Browsing by Author "Yokoyama, Y"

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    Beryllium isotope signatures of ice shelves and sub-ice shelf circulation
    (Elsevier, 2019-01-01) White, DA; Fink, D; Post, AL; Simon, KJ; Galton-Fenzi, BK; Foster, S; Fujioka, T; Jeromson, MR; Blaxell, M; Yokoyama, Y
    Be isotopes are a useful tracer of sediment source and transport pathways but have not been widely tested in glacio-marine environments. We measured Be isotopes in a range of depositional environments from open marine, sub-ice shelf and subglacial settings throughout Prydz Bay, one of Antarctica's largest ice drainage systems. We find that strong sub-ice shelf and bottom current circulations can advect 10Be-rich open marine sediments into an ice shelf cavity, and 10Be-poor terrestrial sediments onto the continental shelf at the ice shelf outflow, meaning that 10Be concentrations reflect sub-ice shelf circulation patterns rather than depositional environment. However, HCl-extractable 10Be/9Be ratios can provide a more robust discrimination of sediment deposited in open marine and sub-ice shelf settings. Thus, Be isotopes are a useful tracer of both environmental setting and sub-ice shelf circulation strength in both modern and paleo-ice sheet margins. Crown Copyright © 2018 Published by Elsevier B.V.
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    Chronostratigraphy of sediment cores from Lake Selina, southeastern Australia: radiocarbon, optically stimulated luminescence, paleomagnetism, authigenic beryllium isotopes and elemental data
    (Elsevier B. V., 2022-06) Lisé-Pronovost, A; Fletcher, MS; Simon, Q; Jacobs, Z; Gadd, PS; Herriers, AIR; Yokoyama, Y
    This Data in Brief paper comprises dataset obtained for sediment cores collected from Lake Selina, located in the West Coast Range of Tasmania, Australia. Datasets include radiocarbon and optically stimulated luminescence age estimates, elemental composition, beryllium isotopes, magnetic properties and the paleomagnetic record measured on the cores assigned as TAS1402 (Location: Tasmania, Year: 2014, Site number: 02). The multi-proxy dataset was used to develop a chronostratigraphy for the 5.5 m and 270,000 year old record. See Lisé-Pronovost et al. (2021) (10.1016/j.quageo.2021.101152) for interpretation and discussion. The data presented in this study serve as an archive for future studies focusing on Earth system dynamics and the timeline and linkages of environmental changes across Tasmania, the Southern Hemisphere and at a global scale. 2022 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND licence.
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    Coral U-series ages: U in corals and in the oceans
    (Elsevier, 2007-07) Esat, TM; Yokoyama, Y
    A by product of U-series dating of corals is the 234U/238U ratio in the oceans at the time the coral was growing. The expectation is that this ratio remains invariant in the oceans over the U-series time scale, of at least 500,000 years, and hence it can be used to detect itinerant U or Th through the sample over time. Often, a suite of corals from a reef section show an intriguing, approximate 1:1, correlation between the 234U/238U ratio and age. This pattern can arise by the addition of almost equal amounts of external 230Th and 234U into the coral over an extended period. Currently, the favoured explanation postulates addition of equal amounts of 230Th and 234Th. The latter has a very short half life of about 25 days but the advantage that during aqueous transport it should not fractionate from 230Th. There are at least three prescriptions for removing the extraneous added components, based on the difference between the measured and expected 234U/238U ratio, to derive a corrected or “true” age for the sample. However, there is growing evidence that 234U/238U may not have been invariant in the oceans through Glacial-Interglacial transitions. In addition, some of the correlated data sets cross over to values of 234U/238U lower than in the present ocean thus requiring both the removal and addition of 230Th and 234Th, in neighbouring corals, which is not easy to explain. We will present data from Huon Peninsula coral reefs that show much more complex behaviour than the so far recognised 1:1 linear trend. Firstly, during major sea-level transitions the 234U/238U ratio shifts from low to high values in step with rising sea-levels. There are several well documented examples of this type of behaviour. Secondly, the variability in 234U/238U is amplified during rapid changes in sea-level. Candidates for this type of behaviour occur during rapid sea-level changes associated with Heinrich-events and also during the OIS6 to OIS5 transition. At these times, there is a direct correlation between sea-level change and U as the 234U/238U ratio shifts between high and low values in step with sea-level. This type of behaviour cannot be explained by extraneous 230Th-234Th infusion or through any other plausible diagenetic process. Conversely, these results cast doubts on the 230Th-234Th addition explanation for the 1:1 234U/238U-age trend in the data and more so on the so-called corrected ages calculated from them. © 2007 Published by Elsevier Ltd.
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    Coupled uranium isotope and sea-level variations in the oceans
    (Elsevier, 2010-12-15) Esat, TM; Yokoyama, Y
    Globally, rivers supply uranium to the oceans with excess 234U relative to secular equilibrium and 234U taken-up by corals can be used for dating. In addition, the 234U abundance in sea water, at the time the coral was growing, can be measured independently. The veracity of U-series ages used in determining past sea-level variations is dependent on selecting pristine corals free from diagenetic alteration. A quantitative test for alteration assumes invariant 234U abundances in the oceans for at least the past half a million years and results from samples outside of a narrow range in modern ocean 234U abundance are excluded from data sets. Here, we have used previously published data to show that 234U in the oceans appears to be variable and directly related to changes in sea-level, not only over long glacial–interglacial timescales but also at very short, centennial timescales. Most of the previously discarded data can be used to provide valuable additional sea-level information. The process permits a unique insight into the interplay between sources and sinks of uranium in the oceans mediated by sea-level changes at rates far faster than previously thought possible. Similar, rapid sea-level, forcing of other trace element abundances in the oceans is likely. © 2010, Elsevier Ltd.
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    Insights into subtropical Australian aridity from Welsby Lagoon, North Stradbroke Island, over the past 80,000 years
    (Elsevier, 2020-04-15) Lewis, RJ; Tibby, J; Arnold, LJ; Barr, C; Marshall, JC; McGregor, GB; Gadd, PS; Yokoyama, Y
    Terrestrial sedimentary archives that record environmental responses to climate over the last glacial cycle are underrepresented in subtropical Australia. Limited spatial and temporal palaeoenvironmental record coverage across large parts of eastern Australia contribute to uncertainty regarding the relationship between long-term climate change and palaeoecological turnover; including the extinction of Australian megafauna during the late Pleistocene. This study presents a new, high-resolution, calibrated geochemical record and numerical dating framework from Welsby Lagoon, a wetland from North Stradbroke Island that records key periods of late Pleistocene environmental change. Single-grain optically stimulated luminescence and radiocarbon dating are integrated into a Bayesian age-depth model for the sedimentary sequence spanning Marine Isotope Stage (MIS) 5 to the present. Scanning micro X-ray fluorescence (XRF) and bulk sediment XRF assays are used to infer past dust dynamics, with changes in the abundance of silica and potassium interpreted as proxies for aridity across local and regional sources. Variations in dust flux were contemporaneous with hydrological change, concordant with changes in vegetation cover on the island and, relate to deflation events at major dust source regions on the Australian continent. The Welsby Lagoon record supports the notion of a variable MIS4 within which an increased dust flux (71–67 ka), may be indicative of drier climate. Additionally, the record also shows a lower dust flux through the Last Glacial Maximum (LGM) than is evident in other Australian aeolian records. However, this low LGM flux is attributed to the wetland’s evolution, rather than a reduction in total dust flux. ©2020 Elsevier Ltd
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    Issues in radiocarbon and U-series dating of corals from the last glacial period
    (Elsevier, 2008-08) Esat, TM; Yokoyama, Y
    Radiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated records. In spite of considerable effort to provide a reliable radiocarbon calibration curve beyond 25,000 years, the majority of the data show large atmospheric radiocarbon peaks which are inconsistent both in magnitude and timing between different determinations. The results of the most recent work [Chiu, T.-C., Fairbanks, R.G., Mortlock, R.A., Bloom, A.L., 2005. Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals. Quaternary Science Reviews 24 (16-17), 1797-1808], from Araki Island fossil corals, indicate a monotonic variation from about 33 to 49 ka, with no radiocarbon peaks, but with some gaps in the data. The difference between this and previous results, from fossil corals, has been attributed to selection of better-quality samples and rigorous analytical methods. However, previous results from Huon Peninsula [Yokoyama, Y., Esat, T.M., Lambeck, K., Fifield, L.K., 2000. Last ice age millennial scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 (3), 383-401; Cutler, K.B., Gray, S.C., Burr, G.S., Edwards, R.L., Taylor, F.W., Cabioch, G., Beck, J.W., Cheng, H., Moore, J., 2004. Radiocarbon calibration and comparison to 50 kyr BP with paired C-14 and Th-230 dating of corals from Vanuatu and Papua New Guinea. Radiocarbon 46 (3), 1127-1160] show radiocarbon peaks exclusively located within the gaps in the Araki data. The timing of the gaps are not random, but appear to be related to severe climate and sea-level variations associated with Heinrich events initiated in the North Atlantic. We propose that the Huon and Araki data sets are complementary rather than exclusive and that the absence of coral growth at Araki Island during Heinrich events presumably reflect local adverse conditions for coral growth. © 2007, Elsevier Ltd.
