Sr Concentration Data Research Articles

Strontium concentration spikes as evidence for fluid-driven carbonate dissolution during subduction

Carbonate dissolution, the process by which carbonate minerals are stoichiometrically dissolved by an aqueous fluid, likely plays an important role in liberating carbon from subducting slabs and may thus make significant contributions to global carbon fluxes. It is therefore necessary to understand the fluid infiltration and flow geometries associated with this process. Despite its widespread use as a powerful tracer of fluid-rock interaction, strontium (Sr) has yet to be widely applied to carbonate dissolution scenarios. Previous work has identified a partially altered metacarbonate layer from Syros, Greece, that has undergone significant carbonate dissolution leading to ∼90% CO2 loss (Ague and Nicolescu, 2014). A bulk-rock compositional profile along this layer has a distinctive spike-shaped Sr profile. Sr concentrations are uniform in the altered region behind the reaction front, increase to form a spike just ahead of the front, and taper off to a baseline value moving farther into the unaltered metacarbonate rock. The origin of this type of spike shape and its potential utility for better understanding carbonate dissolution processes remain unaddressed. Here, we present a numerical diffusion-reaction model which demonstrates that both carbonate dissolution and multidimensional transport of Sr, in which the primary advective flow direction is perpendicular to the propagation of the diffusion front, are required to produce this shape. This model, as well as newly acquired Sr concentration data for calcite (former aragonite) and epidote, also indicate that the infiltration of a fluid with an elevated Sr concentration is necessary. We suggest that such spike-shaped features may aid in identifying carbonate dissolution in other settings, such as vein-selvage systems in metacarbonate rocks or reaction rinds on carbonate-bearing mélange blocks.

Waters from the Djiboutian Afar: A Review of Strontium Isotopic Composition and a Comparison with Ethiopian Waters and Red Sea Brines

Drinking water is scarce in Djibouti because of the hot desert climate. Moreover, seawater intrusion or fossil saltwater contamination of the limited number of freshwater aquifers due to groundwater overexploitation affect those who live close to the coastline (~80% of the population). Despite this, the geothermal potential of the country’s plentiful hot springs could resolve the increasing electricity demand. Strontium isotopes (87Sr/86Sr) are routinely used to determine sources and mixing relationships in geochemical studies. They have proven to be useful in determining weathering processes and quantifying endmember mixing processes. In this study, we summarise and reinterpret the 87Sr/86Sr ratio and Sr concentration data of the groundwater collected to date in the different regions of the Djibouti country, trying to discriminate between the different water sources, to evaluate the water/rock ratio and to compare the data with those coming from the groundwater in the neighbouring Main Ethiopian Rift and the Red Sea bottom brine. New preliminary data from the groundwater of the Hanlé-Gaggadé plains are also presented.

Calcium isotope constraints on the marine carbon cycle and CaCO3 deposition during the late Silurian (Ludfordian) positive δ13C excursion

This study investigates calcium isotope variations (δ44/40Ca) in late Silurian marine carbonates deposited in the Prague Basin (Czech Republic), which records one of the largest positive carbon isotope excursion (CIE) of the entire Phanerozoic, the mid-Ludfordian CIE, which is associated with major climatic changes (abrupt cooling) and global sea-level fluctuations. Our results show that during the onset of the CIE, when δ13C increases rapidly from ∼0‰ to ∼8.5‰, δ44/40Ca remains constant at about 0.3±0.1‰ (relative to NIST 915a), while 87Sr/86Sr in well-preserved carbonates are consistent with a typical Ludfordian seawater composition (ranging from ∼0.70865 to ∼0.70875). Such decoupling between δ13C and δ44/40Ca trends during the onset of the CIE is consistent with the expected order-of-magnitude difference in the residence times of Ca (∼106yr) and C (∼105yr) in the open ocean, suggesting that the mid-Ludfordian CIE was caused by processes where the biogeochemical pathways of C and Ca in seawater were mechanistically decoupled. These processes may include: (i) near shore methanogenesis and photosynthesis, (ii) changes in oceanic circulation and stratification, and/or (iii) increased production and burial of organic C in the global ocean. The latter, however, is unlikely due to the lack of geological evidence for enhanced organic C burial, and also because of unrealistic parameterization of the ocean C cycle needed to generate the observed CIE over the relatively short time interval. In contrast, higher up in the section where δ13C shifts back to pre-excursion baseline values, there is a correlated shift to higher δ44/40Ca values. Such coupling of the records of Ca and C isotope changes in this part of the study section is inconsistent with the abovementioned differences in oceanic Ca and C residence times, indicating that the record of δ44/40Ca changes does not faithfully reflect the evolution of the oceanic Ca reservoir, but rather some local processes in the Prague Basin. These can be related to restricted elemental/sediment cycling involving mixing of isotopically distinct Ca sources and carbonate polymorphs (calcite vs. aragonite), and/or possible kinetic Ca isotope effects due to changes in the rate of marine carbonate formation. Evidence supporting the ‘kinetic’ effect in the studied mid-Ludfordian carbonates is indicated by correlated δ44/40Ca and Sr-concentration data (rs=−0.76, p<0.001, n=41) yielding a slope of −0.00097, which is indistinguishable from the ‘kinetic’ slope of abiotic calcite precipitation. Kinetic processes are integral to the model of rapid carbonate precipitation recently proposed by Kozłowski (2015), to explain the origin of the mid-Ludfordian CIE, involving intense methanogenesis/photosynthesis in near shore settings coupled with rapid CaCO3 precipitation (i.e., massive whitings events) and eustatically-controlled carbonate hypersaturation of seawater. More Ca isotope studies are needed to shed light on the question of whether kinetics or mineralogy controls the coupled variations in carbonate δ44/40Ca and δ13C records observed in this study and other large positive CIEs in geological record.

