Harald Strauss

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 ABUNDANCE OF 13C IN MARINE ORGANIC MATTER AND ISOTOPIC FRACTIONATION IN THE GLOBAL BIOGEOCHEMICAL CYCLE OF CARBON DURING THE PAST 800 MA

Abstract New records of the abundance of 13 C in marine organic matter have been compiled for (i) the later Neoproterozoic, from 800 to 543 Ma (346 analyses), (ii) the Cambrian through the Jurassic (1616 analyses), and (iii) the Cretaceous and Cenozoic (2493 analyses). Comparison of these to existing compilations of the abundance of 13 C in sedimentary carbonates has allowed development of a record of the isotopic fractionation (≡eTOC) accompanying the production and burial of organic material. Over time, globally averaged values of eTOC have fallen in three ranges: (i) greater than 32‰ and apparently indicative of significant inputs from sulfide-oxidizing or other chemoautotrophic bacteria, notably during late Proterozoic interglacials at 752, 740–732, and 623–600 Ma; (ii) between 28 and 32‰ and indicative of maximal fractionation of carbon isotopes by phytoplanktonic producers, during the Neoproterozoic from 800 to 750 and from 685 to 625 Ma and during the Phanerozoic up to the early Oligocene; and (iii) less than 28‰, probably reflecting a reduction of primary fractionation by some combination of low levels of CO2, rapid rates of growth, and high ratios of cellular volume to surface area during Neoproterozoic glaciations (740, 720, and 575 Ma) and since the early Oligocene. Evidence of similar variations during the Ordovician and Gondwanan glaciations is absent. The decline in eTOC since the early Oligocene, from 30 to 22‰, has been nearly linear. The structure of the record of eTOC suggests that the maximal isotopic fractionation between dissolved CO2 and primary biomass has consistently been 25‰. Overall, the records provide compelling evidence that values of eTOC have varied widely and that the long-term average fractionation is roughly 30‰.

Ferruginous Conditions Dominated Later Neoproterozoic Deep-Water Chemistry

Earths surface chemical environment has evolved from an early anoxic condition to the oxic state we have today. Transitional between an earlier Proterozoic world with widespread deep-water anoxia and a Phanerozoic world with large oxygen-utilizing animals, the Neoproterozoic Era [1000 to 542 million years ago (Ma)] plays a key role in this history. The details of Neoproterozoic Earth surface oxygenation, however, remain unclear. We report that through much of the later Neoproterozoic (<742 ± 6 Ma), anoxia remained widespread beneath the mixed layer of the oceans; deeper water masses were sometimes sulfidic but were mainly Fe2+-enriched. These ferruginous conditions marked a return to ocean chemistry not seen for more than one billion years of Earth history.

The isotopic composition of sedimentary sulfur through time

Abstract Our knowledge of the isotopic composition of sedimentary sulfur and its evolution through time is based on the studies of marine evaporitic sulfate deposits and sedimentary pyrite, the latter mostly formed as a result of bacterial sulfate reduction. Traditionally, their isotope records have been utilized to model and interpret the global sulfur cycle on Earth. The significance, potential and limitations of the presently available sulfur isotope data for the Phanerozoic and the Neoproterozoic with respect to modern concepts of high-resolution isotope stratigraphy are critically evaluated. In general, the sulfate and sulfide isotope records require further systematic research, particularly in order to improve the inadequate, irregular time spacing of results. The potential of sulfur isotopic investigations of sedimentary sulfides from time boundaries is rather limited. Frequently, local depositional and/or diagenetic effects appear to dominate over potentially present global signals.

Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry

The evolution of the Earth’s atmosphere is marked by a transition from an early atmosphere with very low oxygen content to one with an oxygen content within a few per cent of the present atmospheric level. Placing time constraints on this transition is of interest because it identifies the time when oxidative weathering became efficient, when ocean chemistry was transformed by delivery of oxygen and sulphate, and when a large part of Earth’s ecology changed from anaerobic to aerobic. The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than ∼2.45 billion years (2.45 Gyr) old and the disappearance of this signal in younger sediments is taken as one of the strongest lines of evidence for the transition from an anoxic to an oxic atmosphere around 2.45 Gyr ago. Detailed examination of the sulphur isotope record before 2.45 Gyr ago also reveals early and late periods of large amplitude non-mass-dependent signals bracketing an intervening period when the signal was attenuated. Until recently, this record has been too sparse to allow interpretation, but collection of new data has prompted some workers to argue that the Mesoarchaean interval (3.2–2.8 Gyr ago) lacks a non-mass-dependent signal, and records the effects of earlier and possibly permanent oxygenation of the Earth’s atmosphere. Here we focus on the Mesoarchaean interval, and demonstrate preservation of a non-mass-dependent signal that differs from that of preceding and following periods in the Archaean. Our findings point to the persistence of an anoxic early atmosphere, and identify variability within the isotope record that suggests changes in pre-2.45-Gyr-ago atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere.

