Search for: All records

Abstract Experiments have been conducted in which CO₂gases with varying C and O isotopic compositions and with stochastic and nonstochastic Δ₄₇values have been allowed to equilibrate with phosphoric acid of two concentrations in reaction vessels of varying dimensions at temperatures of 25 and 90 °C. Rates of¹³C¹⁸O and¹⁸O exchange between the CO₂and the phosphoric acid varied as a function of the length of exposure, volume of reaction vessel, acid strength, and difference of the initial Δ₄₇and δ¹⁸O values of the CO₂from theoretical equilibrium values. The Δ₄₇values were also altered by heated stainless steel surfaces such as those found within the Kiel device and other preparation systems. These results have been used to explain variations in the differences in the fractionation between 25 and 90 °C reported for calcite by different workers as well as differences in the slopes between temperature and Δ₄₇values produced by reacting samples at different temperatures (25 and 90 °C). 

Abstract Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio‐chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from theCIROS‐1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore‐filling blocky fabric throughout the core. Oxygen isotope compositions (δ¹⁸O = −30·0 to −8·6‰ Vienna Pee‐Dee Belemnite) and clumped isotope temperatures (TΔ₄₇ = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ¹⁸O_w) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluidδ¹⁸O_wvalues are consistent with those of pore water, previously identified as cryogenic brine in the nearbyAND‐2A core. Petrographic and geochemical data suggest that aragonite cement in theCIROS‐1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in theCIROS‐1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.