Karem Azmy

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.

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.

Oxygen and carbon isotopic composition of Silurian brachiopods: Implications for coeval seawater and glaciations

We collected 236 calcitic brachiopod shells, covering the entire Silurian Period (~ 30 m.y.), at high temporal resolution from stratotype sections from Anticosti Island (Canada), Wales (United Kingdom), the Oslo region (Norway), Gotland (Sweden), and Podolia (Ukraine), Estonia, Latvia, and Lithuania. Data from petrography, scanning electron microscopy, cathodoluminescence, isotopes, and trace elements all confirmed that there was excellent preservation in most shells, thus arguing for retention of primary isotope signals; exceptions were samples from the Oslo region. The d18O and d13C values for the well-preserved samples range from –2‰ to –6.5‰ and from –1‰ to 7.5‰ (Peedee belemnite), respectively. In terms of temporal trends, oxygen and carbon isotopes vary in parallel, with a slight decrease with declining age of ~1‰ through Silurian time, with superimposed short-term oscillations that are negatively correlated with sea-level changes. Three successive positive d18O shifts in early Aeronian, latest Aeronian, and early Wenlock time correlate with sea-level lowstands and with glacial diamictite deposits in the Amazon Basin and in Africa. The high d18O and d13C values are attributed to cold episodes with low sea levels and low values to warm episodes with high sea levels. During warm Silurian episodes, the d18O value of seawater is suggested to have been about –3.5‰ standard mean ocean water (SMOW) and the global tropical temperatures of about 20–30 °C, similar to the present-day values in summer. During glacial episodes, seawater is proposed to have had a d18O value of ~–2.5‰ (SMOW) and temperature of about 14–23 °C, comparable to tropical temperatures proposed for the last glacial episode (14 to 28 °C). Three positive d13C peaks, in early and late Wenlock and late Ludlow time, likely of regional to global significance, appear to coincide with sea-level lowstands, but we are as yet unable to propose a convincing causative geologic scenario that would explain their origin.

Dolomitization and isotope stratigraphy of the Vazante Formation, Sao Francisco Basin, Brazil

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between 4 m (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The 18 Oa nd 13 C values vary from − 0.1 to − 14.3‰ (PDB) and 0.2 to − 2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, 18 O, 87 Sr/ 86 Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The 13 C values show only small variations at the base of the formation, followed by a major negative plunge ( 4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87 Sr/ 86 Sr values of 0.70614–0.70734. The lowest 87 Sr/ 86 Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative 13 C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The 34 S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment.

Silurian strontium isotope stratigraphy

A sample set of 164 calcitic brachiopod shells, covering the entire Silurian Period (∼ 30 m.y.) with a resolution of about 0.7 m.y., was collected from stratotype sections at Anticosti Island (Canada), Wales (United Kingdom), Gotland (Sweden), Podolia (Ukraine), Latvia, and Lithuania. They show 87Sr/86Sr values ranging from 0.707930 to 0.708792 that progressively increase with time. This may indicate an increasing riverine flux of radiogenic Sr into the ocean from weathering of continental sialic rocks due to progressive warming of the climate. Exceptionally high increases in 87Sr/86Sr values were observed in early Llandovery (Rhuddanian), late Llandovery (Telychian), and late Ludlow (Gorstian-Ludfordian boundary) samples. Partial linear regressions, based on a stepwise climbing pattern, with local drops around the Llandovery-Wenlock boundary and in latest Ludlow time, were used to estimate relative ages with a resolution of about ±2 biozones (∼1.5–2 m.y.). The Sr-isotope curve shows distinct inflection points in earliest Wenlock and mid-Pridoli time. These may be used to correlate the Llandovery-Wenlock boundary in the United Kingdom, Gotland, and Lithuania, and the Kaugatuma-Ohesaare boundary in the Baltic states and Podolia.

Heavy metal contamination from gold mining recorded in Porites lobata skeletons, Buyat-Ratototok district, North Sulawesi, Indonesia

Shallow marine sediments and fringing coral reefs of the Buyat-Ratototok district of North Sulawesi, Indonesia, are affected by submarine disposal of tailings from industrial gold mining and by small-scale gold mining using mercury amalgamation. Between-site variation in heavy metal concentrations in shallow marine sediments was partially reflected by trace element concentrations in reef coral skeletons from adjacent reefs. Corals skeletons recorded silicon, manganese, iron, copper, chromium, cobalt, antimony, thallium, and lead in different concentrations according to proximity to sources, but arsenic concentrations in corals were not significantly different among sites. Temporal analysis found that peak concentrations of arsenic and chromium generally coincided with peak concentrations of silica and/or copper, suggesting that most trace elements in the coral skeleton were incorporated into detrital siliciclastic sediments, rather than impurities within skeletal aragonite.

