Sea surface temperature changes in the Okhotsk Sea and adjacent North Pacific during the last glacial maximum and deglaciation

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Abstract

We estimated sea surface temperatures (SSTs) in the western Okhotsk Sea during the last glacial maximum (LGM) and the last glacial termination from measurements of the alkenone unsaturation index (UK′37), the Tetra Ether indeX of tetraethers consisting of 86 carbon atoms (TEX86), and TEX86L in piston core sediments, which reveal the climate response of this region to different types of forcings. TEX86L is a modification of TEX86 proposed for high-latitude ocean regions. The TEX86L-derived temperatures were not different from the alkenone-derived temperatures from the same and nearby sediment samples. During the LGM, alkenone-derived temperatures in the Okhotsk Sea were relatively warm. These and similarly warm alkenone-derived temperatures found at many other sites in the western North Pacific may reflect a shift in the season of biomarker production from early summer and autumn to mid-summer during the LGM as a result of the expansion of the season of sea-ice cover. Empirical orthogonal function (EOF) analysis of alkenone-derived temperatures at 14 sites from the North Pacific suggested substantial cooling between 20 and 14 cal kyr BP during the deglaciation at 11 of the sites (including 4 sites in the Okhotsk Sea). This dominant EOF mode suggests either an overall seasonal bias in alkenone-derived temperatures or potential shifts in the Haptophyseae production season. The second EOF mode of variability indicated a deglacial warming trend punctuated by millennial-scale cold events, associated with reorganizations of the Atlantic Meridional Overturning Circulation. The SST pattern corresponding to this EOF mode is indicative of millennial-scale meridional shifts of the axis of the westerly jet, which may have affected heat transport across the Kuroshio. Reconstructed temperature changes, in particular in the Okhotsk Sea, provide insight into the conditions that modulated the ventilation history of intermediate waters in the North Pacific.

Introduction

The mid- to high-latitude region of the western–central North Pacific – including the Kuroshio–Oyashio transition area and marginal seas (the Okhotsk, Japan, and Bering Seas) – is a key area for understanding present-day climate variability of the eastern Asian continent (Yasuda, 2003). An important climate variability pattern in this region is the Pacific Decadal Oscillation (PDO), which relates the intensity of the Aleutian Low to changes in marine ecosystems (Mantua et al., 1997). The PDO is estimated as the leading empirical orthogonal function (EOF) mode of the monthly mean Pacific sea surface temperature (SST) anomaly in the region from 20°N poleward and has a 20–30 year periodicity. A positive (negative) PDO is associated with a strengthened (weakened) Aleutian Low, which intensifies (reduces) cold air outbreaks over the western North Pacific, but, in contrast, causes warmer (colder) air to be advected poleward over the eastern North Pacific. The western–central Pacific is also characterized by intense east–west thermal fronts, especially on the western side of the Emperor Seamount Chain (Roden et al., 1982). Glacial to interglacial changes in the surface circulation and hydrography of this area probably affected the climate of the Asian continent and of Japan (Chinzei et al., 1987, Sawada and Handa, 1998).

Surface seawater conditions such as salinity and temperature influence the intensity of downwelling in the ocean's interior. The formation of Dense Shelf Water (DSW; Martin et al., 1998) on the continental shelf of the northeastern Okhotsk Sea is affected by changes in autumn SST (Ogi et al., 2001), sea surface salinity (SSS), and sea-ice extent (Sakamoto et al., 2005). The DSW is critical as a source of Okhotsk Sea Intermediate Water (OSIW), which in turn is a key component of North Pacific Intermediate Water (NPIW; Tally and Nagata, 1995), an important transient carbon reservoir (Tsunogai et al., 1992).

According to the alkenone record derived from sediments from the Okhotsk Sea, SST and SSS varied in phase with Dansgaard–Oeschger (D–O) events during the last glaciation (Harada et al., 2008). D–O events are millennial-scale warm–cold fluctuations recorded by δ18O in the Greenland ice sheet (e.g., Grootes and Stuiver, 1997). D–O events are associated with variations in the sea-ice extent as reconstructed from the amount of ice-rafted debris (IRD) identified in Okhotsk Sea sediments deposited during the last glaciation (Sakamoto et al., 2006). Millennial-timescale variation of SST, SSS, and sea-ice extent probably affected the formation of DSW, OSIW, and NPIW during the last glaciation and deglaciation. Although syntheses based on compilations of reconstructed last glacial maximum (LGM) and deglacial SSTs for the North Pacific (Kiefer and Kienast, 2005) and the Multi-proxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project (MARGO Project Members, 2009) for the global ocean have been published recently, variations in mixed-layer temperatures in the mid- to high-latitude region of the western–central North Pacific and its marginal seas have not been well studied on glacial–interglacial timescales.

