Seawater Strontium Research Articles

Marine barite and celestite saturation in seawater

The stoichiometric solubility product , K sp,T *, of barite and celestite in seawater has been calculated using thermodynamic constants, K s0 , and the activity coefficients for barium, strontium , and sulfate in seawater. An equation of the form: ln K sp,T *=A+B ln T+ C T +DS n has been used. The constants A , B , C , D and n are derived from the calculated stoichiometric (or total) solubility product of barite and celestite in seawater as a function of temperature and salinity. T is the absolute temperature (K) and S is the salinity. The effect of pressure on K sp,T * is also calculated. Comparing the solubility products determined from this equation and the pressure effect equation to the distribution of Ba, Sr and SO 4 in seawater, we conclude that the upper surface water of the Southern Ocean is likely supersaturated with respect to pure barite, in agreement with Jeandel et al. [Jeandel, C., Dupre, B., Lebaron, G., Monnin, C., Minster, J.F., 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 43, 1–31.] and Monnin et al. [Monnin, C., Jeandel, C., Cattaldo, T., Dehairs, F., 1999. The marine barite saturation state of the world oceans. Mar. Chem. 65, 253–261.] and that the oceanic water column is typically <30% saturated with respect to celestite. The model, which includes the thermodynamic solid–solution behavior of barite in seawater at 25°C and 1 atm, suggests that this mineral may contain up to 13 mol% SrSO 4 at equilibrium. Accordingly, we have determined the stoichiometric solubility products of strontian barite as a function of salinity and temperature: ln Ba K sp ′=247.88−38.333 ln T− 15421 T +1.27S 0.3 Using our model results for the total solubility product of the Sr-barite phase and seawater Ba and SO 4 concentration data, we conclude that the maximum saturation level of the oceans with respect to marine barite is 63% in the North Atlantic, 88% in the Indian Ocean, and 111% in the North Pacific. The depth of this maximum saturation level is shallower in the Atlantic Ocean (about 1000 m) than in the Pacific and Indian oceans (about 2000 m).

Seawater strontium and Sr/Ca variability in the Atlantic and Pacific oceans

Seawater Sr and Sr/Ca exhibit spatial gradients of 2–3% globally, with the deep ocean more enriched relative to the surface. In latitudinal transects, the highest surface values are found at high latitudes and associated with areas of upwelling. A pronounced upper ocean vertical Sr gradient is attributable to the production of celestite skeletons by surface-dwelling acantharia, coupled to a shallow dissolution cycle. The upper ocean residence time of Sr with respect to celestite cycling is much shorter than its global oceanic residence time. Although the magnitude of seawater Sr/Ca variability is relatively small, it is significant with respect to high-precision paleoceanographic applications. Sr/Ca gradients in the contemporary ocean also complicates evaluating Quaternary changes in seawater Sr/Ca that may have resulted from other processes, such as aragonite recrystallization during sea-level low stands.

The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hy-normative transitional basalts, through alkali basalts, to basanites. Clinopyroxene in the mafic rocks varies in composition from Al, Ti-poor diopside to Al, Ti-rich augite. These two populations can be present simultaneously in the same sample and even in the same crystal, suggesting polybaric fractionation in the pressure range 0–4 kbar, or mixing between basaltic magmas with different degrees of alkalinity. On the basis of their major and trace element and Sr–Nd–Pb isotope composition and age of eruption, two groups of basalts are distinguished: a high TiO2–P2O5 group, erupted before 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, separated by a caldera collapse. The felsic volcanic rocks have compositions ranging from comenditic trachyte to comendite and pantelleritic trachyte to pantellerite, with progressively increasing peralkalinity. The Sr–Nd isotope compositions of most of the felsic volcanic rocks are similar to those of the mafic volcanic rocks, except for some very Sr-poor pantellerites, which show post-depositional exchange with seawater strontium. On the basis of their petrographic and geochemical characteristics, Sr–Nd–Pb isotope data, computer modelling and geological observations, it is suggested that the mafic volcanic rocks represent a number of different alkaline parental magmas from which the felsic volcanic rocks were derived via prolonged, closed-system fractional crystallization. The source region for the parental magmas was heterogeneous, and may have involved at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and a HIMU-like enriched component. A further enriched component, similar to the Enriched Mantle 1 (EM 1) component, could also have been involved. According to geophysical data, the lithosphere is thinned beneath the island, and the asthenospheric mantle rises to a depth of 60 km. Rare earth element data require residual garnet in the source and constrain the melting process to a depth of 70–80 km. The petrological and geochemical data suggest that the mafic magmas are generated within the asthenospheric mantle, from a deep plume bringing the HIMU–EM 1 isotopic and trace element signatures. Interaction of these OIB-like magmas with the shallower asthenospheric mantle, providing a depleted MORB signature, gives rise to magmas with the observed isotopic and geochemical characteristics.

