Elsevier

Marine Micropaleontology

Volume 49, Issue 4, November 2003, Pages 335-364
Marine Micropaleontology

Spatial and temporal variability of surface water in the Kuroshio source region, Pacific Ocean, over the past 21,000 years: evidence from planktonic foraminifera

https://doi.org/10.1016/S0377-8398(03)00062-8Get rights and content

Abstract

The Kuroshio Current is the major western boundary current of the North Pacific Ocean and has had a large impact on surface water character and climate change in the northwestern Pacific region. The Kuroshio Current becomes a distinctive surface flow in the Ryukyu Arc region after diverging from the North Equatorial Current and passing through the Okinawa Trough. Therefore, the Ryukyu Arc area can be called the Kuroshio source region. We reconstructed post-21-ka time–space changes in surface water masses in the Ryukyu Arc region using 15 piston cores which were dated by planktonic δ18O stratigraphy and AMS 14C ages. Our analysis utilized spatial and temporal changes in planktonic foraminiferal assemblages which were classified into the Kuroshio, Subtropical, Coastal, and Cold water groups on the basis of modern faunal distributions in the study region. These results indicate that the Kuroshio Current and adjacent surface water masses experienced major changes during: (1) the Last Glacial Maximum (LGM), and (2) the so-called Pulleniatina minimum event (PME) from ∼4,500 to 3,000 yr BP. The Kuroshio LGM event corresponds to severe global cooling and is marked by decreases in planktonic δ18O values and estimated sea-surface temperature (SST) with the dominance of the Cold water group of planktonic foraminifera. Cooling within the Kuroshio source region was enhanced during the LGM event because the Kuroshio Current was forced eastward due to the formation of a land bridge between Taiwan and the southern Ryukyu Arc which prohibited its flow into the Okinawa Trough. Except for the severe reduction and disappearance of the Pulleniatina group, no clear cooling signal was identified during the PME based on δ18O values, estimated SST values and variations in the composition of planktonic foraminiferal faunas. The PME assemblages are marked by high abundances of Neogloboquadrina dutertrei, a distinctive Kuroshio type species, along with other species assigned to the Coastal and Central water groups. Subtle ecological differences exist between Pulleniatina obliquiloculata and N. dutertrei; i.e. P. obliquiloculata exhibits lower rates of reproduction under conditions of lower primary productivity in the central Equatorial Pacific Ocean. El Niño-like conditions in the Equatorial Pacific Ocean result in lower rates of surface transport in the Kuroshio Current. In turn, this response triggers lower rates of primary productivity in central equatorial surface waters as well as in the upstream Kuroshio source region, ultimately resulting in a lower abundance of P. obliquiloculata. Thus, we interpret the PME as a possible proxy signal of El Niño-like conditions and enhancement of the El Niño Southern Oscillation climate system after the PME in the tropical and sub-tropical Pacific Ocean.

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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