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  • Inverse method  (2)
  • Inverse modeling  (1)
  • 2015-2019  (3)
  • 1
    Publication Date: 2016-05-12
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 30 (2015): 1470-1489, doi:10.1002/2014PA002743.
    Description: The ocean circulation modifies mixed layer (ML) tracer signals as they are communicated to the deep ocean by advection and mixing. We develop and apply a procedure for using tracer signals observed “upstream” (by planktonic foraminifera) and “downstream” (by benthic foraminifera) to constrain how tracer signals are modified by the intervening circulation and, by extension, to constrain properties of that circulation. A history of ML equilibrium calcite δ18O (δ18Oc) spanning the last deglaciation is inferred from a least-squares fit of eight benthic foraminiferal δ18Oc records to Green's function estimated for the modern ocean circulation. Disagreements between this history and the ML history implied by planktonic records would indicate deviations from the modern circulation. No deviations are diagnosed because the two estimates of ML δ18Oc agree within their uncertainties, but we suggest data collection and modeling procedures useful for inferring circulation changes in future studies. Uncertainties of benthic-derived ML δ18Oc are lowest in the high-latitude regions chiefly responsible for ventilating the deep ocean; additional high-resolution planktonic records constraining these regions are of particular utility. Benthic records from the Southern Ocean, where data are sparse, appear to have the most power to reduce uncertainties in benthic-derived ML δ18Oc. Understanding the spatiotemporal covariance of deglacial ML δ18Oc will also improve abilities of δ18Oc records to constrain deglacial circulation.
    Description: 2016-05-12
    Keywords: Oxygen isotopes ; Inverse modeling ; Deglaciation ; Tracers ; Ocean circulation ; Green's function
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 2
    Publication Date: 2018-01-10
    Description: © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 125 (2017): 106-128, doi:10.1016/j.dsr.2017.05.003.
    Description: The high particle reactivity of thorium has resulted in its widespread use in tracing processes impacting marine particles and their chemical constituents. The use of thorium isotopes as tracers of particle dynamics, however, largely relies on our understanding of how the element scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k1), Th desorption (k−1), bulk particle degradation (β-1), and bulk particle sinking speed (w) along the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at these stations appear to be generally dominated by radioactive production and decay sorption reactions, particle degradation, and particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model to data of dissolved and particulate 228,230,234Th, 228Ra, particle concentrations, and 234,238U estimates based on salinity, using a nonlinear programming technique. We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000 m and (1.4–1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., , k1 ∝ Pb where b ≥ 1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K = k1/(k-1 + β-1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that decreases with P, in line with a particle concentration effect (dKD/dP 〈 0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.
    Description: We acknowledge the U.S. National Science Foundation for providing funding for this study (grant OCE-1232578) and for U.S. GEOTRACES North Atlantic section ship time, sampling, and data analysis. The U.S. NSF also supported the generation of 230Th data (OCE-0927064 to LDEO, OCE-O092860 to WHOI, and OCE-0927754 to UMN) and 228,234Th data (OCE-0925158 to WHOI).
    Keywords: GEOTRACES ; Thorium ; Particle Concentration Effect ; Single-particle class model ; Inverse method
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
    Publication Date: 2018-07-08
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 128-151, doi:10.1002/2017PA003174.
    Description: We present a synthesis of 1,361 deep‐sea radiocarbon data spanning the past 40 kyr and computed (for 14C‐dated records) from the same calibration to atmospheric 14C. The most notable feature in our compilation is a long‐term Δ14C decline in deep oceanic basins over the past 25 kyr. The Δ14C decline mirrors the drop in reconstructed atmospheric Δ14C, suggesting that it may reflect a decrease in global 14C inventory rather than a redistribution of 14C among different reservoirs. Motivated by this observation, we explore the extent to which the deep water Δ14C data jointly require changes in basin‐scale ventilation during the last deglaciation, based on the fit of a 16‐box model of modern ocean ventilation to the deep water Δ14C records. We find that the fit residuals can largely be explained by data uncertainties and that the surface water Δ14C values producing the fit are within the bounds provided by contemporaneous values of atmospheric and deep water Δ14C. On the other hand, some of the surface Δ14C values in the northern North Atlantic and the Southern Ocean deviate from the values expected from atmospheric 14CO2 and CO2 concentrations during the Heinrich Stadial 1 and the Bølling‐Allerød. The possibility that deep water Δ14C records reflect some combination of changes in deep circulation and surface water reservoir ages cannot be ruled out and will need to be investigated with a more complete model.
    Description: U.S. National Science Foundation Grant Number: OCE‐1301907
    Description: 2018-07-08
    Keywords: Last deglaciation ; Ocean ventilation ; Data synthesis ; Radiocarbon ; Inverse method
    Repository Name: Woods Hole Open Access Server
    Type: Article
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