Oxygen isotope evolution of Phanerozoic seawater

Abstract A compilation of over 2000 measurements of 18O and 13C on Phanerozoic low-Mg calcite shells, such as brachiopods, belemnites and oysters, delineates secular 18O/16O and 13C/12C variations that are similar to those previously described for whole rocks. The trend for the δ18O suggests about ∼5±2‰ enrichment from the Cambrian to today. In contrast, the δ13C rise during the Paleozoic is followed by its decline in the Mesozoic and Cenozoic. Optical (textural) and chemical criteria suggest that the interior “secondary” layer of the brachiopod shells, the material that carries these signals, is well preserved in many samples and the extracted secular isotopic trends are therefore a primary feature of the geologic record. The similarity of the δ18O/δ13C isotope patterns in ancient and modern brachiopods also supports such an interpretation. In our view, the 18O enrichment in progressively younger samples is principally, although not exclusively, a reflection of the evolving 18O/16O composition of seawater. If so, a delineation of this trend may ultimately result in development of a valuable paleoclimatic and paleoceanographic tracer for the Phanerozoic.

Water column anoxia, enhanced productivity and concomitant changes in δ13C and δ34S across the Frasnian–Famennian boundary (Kowala — Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Frasnian–Famennian boundary section at Kowala (Holy Cross Mountains/Poland) shows that the lower water column was oxygen-deficient during late Frasnian and early Famennian times. The abundance and carbon isotopic composition of diaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove that euxinic waters reached into the photic zone, at least episodically. Total organic carbon (TOC) contents show two maxima that are time-equivalent to the Kellwasser horizons deposited in shallower water settings. Enhanced TOC concentrations are explained by a higher primary productivity, presumably as a consequence of an enhanced nutrient supply from the continent. The increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to TOC increased at the Frasnian–Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27‰ excursion across the boundary. The observation that the δ34S values of organic-bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source, likely early diagenetic sulfide. A change in the δ13C of total dissolved inorganic carbon as a consequence of an enhanced burial of 12C-enriched organic carbon is indicated by a +3‰ excursion measured for TOC as well as for individual n-alkanes and isoprenoids. The burial of large amounts of organic carbon is expected to result in a decrease in pCO2 and should affect the photosynthetic carbon isotope fractionation (ep). The fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values, as a consequence of high atmospheric and oceanic-dissolved CO2 concentrations during the Devonian.

Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition

The Archean-Proterozoic transition is characterized by the widespread deposition of organic-rich shale, sedimentary iron formation, glacial diamictite, and marine carbonates recording profound carbon isotope anomalies, but notably lacks bedded evaporites. All deposits refl ect environmental changes in oceanic and atmospheric redox states, in part associated with Earth’s earliest ice ages. Time-series data for multiple sulfur isotopes from carbonateassociated sulfate as well as sulfi des in sediments of the Transvaal Supergroup, South Africa, capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric massindependent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event (ca. 2.3 Ga).

Geological evolution from isotope proxy signals — sulfur

Abstract A currently emerging sulfur isotope record for Phanerozoic seawater, based on structurally substituted sulfate in stratigraphically well constrained biogenic carbonates, allows the detailed assessment of secular variations within the global sulfur cycle and the interaction between the sulfur and carbon cycles. It is superior to the evaporite-based dataset because it enables sampling of the entire biostratigraphic column. Discrete biological and environmental signals can be deciphered from a somewhat “noisy” sulfur isotope record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around −51‰ which appears to be constant throughout the entire Phanerozoic. Observable large spreads of δ 34 S sulfide for any given sedimentary unit are caused by environmental parameters, such as type and availability of organic carbon or availability of sulfate. In particular, the growing importance of land plants and their impact on the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition.

Active Microbial Sulfur Disproportionation in the Mesoproterozoic

The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34S/32S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.