Spatial and temporal variability of surface water in the Kuroshio source region, Pacific Ocean, over the past 21,000 years: evidence from planktonic foraminifera
Introduction
The extent of cool tropical surface water in the Pacific Ocean during the glacial periods remains a controversial topic. However, it is clear that cooling would have significantly affected a variety of important oceanic and atmospheric processes and patterns as emphasized by Andreasen and Ravelo (1997). This study focuses on apparent changes in sea surface temperature (SST) and circulation within the source region of the Kuroshio Current in the western Pacific Ocean during: (1) the Last Glacial period, and (2) the later so-called ‘Pulleniatina minimum event’ (PME).
The Kuroshio Current represents the major western boundary current of the North Pacific Ocean and variations in its character and behavior have immediate consequences for the ocean and climate processes in the entire northwestern Pacific region. Recently, several authors have suggested that SST in the marginal seas of the western Pacific Ocean cooled by ∼5.0°C during the Last Glacial period (e.g. Moore et al., 1980, Linsley et al., 1985, Thunell et al., 1994, Martinez et al., 1997, Pflaumann and Jian, 1999). The latter value stands in contrast to the estimated ∼2.0°C variation in SST thought to have characterized the open tropical Pacific Ocean during the Last Glacial period (e.g. Moore et al., 1980, CLIMAP Project Members, 1981, Ohkohchi et al., 1994). The differences in cooling of SST in marginal seas vs. the open ocean likely reflect the higher sensitivity of marginal seas to global cooling and the associated large scale changes in atmospheric and ocean circulation. The results of our study demonstrate that changing configurations (e.g. geography) of marginal seas in the western Pacific region during the Last Glacial Maximum (LGM) also represent an important factor which forced variations in the character and circulation of the adjacent Pacific Ocean.
In particular, Ujiié et al. (1991) and Ahagon et al. (1993) indicate that the path of the Kuroshio Current was forced to migrate eastward during the LGM due to the emergence of the Taiwan–Ryukyu land bridge, a major geographic barrier separating the East China Sea from the open Pacific Ocean. An analysis of stable isotopes and frequency variations of the Pulleniatina group in 17 piston cores from the Ryukyu Arc region further details this critical change in the course of the Kuroshio Current (Ujiié and Ujiié, 1999). In addition, a brief period of Holocene change in the Kuroshio Current, the PME, has been identified in this same area by Jian et al. (1996), Li et al. (1997) and Ujiié and Ujiié (1999).
Most recently, Ujiié and Ujiié (2000) used factor analysis to compare surface water mass properties (e.g. temperature, salinity and chrolophyll) with planktonic foraminiferal assemblages in 52 surface sediment samples collected in the Ryukyu Arc region. Their works resulted in planktonic foraminiferal assemblages being classified into four groups (Kuroshio, Subtropical, Coastal and Cold water groups). In this report, we use the planktonic foraminiferal groupings of Ujiié and Ujiié (2000) along with a more detailed analysis of planktonic foraminiferal assemblages to establish variations in SST within the Kuroshio Current and surrounding water masses during the post-∼20-kyr period.
