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Evolution of the Indian summer monsoon and terrestrial vegetation in the Northeastern Indian region during the mid-Pleistocene transition (INTERMILAN)

Subject Area Palaeontology
Term from 2016 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 320220536
 
Final Report Year 2020

Final Report Abstract

3. Summary (Zusammenfassung) 3.1 Presentation of key scientific findings (and any potential applications) (Allgemeinverständliche Darstellung der wichtigsten wissenschaftlichen Fortschritte (Anwendungsaspekte)) Until now there were no long high-resolution palynological records available covering several glacial/interglacial cycles in the northern Bay of Bengal. The proposed study is providing for the first time unique orbital scale changes in vegetation and terrestrial hydrology in northeastern India, as it is based on the study of pollen from sediments older than 600 ka, when other pollen records are not available as well as changes in surface ocean conditions deduced from organic-walled dinocysts. This unique palynological record with well-preserved palynomorphs covering the period between ~1200 and ~480 kyr offers a detailed land-ocean-atmosphere correlation offshore the Ganges–Brahmaputra River Delta. Furthermore, our results reveal clearly that ISM strength was weak during glacial maxima and generally stronger during interglacials answering most of the initial specific questions raised in the proposal: • what floral and vegetation changes took place in the northeastern Indian region during this time interval (during the MPT)? When do more changes occur in the vegetation? Throughout the sequence, pollen record shows the dominance of three distinct vegetation types: grassland plants such as Poaceae and Cyperaceae, coastal plant communities such as mangrove (mainly represented by Rhizophora) and Chenopodiaceae/Amaranthaceae (Cheno-Ams). Humid deciduous tropical forest such as Alchornea and Celtis and mountainous vegetation represented by Podocarpus are also found. Our results reveal that halophytic vegetation such as Cheno-Ams, tolerating highly saline environments and having high colonizing abilities; expand considerably along with grass vegetation (Poaceae) during glacial maxima while mangrove trees develop mainly during Interglacials due to increased river runoff promoted by higher moisture availability in the coast associated to stronger ISM. Additionally, fresh water algae colonies exhibit high concentration values during Interglacials caused by river runoff and direct precipitation over the coast while charred particle-pollen ratios increase during glacial peaks (we are using this ratio rather than the charcoal concentration in order to overcome the dilution effect) suggesting drier conditions. • What are the climatic implications of the vegetation change? Taken together our results show a strong alternation between halophytic and dry plants association during glacials and humid vegetation during Interglacials suggesting a respective change from dry to wet climatic conditions associated with weaker / stronger monsoon precipitations over the study area in agreement with previous reconstructions (Gebregiorgis et al., 2018). • What changes took place in the coastal/intertidal plant communities? Are peaks of mangrove and saltmarshes able to document sea-level changes during interglacial/glacial transitions? One of the most interesting feature in the presented palynological record is the directional alternation of three pollen families that compare nicely with the global sea-level reconstructions from Bintanja & van de Wal (2008), in the following order: Poaceae, Cheno-Ams and Rhizophora. This comparison shows that when the sea level was∼80–120 m lower than today, the exposed shelf allowed the grass (Poaceae) and Chen- Ams to expand substantially while mangrove develop when sea-level is higher along with strong monsoonal precipitation during interglacials. The coastal dynamics in the Ganges–Brahmaputra Delta related to fluctuations in the sea level and available local moisture have played a major role in modulating the local coastal plant community by favoring or reducing the expansion of salt marsh vegetation and mangroves. Our new palynological record has great ecological significance, in as much as it deals with intertidal ecosystems that have not been intensively studied. It offers an important complement to previously published paleorecords from the region and highlights the contrasting processes (monsoon and/or sea-level) to which vegetation have responded. • What sea-surface ecological changes took place during the past 1.2 Myr? What are the timings of salinity changes as well as stratification and surface productivity as inferred from dinocysts? Dinoflagellate cyst assemblages were dominated by autotrophic dinocysts, mainly Spiniferites and L. machaerophorum during Interglacials along with a striking increase of cysts from the river plume assemblage. The occurrence of these assemblages indicate nutrient-rich, warm and stratified surface water conditions (Marret and Zonneveld, 2003, Bouimetarhan et al., 2013, Zonneveld et al., 2013) influenced by enhanced river discharge due to increased precipitation in the drainage basin of the Ganges– Brahmaputra River that supplied large amounts of terrigenous material and nutrients to the continental margin during Interglacial. The development of open ocean species Impagidinium indicative of oligotrophic surface waters and oxygen rich bottom waters show the response of dinocyst assemblages to higher sea-level during Interglacials. Additionally, L. machaerophorum-dominated assemblage along with high concentrations of fresh water algae colonies and the expansion of Rhizophora suggest a trend towards less saline and more stratified surface waters conditions (Bouimetarhan et al., 2018) in response to higher monsoonal precipitation on land due to the ISM intensification during Interglacials. • What is the impact of ocean surface conditions and Indian Ocean SST variability on terrestrial environments, especially on the hydrological cycle and vegetation of the studied area? According to our palynological records, sea level has influenced both the dinocyst assmblages and the coastal vegetation in the Ganges–Brahmaputra delta. This might suggest the sensitivity of ISM precipitation controlled by the mean state and frequency of the global ice-volume thus, affecting the tropical vegetation, hydrologic variability and surface water conditions after 900 kyr following the 100 kyr rhythm of the Late Pleistocene ice ages. However, we observe that large-scale changes in vegetation correlate very well with d18O of benthic foraminifera suggesting the ecological and environmental response to Indian Ocean SST rather than an astronomically controlled 100-kyr pacing. 3.2 Any surprises encountered in the course of the project and in the results obtained („Überraschungen“ im Projektverlauf und bei den Ergebnissen) - not applicable -

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