Dieter Buhl

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.

The Sudbury Structure (Ontario, Canada): a tectonically deformed multi-ring impact basin

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.

Strontium isotope stratigraphy of the Middle Devonian: Brachiopods and conodonts

A set of 145 strontium isotope ratios, based on skeletal components of Emsian to Frasnian age, reveals a detailed structure of the seawater 87Sr/86Sr curve that can ultimately serve as a tool for “high resolution isotope stratigraphy” in the Devonian. Brachiopod and conodont samples were collected in the Eifel region of Germany, the area around the global stratotype for the Middle Devonian, with an apparent average temporal resolution in the 105 y range. Preservation of the brachiopod shell material has been assessed by optical microscopy, SEM, and cathodoluminescence and only the better preserved internal (“secondary”) layer of the shell has been utilized for strontium isotope measurements. The 87Sr/86Sr curve shows a short decline from 0.7081 to 0.7078 through the late Emsian, followed by short-term fluctuations with an amplitude of up to 10−4 around a mean of ~0.7078 during the Eifelian and Givetian, and a rise from late Givetian into the Late Devonian. The above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records, due to erosional and nondepositional events, but a direct confirmation for such a scenario can be claimed only for the latest Eifelian otomari-event. Insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes. Conodont skeletal elements, even if well preserved at CAI < 2.5, yielded results that are mostly enriched in radiogenic 87Sr, if compared to coeval brachiopods. Although this enrichment is usually within the 10−5 range, conodonts, despite their superior stratigraphic resolution, should be utilized only as a material of second choice in cases where brachiopods are rare or absent.

Nature and Nurture: Environmental Isotope Story of the River Rhine

The total dissolved load of the Rhine river increases downstream due to the combined impact of natural and pollution loads. The natural background, controlled by geology, is soon swamped by pollution, particularly from salt and coal mining activities in Alsace and the Ruhr area. The Rhine is also a net source of CO2 from oxidation of excess organic productivity that is fuelled by nutrient pollution, a problem even more serious for some of its tributaries.

High precision calcium isotope ratio measurements using a magnetic sector multiple collector inductively coupled plasma mass spectrometer

Calcium isotope abundances were measured using a Finnigan Neptune magnetic sector multiple collector inductively coupled plasma mass spectrometer capable of resolving all molecular isobaric interferences from 42Ca, 43Ca, 44Ca, 46Ca and 48Ca. Scattering events caused by the intense 40Ar+ ion beam did not contribute to the uncertainty in the baseline of the calcium mass spectrum. Quantitative separation of the calcium from the sample matrix was carried out to ensure that the measurements were independent of the sample type. In addition, thorough desolvation of the aerosol was found to have a significant effect on the stability and sensitivity of the method. The stable mass bias of the instrument enabled normalization of the measured isotope abundance ratios relative to standard reference materials. Delta values including δ44Ca/43Ca, δ44Ca/42Ca and δ48Ca/42Ca were measured with external reproducibilities better than ±0.2‰ (2s) from 10 ppm solutions of calcium-containing materials, including sea-water and biogenic and non-biogenic marine carbonates.

Catastrophic extinction of Caribbean rudist bivalves at the Cretaceous-Tertiary boundary

Strontium isotope ratios ( 87 Sr/ 86 Sr) in pristine low-Mg calcite of shells of rudist bivalves from the Titanosarcolites limestones exposed in the Central, Maldon, and Marchmont inliers of Jamaica indicate that species-rich rudist-coral associations persisted into the latest Maastrichtian (66–65 Ma). This finding contradicts the currently accepted hypothesis of stepwise extinction of rudist bivalves in the middle Maastrichtian and argues for a catastrophic, impact-related demise of Caribbean Cretaceous reefal ecosystems at the Cretaceous-Tertiary boundary.

