Seawater Strontium Research Articles

Fast direct determination of strontium in seawater using high-performance chelation ion chromatography

A new method for the fast direct chromatographic determination of strontium in seawater has been developed. Two coupled silica monolithic columns (total dimensions 200×4.6mm I.D.) chemically modified with iminodiacetic acid (IDA) functional groups were used for baseline separation of all alkaline-earth metals in seawater in less than 4min at flow rate of 4.0mLmin−1, using an eluent containing 0.25M sodium chloride and 2mM glycolic acid, pH 5.11. Several colour-forming ligands, including o-cresolphthalein complexone (o-CPC), methylthymol blue (MTB), xylenol orange (XO), 4-(2-pyridylazo)resorcinol (PAR) and ZnEDTA–PAR were evaluated as reagents for post-column reaction (PCR) based spectrophotometric detection. Sensitivity of the developed method for strontium was at the low mgL−1 level in undiluted seawater samples, in the presence of excessive amounts of calcium and magnesium, using post-column reaction detection with o-CPC at 570nm. The method allowed quantitative determination of strontium in seawater samples (LOD 3.23±0.15mgL−1, sr=4.6%), with direct injection of just 4μL of seawater onto the chromatographic column.

Selective extinction of Triassic marine reptiles during long-term sea-level changes illuminated by seawater strontium isotopes

The relationship between cycles of sea-level change and marine diversity has long been the subject of debate. Large predators may be particularly sensitive to changes in habitat availability and marine productivity driven by changes in sea-level, especially those dependent on nearshore benthic food resources. To test this relationship, we compared the proportional diversity of differing marine reptile ecotypes through the Triassic with the isotopic composition of seawater strontium (87Sr/86Sr), a geochemical index linked to tectonically controlled sea-level change. The proportional abundance of marine reptiles adapted toward a diet of shelled prey rose during times of rapid sea-level rise and fell during times of rapid sea-level fall, while open water forms were more resistant to these changes. Our results indicate that the rate of sea-level change, rather than the absolute magnitude of sea-level or flooded shelf area, played a role in shaping patterns of ecological diversification and ecologically selective extinction during the Triassic. The link between the isotopic composition of seawater strontium and the evolution of marine reptiles demonstrates that sea-level change played an important role in the structuring of marine ecosystems over geologic time.

An emplacement mechanism for the mega‐block zone within the Chicxulub crater, (Yucatán, Mexico) based on chemostratigraphy

Abstract– To better constrain the emplacement mechanism of the so‐called “mega‐block zone,” a structurally complex unit of target rocks within the Chicxulub impact structure, the stratigraphic coherence of this zone is tested using its strontium isotopic composition. Forty‐eight samples across the 616 m sequence of deformed Cretaceous rocks in the lower part of the Yaxcopoil‐1 core, drilled by ICDP in 2002, were analyzed for their 87Sr/86Sr isotope ratio. The oceanic anoxic event 2 (OAE2 event), located near the base of the core forms the only stratigraphic anchor point. From this point upward to approximately 1050 m depth, the 87Sr/86Sr trend shows small oscillations, between approximately 0.7074 and 0.7073, characteristic of Cenomanian to Santonian values. This is followed by an increase to approximately 0.7075, similar to the one reported in the seawater strontium curve during the Campanian. Scattered Sr isotope ratios are attributed to local diagenetic effects, such as those expected from the possible presence of hot, impact‐induced dikes and hydrothermal fluid flow, originating from the thick central melt sheet. The absence of Upper Maastrichtian Sr isotope values may result from the removal of upper target lithologies during the impact cratering process. Based on these results, the displaced Cretaceous sequence in Yax‐1 appears to have preserved its stratigraphic coherence. During the modification stage, it probably moved as a whole into the annular basin during collapse of the crater wall, thereby breaking up into discrete units along previously weakened detachment zones. This model is consistent with the emplacement mechanism postulated by Kenkmann et al. (2004).

