Carbonate clumped isotope thermometry is a useful tool practised in studies of temperature history and fluid composition of surface and subsurface environments, with application to both inorganic and biological precipitates. Its measurements are based upon the propensity with which 13C and 18O isotopes, within a carbonate mineral, are bound to one another, in relation to a stochastic distribution. The quantity of these 13C-18O bonds (commonly referred to as “clumps”) is determined by gas source mass spectrometry on CO2 produced from acid digestion of carbonate minerals, and is controlled by physiochemical parameters of the solution at the time of mineral precipitation. If equilibrium is reached, then 13C-18O abundance, measured against the stochastic distribution and represented by the variable Δ47, can be used to measure the temperature of precipitation of the carbonate without the need to characterize the isotopic composition of coeval fluids. However, long-term reproducibility of these measurements is a critical factor contributing to uncertainties in all calibrations and applications. Here we discuss the impact of using different standardization procedures on the accuracy and precision of Δ47 measurements, as compared across three mass spectrometers with four different configurations within the Tripati Lab at UCLA. Specifically, we assess the long-term reproducibility of carbonate standard Δ47 values across mass spectrometers, using a correction scheme that incorporates either gas and carbonate standards of known composition, or both, and the impact of different approaches for characterizing instrument drift (i.e., averaging for an interval or using a moving window). We also recommend best practices to promote reproducibility.
Effective, considerate shale play water management support operations and protect the environment. A parameter often overlooked is total dissolved solids (TDS). Knowledge of TDS is important to meet these dual goals. Subsurface TDS typically increases with depth. However, produced-water samples from the Eagle Ford Shale show a strong TDS decrease by a factor of ~10 with increasing well depth (~200,000 ppm at ~2.5 km to 18,000 ppm at ~3.6 km). Water stable isotopes strongly suggest that the low TDS is not due to dilution by meteoric water. Rather, it is attributed to smectite-to-illite conversion, in which the smectite interlayer water is released into the pore space. Depth, temperature, and other related indicators (source for K, excess silica) support such a mechanism. In addition, water-isotope patterns and 87Sr/86Sr ratios suggest a conversion operating in a closed system. Order-of-magnitude calculations show that the 8% of mixed-layer clay present on average in the Lower Eagle Ford Shale is sufficient to dilute brines to observed levels. Stakeholders could then have a more optimistic outlook on water recycling and on using produced water for other uses (irrigation, municipal) because the low salinity is an intrinsic property of the formation rather than due to short-term mixing.
Tidal salt marshes are the most productive “Blue Carbon” ecosystem and play a significant role in the Global Carbon Cycle (Mcleod et al., 2011, Chung et al., 2011). Salt marshes account for 75% of the organic carbon (C) found in “Blue Carbon” systems, yet cover less than 1% of Earth’s surface (Hopkinson et al., 2012, Howard et al., 2014). They have a high C storage capacity due to a continuous sediment C accumulation rate (CAR) greater than that of any other “Blue Carbon” ecosystem (Murray et al., 2011, Chmura, 2013, Ouyang and Lee, 2014). However, Global estimates of salt marsh C-stocks and CAR are subject to large uncertainties (Duarte et al, 2013, Chastain et al, 2018). The Delaware Bay (DB) salt marshes have been developing for ~2000 years. When these systems are degraded they become a potential source of C-emissions. 8.85 km2 of salt marsh has converted to open water between 1996-2010 and future losses are estimated to reach 5 km2/yr by 2100 (Partnership for the Delaware Estuary, 2017). Conversion could outpace C storage if the depth of erosion is ≥ the thickness of the marsh sediments (Theuerkauf et al., 2015). Most salt-marsh sediment C-stock assessments are reported within the top 1 m of the sediment column (Ouyang and Lee, 2014), thereby representing ~ 100 years of salt-marsh accumulation as compared to the actual 1-6 m sediment sequences accumulated throughout the life span of most U.S. Mid-Atlantic regional salt marshes (Nikitina et al., 2015, Kirwan et al., 2013, Kemp et al., 2013). We estimate the average thickness of the DB salt marsh sediments is 2.6 m, C-stock is 0.1020 MgC/m2 and salt marsh C-stock loss over the 14 yr period is ~0.9 TgC (3.3MMT CO2 equivalents). As this critical “Blue Carbon” habitat reportedly declines, the resulting CO2 degassing flux has a significant impact on the Global Carbon Budget contributing to climate change and ocean acidification (Cai W-J, 2011). Recognition of this sink-to-source conversion emphasized the need for more accurate stock estimates and risk assessments based on estimates of CO2 emissions from lost and degraded salt marshes (Lovelock et al., 2017). The results show that the DB salt marshes sequester significant amounts of C, suggesting that C-stock assessments focused on the top 1 m of sediment underestimate the total C-stock and potential C-emissions by more than three-fold
The overturning tropical Pacific circulation known as the Walker circulation embodies complex interactions between large-scale circulations, deep and shallow convection, stratocumulus clouds, and microphysical cloud processes. The large and multi-scale nature of the Walker circulation has made high resolution modeling costly, while disentangling the relevant circulations and processes in a global model with more parameterizations is often challenging. This work uses the framework of the Walker Circulation as a unifying experiment for both high-resolution and global models with the goal of identifying how deep tropical convective heating and low-level clouds interact with and are influenced by the circulations in which they are embedded. A high resolution model with explicit convection (1km and 2km grid-spacing) is used to examine the system free of the complications inherent in convective parameterizations. The same model is also used at GCM-like resolutions with parameterized convection (25km and 100km grid-spacing) to gain insight into how the clouds and circulations interact in a GCM configuration. We define the idealized Walker circulation with a prescribed sea surface temperature dipole pattern, no rotation, uniform insolation, fully interactive radiation, and a channel domain (100km x 4000km). All simulations use the the same nonhydrostatic dynamical core (FV3) with the physics based on those in the AM4 GFDL atmospheric model. We find large differences in the total condensate between the high-resolution model and the GCM with the high-resolution model tending to have less low-level condensate but more condensate in the deep convective regions. This is reflected in the relative humidity fields as well. The parameterized entrainment of deep convection and the feedbacks of low-level tropical clouds are both leading factors contributing to the large spread of the climate sensitivity. With this in mind experiments are performed with the GCM in which the lateral mixing rate of deep convective plumes is varied. In addition, the detailed representation of cloud fraction between the two models is investigated. Our goal is to determine to what extent deep tropical convection can influence remote low-level clouds in regions with a subsiding free troposphere.