- Publication Date:
- NSF-PAR ID:
- Journal Name:
- Elements in geochemical tracers in earth system science
- Sponsoring Org:
- National Science Foundation
More Like this
Toward an Improved Understanding of the Marine Barium Cycle and the Application of Marine Barite as a Paleoproductivity ProxyMarine barite (BaSO4) is a relatively ubiquitous, though minor, component of ocean sediments. Modern studies of the accumulation of barite in ocean sediments have demonstrated a robust correlation between barite accumulation rates and carbon export to the deep ocean. This correlation has been used to develop quantitative relationships between barite accumulation rates and export production and is used to reconstruct export production in the geologic past, particularly during times of dynamic changes in the carbon cycle. We review the processes that affect the formation and preservation of marine barite, as well as those controlling the relationship between the barium (Ba) and carbon biogeochemical cycles. Additionally, we take a new approach to modeling the marine Ba cycle as a two-box model, specifically evaluating Ba utilization in the surface ocean and refining the equation describing the relationship between export production and barite formation. We compare these new results with past modeling efforts. The new model demonstrates that increases in export production can lead to sustained increases in barite accumulation in marine sediments without resulting in complete surface water Ba depletion, which is distinctly different from previous modeling results.
Changes in the concentration and isotopic composition of the major constituents in seawater reflect changes in their sources and sinks. Because many of the processes controlling these sources and sinks are tied to the cycling of carbon, such records can provide insights into what drives past changes in atmospheric carbon dioxide and climate. Here, we present a stable strontium (Sr) isotope record derived from pelagic marine barite. Our δ88/86Sr record exhibits a complex pattern, first declining between 35 and 15 million years ago (Ma), then increasing from 15 to 5 Ma, before declining again from ~5 Ma to the present. Numerical modeling reveals that the associated fluctuations in seawater Sr concentrations are about ±25% relative to present-day seawater. We interpret the δ88/86Sr data as reflecting changes in the mineralogy and burial location of biogenic carbonates.
Cadmium is a trace metal of interest in the ocean partly because its concentration mimics that of phosphate. However, deviations from the global mean dissolved Cd/PO 4 relationship are present in oxygen deficient zones, where Cd is depleted relative to phosphate. This decoupling has been suggested to result from cadmium sulphide (CdS) precipitation in reducing microenvironments within sinking organic matter. We present Cd concentrations and Cd isotope compositions in organic-rich sediments deposited at several upwelling sites along the northeast Pacific continental margin. These sediments all have enriched Cd concentrations relative to crustal material. We calculate a net accumulation rate of Cd in margin settings of between 2.6 to 12.0 × 10 7 mol/yr, higher than previous estimates, but at the low end of a recently published estimate for the magnitude of the marine sink due to water column CdS precipitation. Cadmium in organic-rich sediments is isotopically light ( δ 114/110 Cd NIST-3108 = +0.02 ± 0.14‰, n = 26; 2 SD) compared to deep seawater (+0.3 ± 0.1‰). However, isotope fractionation during diagenesis in continental margin settings appears to be small. Therefore, the light Cd isotope composition of organic-rich sediments is likely to reflect an isotopically light source of Cd.more »
Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impactMass extinction at the Cretaceous–Paleogene (K-Pg) boundary coin- cides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we docu- ment a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth sys- tem modeling, indicate that a partial ∼50% reduction in global ma- rine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario recon- ciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivitymore »
The effects of growth conditions on the chemistry, structure, electrical leakage, dielectric response, and ferroelectric behavior of Ba 1−x TiO y thin films are explored. Although single-phase, coherently-strained films are produced in all cases, small variations in the laser fluence during pulsed-laser deposition growth result in films with chemistries ranging from BaTiO 3 to Ba 0.93 TiO 2.87 . As the laser fluence increases, the films become more barium deficient and the out-of-plane lattice parameter expands (as much as 5.4% beyond the expected value for Ba 0.93 TiO 2.87 films). Stoichiometric BaTiO 3 films are found to be three orders of magnitude more conducting than Ba 0.93 TiO 2.87 films and the barium-deficient films exhibit smaller low-field permittivity, lower loss tangents, and higher dielectric maximum temperatures. Although large polarization is observed in all cases, large built-in potentials (shifted loops) and hysteresis-loop pinching are present in barium-deficient films – suggesting the presence of defect dipoles. The effects of these defect dipoles on ferroelectric hysteresis are studied using first-order reversal curves. Temperature-dependent current–voltage and deep-level transient spectroscopy studies reveal at least two defect states, which grow in concentration with increasing deficiency of both barium and oxygen, at ∼0.4 eV and ∼1.2 eVmore »