Vegetation and fire variability in the central Cerrados (Brazil) during the Pleistocene-Holocene transition was influenced by oscillations in the SASM boundary belt
Introduction
The Brazilian Cerrados is the second largest biome of the Neotropics, covering about 2 million km2 of tropical South America (Fig. 1A). The vegetation of the Cerrados is classified as a moist savanna and comprises open savannas, savanna woodlands, grasslands and shrubby grasslands, dry forests and riverine forests (Furley, 1999). The wide range of latitudes - 2° to 24°S – across which this biome stretches entails significant climatic variability across the region, but the occurrence of a strong dry season during austral winter (usually spanning four to six months) is common to the whole biome (Oliveira-Filho and Ratter, 2002, Fig. 1B). Rainfall is concentrated in the austral summer months, and is influenced by South American Summer Monsoon (SASM) activity (Silva and Kousky, 2012). The distribution of the Cerrados also coincides with the highly weathered, nutrient-poor soils found in the plateaus of central Brazil (Motta et al., 2002; Oliveira-Filho and Ratter, 2002). In addition to rainfall seasonality and dystrophic soils, fire is an important factor in the maintenance of the physiognomic and floristic diversity of the Cerrados (Hoffmann and Moreira, 2002). Most of the flora found in the Cerrados is adapted to, or dependent on, fire; as such, the origin and diversification of these plant lineages are, to a great extent, related to fire-resistant adaptations (Simon et al., 2009).
Past changes in the structure and composition of vegetation cover in the Cerrados can be considered a result of variations in rainfall amount and seasonality and/or of changes in the fire regime. SASM activity is known to have varied at the millennial scale as a response to variations in Southern Hemisphere (SH) insolation, which occur at orbital cycles of ca. 20 kyr (Kutzbach et al., 2008). SASM activity and SH insolation are positively correlated in that monsoon circulation and precipitation in the SH is enhanced when SH insolation is at a maximum (Cruz et al., 2005; Kutzbach et al., 2008). Increased SASM convective activity is caused by modifications in the interhemispheric temperature gradient – which is significantly controlled by summer insolation – that results in southward shifts of the Intertropical Convergence Zone (ITCZ) and South Atlantic Convergence Zone (SACZ) (Cheng et al., 2013; Schneider et al., 2014), main features of the SASM system. Other mechanisms besides insolation variability have also been documented to affect the interhemispheric temperature gradient, as well as influence the ITCZ/SACZ systems; for example, Heinrich events of massive ice melting in the North Atlantic have also been related to SASM variability (Kutzbach et al., 2008; Stríkis et al., 2015; Rojas et al., 2016).
Late Quaternary speleothem δO18 records from southern Brazil (Botuverá cave (Cruz et al., 2005; Wang et al., 2007)), central-western Brazil (Jaraguá cave (Novello et al., 2017), and central-eastern Brazil (Lapa Grande and Lapa Sem Fim caves (Stríkis et al., 2015, 2018) have been used to track changes in SASM position and activity over the last glacial cycle because they show significant co-variance with SH summer insolation and/or cold episodes in the North Atlantic. These speleothem records show differences that are related to their latitude (from northern to southern Brazil) and, as such, can be used to infer shifts in SASM belt activity and amplitude (Maslin et al., 2011).
Speleothem records show a succession of wet/dry events during the late Pleistocene and Holocene. Dry events with reduced rainfall over central and southern SASM regions were observed during warmer NH conditions (14.7–12.9 kyr BP) (Wang et al., 2007; Novello et al., 2017), while the return of colder NH conditions during the Younger Dryas event (12.9–11.7 kyr BP) resulted in increased moisture across southern, central-eastern and western South America (Wang et al., 2007; Novello et al., 2017; Stríkis et al., 2018). During the early Holocene, the onset of relatively drier conditions in the SASM region was again correlated with warmer NH conditions (Novello et al., 2017; Cruz et al., 2005). From the middle Holocene onwards, a general trend towards wetter conditions was observed in southern Brazil (Cruz et al., 2005). Conversely, in the eastern Amazon, the spelothem record from the Paraíso cave (Wang et al., 2017) shows peak of high humidity around 6000 yr BP and then a tendency of decrease in rainfall amount until the present. In central-eastern Brazil, the Holocene was punctuated by abrupt fluctuations in wet and dry events, with the early and middle Holocene showing longer wet events than the late Holocene (from 4.0 kyr BP onwards) (Stríkis et al., 2011).
