srsample ratio is higher than about 12. The algorithm allows the
simultaneous calculation of 87Sr/86Sr and 88Sr/86Sr ratios. Standard measurements
showed a δ88/86Sr-value
(δ88/86Sr=((88Sr/86Sr)Sample/(88Sr/86Sr)NBS 987)-1)*1000) of
0.39 for the IAPSO seawater standard corresponding to an external reproducibility of ±0.02 (n=12). The
IAPSO δ88/86Sr-value corresponds to a 87Sr/86Sr-ratio of 0.709285(6). Both values
are in accordance with earlier publications (2) and theoretical predictions based on the δ88/86Sr
ratio of seawater and assuming mass-dependent isotope fractionation. Preliminary application of the Sr-
double spike to carbonate samples of the Phanerocoic indicate unexpected δ88/86Sr variations
in the order of about 0.2 to 0.3 ‰ which indicate varying supply of Sr from isotopically distinctively
different sources. Furthermore a direct comparison of double spike TIMS, bracketing standard and laser-
ablation MC-ICP-MS (3) results are in agreement and can be used to discuss limitation and perspectives of
future Sr isotope measurements.
References:
1 Heuser A., Eisenhauer A., Gussone N., Bock B., Hansen B.T. and Nägler Th.F. (2002) Measurement of
Calcium Isotopes (δ 44Ca) Using a Multicollector TIMS Technique. International Journal of Mass
Spectrometry 220, 385-397.
2 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.
3. J. Fietzke, V. Liebetrau, D. Günther, K. Gürs, K. Hametner, K. Zumholz, T. H. Hansteen and A.
Eisenhauer (2008) An alternative data acquisition and evaluation strategy for improved isotope
ratio precision using LA-MC-ICP-MS applied to stable and radiogenic
strontium isotopes in carbonates. Journal of Analytical Atomic Spectrometry (JAAS), doi: 10.1039/b717706b
B11C-0376
A change of Sr cycle in the Ediacaran Ocean: Evidence from radiogenic and stable isotope ratios of Sr, in Three Gorges, South China.
Objective. To decode surface environmental changes and patterns of biological evolution during the Ediacaran Methods employed. We undertook deep drilling in Three Gorges area in South China to obtain continuous and fresh samples without surface alteration and oxidation. 87Sr/86Sr and 88Sr/86Sr ratios of the fresh carbonate rocks were measured with multiple collector-inductively coupled plasma-mass spectrometric techniques. We discuss the surface environmental change in the Ediacaran by comparing the Sr isotope ratios with 13C/12C and 18O/16O. Result. The chemostratigraphy of 87Sr/86Sr ratios of the drilled samples displays a smooth curve and two large positive shifts in the Ediacaran. The combination of the detailed chemostratigraphies of 13C/12C, 18O/16O, and 87Sr/86Sr enable us to find connections among them and indicates that the first large positive shift of 87Sr/86Sr is slightly preceded by negative 13C/12C and positive 18O/16O excursions. The second large positive shift of 87Sr/86Sr is simultaneous with positive 13C/12C shift and clearly precedes next negative 13C/12C excursion. Considering Mn content and Fe content of carbonate rocks, which respond to redox in the ocean, the first positive shift can be explained by Gaskiers glaciation. Exposed surface of continental crust were increased by regression accompanied by Gaskiers glaciation and enhanced weathering rate by rivers increased seawater 87Sr/86Sr. The second positive shift of 87Sr/86Sr is a long-term fluctuation. We speculate enhanced weathering rate, resulting from convergence of Gondwana supercontinent, as a cause of the second positive shift of 87Sr/86Sr. Chemostratigraphy of 88Sr/86Sr also displays a smooth curve. Before Gaskiers glaciation, 88Sr/86Sr curve have a negative correlation with 87Sr/86Sr fluctuation. We interpret that this negative correlation is explained by mass-dependent fractionation. When Sr are increased in ocean (high 87Sr/86Sr), 86Sr are selectively taken in carbonate (low 88Sr/86Sr). However, after the Gaskiers glaciation, the 88Sr/86Sr are high although 87Sr/86Sr ratios keep high values. This is not explained by only mass-dependent fractionation. It implies that output of Sr from the ocean always exceeds input of the Sr from continental weathering. We found a change of the Sr cycle in the ocean in Gaskiers glaciation or thereabout.
