Last Glacial Maximum

Introduction

During the LGM, the summer monsoons were extremely weak and the winter monsoons dominated LGM climate. While the strength of the monsoon is predicted from solar insolation estimates, the paleoclimate record of Southeast Asia confirms the weakening of the summer monsoons and the strengthening of the winter monsoons. While a weaker summer monsoon means colder conditions on land, humidity varies regionally depending on proximity to moisture sources. Based on pollen records (Sun et al, 2004), glacial extent (Shi, 2002), and ice core records (Shi, 2002), temperatures dropped 6-9 C on the Tibetan Plateau, 5-11 C in Eastern China, and 6-8 C in Japan. Modelling shows similar temperature changes in the highland and eastern coastal areas (Yu et al, 2003). The d18O record from the Guliya ice core at an altitude of 6200 m a.s.l. on the west Kunlun Mountains on the northern margin of the Tibetan Plateau, indicates that the temperature fluctuations during the last 125,000 years are consistent with glacial cycles in the Northern Hemisphere ice sheets and oceans (Thompson et al, 1997).

Foraminiferal assemblages show that sea surface temperature changes during the LGM were less remarkable. In the Arabian Sea and Indian Ocean, temperatures were fairly comparable to modern values (Trend-Staid & Prell, 2002). In the South China Sea, however, temperatures were 6-10 C cooler during the LGM due to a lower sea level cutting off the Sea's southern connection to warm tropical waters (Wang & Sun, 1994). Trend-Staid & Prell (2002) note that although sea surface temperatures during the LGM did not vary much from modern values, the SST gradient from the equator to midlatitudes did increase leading to more intense zonal winds and ocean circulation patterns.

Precipitation

While land temperatures were dramatically lower during the LGM, humidity varied locally. While the weakened summer monsoon and stronger winter monsoons would have brought less precipitation to the Asian landmass, local interactions with the westerlies and orographic precipitation provided humidity to northwestern China/Mongolia and a moisture-laden Northeast Winter Monsoon (at low latitudes) supported tropical vegetation on the exposed Sunda shelf (Sun et al, 2000). Paleolake levels (figure to the right, Yu et al, 2003), glacier equilibrium line estimates (Shi, 2002), and paleosol-loess sequences (Rutter et al, 2003) confirm wetter conditions in the High Asia/Mongolian region and near the Fennoscandian icesheet. Also note the change the trajectory of the boundary between wet and dry conditions between the present and the LGM (figure to the right).

Dry Eastern China

Extreme fluctuations in atmospheric circulation due to massive continental glaciers lead to regional variability in sensitivity to forcings (Bush et al, 2004). The Asian interior (35-65 N) was drier than today because of a combination of orographic blocking of the trade westerlies by the Fennoscandian ice sheet (Bust et al, 2004) and decreased strength of the Pacific Summer Monsoon coupled with increased continentality (Wang and Sun, 1994). This increased aridity lead to expansion of the Taklimakan and Gobi deserts inferred from increased sand content in loess (Bush et al, 2002). On the other hand, decreased humidity over the Chinese Loess Plateau, evidenced by decreased chemical weathering (Rutter et al, 2003) and the decreased paleolake levels across eastern China (Yu et al, 2003), was driven primarily by the weakened summer monsoon (Bush et al, 2004).

The South China and East China Seas and the Sea of Japan are relatively shallow seas on continental shelf. The lower sea level of the LGM (120 m) exposed 1.5 million km^2 of continental shelf (Wang & Sun, 1994). The smaller South China Sea became isolated from the warm tropical water of the Kuroshio current and SST dropped 6-10C (planktonic foraminiferal data)(Wang & Sun, 1994). A lower pressure gradient from land to sea lead to a weaker summer monsoon. The weaker summer monsoon, a decline in sea surface temperature, and increased continentality from the exposure of continental shelf all contributed to the drier conditions in Eastern China.

