Seminar: "Reconstructing rainfall seasonality in the tropics", by Petter Hällberg

The Ocean's Challenge groups are organizing a new seminar.

Seminar: "Reconstructing rainfall seasonality in the tropics".

Speaker: Petter Hällberg, Stockholm University

Date: Thursday June 6th 2024
Time: 13:00 (30 minutes plus Q+A)
Room: Z/033 and online https://us02web.zoom.us/j/81681944336?pwd=VlpBNnl6M1l3cUJaN3BROTI3N1I3Zz09
 

"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), as well as biomass burning using levoglucosan concentrations in the core.

Finally, we compare our proxy results to a transient climate model simulation (MPI-ESM1.2) to pinpoint the mechanism for seasonality changes. We find that algal dD indicates stronger Indonesian-Australian Summer Monsoon (IASM) 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 strengthened IASM, consistent with the modeling results. 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. These results resolve the seasonal versus annual components of past rainfall variability in the Indo-Pacific Warm Pool region, and highlights the importance of considering seasonality in rainfall reconstructions."