Porewater Sr Research Articles

Volcanogenic aluminosilicate alteration drives formation of authigenic phases at the northern Hikurangi margin: Implications for subseafloor geochemical cycles

Alteration of volcanogenic aluminosilicates (VAs) in marine sediments is recognized as critical in regulating geochemical cycles and sustaining the oceanic deep biosphere, but rates of VA alteration and its associated authigenic mineral formation are not commonly reported. Here we present results on analyses of sediments and pore water recovered from the upper 150 mbsf of four sites drilled on the northern Hikurangi margin during IODP Expeditions 372 and 375. Petrographic analyses show that volcanogenic materials (glass shards, feldspar, volcanic lithoclasts) constitute important components (15–45 wt%) of the hemipelagic mud, and reveal ongoing glass alteration with accompanying authigenic phase formation. A reaction-transport model constrained by pore water Sr, 87Sr/86Sr, Ca, Mg, and Si was applied to simulate VA diagenetic reactions. Our model results yield VA alteration rates of 0.047–0.64 mmol Sr m−2 yr−1, with substantially higher values at Sites U1517 and U1520 that experienced rapid sediment emplacement. In addition, our simulations show that >99% of the dissolved Si generated by VA alteration is fixed in silica cement and authigenic clay, and that ∼50% of Ca incorporated in the authigenic carbonate is supplied by VA alteration. First-order estimates suggest that, in addition to authigenic carbonate precipitation, authigenic clay formation may represent an important sink for dissolved Mg. This study quantitatively examines the linkage between VA alteration and formation of authigenic phases, highlights its role in subsurface geochemical cycles, and indicates that slope instability may play an important role in promoting VA diagenesis.

Biogeochemical Controls on Strontium Fate at the Sediment-Water Interface of Two Groundwater-Fed Wetlands with Contrasting Hydrologic Regimes.

Radioactive strontium (Sr) is a common groundwater contaminant at many nuclear sites. Its natural retention in groundwater-fed wetlands is an attractive remediation strategy. However, at present, the biogeochemical mechanisms controlling Sr transport at the sediment-water interface are poorly understood. In this field study, Sr fate was investigated in two wetlands with contrasting vegetation and hydrologic regimes. The marsh was an open-water wetland with constant water table and no emergent vegetation. The swamp was vegetated with fluctuating water levels and a thick mat of submerged cattail litter in the water column. High-resolution porewater Sr concentrations and solid-phase sediment Sr species revealed distinct profiles between the two wetlands. The marsh exhibited a strongly reduced environment and sharp concentration peaks at the sediment-water interface. In contrast, the smaller concentration gradients of the swamp resulted in a reduced flux of Sr to the surface water. The organic fraction of the sediment dominated Sr retention compared to the inorganic iron and manganese oxides. However, the marsh had a significant fraction of recalcitrant Sr presumably due to its incorporation into sulfur and/or carbonate minerals. These results suggest that vegetated wetlands with fluctuating hydrologic regimes could act as efficient sinks for Sr pollution.

Diagenetic incorporation of Sr into aragonitic bivalve shells: implications for chronostratigraphic and palaeoenvironmental interpretations

AbstractAragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected 87Sr/86Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4·5 ± 0·6 mole% SrCO3; ca 25 mmol mol−1 Sr/Ca) and was depleted in 18O and 13C compared to the middle layer, both features inconsistent with pristine material. The δ18O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite‐bearing interval suggesting that Sr‐isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn‐Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material.

A study of celestine equilibrium in marine sediments using the entire ODP/IODP porewater data base