The Messinian marine to nonmarine gypsums of Jumilla (Northern Betic Cordillera, SE Spain): Isotopic and Sr concentration constraints on the origin of parent brines

The origin of the Messinian Hoya de la Sima (HS) gypsum (Betic foreland) is constrained using 87Sr/86Sr, δ34S, Sr concentration, and petrographic data. The Lower and Middle HS units consist of subaqueous vertically-aligned and stromatolitic selenites, the latter containing unusual microbial depositional textures. The Upper Unit consists of very-shallow-water bioturbated lenticular gypsum with Paracamelus ichnites. 87Sr/86Sr and δ34S indicate precipitation from predominantly marine waters, with upward increasing continental influence. Mixing models between Messinian seawater and continental water that dissolved Triassic evaporites show that the percentages of seawater required to explain the measured 87Sr/86Sr are analogous to the percentages obtained using δ34S, supporting precipitation from such mixtures. 87Sr/86Sr and δ34S of Lower HS selenites resemble those of the Primary Lower Gypsum (PLG) of the Messinian Salinity Crisis (MSC), in both cases indicating precipitation from seawater–continental water mixtures in which most Sr and SO4 were supplied by Messinian seawater. In the Lower HS selenites, Sr concentrations indicate contributing continental waters with Sr/Ca ratios similar to seawater. However, Sr concentrations of PLG selenites from other Betic basins (Bajo Segura, Sorbas indicate parent waters with Sr/Ca ratios lower than seawater. If the Sr contents of the betic PLG selenites are representative, it is unlikely that the Lower HS selenites represent the PLG. However, we cannot completely discard that option since different LPG subbasins could have had variable Sr/Ca. The HS gypsums formed coevally to diapirism of Triassic evaporites, in a restricted lagoonal basin developed during or slightly after a phase of strike-slip faulting in the Betic Cordillera. More general implications of this work are that Sr concentrations, combined with 87Sr/86Sr and δ34S data, provide key constraints on the origin of parent brines, and using Sr concentrations as paleosalinity proxy in gypsum precipitated from seawater–continental water mixtures is inconclusive because they largely depend on the initial Sr/Ca ratio of the brine. This study also illustrates that the electron microprobe is ideal to obtain reliable, spatially-resolved, Sr concentration data in selenitic gypsum. These data, in combination with fluorescence petrography, provide valuable criteria to discriminate primary depositional from diagenetic gypsum, a critical step in the interpretation of geochemical data.

Environmental controls on the boron and strontium isotopic composition of aragonite shell material of cultured &amp;lt;i&amp;gt;Arctica islandica&amp;lt;/i&amp;gt;

