Rosemary C. Capo

Strontium isotopes as tracers of ecosystem processes: theory and methods

Abstract The strontium (Sr) isotope method can be a powerful tool in studies of chemical weathering and soil genesis, cation provenance and mobility, and the chronostratigraphic correlation of marine sediments. It is a sensitive geochemical tracer, applicable to large-scale ecosystem studies as well as to centimeter-scaled examination of cation mobility within a soil profile. The 87Sr/86Sr ratios of natural materials reflect the sources of strontium available during their formation. Isotopically distinct inputs from precipitation, dryfall, soil parent material, and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem. The isotopic compositions of labile (soil exchange complex and soil solution) strontium and Sr in vegetation reflect the sources of cations available to plants. Strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium. The extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method. In this paper, we review the geochemistry and isotopic systematics of strontium, and discuss the use of this method as a tracer of earth surface processes.

Geochemical and Strontium Isotope Characterization of Produced Waters from Marcellus Shale Natural Gas Extraction

Extraction of natural gas by hydraulic fracturing of the Middle Devonian Marcellus Shale, a major gas-bearing unit in the Appalachian Basin, results in significant quantities of produced water containing high total dissolved solids (TDS). We carried out a strontium (Sr) isotope investigation to determine the utility of Sr isotopes in identifying and quantifying the interaction of Marcellus Formation produced waters with other waters in the Appalachian Basin in the event of an accidental release, and to provide information about the source of the dissolved solids. Strontium isotopic ratios of Marcellus produced waters collected over a geographic range of ~375 km from southwestern to northeastern Pennsylvania define a relatively narrow set of values (ε(Sr)(SW) = +13.8 to +41.6, where ε(Sr) (SW) is the deviation of the (87)Sr/(86)Sr ratio from that of seawater in parts per 10(4)); this isotopic range falls above that of Middle Devonian seawater, and is distinct from most western Pennsylvania acid mine drainage and Upper Devonian Venango Group oil and gas brines. The uniformity of the isotope ratios suggests a basin-wide source of dissolved solids with a component that is more radiogenic than seawater. Mixing models indicate that Sr isotope ratios can be used to sensitively differentiate between Marcellus Formation produced water and other potential sources of TDS into ground or surface waters.

SOURCES OF STRONTIUM AND CALCIUM IN DESERT SOIL AND CALCRETE

The carbon-cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely how much calcium is derived from atmospheric sources compared with that from the chemical weathering of silicate minerals. In the petrocalcic horizon (calcrete) of a Pleistocene soil from the USDA‐SCS Desert Project area near Las Cruces, NM, approximately 1.5 g Ca=cm 3 has been added, with an associated expansion of the profile of200%. Strontium isotope values for the labile cations and carbonate from the A, B and K soil horizons have 87 Sr= 86 Sr values that range from 0.7087 to 0.7093, similar to the values for easily soluble local dust and rain. The parent material, non-calcareous Camp Rice alluvial sediment, has a 87 Sr= 86 Sr ratio of0.7165. Mixing calculations indicate a minimum atmospheric contribution to soil carbonate calcium of 94%; the more likely scenarios indicate at least 98% of the Ca originated from atmospheric input. The variations in 87 Sr= 86 Sr ratios of soil silicate (0.7131 to 0.7173) are consistent with weathering of volcanogenic sediments and neoformation of clay minerals in the petrocalcic horizon. Moreover, the Sr isotope data suggest that 50‐70% of silicate in the uppermost 25 cm of the profile could be atmosphere-derived. The isotopic composition of labile strontium in the A horizon and the mass distribution of silicon and calcium indicate that the uppermost portion of the profile is the present zone for the release of cations due to silicate weathering. Steady-state models of the whole profile yield a Sr weathering flux ranging from200 to 400 m gc m 2 Ma 1 . The results indicate that both the present-day and long-term contribution of calcium from silicate weathering is less than 2% of that supplied from the atmosphere, and confirm that desert soil formation is not a significant sink for atmospheric carbon.

