Showing papers by "Jess F. Adkins published in 2020"

Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP)

Abstract: The concentration of radiocarbon (14C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their 14C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0–55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 14C curve and reconstructed changes in CO2 obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric 14C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric 14C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for ΔR, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/.

The ECCO-Darwin Data-Assimilative Global Ocean Biogeochemistry Model: Estimates of Seasonal to Multidecadal Surface Ocean pCO2 and Air-Sea CO2 Flux

Abstract: Quantifying variability in the ocean carbon sink remains problematic due to sparse observations and spatiotemporal variability in surface ocean pCO₂. To address this challenge, we have updated and improved ECCO‐Darwin, a global ocean biogeochemistry model that assimilates both physical and biogeochemical observations. The model consists of an adjoint‐based ocean circulation estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium and an ecosystem model developed by the Massachusetts Institute of Technology Darwin Project. In addition to the data‐constrained ECCO physics, a Green’s Function approach is used to optimize the biogeochemistry by adjusting initial conditions and six biogeochemical parameters. Over seasonal to multi‐decadal timescales (1995–2017), ECCO‐Darwin exhibits broad‐scale consistency with observed surface ocean pCO₂ and air‐sea CO₂ flux reconstructions in most biomes, particularly in the subtropical and equatorial regions. The largest differences between CO₂ uptake occur in subpolar, seasonally‐stratified biomes, where ECCO‐Darwin results in stronger winter uptake. Compared to the Global Carbon Project OBMs, ECCO‐Darwin has a time‐mean, global‐ocean CO2 sink (2.47 ± 0.50 Pg C year⁻¹) and interannual variability that are more consistent with interpolation‐based products. Compared to interpolation‐based methods, ECCO‐Darwin is less sensitive to sparse and irregularly‐sampled observations. Thus, ECCO‐Darwin provides a basis for identifying and predicting the consequences of natural and anthropogenic perturbations to the ocean carbon cycle, as well as the climate‐related sensitivity of marine ecosystems. Our study further highlights the importance of physically‐consistent, property‐conserving reconstructions, as are provided by ECCO, for ocean biogeochemistry studies.

Variability in sulfur isotope records of Phanerozoic seawater sulfate

Abstract: The δ³⁴S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ³⁴S values must be derived from proxy materials that are impacted differently by depositional and postdepositional processes. We produced new time series estimates for the δ³⁴S value of seawater sulfate by combining 6,710 published data from three sedimentary archives—marine barite, evaporites, and carbonate‐associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ³⁴S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high‐frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ³⁴S proxies, indicating that postdepositional processes obscure detailed knowledge of seawater sulfate's δ³⁴S value deeper in time.

Stable Isotope Analysis of Intact Oxyanions Using Electrospray Quadrupole-Orbitrap Mass Spectrometry

Abstract: The stable isotopes of sulfate, nitrate, and phosphate are frequently used to study geobiological processes of the atmosphere, ocean, as well as land. Conventionally, the isotopes of these and other oxyanions are measured by isotope-ratio sector mass spectrometers after conversion into gases. Such methods are prone to various limitations on sensitivity, sample throughput, or precision. In addition, there is no general tool that can analyze several oxyanions or all the chemical elements they contain. Here, we describe a new approach that can potentially overcome some of these limitations based on electrospray hyphenated with Quadrupole Orbitrap mass spectrometry. This technique yields an average accuracy of 1–2‰ for sulfate δ³⁴S and δ¹⁸O and nitrate δ¹⁵N and δ¹⁸O, based on in-house and international standards. Less abundant variants such as δ¹⁷O, δ³³S, and δ³⁶S, and the ³⁴S–¹⁸O “clumped” sulfate can be quantified simultaneously. The observed precision of isotope ratios is limited by the number of ions counted. The counting of rare ions can be accelerated by removing abundant ions with the quadrupole mass filter. Electrospray mass spectrometry (ESMS) exhibits high-throughput and sufficient sensitivity. For example, less than 1 nmol sulfate is required to determine ¹⁸O/³⁴S ratios with 0.2‰ precision within minutes. A purification step is recommended for environmental samples as our proposed technique is susceptible to matrix effects. Building upon these initial provisions, new features of the isotopic anatomy of mineral ions can now be explored with ESMS instruments that are increasingly available to bioanalytical laboratories.

The carbonic anhydrase activity of sinking and suspended particles in the North Pacific Ocean

Abstract: The enzyme carbonic anhydrase (CA) is crucial to many physiological processes involving CO₂, from photosynthesis and respiration, to calcification and CaCO₃ dissolution. We present new measurements of CA activity along a North Pacific transect, on samples from in situ pumps, sediment traps, discreet plankton samples from the ship's underway seawater line, plankton tows, and surface sediment samples from multicores. CA activity is highest in the surface ocean and decreases with depth, both in suspended and sinking particles. Subpolar gyre surface particles exhibit 10× higher CA activity per liter of seawater compared to subtropical gyre surface particles. Activity persists to 4700 m in the subpolar gyre, but only to 1000 m in the subtropics. All sinking CA activity normalized to particulate organic carbon (POC) follows a single relationship (CA/POC = 1.9 ± 0.2 × 10⁻⁷ mol mol⁻¹). This relationship is consistent with CA/POC values in subpolar plankton tow material, suspended particles, and core top sediments. We hypothesize that most subpolar CA activity is associated with rapidly sinking diatom blooms, consistent with a large mat of diatomaceous material identified on the seafloor. Compared to the basin‐wide sinking CA/POC relationship, a lower subtropical CA/POC suggests that the inventory of subtropical biomass is different in composition from exported material. Pteropods also demonstrate substantial CA activity. Scaled to the volume within pteropod shells, first‐order CO₂ hydration rate constants are elevated ≥ 1000× above background. This kinetic enhancement is large enough to catalyze carbonate dissolution within microenvironments, providing observational evidence for CA‐catalyzed, respiration‐driven CaCO₃ dissolution in the shallow North Pacific.

A Mechanistic Study of Carbonic Anhydrase-Enhanced Calcite Dissolution

Abstract: Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001; Subhas et al., 2017), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA‐bearing solution. CA is found to etch the calcite surface only when in extreme proximity (~ 1 nm) to the mineral. Subsequently, the CA induced etch pits create step edges that serve as active dissolution sites. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO₂ hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO₃ and alkalinity cycles.