Dissertation Christian Leipe
Total Page：16
File Type：pdf, Size：1020Kb
Insights into late Quaternary vegetation and climate dynamics in Monsoon Asia obtained from numerical pollen-based reconstructions Doctoral thesis Christian Leipe Insights into late Quaternary vegetation and climate dynamics in Monsoon Asia obtained from numerical pollen-based reconstructions Doctoral thesis submitted in fulfilment of the requirements for the academic degree Doctor rerum naturalium (Dr. rer. nat.) to the Department of Earth Sciences of the FREIE UNIVERSITÄT BERLIN by Dipl.-Geogr. Christian Leipe Berlin, May 2014 Front cover figure: Schematic illustration of the mean main wind directions (arrows) after Kalnay et al. (1996) and the 400 mm precipitation isoline (red line) after New et al. (2002) in Monsoon Asia during summer (June–September). 1st Reviewer: Prof. Dr. Pavel E. Tarasov 2nd Reviewer: Prof. Dr. Frank Riedel Date of defence: 30 June 2014 Abstract Abstract The Asian monsoon is one of the major components of the world’s climate system. The monsoon circulation during the warm half of the year (the Asian summer monsoon) brings significant amounts of precipitation – mainly between May and September – to a vast area of South and East Asia stretching from the western Arabian Sea to the southern Russian Far East. The Asian summer monsoon is generally subdivided into the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM). Numerous studies have demonstrated that both subsystems have varied significantly over different time scales during the late Quaternary. In order to predict potential future climate dynamics in Monsoon Asia and to provide essential information to build adaptation strategies, it is crucial to understand the spatiotemporal patterns and the magnitude of past variations in the Asian monsoon activity and their relation to other components of the global climate system as well as extraterrestrial forcing factors. Despite considerable progress during the last decades, many issues are still unresolved. To improve existing climate model simulations and to broaden the knowledge about the Asian summer monsoon evolution, additional palaeoclimate proxy studies are required from both subdomains. This study mainly focuses on the reconstruction of past vegetation and climate dy- namics in different regions of Monsoon Asia using numerical approaches (biome reconstruction method and modern analogue technique) based on fossil and modern pollen assemblages. The set of employed fossil pollen records include one from a high- alpine lake (north-western Himalayas, India) located within the ISM domain spanning the last ca. 12 ka and two from peat bogs located within the EASM domain spanning the last ca. 5.5 ka (south-western Hokkaido, Japan) and ca. 44 ka (north-western Sakhalin, Russia). All three climate archives are situated close to the modern summer monsoon limit. The results suggest that the initial post-glacial ISM strengthening temporally coin- cides with the termination of the Younger Dryas. The Holocene moisture optimum in the north-western Himalayas prevailed from ca. 11–9.6 cal ka BP. The moisture evolution over the Holocene is marked by a gradual decline, which parallels the southward migration of the mean summer position of the Intertropical Convergence Zone in response to the orbitally induced decrease in summer insolation. The results of the quantitative reconstruction indicate considerable fluctuations in annual precipitation, which was during the wettest interval (early Holocene) ca. 430 mm higher and during the driest interval (late Holocene) ca. 35 mm lower compared to modern conditions (ca. 250 mm). Correlation with other palaeoclimate proxy records suggests that the decrease in precipitation in the regions at the northern limit of the ISM was greater than in the southern parts of the ISM domain. i Abstract There is evidence that likely due to Northern Hemisphere ice sheet boundary condi- tions, the southern ISM domain received more westerly-derived winter precipitation during the early Holocene than during the late Holocene. During the late Holocene, moisture availability is slightly increasing, which is likely a result of strengthened winter westerly disturbances. Holocene centennial-scale intervals of enhanced aridity, which are in concert with North Atlantic cold climate (i.e. Bond) events, probably indicate reduced winter westerly airflow leading to decreased winter precipitation in the north-western Himalayas. As inferred from the quantitative climate reconstructions, the climate conditions in the northern EASM domain during Marine Isotope Stage 3 were only slightly colder and drier than at present. A significantly strong trend towards climate deterioration is documented during Heinrich event 4. Like in the north-western Himalayas, the