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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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    Southern Ocean control of silicon stable isotope distribution in the deep Atlantic Ocean (2012)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Geochemical Cycles 26 (2012): GB2035, doi:10.1029/2011GB004141.
    Description: The fractionation of silicon (Si) stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon (δ30Si) dissolved in seawater a sensitive tracer of the oceanic biogeochemical Si cycle. We present a high-precision dataset that characterizes the δ30Si distribution in the deep Atlantic Ocean from Denmark Strait to Drake Passage, documenting strong meridional and smaller, but resolvable, vertical δ30Si gradients. We show that these gradients are related to the two sources of deep and bottom waters in the Atlantic Ocean: waters of North Atlantic and Nordic origin carry a high δ30Si signature of ≥+1.7‰ into the deep Atlantic, while Antarctic Bottom Water transports Si with a low δ30Si value of around +1.2‰. The deep Atlantic δ30Si distribution is thus governed by the quasi-conservative mixing of Si from these two isotopically distinct sources. This disparity in Si isotope composition between the North Atlantic and Southern Ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate (δ15N-NO3). We infer that the meridional δ30Si gradient derives from the transport of the high δ30Si signature of Southern Ocean intermediate/mode waters into the North Atlantic by the upper return path of the meridional overturning circulation (MOC). The basin-scale deep Atlantic δ30Si gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes, which fractionates Si isotopes between the abyssal and intermediate/mode waters formed in the Southern Ocean.
    Description: This work was supported by Swiss National Science Foundation grants 200021-116473 and 200020-130361.
    Description: 2012-12-19
    Keywords: Atlantic Ocean ; Southern Ocean ; Silicon isotopes
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    What can we learn from X-ray fluorescence core scanning data? A paleomonsoon case study (2020)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 21(2), (2020): e2019GC008414, doi:10.1029/2019GC008414.
    Description: X‐ray fluorescence (XRF) core scanning of marine and lake sediments has been extensively used to study changes in past environmental and climatic processes over a range of timescales. The interpretation of XRF‐derived element ratios in paleoclimatic and paleoceanographic studies primarily considers differences in the relative abundances of particular elements. Here we present new XRF core scanning data from two long sediment cores in the Andaman Sea in the northern Indian Ocean and show that sea level related processes influence terrigenous inputs based proxies such as Ti/Ca, Fe/Ca, and elemental concentrations of the transition metals (e.g., Mn). Zr/Rb ratios are mainly a function of changes in median grain size of lithogenic particles and often covary with changes in Ca concentrations that reflect changes in biogenic calcium carbonate production. This suggests that a common process (i.e., sea level) influences both records. The interpretation of lighter element data (e.g., Si and Al) based on low XRF counts is complicated as variations in mean grain size and water content result in systematic artifacts and signal intensities not related to the Al or Si content of the sediments. This highlights the need for calibration of XRF core scanning data based on discrete sample analyses and careful examination of sediment properties such as porosity/water content for reliably disentangling environmental signals from other physical properties. In the case of the Andaman Sea, reliable extraction of a monsoon signal requires accounting for the sea level influence on the XRF data.
    Description: The staff at the Bremen Core Repository is thanked for their help with core handling and Sam Müller at the University of Kiel provided technical assistance with the XRF scanner. We thank two anonymous reviewers for their insightful comments that improved the manuscript significantly. This work was partially funded through DFG Grant HA 5751/3. P. A. and K. N.‐K. acknowledge support from UK‐IODP and Natural and Environment Research Council, UK. The authors express their thanks to all those who contributed to the success of the National Gas Hydrate Program Expedition 01 (NGHP01) and Expedition 353. The data set supporting the conclusions of this article is available in the PANGEA repository (doi: https://doi.pangaea.de/10.1594/PANGAEA.910533).
    Description: 2020-07-10
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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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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  • 5
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    Provenance and weathering of clays delivered to the Bay of Bengal during the middle Miocene: linkages to tectonics and monsoonal climate (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bretschneider, L., Hathorne, E. C., Huang, H., Luebbers, J., Kochhann, K. G. D., Holbourn, A., Kuhnt, W., Thiede, R., Gebregiorgis, D., Giosan, L., & Frank, M. Provenance and weathering of clays delivered to the Bay of Bengal during the middle Miocene: linkages to tectonics and monsoonal climate. Paleoceanography and Paleoclimatology, 36(2), (2021): e2020PA003917, https://doi.org/10.1029/2020PA003917.