A summary of strontium and oxygen isotope variation in archaeological human tooth enamel excavated from Britain

This paper presents a compilation of strontium and oxygen isotope data from human tooth enamel that has been produced at NERC Isotope Geosciences Laboratory over the last c.15 years. These many and often small studies are here combined to provide an overview of data from Britain. The strontium isotope composition ranges between 0.7078 and 0.7165 (excluding individuals deemed to be of non-British origin). The median Sr concentration is 84 ppm but there is a vector of increasing Sr concentrations related to seawater strontium isotope composition that is seen in individuals predominantly from the west coast of Scotland attributed to the used of kelp as a fertilizer. The oxygen isotope data is normally distributed with a mean value of 17.7‰ ± 1.4‰ (2SD n = 615). Two sub-populations of local individuals have been identified that provide control groups for human enamel values from the eastern side of Britain where there are lower rainfall levels: 17.2‰ ± 1.3‰, (2SD, n = 83) and western area of Britain where rainfall levels are higher = 18.2‰ ± 1‰, (2SD, n = 40). These data make it possible to make direct comparisons of population means between burial populations and the control dataset to assess commonality of origin.

Variability of the fields of concentration of microelements and strontium in seawater near the South Coast of the Crimea in 2002–2007

We study the space-and-time variability of the concentrations of 27 microelements (Sc, Cr, Mn, Fe, Ni, Co, Cu, Zn, As, Mo, Ag, Cd, Sn, Se, Sb, Ba, La, Ce, Sm, Eu, Tb, Hf, Hg, Pb, Au, Th, and U) and Sr in the sea, river, and waste waters of the South Coast of Crimea in 2002–2007. Parallel with the data about elements studied earlier, we present, for the first time, the data of investigations of the contents of rareearth elements (Lu, Nd and Yb) whose concentrations in the major part of the analyzed samples of seawater were lower than the limit of their quantitative determination. We present the ranges of the concentrations of elements and perform their comparison with the data of the previous period of monitoring.

Possible evidence for a large decrease in seawater strontium/calcium ratios and strontium concentrations during the Cenozoic

Few constraints exist on the major element chemistry of ancient oceans. Turritellid marine snails precipitate aragonitic shells and are abundant in the Cenozoic fossil record, and therefore may be ideal for reconstructing past seawater chemistry. Here we report strontium to calcium (Sr/Ca) ratios of modern and early Cenozoic turritellid shells that still retain an aragonitic mineralogy. By applying partition coefficients determined from modern specimens to data for Paleocene and Eocene shells, we calculate paleo-seawater Sr/Ca values. The high Sr/Ca values recorded in fossil shells may imply that seawater Sr/Ca ratios varied from at least 9.5–15.2 mmol/mol, much higher than modern average seawater values (8.6 mmol/mol). High paleo-seawater Sr/Ca ratios may indicate that substantial changes in the biogeochemical cycling of Sr have occurred over the past ~ 37 million years. A decrease in seawater strontium concentrations could have arisen from an increase in the proportion of weathered radiogenic silicate rocks relative to carbonates, reducing the riverine flux of strontium to the oceans, or changes in the exposure and erosion of aragonitic carbonates on tropical shelves due to variations in sea level.

Barium partitioning in coccoliths of Emiliania huxleyi

The coccolithophore Emiliania huxleyi was grown in seawater under different Ba concentrations. The relationship of coccolith Ba/Ca ratio and seawater Ba/Ca ratio was found to be linear. The linear regression yields the apparent Ba exchange coefficient of 0.10. Our data support a recently proposed generic model (Langer G., Gussone N., Nehrke G., Riebesell U., Eisenhauer A., Kuhnert H., Rost B., Trimborn S., and Thoms S. (2006) Coccolith strontium to calcium ratios in Emiliania huxleyi: the dependence on seawater strontium and calcium concentrations. Limnol. Oceanogr. 51, 310–320.) developed for explaining apparent exchange coefficients of metabolically inert divalent trace metals, such as Sr, in E. huxleyi. This model represents the first approach combining cell physiological processes and data from inorganic precipitation experiments, which quantitatively explains coccolith apparent Sr and Ba exchange coefficients.

