Vegetation and fire responses to late Quaternary climate changes in the neotropics: a palynological and microcharcoal approach

Author(s)

Thomas Kenji Akabane

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2024-07-23

Examining board members

Paulo Eduardo de Oliveira; Ana Carolina Oliveira de Queiroz Carnaval; Carlos Dapolito Junior; Valdir Felipe Novello; Luciana Figueiredo Prado

• Advisor: Paulo Eduardo de Oliveira; Cristiano Mazur Chiessi

Abstract

During the Quaternary, long-(orbital) and short-(millennial) term climate fluctuations produced extensive environmental changes in the Neotropics. Understanding the mechanisms controlling these responses is essential for assessing future impacts of ongoing global climate changes. Important gaps in the knowledge of Neotropical climate-vegetation-fire interactions occur due to the scarcity of continuous high-resolution (sub-millennial) records. This thesis explores Neotropical vegetation and fire responses to late Quaternary climate changes through three studies. (1) The first study is based on a comprehensive compilation of 249 pollen and 127 charcoal records from the Neotropics that allows investigating trends of tree cover and fire regime over the last 21 kyr. We show that the primary climate controls varied in space and time. In general, during the Pleistocene, temperature and atmospheric CO2 were the main limiting factors of tree cover in the sub- and extra-tropical latitudes, while fire activity was limited by low biomass availability. Meanwhile, in tropical regions, both vegetation and fire responded to precipitation. During the Holocene, when temperature and CO2 fluctuations became less pronounced, precipitation acted as the main driver of environmental changes. Notably, the late Holocene is marked by the expansion of anthropogenic impacts on the landscape. (2,3) The second and third studies are based on new high-resolution pollen and charcoal analyses of marine sediment cores. These records provide information on vegetation and fire regime changes over large drainage basins and on a direct land-sea correlation. (2) The second study consists in palynological and microcharcoal analyses spanning the last 25 kyr of marine sediment core GeoB16224-1 (6°39.38N, 52°04.99W), retrieved from a site under the influence of the Amazon sedimentary plume. During the Last Glacial Maximum, palynological analysis indicates that the Amazon basin remained predominantly covered by forests despite significant floristic changes driven by the expansion of cold-adapted taxa towards the lowlands. These settings were also favored by a weak fire regime. During Heinrich stadial 1 (1814.8 ka), a period of slowdown of the Atlantic Meridional Overturning Circulation (AMOC), seasonally-dry taxa increased in abundance, likely due to the reduced precipitation over northern Amazonia. Despite marked changes in composition, forests were in general resilient. These data together with climate and vegetation modelling provide further insights into the interaction of two key tipping elements of the climate system, the Amazon rainforest and the AMOC. These findings are particularly important as the AMOC may reach a critical threshold in the near future, leading to substantial changes in Amazonian precipitation. (3) The third study is based on microcharcoal analysis of marine sediment core M125-95-3 (10°56.7S, 36°12.342W) retrieved under the influence of the São Francisco River terrigenous input and spanning the last 71 kyr. This fire record shows changes on both orbital- and millennial-scale related to changes in vegetation and climate. Intensification in the fire regime correlates with an expansion of open savanna formations. Orbital-scale changes were likely influenced by a combination of obliquity and insolation. High austral summer insolation led to drier conditions over most of the basin, strengthening the fire regime. During periods of minimum (maximum) obliquity, fire activity increased (decreased) due to seasonality changes promoting the expansion (retraction) of savannas. Short-term (millennial) variability, primarily associated with increased regional precipitation during Heinrich stadials, caused decreases in basin-wide fire activity due to fuel-moisture limited conditions. In sum, these three studies that compose this thesis provide key insights into the mechanistic links between climate, vegetation, and fire, that operated during the late Quaternary in the Neotropics, and point towards the potential impacts of future climate changes on the environment. (AU)

• FAPESP's process: 19/19948-0 - Influence of Late Pleistocene climate changes on the Amazon vegetation
• Grantee: Thomas Kenji Akabane
• Support Opportunities: Scholarships in Brazil - Doctorate