The present study aims to discuss how SASM variability has affected the fire activity and vegetation of the central Cerrados during the Pleistocene/Holocene transition and the early Holocene. Our discussion is based on a new high-resolution pollen, charcoal and XRF record from Lagoa Feia ("Lake Feia" in Portuguese), which is located near Brasília in the core region of the Brazilian Cerrados. Lagoa Feia represents the first record from the central area of the Cerrados that covers the early Holocene, as previously published records from this region include hiatuses or no pollen record at the end of the Pleistocene and early Holocene (Salgado-Labouriau et al., 1997; Barberi et al., 2000). The Lagoa Feia record provides unprecedented insight into landscape and rainfall variability in central Brazil. We have assessed the regional scope of these variations, along with their implications for SASM boundary belt oscillations based on comparisons with other pollen and paleoclimatic records from the Cerrados and adjacent regions.
Section snippets
Vegetation and climate in the Cerrados
The Brazilian Cerrados extends from the equatorial zone to around 24° south latitude in Central Brazil. As such, the Cerrados includes transitional areas with four other Brazilian biomes, namely, the Amazon Forest to the north, the Caatinga to the northeast, the Atlantic Forest to the east and the Pantanal to the west (Fig. 1A).
The Cerrados demonstrates high physiognomic diversity as the region includes forests, savannas and grasslands. The Cerrados contains two different kinds of forest
Settings
Lagoa Feia is an elongated lake about 3 km long in the NE-SW direction and about 140 m wide. It is located in the city of Formosa (Goiás State), which is close to the Brazilian capital, Brasília. It is connected to a small drainage upstream and is the source of the Rio Preto (Fig. 1D). The water is about 5 m deep at the center of the lake, with aquatic plants currently covering a large part of the lake’s surface.
Lagoa Feia is located on a plateau at an altitude of around 900 m that is composed
Core sedimentology, XRF scanning results and chronology
Core LF15-2 demonstrated three distinct sediment layers: the sediment between 500 and 437 cm was composed of fine to coarse siliciclastic sand, with clay content increasing towards the top of the layer; the sediment between 437 and 415.5 cm was composed of black clay; while the sediment from 415.5 cm to the top of the core was mainly composed of organic sediment with vegetal fibers (Fig. 2).
The XRF core scanning results also revealed three main depositional units that corresponded to the three
Paleoecological interpretation of the Lagoa Feia record
The observed changes in the geochemical, pollen and charcoal aspects of the Lagoa Feia record describe significant changes in lake, vegetation and fire regime dynamics.
Lacustrine deposition at Lagoa Feia began around 19,000 cal yr BP, right after the Last Glacial Maximum (21–19 kyr). Lake formation could be explained by local subsidence of the plateau due to sub-superficial limestone dissolution associated with superficial weathering (Ferraz-Vicentini, 1999). These processes would have raised
Conclusions
The Pleistocene-Holocene transition and early Holocene are characterized by a succession of wet and dry episodes related to shifts in the SASM boundary belt. These shifts in rainfall distribution affected biodiversity, vegetation structure and fire activity in the central Cerrados.
During the last 13 kyr, three main phases were identified from pollen records from central Brazil. The first phase, between 13,000 and 11,200 cal yr BP, showed noticeably increased SASM activity in central Brazil,
Acknowledgements
This work was supported by UMR ISEM and IRD and is part of the projects - Dimensions of biodiversity (FAPESP (BIOTA 444 2013/50297-0), NSF (DEB 1343578) and NASA) and the Labex-CEBA. We thank Anne-Lise Develle at the Laboratoire Environnement Dynamique Territoire Montagne (EDYTEM) (University of Savoie, CNRS) for her assistance in the XRF scanning analyses, Vania Pivello (Ib-USP) for her assistance in the fieldwork, Boromir Bogumil for filming the fieldwork at Lagoa Feia (film available at: //www.youtube.com/watch?v=sz3zV7QC_H0
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