The stability of celestine (SrSO 4) in marine sediments has been investigated through the calculation of its saturation index at the in situ temperature and pressure, using the entire ODP/IODP porewater composition data base (14,416 samples recovered from sediments collected during 95 ODP and IODP Legs) that has been thoroughly corrected for missing data and inconsistencies. The porewater in situ pressure has been obtained from depth data, and the in situ sample temperature has been calculated from the bottom seawater temperature and from the measured thermal gradient. When the latter is unavailable, a default value of 35 °C/km or an assumed gradient similar to that of a site located close to the study area has been used. Molarities have been converted to molalities through the calculation of the porewater densities, which have represented by an empirical function of the total dissolved solids load up to 150 g/L. Ocean bottom waters are largely undersaturated with respect to celestine. Yet sediment porewater saturation is not uncommon: it is reached in 83 boreholes (i.e. about 10% of all boreholes) drilled during 22 ODP/IODP Legs. Celestine equilibrium can be reached through two different non-exclusive causes: a strontium increase in porewater linked to the dissolution of Sr-rich aragonite or biogenic calcite and precipitation of Sr-poor calcite (carbonate recrystallization), or an increase in the strontium and sulfate porewater concentrations linked to a salinity increase due to the presence of brines. Sediments at most of the sites exhibiting celestine equilibrium share common characteristics such as a carbonate-rich lithology (typically higher than 80 wt.%) and a low organic carbon content (generally below 1 wt.%). These results indicate that modification of porewaters during burial diagenesis can easily lead to celestine saturation, especially in carbonate-rich sediments. We then briefly discuss former interpretations of the presence of celestine in ancient and recent marine sediments, as well as the consequences of the incorporation of celestine formation in diagenetic models using the porewater Sr content and Sr isotopic composition.

Feldspar dissolution rates in the Topopah Spring Tuff, Yucca Mountain, Nevada

Abstract Two different field-based methods are used here to calculate feldspar dissolution rates in the Topopah Spring Tuff, the host rock for the proposed nuclear waste repository at Yucca Mountain, Nevada. The center of the tuff is a high silica rhyolite, consisting largely of alkali feldspar (∼60 wt%) and quartz polymorphs (∼35 wt%) that formed by devitrification of rhyolitic glass as the tuff cooled. First, the abundance of secondary aluminosilicates is used to estimate the cumulative amount of feldspar dissolution over the history of the tuff, and an ambient dissolution rate is calculated by using the estimated thermal history. Second, the feldspar dissolution rate is calculated by using measured Sr isotope compositions for the pore water and rock. Pore waters display systematic changes in Sr isotopic composition with depth that are caused by feldspar dissolution. The range in dissolution rates determined from secondary mineral abundances varies from 10 −16 to 10 −17 mol s −1 kg tuff −1 with the largest uncertainty being the effect of the early thermal history of the tuff. Dissolution rates based on pore water Sr isotopic data were calculated by treating percolation flux parametrically, and vary from 10 −15 to 10 −16 mol s −1 kg tuff −1 for percolation fluxes of 15 mm a −1 and 1 mm a −1 , respectively. Reconciling the rates from the two methods requires that percolation fluxes at the sampled locations be a few mm a −1 or less. The calculated feldspar dissolution rates are low relative to other measured field-based feldspar dissolution rates, possibly due to the age (12.8 Ma) of the unsaturated system at Yucca Mountain; because oxidizing and organic-poor conditions limit biological activity; and/or because elevated silica concentrations in the pore waters (∼50 mg L −1 ) may inhibit feldspar dissolution.

Temperature dependence of the strontium distribution coefficient in calcite; an experimental study from 408 degrees to 2008 degrees C and application to natural diagenetic calcites

ABSTRACT The temperature dependence of Sr coprecipitation with calcite was determined experimentally in solutions with Sr/Ca ratios and ionic strengths closely resembling marine pore fluids. Aragoniteto-calcite and dolomite-to-calcite transformations were conducted over a temperature range from 40° to 200°C. Temperature dependence of the distribution coefficient of strontium in calcite (DcSr) is significant. DcSr in aragonite-to-calcite experiments varied from 0.046 at 40°C to 0.068 at 200°C. DcSr in dolomite-to-calcite experiments varied from 0.034 at 40°C to 0.062 at 200°C. Experimental values of DcSr are somewhat dependent on the precursor phase. The experimental results are applied to natural diagenetic and hydrothermal calcites recovered from the sediment-covered Middle Valley part of the Juan de Fuca Ridge spreading center (Ocean Drilling Program Holes 857C and 858D). Low-Mg calcites from Hole 858D have Sr contents appropriate for their present-day in situ temperatures and pore-water Sr values. In contrast, high-Mg calcites from Hole 858D that precipitated from supersaturated pore waters at inferred high growth rates have elevated Sr contents relative to calculated equilibrium values. The Sr contents of calcites from Hole 857C are close to predicted equilibrium values in some instances but are substantially different in others. Deviations from predicted equilibrium Sr contents, especially in nodules, are attributed to the formation of mixed generations of carbonate precipitating with increasing burial depths and temperatures through pore waters with variable Sr composition.

