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Paleoclimate and seasonality on Sumatra during the Late Glacial and Holocene: Insights from biomarkers and climate model simulations
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0002-7799-8871
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 [en]
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: urn:nbn:se:su:diva-227455ISBN: 978-91-8014-715-6 (print)ISBN: 978-91-8014-716-3 (electronic)OAI: oai:DiVA.org:su-227455DiVA, id: diva2:1844267
Public defence
2024-05-03, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2017-04430Available from: 2024-04-10 Created: 2024-03-13 Last updated: 2024-03-27Bibliographically approved
List of papers
1. Seasonal aridity in the Indo-Pacific Warm Pool during the Late Glacial driven by El Niño-like conditions
Open this publication in new window or tab >>Seasonal aridity in the Indo-Pacific Warm Pool during the Late Glacial driven by El Niño-like conditions
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2022 (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.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-207897 (URN)10.5194/cp-18-1655-2022 (DOI)000826364300001 ()2-s2.0-85134567959 (Scopus ID)
Available from: 2022-08-23 Created: 2022-08-23 Last updated: 2024-03-13Bibliographically approved
2. Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra
Open this publication in new window or tab >>Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra
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2023 (English)In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 186, article id 104702Article in journal (Refereed) Published
Abstract [en]

Branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in sedimentary archives are increasingly used for paleotemperature reconstructions due to their strong correlation with mean annual air temperature. However, environmental factors can influence the brGDGT producing bacterial community, potentially affecting the brGDGT-temperature relationship and introducing fundamental errors in reconstructions. Here we assess the reliability of the methylation index of brGDGTs (MBT ' 5ME) in sediments as a paleotemperature proxy by tracking provenance differences based on brGDGT fractional abundances in a short lake core, two peat cores and surface soils on Sumatra (n = 333 in total). Then, we attempt to reconstruct the Holocene paleotemperatures on Sumatra using the two peat cores. Our results indicate distinct brGDGT and H-shaped brGDGT (H-GDGT) compositions in soil, lake and peat environments, suggesting production by different bacterial communities. We introduce a new index, the isomerization of H-GDGTs (IRH) that can distinguish between these environments. In an 11,000-year long peat core from Diatas, we find that brGDGT composition changes are dominated by bacterial community shifts rather than temperature changes. In contrast, a core from the nearby Padang peatland can be robustly used for a brGDGT-based paleotemperature reconstruction since there are no signs of past environmental or brGDGT source shifts. The results from Padang indicate a gradual warming trend over the past 8,000 years, consistent with climate model simulations and nearby sea surface temperature reconstructions. However, current MBT ' 5ME calibrations yield larger warming trends compared to simulations and other proxy studies, suggesting the need for tropical and/or peat-specific brGDGT temperature calibrations. Our findings demonstrate the importance of assessing environmental shifts and bacterial source community changes when employing brGDGT paleothermometry. The methodological framework outlined in this study can be used in future research for reliable down-core brGDGT temperature reconstructions. Our proxy reconstruction over the past 8,000 years offers novel insights into the Holocene temperature evolution from a region with low climate seasonality.

Keywords
brGDGTs, H-GDGTs, Bacterial community shifts, Paleothermometry, Holocene temperature conundrum, Peat core
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-226557 (URN)10.1016/j.orggeochem.2023.104702 (DOI)001111774300001 ()2-s2.0-85176231528 (Scopus ID)
Available from: 2024-02-14 Created: 2024-02-14 Last updated: 2024-03-13Bibliographically approved
3. Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: insights from δD, alkanes and GDGTs
Open this publication in new window or tab >>Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: insights from δD, alkanes and GDGTs
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Rainfall seasonality in the tropics has a substantial impact on both ecosystems and human livelihoods. Yet, reconstructions of past rainfall variability have so far generally been unable to differentiate between annual and seasonal precipitation changes. Past variations in seasonality are therefore largely unknown. Here, we disentangle hydrogen isotopic (dD) signals from terrestrial leaf waxes and algae in an 8000-year peat core from Sumatra, which reflect annually integrated versus wet season signals, respectively. We validate these results using lipid biomarkers by reconstructing vegetation dynamics via n-alkane distributions and peatland hydrological conditions using glycerol dialkyl glycerol tetraethers (GDGTs). Finally, we reconstruct biomass burning using levoglucosan concentrations in the core. Algal dD indicate stronger winter (~DJF) monsoon precipitation in the Mid-Holocene, between 8-4.2 ka BP. A period of alternating flooding, droughts and wildfires is reconstructed between 6-4.2 ka BP, implicating very strong monsoonal precipitation and drying out and burning during a longer and intensified dry season. We attribute this strong rainfall seasonality in the Mid-Holocene mainly to orbitally forced insolation seasonality, and a strenghtened Indonesian-Australian summer monsoon. In terms of annually integrated conditions, terrestrial plant dD, vegetation composition and GDGTs all indicate wetter conditions peaking between 3-4.5 ka BP, preceded by drier conditions, and followed by drastic and rapid drying in the late Holocene from around 2.8 ka BP. Our multiproxy annual precipitation reconstruction thereby indicates the wettest overall conditions approximately 1500-2000 years later than a nearby speleothem d18O record, which instead follows the dD recorded by algae in our record. We therefore hypothesize that speleothem reconstructions predominantly record the wet season isotopic signal, due to cave groundwater recharge occurring mainly after heavy precipitation. These results resolve the seasonal versus annual components of past rainfall variability in the Indo Pacific Warm Pool region, and highlight the importance of considering seasonality in rainfall reconstructions.

National Category
Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-227454 (URN)
Available from: 2024-03-13 Created: 2024-03-13 Last updated: 2024-03-13

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