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    Late holocene sea-level fall and turn-off of reef flat carbonate production: rethinking bucket fill and coral reef growth models
    (GeoScienceWorld, 2015-02-01) Harris, DL; Webster, JM; Vila-Concejo, A; Hua, Q; Yokoyama, Y; Reimer, PJ
    Relative sea-level rise has been a major factor driving the evolution of reef systems during the Holocene. Most models of reef evolution suggest that reefs preferentially grow vertically during rising sea level then laterally from windward to leeward, once the reef flat reaches sea level. Continuous lagoonal sedimentation (“bucket fill”) and sand apron progradation eventually lead to reef systems with totally filled lagoons. Lagoonal infilling of One Tree Reef (southern Great Barrier Reef) through sand apron accretion was examined in the context of late Holocene relative sea-level change. This analysis was conducted using sedimentological and digital terrain data supported by 50 radiocarbon ages from fossil microatolls, buried patch reefs, foraminifera and shells in sediment cores, and recalibrated previously published radiocarbon ages. This data set challenges the conceptual model of geologically continuous sediment infill during the Holocene through sand apron accretion. Rapid sand apron accretion occurred between 6000 and 3000 calibrated yr before present B.P. (cal. yr B.P.); followed by only small amounts of sedimentation between 3000 cal. yr B.P. and present, with no significant sand apron accretion in the past 2 k.y. This hiatus in sediment infill coincides with a sea-level fall of ∼1–1.3 m during the late Holocene (ca. 2000 cal. yr B.P.), which would have caused the turn-off of highly productive live coral growth on the reef flats currently dominated by less productive rubble and algal flats, resulting in a reduced sediment input to back-reef environments and the cessation in sand apron accretion. Given that relative sea-level variations of ∼1 m were common throughout the Holocene, we suggest that this mode of sand apron development and carbonate production is applicable to most reef systems. Copyright © 2020 Geological Society of America
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    Modern and possible paleotsunami deposits in Samenoura, Sanriku Coast, and their relation to tsunami source mechanisms
    (Japan Geoscience Union Meeting, 2014-05-02) Sugawara, D; Nishimura, Y; Goto, K; Goff, JR; Jaffe, BE; Richmond, B; Chagué-Goff, C; Szczuciński, W; Yokoyama, Y; Miyairi, Y; Sawada, C
    Samenoura is situated in the bay head of a small inlet on the Pacific coast of Oshika Peninsula, one of the nearest places to the epicenter of the 2011 Tohoku-oki Earthquake. According to the Joint Survey Group, wave heights were measured at more than 20 m near the coastline. This area was severely damaged as a result of both co-seismic subsidence and tsunami inundation. We carried out field surveys of the Tohoku-oki and paleotsunami deposits at Samenoura in March, May and October 2013. Sandy deposits laid down by the Tohoku-oki tsunami were up to 20 cm thick at locations with an elevation greater than 10 m, and were several cm thick within the forest higher up. The tsunami deposit also contained numerous shell fragments and foraminifera. Although some possible sources of the tsunami deposits can be attributed to narrow sandy beaches near the study area, the deposition of such a thick sandy deposit is more or less enigmatic, considering the steep Ria-type coastal topography.Using a gouge auger and geoslicer, we found at least two sand layers intercalated within muddy sediments. A volcanic ash layer, which corresponds to the AD 915 Towada-a tephra, was also identified from a horizon between these sand layers. The underlying sand layer was most probably laid down by the 869 Jogan earthquake tsunami, one of the large-scale events known to have affected the region. Previous studies of the Jogan tsunami have proposed several possible source models that involve an interplate thrust earthquake. Given that the local bathymetry and topography of Samenoura Bay may be sensitive to the waveform of a large-scale tsunami, paleotsunami deposits found from this area may be the key to determining the source mechanisms of events on the Sanriku Coast.In this presentation, the possible correlation of the sandy deposits with known paleotsunami events based on detailed radiocarbon dating is discussed. The hydrodynamic character and processes of tsunami sediment erosion and deposition in Samenoura Bay are analyzed using numerical modeling of both interplate and outer-rise earthquake scenarios. © Japan Geoscience Union Meeting, 2014.
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    Prospects for the new frontiers of earth and environmental sciences
    (Elsevier, 2008-08) Yokoyama, Y; Matsuzaki, H; Esat, TM
    One of the major advances in environmental geochemistry, over the past two decades, has been the introduction of Accelerator-based Mass Spectrometry (AMS) for radiocarbon dating and for investigating terrestrial surface processes through trace quantities of cosmic-ray produced nuclides. During October 2006, a symposium at the University of Tokyo celebrated 50,000 hours of AMS operations at the Tokyo “Micro Analysis Laboratory, Tandem Accelerator (MALT)”. MALT is one of 10 current AMS facilities in Japan but the only one capable of analyzing multiple nuclides. More than 20 talks and over 30 posters were presented covering a diverse range of AMS studies, including, radiocarbon dating, measurements in ice cores, progress in instrumentation, and analyses of in-situ-produced nuclides. © 2008, Elsevier Ltd.