The strontium isotopic composition of Ordovician and Silurian brachiopods and conodonts: Relationships to geological events and implications for coeval seawater

utilized for strontium isotope measurements. The results document a gradual decrease in 87 Sr/ 86 Sr, from 0.7091 to 0.7087, from Tremadoc to Llandeilo, a sharp decline to 0.7078 during the late Llandeilo-early Caradoc; little change during Caradoc and the Ashgil; and a steady rise to 0.7087 through the Silurian. These long-term (10 7 yr) variations, with magnitudes in the range of 10 23 , are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering. The gradual decrease in 87 Sr/ 86 Sr ratio during the Early Ordovician may record the reduction in uplift and weathering rates due to waning of the Pan-African orogenies. The rapid decline near the Llandeilo/Caradoc boundary suggests a strong hydrothermal flux likely due to increased sea-floor spreading and a possible superplume event. The latter may have caused the prominent transgressive phase, the largest in the Phanerozoic, which would have muted continental flux input. Although the Caradoc was the main interval for the Taconic Orogeny, its impact on the Sr continental flux may have been delayed until the early Llandovery. This effect, complemented by reworking of glacial deposits near the Ordovician-Silurian boundary and enhanced by phases of the Silurian Salinic Orogeny, may have combined to give the progressive increase in the strontium isotope ratio through the Silurian. The scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes. Copyright

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.

Strontium- and neodymium-isotopic characteristics of a heterolithic breccia in the basement of the Sudbury impact structure, Canada

Abstract Rb Sr and Sm Nd measurements were undertaken on Footwall Breccia samples originating from Levack Township in the North Range of the Sudbury Structure. The granitic to quartzdioritic matrix of this heterolithic breccia, which contains shocked rock and mineral fragments, yields a Rb Sr isochron age of 1.825 ± 0.021 Ga (ISr = 0.70356 ± 0.00005; 2σ errors), based on the analysis of 3–6 mm thick adjacent whole-rock slabs from a large hand specimen. This age is interpreted as a cooling age after the thermal metamorphism of the Footwall Breccia by the 1.85 Ga old Sudbury Igneous Complex, which had caused temperatures in excess of 1000°C in the matrix near the contact. Mafic rock fragments in this breccia were excluded from the Sr isotope exchange at that time. The Sm Nd system of the identical small whole-rock samples was only partly disturbed by the Sudbury Igneous Complex. The data points scatter around a 2.05 ± 0.49 Ga regression line without age significance. Plagioclase, K-feldspar, and mafic mineral phases of the breccia matrix, combined with the corresponding whole rocks, display a range in Rb Sr ages from 1.67 ± 0.08 Ga to 1.430 ± 0.015 Ga, obviously associated with metasomatic processes in the Footwall Breccia during the declining Penokean metamorphism. Strontium and neodymium model parameters of the Footwall Breccia matrix near Levack show little variation: eSrt = 16.33 to 18.07, eNdt = −10.69 to −11.79 (t = 1.86 Ga), and mean TNdDM = 2.74 ± 0.04 Ga. Based on these results, bedrocks exposed in the South Range and rocks now forming the sedimentary filling of the Sudbury basin can be excluded as major components in the matrix of the analyzed Footwall Breccia. The convergence of neodymium model ages with U Pb ages on zircons from the Levack gneiss, which forms the crystalline basement adjacent to the Footwall Breccia in the Levack area, support an exclusively local origin for matrix and fragments of this breccia. It is proposed that the Footwall Breccia is part of the uplifted floor of the Sudbury crater.

On the significance of crater ages : new ages for Dellen (Sweden) and Araguainha (Brazil)

Samples of a few grams in weight from the impact melt at the Dellen crater (Sweden) and the Araguainha dome (Brazil) were analyzed by Rb-Sr and Sm-Nd techniques. For Dellen, pure labradorite, orthopyroxene, and glass separates from a clast-free impact melt rock with holohyaline ground mass yield an internal Rb-Sr isochron with 89.0 ± 2.7 Ma (Isr = 0.83571 ±0.00007; 2σ), compared to Ar-Ar plateau ages of 102 ± 3.2 Ma and 109.6 ±2.0 Ma published for total melt rocks. Various whole-rock values not plotting on this Rb-Sr isochron indicate primary variations in 87Sr86Sr over the melt sheet. The age of the Araguainha dome (Brazil) is bracketed by the Rb-Sr model age of 243 ± 19 Ma (ISr = 0.7226 ± 0.0005), for altered cordierite in the impact melt, and by the internal Rb-Sr isochron age of 449 ± 9 Ma (ISr = 0.7140 ± 0.0007), for an uplifted granite (shock stage Ib). In melt rocks from both impact structures 143Nd144Nd was not homogenized during total rock melting: Sm-Nd data either scatter around the primary isochron (Araguainha) or point to a partial resetting (Dellen). This data array may be caused by tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system of the impact melt rocks. The significant difference between the new isochron age for Dellen and Ar-Ar “ages” show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy.