Frank Bruhn

87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater

A total of 2128 calcitic and phosphatic shells, mainly brachiopods with some conodonts and belemnites, were measured for their , and values. The dataset covers the Cambrian to Cretaceous time interval. Where possible, these samples were collected at high temporal resolution, up to 0.7 Ma (one biozone), from the stratotype sections of all continents but Antarctica and from many sedimentary basins. Paleogeographically, the samples are mostly from paleotropical domains. The scanning electron microscopy (SEM), petrography, cathodoluminescence and trace element results of the studied calcitic shells and the conodont alteration index (CAI) data of the phosphatic shells are consistent with an excellent preservation of the ultrastructure of the analyzed material. These datasets are complemented by extensive literature compilations of Phanerozoic low-Mg calcitic, aragonitic and phosphatic isotope data for analogous skeletons. The oxygen isotope signal exhibits a long-term increase of from a mean value of about −8‰ (PDB) in the Cambrian to a present mean value of about 0‰ (PDB). Superimposed on the general trend are shorter-term oscillations with their apexes coincident with cold episodes and glaciations. The carbon isotope signal shows a similar climb during the Paleozoic, an inflexion in the Permian, followed by an abrupt drop and subsequent fluctuations around the modern value. The ratios differ from the earlier published curves in their greater detail and in less dispersion of the data. The means of the observed isotope signals for , , and the less complete (sulfate) are strongly interrelated at any geologically reasonable (1 to 40 Ma) time resolution. All correlations are valid at the 95% level of confidence, with the most valid at the 99% level. Factor analysis indicates that the , , and isotope systems are driven by three factors. The first factor links oxygen and strontium isotopic evolution, the second and , and the third one the and . These three factors explain up to 79% of the total variance. We tentatively identify the first two factors as tectonic, and the third one as a (biologically mediated) redox linkage of the sulfur and carbon cycles. On geological timescales (≥1 Ma), we are therefore dealing with a unified exogenic (litho-, hydro-, atmo-, biosphere) system driven by tectonics via its control of (bio)geochemical cycles.Abstract A total of 2128 calcitic and phosphatic shells, mainly brachiopods with some conodonts and belemnites, were measured for their δ 18 O , δ 13 C and 87 Sr / 86 Sr values. The dataset covers the Cambrian to Cretaceous time interval. Where possible, these samples were collected at high temporal resolution, up to 0.7 Ma (one biozone), from the stratotype sections of all continents but Antarctica and from many sedimentary basins. Paleogeographically, the samples are mostly from paleotropical domains. The scanning electron microscopy (SEM), petrography, cathodoluminescence and trace element results of the studied calcitic shells and the conodont alteration index (CAI) data of the phosphatic shells are consistent with an excellent preservation of the ultrastructure of the analyzed material. These datasets are complemented by extensive literature compilations of Phanerozoic low-Mg calcitic, aragonitic and phosphatic isotope data for analogous skeletons. The oxygen isotope signal exhibits a long-term increase of δ 18 O from a mean value of about −8‰ (PDB) in the Cambrian to a present mean value of about 0‰ (PDB). Superimposed on the general trend are shorter-term oscillations with their apexes coincident with cold episodes and glaciations. The carbon isotope signal shows a similar climb during the Paleozoic, an inflexion in the Permian, followed by an abrupt drop and subsequent fluctuations around the modern value. The 87 Sr / 86 Sr ratios differ from the earlier published curves in their greater detail and in less dispersion of the data. The means of the observed isotope signals for 87 Sr / 86 Sr , δ 18 O , δ 13 C and the less complete δ 34 S (sulfate) are strongly interrelated at any geologically reasonable (1 to 40 Ma) time resolution. All correlations are valid at the 95% level of confidence, with the most valid at the 99% level. Factor analysis indicates that the 87 Sr / 86 Sr , δ 18 O , δ 13 C and δ 34 S isotope systems are driven by three factors. The first factor links oxygen and strontium isotopic evolution, the second 87 Sr / 86 Sr and δ 34 S , and the third one the δ 13 C and δ 34 S . These three factors explain up to 79% of the total variance. We tentatively identify the first two factors as tectonic, and the third one as a (biologically mediated) redox linkage of the sulfur and carbon cycles. On geological timescales (≥1 Ma), we are therefore dealing with a unified exogenic (litho-, hydro-, atmo-, biosphere) system driven by tectonics via its control of (bio)geochemical cycles.