The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hy-normative transitional basalts, through alkali basalts, to basanites. Clinopyroxene in the mafic rocks varies in composition from Al, Ti-poor diopside to Al, Ti-rich augite. These two populations can be present simultaneously in the same sample and even in the same crystal, suggesting polybaric fractionation in the pressure range 0–4 kbar, or mixing between basaltic magmas with different degrees of alkalinity. On the basis of their major and trace element and Sr–Nd–Pb isotope composition and age of eruption, two groups of basalts are distinguished: a high TiO2–P2O5 group, erupted before 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, separated by a caldera collapse. The felsic volcanic rocks have compositions ranging from comenditic trachyte to comendite and pantelleritic trachyte to pantellerite, with progressively increasing peralkalinity. The Sr–Nd isotope compositions of most of the felsic volcanic rocks are similar to those of the mafic volcanic rocks, except for some very Sr-poor pantellerites, which show post-depositional exchange with seawater strontium. On the basis of their petrographic and geochemical characteristics, Sr–Nd–Pb isotope data, computer modelling and geological observations, it is suggested that the mafic volcanic rocks represent a number of different alkaline parental magmas from which the felsic volcanic rocks were derived via prolonged, closed-system fractional crystallization. The source region for the parental magmas was heterogeneous, and may have involved at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and a HIMU-like enriched component. A further enriched component, similar to the Enriched Mantle 1 (EM 1) component, could also have been involved. According to geophysical data, the lithosphere is thinned beneath the island, and the asthenospheric mantle rises to a depth of 60 km. Rare earth element data require residual garnet in the source and constrain the melting process to a depth of 70–80 km. The petrological and geochemical data suggest that the mafic magmas are generated within the asthenospheric mantle, from a deep plume bringing the HIMU–EM 1 isotopic and trace element signatures. Interaction of these OIB-like magmas with the shallower asthenospheric mantle, providing a depleted MORB signature, gives rise to magmas with the observed isotopic and geochemical characteristics.

Effects of Quaternary Sea Level Cycles on Strontium in Seawater

The effects of Quaternary sea level changes on the Sr budget of the ocean are investigated using coupled numerical models of the seawater Sr and Ca budgets. Glacial/interglacial sea level cycles influence the Sr concentration of seawater directly through the periodic exposure and weathering of aragonite on continental shelves and indirectly by modulating the location and extent of carbonate deposition in the ocean. As sea level recedes, brief pulses of Sr are added to the ocean from recrystallization of shelf aragonite; this flux at its maximum is nearly ten times larger than the combined riverine and hydrothermal influx. The large shelf recrystallization fluxes during glacial maxima produce rapid increases in the Sr/Ca ratio of seawater despite the long residence time of Sr in the ocean. Variations in the ratio of aragonitic vs. calcitic sedimentation also influence on the Sr content of seawater because the Sr partitioning coefficient is higher in aragonite; however, this effect is minor compared to shelf aragonite recrystallization. Based on a variety of sensitivity tests, the amplitude of Sr/Ca variations ranges from 1 to 3 % and is maximized by constant rates of total carbonate deposition, extensive aragonite recrystallization, and carbonate budgets which include dissolution of shelf carbonates. Such variation is sufficient to produce up to 1.5°C errors in paleotemperatures calculated from coral Sr/Ca ratios since the last glacial maximum.