Strontium isotopic evidence on the chemical evolution of pore waters in the Milk River Aquifer, Alberta, Canada

Strontium isotope ratios were measured on 13 rock, 18 leachate and 28 pore-water samples from the Milk River aquifer, the confining argillaceous formations, and the glacial till mantling the recharge area. Strontium isotope ratios ( 87Sr/ 86Sr) of pore waters from the aquifer, confining units, and the glacial till ranged from 0.7069 to 0.7082. The 87Sr/ 86Sr ratios in aquifer pore waters decrease with increasing distance from the aquifer recharge area, and this is interpreted to be the result of mixing and water–rock interaction within the aquifer. The solute composition of the recharging groundwater is modified by the local lithology, causing distinct geochemical patterns along different flow paths within the aquifer. Whole-rock 87Sr/ 86Sr ratios indicate that the shales and till are generally more radiogenic than the aquifer sandstone. The authigenic carbonate cements and rock-forming minerals comprising the major lithologic units had little apparent influence on the pore-water Sr chemistry. Carbonate cement leachates from the till and the aquifer sandstone are more radiogenic than those from the confining shale formations. Feldspar separates from the aquifer sandstone have relatively radiogenic Sr isotope ratios, whereas bentonites from the Milk River and Colorado Shale Formations have whole-rock and leachate Sr isotope ratios that are relatively unradiogenic. Ratios of most Milk River aquifer pore waters are lower than those of any leachates or whole rocks analyzed, except the bentonites. The 87Sr/ 86Sr ratios of exchangeable Sr in the bentonites are similar to ratios found in the more evolved pore waters. Simple rock–water interaction models calculated for the whole-rock, leachate, and exchangeable-ion/pore-water pairs indicate that ion exchange with bentonite clays within the Milk River and Colorado Shale Formations appears to influence the isotopic evolution of the pore-water Sr in each of these units.

Models for the coupled Sr-sulfate budget in deep-sea carbonates

A numerical model for the Sr and sulfate budgets during deposition of deep-sea carbonate sediments was developed and used to fit porewater Sr 2+ and SO 4 2− data from DSDP Sites 593, 590, 574, 573, 572, 516, and 289. The Sr and sulfate budgets are coupled whenever the porewaters become saturated with respect to celestite (SrSO 4). Models for the porewater Sr and sulfate at Sites 593, 590, 516, and 289 suggest that porewater Sr concentrations are controlled by the precipitation of celestite and predict maximum celestite concentrations of the order of 0.02 wt% for sediments at Sites 593, 590, and 289, and trace amounts of celestite at Site 516. The actual amount of celestite predicted by the model is not well constrained because it depends on the as yet poorly known value of the equilibrium distribution coefficient for Sr between calcite and porewaters. The fact that celestite has been found in sediments from Sites 590 and 593 is used to derive an upper bound for the Sr distribution coefficient, that suggests that all laboratory determined values for the Sr distribution coefficient between calcite and porewater are too large. The occurrence of celestite at Sites 593, 590, 516, and 289 does not significantly change earlier estimates of the rates of solution-reprecipitation of calcite and its consequences in terms of shifting the Sr content or Sr isotopic composition of the bulk sediment from what the sediment incorporated when originally deposited. The sulfate data at Sites 593 and 590 are best fit by invoking a flow of porewater within the sediments of the order of 75 m/Ma. The Sr and sulfate concentrations in porewaters from Sites 572, 573, and 574 never reach celestite saturation and thus the two budgets are decoupled. Both the Sr and the sulfate data require that all three Sites have abundant unaltered seawater flowing in the underlying basement and can only be modeled if seawater concentrations of Sr and sulfate are imposed as boundary conditions at both the sediment-water interface and the bottom of the section. Model sulfate profiles for Sites 572, 573, and 574 confirm prior estimates of the rate of porewater flow at these Sites that were made using Sr data.