Dolomitization of the Lower Ordovician Aguathuna Formation carbonates, Port au Port Peninsula, western Newfoundland, Canada: implications for a hydrocarbon reservoir

The Lower Ordovician Aguathuna Formation (∼100 m thick) is formed of shallow-marine carbonates, which constitute the uppermost part of the St. George Group of western Newfoundland. Sedimentation was paused by a major subaerial exposure (St. George Unconformity), which likely developed a significant pore system in the underlying carbonates by meteoric dissolution. The sequence has been affected by multiphase dolomitization that caused complex changes in the rock porosity. The Aguathuna dolomites are classified into three main generations ranging in crystal size between ∼4 µm and 2 mm. The occurrence of fabric-retentive dolomicrites implies that dolomitization likely started during the early stages of diagenesis. Although dolomitization is pervasive in the upper part of the formation and significantly occludes the pores, some intervals in the lower part have higher porosity. The development of lower permeable layers overlain by an impermeable (seal) cap suggests a possible potential diagenetic trap. Unlike sabkha deposits, the Aguathuna carbonates do not have evaporite interlayers. Furthermore, the low Sr contents (∼96 ppm) and the δ18O values of earlier dolomites (–3.3‰ to –6.9‰ VPDB (Vienna Pee Dee Belemnite)) are also difficult to reconcile with a brine origin. The Sr/Ca molar ratios (0.0067–0.0009), calculated for the earliest dolomitizing fluid, suggest a modified seawater origin, likely mixed sea and meteoric waters. The least radiogenic 87Sr/86Sr values of the earliest dolomite are consistent with those of early Ordovician seawater, which supports an early-stage diagenesis. Petrography, geochemistry, and fluid inclusions of the late dolomites suggest precipitation at higher temperatures (∼73–95 °C) in deeper burial environments from hydrothermal solutions.

Is mid-late Paleozoic ocean-water chemistry coupled with epeiric seawater isotope records?

Isotopes of epeiric sea carbonates are used to construct seawater records for modeling global changes in Paleozoic ocean chemistry, climate, and for intercontinental correlation. We present for the first time geochemical results of Paleozoic brachiopods (biogenic low-Mg calcite, bLMC) from open-ocean Permian–Carboniferous seamounts of Japan situated in the tropical mid-Panthalassic Ocean. Strontium isotope values of bLMC from the Panthalassic and Paleotethys Oceans are coupled with those of coeval specimens from epeiric seas of North America, Europe, and Russia ( p = 0.393), but not with those of epeiric sea whole rocks (matrix aragonite/calcite, mAC; p = 0.029) and conodonts (biogenic apatite, bA; p = 0.031). Oxygen isotope values of bLMC from the Panthalassic and Paleotethys exhibit mixed results with studies of counterparts from epeiric seas ( p = 0.596) reflecting overprinting of local environmental conditions on global trends. Carbon isotope values of bLMC and mAC from the Panthalassic and Paleotethys Oceans are generally dissimilar to those of coeval material from epeiric seas of North America, Europe, and Russia ( p = 0.001 and 0.002, respectively). Factors such as water mass stratification, evaporation, dilution, depth, temperature, carbon burial and/or oxidation variations, and syndepositional diagenesis within the local environment probably influenced the chemistry of the fauna and accumulating sediments. This decoupling of carbon and oxygen isotope values from the open ocean with those from epeiric seas makes questionable the use of isotope results from epeiric seas for international correlation, constructing global seawater records, determining fluxes in the global carbon cycle, and for modeling climate changes and subsequently atmospheric carbon dioxide levels.