A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth

The strontium isotope composition of seawater is strongly influenced on geological time scales by changes in the rates of continental weathering relative to ocean crust alteration. However, the potential of the seawater 87 Sr/ 86 Sr curve to trace globally integrated chemical weathering rates has not been fully realised because ocean 87 Sr/ 86 Sr is also in- fluenced by the isotopic evolution of Sr sources to the ocean. A preliminary attempt is made here to normalise the seawa- ter 87 Sr/ 86 Sr curve to plausible trends in the 87 Sr/ 86 Sr ratios of the three major Sr sources: carbonate dissolution, sili- cate weathering and submarine hydrothermal exchange. The normalised curve highlights the Neoproterozoic-Phanerozoic transition as a period of exceptionally high continental influ- ence, indicating that this interval was characterised by a tran- sient increase in global weathering rates and/or by the weath- ering of unusually radiogenic crustal rocks. Close correlation between the normalised 87 Sr/ 86 Sr curve, a published seawa- ter 34 S curve and atmospheric pCO2 models is used here to argue that elevated chemical weathering rates were a ma- jor contributing factor to the steep rise in seawater 87 Sr/ 86 Sr from 650 Ma to 500 Ma. Elevated weathering rates during the Neoproterozoic-Cambrian interval led to increased nutri- ent availability, organic burial and to the further oxygenation of Earth's surface environment. Use of normalised seawater 87 Sr/ 86 Sr curves will, it is hoped, help to improve future geo- chemical models of Earth System dynamics.

A normalised seawater strontium isotope curve and the Neoproterozoic-Cambrian chemical weathering event

A normalised seawater strontium isotope curve and the Neoproterozoic-Cambrian chemical weathering event

87Sr/ 86Sr record of Permian seawater

A population of 169 Permian articulate brachiopod shells was analysed for their 87Sr/ 86Sr ratios. 51 of these, characterised as well preserved and stratigraphically well defined, are utilized for delineation of the Permian seawater strontium isotope trend. The 87Sr/ 86Sr curve shows values of about 0.7080 in the lowermost Permian (Asselian), followed by a gradual decline to 0.70685 in the Capitanian and a minor rise in the Dzhulfian and lower Dorashamian to values of 0.70715 just below the Permian-Triassic boundary. The Early Permian decrease in the strontium isotope curve that commences in the early Sakmarian is coincident with the advancing deglaciation of the Gondwana and with the increased aridity in large parts of the Pangaea. These factors may have led to a reduced continental weathering of Rb-rich silicate rocks, and thus to the decline in seawater 87Sr/ 86Sr. Starting with the Artinskian, the opening of the Neotethys and the associated widespread basaltic volcanism supplied low radiogenic strontium to seawater from an enhanced hydrothermal flux. In the Capitanian, basaltic volcanism in the entire Palaeotethys ceased and this may have been the reason for a slightly more radiogenic seawater isotopic composition during the Lopingian. Higher input of riverine Sr due to expansion of humid areas may have been a contributory factor.

Isotopic microanalysis of seawater strontium in biogenic calcite to assess subsequent rehomogenisation during metamorphism

Precise LAM-MC-ICPMS in situ microanalyses of 87Sr/ 86Sr ratios in biogenic calcite of the bivalve ‘ Atomodesma’ are reported for 14 horizons in low-grade metasediments of late Early Permian to late Late Permian age in South Island, New Zealand. ‘ Atomodesma’ shells (and associated skeletal echinoderm calcite) in zeolite and possibly to lower prehnite-pumpellyite mineral facies material yield isotopic data that are interpreted as primary seawater 87Sr/ 86Sr values. Data for higher metamorphic grade (probable upper prehnite-pumpellyite to lawsonite-albite facies) samples consistently show evidence of Sr isotopic homogenisation between ‘ Atomodesma’ calcite and host matrix and thus seawater 87Sr/ 86Sr values are not retained in shell debris under any circumstances.

Global correlation for strontium isotope curve in the Late Cretaceous of Tibet and dating marine sediments

87Sr/86Sr ratios of marine carbonate samples collected from a sedimentary section of the Late Cretaceous in the south of Tibet were measured. Based on the absence of cathodo-luminescence and a very low Mn/Sr ratio (average 0.06) of the samples, it is thought that they contain information on the original seawater strontium isotope composition. The strontium isotope evolution curve of the Late Cretaceous in Tibet we established here, is consistent with other coeval curves from Europe, North America and Antarctica, supports the notion that the strontium isotope composition of seawater is governed by global events, which provides a new approach for the inter-continental and inter-basinal correlations of Late Cretaceous in the area and is a complementarity for biostratigraphy. In addition, we attempt to determine the age of the boundaries for Campanian/Santonian and Maastrichtian/Campanian by87Sr/86Sr ratios for Gamba section in southern Tibet. The two boundaries are located in the thickness of 217 m (83.5 Ma) and 291 m (71.3 Ma), respectively.