onset of Holocene climate amelioration parallels the Younger Dryas termination. While the moisture optimum appears to have occurred in the northern EASM domain during the early Holocene, the thermal optimum was reached during the middle Holocene. Together with the results from Hokkaido, the findings from Sakhalin indicate that the Holocene climate conditions of these islands were considerably influenced by ocean currents. Palynological and geomorphological analyses suggest that the evolution of the Bronze Age Harappan Civilisation of the greater Indus Valley was linked to short- and long-term climate trends. The gradual decrease in precipitation probably caused crop yields to fall, which promoted the establishment of the mature phase (4.5–3.9 cal ka BP) urban centres to provide an infrastructure for storage, protection, administration, and redistribution of staple crops. Additional pronounced dry spells at ca. 4 and 3.2 cal ka BP in combination with more frequent El Niño–Southern Oscillation-related interannual monsoon fluctuations during the late Holocene probably further hamper sufficient food supply that may have caused the protracted deurbanisation after ca. 4 cal ka BP and eventual demise of the sophisticated Harappan Civilisation between ca. 3.5–3 cal ka BP. The results of the case studies presented in this thesis yield new insights into late Quaternary Asian monsoon dynamics, possible past climate–human interactions and demonstrate the potential of numerical pollen-based approaches to determine the magnitude of past climate change. They also point out the need for additional palaeocli- mate proxy records from different parts of South and East Asia to enhance the understand- ing of the Asian monsoon system and the underlying driving mechanisms as well as additional modern pollen reference samples to improve numerical pollen-based vegetation and climate reconstructions. ii Zusammenfassung Zusammenfassung Der asiatische Monsun ist ein Hauptbestandteil des globalen Klimasystems. Wäh- rend der warmen Jahreshälfte verursacht die Monsunzirkulation (asiatischer Sommermon- sun) beträchtliche Niederschläge – hauptsächlich zwischen Mai und September – in einem großen Gebiet Süd- und Ostasiens, welches sich in etwa vom westlichen Arabischen Meer bis zum südlichen Teil des russischen fernen Ostens erstreckt. Der asiatische Sommermon- sun ist grundsätzlich unterteilt in den indischen Sommermonsun (ISM) und den ostasiatischen Sommermonsun (EASM). Zahlreiche Studien haben gezeigt, dass beide Subsysteme während des Spätquartärs wesentlichen Änderungen bezüglich ihrer Intensität auf unterschiedlichen Zeitskalen unterworfen waren. Um robuste Vorhersagen über mögliche zukünftige Änderungen im asiatischen Monsunsystem treffen zu können, welche die Grundlagen zur Definition von geeigneten Anpassungsstrategien bilden, ist es unerlässlich die raum-zeitlichen Muster und das Ausmaß vergangener Variabilitäten der Monsunaktivität und deren Beziehung zu anderen Bestandteilen des weltweiten Klimasystems sowie extraterrestrische Einflussfaktoren zu verstehen. Trotz maßgeblicher Fortschritte in den letzten Jahrzehnten sind viele Fragen nach wie vor unbeantwortet. Um die Leistungsfähigkeit bestehender Klimamodelle zu verbessern und das Verständnis der Evolution des asiatischen Sommermonsuns zu erweitern, sind weitere Proxy-basierte Paläoklimastudien in beiden Teilgebieten (ISM und EASM) erforderlich. Diese Arbeit befasst sich hauptsächlich mit der Rekonstruktion vergangener Vegeta- tions- und Klimaänderungen in verschiedenen Regionen Monsun-Asiens unter Verwendung numerischer Rekonstruktionsmethoden („biome reconstruction me- thod“ und „modern analogue technique“) auf der Grundlage fossiler und rezenter Pollenspektra. Das Set der verwendeten fossilen Pollenprofile beinhaltet ein Profil aus einem hochalpinen See (nordwestlicher Himalaya, Indien) aus dem Einflussbereich des ISM, welches den Zeitraum der letzten ca. 12 ka umfasst und zwei weitere Profile aus Torfmooren aus dem Bereich des EASM, die die letzten ca. 5.5 ka (südwestliches Hokkaido, Japan) bzw. ca. 44 ka (nordwestliches Sachalin, Russland) repräsentieren. Alle drei Klimaarchive befinden sich an der nördlichen Grenze des heutigen Einflussbereichs des asiatischen Summermonsuns und sind deshalb besonders gut geeignet, um Erkenntnisse über vergangene Schwankungen in der Monsunintensität zu erlangen. Die Ergebnisse der Untersuchungen am Pollenprofil aus dem nordwestlichen Hima- laya zeigen, dass der postglaziale Anstieg der ISM-Aktivität zeitlich mit dem Ende der Jüngeren Dryas übereinstimmt. Das holozäne Feuchtigkeitsoptimum datiert zwischen ca. 9.6–11 cal ka BP. Die Entwicklung der holozänen Feuchtigkeits- bzw. Niederschlagsinten- sität ist gekennzeichnet von einer sukzessiven Abnahme, die mit der südwärts