Alexander Heuser

Model for kinetic effects on calcium isotope fractionation (δ44Ca) in inorganic aragonite and cultured planktonic foraminifera

The calcium isotope ratios (δ44Ca = [(44Ca/40Ca)sample/(44Ca/40Ca)standard −1] · 1000) of Orbulina universa and of inorganically precipitated aragonite are positively correlated to temperature. The slopes of 0.019 and 0.015‰ °C−1, respectively, are a factor of 13 and 16 times smaller than the previously determined fractionation from a second foraminifera, Globigerinoides sacculifer, having a slope of about 0.24‰ °C−1. The observation that δ44Ca is positively correlated to temperature is opposite in sign to the oxygen isotopic fractionation (δ18O) in calcium carbonate (CaCO3). These observations are explained by a model which considers that Ca2+-ions forming ionic bonds are affected by kinetic fractionation only, whereas covalently bound atoms like oxygen are affected by kinetic and equilibrium fractionation. From thermodynamic consideration of kinetic isotope fractionation, it can be shown that the slope of the enrichment factor α(T) is mass-dependent. However, for O. universa and the inorganic precipitates, the calculated mass of about 520 ± 60 and 640 ± 70 amu (atomic mass units) is not compatible with the expected ion mass for 40Ca and 44Ca. To reconcile this discrepancy, we propose that Ca diffusion and δ44Ca isotope fractionation at liquid/solid transitions involves Ca2+-aquocomplexes (Ca[H2O]n2+ · mH2O) rather than pure Ca2+-ion diffusion. From our measurements we calculate that such a hypothesized Ca2+-aquocomplex correlates to a hydration number of up to 25 water molecules (490 amu). For O. universa we propose that their biologically mediated Ca isotope fractionation resembles fractionation during inorganic precipitation of CaCO3 in seawater. To explain the different Ca isotope fractionation in O. universa and in G. sacculifer, we suggest that the latter species actively dehydrates the Ca2+-aquocomplex before calcification takes place. The very different temperature response of Ca isotopes in the two species suggests that the use of δ44Ca as a temperature proxy will require careful study of species effects.

Separation of Mg, Ca and Fe from geological reference materials for stable isotope ratio analyses by MC-ICP-MS and double-spike TIMS

Near quantitative separation of analyte elements from the sample matrix is commonly required to obtain precise and accurate stable isotope data, especially if MC-ICP-MS is used in conjunction with the standard-sample bracketing (SSB) technique for mass bias drift correction. Here, we report a robust procedure that allows for the combined chemical separation of Mg, Ca and Fe, using ion-exchange columns that contain 1 ml AG50W-X8 (200–400 mesh) cation exchange resin. Magnesium was separated for isotope ratio analyses from many geological sample types by a single pass through this column. For Mg purification, Be, Ti, Mn, Fe and Al are selectively eluted using dilute HF and an acetone–HCl mixture. The separation of Mg from Ca-dominated carbonate samples and/or a combination of Mg, Ca and Fe separation from the same sample aliquot, is achieved by the adsorption of Ca and Fe onto the ion-exchange resin from 10 M HCl. Following purification, geological reference materials, including water, bone, carbonate and sediment samples, igneous and sedimentary rocks and a chondritic meteorite were analysed by MC-ICP-MS (Mg and Fe isotopes) and double-spike TIMS (Ca isotopes). Average external repeatabilities were ±0.16‰ for 26Mg/24Mg, ±0.26‰ for 44Ca/40Ca and ±0.05‰ for 56Fe/54Fe (2sd; n ≥ 5). Comparison with published data documents the accuracy of the results. For Mg isotope analyses using SSB-MC-ICP-MS, matrix-induced mass bias effects were studied using element additions. The artificial matrices left a memory in subsequent standard analyses, likely due to depositions on the cones. This observation allowed for the detection of matrix effects in unknown samples. Finally, the current status of Mg and Ca zero-delta reference materials is briefly discussed.

Calcium isotope (δ44/40Ca) variations of Neogene planktonic foraminifera

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.