    Description: Tectonics and regional monsoon strength control weathering and erosion regimes of the watersheds feeding into the Bay of Bengal, which are important contributors to global climate evolution via carbon cycle feedbacks. The detailed mechanisms controlling the input of terrigenous clay to the Bay of Bengal on tectonic to orbital timescales are, however, not yet well understood. We produced orbital‐scale resolution geochemical records for International Ocean Discovery Program Site U1443 (southern Bay of Bengal) across five key climatic intervals of the middle to late Miocene (15.8–9.5 Ma). Our new radiogenic Sr, Nd, and Pb isotope time series of clays transported to the Ninetyeast Ridge suggest that the individual contributions from different erosional sources overall remained remarkably consistent during the Miocene despite major tectonic reorganizations in the Himalayas. On orbital timescales, however, high‐resolution data from the five investigated intervals show marked fluctuations of all three isotope systems. Interestingly, the variability was much higher within the Miocene Climatic Optimum (around 16–15 Ma) and across the major global cooling (~13.9–13.8 Ma) until ~13.5 Ma, than during younger time intervals. This change is attributed to a major restriction on the supply of High Himalayan erosion products due to migration of the peak precipitation area toward the frontal domains of the Himalayas and the Indo‐Burman Ranges. The transient excursions of the radiogenic isotope signals on orbital timescales most likely reflect climatically driven shifts in monsoon strength.
    Description: This research used samples and data provided by the International Ocean Discovery Program and was funded by the German Research Foundation (DFG) (grants HA 5751/6‐1 and HA 5751/6‐2, KU 649/36‐1, and TH 1317‐8 and TH 1317‐9). Open access funding enabled and organized by Projekt DEAL.
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
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    Enhanced late miocene chemical weathering and altered precipitation patterns in the watersheds of the Bay of Bengal recorded by detrital clay radiogenic isotopes (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bretschneider, L., Hathorne, E. C., Bolton, C. T., Gebregiorgis, D., Giosan, L., Gray, E., Huang, H., Holbourn, A., Kuhnt, W., & Frank, M. Enhanced late miocene chemical weathering and altered precipitation patterns in the watersheds of the Bay of Bengal recorded by detrital clay radiogenic isotopes. Paleoceanography and Paleoclimatology, 36(9), (2021): e2021PA004252, https://doi.org/10.1029/2021PA004252.
    Description: The late Miocene was a period of declining CO2 levels and extensive environmental changes, which likely had a large impact on monsoon strength as well as on the weathering and erosion intensity in the South Asian Monsoon domain. To improve our understanding of these feedback systems, detrital clays from the southern Bay of Bengal (International Ocean Discovery Program Site U1443) were analyzed for the radiogenic isotope compositions of Sr, Nd, and Pb to reconstruct changes in sediment provenance and weathering regime related to South Asian Monsoon rainfall from 9 to 5 Ma. The 100 kyr resolution late Miocene to earliest Pliocene record suggests overall low variability in the provenance of clays deposited on the Ninetyeast Ridge. However, at 7.3 Ma, Nd and Pb isotope compositions indicate a switch to an increased relative contribution from the Irrawaddy River (by ∼10%). This shift occurred during the global benthic δ13C decline, and we suggest that global cooling and increasing aridity resulted in an eastward shift of precipitation patterns leading to a more focused erosion of the Indo-Burman Ranges. Sr isotope compositions were decoupled from Nd and Pb isotope signatures and became more radiogenic between 6 and 5 Ma. Grassland expansion generating thick, easily weatherable soils may have led to an environment supporting intense chemical weathering, which is likely responsible for the elevated detrital clay 87Sr/86Sr ratios during this time. This change in Sr isotope signatures may also have contributed to the late Miocene increase of the global seawater Sr isotope composition.
    Description: This research used samples and data provided by the International Ocean Discovery Program and was funded by the German Research Foundation (DFG) (grants HA 5751/6-1 & -2). C. T. Bolton acknowledges funding from the French ANR project iMonsoon (ANR-16-CE01-0004-01) and IODP France. W. Kuhnt acknowledges funding from the DFG (grant Ku649/36-1).
    Keywords: Clay radiogenic isotopes ; Late Miocene ; South Asian Monsoon ; Chemical weathering
    Repository Name: Woods Hole Open Access Server
    Type: Article
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