Isotopic constraints on the Cenozoic evolution of the carbon cycle

In the last few years, several models have been built to explore the Cenozoic evolution of the carbon and strontium cycles. Of particular interest is the study of the impact on the carbon cycle of major mountain uplifts such as the Himalayan orogeny. To explain the Cenozoic increase in the measured seawater strontium isotopic ratio, it was recently proposed that the Himalayan uplift could be responsible for an enhanced consumption of atmospheric CO 2 by continental silicate weathering. Here, a new model of the carbon cycle evolution over Cenozoic times is presented. It calculates the various fluxes involved in the organic and inorganic components of the carbon cycle from the seawater δ 13C, the biological isotopic fractionation in the ocean and the seafloor spreading rate. The model equilibrates the budgets of the carbon and alkalinity cycles on the million year timescale, assuming as many previous investigators that the system remains close to equilibrium. The validity of this equilibrium approximation is examined critically. Various sensitivity experiments are performed in order to test the impact of the model parameters on the results. The calculated history of the carbonate deposition rate is consistent with the available reconstruction. The continental silicate weathering rate calculated by the model appears to be widely insensitive to the model parameters, showing three distinct evolutions over the Cenozoic. The model indeed suggests a time of relative constancy of the silicate weathering flux before 40 Ma, followed by a period of slow decrease until 15 Ma and finally a marked increase up to the present. In a progressively cooler world, this evolution may be interpreted as a change from a `chemically' controlled to a `physically' controlled weathering regime. The evolution of continental silicate weathering thus partly appears decoupled from the increase in the observed seawater strontium isotopic ratio. For this reason, the evolution of the calculated riverine 87Sr/ 86Sr ratio shows a strong increase over the Cenozoic, from about 0.710 to 0.712. However, this increase may largely be reduced by considering the recycling of a pelagic carbonate reservoir increasing over the Cenozoic or by assuming that seafloor basalt weathering is a CO 2- or climate-dependent process.

Analysis of the Essential Nutrient Strontium in Marine Aquariums by Atomic Absorption Spectroscopy: An Undergraduate Analytical Chemistry Laboratory Exercise

An undergraduate atomic absorption spectroscopy (AAS) laboratory experiment is presented involving the analysis of the essential nutrient strontium in a real-life sample, sea water. The quantitative analysis of strontium in sea water is a problem well suited for an undergraduate analytical chemistry laboratory. Sea water contains numerous components which prevent the direct quantitative determination of strontium. Students learn first hand about the role of interferences in analytical measurements, and about the method of standard addition which is used to minimize these effects. This laboratory exercise also introduces undergraduate students to practical problems associated with AAS. We encourage students as a part of this experiment to collect and analyze marine water samples from local pet shops.

An evaluation of strontium isotopic dating of ferromanganese oxides in a marine hydrogenous ferromanganese crust

In order to assess the utility of dating marine, hydrogenous ferromanganese crusts with strontium isotopes, we conducted a series of leaching experiments on samples selected from several stratigraphic layers of a well-preserved 9.5-cm-thick crust from the Pacific Ocean. Initially, using a method similar to that of previous studies, acetic-acid-leached residues of twelve subsamples were leached with HCl and the leachates analyzed for their isotopic composition. The results did not make stratigraphic sense, so we carried out two sequences of more detailed leaches on subsamples from four layers representing the chemical and textural spectrum observed in the crust. In one sequence, mineral-specific leaches were selected for optimal extraction of strontium associated with the respective manganese and iron oxide phases, the main compositional endmembers within ferromanganese crusts. Strontium isotopic ratios for these leaches were compared with those from the other sequence, involving acetic acid plus HCl leaches on the same material. Residues remaining at the end of each leaching sequence were also analyzed. It is evident from the results that HCl extracts strontium associated with both phosphatic material and aluminosilicate detritus present within the ferromanganese layers. 87Sr 86Sr ratios obtained on the manganese oxides indicate that the strontium within the oxides exchanges with seawater strontium throughout the history of the deposit, regardless of the porosity of the layer. Assuming that this crust is representative of other marine, hydrogenous crusts, strontium isotopes appear to be of little value for the direct dating of ferromanganese oxide deposits.

