Strengthening Of Summer Monsoon Research Articles

Chinese loess record of monsoon climate during the last glacial–interglacial cycle

The thick loess–paleosol succession of central China contains a detailed record of changes in East Asian monsoon climate since the late Miocene Epoch. The alternating loess–paleosol stratigraphy closely resembles the marine oxygen-isotope record, implying that episodic dust deposition and pedogenesis are in phase with global ice-volume fluctuations and controlled by variations in solar radiation reaching the land surface. Deposits equivalent to marine isotope stages (MIS) 5 through 1 have been dated by 14C, 10Be, luminescence, and magnetic susceptibility and grain-size models. Malan Loess (MIS 4–2) decreases in thickness southeastward from >30 m in the western Loess Plateau to <5 m east of Xi'an. Downwind, Asian dust is found in Japan, the North Pacific Ocean, and the Greenland Ice Sheet, where it has a chemical signature that points to a primary source in western China. During glacial times, the mean dust flux in the central Loess Plateau reached 0.35 mm/year, whereas in interglacial times it fell to <0.1 mm/year. Times of high dust influx and reduced soil development coincided with strengthened winter monsoon conditions, which are inferred from variations in loess grain size. Times of decreased dust accumulation and strong pedogenesis, marked by high values of magnetic susceptibility, represent times of strengthened summer monsoon conditions. High-frequency fluctuations in dust influx during deposition of the Malan Loess include peaks that likely correlate with North Atlantic Heinrich events. The Younger Dryas oscillation is recorded in Chinese eolian and organic deposits by a shift from cool, humid late-glacial conditions to cold and dry conditions, followed by a return to milder, humid climate at the beginning of the Holocene. Warm, moist climate, associated with a strong summer monsoon, characterized the first half of the Holocene Epoch when a widespread paleosol complex developed on the Loess Plateau. Increased dust deposition during the middle to late Holocene marked a shift to cooler, drier conditions.

Late Glacial and Holocene vegetation history and paleoclimate of the Kaibab Plateau, Arizona

Sediment cores spanning the last 13,500 calendar years (cal yr) were obtained from two lakes (Fracas Lake, 2518 m; Bear Lake, 2778 m) on the Kaibab Plateau in northern Arizona. Pollen and plant macrofossil records indicate that before ∼12,900 cal yr B.P., high elevation landscapes of the Kaibab Plateau near Bear Lake were covered by alpine tundra, while lower elevations near Fracas Lake were occupied by Picea woodland. At ∼12,900 cal yr B.P., Picea engelmannii and Abies lasiocarpa forest expanded upward to occupy the top of the plateau. Pinus ponderosa arrived near Fracas Lake 11,000 cal yr B.P., replacing Picea forests. Since then, Pinus ponderosa forest has dominated the Fracas Lake area. Pinus ponderosa did not appear at Bear Lake until ∼9730 cal yr B.P. Mixed forests of Picea (mainly Picea pungens), Abies lasiocarpa, Pinus ponderosa, and Pseudotsuga (after ∼8000 cal yr B.P.) grew near Bear Lake for the remainder of the Holocene. Picea engelmannii populations reexpanded near Bear Lake after 4000 cal yr B.P. Charcoal records indicate that fire probably helped Pinus ponderosa to become established near Bear Lake. Climate changes on the Kaibab Plateau since the Late Glacial were inferred from lake levels and vegetation patterns. The Late Glacial (>11,000 cal yr B.P.) was cold and probably wet. The early Holocene (11,000 to ∼8000 cal yr B.P.) was cooler than today and may have been the wettest period. Fracas Lake and Bear Lake were probably deepest then. During this period, a strengthened summer monsoon brought in more moisture from the eastern Pacific Ocean and the Gulf of Mexico. During the dry and warm mid-Holocene, Fracas Lake and Bear Lake experienced water-level declines. The late Holocene was relatively wet and cool again, and aquatic plants were abundant in the two lakes. Increasing effective moisture in the late Holocene was related to decreasing summer insolation.

