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Changes in Monsoonal Precipitation and Atmospheric Circulation During the Holocene Reconstructed from Stalagmites from Northeastern India

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Changes in Monsoonal Precipitation and Atmospheric Circulation During the Holocene Reconstructed from Stalagmites from Northeastern India Changes in Monsoonal Precipitation and Atmospheric Circulation During the Holocene Reconstructed from Stalagmites from Northeastern India Dissertation Zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.) in der Wissenschaftsdisziplin Geologie eingereicht von Sebastian Breitenbach an der Mathematisch-Naturwissenschaftlichen Fakultat¨ der Universitat¨ Potsdam Marz¨ 2009 This work is licensed under a Creative Commons License: Attribution - Share Alike 3.0 Germany To view a copy of this license visit http://creativecommons.org/licenses/by-sa/3.0/de/deed.en Published online at the Institutional Repository of the University of Potsdam: URL http://opus.kobv.de/ubp/volltexte/2009/3780/ URN urn:nbn:de:kobv:517-opus-37807 [http://nbn-resolving.org/urn:nbn:de:kobv:517-opus-37807] Abstract Recent years witnessed a vast advent of stalagmites as palaeoclimate archives. The multitude of geo- chemical and physical proxies and a promise of a precise and accurate age model greatly appeal to palaeoclimatologists. Although substantial progress was made in speleothem-based palaeoclimate research and despite high-resolution records from low-latitudinal regions, proving that palaeoenvi- ronmental changes can be archived on sub-annual to millennial time scales our comprehension of climate dynamics is still fragmentary. This is in particular true for the summer monsoon system on the Indian subcontinent. The Indian summer monsoon (ISM) is an integral part of the intertrop- ical convergence zone (ITCZ). As this rainfall belt migrates northward during boreal summer, it brings monsoonal rainfall. ISM strength depends however on a variety of factors, including snow cover in Central Asia and oceanic conditions in the Indic and Pacific. Presently, many of the fac- tors influencing the ISM are known, though their exact forcing mechanism and mutual relations remain ambiguous. Attempts to make an accurate prediction of rainfall intensity and frequency and drought recurrence, which is extremely important for South Asian countries, resemble a puzzle game; all interaction need to fall into the right place to obtain a complete picture. My thesis aims to create a faithful picture of climate change in India, covering the last 11,000 ka. NE India represents a key region for the Bay of Bengal (BoB) branch of the ISM, as it is here where the monsoon splits into a northwestward and a northeastward directed arm. The Meghalaya Plateau is the first barrier for northward moving air masses and receives excessive summer rainfall, while the winter season is very dry. The proximity of Meghalaya to the Tibetan Plateau on the one hand and the BoB on the other hand make the study area a key location for investigating the interaction between different forcings that governs the ISM. A basis for the interpretation of palaeoclimate records, and a first important outcome of my thesis is a conceptual model which explains the observed pattern of seasonal changes in stable isotopes (δ18O and δ2H) in rainfall. I show that although in tropical and subtropical regions the amount effect is commonly called to explain strongly depleted isotope values during enhanced rainfall, alone it cannot account for observed rainwater isotope variability in Meghalaya. Monitoring of rainwater isotopes shows no expected negative correlation between precipitation amount and δ18O of rainfall. In turn I find evidence that the runoff from high elevations carries an inherited isotopic signature into the BoB, where during the ISM season the freshwater builds a strongly depleted plume on top of the marine water. The vapor originating from this plume is likely to ‘memorize’ and transmit further very negative δ18O values. The lack of data does not allow for quantification of this ‘plume effect’ on isotopes in rainfall over Meghalaya but I suggest that it varies on seasonal to millennial timescales, depending on the runoff amount and source characteristics. The focal point of my thesis is the extraction of climatic signals archived in stalagmites from NE India. High uranium concentration in the stalagmites ensured excellent age control required for successful high-resolution climate reconstructions. Stable isotope (δ18O and δ13C) and grey-scale data allow unprecedented insights into millennial to seasonal dynamics of the summer and winter 18 monsoon in NE India. ISM strength (i. e. rainfall amount) is recorded in changes in δ Ostalagmites. The δ13C signal, reflecting drip rate changes, renders a powerful proxy for dry season conditions, and shows similarities to temperature-related changes on the Tibetan Plateau. A