Strontium and neodymium isotopic analyses of marine barite separates

Strontium and neodymium isotopic data are reported for barite samples chemically separated from Late Miocene to Pliocene sediments from the eastern equatorial Pacific. At a site within a region of very high productivity close to the equator, 87Sr/ 86Sr ratios in the barite separates are indistinguishable from those of foraminifera and fish teeth from the same samples. However, at two sites north of the productivity maximum barite separates have slightly, but consistently lower (averaging 62 × 10 −6) ratios than the coexisting phases, although values still fall within the total range of published values for the contemporaneous seawater strontium isotope curve. We examine possible causes for this offset including recrystallization of the foraminifera, fish teeth or barite, the presence of non-barite contaminants, or incorporation of older, reworked deep-sea barite; the inclusion of a small amount of hydrothermal barite in the sediments seems most consistent with our data, although there are difficulties associated with adequate production and transportation of this phase. Barite is unlikely to replace calcite as a preferred tracer of seawater strontium isotopes in carbonate-rich sediments, but may prove a useful substitute in cases where calcite is rare or strongly affected by diagenesis. In contrast to the case for strontium, neodymium isotopic ratios in the barite separates are far from expected values for contemporary seawater, and appear to be dominated by an (unobserved) eolian component with high neodymium concentration and low 143Nd/ 144Nd. These results suggest that the true potential of barite as an indicator of paleocean neodymium isotopic ratios and REE patterns will be realized only when a more selective separation procedure is developed.

Extinction of inoceramid bivalves in Maastrichtian strata of the Bay of Biscay region of France and Spain

Maastrichtian strata of the Zumaya-Algorta Formation of southwestern France and northeastern Spain record a major pulse of extinction among inoceramid bivalves well before the Cretaceous–Tertiary (K–T) boundary. Inoceramids are the most abundant macrofossils preserved in the study sections; at least six species ofInoceramus[I. (Endocostea)aff.I.(En.) balticusGiers,I.(En.) pteroidesGiers,I.(Platyceramus)aff.I.(Pl.) cycloidesWegner,I.(Trochoceramus) nahorianensisKociubynskij,I.(Tr.) morganiSornay, andI.(?)goldfussianusd'Orbigny] are common to abundant in lower Maastrichtian strata. However, all six species disappear over a few tens of meters of section near the base of the upper Maastrichtian, as defined by the first appearance of the planktonic foraminiferAbathomphalus mayaroensis. Tenuipteria argentea(Conrad), which has not been recovered from the lower Maastrichtian portions of the sections, occurs at low abundances through the upper Maastrichtian, disappearing within 10 cm of the K–T boundary.The mid-Maastrichtian extinction interval among inoceramids occurs within the upperGlobotruncana gansserito lowerAbathomphalus mayaroensisplanktonic foraminiferal zone, in nannofossil zone 24 to 25A, in theAnapachydiscus fresvillensisammonite zone, in magnetochron 31N, and near the base of a change in slope of the seawater strontium curve, all as recognized by previous studies in one or more of the study sections. Whereas the new data presented here are not global in extent, the observed distribution of inoceramids may be the local manifestation of global oceanic changes during the mid-Maastrichtian.

Strontium isotopic variations in Jurassic and Cretaceous seawater

A high-resolution seawater strontium isotope curve has been generated through the analysis of well-dated and well-preserved belemnites and oysters from the Middle and Upper Jurassic and the Lower Cretaceous of Great Britain. Analysis of Fe and Mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered. The strontium isotope curve remains relatively flat through the Aalenian and early Bajocian, rapidly descends through the late Bajocian and Bathonian, and reaches a minimum in the Callovian and Oxfordian. It then begins a rapid increase in the Kimmeridgian and Portlandian that continues through much of the early Cretaceous. The curve levels off in the Barremian, suddenly dips downwards in the Aptian, and recovers gradually through the Albian. The strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of Great Britain using the techniques of strontium isotope stratigraphy. Model results indicate that much of the variation in seawater 87Sr 86Sr between 120 and 40 Ma can be explained by changing the intensity of mid-ocean ridge hydrothermal fluxes proportionally to estimated mid-ocean ridge crustal generation rates. It is also possible that the variations during the rest of the Mesozoic and the Permian are primarily reflections of changing hydrothermal inputs. The model results have several important implications. First, they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs. Second, they suggest that from at least the Aptian through the Eocene variations in continental weathering were minimal. This heightens the importance of the rapid rise in seawater 87Sr 86Sr beginning ~ 40 Ma as a significant transition to an extended period of increasing fluvial 87Sr fluxes continuing to the present. Finally, the results suggest that several documented short-term excursions towards lower 87Sr 86Sr in the latest Triassic, Pliensbachian-Toarcian, Callovian-Oxfordian, Aptian-Albian, and Cenomanian-Turonian are interpretable as pulses of seafloor hydrothermal activity. If so, the strontium isotope record offers a means of constraining the timing, duration, and magnitude of known or proposed hydrothermal events in the geological record.