Therefore, the purpose of this study was to clarify the interaction between atmospheric circulation, the sea-surface environment, and circulation in the intermediate–deep ocean in the western–central North Pacific and its marginal seas in response to local and remote forcings during the last glacial to deglacial period. Here, we present SST changes from 24 to 10 kyr BP derived from two types of sedimentary biomarker from the Okhotsk Sea: the UK′37 index of long-chain alkenones (Brassell et al., 1986, Prahl and Wakeham, 1987) and the Tetra Ether indeX of tetraethers consisting of 86 carbon atoms (TEX86), which is based on the number of cyclopentyl moieties in the isoprenoid glycerol dialkyl glycerol tetraether (GDGT) lipids (Schouten et al., 2002). We also compare SSTs from the Okhotsk Sea with those from the western and central North Pacific thermal front areas, the Kuroshio–Oyashio transition area, and the Japan Sea. We discuss changes in these temperature data in response to warm and cold global events associated with atmospheric circulation changes during the LGM and the last deglaciation, including Heinrich Event 1 (H1, 17.5–14.6 kyr BP), the Bølling–Allerød period (B–A, 14.6–12.8 kyr BP; Yu and Eicher, 2001), and the Younger Dryas (YD, 12.8–11.5 kyr BP; Muscheler et al., 2008).

Section snippets

Study area and sediment core locations

The study area is shown in Fig. 1. In this study we used data from sediment cores collected at 20 sites, including some previously reported results; cores from sites 1, 7, and 8 were first analyzed as part of this study. Table 1 shows specific cruise and location information for all cores, including references where the original data have been published. Core site 1 is in the northwestern Okhotsk Sea beneath the East Sakhalin Current (ESC) at the surface and OSIW at intermediate depth (Itoh et

Materials and methods

The sediment cores analyzed for alkenone and TEX86 in this study were XP07 C9 (site 1), MD01-2412 (site 7), and MR06-04 PC-4 (site 8), all collected from the western Okhotsk Sea (Fig. 1). The sediments of each core consist of siliceous sandy mud containing calcareous fossils, such as foraminifera.

Specimens of the planktonic foraminifer Globigerina bulloides or Neogloboquadrina pachyderma were picked from cores MR06-04 PC-4 and XP07 C9 for measurement of 14C/12C by accelerated mass spectrometry

Results

We compared the alkenone- and TEX86L-derived temperature profiles obtained in this study with previously published temperature data collected from the Okhotsk Sea (Fig. 3A), the western–central North Pacific (Fig. 3B), and the Japan Sea and Kuroshio region (Fig. 3C). Table 1 shows minimum and maximum values of alkenone- and TEX86L-derived temperatures during the LGM, H1, B–A, and YD. In the case of site 7, the alkenone- and TEX86L-derived temperatures were determined from the same sediment

Core-top temperatures recorded in biomarkers and the difference between alkenone- and TEX86L-derived temperatures

In the present-day Okhotsk Sea, the stratification throughout the surface and subsurface layers causes there to be a steep temperature gradient, and the temperature difference between the surface and 30 m depth is 7–10 °C from early summer to autumn (June–November) (Rostov et al., 2003). The seasonality of alkenone production is not homogeneous. A comparison of time series data obtained by a sediment trap experiment and satellite SST data (Seki et al., 2007) has shown that both the dominant

Conclusions

We compared alkenone- and TEX86L-derived temperature changes between the Okhotsk Sea and the western–central North Pacific Ocean and the Japan Sea during the LGM and the last deglaciation. Our understanding of these periods as inferred from the biomarker records can be summarized as follows:

  • (1)

    Within calibration error, the TEX86L-derived temperatures were the same as alkenone-derived temperatures in samples from the same sediment core and in cores from two sites located near each other throughout

Acknowledgments

We are grateful to Captain Akamine and the crew of R/V Mirai for their help with sediment collection in the Okhotsk Sea during the MR06-04 cruise. We also thank the crew of R/V Professor Khromov, operated by the Far Eastern Regional Hydrometeorological Research Institute, Russia, for their help with sediment collection in the Okhotsk Sea. This work was supported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Research Joint Project between Japan and Pacific

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