Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: insights from δD, alkanes and GDGTs
Show others and affiliations
(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: urn:nbn:se:su:diva-227454OAI: oai:DiVA.org:su-227454DiVA, id: diva2:1844255
Available from: 2024-03-13 Created: 2024-03-13 Last updated: 2024-03-13
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

Open Access in DiVA

No full text in DiVA

Search in DiVA

By author/editor
Hällberg, Petter
Climate Research

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 278 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf