Paleoclimatology and paleoceanography of the East Asian monsoon system, the Japan Sea, and marginal basins

Onset and evolution of millennial-scale variability of Asian monsoon and its possible linkage with uplift of the Himalaya and Tibetan Plateau

Elucidation of the processes and mechanisms of millennial-scale abrupt climatic changes (e.g., DOCs) is one of the most important subjects outlined in the IODP Initial Science Plan “Earth, Oceans, and Life.” Such millennial-scale climatic variability is a global phenomenon and is characterized by complex interactions among the atmosphere, ocean, cryosphere, and biosphere (e.g., Bond et al., 1993; Broecker et al., 1990). However, the spatial patterns, amplification, and propagation mechanisms and ultimate driving forces of DOCs are still not well understood. Reconstructing the evolution of DOCs within a precise temporal and spatial context is necessary to solve these problems.

The East Asian and Indian summer monsoon have varied significantly in association with DOCs (Schulz et al., 1998; Tada et al., 1999; Wang et al., 2001, 2008; Tada, 2004). In addition, climate and oceanography in the western Mediterranean Sea and surrounding areas varied in association with DOCs, with the variation characterized by a northward shift and intensification of the Westerly Jet during the Dansgaard–Oeschger stadials (Moreno et al., 2002; Cacho et al., 2002; Sánchez Goñi et al., 2002). Shifts in circulation of the Westerly Jet are also reported from North America, with its southward shift during Dansgaard–Oeschger stadials (Asmerom et al., 2010; Wagner et al., 2010).

Nagashima et al. (2007, 2011) showed that the Westerly Jet path over the Japan Sea also shifted southward during Dansgaard-Oeschger stadials. Together with the western Mediterranean and North American results described above, this suggests that the Westerly Jet circulation mode changed in association with DOCs. Because the inferred movement of the Westerly Jet axis is opposite between the Mediterranean Sea and what is documented in North America and the Japan Sea (Fig. F2), the shift could be characterized by different mode of meandering rather than a simple north–south shift analogous to the seasonal cycle (Fig. F3). Ono et al. (1998) suggested that the Westerly Jet flowed along the southern side of the HTP during marine isotope stage (MIS) 2, based on eolian quartz provenance in East Asia inferred from electron spin resonance (ESR) signal intensity. If correct, the topographic effect of the HTP could be critical for causing a different course of the Westerly Jet circulation and its shift in association with DOCs. It is even possible that HTP uplift could have created multiple stable modes of Westerly Jet circulation and triggered the onset of DOC-type climatic variability (Tada, 2004).

According to the study of IRD in the North Atlantic, millennial-scale IRD events are recognized at least since 1.4 Ma (Jansen et al., 2000; Raymo et al., 1998) and could be as old as 2.5 Ma (Mc Intyre et al., 2001; Becker et al., 2006). However, it is not well constrained when such millennial-scale variability started, how it evolved through time, and what is its relationship with the onset and evolution of the orbital-scale variability. Furthermore, the timing and mode of uplift of the HTP is still being debated (e.g., Copeland, 1997; Tapponnier et al., 2001; Royden et al., 2008; Wang et al., 2008), but results of recent studies suggest that HTP uplift started as early as 50 Ma and that the southern and central parts of Tibet approximately reached their present height by 34 Ma (e.g., Rowley and Currie, 2006). Records from inland Asia also demonstrate that uplift of northern Tibet started around 3.6 Ma (Zheng et al., 2000; Li et al., 1997) and that uplift of Himalaya restarted during the late Pliocene to Pleistocene and continued until the present (Jain et al., 2000; Sakai et al., 2002; Vance et al., 2003; Wobus et al., 2003). Because uplift of these areas should have enhanced the extent and altitude of the topographic barrier against the Westerly Jet, it is possible that the course and intensity of the Westerly Jet were also influenced significantly (Rea et al., 1998). Thus, results of recent paleoclimatic and tectonic studies seem concordant with the idea that HTP uplift amplified the DOC-type millennial-scale variability of Westerly Jet circulation through the topographic effect.

Recent climatic model simulation studies that evaluate the impact of HTP uplift on the onset and evolution of Asian monsoon and its variability suggest that desertification in inland Asia and the EASM and EAWM and their orbital-scale variability neither started nor evolved simultaneously (An et al., 2001; Liu and Yin, 2002; Abe et al., 2003, 2004; Kitoh, 2004). According to the models, the Indian monsoon and EASM intensified from 3.6 to 2.6 Ma and were stronger than the modern monsoon during the relatively early stage of the uplift. Intensities slightly decreased during the later stages of uplift, whereas the EAWM intensified only after ~50% of the present height of Tibet was attained and increased from 3.6 Ma, continuing to do so after 2.6 Ma. Temporal variations of the summer and winter monsoon proxies in Chinese loess seem consistent with these climatic simulation results (An et al., 2001), although questions remain about the validity of the winter monsoon proxies used (grain size of loess and paleosol) (Ding et al., 2005).