On the celestite-secreting Acantharia and their effect on seawater strontium to calcium ratios

Significant variations in the Sr/Ca in waters from the eastern Indian Ocean in the vicinity of Australia, both from the surface and from shallow depth profiles, are documented. The strontium sulfate-secreting protozoans Acantharia, which are common in the upper 400 m of the oceans, especially at low latitudes, contribute substantially to changes in the Sr/Ca of oceanic waters by extracting Sr for the formation of their skeletons. Below such depths, these organisms dissolve and the Sr/Ca of seawater regains its ‘conservative’ nature. This mechanism accounts for some of the variability of the Sr/Ca near the surface of the ocean, but this may still be found at depth as surface waters become entrained at greater depths. It is argued here that the noted Sr/Ca variations may explain discrepancies between coral data sets from different parts of the oceans, and calls for caution when reconstructing sea-surface temperatures from the Sr/Ca of corals.

Optimization of a Mobile Phase for Monitoring Strontium in Seawater Using Non‐suppressed Ion Chromatography

A non‐suppressed ion chromatographic (IC) method has been developed for the monitoring of strontium in the presence of a large excess of alkali and alkaline earths. The mobile phase consisting of ethylenediamine (EDA) and organic acid as a complexing agent has been optimized in combination with a high‐speed cationic column. The reported method has been tested, successfully, to monitor strontium in coastal Arabian seawater with a coefficient of variation 2 (n = 5). The reported results have been compared with ICP‐OES analysis and are in agreement.

Imbalance in the oceanic strontium budget

Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation.

Aftermath of a snowball Earth

Using a simple 3‐box model of the ocean‐atmosphere system, we simulate the cycling of carbon and strontium in the aftermath of a global glaciation. Model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pCO2 conditions as well as ocean mixing, air‐sea gas exchange, and biological productivity. The δ13C of the first carbonate precipitated after the glaciation depends on the pCO2, temperature, the saturation state of the surface ocean, and kinetic effects associated with mineral precipitation. With no biological productivity, the model produces δ13C values between +1‰ and −3‰, consistent with observations. This is in direct contradiction with arguments by Kennedy et al. [2001a], who suggest that the δ13C value of dissolved carbon in a snowball ocean (and directly afterward) must be −5‰. Kennedy et al. assume the carbon isotope cycle is in steady state, which does not apply to a global glaciation, and also neglect any effect of high pCO2 on the carbonate chemistry of seawater. A major difference between our findings and the qualitative predictions of Hoffman et al. [1998] is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves. As a result, the ∼2‰ drop in the δ13C observed in the cap dolostone is unlikely to be the product of Rayleigh distillation of atmospheric CO2 via silicate weathering. Instead, we interpret the ∼2‰ drop in the δ13C values as indicative of an increase in sea surface temperature which lowers the fractionation between CO2 and carbonate. Kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important. Rayleigh distillation of atmospheric CO2 via silicate weathering is a viable explanation for the continued drop in the δ13C values in the limestone sequence above the cap dolostone, with biological productivity and carbonate weathering driving a slow increase in δ13C values once pCO2 levels decline. Our study also simulates the cycling of strontium in seawater. In contrast to the finding of Jacobsen and Kaufman [1999] and Kennedy et al. [2001a], model simulations show a drop in 87Sr/86Sr of less than 0.001 during 5 million years of global glaciation and an increase of less than 0.001 over the entire episode of silicate weathering. Our calculations emphasize the importance of considering the changes in seawater chemistry due to high pCO2 in evaluating the Snowball Earth hypothesis.

