494 research outputs found
Developing proxies to constrain redox gradients in terminal Ediacaran oceans
There is a long-standing interest in the relationship between the rise of early Metazoans and changes in the redox structure of the oceans. As such, there is a need for reliable geochemical proxy archives that record palaeo-redox. Before we can use proxies in deep time we must understand their application in modern environments, and ensure pristine seawater signals can be extracted effectively. We investigate the sulfur cycle in the modern ocean, using new data from minor sulfur isotopes to constrain the proportional pyrite burial flux - a key control on atmospheric oxygen regulation through time - to between 20 and 35%. Ce anomalies in rare earth element patterns record redox information, and we develop the leaching methods for extracting pristine signals from carbonates. We suggest that a partial leach in nitric acid reduces the risk of contamination. We apply multiple redox proxies (Fe-speciation, TOC, carbon isotopes, CAS-pyrpaired sulfur isotopes and Ce anomalies) to terminal Ediacaran carbonates from the Nama Group, Namibia, to reconstruct the redox structure of the Nama Group and its relationship to the distribution of biomineralising Metazoans. We generate a holistic redox model that distinguishes between anoxic, intermediate and fully oxygenated waters, including identification of manganous conditions using novel observations of positive Ce anomalies. We distinguish between spatial and temporal variability in redox using nine sections from variable relative water depths. Dynamic redox conditions are associated with small, monospecific communities of Metazoans in short-lived horizons. Metazoans are largely absent from low oxygen manganous waters, whereas fully oxic waters host large, complex Metazoan communities. We suggest that redox exerted an important control on the ecological structure of terminal Ediacaran Metazoan communities
Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic
A hypothesized rise in oxygen levels in the Neoproterozoic, dubbed the Neoproterozoic Oxygenation Event, has been repeatedly linked to the origin and rise of animal life. However, a new body of work has emerged over the past decade that questions this narrative. We explore available proxy records of atmospheric and marine oxygenation and, considering the unique systematics of each geochemical system, attempt to reconcile the data. We also present new results from a comprehensive COPSE biogeochemical model that combines several recent additions, to create a continuous model record from 850 to 250 Ma. We conclude that oxygen levels were intermediate across the Ediacaran and early Palaeozoic, and highly dynamic. Stable, modern-like conditions were not reached until the Late Palaeozoic. We therefore propose that the terms Neoproterozoic Oxygenation Window and Palaeozoic Oxygenation Event are more appropriate descriptors of the rise of oxygen in Earth's atmosphere and oceans
Microscopic two-nucleon overlaps and knockout reactions from C
The nuclear structure dependence of direct reactions that remove a pair of
like or unlike nucleons from a fast C projectile beam are considered.
Specifically, we study the differences in the two-nucleon correlations present
and the predicted removal cross sections when using -shell shell-model and
multi- no-core shell-model (NCSM) descriptions of the two-nucleon
overlaps for the transitions to the mass =10 projectile residues. The NCSM
calculations use modern chiral two-nucleon and three-nucleon (NN+3N)
interactions. The -removal cross sections to low-lying =0, B
final states are enhanced when using the NCSM two-nucleon amplitudes. The
calculated absolute and relative partial cross sections to the low energy
B final states show a significant sensitivity to the interactions used,
suggesting that assessments of the overlap functions for these transitions and
confirmations of their structure could be made using final-state-exclusive
measurements of the -removal cross sections and the associated momentum
distributions of the forward travelling projectile-like residues.Comment: 9 pages, 7 figure
Relating breakup and incomplete fusion of weakly-bound nuclei through a classical trajectory model with stochastic breakup
A classical dynamical model that treats break-up stochastically is presented
for low energy reactions of weakly-bound nuclei. The three-dimensional model
allows a consistent calculation of breakup, incomplete and complete fusion
cross sections. The model is assessed by comparing the breakup observables with
CDCC quantum mechanical predictions, which are found to be in reasonable
agreement. Through the model, it is demonstrated that the breakup probability
of the projectile as a function of its distance from the target is of primary
importance for understanding complete and incomplete fusion at energies near
the Coulomb barrier.Comment: Accepted in Physical Review Letter
Probing halo nucleus structure through intermediate energy elastic scattering
This work addresses the question of precisely what features of few body
models of halo nuclei are probed by elastic scattering on protons at high
centre-of-mass energies. Our treatment is based on a multiple scattering
expansion of the proton-projectile transition amplitude in a form which is well
adapted to the weakly bound cluster picture of halo nuclei. In the specific
case of Li scattering from protons at 800 MeV/u we show that because
core recoil effects are significant, scattering crosssections can not, in
general, be deduced from knowledge of the total matter density alone.