Section snippets
Geographic and oceanographic setting
The Ryukyu Arc region is located in the northwestern region of the Pacific Ocean (Fig. 1a). The Ryukyu Arc proper separates the East China Sea from the open Pacific Ocean and extends ∼1200 km between Taiwan and the island of Kyushu, Japan (Fig. 1b). The East China Sea includes a broad continental shelf and the adjacent Okinawa Trough which reaches water depths of more than 2000 m. The Ryukyu Arc can be subdivided geographically and geologically into northern, central and southern sections by
Materials
For this study, we chose 11 piston cores from a set of 17 piston cores originally studied by Ujiié and Ujiié (1999) and used an additional four cores with the goal of detailing down-core changes in planktonic foraminiferal assemblages in the Ryukyu Arc region during the post-LGM (e.g. the last 21,000 years). Locations and water depths of the 15 cores studied are listed in Table 1 with locations shown in Fig. 1b. Six piston cores collected in the Okinawa Trough area were located directly under
Methods
Stable oxygen and carbon isotope ratios (δ18O and δ13C vs. PeeDee Belemnite, respectively) were measured using ∼40 specimens of planktonic foraminifera Globigerinoides sacculifer larger than 250 μm. Two centimeter-thick sediment samples were collected at 10-cm intervals in each core. We used specimens of Globigerinoides ruber for our measurements in core RN88-PC5 due to insufficient numbers of G. sacculifer. We measured the isotope ratios using a Finnigan MAT delta E mass spectrometer at the
Planktonic δ18O fluctuation and SST
Planktonic δ18O values in 12 cores located in slope areas of the Okinawa Trough and Ryukyu Trench decrease toward the Recent (e.g. toward core-tops), likely reflecting post-glacial warming (Fig. 2a,b). Alternatively, δ18O values increase in the oldest portions of six of the cores which penetrated sediments representing the Last Glacial period (cores RN93-PC8, MD98-2193 and RN93-PC3 from the Okinawa Trough and cores RN93-PC12, RN94-PC6 and RN94-PC3 from the Ryukyu Trench slope). The AMS 14C ages
Time-series fluctuations of planktonic foraminiferal assemblages
As noted earlier, Ujiié and Ujiié (2000) analyzed the relationships between Recent planktonic foraminifera and the key oceanographic parameters in the Ryukyu Arc region and classified assemblages into the Subtropical, Kuroshio, Coastal, and Cold water groups (e.g. groups A–D). These same groupings are utilized in our analysis of fossil assemblages in this study. The following sections summarize the results of our analysis of time-series fluctuations and spatial-temporal variations of the four
Discussion
Planktonic foraminiferal distributions indicate that the drastic changes of the Kuroshio Current and surrounding surface water masses occurred twice during the past ∼21,000 years including the Last Glacial period and the shorter PME from ∼4,500 to 3,000 yr BP. However, there are significant differences between the fluctuations of planktonic δ18O values during these two periods; values clearly decreased during the former period whereas they did not exhibit significant change during the PME (Fig.
Conclusions
The major conclusions of our analysis can be summarized as follows.
(1) The Kuroshio Current was prevented from flowing into the Okinawa Trough during the LGM by a land bridge formed between Taiwan and the southern Ryukyu Arc as a product of the associated low stand of sea level. This geographic condition affected heat transport within the Kuroshio source region leading to lowered planktonic δ18O and SST values and the appearance of Cold water species, despite only minimal variations in SST in
Acknowledgements
This work was mainly carried out by Y.U. for the obtaining of a Ph.D. Thesis at the Ocean Research Institute, University of Tokyo, under the supervision of A.T. We like to thank Tadamichi Oba and Michiyo Shimamura of Hokkaido University for facilitating the stable isotope measurement of two cores. Eiji Matsumoto, Nagoya University, financially supported the AMS 14C age dating of Core MD98-2193 by K.O. We express our appreciation to Hodaka Kawahata, Geological Survey of Japan, and Franck
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2021, Quaternary Science ReviewsCitation Excerpt :Using an ensemble of high-resolution ocean model simulations, Vogt-Vincent and Mitarai (2020) concluded that the core of the Kuroshio midstream has remained in the OT during the past changes in sea level, climate, and tectonics during the late Quaternary. Despite the different views of whether the mainstream of KC has remained in the OT during the last glacial or deglacial periods, most studies agree that the KC is a persistent hydrodynamic feature in the OT during the Holocene, although the timing of the onset of KC intensification varied between ∼7 ka and 15 ka among studies (Jian et al., 2000; Li et al., 2020; Lim et al., 2017; Shi et al., 2014; Ujiié et al., 2003; Zheng et al., 2016). The Hg and antimony (Sb) in marine sediments of what we measured in this study could be derived from few anthropogenic of the post-industrialization era, and most natural, including submarine hydrothermal and volcanic emissions, rivers, atmospheric aerosols, etc. (Filella, 2011; Fitzgerald and Lamborg, 2014; Fitzgerald et al., 2007).