Effects of rainfall on weathering rate, base cation provenance, and Sr isotope composition of Hawaiian soils

A climate transect across the Kohala Peninsula, Hawaii provides an ideal opportunity to study soil processes and evolution as a function of rainfall. The parent material is the ∼150 ka Hawi alkali basalt aa flow, and median annual precipitation (MAP) changes from ∼16 cm along the west coast to ∼450 cm in the rain forest near the crest of the peninsula. We measured labile (plant-available) base cation concentrations and 87Sr/86Sr ratios of labile strontium and silicate residue from soil profiles across the transect from 18 to 300 cm MAP. Depletion of labile cations and a shift in labile 87Sr/86Sr ratios toward rainwater values with increasing rainfall clearly show the transition from a mineral-supported to a rainwater-supported cation nutrient budget. In contrast, increases in soil silicate residue 87Sr/86Sr values with increasing MAP result primarily from input of exogenous eolian material (dust derived from Asian loess), with a greater dust fraction at the high MAP sites due to aerosol washout. Most of the soil silicate strontium in high-MAP sites is still derived from the original parent material, but the shallower portions of profiles can be dust-dominated. The variations in labile 87Sr/86Sr with rainfall allow us to calculate weathering rates as a function of MAP. The primary uncertainty is the degree to which Sr in rainwater actually interacts with the labile cation reservoir before being flushed from the system; mass balance calculations for the 150 ka evolution of the profile suggest that only on the order of 5 to 50% of rainwater strontium exchanges with the labile reservoir. Our models suggest that the present-day supply of strontium by weathering increases steadily with rainfall in the low-MAP (<140 cm) sites, then decreases dramatically as the soils become depleted in weatherable parent material. This implies that the initial weathering rate of the high-MAP sites was very high, and that there may be some change in soil weathering behavior in the 100 to 160 cm MAP range. Weathering rates calculated from the labile 87Sr/86Sr are on the same order as other estimates for chemical denudation rates of basaltic terrains.

The importance of sea spray to the cation budget of a coastal Hawaiian soil: a strontium isotope approach

Soil nutrients such as Ca, Mg, and K are traditionally thought to be derived primarily from rock weathering. Here we show that sea spray is a significant source of nutrient elements to modern and buried soils developed on <30,000-year-old Pahala Ash deposits 50 m from the coast at South Point, Hawaii. The soil profiles evolved in a semi-arid climate and have always been above sea level and the water table. Rhizoliths (fossilized root traces) and horizontal laminated carbonate sheets found in buried soils are composed of high-Mg calcite (up to 14 mol% MgCO3). Differences in strontium isotopic composition between marine aerosols (87Sr/86Sr=0.7092) and tephra parent material (∼0.7035) allow quantification of cation sources to the labile soil reservoir and to pedogenic carbonate. Mixing equations indicate that 50–80% of labile soil Sr and approximately half of carbonate Sr was derived from marine sources. Using the Sr isotopic signatures and Sr/Ca ratios of seawater and tephra as end members, we determined that up to 2/3 of the Ca in the labile reservoir and up to 1/3 of Ca in the carbonates has a marine origin. Carbonate 87Sr/86Sr ratios are fairly constant with depth, but labile 87Sr/86Sr ratios indicate decreasing sea spray aerosol influence with depth. This trend could be due either to sequestering of aerosol-derived Sr in the upper part of the profile or to lower aerosol input in the past due to lower sea level. The unusual occurrence of high-Mg pedogenic calcite probably results from high labile Mg/Ca ratios during earlier stages of weathering, coupled with rapid calcite precipitation during soil pore water evaporation.

Quantitative strontium isotope models for weathering, pedogenesis and biogeochemical cycling

Abstract Isotopes of strontium (Sr) are a useful tracer for weathering, atmospheric fluxes, cation biocycling, and pedogenesis. We present basic models for application of strontium isotopes to the soil–vegetation–atmosphere system. The mathematical formulations fall into the general categories of: (1) steady-state models, in which isotopic ratios remain constant over the time scale of interest; and (2) time-dependent models, in which isotope ratios change through time. In the steady-state models, fluxes of Sr and other elements to the system are constant. Steady-state models can be used to infer short-term weathering rates from river and stream isotope compositions, to determine fluxes to a single- or multiple-layer soil exchange/solution system, and to quantify nutrient fluxes to vegetation. Time-dependent models involve a change in isotopic ratio from some initial value to a new value over the time period of interest; in some cases, the change may represent a shift from one steady-state situation to a new one after a shift in one or more of the fluxes feeding the system. Examples of applications of time-dependent models include identifying the dominant cation sources to an evolving soil exchange/solution system, and calculating weathering rates by measuring the isotopic compositions of primary soil minerals. We use time-dependent models to explain differences in the isotopic ratios of labile and carbonate Sr from arid sites in Hawaii (with a basalt parent material isotopic signature) and New Mexico (with an atmospheric isotopic signature). These models suggest that the difference is due to a combination of low atmospheric strontium fluxes and high weathering rates in the Hawaiian profile compared to the New Mexico calcrete profile.