B11C-0377
Mass-dependent isotopic fractionation and radiogenic isotope variation of Strontium in the Neoproterozoic Doushantuo Formation
We measured both mass-dependent isotope fractionation of delta88Sr (88Sr/86Sr) and radiogenic isotopic variation of Sr (87Sr/86Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The delta88Sr and 87Sr/86Sr compositions showed three remarkable characteristics: (1) high radiogenic 87Sr/86Sr values and gradual decrease in the 87Sr/86Sr ratios, (2) anomalously low delta88Sr values at the lower part cap carbonate, and (3) a clear correlation between 87Sr/86Sr and delta88Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high 87Sr/86Sr and lighter delta88Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO2 content caused sudden precipitation of cap carbonate from the surface seawater with high 87Sr/86Sr and lighter delta88Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal delta88Sr ratios, probably caused gradual decrease of the 87Sr/86Sr ratios of the seawater and deposition of carbonate with normal delta88Sr ratios. The combination of 87Sr/86Sr and delta88Sr isotope systematics gives us new insights on the surface environment.
B11C-0378
Strontium stable isotope variations and the marine strontium cycle
Coupled stable and radiogenic strontium (Sr) isotopes potentially provide key information on the present-day Sr cycle, on variations in the flux and composition of material delivered by continental weathering and hydrothermal exchange at mid-ocean ridges, and on changes in carbonate productivity over time. This study presents high-precision stable (and radiogenic) Sr isotope data (obtained using double-spike TIMs) for rivers, MOR hydrothermal fluids, seawater and preliminary data for a Quaternary marine foraminiferal record. Riverine data, for both basaltic and granitic catchments, indicates that a significant fractionation of Sr stable isotopes may occur during weathering. This fractionation is largely due to the formation of secondary weathering minerals, and results in Sr stable isotope compositions that are both lighter and heavier than seawater, with an overall range of some 1 per mil. High-temperature hydrothermal fluids are slightly lighter than seawater, conistent with mixing between MOR basalt/peridotite sourced fluids and ambient seawater. Data for a Quaternary marine foraminiferal record indicate a very small shift in the Sr stable isotope composition of seawater over this interval, which has implications for changes in the flux and composition of material delivered by continental weathering. These results indicate that, while chronostratigraphy based on shifts in the normalised radiogenic Sr isotope ratio remains a robust technique, information on Sr fluxes into and out of the oceans, and their variation over time (i.e. the cause of the radiogenic Sr variations) are only revealed through reconstruction of both stable and radiogenic marine Sr records.