Strontium isotopes in Early Jurassic seawater

The analysis of well-preserved, well-dated belemnites and oysters from the Jurassic of Great Britain has resulted in a well-constrained, detailed seawater strontium isotope curve for the Early Jurassic. The preservation of fossil low-Mg calcite was monitored using Mn, Fe, δ 13 C, and δ 18 O. Iron was the most useful indicator, with about 75% of the samples containing more than 150 ppm Fe showing 87Sr 86Sr ratios elevated relative to adjacent points on the curve. High Mn concentrations less often correlated with elevated 87Sr 86Sr ratios; however, low Mn concentrations (<25 ppm Mn) were consistently associated with apparently well-preserved 87Sr 86Sr . δ 13 C and δ 18 O proved to be insensitive to diagenesis as it affects 87Sr 86Sr . The principal features of the strontium isotope curve include a rise to about 0.70772 in the latest Triassic and earliest Jurassic (Hettangian). From the Hettangian, the curve begins a roughly linear descent through the Sinemurian and Pliensbachian. Following a small levelling off and increase in the late Pliensbachian, the curve falls rapidly to its Early Jurassic minimum of 0.70706. It then gently increases through the Toarcian until the falciferum zone, where it shows an apparently abrupt increase to 0.70719 before continuing its slow increase to 0.70728 in the Aalenian-Bajocian (Middle Jurassic). This reversal of the downwards trend established in the Sinemurian and Pliensbachian had not been previously identified. The Sinemurian and Pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones (±0.5 to 1 Ma).

Marine barite as a monitor of seawater strontium isotope composition

THE strontium isotope ratio in sea water is influenced by climate, tectonics, weathering and hydrothermal activity at ocean ridges1–4. Its evolution through time, determined primarily by measuring the strontium isotope composition of marine carbonates, holds information about variations in these processes, and is also useful for stratigraphic correlation and dating5–7. Carbonates are absent from some marine sediments such as siliceous oozes and red clays, and can be significantly diagenetically altered in others, especially in Eocene and older sediments. Here we show that marine barite is an effective alternative monitor of seawater 87Sr/86Sr. We find that microcrystals of marine barite separated from Holocene Pacific, Atlantic and Indian Ocean sediments all record the modern seawater 87Sr/86Sr value. Moreover, the 87Sr/86Sr of barite from 25 sediment samples spanning the past 35 Myr falls within the range of published data for carbonates over this time period. We conclude that marine barite reliably records both present and past variations in seawater strontium isotope composition.

Synchronous changes in seawater strontium isotope composition and global climate

THE 87Sr/86Sr ratio of sea water has increased gradually over the past 40 Myr, suggesting a concomitant increase in global chemical weathering rates1–6. Recently, Dia et al.7 analysed a 250-kyr 87Sr/86Sr record, and found superimposed on this gradual increase higher-frequency 87Sr/86Sr variations which appeared to follow a 100-kyr cycle; this periodicity corresponds to one of the prominent cycles in the Earth's orbital parameters, which are known to modulate the patterns of solar insolation and hence climate8–10. The resolution of this record was, however, insufficient to establish the phase relationship between the 87Sr/86Sr variations and global climate cycles. Here we present a high-resolution seawater 87Sr/86Sr record spanning the past 450 kyr. We find that maxima and minima in 87Sr/86Sr coincide with minima and maxima, respectively, in continental ice volume (from the SPECMAP oxygen isotope record20), apparently suggesting that there was less chemical weathering in arid glacial periods than in the more humid interglacials. During glacial–interglacial transitions, however, seawater 87Sr/86Sr changes at a rate of ∼ 1 p.p.m. kyr−1, approximately three times that evaluated by Dia et al.7. Mass-balance calculations illustrate that simple changes in modern chemical weathering regimes cannot fully account for such rapid changes, suggesting that we need to revise current ideas about strontium reservoirs and the mechanisms for exchange between them.