Climate extremes in Loess of China coupled with the strength of deep-water formation in the North Atlantic

The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly developed S4, S5-1 and S5-3 soils (corresponding to the marine δ 18O stages 11, 13, and 15, respectively) as evidence of three episodes of great warmth and two coarse-grained loess units (L9 and L15, corresponding to the marine δ 18O stages 22, 23, 24 and 38, respectively) which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle. Paleopedological, geochemical and magnetic susceptibility data from three loess sections (Xifeng, Changwu and Weinan) suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated mean annual temperature (MAT) of at least 4–6°C higher and a mean annual precipitation (MAP) of 200–300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt (the contact of the monsoonal northward warm-humid air mass with the dry-cold southward one) might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5–3°C and 150–250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region. Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatest/smallest Atlantic–Pacific δ 13C gradients, respectively, indicating their relationships with the strength of Deep Water (NADW) production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 10 4 years.

A correlation between southern and northern hemispheres during the last 0.6 Ma

A comparison of climate records between Chinese Loess Plateau which shows a strengthening of summer monsoon in the last 0.6 Ma and a strengthening of Asia summer monsoon during 35-25 and 4-2.5ka B.P. intervals, and the Australian records which show a strengthening of aridity or desertification suggest that the Australian high leading to the desertification strengthened the Asia summer monsoon in the past through the cross-equator circulation. The synchronous variation in the Holocene Optimum as indicated by Asia and Australia climate records, on the other hand, suggests that the cross-equator East Asia winter monsoon circulation related to the Mongolia high might have influenced the Australia summer monsoon. The interaction of the monsoon climate between southern and northern hemispheres through cross-equator circulation probably started to be obvious since 0.6 Ma B. P.

Holocene environmental changes in the Atacama Altiplano and paleoclimatic implications

Strengthened summer monsoon brought tropical/continental moisture as far south as 25oS during late-glacial and early Holocene times. Precipitation rates in the Altiplano of the western Andes (24oS) increased to 500 mm yr-1 compared to &lt;200 mm yr-1 today. There is evidence of dramatically decreasing lake levels between 8 400 and about 3 000 yr B.P., and conditions drier than today were established. This arid period was interrupted by low-frequency but heavy storms. The monsoon precipitation belt advanced once again in several stages to its current position (200 mm yr-1 isohyeta at 24oS) around 3 000 yr B.P. The reasons for these changes are not known: the variable circulation in the E-Pacific, teleconnections to the northern hemisphere, environmental changes in the source area of the moisture (i.e. tropical continent), or internal forcing due to changes in the radiation budget of the Altiplano are considered as possible explanations.

Episode of Strengthened Summer Monsoon Climate of Younger Dryas Age on the Loess Plateau of Central China

The Baxie loess section, just east of the Tibetan Plateau, contains evidence showing that the Asian monsoon climate experienced an abrupt reversal near the end of the last glacial age. Rapid deposition of dust under cool, dry full-glacial conditions gave way to an interval of soil development and reduced dust influx attributed to a strengthening of the warm, moist summer monsoon. A subsequent abrupt increase in dust deposition, a response to a weakening of the summer monsoon, was later followed by renewed soil formation as summer monsoon circulation again intensified during the early Holocene. By one interpretation, the thin upper loess is a manifestation of the European Younger Dryas oscillation; however, in this case the available 14C ages require either that (1) onset of loess deposition lagged the beginning of the Younger Dryas event in Europe by as much as 2000 calibrated 14C years or (2) all the 14C ages are too young, possibly due to contamination. Alternatively, the late-glacial paleosol, the top of which is synchronous with the abrupt end of the late-glacial δ18O anomaly in the Dye 3 Greenland ice core, records the Younger Dryas event. Such an interpretation is consistent with general circulation model simulations of Younger Dryas climate that show strong seasonality and a strengthened summer monsoon, and with marine cores from the western Pacific Ocean that contain evidence of pronounced cooling of surface waters during Younger Dryas time.