B11C-0379
Calcium Isotope Systematics of Diagenetically Altered Carbonates: Example from the Proterozoic Carbonates of Transvaal Supergroup, South Africa
We analyzed mass-dependent (δ44/40Ca) and radiogenic (εCa) calcium isotope variations of diagenetically altered carbonates collected from the Duitschland Formation (~2.45 Ga) of the Transvaal Supergroup in a vicinity of the younger Bushveld Igneous Complex (Frauenstein, 2005, PhD Thesis, Ruhr Univ. Bochum). Textural, trace element and isotope data measured on these samples provide convincing evidence for extensive post-depositional alteration and diagenetic resetting. Samples selected for the Ca isotope study have Mn/Sr ratios from 0.8 to 33, 87Sr/86Sr from 0.704 to 0.719 and their δ18O and δ18C scatter from -20 to -2.8‰ and from 9.7 to -1.1‰, respectively. The δ44/40Ca (NIST) of carbonates range from 0.3 to 1.3‰ and their εCa indicate no radiogenic 40Ca excesses larger than the analytical uncertainty of ~1.5 ε-unit, confirming that the δ44/40Ca variation is exclusively due to mass-dependent fractionation. There is a difference between δ44/40Ca of limestones and dolostones, the former range from ~0.3 to 1.2‰ and the latter cluster tightly around 1.1‰. Using Mn/Sr as an index for diagenetic alteration (Brand and Veizer, 1980, J. Sed. Petrol., 50, 1219-1236) the δ44/40Ca of limestones becomes progressively heavier with an increasing degree of alteration (δ44/40Ca vs. Mn/Sr, r = .84, p < .01). The δ44/40Ca and Mn/Sr of dolostones plot in the area occupied by 'most altered' limestones, perhaps suggesting that these dolomites might be of diagenetic origin. The best preserved or 'least altered' limestone with the lowest Mn/Sr and δ44/40Ca show also the least radiogenic 87Sr/86Sr of 0.7042 that is in a range of the accepted Sr isotope composition of coeval seawater (Shield and Veizer, 2002, Geochem., Geophys., Geosys., 3, 1-12). The slope of δ44/40Ca vs. Sr content in studies carbonates of about -0.00077 is very close to the slope documented for diagenetically altered skeletal carbonates (-0.00085; Steuber and Buhl, 2006, Geochim. Cosmochim. Acta, 70, 5507-5521) or kinetic precipitates (-0.00094; Tang et al., 2008, Geochim. Cosmochim. Acta, 72, 3733-3745), yet it is much steeper compared to that of well-preserved Phanerozoic brachiopods (-0.00028; Farkas et al., 2008 Geochim. Cosmochim. Acta, 71, 5117-5134). Thus, care should be exercised when interpreting δ44/40Ca data of ancient marine carbonates in terms of paleo-seawater composition and they should always be reported with supporting trace element data. Finally, we propose that in a suite of coeval marine limestones, samples with the lowest δ44/40Ca, Mn/Sr and 87Sr/86Sr should, in most cases, represent the least altered components.
B11C-0380
Interpreting the Marine Calcium Isotope Record: Influence of Reef Builders
The calcium isotopic composition of seawater as recorded in brachiopod shells varied substantially during the Paleozoic (Farkas et al. 2007, Geochim. Cosmochim. Acta, 71, 5117-5134). The most prominent feature of the record is an excursion to higher 44Ca/40Ca values that started during the Early Carboniferous and lasted until the Permian. The shift occurred shortly after the transition from a calcite-sea to an aragonite-sea (Sandberg 1983, Nature 305, 19-22; Stanley and Hardie 1998, Pal3, 144, 3-19). It therefore has been interpreted to reflect a change in the average calcium isotope fractionation of carbonates produced in the oceans. Aragonite is depleted by about 0.6 permil in 44Ca/40Ca compared to calcite (Gussone et al. 2005, Geochim. Cosmochim. Acta, 69, 4485-4494). Consequently a transient shift from calcite dominated to an aragonite dominated calcium carbonate sedimentation could have caused the observed 0.5 permil isotope shift. We compare the marine calcium isotope record with a new compilation of the Phanerozoic trends in the skeletal mineralogy of marine invertebrates (Kiessling et al. 2008, Nature Geoscience, 1, 527-530). The compilation is based on data collected in the PaleoReef database and the Paleobiology Database, which include information on Phanerozoic reef complexes and taxonomic collection data of Phanerozoic biota, respectively. We find a strong positive correlation between the calcium isotope ratios and the abundance of aragonitic reef builders from the Silurian until the Permian at a sample resolution of about 10 million years. The two records, however, diverge in the Triassic, when reefs were dominated by aragonite but the calcium isotope values remained at a relatively low level. We also find a good correlation between calcium isotopes and the proportion of aragonite in the general record of Phanerozoic biota. However, in this case the records start to diverge already in the latest Carboniferous. The observations suggest that the Paleozoic calcium isotope record was indeed strongly controlled by the skeletal mineralogy of the major carbonate producers. The collapse of the correlation in the late Paleozoic points to the evolution of new biocalcification mechanisms that allowed a more strict biological control of the calcium fluxes in the calcifying biota (sensu Gussone et al. 2006, Geology, 34, 625-628).
