Yokoyama Yusuke

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  • Article
    Coral record of younger dryas chronozone warmth on the Great Barrier Reef
    (American Geophysical Union, 2020-12-11) Brenner, Logan D. ; Linsley, Braddock K. ; Webster, Jody M. ; Potts, Donald C. ; Felis, Thomas ; Gagan, Michael K. ; Inoue, Mayuri ; McGregor, Helen V. ; Suzuki, Atsushi ; Tudhope, Alexander W. ; Esat, Tezer M. ; Thomas, Alexander L. ; Thompson, William G. ; Fallon, Stewart ; Humblet, Marc ; Tiwari, Manish ; Yokoyama, Yusuke
    The Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleo‐application of Isopora coral‐derived SST calibrations to a suite of 25 previously published fossil Isopora from the central GBR spanning ~25–11 kyr BP. The resultant multicoral Sr/Ca‐ and δ18O‐derived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5–5.5°C at Noggin Pass (~17°S) and ~7–8°C at Hydrographer's Passage (~20°S) (Sr/Ca‐derived) during the LGM, in line with previous estimates (Felis et al., 2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9–11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with near‐modern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil Isopora SSTA data from the GBR provide new insights into the deglacial reef response, with near‐modern warming during the YDC, since the LGM.
  • Article
    Dynamic millennial-scale climate changes in the northwestern Pacific over the past 40,000 years
    (American Geophysical Union, 2010-12-03) Chen, Min-Te ; Lin, Xiaopei ; Chang, Yuan-Pin ; Chen, Y.-C. ; Lo, L. ; Shen, Chuan-Chou ; Yokoyama, Yusuke ; Oppo, Delia W. ; Thompson, William G. ; Zhang, Rong
    Ice core records of polar temperatures and greenhouse gases document abrupt millennial-scale oscillations that suggest the reduction or shutdown of thermohaline Circulation (THC) in the North Atlantic Ocean may induce the abrupt cooling in the northern hemisphere. It remains unknown, however, whether the sea surface temperature (SST) is cooling or warming in the Kuroshio of the Northwestern Pacific during the cooling event. Here we present an AMS 14C-dated foraminiferal Mg/Ca SST record from the central Okinawa Trough and document that the SST variations exhibit two steps of warming since 21 ka — at 14.7 ka and 12.8 ka, and a cooling (∼1.5°C) during the interval of the Younger Dryas. By contrast, we observed no SST change or oceanic warming (∼1.5–2°C) during the episodes of Northern Hemisphere cooling between ∼21–40 ka. We therefore suggest that the “Antarctic-like” timing and amplitude of millennial-scale SST variations in the subtropical Northwestern Pacific between 20–40 ka may have been determined by rapid ocean adjustment processes in response to abrupt wind stress and meridional temperature gradient changes in the North Pacific.
  • Article
    Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum
    (Nature Publishing Group, 2014-06-17) Felis, Thomas ; McGregor, Helen V. ; Linsley, Braddock K. ; Tudhope, Alexander W. ; Gagan, Michael K. ; Suzuki, Atsushi ; Inoue, Mayuri ; Thomas, Alexander L. ; Esat, Tezer M. ; Thompson, William G. ; Tiwari, Manish ; Potts, Donald C. ; Mudelsee, Manfred ; Yokoyama, Yusuke ; Webster, Jody M.
    Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and δ18O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1–2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.
  • Article
    Monsoon hydrography and productivity changes in the East China Sea during the past 100,000 years : Okinawa Trough evidence (MD012404)
    (American Geophysical Union, 2009-08-29) Chang, Yuan-Pin ; Chen, Min-Te ; Yokoyama, Yusuke ; Matsuzaki, Hiroyuki ; Thompson, William G. ; Kao, Shuh-Ji ; Kawahata, Hodaka
    We analyzed the high-resolution foraminifer isotope records, total organic carbon (TOC), and opal content from an Okinawa Trough core MD012404 in order to estimate the monsoon hydrography and productivity changes in the East China Sea (ECS) of the tropical western Pacific over the past 100,000 years. The variability shown in the records on orbital time scales indicates that high TOC intervals coincide with the increases of boreal May–September insolation driven by precession cycles (∼21 ka), implying a strong connection to the variations in monsoons. We also observed possibly nearly synchronous, millennial-scale changes of the ECS surface hydrography (mainly driven by salinity changes but also by temperature effects) and productivity coincident with monsoon events in the Hulu/Dongge stalagmite isotope records. We found that increased freshening and high productivity correlate with high monsoon intensity in interstadials. This study suggests that the millennial-scale changes in monsoon hydrography and productivity in the ECS are remarkable and persistent features over the past 100,000 years.
  • Article
    Nutritional sources of meio- and macrofauna at hydrothermal vents and adjacent areas: Natural-abundance radiocarbon and stable isotope analyses
    (Inter Research, 2019-07-18) Nomaki, Hidetaka ; Uejima, Yuki ; Ogawa, Nanako O. ; Yamane, Masako ; Watanabe, Hiromi K. ; Senokuchi, Reina ; Bernhard, Joan M. ; Kitahashi, Tomo ; Miyairi, Yosuke ; Yokoyama, Yusuke ; Ohkouchi, Naohiko ; Shimanaga, Motohiro
    Deep-sea hydrothermal vents host unique marine ecosystems that rely on organic matter produced by chemoautotrophic microbes together with phytodetritus. Although meiofauna can be abundant at such vents, the small size of meiofauna limits studies on nutritional sources. Here we investigated dietary sources of meio- and macrofauna at hydrothermal vent fields in the western North Pacific using stable carbon and nitrogen isotope ratios (δ13C, δ15N) and natural-abundance radiocarbon (Δ14C). Bacterial mats and Paralvinella spp. (polychaetes) collected from hydrothermal vent chimneys were enriched in 13C (up to -10‰) and depleted in 14C (-700 to -580‰). The δ13C and Δ14C values of dirivultid copepods, endemic to hydrothermal vent chimneys, were -11‰ and -661‰, respectively, and were similar to the values in the bacterial mats and Paralvinella spp. but distinct from those of nearby non-vent sediments (δ13C: ~-24‰) and water-column plankton (Δ14C: ~40‰). In contrast, δ13C values of nematodes from vent chimneys were similar to those of non-vent sites (ca. -25‰). Results suggest that dirivultids relied on vent chimney bacterial mats as their nutritional source, whereas vent nematodes did not obtain significant nutrient amounts from the chemolithoautotrophic microbes. The Δ14C values of Neoverruca intermedia (vent barnacle) suggest they gain nutrition from chemoautotrophic microbes, but the source of inorganic carbon was diluted with bottom water much more than those of the Paralvinella habitat, reflecting Neoverruca’s more distant distribution from active venting. The combination of stable and radioisotope analyses on hydrothermal vent organisms provides valuable information on their nutritional sources and, hence, their adaptive ecology to chemosynthesis-based ecosystems.