Wet Mongolia

Bush et al (2004) suggest orographic precipitation triggered by the Fennoscandian icesheet increased precipitation and snow melt in proximity to the icesheet margin. Shi (2002) suggests that High Asia was able to maintain high lake levels and growing glaciers due to the southern deflection of the westerlies into the area, coupled with lower temperatures (i.e. less evaporation). Due to the southerly deflection of the westerlies by the Northern Hemisphere icesheets into the Plateau region, western parts of the Plateau probably had less of a drop and less fluctuation in annual precipitation (Shi, 2002).

Glaciation during LGM

The present glacial area in the southeast Tibetan Plateau is about 13,2000 km^2, while it is estimated to have been about 15x larger than that (199,000 km^2) during the LGM (Shi, 2002).

During the LGM, colder conditions (6-9 C) depressed the equilibrium line altitude of mountain glaciers on the Tibetan Plateau by 300-1000 m (Shi, 2002). According to Shi (2002), differences in the ELA are due to differences in precipitation. In areas where precipitation wasn't as dramatically lowered during the LGM (the western Mongolian region and southernmost Himalayas), mountain glaciers were able to lower the maximum extent. Shi (2002) also argues that cold temperatures could have caused some glaciers to develop into extreme continental-type glaciers, frozen to their beds and prevented from moving to lower altitudes as quickly.

There is some debate as to whether an extensive ice sheet covered the Tibetan Plateau. The consensus seems to be for more dispersed mountain glaciers, but certain scholars (in particular, Kuhle, 1998) favor one wide ice sheet.

Vegetation Changes

Unsurprisingly, dramatic changes in temperature and precipitation lead to shifts in ecological regimes. As seen in the figure to the right, pollen and macrofauna records suggest an expansion of tundra and desert to the southeast in China, with a complete disappearance of rainforest in China (Wang et al, 1994). Modelling suggests that the Winter Monsoon strengthened during the LGM due to a higher pressure difference between the Aleutian low and the Mongolian Plateau high (Yu et al, 2003; Lu et al, 2004). This may have contributed to the southward shift in the desert vegetation.

Higher basin sedimentation rates (Agnihotri et al, 2003) and higher dust flux (Nilson & Lehmkuhl, 2001) also suggest that there were dramatic changes in vegetation and surface stabilization during the LGM.

Pollen assemblages from a core in the northern South China Sea (Core 17940) show climate was in general much colder and drier at the LGM (Sun et al, 2000). The terrestrial pollen record suggests quasicyclic alternation between a cold, humid phase with a more temperate and drier phase. This is seen in the alternation between expansion of cold tolerant montane conifer forest southeastward and expansion of an herb-dominated grassland (Sun et al, 2000).

Dropping sea levels and subsequent exposure of 390 million km^2 of continental shelf created a new ecological niche into which vegetation could expand. At lowstand, (see below figure from Sun et al, 2000), the northern continental shelf was dominated by Artemesia-dominated grasslands (Sun et al, 2000). 30-50% Artemesia pollen suggests precipitation ranging from 300-500 mm (fairly dry) with average July temperatures of 15-24 C (much colder than today). In contrast, the southern shelf was dominated by tropical lowland forest and mangroves (Sun et al, 2000). The authors suggest that the tropical vegetation was maintained by moisture from the Northeastern Winter Monsoon allowing for cooler but not drier conditions during the LGM.

Weathering Intensity of Land

With weaker summer monsoons, decreases in precipitation and temperature and higher rates of atmospheric dust deposition lead to less developed soil profiles during the LGM. Alternating beds of loess and paleosols provide a direct terrestrial fingerprint of glacial and interglacial climates and because they correlate, in general, with oxygen isotope records from deep sea cores (Porter and An, 1995), they have been used as a proxy for global climate. On the Loess Plateau, the Malan Loess contains the LGM record (Derbyshire et al, 1997).