sub-annual grey-scale profile supports a concept of lower drip rate and slower stalagmite growth during dry conditions. During the Holocene, ISM followed a millennial-scale decrease of insolation, with decadal to cen- tennial failures resulting from atmospheric changes. The period of maximum rainfall and enhanced seasonality corresponds to the Holocene Thermal Optimum observed in Europe. After a phase of rather stable conditions, ∼4.5 kyr ago, the strengthening ENSO system dominated the ISM. Strong El Ni˜noevents weakened the ISM, especially when in concert with positive Indian Ocean dipole events. The strongest droughts of the last ∼11 kyr are recorded during the past 2 kyr. ii Using the advantage of a well-dated stalagmite record at hand I tested the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to detect sub-annual to sub- decadal changes in element concentrations in stalagmites. The development of a large ablation cell allows for ablating sample slabs of up to 22 cm total length. Each analyzed element is a potential proxy for different climatic parameters. Combining my previous results with the LA- ICP-MS-generated data shows that element concentration depends not only on rainfall amount and associated leaching from the soil. Additional factors, like biological activity and hydrogeochemical conditions in the soil and vadose zone can eventually affect the element content in drip water and in stalagmites. I present a theoretical conceptual model for my study site to explain how climatic signals can be transmitted and archived in stalagmite carbonate. Further, I establish a first 1500 year long element record, reconstructing rainfall variability. Additionally, I hypothesize that volcanic eruptions, producing large amounts of sulfuric acid, can influence soil acidity and hence element mobilization. iii Zusammenfassung Stalagmiten erfuhren in den letzten Jahren vermehrt Aufmerksamkeit als bedeutende Pal¨aoklima- Archive. Pal¨aoklimatologen sind beeindruckt von der grossen Zahl geochemischer und physikalischer Indikatoren (Proxies) und der M¨oglichkeit, pr¨azise absolute Altersmodelle zu erstellen. Doch ob- wohl substantielle Fortschritte in der speleothem-basierten Klimaforschung gemacht wurden, und trotz hochaufgel¨oster Archive aus niederen Breiten, welche zeigen, das Umweltver¨anderungen auf Zeitskalen von Jahren bis Jahrtausenden archiviert und rekonstruiert werden k¨onnen, bleibt unser Verst¨andnis der Klimadynamik fragmentarisch. Ganz besonders gilt dies fur¨ den Indischen Som- mermonsun (ISM) auf dem Indischen Subkontinent. Der ISM ist heute als ein integraler Bestandteil der intertropischen Konvergenzzone verstanden. Sobald dieser Regengurtel¨ w¨ahrend des borealen Sommer nordw¨arts migriert kann der ISM seine feuchten Luftmassen auf dem Asiatischen Festland entladen. Dabei h¨angt die St¨arke des ISM von einer Vielzahl von Faktoren ab. Zu diesen geh¨oren die Schneedicke in Zentralasien im vorhergehenden Winter und ozeanische Bedingungen im Indis- chen und Pazifischen Ozean. Heute sind viele dieser Faktoren bekannt. Trotzdem bleiben deren Mechanismen und internen Verbindungen weiterhin mysteri¨os. Versuche, korrekte Vorhersagen zu Niederschlagsintensit¨at und H¨aufigkeit oder zu Durreerreignissen¨ zu erstellen ¨ahneln einem Puzzle. All die verschiedenen Interaktionen mussen¨ an die richtige Stelle gelegt werden, um ein sinnvolles Bild entstehen zu lassen. Meine Dissertation versucht, ein vertrauenswurdiges¨ Bild des sich wan- delnden Holoz¨anen Klimas in Indien zu erstellen. NE Indien ist eine Schlusselregion¨ fur¨ den ¨ostlichen Arm des ISM, da sich hier der ISM in zwei Arme aufteilt, einen nordwestw¨arts und einen nordostw¨arts gerichteten. Das Meghalaya Plateau ist das erste Hindernis fur¨ die sich nordw¨arts bewegenden Luftmassen und erh¨alt entsprechend exzessive Niederschl¨age w¨ahrend des Sommers. Die winterliche Jahreszeit dagegen ist sehr trocken. Die N¨ahe zum Tibetplateau einerseits und der Bucht von Bengalen andererseits determinieren die Schlussel-¨ position dieser Region fur¨ das Studium der Interaktionen der den ISM beeinflussenden Kr¨afte. Ein Fundament fur¨ die Interpretation der Pal¨aoklimarecords und ein erstes wichtiges Ergebnis meiner Arbeit ist ein konzeptuelles Modell, welches die beobachteten saisonalen Ver¨anderungen stabiler Isotope (δ18O und δ2H) im Niederschlag erkl¨art. Ich zeige, das obwohl in tropischen und sub- tropischen Regionen meist der amount effect zur Erkl¨arung stark negativer Isotopenwerte w¨ahrend starker Niederschl¨age herangezogen wird, dieser allein nicht ausreicht, um die Isotopenvariabilit¨at im Niederschlag Meghalaya’s zu erkl¨aren. Die Langzeitbeobachtung der Regenwasserisotopie zeigt keine negative Korrelation zwischen Niederschlagsmenge und δ18O. Es finden sich Hinweise, das der Abfluss aus den Hochgebirgsregionen Tibets und des Himalaya eine Isotopensignatur an das Oberfl¨achenwasser der Bucht
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