Abstract. Ocean acidification, the decrease in ocean pH associated with increasing atmospheric CO2, is likely to impact marine organisms, particularly those that produce carbonate skeletons or shells. Therefore, it is important to investigate how environmental factors (seawater pH, temperature and salinity) influence the chemical compositions in biogenic carbonates. In this study we report the first high-resolution strontium (87Sr / 86Sr and δ88 / 86Sr) and boron (δ11B) isotopic values in the aragonite shell of cultured Arctica islandica (A. islandica). The 87Sr / 86Sr ratios from both tank water and shell samples show ratios nearly identical to the open ocean, which suggests that the shell material reflects ambient ocean chemistry without terrestrial influence. The 84Sr–87Sr double-spike-resolved shell δ88 / 86Sr and Sr concentration data show no resolvable change throughout the culture period and reflect no theoretical kinetic mass fractionation throughout the experiment despite a temperature change of more than 15 °C. The δ11B records from the experiment show at least a 5‰ increase through the 29-week culture season (January 2010–August 2010), with low values from the beginning to week 19 and higher values thereafter. The larger range in δ11B in this experiment compared to predictions based on other carbonate organisms (2–3‰) suggests that a species-specific fractionation factor may be required. A significant correlation between the ΔpH (pHshell − pHsw) and seawater pH (pHsw) was observed (R2 = 0.35), where the pHshell is the calcification pH of the shell calculated from boron isotopic composition. This negative correlation suggests that A. islandica partly regulates the pH of the extrapallial fluid. However, this proposed mechanism only explains approximately 35% of the variance in the δ11B data. Instead, a rapid rise in δ11B of the shell material after week 19, during the summer, suggests that the boron uptake changes when a thermal threshold of &gt; 13 °C is reached.

Artificial radionuclides database in the Pacific Ocean: HAM database.

The database “Historical Artificial Radionuclides in the Pacific Ocean and its Marginal Seas”, or HAM database, has been created. The database includes Sr, Cs, and Pu concentration data from the seawater of the Pacific Ocean and its marginal seas with some measurements from the sea surface to the bottom. The data in the HAM database were collected from about 90 literature citations, which include published papers; annual reports by the Hydrographic Department, Maritime Safety Agency, Japan; and unpublished data provided by individuals. The data of concentrations of Sr, Cs, and Pu have been accumulating since 1957—1998. The present HAM database includes 7737 records for Cs concentration data, 3972 records for Sr concentration data, and 2666 records for Pu concentration data. The spatial variation of sampling stations in the HAM database is heterogeneous, namely, more than 80% of the data for each radionuclide is from the Pacific Ocean and the Sea of Japan, while a relatively small portion of data is from the South Pacific. This HAM database will allow us to use these radionuclides as significant chemical tracers for oceanographic study as well as the assessment of environmental affects of anthropogenic radionuclides for these 5 decades. Furthermore, these radionuclides can be used to verify the oceanic general circulation models in the time scale of several decades.

Geochemistry of biotite in metabentonites as an age discriminant, indicator of regional magma sources and potential correlating tool

AbstractThe chemical composition of unaltered biotites extracted from 18 Lower Palaeozoic metabentonites from Sweden, Norway and Scotland was determined. Discrimination based on FeO, MgO and TiO2proved able to separate the metabentonites on the basis of their stratigraphical age. A triangular discrimination diagram using MgO, FeO and Al2O3classified the biotites as calc-alkaline. Independent support for the discriminant power of FeO, MgO and TiO2in biotite comes from Sr isotope ratios and Sr concentration data from cognate apatite crystals. Within this dataset Caradocian and Llandoverian metabentonites have unique biotite compositions, while Wenlockian samples can be characterized to a lesser degree. Biotite composition also supported the correlation of a pair of metabentonites from Norway known to form part of the same horizon. Biotite in metabentonites preserves the chemical signature of the host magmas in which it crystallized and can be used to discriminate between metabentonites of different ages and provenance.

Analysis of geological Sr isotope markers in fish otoliths with subannual resolution using laser ablation-multicollector-ICP-mass spectrometry

In this pilot study, laser ablation-multicol- lector-inductively coupled plasma-mass spectrome- try (LA-MC-ICP-MS) and Sr isotope geochemistry were applied to fish life-history studies through the micro-analysis of Sr abundance and 87 Sr/ 86 Sr in the otoliths of an anadromous fish. With a laser sampling area of 70-lm diameter, LA-MC-ICP-MS of a deep-sea coral gave precision values for 87 Sr/ 86 Sr determina- tion of approximately 0.04% relative standard devia- tion with an accuracy to <1r of seawater. Analyses of different sides of the same otolith and different oto- liths from the same fish gave reproducible results overall, which suggested that at the Sr concentrations found in the freshwater portions of the otoliths (ca. 500 lg/g) LA-MC-ICP-MS can be reliably employed. Subtle isotopic differences occurred between a pair of otoliths from one fish, which may be due either to sampling resolution limitations imposed by the size of the laser spot, or to asymmetrical otolith growth which incorporated environmental Sr in one otolith but not the other at certain times. New information about fish provenance which could not be gained from Sr concentration data alone included insights into the movements of individuals between isotopically contrasting freshwater environments. LA-MC-ICP- MS of calcified tissues, in combination with Sr sotope geochemistry, has the potential to retrieve new data about fish life-history movements which complement the information from other techniques.