B11C-0381
Calcium Isotope Fractionation in mid-ocean ridge hydrothermal systems
The purpose of this study is to investigate Ca isotope fractionation during hydrothermal alteration of mid- ocean ridge basalts. Both high and low temperature hydrothermal fluids are enriched in calcium relative to seawater reflecting its derivation from the ocean crust. Indeed, hydrothermal alteration at ridge-crests and at ridge-flanks provides significant amounts of Ca and affects the Ca isotopic composition of the oceans. It is commonly assumed that there is no significant fractionation of Ca isotopes between the oceanic crust and hydrothermal fluids at the high temperatures of water-rock interaction. But this assumption was recently challenged by Amini et al. (2008 GCA), who showed that Ca isotope fractionation occurs during hydrothermal anhydrite precipitation. In fact, there are numerous reasons to suspect that Ca isotope fractionation might occur along the hydrothermal flow path. Ca is transported as a chloro-complex in vent fluids, its concentration being affected by leaching and/or partial dissolution of silicates, precipitation of secondary sulfate and silicate mineral phases and phase separation. In ridge-crest high temperature fluids the Ca concentration is governed by the extent to which vapor and brine phases have been separated and by equilibration with secondary mineral phases (mainly clinozoisitic epidote and albite-anorthite). In contrast, in ridge-flank low temperature hydrothermal fluids the Ca concentration is affected by the extent to which it is leached from basalt via Mg fixation. Both equilibrium and/or kinetic isotope fractionation effects could be expected in the high temperature fluids. The direction and magnitude of fractionation is difficult to predict a priori. On the other hand, incomplete leaching reactions occurring at low temperatures are likely to be kinetic in nature and may produce hydrothermal fluids isotopically lighter than the basalts. The measured δ44CaSW values for the high temperature fluids collected along the Juan de Fuca Ridge are distinctly low and range from -0.88 to -1.04 permil. When corrected for seawater contribution the δ44Ca values for the high temperature fluids range from -1.05 to -1.66 permil. The measured δ44CaSW values for the ridge-flank low temperature fluids collected from ODP 1026B and Baby Bare range from -0.79 to -0.87 permil; when corrected for seawater contribution these values range from -0.97 to -1.03 permil. It is evident from the data that Ca isotopes do fractionate during the high temperature hydrothermal alteration of MORB (δ44CaSW = -1.0 permil) and that the resulting fluids are lighter than the basalts by as much as 0.66 permil. Surprisingly, the low temperature fluids do not show any fractionation of Ca isotopes.
B11C-0382
Controls on Calcium Isotope Fractionation in Cultured Foraminifera
Calcium isotopes have recently emerged as an important tool to study the biomineralization pathways and processes in foraminifera. We analyzed calcium isotopes in planktonic and benthic foraminifera grown under controlled laboratory conditions at different salinity, temperature and pH values. Our results indicate that calcium isotope fractionation in foraminifera is controlled by more than one environmental parameter, requiring a common mechanism to explain the observed trends. There is a significant negative correlation between calcium isotope fractionation and the distribution coefficient of strontium in planktonic foraminifera, which has the same slope (within error) as that in inorganic calcite (Tang et al, 2008). In analogy to these inorganic experiments, calcium isotopic fractionation and Sr/Ca ratios in planktonic foraminifera appear to be mainly controlled by the precipitation rate. However, the two regressions (inorganic vs. foraminiferal) have a small but constant offset from each other by about 0.2 permil in delta(44Ca/40Ca) for a given D(Sr). This offset is presumably due to a vital effect that can be modeled via Rayleigh distillation from an internal biomineralization reservoir (Elderfield et al., 1996). Our preliminary results suggest that such a reservoir behaves as a semi-open system, wherein only less than 25 percent of the calcium taken up from seawater is being utilized for calcification. Elderfield, H., Bertram, C.J., Erez, J. 1996. A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate. Earth Planet. Sci. Lett., 142:409-423. Tang J., Dietzel M., Boehm F., Koehler S.J., Eisenhauer A. 2008. Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochim. Cosmochim. Acta, 72:3733-3745.