Reevaluation of Early Oligocene, Eocene, and Paleocene Seawater Strontium Isotope Ratios Using Outcrop Samples from The U.S. Gulf Coast

The Cenozoic strontium isotope (87Sr/86Sr) variation in seawater is largely defined using Deep Sea Drilling Project (DSDP) samples. Examination of non‐DSDP results, particularly for the Paleogene, suggest that the original seawater curve of Burke et al. (1982) may lie above the true seawater ratio during this time. To investigate this possibility, we analyzed the 87Sr/86Sr for 230 Paleogene rock and shell samples from the 83 sites in Alabama, Mississippi, and Louisiana. Many of the samples are from classic sections on which the Gulf Coast Paleogene stratigraphy was originally defined. Multiple analyses of samples from these units show very little scatter, and in most cases, precise agreement is obtained among geographically separated collection sites. We believe that analysis of multiple samples from a unit of established age, collected at more than one site, yields results that most accurately define the actual seawater Sr isotope ratio at any given time. Results from these units define a seawater variation line that tracks well within the range of DSDP data, but is generally below the original seawater line of Burke et al. (1982). It is often assumed that samples from continental settings will exchange with high‐ratio Sr thereby raising 87Sr/86Sr values. Our Gulf Coast samples, which are derived from a continental setting, yield individual 87Sr/86Sr values that are both higher and lower than the estimated contemporaneous seawater ratio. An analysis of our Gulf Coast data and DSDP results show that the direction of isotopic exchange cannot be confidently predicted for individual samples based solely on their derivation from either continental or oceanic settings.

Himalayan Tectonics, Weathering Processes, and the Strontium Isotope Record in Marine Limestones

The time evolution of the isotopic composition of seawater strontium (the ratio of strontium-87 to strontium-86) over the last 500 million years has the form of an asymmetric trough. The values are highest in the Cambrian and Recent (0.7091) and lowest in the Jurassic (0.7067). Superimposed on this trend are a number of smaller oscillations. Consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only Himalayan-style continental collisions can influence the isotope ratio on the scale observed. The contemporary Himalayan orogeny is by far the largest since the late Precambrian Pan-African event that produced the high in the Cambrian.

Seawater strontium isotopic perturbation at the Permian-Triassic boundary, West Spitsbergen, and its implications for the interpretation of strontium isotopic data

Brachiopod shell samples from the Kapp Starostin Formation, West Spitsbergen, provide evidence for a large and rapid drop in seawater strontium isotopic ratios close to the Permian-Triassic boundary. The strontium isotopic shift is associated with similarly dramatic declines in carbon and oxygen isotope curves recorded in the same samples. This pattern can be explained by a paleoceanographic model that we have proposed previously, a model in which there is replacement of a largely stagnant, stratified ocean by a vigorously mixed one at the Permian-Triassic boundary.

The isotopic composition of strontium and sulfur in seawater of Late Permian (Zechstein) age

The Paleozoic minima of 87 Sr 86 Sr ratios and δ 34S values in seawater are observed in anhydrite layers of the Stassfurt (A-2) and Aller (A-4) cycles of evaporation of Late Permian age in Germany. These minima were probably caused by extremely low rates of continental runoff and bacterial sulfate reduction. Thereafter conditions changed suddenly, with a steeply increasing supply of radiogenic Sr and of nutrients into seawater. The latter induced a large growth in the production of organic material, in bacterial activity in sediments, and in the flow of isotopically heavy sulfur back into the oceans. This large change in the cycles of S and Sr (as well as of C and O) within the relatively short Zechstein period (2–5 Ma) reflects a considerable modification of the plate tectonic and environmental conditions shortly before the beginning of the Mesozoic era. The Ochoan deposition in the USA is probably of the same age as the Stassfurt-Leine evaporation in W Europe.

Seawater Strontium Isotopic Variations from 2.5 Million Years Ago to the Present

Measurements of marine carbonate samples indicate that during the past 2.5 million years the (87)Sr/(86)Sr ratio of seawater has increased by 14 x 10(-5). The high average rate of increase of (87)Sr/(86)Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as +/-30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.

Seawater strontium isotopic variations from 2.5 million years ago to the present: Capo, R.C. and D.J. DePaolo, 1990. Science, 249(4964):51–55

Seawater strontium isotopic variations from 2.5 million years ago to the present: Capo, R.C. and D.J. DePaolo, 1990. Science, 249(4964):51–55