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  • 1
    Publication Date: 2020-01-21
    Type: Dataset
    Format: text/tab-separated-values, 355 data points
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  • 2
    Publication Date: 2020-01-18
    Type: Dataset
    Format: text/tab-separated-values, 22 data points
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  • 3
    Publication Date: 2020-01-18
    Type: Dataset
    Format: text/tab-separated-values, 22 data points
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  • 4
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    In:  Supplement to: Vidal, Laurence; Schneider, Ralph R; Marchal, Olivier; Bickert, Torsten; Stocker, Thomas F; Wefer, Gerold (1999): Link between the North and South Atlantic during the Heinrich events of the last galcial period. Climate Dynamics, 15(12), 909-919, https://doi.org/10.1007/s003820050321
    Publication Date: 2020-01-18
    Description: High resolution benthic oxygen isotope records combined with radiocarbon datings, from cores retrieved in the North, Equatorial, and South Atlantic are used to establish a reliable cronostratigraphy for the last 60 ky. This common temporal framework enables us to study the timing of the sub-Milankovitch climate variability in the entire surface Atlantic during this period, as reflected in planktonic oxygen isotope records. Variations in sea surface temperatures in the Equatorial and South Atlantic reveal two warm periods during the mid-stage 3 which are correlated to the warming observed in the North Atlantic after Heinrich events (HL) 5 and 4. However, the records show that the warming started about 1500 y earlier in the South Atlantic. A zonally averaged ocean circulation model simulates a similar north-south thermal antiphasing between the latitudes of our coring sites, when pertubated by a freshwater flux anomaly. We infer that the observed phase relationship between the northern and the southern Atlantic is related to periods of reduced NADW production in the North Atlantic, such as during HL5 and HL4.
    Type: Dataset
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  • 5
    Publication Date: 2017-01-04
    Description: Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 90 (2012):126–148, doi:10.1016/j.gca.2012.05.009.
    Description: The ability of paired measurements of thorium isotope activity and particle concentration to constrain rate constants of sorption reactions and particle dynamics in the ocean is examined. This study is motivated by GEOTRACES and other sampling programs where Th and particle data are gathered in various oceanic environments. Our approach relies on inversions with a model of trace metal and particle cycling in the water column. First, the model is used to simulate vertical profiles of (i) the activity of three Th isotopes (228,230,234Th) in the dissolved phase, small suspended particles, and large sinking particles, and (ii) the concentration of small and large particles. The simulated profiles are then subsampled and corrupted with noise to generate a pseudo data set. These data are combined with the model with arbitrary values of rate constants of Th adsorption, Th desorption, particle sinking, particle remineralization, and particle (dis)aggregation in an effort to recover the actual values used to generate the data. Inversions are performed using a least-squares technique with varying assumptions about data noise, data sampling, and model errors. We find that accurate and precise recovery of rate parameters is possible when all data have a relative error of less than 20%, vertical sampling is dense enough to resolve activity and concentration gradients, and model errors are negligible. Estimating cycling rates from data with larger errors and (or) at locations where model assumptions are not tenable would remain challenging. On the other hand, the paired data set would improve significantly the relative precision of rate parameters compared to that of prior estimates (⩾100%), even with current data uncertainties and significant model errors. Based on these results, we advocate the joint measurement of all three Th isotopes, 228Ra, and particles collected by in situ filtration within GEOTRACES and other sampling programs targeted at the study of particle processes in the ocean.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 6
    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
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  • 7
    Publication Date: 2017-12-14
    Description: © The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 113 (2016): 57-79, doi:10.1016/j.dsr.2016.03.008.
    Description: Thorium is a highly particle-reactive element that possesses different measurable radio-isotopes in seawater, with well-constrained production rates and very distinct half-lives. As a result, Th has emerged as a key tracer for the cycling of marine particles and of their chemical constituents, including particulate organic carbon. Here two different versions of a model of Th and particle cycling in the ocean are tested using an unprecedented data set from station GT11-22 of the U.S. GEOTRACES North Atlantic Section: (i) 21 228;230;234Th activities of dissolved and particulate fractions, (ii) 228Ra activities, (iii) 234;238U activities estimated from salinity data and an assumed 234U/238U ratio, and (iv) particle concentrations, below a depth of 125 m. The two model versions assume a single class of particles but rely on different assumptions about the rate parameters for sorption reactions and particle processes: a first version (V1) assumes vertically uniform parameters (a popular description), whereas the second (V2) does not. Both versions are tested by fitting to the GT11-22 data using generalized nonlinear least squares and by analyzing residuals normalized to the data errors. We find that model V2 displays a significantly better fit to the data than model V1. Thus, the mere allowance of vertical variations in the rate parameters can lead to a significantly better fit to the data, without the need to modify the structure or add any new processes to the model. To understand how the better fit is achieved we consider two parameters, K = k1=(k-1 + β-1) and K/P, where k1 is the adsorption rate constant, k-1 the desorption rate constant, β-1 the remineralization rate constant, and P the particle concentration. We find that the rate constant ratio K is large (≥0.2) in the upper 1000 m and decreases to a nearly uniform value of ca. 0.12 below 2000 m, implying that the specific rate at which Th attaches to particles relative to that at which it is released from particles is higher in the upper ocean than in the deep ocean. In contrast, K/P increases with depth below 500 m. The parameters K and K/P display significant positive and negative monotonic relationship with P, respectively, which is collectively consistent with a particle concentration effect.