B11C-0383
Intra Shell Variability Of Trace Element Distributions In Planktic Foraminifera From Plankton Tows And Sediment Traps
Trace element incorporation into foraminiferal carbonate is controlled by physical and chemical conditions in the marine environment and can thus be used to reconstruct past changes in these conditions over time. To better understand the incorporation of trace elements we have performed analysis on Globigerina bulloides and Globorotalia inflata using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Tests were collected from the upper water column off northwest Africa by depth stratified plankton tows and by sediment traps from the same station, which allows for direct comparison of trace element to Ca ratios with in-situ environmental conditions (e.g. temperature or carbonate ion concentration). Our study documents the Mg/Ca and Sr/Ca composition and variability during the foraminiferal descent through the water column, i.e. within the calcification depth habitat. Depth profiles through the chamber walls show a large variability within individual chambers, and within the test as a whole, especially for Mg/Ca. For both species, average ratios tend to increase from juvenile chambers towards the final chambers, which, for the species G. inflata is difficult to reconcile with the vertical migration to deeper levels in the water column during its life cycle as indicated by oxygen isotopes. We further discuss the differences between upper water column ratios from plankton tows and the corresponding sediment trap samples, as well as the observed trends in the trace element composition and shell size.
B11C-0384
Characteristics (Delta44/40Ca, Mg/Ca and Sr/Ca) of Mytilus edulis and Arctica islandica Shells formed in a Temperature-Salinity Matrix
We investigated the influence of temperature (5°C to 15°C (A. islandica) resp. 25°C (M. edulis)) and salinity (15 to 35 psu) regimes on the calcium (Ca) isotope fractionation (Δ44/40Ca) and on Mg/Ca and Sr/Ca in cultured bivalves (Mytilus edulis and Arctica islandica). In an orthogonal 2-factorial (temperature vs. salinity) experiment, the bivalves were allowed to grow for 15 weeks under tightly controlled conditions and then probed and analysed by thermal ionisation mass spectrometry (TIMS) and optical emission spectrometry (ICP-OES). Several interactions between the factors temperature and salinity with respect to their influence on bivalve shell parameters could be found. However, with the exception of Sr/Ca data, the variation of measured shell characteristics between individual bivalves was high. The Sr/Ca - salinity proxy seems to be the most reliable (linear. regression, M. edulis: Sr/Ca = -0.0283*sal + 1.7967, R2 = 0.81, p < 0.001), even though, in A. islandica shells, it can be blurred by temperature effects at low salinities (lin. regr. at 5°C: p > 0.05, at 10°C: Sr/Ca = - 0.061*sal + 3.13, R2 = 0.93, p < 0.001, at 15°C: Sr/Ca = -0.066*sal + 3.34, R2 = 0.92, p < 0.001). In M. edulis shells Mg/Ca ratios related well with seawater temperature (lin. regr.: Mg/Ca = 0.642*t - 0.107, R2 = 0.81, p < 0.001). Ca isotope ratios only in A. islandica related significantly with temperature (lin. regr.: Δ44/40Ca = 0.016*t – 1.26, R2 = 0.29, p < 0.01) but due to an interacting salinity effect at 10°C (lin. regr.: Δ44/40Ca = -0.0148*sal – 0.738, R2 = 0.62, p = 0.002) the temperature model can only explain a limited part of the variation. Overall, the calcitic shells of M. edulis appear to provide the better element ratio proxies (Mg/Ca for temperature and Sr/Ca for salinity) and the aragonitic shells of A. islandica have the better Ca isotope - temperature proxy.
