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  • 1
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    Th-230 normalization : an essential tool for interpreting sedimentary fluxes during the late Quaternary (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA1018, doi:10.1029/2003PA000939.
    Description: There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.
    Description: R. Francois and M. P. Bacon acknowledge support from the National Science Foundation. M. Frank thanks the Swiss Science Foundation for support.
    Keywords: Paleoflux ; Sediment focusing ; Paleoproductivity
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Export fluxes of calcite in the eastern equatorial Pacific from the Last Glacial Maximum to present (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2018, doi:10.1029/2003PA000986.
    Description: The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for “fraction of calcite preserved.” This method produced very similar results at the four sites and revealed that the export flux of calcite was 30–50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.
    Description: This research was partly supported by NSF grant OCE-0095617 and funds from the Northern Illinois University Graduate School (Loubere); the NASA Michigan Space Grant Consortium Seed Grant for summer, 2001 for 230Th analyses at WHOI (Mekik); the French Ministere de l’Education Nationale, de la Recherche et de la Technologie, and a EURODOC grant from the Region Rhone-Alpes (Pichat).
    Keywords: Calcite fluxes ; Eastern equatorial Pacific ; Glacial-interglacial
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Lower export production during glacial periods in the equatorial Pacific derived from (231Pa/230Th)xs,0 measurements in deep-sea sediments (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA4023, doi:10.1029/2003PA000994.
    Description: The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25′S, 146°24′E, 1900 m) and eastern (Ocean Drilling Program Leg 138 Site 849, 0°11.59′N, 110°31.18′W, 3851 m) equatorial Pacific display similar variability over the last 85,000 years, i.e., from isotopic stages 1 to 5a, with systematically higher values during the Holocene, isotopic stage 3, and isotopic stage 5a, and lower values, approaching the production rate ratio of the two isotopes (0.093), during the colder periods corresponding to isotopic stages 2 and 4. We have also measured the 230Th-normalized biogenic preserved and terrigenous fluxes, as well as major and trace elements concentrations, in both cores. The (231Pa/230Th)xs,0 results combined with the changes in preserved carbonate and opal fluxes at the eastern site indicate lower productivity in the eastern equatorial Pacific during glacial periods. The (231Pa/230Th)xs,0 variations in the western equatorial Pacific also seem to be controlled by productivity (carbonate and/or opal). The generally high (231Pa/230Th)xs,0 ratios (〉0.093) of the profile could be due to opal and/or MnO2 in the sinking particles. The profiles of (231Pa/230Th)xs,0 and 230Th-normalized fluxes indicate a decrease in exported carbonate, and possibly opal, during isotopic stages 2 and 4 in MD97-2138. Using 230Th-normalized flux, we also show that sediments from the two cores were strongly affected by sediment redistribution by bottom currents suggesting a control of mass accumulation rates by sediment focusing variability.
    Description: SP funding for this research was provided by grants from the French Minister of Research and a EURODOC grant of the Re´gion Rhoˆne-Alpes (SAFIR-980065327). SP also gratefully acknowledges the financial support of the WHOI Geology and Geophysics Dept. This work was also supported by a CNRS-NSF grant (SP and KWWS). The contribution of JFM to this study was supported in part by the US NSF and by WHOI OCCI and Mellon awards.
    Keywords: (Pa-231/Th-230)xs,0 ; Export productivity ; Pacific
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Glacial‐interglacial circulation changes inferred from 231Pa/230Th sedimentary record in the North Atlantic region (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA2204, doi:10.1029/2008PA001696.
    Description: Studies from the subtropical western and eastern Atlantic Ocean, using the 231Pa/230Th ratio as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation (MOC) over the last deglaciation. In this study, we present a compilation of existing and new sedimentary 231Pa/230Th records from North Atlantic cores between 1710 and 4550 m water depth. Comparing sedimentary 231Pa/230Th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the North Atlantic during the last 20,000 years. The 231Pa/230Th ratio measured in upper Holocene sediments indicates slow water renewal above ∼2500 m and rapid flushing below, consistent with our understanding of modern circulation. In contrast, during the Last Glacial Maximum (LGM), Glacial North Atlantic Intermediate Water (GNAIW) drove a rapid overturning circulation to a depth of at least ∼3000 m depth. Below ∼4000 m, water renewal was much slower than today. At the onset of Heinrich event 1, transport by the overturning circulation declined at all depths. GNAIW shoaled above 3000 m and significantly weakened but did not totally shut down. During the Bølling‐Allerød (BA) that followed, water renewal rates further decreased above 2000 m but increased below. Our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode, with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the LGM but less accentuated. MOC during the Younger Dryas appears very similar to BA down to 2000 m and slightly slower below.
    Description: The LSCE-WHOI cooperation has been supported by a NSF-CNRS cooperative grant NSF INT-0233483. Analytical measurements in LSCE have been supported by French Programme National d’Etude de la Dynamique du Climat, Commissariat a` l’Energie Atomique, and Centre National de la Recherche Scientifique. The participation of J.F.M. in this project was supported in part by grants from the U.S.-NSF, WHOI-OCCI, and the Gary Comer Science and Education Foundation. R.F.’s participation was supported by grants from NSERC and the Canadian Foundation for Climate and Atmospheric Science.
    Keywords: MOC
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
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    Application of an inverse method to interpret 231Pa/230Th observations from marine sediments (2011)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 26 (2011): PA1212, doi:10.1029/2010PA002022.
    Description: Records of 231Pa/230Th from Atlantic sediments have been interpreted to reflect changes in ocean circulation during the geologic past. Such interpretations should be tested with due regard to the limited spatial coverage of 231Pa/230Th data and the uncertainties in our current understanding of the behavior of both nuclides in the ocean. Here an inverse method is used to evaluate the information contained in 231Pa/230Th compilations for the Holocene, Last Glacial Maximum (LGM), and Heinrich Event 1 (H1). An estimate of the abyssal circulation in the modern Atlantic Ocean is obtained by combining hydrographic observations and dynamical constraints. Then sediment 231Pa/230Th data for each time interval are combined with an advection-scavenging model in order to determine their (in)consistency with the modern circulation estimate. We find that the majority of sediment 231Pa/230Th data for the Holocene, LGM, or H1 can be brought into consistency with the modern circulation if plausible assumptions are made about the large-scale distribution of 231Pa and about model uncertainties. Moreover, the adjustments in the data needed to reach compatibility with a hypothetical state of no flow (no advection) are positively biased for each time interval, suggesting that the 231Pa/230Th data (including that for H1) are more consistent with a persistence of some circulation than with no circulation. Our study does not imply that earlier claims of a circulation change during the LGM or H1 are inaccurate, but that these claims cannot be given a rigorous basis given the current uncertainties involved in the analysis of the 231Pa/230Th data.
    Description: O.M. acknowledges the support from the U.S. National Science Foundation. J.F.M. acknowledges support from the U.S. National Science Foundation and the Comer Research and Education Foundation.
    Keywords: Pa-231/Th-230 ; Meridional overturning circulation ; Inverse method ; Heinrich Event
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    Comment on “Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?” by M. Lyle et al. (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 22 (2007): PA1216, doi:10.1029/2005PA001235.
    Keywords: Paleoflux ; Th-230
    Repository Name: Woods Hole Open Access Server
    Type: Article
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