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Papers by Sylvia Riechelmann

Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are of... more Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.

ABSTRACT The Magnesium isotope proxy in Earth surface research is still underexplored. Recently, ... more ABSTRACT The Magnesium isotope proxy in Earth surface research is still underexplored. Recently, field and laboratory experiments have shed light on the complex suite of processes affecting Mg isotope fractionation in continental weathering systems. Magnesium-isotope fractionation in speleothems depends on a series of factors including biogenic activity and composition of soils, mineralogy of hostrock, changes in silicate versus carbonate weathering ratios, water residence time in the soil and hostrock and disequilibrium factors such as the precipitation rate of calcite in speleothems. Furthermore, the silicate (here mainly Mg-bearing clays) versus carbonate weathering ratio depends on air temperature and rainfall amount, also influencing the soil biogenic activity. It must be emphasized that carbonate weathering is generally dominant, but under increasingly warm and more arid climate conditions, silicate weathering rates increase and release 26Mg-enriched isotopes to the soil water. Furthermore, as shown in laboratory experiments, increasing calcite precipitation rates lead to elevated delta26Mg ratios and vice versa. Here, data from six stalagmite time-series Mg-isotope records (Thermo Fisher Scientific Neptune MC-ICP-MS) are shown. Stalagmites from caves in Morocco, Germany and Peru are presented. The lowest mean Mg-isotope compositions are found in two Pleistocene Moroccan stalagmites (delta26Mg: -4.26 ± 0.07‰ and -4.17 ± 0.15‰). The cyclical shifts in both stalagmites are best explained by periods of increasing and decreasing aridity. In contrast, Holocene Peruvian stalagmites (0 to 14 ka) show a high mean delta26Mg-value of -3.96 ± 0.04‰ and a very low level of variability in time. This is probably due to the equatorial climate lacking significant variations in temperature and/or rainfall amount. Changes in precipitation rate show effects in stalagmites from western Germany and Peru resulting in a small variability exceeding the error of the delta26Mg-values. Stalagmites from Western Germany (BU 4 mean delta26Mg: -4.20 ± 0.10‰; AH-1 mean delta26Mg: -4.01 ± 0.07‰) are, in terms of the factors that control isotope fractionation, complex. This is because factors such as precipitation rate, changes in silicate versus carbonate weathering ratios, air temperature and rainfall amount interfere in a highly complicated manner.

European geosciences union general assembly, 2017

Chemical Geology

Abstract Carbonate hardparts of marine organisms are frequently explored archives of their ambien... more Abstract Carbonate hardparts of marine organisms are frequently explored archives of their ambient seawater composition. Among the various materials used, the hardparts of molluscs are particularly relevant because their accretionary growth allows for the compilation of time-resolved proxy data sets. Here, we explore to which extent the calcium isotope values of aragonitic cephalopod hardparts represent proxies for seawater composition or record a trophic level signal. In a first step, we collect Ca isotope data from a diverse set of modern cephalopods, including Nautilus, Sepia and Spirula. These taxa differ in their external/internal hardparts, life spans, trophic levels, habitat depth, hinterland geology, and resulting runoff. In a second step, we have put these concepts to the test and explore the calcium isotope signatures of Lower Cretaceous (lower Albian) well-preserved cephalopods (nautiloids, ammonoids and belemnites) and bivalve shells from Madagascar. In the sense of a methodological comparison, two measurement approaches (double spike (δ44/40Ca, TIMS) and bracketing standard (δ44/42Ca, MC-ICP-MS) techniques are applied. Calcium from organic matter and intra-shell variations on bulk δ44/40Ca isotope values are explored. Four protocols (H2O2, NaOH, HClO4, incineration up to 250 °C including subsequent rinsing with deionised water and Ca removal) are applied to destroy organic matter in the shell material and to dissolve any non‑carbonate bound Ca, i.e., its metastable secondary Ca reservoir. These are equally successful, and the Ca isotope values of the purified carbonate phase plot on the equilibrium mass fractionation line. Samples not treated to destroy or remove their organic matter do not fall on the equilibrium mass fractionation line. Limitations of sample pre-treatment are reflected by a slight shift towards lower values in the case of incineration experiments at 650 °C. This feature is best explained in the context of a mineralogical phase transformation of aragonite to calcite. Calcium isotope values show no significant variability when sampling hardparts with different microstructures (callus, nacre, prismatic layer) and mineralised during different ontogenetic phases (outer shell, septa). Perhaps most relevant, mean δ44/40Ca (‰ SRM-915a) isotope values for Nautilus (0.72‰ ±0.19 2SD), Sepia (0.79‰ ±0.18 2SD), and Spirula (0.50‰ ±0.18 2SD) lack evidence for a significant control by trophic levels. Nitrogen isotope data and known diets for all three cephalopods serve as a litmus test for these data, which do not correlate with this dietary proxy information. These results are considered encouraging and suggest that Ca isotope values of cephalopod shells are proxies for their ambient seawater Ca isotope signatures. According to our data, however, it remains unclear to which extend cephalopod hardparts record the Ca isotopic composition of past seawater. Specifically, the data from three different modern species point to a mean Δ44/40Ca(ceph-seawater) fractionation of −1.21‰ (± 0.15 2SD). Based on our work, we present a best practice guide for cephalopod Ca isotope analysis and interpretation.

The EGU General Assembly, 2018

Zeitschrift der Deutschen Gesellschaft für Geowissenschaften

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

Geochemistry, Geophysics, Geosystems

Geochimica et Cosmochimica Acta

Geophysical Research Letters

Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are of... more Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.