Chapter 48 Neoproterozoic successions of the São Francisco Craton, Brazil: the Bambuí, Una, Vazante and Vaza Barris/Miaba groups and their glaciogenic deposits

Abstract The Neoproterozoic successions of the São Francisco Craton are primarily represented by the Bambuí and Una groups, deposited in cratonic epicontinental basins, and by the Vazante and Vaza Barris/Miaba groups, which accumulated on passive margins on the edges of the craton. The epicontinental basins comprise three megasequences: glaciogenic, carbonate platform (marine) and dominantly continental siliciclastics. Possible correlative sequences are observed in the passive margin deposits. At least two major transgressive–regressive sea-level cycles occurred during the evolution of the carbonate megasequence, which lies above glaciomarine diamictites of probable early Cryogenian (i.e. Sturtian) age. C, O, Sr and S isotope trends from analyses of well-preserved samples, together with lithostratigraphic observations, provide reasonable correlations for most of the Neoproterozoic successions of the São Francisco Craton. The 87Sr/86Sr record of these successions, ranging from 0.70769 to 0.70780, supports the proposed correlation with the Bambuí, Una and Vaza/Barris successions, and with the basal units of the Vazante Group. In addition, C-isotope positive excursions ranging from +8.7 to +14‰ and negative excursions from –5.7 to –7‰ VPDB in the Bambuí, Una and Vaza-Barris successions provide key markers for correlations. The precise ages of the sedimentation in these successions remains a matter of debate, but organic shales of two units of the Vazante Group have been dated by Re–Os techniques in two different laboratories, both yielding Mesoproterozoic ages. The Neoproterozoic and Mesoproterozoic successions preserve significant glaciogenic deposits.

Dolomites of the Boat Harbour Formation in the Northern Peninsula, western Newfoundland, Canada: Implications for dolomitization history and porosity control

The Boat Harbour Formation constitutes the upper part of the Lower Ordovician St. George Group on the Northern Peninsula, western Newfoundland. It ranges in thickness from 140 m (459 ft) at Main Brook to 96 m (315 ft) at Daniels Harbour. Dolomitization of the carbonate sequence is more pervasive in the lower 30 to 40 m (98 to 131 ft) at Main Brook, whereas at Daniels Harbour, the section is entirely dolomitized. Petrography suggests that the Boat Harbour Formation has been affected by three phases of dolomitization. The earliest (near surface or synsedimentary) phase is D1 dolomicrite (4–55 m), which exhibits dull to no luminescence. It commonly occurs as laminae-capping cycles and as breccias in the younger dolomite phases. It has low Sr (228 30 ppm) and an average 18O value of 6.0 0.8 (Vienna Peedee belemnite [VPDB]) in the Main Brook section but more depleted signatures for 18O of 8.8 1 (VPDB) and lower Sr contents (45 8 ppm) in the Daniels Harbour section. The geochemical composition suggests that D1 was developed from fluids of a mixture of meteoric and marine waters. The midburial phase D2 dolomite consists of coarse planar subeuhedral crystals (30–400 m) that show concentric cathodoluminescence zoning and are also crosscut by microstylolites. Its 18O values range between 6.6 1.3 (VPDB) at Main Brook and 9.0 0.5 (VPDB) at Daniels Harbour. This dolomite likely precipitated from fluids that circulated through crustal rocks with progressive burial (Th value of 114C 11C and salinity value of 23 1.8 eq. wt. % NaCl). The late-stage D3 dolomite has large and coarse nonplanar crystals (125 m–7 mm) that exhibit sweeping extinction under crossed polars, which is characteristic of saddle dolomite and also sometimes shows thin brightly luminescent rims. It was likely precipitated during deeper burial in pulses and from hot fluids (Th values of 148C 19C and 115C 19.6C and mean salinities of 23 2 and 22 2 eq. wt. % NaCl at Main Brook and Daniels Harbour, respectively). This is also supported by their relatively more depleted 18O (11.1 1.2 and 12.3 1.4 VPDB, respectively) and low Sr contents (88 36 and 38 5.9 ppm, respectively). Porosity in the Boat Harbour Formation is mainly associated with the midburial D2 dolomite. Intercrystalline porosity is the dominant type, and it ranges in the formation from less than 1% to 8% at Main Brook and from 7% to 12% at Daniels Harbour. Vugs are less common but are associated with D3 dolomite. The porous zone in the formation at Main Brook starts approximately 10 to 15 m (33 to 49 ft) below the lower Boat Harbour disconformity and extends down to the lower formational boundary, whereas porous zones in the formation at Daniels Harbour are indiscriminately distributed throughout the section.