We advocate that the optical potential concept for the scattering of halo
nuclei on protons should be avoided and that the multiple scattering series for
the full transition amplitude should be used instead.Comment: 8 pages REVTeX, 1 eps figure, accepted for publication in Phys. Rev.
Coulomb and nuclear breakup effects in the single neutron removal reaction 197Au(17C,16C gamma)X
We analyze the recently obtained new data on the partial cross sections and
parallel momentum distributions for transitions to ground as well as excited
states of the 16C core, in the one-neutron removal reaction 197Au(17C,16C
gamma)X at the beam energy of 61 MeV/nucleon. The Coulomb and nuclear breakup
components of the one-neutron removal cross sections have been calculated
within a finite range distorted wave Born approximation theory and an eikonal
model, respectively. The nuclear contributions dominate the partial cross
sections for the core excited states. By adding the nuclear and Coulomb cross
sections together, a reasonable agreement is obtained with the data for these
states. The shapes of the experimental parallel momentum distributions of the
core states are described well by the theory.Comment: Revtex format, two figures included, to appear in Phys. Rev. C.
(Rapid communications
Two-nucleon correlation effects in knockout reactions from 12C
Reactions that involve the direct and sudden removal of a pair of like or
unlike nucleons from a fast projectile beam by a light target nucleus are
considered. Specifically, we study the three two-nucleon removal channels from
{C} that populate final states in the {Be}, {B} and
{C} reaction residues. The calculated two-nucleon removal cross sections
and the residue momentum distributions are compared with available high energy
data at 250, 1050 and 2010 MeV per nucleon, data that are inclusive with
respect to the bound final-states of the residues. The measured -removal
cross sections only are significantly greater than the values calculated,
suggesting that the reaction mechanism observes enhanced spatial
correlations compared to those present in the shell-model wave functions.Comment: 10 pages, 5 figures - Accepted Physical Review
Four-body continuum-discretized coupled-channels calculations using a transformed harmonic oscillator basis
The scattering of a weakly bound three-body system by a target is discussed.
A transformed harmonic oscillator basis is used to provide an appropriate
discrete and finite basis for treating the continuum part of the spectrum of
the projectile. The continuum-discretized coupled channels framework is used
for the scattering calculations. The formalism is applied to different
reactions, 6He+12C at 229.8 MeV, 6He+64Zn at 10 and 13.6 MeV, and 6He+208Pb at
22 MeV, induced by the Borromean nucleus 6He. Both the Coulomb and nuclear
interactions with a target are taken into account.Comment: Published in Phys. Rev.
Non-adiabatic corrections to elastic scattering of halo nuclei
We derive the formalism for the leading order corrections to the adiabatic
approximation to the scattering of composite projectiles. Assuming a two-body
projectile of core plus loosely-bound valence particle and a model (the core
recoil model) in which the interaction of the valence particle and the target
can be neglected, we derive the non-adiabatic correction terms both exactly,
using a partial wave analysis, and using the eikonal approximation. Along with
the expected energy dependence of the corrections, there is also a strong
dependence on the valence-to-core mass ratio and on the strength of the
imaginary potential for the core-target interaction, which relates to
absorption of the core in its scattering by the target. The strength and
diffuseness of the core-target potential also determine the size of the
corrections. The first order non-adiabatic corrections were found to be smaller
than qualitative estimates would expect. The large absorption associated with
the core-target interaction in such halo nuclei as Be11 kills off most of the
non-adiabatic corrections. We give an improved estimate for the range of
validity of the adiabatic approximation when the valence-target interaction is
neglected, which includes the effect of core absorption. Some consideration was
given to the validity of the eikonal approximation in our calculations.Comment: 14 pages with 10 figures, REVTeX4, AMS-LaTeX v2.13, submitted to
Phys. Rev.
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