ABSTRACT The Magnesium isotope proxy in Earth surface research is still underexplored. Recently, ... more ABSTRACT The Magnesium isotope proxy in Earth surface research is still underexplored. Recently, field and laboratory experiments have shed light on the complex suite of processes affecting Mg isotope fractionation in continental weathering systems. Magnesium-isotope fractionation in speleothems depends on a series of factors including biogenic activity and composition of soils, mineralogy of hostrock, changes in silicate versus carbonate weathering ratios, water residence time in the soil and hostrock and disequilibrium factors such as the precipitation rate of calcite in speleothems. Furthermore, the silicate (here mainly Mg-bearing clays) versus carbonate weathering ratio depends on air temperature and rainfall amount, also influencing the soil biogenic activity. It must be emphasized that carbonate weathering is generally dominant, but under increasingly warm and more arid climate conditions, silicate weathering rates increase and release 26Mg-enriched isotopes to the soil water. Furthermore, as shown in laboratory experiments, increasing calcite precipitation rates lead to elevated delta26Mg ratios and vice versa. Here, data from six stalagmite time-series Mg-isotope records (Thermo Fisher Scientific Neptune MC-ICP-MS) are shown. Stalagmites from caves in Morocco, Germany and Peru are presented. The lowest mean Mg-isotope compositions are found in two Pleistocene Moroccan stalagmites (delta26Mg: -4.26 ± 0.07‰ and -4.17 ± 0.15‰). The cyclical shifts in both stalagmites are best explained by periods of increasing and decreasing aridity. In contrast, Holocene Peruvian stalagmites (0 to 14 ka) show a high mean delta26Mg-value of -3.96 ± 0.04‰ and a very low level of variability in time. This is probably due to the equatorial climate lacking significant variations in temperature and/or rainfall amount. Changes in precipitation rate show effects in stalagmites from western Germany and Peru resulting in a small variability exceeding the error of the delta26Mg-values. Stalagmites from Western Germany (BU 4 mean delta26Mg: -4.20 ± 0.10‰; AH-1 mean delta26Mg: -4.01 ± 0.07‰) are, in terms of the factors that control isotope fractionation, complex. This is because factors such as precipitation rate, changes in silicate versus carbonate weathering ratios, air temperature and rainfall amount interfere in a highly complicated manner.

European geosciences union general assembly, 2017

Chemical Geology

Abstract Carbonate hardparts of marine organisms are frequently explored archives of their ambien... more Abstract Carbonate hardparts of marine organisms are frequently explored archives of their ambient seawater composition. Among the various materials used, the hardparts of molluscs are particularly relevant because their accretionary growth allows for the compilation of time-resolved proxy data sets. Here, we explore to which extent the calcium isotope values of aragonitic cephalopod hardparts represent proxies for seawater composition or record a trophic level signal. In a first step, we collect Ca isotope data from a diverse set of modern cephalopods, including Nautilus, Sepia and Spirula. These taxa differ in their external/internal hardparts, life spans, trophic levels, habitat depth, hinterland geology, and resulting runoff. In a second step, we have put these concepts to the test and explore the calcium isotope signatures of Lower Cretaceous (lower Albian) well-preserved cephalopods (nautiloids, ammonoids and belemnites) and bivalve shells from Madagascar. In the sense of a methodological comparison, two measurement approaches (double spike (δ44/40Ca, TIMS) and bracketing standard (δ44/42Ca, MC-ICP-MS) techniques are applied. Calcium from organic matter and intra-shell variations on bulk δ44/40Ca isotope values are explored. Four protocols (H2O2, NaOH, HClO4, incineration up to 250 °C including subsequent rinsing with deionised water and Ca removal) are applied to destroy organic matter in the shell material and to dissolve any non‑carbonate bound Ca, i.e., its metastable secondary Ca reservoir. These are equally successful, and the Ca isotope values of the purified carbonate phase plot on the equilibrium mass fractionation line. Samples not treated to destroy or remove their organic matter do not fall on the equilibrium mass fractionation line. Limitations of sample pre-treatment are reflected by a slight shift towards lower values in the case of incineration experiments at 650 °C. This feature is best explained in the context of a mineralogical phase transformation of aragonite to calcite. Calcium isotope values show no significant variability when sampling hardparts with different microstructures (callus, nacre, prismatic layer) and mineralised during different ontogenetic phases (outer shell, septa). Perhaps most relevant, mean δ44/40Ca (‰ SRM-915a) isotope values for Nautilus (0.72‰ ±0.19 2SD), Sepia (0.79‰ ±0.18 2SD), and Spirula (0.50‰ ±0.18 2SD) lack evidence for a significant control by trophic levels. Nitrogen isotope data and known diets for all three cephalopods serve as a litmus test for these data, which do not correlate with this dietary proxy information. These results are considered encouraging and suggest that Ca isotope values of cephalopod shells are proxies for their ambient seawater Ca isotope signatures. According to our data, however, it remains unclear to which extend cephalopod hardparts record the Ca isotopic composition of past seawater. Specifically, the data from three different modern species point to a mean Δ44/40Ca(ceph-seawater) fractionation of −1.21‰ (± 0.15 2SD). Based on our work, we present a best practice guide for cephalopod Ca isotope analysis and interpretation.

The EGU General Assembly, 2018

Zeitschrift der Deutschen Gesellschaft für Geowissenschaften

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

Geochemistry, Geophysics, Geosystems

Geochimica et Cosmochimica Acta

Geophysical Research Letters