Fluid flow in deep-sea carbonates: estimates based on porewater Sr

Fluid flow of up to 200 m/myr is documented in thick (about 500 m) deep-sea carbonate sections by numerical modeling of the porewater Sr 2+ and 87Sr/ 86Sr profiles at DSDP Sites 572, 573 and 574. These sites were selected as likely candidates for significant fluid flow in the sedimentary part of the section because it had already been shown that they are underlain by rapidly flowing fresh seawater in the basaltic basement. The nature of the exercise is to fit the porewater Sr properties at these sites by finding a common rate of Sr exchange as a function of sediment age and a local rate of vertical fluid flow. The flow rates determined in this way are 20 m/myr upwards at Site 574, 20 m/myr downwards at Site 573 and 200 m/myr downwards at Site 572. Because of uncertainties of about a factor of 2 in the assumption of a common rate of Sr exchange at all sites, and trade-offs between the Sr exchange and fluid flow in determining the shape of Sr 2+ and 87Sr/ 86Sr profiles at a given site, the precision of the method is of the order of 10 m/myr. Thus at least at Site 572, the Sr properties of the interstitial waters provide a convincing measure of vertical fluid flow through a thick carbonate section.

Diagenesis and Sr isotopic evolution of seawater using data from DSDP 590B and 575

New Sr isotopic and concentration analyses of bulk carbonate and porewaters from a carbonate-rich Miocene section sampled at DSDP Site 575 are reported and used, along with our earlier data from a section of similar age at DSDP Site 590B, in a numerical model to determine for each site the rate of Sr exchange during diagenesis and the degree to which this amount of exchange has shifted the 87Sr 86Sr of the bulk carbonate from the ratio each increment of sediment had when first deposited. Despite the fact that the two sites have very different water depth (4536 m at 575 vs. 1299 m at 590B), different sedimentation rate ( ∼ 10 m/My at 575 vs. ∼ 40 m/My at 590B), and significantly different porewater Sr 2+ gradients, we find that the rate of Sr exchange as a function of sediment age is almost indistinguishable between the two sites. The rate of Sr exchange determined at the two sites is such that the resulting shift in bulk carbonate 87Sr 86Sr due to diagenesis is small compared to the total range of 87Sr 86Sr values measured, but large compared to the analytical uncertainty of the individual isotopic ratio measurements. By taking this shift into account we reconstruct the original 87Sr 86Sr of each increment of carbonate sampled, which when plotted as a function of age becomes our best estimate of the Sr isotopic evolution of seawater. Because Sr is very well mixed in the ocean, at any given time there is a single worldwide value of seawater 87Sr 86Sr . Therefore, if we are quantitatively accounting for the effect Sr exchange, we should find the same seawater evolution curve regardless of what DSDP Site is used. When we compare the observed bulk carbonate 87Sr 86Sr vs. age at the two sites they are seen to differ by amounts that are sometimes large compared to the analytical uncertainties of the measurements. However, when these data are corrected for the post-depositional Sr isotopic shifts predicted by our diagenetic model, we find almost perfect agreement. This agreement suggests that we have made a realistic determination of the rate of Sr exchange and its consequences in terms of shifting the 87Sr 86Sr of the bulk carbonate, and more importantly, that Sr isotopes can be used to correlate marine sediments with an accuracy comparable to the very small analytical uncertainties of modern isotopic measurements.

Rb-Sr dating of diagenesis and source age of clays in Upper Devonian black shales of Texas

The Rb-Sr method can be used to date diagenetic events that have affected the illitic clays commonly found in sedimentary rocks. Diagenetic 1Md illite forms in equilibrium with pore-water Sr and appears to be a stable phase that is able to maintain a closed chemical system even under deep burial conditions. Meaningful analytical data can be obtained after careful size fractionation to separate coarse-grained, detrital 2M illite from finer-grained, diagenetic 1Md illite, followed by removal of exchangeable Rb and Sr with ammonium acetate. Rb-Sr data from the µ fraction of widely separated samples of Upper Devonian black shales in Texas fall on a single isochron having an age of 302 ± 4 m.y. Diagenetic illite must have formed rapidly in response to activity in the nearby Ouachita orogenic belt. In the middle Pennsylvanian, none of the shales was buried more than a few hundred metres, and so clay diagenesis may have been initiated by updip migration of hot brines out of the Ouachita tectonic zone. Later burial to as much as 7 km did not result in further diagenetic changes affecting the Rb-Sr system. Coarse-grained illite (2–1 µ ) from the west Texas Woodford Shale records an isochron age of the source region of 539 ± 18 m.y. This is an age not commonly encountered in rocks of North America, suggesting a derivation of Woodford clays from the southern continental landmass.