Ján Veizer

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.

Chemical Diagenesis of a Multicomponent Carbonate System--1: Trace Elements

ABSTRACT Theoretical considerations (i.e., partition coefficients, water/rock ratio, chemistry of interstitial meteoric water) of elemental behaviour during diagenetic stabilization with meteoric waters suggests that it leads to a decrease in strontium, sodium, and possibly magnesium and an increase in manganese, iron, and zinc in progressively altered carbonates. Such elemental behaviour is exhibited by the different carbonate components of the Mississippian Burlington Limestone of Iowa and Missouri and the Silurian Read Bay Formation of Arctic Canada. In the Burlington Limestone the rock matrix (e.g., biosparite), the enclosed crinoids, and to some degree the rugose corals are chemically similar. The crinoid ossicles have average strontium content of 160 ppm, rugose corals 180 ppm, and the enclosing biosparite 120 ppm. In contrast, in the Read Bay Formation each of the above mentioned components has a specific chemistry, with 210 ppm strontium for crinoids, 780 ppm for rugose corals, and 360 ppm for their enclosing micrite matrix. These chemical trends are accompanied by textural changes of the host carbonate sediments. In the Burlington Lime tone this involves the presence of depositional sparite, whereas in the Read Bay Formation this increase in textural maturity involves the transition from micrite to microspar to minor pseudospar and sparite. The combination of these textural trends with the elemental patterns shows that the degree with which a particular carbonate component approaches either the open system or the partly closed system equilibrium is dictated by its respective mineralogical stability and the water/rock ratio. While the results show that the carbonate assemblage may act as a completely open diagenetic system (e.g., Burlington Limestone), available data for the majority of studied sequences (e.g., Read Bay Formation) suggest that diagenetic equilibration ceases while some original depositional differences in chemical composition are still preserved. This implies that diagenetic stabilization proceeds through partly closed reaction zones on solid-liquid interfaces. Transfer of the chemical and textural inform tion from the dissolving phase (original sedimentary carbonate particle) to the precipitating phase (diagenetic carbonate component) proceeds via a Messenger Film water in the reaction zone, which is in disequilibrium with the meteoric bulk aquifer water. Thus the chemical composition of carbonate components of ancient limestones may serve as a potential tool for evaluating the degree of diagenesis and for deducing the original mineralogy of the different stabilized carbonate phases. Application of this trace element model suggests that Paleozoic crinoids were composed originally of metastable high-magnesium calcite, rugose corals were composed originally of stable low-magnesium calcite or high-magnesium calcite with low Mg2+ content, and micrite was origially aragonite lime mud.

The nature of O18/O16 and C13/C12 secular trends in sedimentary carbonate rocks.

Abstract Measurements on 170 carbonate rocks show decreasing δO 18 of similar magnitude for both limestones and dolomites over a time span of ~ 2800 m.y. The Proterozoic dolomites are on average heavier by ~5%. in δO 18 than their coeval limestones. The data may indicate displacement in δC 13 of about 3%. at approximately 570 m.y. ago, with Precambrian carbonates being heavier in δC 13 at a given δO 18 level than their Phanerozoic counterparts. The compilation of the previously published data is consistent with the above described features.

Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon

Atmospheric carbon dioxide concentrations are believed to drive climate changes from glacial to interglacial modes, although geological and astronomical mechanisms have been invoked as ultimate causes. Additionally, it is unclear whether the changes between cold and warm modes should be regarded as a global phenomenon, affecting tropical and high-latitude temperatures alike, or if they are better described as an expansion and contraction of the latitudinal climate zones, keeping equatorial temperatures approximately constant. Here we present a reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past ∼550 Myr) from our database of oxygen isotopes in calcite and aragonite shells. The data indicate large oscillations of tropical sea surface temperatures in phase with the cold–warm cycles, thus favouring the idea of climate variability as a global phenomenon. But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric carbon dioxide concentrations. The results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.

87Sr/86Sr composition of seawater during the Phanerozoic

Abstract 87 Sr 86 Sr measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87Sr/86Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low 87 Sr 86 Sr in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.

Geochemistry of brachiopods: Oxygen and carbon isotopic records of Paleozoic oceans

Abstract Combined trace element and isotope studies of 319 brachiopods, covering the Ordovician to Permian time span, show that δ13C and δ18O in well preserved specimens varied during the Paleozoic. The overall δ13C secular trend is in accord with the previously published observations, but its details are obscured by vital isotopic fractionation effects at generic level. Nonetheless, the results suggest that the negative correlation between marine δ13Ccarbonate and δ34Ssulphate deteriorates at time scales of ⩽ 106 years, due to the long residence time, and thus slow response, of SO42− in the ocean. For oxygen isotopes, all Devonian and older specimens have δ18O of ⩽ −4%, while the well preserved Permian samples have near-present day δ18O of about −1% (PDB). This isotopic dichotomy is probably not due to post-depositional phenomena, salinity, or biogenic fractionation effects. This leaves open the perennial arguments for a change in 18 O 16 O of sea water versus warmer ancient oceans. The present data are difficult to explain solely by the temperature alternative. The coincidence of the proposed shift in δ18O with the large Late Paleozoic changes in marine 87 Sr 86 Sr , 13 C 12 C , 34 S 32 S , and “sea level stands” argues for a tectonic cause and for a change in 18 O 16 O of sea water, although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites. Whatever the precise cause, or combination of causes, the implications for tectonism and/or paleoclimatology are of first order significance.

Basement and Sedimentary Recycling and Continental Evolution

The present day cumulative distribution of areas of continental basement age provinces (as defined by radiometric estimates) follows an exponential function. A similar distribution is displayed by the reserves of their associated economic deposits and by the thickness and areal distribution data for the overlying sediments. This exponential pattern, similar to isotope decay systematics, is a result of recycling (s.l.). Each theoretically possible growth model must be complemented by an appropriate average recycling constant in order to generate the observed present day distribution. Computer simulation of the system suggests that: (a) the theoretical recycling (radiometric rejuvenation) constant b for the continental basement is

Strontium isotope evolution of Late Permian and Triassic seawater

\leq 6.5 \times 10^{-4}/10^{6}a