A Cenozoic seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes

A Cenozoic multi-species record of benthic foraminiferal calcite Sr/Ca has been produced and is corrected for inter-specific offsets (typically less than 0.3 mmol/mol) and for the linear relationship between decreasing benthic foraminiferal Sr/Ca and increasing water depth. The water depth correction, determined from Holocene, Late Glacial Maximum and Eocene paleowater-depth transects, is ∼0.1 mmol/mol/km. The corrected Cenozoic benthic foraminiferal Sr/Ca record ranges from 1.2 to 2.0 mmol/mol, and has been interpreted in terms of long-term changes in seawater Sr/Ca, enabling issues related to higher-resolution variability in Sr/Ca to be ignored. We estimate that seawater Sr/Ca was ∼1.5 times modern values in the late Cretaceous, but declined rapidly into the Paleogene. Following a minimum in the Eocene, seawater Sr/Ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late Miocene. By assuming scenarios for changing seawater calcium concentration, and using published carbonate accumulation rate data combined with suitable values for Sr partition coefficients into carbonates, the seawater Sr/Ca record is used to estimate global average river Sr fluxes. These fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity/tectonic outgassing to calculate changes in global average river 87Sr/ 86Sr through the Cenozoic. The absolute magnitude of Sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties. Nevertheless, our results suggest that river Sr flux increased from 35 Ma to the present day (roughly two-fold) accompanied by an overall increase in 87Sr/ 86Sr (by ∼0 to 0.001). Between 75 and 35 Ma, river 87Sr/ 86Sr also increased (by ∼0.001 to 0.002) but was accompanied by a decrease (two- to three-fold) in river Sr flux.

Seawater strontium isotope curve at the Silurian/Devonian boundary: a study of the global Silurian/Devonian boundary stratotype

Present application of 87Sr/ 86Sr chemostratigraphy to detailed stratigraphical tasks is limited by inaccurate calibration of the general seawater strontium curve to absolute as well as to relative time scales. For this reason, refinement of the general seawater strontium curve has been suggested, using mainly clearly defined global boundary stratotype sections. This study reports the first 87Sr/ 86Sr data from the global Silurian/Devonian boundary stratotype section and fills an existing 1-Ma gap in available data. Generally, the data from the stratotype fit the range interpolated from published 87Sr/ 86Sr data of the general curve, but the slight differences may suggest an existence of a high-order oscillation near the Silurian/Devonian boundary. Higher 87Sr/ 86Sr values in the Devonian part of boundary bed 20 (20-beta) may indicate an exotic material influx of recycled sediment.

Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous

There were three negative seawater strontium-isotope excursions (shifts to lower 87 Sr/ 86 Sr values) during the Jurassic and Cretaceous that were of relatively short duration (5-13 my) and showed a relatively quick recovery to pre-excursion 87 Sr/ 86 Sr ratios. These excursions occurred in the Pliensbachian-Toarcian (Early Jurassic), Aptian-Albian, and Cenomanian-Santonian (Early and Late Cretaceous respec- tively). Each excursion coincided closely in time with an Oceanic Anoxic Event (OAE) marked by sediments unusually rich in organic carbon. The Jurassic OAE occurred at the end of the strontium-isotope excursion, whereas the two Cretaceous OAEs oc- curred at the onset of the accompanying strontium-isotope excursions. The possible causes of these excursions were evaluated by successively examining the changes in the riverine strontium fluxes, riverine 87 Sr/ 86 Sr ratios, or hydrothermal strontium fluxes required to produce each excursion. A range of seawater strontium budgets was used to encompass the uncertainties in modern and ancient cycles. To produce the excursions, we calculate that the riverine strontium fluxes would have had to decrease by 6 to 15 percent or the fluvial 87 Sr/ 86 Sr ratios by 0.00019 to 0.00046. The uncertainties largely stem from the assumed magnitude of the hydrothermal strontium flux at the onset of each excursion. Alternatively, increases in sea-floor hydrothermal activity of 7 to 104 percent could also have produced the strontium-isotope excursions. This large range is due mostly to uncertainties in the relative flux of strontium from axial high-temperature hydrothermal systems and low-temperature off-axis systems. Only a small portion of this range stems from uncertainties in the riverine strontium terms. The possible causes of the excursions were further evaluated by examining several geologic factors that could have affected riverine strontium, including climate change, sealevel, and the eruption of flood basalts. We conclude that neither variations in riverine strontium fluxes nor in 87 Sr/ 86 Sr ratios is the likely cause of the strontium- isotope excursions. The most probable explanation is increased rates of hydrothermal activity related to increased ocean-crust production at the mid-ocean ridges. The close correlation in time between the strontium-isotope excursions and the major Oceanic Anoxic Events (OAEs) is compatible with a causal linkage. We propose that increased ocean-crust production led to enhanced CO2 outgassing and global warming, which in turn led to several processes that acted to make surface ocean waters more productive. However, because OAEs did not occur throughout the proposed periods of enhanced hydrothermal activity, it appears that these processes only preconditioned the oceans for the OAEs: sealevel rise may have been the final trigger. This model explains why all three OAEs did not occur at the same time relative to the onset of excess hydrothermal activity and why OAEs are not associated with every sealevel rise documented in the stratigraphic record.