87Sr86Sr isotopic evolution of Lower Carboniferous seawater: Dinantian of western Europe

Abstract The chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system. The original seawater signature can be measured only indirectly. For the Palaeozoic, samples with the greatest utility for such an approach are the secondary layers of articulate brachiopods. For a correct interpretation of isotope data, samples that have suffered alteration by diagenetic processes must be avoided. Trace element analysis is frequently utilized as an analytical tool. The present study deals with the delineation of a detailed 87 Sr 86 Sr - curve for the Dinantian. This, in turn, enables us to search for causative factors of the observed isotopic oscillations. A total of 175 Lower Carboniferous (Dinantian) brachiopod shells from a variety of western European locations have been analysed for their 87 Sr 86 Sr ratios. For 152 of these shells, quantitative geochemical tests on diagenetic alteration, based on trace element repartitioning during recrystallisation, have been performed. The concentrations of Ca, Mg, Sr, Mn and Fe were routinely measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), analysing the diluted phosphoric acid that remained after carbonate dissolution for stable isotope gas preparation. In addition, 71 samples have been analysed by proton-induced X-ray emission spectroscopy (micro-PIXE). Despite large differences in the analysed masses (3–6 mg for ICP-AES; as low as 10 ng for PIXE) both methods yield comparable data for the Sr and Mn contents. Any discrepancies can be related to inhomogeneities of the shells (e.g. punctae). Of these shells 73 have Sr and Mn contents of > 600 ppm and The Sr isotope record of the Dinantian seawater is characterised by a decline in 87 Sr 86 Sr ratio from 0.7082 at the Devonian/Carboniferous transition to 0.7076 in the mid-Visean. Superimposed on this trend are higher-order fluctuations with a periodicity in the Ma range. The Dinantian seawater curve may potentially serve as a geochronological and correlation tool, particularly for the Hastarian to lower Chadian interval, where the attainable resolution is ∼1 Ma. This is better than the resolution available by biostratigraphy. The higher-order wiggles, for the most part, can be generated by changing the riverine flux of Sr or its isotopic ratio within reasonable geological scenarios. Some ‘spikes’, however, are too large and too short to be explained by the above phenomena. These apparent spikes may reflect, instead, the fact that their duration has been underestimated due to unreliable geochronology and/or the presence of unrecognised hiatuses.

PIXE microprobe for geoscience applications

Abstract Microprobe analysis with PIXE (proton induced X-ray emission) is a well known analytical method with detection limits at the ppm level. It is frequently used for geoscience applications. To improve the versatility and reliability, the Bochum microprobe facility has been reconstructed incorporating a new data acquisition system, a beam dose monitor, and a new slit system.

Diagenetic history of sedimentary carbonates: Constraints from combined cathodoluminescence and trace element analyses by micro-PIXE

Abstract Trace element data for a succession of cements can contribute to the reconstruction of the diagenetic history of sediments. In the case of carbonate cements, cathodoluminescence (CL) microscopy has been successfully utilized for qualitative description of different cementation environments. In order to interpret correctly CL colour, trace element data other than Mn and Fe are often required. Due to frequent microscale zonation of trace elements in natural calcites and due to their low threshold concentration for CL activation, microsampling techniques with high spatial resolution and low detection limits are required. In this case study, trace element concentrations have been correlated with the CL behaviour of carbonate cements using the Bochum proton microprobe (PIXE) and a hot cathode luminescence device. The CL patterns, including the fine zonation, correlate well with the trace element distribution.

Diagenetic alteration of calcitic fossil shells: Proton microprobe (PIXE) as a trace element tool

Abstract Trace element contents (Mn, Fe, Sr) of a set of Lower Carboniferous brachiopod shell fragments have been measured using the Bochum proton microprobe. The resulting data are in good agreement with ICP measurements of the same shells and serve as a quantitative criterion to constrain the preservation state of the sample material for oxygen isotope analyses. They thus help to interpret correctly the isotope data. Linescans and elemental maps yield information on the lateral distribution of trace elements.

Trace element concentrations in conodonts measured by the Bochum proton microprobe

Abstract The Proton Microprobe at Bochum has been used to study trace element contents and elemental distributions in a suite of Devonian and Triassic conodonts, skeletal parts of marine invertebrates composed of the CO3- and F-rich modification of apatite (francolite). The conodonts contain a plethora of substituting elements (Mn, Fe, Sr, Y, light REE) at high concentrations, and linescans and elemental maps reveal preferential enrichments of many of these elements around skeletal rims. This observation tends to support the concept of post-depositional chemical alteration of the conodonts, but within an environment that still reflects the ambient marine conditions. Consequently, despite the high degree of substitution in the apatite structure, the Sr isotope signals of ancient seawater may still be retained by the samples.

Wide-range scanning and lens correction of an ion microprobe

Abstract The expected performance of a wide range scanning unit designed for an ion microprobe using a superconducting solenoid lens is investigated. Since the resulting spot size is mainly limited by chromatic aberration, the scanning unit should be installed close to the center of the solenoid lens to achieve optimal ion optical properties. Furthermore, the deflection system should consist of two independent controllable multipole units that correct an axial astigmatism introduced by a residual asymmetry of the solenoid lens.