    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.
    Description: 2017-03-31
    Keywords: GEOTRACES ; North Atlantic ; Thorium ; Particles ; Reversible Exchange ; Model ; Inverse Method
    Repository Name: Woods Hole Open Access Server
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  • 8
    Publication Date: 2018-06-29
    Description: Author Posting. © The Author(s), 2018. 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 Geochimica et Cosmochimica Acta 233 (2018): 115-134, doi:10.1016/j.gca.2018.04.035.
    Description: The dependence of thorium scavenging by particles on particle composition is examined at selected stations of the U.S. GEOTRACES North Atlantic Section (GA03). Scavenging is here described by the apparent, first-order rate constant of Th adsorption onto particles (k1), as estimated from an inversion of Th radioisotope and radioactive parent data. Our k1 estimates are regressed against particle phase data using two different models. Model I considers biogenic particles (POC+PIC+bSi), lithogenic particles, Mn (oxyhydr)oxides, and Fe (oxyhydr)oxides as regressors, and k1 as the regressand. Model II considers ln(POC+PIC+bSi), ln(lithogenic particles), ln(Mn (oxyhydr)oxides), and ln(Fe (oxyhydr)oxides) as regressors, and ln(k1) as the regressand, where ln() denotes the natural logarithm. Thus, models I and II posit that the effects of particle phases on k1 are, respectively, additive and multiplicative. These models are applied to three groups of stations: (i) all selected stations, (ii) stations west of theMauritanian upwelling region (“western stations”), and (iii) stations within that region (“eastern stations”). We find that model II appears to better describe the effect of particle composition on k1 than model I. Particle composition explains a larger fraction of the variance of k1 for the eastern stations (R2 = 0.60 for model I and 0.67 for model II) than for the western stations (R2 = 0.26 for model I and 0.39 for model II). When considering all stations, the variance of k1 explained by particle composition is intermediate (R2 = 0.50 for model I and 0.51 for model II). According to model II, the variance of k1 explained by particle composition is predominantly due to biogenic particles at the eastern stations and to Mn (oxyhydr)oxides at the western stations. Additionally, we find that particle composition does not explain a significantly different proportion of variance of k1 than particle concentration. It is thus concluded that, at our selected stations, (i) biogenic particles andMn (oxyhydr)oxides more strongly influence Th scavenging than any other phases considered, and (ii) particle composition and particle concentration have comparable effects on this process.
    Description: We acknowledge the U.S. National Science Foundation for supporting this study (grant OCE-1232578) and the U.S. GEOTRACES North Atlantic section ship time, sampling, and data analysis.
    Repository Name: Woods Hole Open Access Server
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  • 9
    Publication Date: 2017-01-04
    Description: Author Posting. © Blackwell, 2007. This is the author's version of the work. It is posted here by permission of Blackwell for personal use, not for redistribution. The definitive version was published in Tellus A 59 (2007): 141–154, doi:10.1111/j.1600-0870.2006.00193.x.
    Description: The 'Sandström theorem' as interpreted by Jeffreys (1925) is that in a flow maintained by a temperature difference, the pathline from the cold region to the warm region must lie below the return path. A formal demonstration of the argument for a rotating fluid requires three assumptions about the relative circulation around a closed material line: (i) the flow is steady, (ii) the closed material line is a closed streamline and (iii) the work done by friction along the streamline is negative. The argument extends to unsteady flows, thereby relaxing (i-ii), if the absolute circulation along the material line is a bounded function of time - a condition that is met for flows with small Rossby number. Its validity for time-periodic two-dimensional flows of horizontal convection is verified numerically. Poincaré sections reveal the presence of chaotic particle transport in these flows, even though the Eulerian velocity fields have a simple time dependence. In spite of chaotic advection, particle motion is in general downwards in the cold region and upwards in the warm region of the fluid, which is consistent with the flow shape envisioned by Jeffreys. This paper gives support to the validity of his argument for the unsteady case and enhances its relevance for the dynamical interpretation of the basic structure of geophysical flows.
    Description: This work has been supported in part by a grant from the U.S. National Science Foundation
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
    Format: 2797816 bytes
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  • 10
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 37 (2007): 394-407, doi:10.1175/jpo3018.1.
    Description: The ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.
    Description: The first author is supported by the NOAA Postdoctoral Program in Climate and Global Change and GG by the National Ocean Partnership Program (ECCO). Author OM acknowledges support from the National Science Foundation.
    Keywords: Tracers ; Transport ; Paleoclimatology ; Ocean models
    Repository Name: Woods Hole Open Access Server
    Type: Article
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