Dating the 840–544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models

We construct a time scale for the 840–544 Ma Neoproterozoic interval from isotopic variation of δ 13C carbonate and δ 13C organic, 87Sr/ 86Sr, and δ 34S sulfate in seawater measured from reference columns in Canada and Australia. We distinguish 18 features (Z–I) in the δ 13C carbonate and δ 13C organic curves: two intervals of well-defined variation in 87Sr/ 86Sr; and two peaks in the variation of δ 34S sulfate. Newly acquired isotopic data in Australia enable correlation with Canada: the Gillen Member of the Bitter Springs Formation, estimated to be about 840 Ma, is correlated with the upper Shaler Supergroup; the Sturtian glacials, about 700 Ma, with the Rapitan glacials; and the Marinoan glacials, about 600 Ma, with the Ice Brook glacials. We recognize only these two major glaciations, and possibly a third minor glaciation, at 570 Ma. Columns in Poland, Namibia, Iran, and Siberia, and possibly Oman and Mongolia provide correlation by δ 13C, and in Svalbard, Siberia, Oman, and Mali by 87Sr/ 86Sr. The inter-glacial (700–600 Ma) peak of δ 34S sulfide enables correlation among Australia, Namibia, and China. These correlations allow calibration of the resultant stratigraphy against time using the best available dates from a number of regions; we make simple linear interpolations between those dates. While recognizing that this can be no more than a rough approximation of the true ages away from the calibration points, the resultant age estimates have the merit of suggesting numerous tests of our stratigraphic scheme. The Neoproterozoic part of the 87Sr/ 86Sr curve resembles in range the Phanerozoic part, but of δ 13C and δ 34S sulfate do not: the 20.5‰ amplitude of δ 13C and 26.5‰ of δ 34S greatly exceed the Phanerozoic 7.5 and 17‰, reflecting radically reduced net carbon and sulfur fluxes in younger times. The earliest Phanerozoic explosion of organisms with carbonate skeletons and the proliferation of bioturbating organisms are coincident with the onset of a C-cycle with isotopic fluctuations damped in both frequency and amplitude. The Neoproterozoic glaciations at 700 and 600 Ma are marked by negative δ 13C carbonate and correspondingly depleted δ 13C organic and lower 87Sr/ 86Sr. The minimum 87Sr/ 86Sr at 840 Ma reflects the input to an ephemeral epeiric sea of excess 86Sr from extensive mafic volcanics in Australia and possibly Canada. The onset of higher values at 600 Ma corresponds to the early Pan-African and Cadomian amalgamation, and internal deformation, uplift, and erosion in Antarctica–Australia. The wealth of new data and the time framework they suggest allow us to build on the work of earlier authors and make a fresh attempt at explaining some of the major features of Neoproterozoic history, particularly in the interval from 700 Ma to the base of the Cambrian. Quantitative modeling is beyond the scope of our study, but we offer explanations for some of the isotopic features we have documented. Two icehouse states were preceded by massive sequestering of CO 2 and accompanied by catastrophic declines in biological productivity. During and immediately after the older, Sturtian, glaciation, the deeper parts of the ocean were anoxic and contained sufficient ferrous iron to sequester very large amounts of sulfur derived from bacterial reduction of sulfate; the evidence suggests that the resultant huge shift in the sulfur isotropic composition was global and accompanied by the reduction of as much as half the sulfate in the anoxic parts of the oceans. The effects of the Pan-African orogeny include mountain building and a high rate of sedimentation, which resulted in the burial of large amounts of organic matter and concomitant oxygenation of the hydrosphere and atmosphere. This reinforced a trend in oxygenation that began before the second, Marinoan, glaciation. A second huge sulfur isotope anomaly accompanies the tectonism, and has been explained previously as possibly resulting from the desiccation and flushing of evolving ocean basins. This may be linked to a remarkable carbon isotope anomaly immediately preceding the Cambrian: the anomaly could be due to release of methane from oceanic clathrates de-stabilized by combined sea level fall and global warming resulting from volcanic release of CO 2.