Pedogenic origin of dolomite in a basaltic weathering profile, Kohala peninsula, Hawaii

We document stoichiometric dolomite occurring in a nonsaline Quaternary soil on the Kohala peninsula, northwestern Hawaii. Geologic constraints and geochemical and isotopic data confirm that this dolomite is not the result of marine influence or wind-blown dust. The strontium isotopic composition of the dolomite (87Sr/87Sr = 0.7045–0.7048) is indicative of its derivation primarily from the weathering of basaltic parent material rather than from meteoric water or seawater. Infiltration of soil waters with elevated Mg/Ca (>1) derived from alteration of ferromagnesian minerals such as olivine likely led to dolomitization of early-precipitated soil calcite and/or to direct dolomite precipitation in the profile. This demonstrates that well-ordered dolomite can form in a nonmarine environment at temperatures <100 °C without undergoing burial diagenesis.

Development of Magnesian Carbonates in Quaternary Soils on the Island of Hawaii

ABSTRACT Pedogenic carbonate minerals have progressively developed on basalt in a Quaternary weathering chronosequence (5-350 ka) on the island of Hawaii. The sites were isolated from direct contact with seawater or the water table, and were minimally influenced by eolian input throughout their history. The carbonates consist chiefly of high-magnesium calcite (HMC) and well-ordered stoichiometric dolomite. Pedogenic development of such minerals is rare, and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates. 87Sr/86Sr ratios of chronosequence carbonates range from 0.7038 to 0.7048. The data indicate that > 75% of the strontium was derived from the weathering of volcanic parent material, even in the oldest soils, where few primary minerals are available to provide Ca and Mg. This is in contrast to continental calcretes, which are commonly dominated by atmospheric inputs and are generally composed of low-Mg calcite (LMC). Stable-isotope data are consistent with carbonate precipitation from soil waters in an arid environment. The magnesium content of the carbonate increases with substrate age, and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early-formed LMC and HMC. Opal was detected in young (5-10 ka) samples, and gibbsite is present in older samples, indicating a shift from silica-dominated to aluminum-dominated soil solution. High concentrations of dissolved silica and aluminum during early weathering likely precluded formation of Mg-rich secondary clay minerals such as smectite or palygorskite. Weathering of basalt with a molar Mg/Ca ratio of 0.8-1.5, and the absence of secondary magnesian silicates, can account for elevated soil-water Mg/Ca ratios (0.7-1.0 in soils > 70 ka) that led to magnesian carbonate formation. Over time, the formation of a clay-rich soil profile prevented rapid evaporation of soil water, possibly facilitating crystallization of stoichiometric dolomite.

Strontium Isotopes Test Long-Term Zonal Isolation of Injected and Marcellus Formation Water after Hydraulic Fracturing

One concern regarding unconventional hydrocarbon production from organic-rich shale is that hydraulic fracture stimulation could create pathways that allow injected fluids and deep brines from the target formation or adjacent units to migrate upward into shallow drinking water aquifers. This study presents Sr isotope and geochemical data from a well-constrained site in Greene County, Pennsylvania, in which samples were collected before and after hydraulic fracturing of the Middle Devonian Marcellus Shale. Results spanning a 15-month period indicated no significant migration of Marcellus-derived fluids into Upper Devonian/Lower Mississippian units located 900-1200 m above the lateral Marcellus boreholes or into groundwater sampled at a spring near the site. Monitoring the Sr isotope ratio of water from legacy oil and gas wells or drinking water wells can provide a sensitive early warning of upward brine migration for many years after well stimulation.

A method for generating uniform size-segregated pyrite particle fractions

BackgroundStandardized sample preparation techniques allow comparison of pyrite dissolution experiments under diverse conditions. Our objective was to assess dry and wet sieving preparation methodologies, and to develop a reproducible technique that yields uniformly size-distributed material within a limited size range of interest.ResultsHere, we describe a wet sieving preparation method that successfully concentrates pyrite particles within a 44–75 μm diameter range. In addition, this technique does not require a post-processing cleanup step to remove adhering particles, as those particles are removed during the procedure. We show that sample preparation methods not only affect the pyrite size distribution, but also apparent dissolution rates.ConclusionThe presented methodology is non-destructive to the sample, uses readily available chemical equipment within the laboratory, and could be applied to minerals other than pyrite.