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Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra
Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).ORCID iD: 0000-0002-7799-8871
Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). University of Helsinki, Finland.ORCID iD: 0000-0002-4768-9832
Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). University of Bayreuth, Germany.
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Number of Authors: 82023 (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.

Place, publisher, year, edition, pages
2023. Vol. 186, article id 104702
Keywords [en]
brGDGTs, H-GDGTs, Bacterial community shifts, Paleothermometry, Holocene temperature conundrum, Peat core
National Category
Ecology
Identifiers
URN: urn:nbn:se:su:diva-226557DOI: 10.1016/j.orggeochem.2023.104702ISI: 001111774300001Scopus ID: 2-s2.0-85176231528OAI: oai:DiVA.org:su-226557DiVA, id: diva2:1837713
Available from: 2024-02-14 Created: 2024-02-14 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, PetterSchenk, FrederikZhang, QiongSmittenberg, Rienk H.

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