In addition, the behavior of the Westerly Jet and its relation with East Asian monsoon during the last 2.6 m.y. has been discussed on the basis of the grain size of Chinese loess (Sun et al., 2003). However, the relation between the Westerly Jet and the East Asian monsoon before 2.6 Ma, as well as the onset timing and evolution process of millennial-scale variability of EASM and EAWM, have never been fully explored.

Potential contributions from Expedition 346

We aim to reconstruct the axial position and intensity of the Westerly Jet through examination and comparison of the eolian dust provenance, flux, and grain size along the latitudinal transect of the Japan Sea. Eolian dust grain size and flux are considered as useful parameters to evaluate the intensity of transport wind and aridity of source areas (e.g., Rea et al., 1985), and reconstruction of the westerly jet axis position may be possible through examination of eolian dust grain size variation along the north–south transect (e.g., Rea and Leinen, 1988). Additionally, approaches such as ESR and quartz crystallinity (e.g., Nagashima et al., 2007, 2011) are likely to be able to resolve contrasts between quartz respectively sourced from Mongolian Gobi (Siberia–Northeastern China), Taklimakan, and the Japanese arc. Application of end-member modeling (e.g., Prins et al., 2000) to the grain size distribution data combined with eolian dust provenance data may further improve our estimation of the relative contribution and grain size distribution of eolian dust from different sources as well as detrital particles delivered from rivers by suspension in the surface water. Other approaches focusing on geochemistry and radiogenic isotopic tracers of terrigenous provenance (e.g., Irino and Tada, 2000, 2002) are likely to be fruitful as well.

We hope to reconstruct the winter monsoon intensity through examination of IRD abundance and distribution along the northern latitudinal transect in the Japan Sea. At present, stronger winter monsoon wind produces deep water, called Japan Sea Proper Water (JSPW), through sea ice formation in the northwestern part of the Japan Sea (Talley et al., 2003). Consequently, sea ice formation and deepwater ventilation could reflect winter monsoon intensity. Ikehara (2003) described the occurrence of millennial-scale IRD events in the northern part of the Japan Sea during the last 160 k.y. Correlation of these IRD data with the lightness (L*) profile of Core MD01-2407 suggests that many of these IRD events coincide with the intervals of high L* values, further suggesting intense deepwater ventilation that in turn coincides with Heinrich events (Fig. F4). Grain size studies of the loess–paleosol sequence in China also suggest intensification of winter monsoon during Heinrich events (e.g., Porter and An, 1995).

We aim to reconstruct summer monsoon intensity through examination of surface water temperature and salinity changes and variation in chemistry, mineralogy, and mass accumulation rate of the terrigenous sediment delivered from the Yangtze River at proposed Site ECS-1 on the northeastern margin of the East China Sea. Ijiri et al. (2005) reported the occurrence of light δ18O spikes of planktonic foraminifer Globigerinoides ruber that seem to correspond to interstadials of the DOC. They interpret these spikes as reflecting low-salinity events caused by increased discharge from the Yangtze and Yellow Rivers and subsequent expansion of East China Sea Coastal Water (ECSCW) because no changes in alkenone temperature are observed during these events. It is possible to estimate salinity changes associated with these events more precisely by combining δ18O with Mg/Ca measurements (Kubota et al., 2010). Recent geochemical studies of Ocean Drilling Program (ODP) Site 1145 in the northern South China Sea have also shown the potential usefulness of K/Al and Ba/Si ratios as proxy indicators for summer and winter monsoon intensities, respectively (Wehausen and Brumsack, 2002).

Collectively, these approaches targeting allied aspects of abrupt climate behavior will allow unprecedented reconstruction of the interrelationships between the EASM, EAWM, and Westerly Jet axis movement through the Pliocene–Pleistocene. Assuming appropriate age control, we further aim to compare our enhanced understanding of the East Asian monsoon from the Japan Sea and East China Sea with marine records from the northwestern Atlantic and ice cores from Antarctica as well as with data from the Okhotsk Sea to examine when the millennial-scale climatic linkage between North Atlantic and East Asia began and how it evolved.