Climate and vegetation dynamics during the last deglaciation in Northeastern Brazil inferred from molecular biomarkers and their isotopic composition

The climate of Northeastern Brazil is strongly controlled by the latitudinal migrations and intensity of the intertropical convergence zone (ITCZ), which govern the spatial and temporal distribution of precipitation and, in turn, vegetation and faunal resources that have been critical for human populations. However, the long-term interactions between ITCZ variability, climate and ecosystems are still poorly understood. Here we present a new record based on molecular biomarkers and their isotopic composition documenting the evolution of paleoenvironments in Northeastern Brazil during the last deglaciation.

Sixty samples were collected from the MD23-3670Q core retrieved off the Parnaíba delta during the AMARYLLIS-AMAGASII campaign. Concentrations and carbon isotopic composition (δ¹³C) of molecular biomarkers (n-alkanes, fatty acids, and pentacyclic triterpenes) were determined to reconstruct climate and vegetation dynamics over the 8.9 to 22.2 cal kBP period.

The δ¹³C record of n-C₂₆ fatty acid shows similar variations as those of bulk organic matter (OM) δ¹³C, with an average -6‰ offset. This offset increases during the Bølling-Allerød and Preboreal, reflecting enhanced contributions of marine-derived OM and less terrestrial-derived OM. As a matter of fact, fatty acid δ¹³C values indicate a stronger contribution of C4-vegetation, suggesting drier conditions, during this period. Reversely, lower δ¹³C values indicate a stronger contribution of C3 vegetation, consistent with wetter conditions, during the Heinrich Stadial 1 and the Younger Dryas. Surprisingly, the Average Chain Length of fatty acids suggests reverse interpretation. Gramineae-specific biomarkers are abundant during the Heinrich Stadial 1 but rare during the Younger Dryas although climatic conditions appear close. High levels of Asteraceae biomarkers are abundant during both Heinrich Stadial 1 and Younger Dryas. Finally, taraxerol levels are notable during two episodes included in the Younger Dryas. This might reflect two phases of conditions favorable for the development of mangroves during sea level rise.

Together, these results reveal a complex and sometimes decoupled response of vegetation and coastal ecosystems to deglacial climate variability in Northeastern Brazil, emphasizing the combined influence of ITCZ-driven hydroclimate changes, sea-level fluctuations, and non-linear response of NE Brazil vegetation to climate changes.

We are grateful to the crew of the R/V Marion Dufresne and GENAVIR staff members for their help in collecting AMARYLLIS-AMAGAS II cores. We also acknowledge the Brazilian Navy and the Brazilian National Council for Scientific and Technological Development (CNPq) for granting access to collect and investigate the material taken in Brazilian jurisdictional waters during the AMARYLLIS- AMAGAS II cruise (ANR 17-EURE-0006).

This research was supported by the project ANR SESAME “Human paleoecology, Social and cultural Evolutions among first Settlements in Southern America (ANR 20-CE03-0005).