A pilot study on the use of natural calcium isotope (44Ca/40Ca) fractionation in urine as a proxy for the human body calcium balance

We explored the possibility of using natural calcium (Ca) isotope variations in the urine (delta(44/40)Ca(urine)) as a proxy for the Ca balance in the human body. We chose two test persons extremely different in their health status, gender and age (4-year-old healthy boy and a 60-year-old woman known to suffer from osteoporosis). During a 5 day interval the Ca isotope composition of the individual diet (delta(44/40)Ca(diet)) was monitored for both test persons to be in general agreement to the Ca isotope composition of the normal western European diet ( approximately -1.02+/-0.1 per thousand). However, measurements showed that (1) delta(44/40)Ca(urine) of both test persons are approximately 1.37 and approximately 2.49 per thousand, respectively, heavier than delta(44/40)Ca(diet) and that (2) the delta(44/40)Ca(urine-boy) is approximately 1.1 per thousand heavier when compared to the value of the woman. The individual offset between diet and test persons is interpreted to reflect individual Ca reabsorption rates in the kidneys being the result of Rayleigh type Ca isotope fractionation related to the partitioning of Ca between the glomerular filtrate and filtered residue. The relative difference between delta(44/40)Ca(urine-boy) and delta(44/40)Ca(urine-woman) of approximately 1.1 per thousand may reflect individual differences in the balance of bone mineralization and demineralization processes related to age, gender and health status. By arbitrarily defining an equilibrium value for Delta(44/40)Ca(diet-urine) of -1.93 per thousand being the arithmetic mean of delta(44/40)Ca(urine) for both test persons the measured delta(44/40)Ca(urine) values may be applied to model the individual bone mineralization and demineralization processes in a qualitative way. Note, second order influences of intestinal Ca absorption during sequestration of Ca between intestine and blood have to be subject of further studies.

Noble metals potential of sulfide-saturated melts from the subcontinental lithosphere

The origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic, but ultimately linked to silicate-sulfide liquid immiscibility. The exact composition of pristine sulfide melts has proved extremely difficult to document and understand, largely because of the ephemeral, reactive qualities and small quantities of such melts. Here we report the discovery of Fe-Ni sulfide melt globules highly enriched in noble metals (Pt, Pd, Au; 120 ppm total platinum group elements [PGE]) within an unusual high-Mg andesitic glass (8.2 wt% MgO, 57.3 wt% SiO2) dredged from the southern Mid-Atlantic Ridge, near the Bouvet triple junction. The composition of this glass indicates derivation of its parental silicate melt from a garnet pyroxenite mantle source with pronounced “continental” isotopic (Pb, Sr, Nd, Hf, Os, O) signatures. We infer that the chemical properties of this high-temperature (1250 °C) melt, notably high SiO2 and Ni (310 ppm) contents, promoted sulfide saturation at low pressures in a purely oceanic setting, and propose that this unique example, with its likely origin in the continental lithospheric mantle, may be a useful analogue for incipient Ni-PGE-sulfide melt generation and magmatic PGE enrichment.

Biological fractionation of stable Ca isotopes in Göttingen minipigs as a physiological model for Ca homeostasis in humanS

ABSTRACT In order to investigate fractionation of calcium (Ca) isotopes in vertebrates as a diagnostic tool to detect Ca metabolism dysfunction we analyzed the Ca isotopic composition (δ44/40Ca = [(44Ca/40Ca)sample/(44Ca/40Ca)reference]−1) of diet, faeces, blood, bones and urine from Göttingen minipigs, an animal model for human physiology. Samples of three groups were investigated: 1. control group (Con), 2. group with glucocorticosteroid induced osteoporosis (GIO) and 3. group with Ca and vitamin D deficiency induced osteomalacia (−CaD). In contrast to Con and GIO whose average δ44/40Cafaeces values (0.39 ± 0.13‰ and 0.28 ± 0.08‰, respectively) tend to be lower than their diet (0.47 ± 0.02‰), δ44/40Cafaeces of −CaD (−0.27 ± 0.21‰) was significantly lower than their δ44/40Cadiet (0.37 ± 0.03‰), but also lower than δ44/40Cafaeces of Con and GIO. We suggest that the low δ44/40Cafaeces of −CaD might be due to the contribution of isotopically light Ca from gastrointestinal fluids during gut passage. Assuming that this endogenous Ca source is a common physiologic feature, a fractionation during Ca absorption is also required for explaining δ44/40Cafaeces of Con and GIO. The δ44/40Caurine of all groups are high (>2.0‰) reflecting preferential renal reabsorption of light Ca isotopes. In Göttingen minipigs we found a Ca isotope fractionation between blood and bones (Δ44/40Cablood-bone) of 0.68 ± 0.15‰.

Tracing copper derived from pig manure in calcareous soils and soil leachates by 65Cu labeling.

Copper is used as a growth promoter in animal husbandry, resulting in high Cu concentrations in animal manure. We tested whether Cu would be mobilized in soils receiving excessive loads of manure, both from recently added and from aged fractions. To discriminate between these Cu sources, manure was labeled with (65)Cu. After soil application of 0, 15, and 30 Mg manure ha(-1), leachate was collected in free-draining lysimeters (40 cm depth) under undisturbed soil over a 53 day period. Determining the total amounts of Cu and the fractions of (65)Cu in leachate and the soil profile enabled us to trace the translocation of Cu derived from labeled manure. More than 84% of the applied Cu was retained in the top 2 cm of soil. Less than 0.01% of the applied Cu was detected overall in the leachate. Of this amount, however, 38% (± 8.9 SE) was leached within 8 days after application. The total Cu concentration in leachates (32-164 μg L(-1)) frequently exceeded the Chinese groundwater quality standard of 50 μg L(-1). The added (65)Cu, however, accounted for less than 3.6% of the total Cu leaching load, suggesting that Cu from older sources and/or geological background controls contamination, regardless of current land management.

Biomedical Application of Ca Stable Isotopes

This chapter summarizes the current knowledge on the Ca isotope composition of different tissues and Ca isotope cycling in the human and vertebrate body. The known sites of Ca isotope fractionation and the calcium isotopic composition of different compartments and body fluids are discussed. The first part of this chapter follows the journey of Ca in the body from the input (diet) to the output (urine, feces). Finally, current biomedical applications of Ca isotopes for the detection of bone loss during bed rest studies and during bone cancer are presented.

Biominerals and Biomaterial

Biominerals are important archives for various paleo-environmental proxies, and consequently, the understanding of biomineralisation related element and isotope fractionation is vital for reliable climate reconstructions. The Ca isotope composition of biominerals has been investigated to explore its potential for paleo-environmental reconstructions and to better understand biomineralisation processes. The overall range of Ca isotope fractionation reported for biominerals resembles that of inorganic minerals, but shows different responses to changes of environmental parameters such as fluid composition and growth rates. In this chapter, we review Ca isotope fractionation characteristics of biominerals from different taxa with respect to biomineralisation processes and potential proxy applications. Available data of alkaline earth metal isotope systems (Mg, Sr) are included for comparison.

The Schwarzhorn Amphibolite (Eastern Rätikon, Austria): an Early Cambrian intrusion in the Lower Austroalpine basement

Abstract The Alpine nappe stack in the Penninic-Austroalpine boundary zone in the Rätikon (Austria) contains a 4×1 km tectonic sliver of meta-diorite, known as the Schwarzhorn Amphibolite. It was deformed and metamorphosed in the amphibolite facies and is unconformably overlain by unmetamorphic Lower Triassic sandstone, indicating pre-Triassic metamorphism. Cataclastic deformation and brecciation of the amphibolite is related to normal faulting and block tilting during Jurassic rifting. Zircon dating of the Schwarzhorn Amphibolite using LA-ICP-MS gave a U-Pb age of 529+9/-8 Ma, interpreted as the crystallization age of the protolith. Geochemical characteristics indicate formation of the magmatic protolith in a supra-subduction zone setting. The Cambrian protolith age identifies the Schwarzhorn Amphibolite as a pre-Variscan element within the Austroalpine basement. Similar calc-alkaline igneous rocks of Late Neoproterozoic to Early Cambrian age are found in the Upper Austroalpine Silvretta Nappe nearby and in several other Variscan basement units of the Alps, interpreted to have formed in a peri-Gondwanan active-margin or island-arc setting.