Bay of Bengal and South China Sea

As evidenced in core sediment from the Bay of Bengal (figure to the left), soils during the LGM had more smectite clay (evidence of less mature soil) than kaolinitic clay (evidence of extremely weathered soil) (Colin et al, 1999).

In the north South China Sea, elemental concentrations in the terrigenous sediment also suggest less chemical weathering during the LGM (Wei et al, 2004).

Higher Sedimentation Rates

Ocean sediment cores from the Arabian Sea have higher concentrations of organic matter and nitrogen in spite of the fact that primary productivity rates are lower during the LGM. The authors interpreted this discrepancy to be due to higher preservation due to higher rates of sedimentation (Agnihotri et al, 2003). As previously mentioned, destabilization of surfaces due to loss of vegetation lead to higher sedimentation rates during the LGM in spite of lower precipitation and less surface run-off. In the Agnihotri et al (2003) study, cation ratios of the silicate fraction of the sediment were used as proxies for the amount of detrital material entering the system. Although, in the central LGM, varve thicknesses decrease in an ocean sediment core off of Pakistan suggesting reduced terrigenous input due to more arid conditions (von Rad et al, 2003).

Increased Dust Transport Increases

As seen in the figure to the left, dust fluxes increases dramatically at the LGM. A comparative literature study of Quaternary aeolian dust accumulation concluded that dried lake bed sediment is the dominant dust source of the dust (Nilson & Lehmkuhl, 2001).

Ocean Circulation

While the Arabian Sea and the Bay of Bengal did not experience a large change in temperature during the LGM, their circulation patterns became dramatically different. In the upper figure to the right, modern summer conditions allow for strong upwelling and high productivity in the Arabian Sea. During the LGM, the weaker monsoon allowed the more southerly westerlies to dominate and upwelling was weaker due to a lack of Ekman transport (Ivanova et al, 2003).

A study by Sirocko et al (2000) found higher concentrations of redox-sensitive trace elements (Mo and S, Cr, Mn, Co, Ni, Cu, HREE, etc.) in deep sea cores throughout the Arabian Sea suggesting poor ventilation during the LGM. The low oxygen levels in the deep water of the Arabian Sea are lowest in the northernmost parts of the basin evidenced by the existence of pyrite minerals (Sirocko et al, 2000).

Sediment records suggest that the South China Sea also had a strong oxygen minimum zone during the LGM and stable estuarine circulation after being cut off from the Borneo sea strait due to falling sea levels (Wang et al, 1999). In contrast, however, the authors found high seasonality in the South China Sea and emerging Sunda Shelf.

Locations of Cores in Arabian Sea (From Sirocko et al., 2000)

Ocean Salinity

In most ocean regions, large variations in surface temperature within the paleoclimate record prevent the effects of surface salinity to be isolated and measured. In the northern Indian Ocean, however, where temperature gradients are small, Cullen (1981) was able to estimate surface salinity during the LGM based on foraminiferal assemblages. The study results show that salinity also dramatically increased during the LGM ( modern conditions in figure).

Greater salinity in the northern part of the Bay of Bengal is due to decreasing the amount of freshwater entering the system. Under modern conditions, low salinity can prevail year-round due to the large amount of water dumped into the basin by the Ganges-Brahmaputra River system during the summer monsoon and the Irrawddy River during the winter (Cullen, 1981).

Summary

In the Asian Monsoon Region during the LGM, summer monsoons were substantially weaker and winter monsoons were substantially stronger. These changes in source region and wind direction, lead to colder temperatures and drier conditions on land and changes in oceanic circulation and productivity. Substantial evidence from multiple proxies has identified areas which were not drier during the LGM and debate continues as to the relative roles of decreased evaporation, orographic precipitation and moisture-laden wind belts. Pollen records from ocean sediment cores suggest that southeastern Asia maintained tropical forest while northeastern Asia was dominated by grassland. Further research is needed to verify whether the pollen is from vegetation that was growing on the shelf sediments or was transported from futher inland by rivers, but current evidence suggests that the shelf vegetation was substantial.