B11C-0385
Precise Determination of Ca Isotopic Compositions in Seawater and Carbonates
Multi-collector ICP-MS (MC-ICP-MS) has been recently used for studying the natural variation of calcium isotopic compositions in the geological materials. In order to evaluate such small variations, a fast and high- precision analytical technique for the Ca isotope has been developed by using high-resolution MC-ICP-MS (Neptune, Thermo Fisher) coupled with a desolvation system (APEX-IR, ESI). Two-steps separation technique (Bio-Rad AG50W-X8 and Elchrom Sr-spec) has been successfully applied to separate matrix component from analyzed samples to avoid spectral interferences. The external reproducibility for δ44/42Ca analysis is better than 0.07‰ (2σ) using a standard-sample bracketing method. Carbonates obtained from inorganic precipitation experiments display a rather small temperature- dependent fractionation at 5-40 °C (0.002 and 0.001‰/C for aragonite and calcite, respectively). Seawaters collected from South China Sea (SCS) displays a small but significant vertical variation in δ44/42Ca. Further investigation is focused to evaluate potential controlling mechanism in affecting Ca isotopes in seawater.
B11C-0386
No Impact of Ionic Strength/Salinity on Divalent Cation Fractionation During Inorganic Calcite Precipitation
In order to apply divalent cation fractionation (44Ca/40Ca, Sr/Ca, etc.) during calcium carbonate formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, the CO2 Diffusion Technique (Dietzel et al., 2004, Chem. Geol. 203, 139) was used to crystallize inorganic calcite from aqueous solutions at different ionic strength/salinity by the addition of NaCl. Results show that at 25°C the discrimination of Sr/Ca in the precipitated calcite is controlled by the precipitation rate (R in ìmol/m2/h). The apparent Sr distribution coefficients (log DSr) of calcite are positive linear related to the log R values. However, the data points of log DSr vs. log R fall in the same line for all experiments done at I = 0.0353, 0.292, and 0.8353, respectively. This indicates that Sr/Ca fractionation during inorganic calcite formation is not significantly influenced by ionic strength/salinity. In analogy to Sr/Ca, the 44Ca/40Ca fractionation is not influenced by ionic strength/salinity either. At 25°C the calcium isotope fractionation between calcite and aqueous calcium ions (Δ44/40Ca[calcite-aq] = δ44/40Ca[calcite] - δ44/40Ca[aq]) correlates negatively with log R values for all the experiments, independent of the respective I value. Tang et al. (2008, Geochim. Cosmochim. Acta 72, 3733) reported a negative linear relationship between Δ44/40Ca[calcite-aq] and log DSr during calcite formation, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)aq ratio. The results of our recent experiments at 25°C and various I values indicate that this general behavior is not affected by ionic strength/salinity.
B11C-0387
Lithium Isotopes in Coral Skeletons
The isotopic composition of Li that gets incorporated into marine carbonates has the potential to be a proxy for the amount of Li weathered out of continental rocks and transported into the oceans. Despite this potential to become a proxy for the rate of continental weathering throughout a significant part of geologic time, only a few studies have reported Li isotopic compositions in carbonates and only foraminifera have been used in attempts to reconstruct the paleo-δ7Li of the oceans. Coral skeletal trace-element and stable isotopic compositions have been used extensively to reconstruct the climate variability of the recent past, with mixed results. So far, however, the aragonitic coral skeletons were not used systematically for paleo-δ7Li reconstructions. As a consequence, very little is known about their Li isotopic composition, the effect of diagenesis, and whether corals might provide an accurate proxy for the evolution of oceanic δ7Li. Using the CRPG-CNRS Cameca 1270 ion microprobe (SIMS), we have measured lithium isotopic compositions (7Li/6Li) in shallow-water and deep-sea corals (Porites lutea, Cladocora caespitosa, Lophelia pertusa and Desmophyllum cristagalli). Cladocora caespitosa samples were grown n laboratory under two different pCO2: 416 ∓ 29 ppm and 729 ∓ 30 ppm. We observe no relation between the Li isotopic compositions of corals and ultrastructure, pH, nor pCO2, within error bars. Nevertheless, in this study, we observed a systematic difference between deep-sea azooxanthellate corals and shallow-water zooxanthellate corals. The lack of pH, pCO2 and ultrastructure dependence of Li isotope fractionation in corals indicates that they could potentially be a proxy for reconstruction of paleo-δ7Li of seawater and continental weathering.