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Seasonal aridity in the Indo-Pacific Warm Pool during the Late Glacial driven by El Niño-like conditions
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0002-7799-8871
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Meteorological and Hydrological Institute, Sweden.ORCID iD: 0000-0002-4768-9832
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Number of Authors: 52022 (English)In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 18, no 7, p. 1655-1674Article in journal (Refereed) Published
Abstract [en]

Island South-East Asia (ISEA) is a highly humid region that hosts the world's largest tropical peat deposits. Most of this peat accumulated only relatively recently during the Holocene, suggesting that the climate was drier and/or more seasonal during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period have received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the end of the Late Glacial (14.7–11.7 ka BP), with a focus on the last stadial of the Younger Dryas (12 ka BP). We further simulate the preceding Allerød (13 ka BP) interstadial climate and perform a sensitivity experiment to disentangle the climate impacts due to orbital forcing and Late Glacial boundary conditions against a slowdown of the Atlantic Meridional Overturning Circulation (AMOC). A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy evidence, CESM1 simulates overall drier conditions during the Younger Dryas and Allerød. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality in the Late Glacial is mainly the result of a combination of three factors: (1) large orbital insolation difference on the Northern Hemisphere (NH) between summer and winter, in contrast to the LGM and the present day, (2) a stronger (dry) East Asian winter monsoon caused by a larger meridional thermal gradient and (3) a major reorganization of the Indo-Pacific Walker Circulation with an inverted land-sea circulation and a complete breakdown of deep convection over ISEA in NH winters. The altered atmospheric circulation, sea surface temperature and sea level pressure patterns led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Intertropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period. Our results also suggest that centennial to millennial shifts in AMOC strength modifies the Pacific Ocean hydroclimate via alteration of the position of the ITCZ, and a modulation of the Pacific Walker Circulation. However, Late Glacial AMOC shifts are overall less important than hydroclimate changes due to orbital forcing and boundary condition changes relative to the modern climate.

Place, publisher, year, edition, pages
2022. Vol. 18, no 7, p. 1655-1674
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-207897DOI: 10.5194/cp-18-1655-2022ISI: 000826364300001Scopus ID: 2-s2.0-85134567959OAI: oai:DiVA.org:su-207897DiVA, id: diva2:1689614
Available from: 2022-08-23 Created: 2022-08-23 Last updated: 2024-03-13Bibliographically approved
In thesis
1. Paleoclimate and seasonality on Sumatra during the Late Glacial and Holocene: Insights from biomarkers and climate model simulations
Open this publication in new window or tab >>Paleoclimate and seasonality on Sumatra during the Late Glacial and Holocene: Insights from biomarkers and climate model simulations
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Deep atmospheric convection in the Indo-Pacific Warm Pool (IPWP) is a key driver of the Hadley and Walker Circulations that modulate the Asian-Australian monsoons and the El Niño Southern Oscillation (ENSO). Temperature and rainfall seasonality, i.e., the amount and timing of precipitation, impacts ecosystems, carbon content in soils and peats, and human livelihoods. Yet, past climate variability in the IPWP is poorly constrained. The Maritime Continent, located in the center of the IPWP remains a “quantification desert”, with a scarcity of terrestrial paleoenvironmental reconstructions.

This thesis investigates the evolution of temperature, precipitation amount and seasonality over the Late Glacial (14.7-11.7 ka BP) and the Holocene (last 11.7 ka). This is achieved by combining climate model simulations and lipid biomarker analyses of terrestrial peat archives from Sumatra. Temperature and seasonality were explored by analysis of climate model simulations for the Late Glacial and Holocene. Microbial membrane-derived glycerol dialkyl glycerol tetraethers (GDGTs) were investigated as temperature and hydro-environmental proxies. Using n-alkane distributions, the abundance of algae, aquatic and terrestrial plants was reconstructed and linked to past hydroclimate variability. The hydrogen isotopic composition (dD) of the n-alkanes was then used to disentangle seasonal and annual precipitation signals.

The analysis of Sumatran GDGTs revealed that bacterial community shifts of the GDGT producers had a strong impact on reconstructed temperatures, and that H-shaped branched GDGT isomers are good tracers of such community shifts. The branched GDGT temperature reconstruction indicates gradual warming over the Holocene, consistent with models and nearby marine records.

Rainfall seasonality has shifted drastically over the studied time frame, in particular during the end of the Late Glacial, and between 6-4.2 ka BP. The Late Glacial climate was characterized by a much stronger seasonality, with a cold and dry Asian winter monsoon suppressing atmospheric deep convection in the region. The resulting mean state conditions resembled the atmospheric circulation and sea surface temperature patterns during extreme El Niño events in the modern climate. The Mid-Holocene (6-4.2 ka BP) was characterized by increased seasonality, with alternating droughts and heavy rains due to strong monsoon precipitation and longer dry season.

The Early Holocene was relatively dry. Wetter conditions started around 7-6 ka BP, and peaked at 4.5-3 ka BP. This is consistent with a dD reconstruction on Sulawesi, but 1.5-2 ka later than indicated by speleothem oxygen isotopic (d18O) records on Sumatra and Sulawesi. However, the speleothem records closely follow algal dD values, interpreted here as a seasonal monsoon signal, suggesting that speleothems in the region reflect monsoonal precipitation rather than an annual signal. Rapid drying was reconstructed for the Late Holocene, starting at 3 ka BP, co-occurring with the onset of strengthened ENSO variability. The Late Holocene drying caused drying out and decomposition of peat in one of the studied cores which resulted in a hiatus of 1700 years, highlighting the importance of hydroclimate for peat and carbon accumulation in tropical wetlands.

In conclusion, this dissertation enhances our understanding of past climatic conditions in the Maritime Continent and contributes toward constraining the evolution of temperature, precipitation, and monsoon-driven seasonality over the Late Glacial and Holocene in a region that has a scarce coverage of paleoclimate proxy information. Additionally, the methodological aspects of this thesis advance terrestrial paleoclimatological reconstructions by constraining source shifts of GDGTs and proposing a novel approach to disentangle seasonal and annual precipitation signals from dD.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2024. p. 56
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 389
Keywords
Holocene, Late Glacial, biomarkers, organic geochemistry, climate model, hydrogen isotopes, stable isotopes, paleoclimate, alkanes, GDGT, brGDGT, H-GDGT, bacterial community shifts, paleothermometry, precipitation reconstruction, peat
National Category
Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-227455 (URN)978-91-8014-715-6 (ISBN)978-91-8014-716-3 (ISBN)
Public defence
2024-05-03, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2017-04430
Available from: 2024-04-10 Created: 2024-03-13 Last updated: 2024-03-27Bibliographically approved

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Hällberg, Petter LarsSchenk, FrederikSmittenberg, Rienk H.

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