Environmental variability at the margin of the South American monsoon system recorded by a high-resolution sediment record from Lagoa Dourada (South Brazil)
Zolitschka, Bernd; Lee, An Sheng; Bermúdez, Daniela Piraquive; Giesecke, Thomas
(2021) Quaternary Science Reviews, volume 272, pp. 1 - 20
(Article)
Abstract
High-resolution geochemical and sedimentological data have been obtained for a lacustrine sediment record from Lagoa Dourada (South Brazil). Four distinctly different depositional processes were determined: (1) Suspension fallout of fine-grained minerogenic particles transferred via fluvial activity dominates the Early Holocene and relates to open grassland in the catchment area; (2)
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Activation of the karst hydrological system with deposition of massive sand layers indicates increased precipitation at the onset of the Middle Holocene; (3) Minerogenic sediments are replaced by organic deposition due to wetter climatic conditions with the expansion of woodlands in combination with release of dissolved nutrients due to enhanced pedogenesis during the Middle to Late Holocene; (4) Human-induced land-use change caused destabilization of soils in the catchment area with resulting cultural soil erosion between 1800 and 1950 CE. These depositional trends are linked to intensity variations of the South American Monsoon System (SAMS). Century-long climatic events detected by data of high-resolution XRF scanning confirm this relationship and provide signals of the 8.2 and 4.2 ka events as well as of the Little Ice Age. These events document increased rainfall with complex responses of the environmental system. Our SAMS-induced consequences of past hydroclimatic variability on the environment of South Brazil provide background information for better evaluating model projections of future climate change.
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Keywords: 4.2 ka event, 8.2 ka event, Geochemistry, Holocene, Human impact, Hydroclimate, Little Ice Age (LIA), Soil erosion, South American Monsoon system (SAMS), XRF scanning, Taverne, Global and Planetary Change, Ecology, Evolution, Behavior and Systematics, Archaeology, Archaeology, Geology
ISSN: 0277-3791
Publisher: Elsevier Limited
Note: Funding Information: Comparison with previously published data from Lagoa Dourada: Sediments from Lagoa Dourada (LD91) have been cored for the first time more than 30 years ago (Melo et al., 2003; Moro et al., 2004). However, this record is severely hampered by (1) very general sediment descriptions differing between Melo et al. (2003) and Moro et al. (2004); (2) many data are only available with low spatial resolution, i.e. one or two analyses per meter; and (3) the chronology is based on two radiocarbon dates only providing ages of 11,170 ? 110 BP and 8720 ? 150 BP from 11.9 m and 10.6 m of LD91, respectively. With these dates, a Lateglacial age has been proposed for the basal sediments at 12.2 m. As a result of our study, these ages should be considered cautiously and not without considering a reservoir correction. Despite of these limitations, Melo et al. (2003) provide mineralogical data based on XRD analyses, which document the presence of quartz, kaolinite, illite, pyrite and gypsum. The latter is certainly an artefact and produced after core splitting when pyrite (FeS) oxidized and recrystallized as gypsum (CaSO4). As total carbon percentages of this old record are erroneously low (<0.83%), the presence of pyrite together with gypsum is interpreted as an indicator of dry (semi-arid) climate conditions prior to 8720 BP and as well for a period between 5000 and 3000 BP (Melo et al., 2003). Furthermore, it is argued that the sand horizons are related to semi-arid conditions, when river runoff and thus flooding events did not occur while karstic springs provided water with quartz grains. While our study does not support these assumptions, the detection of a planktonic diatom maximum at 11.8 m by Moro et al. (2004) agrees with LZ C with its high BSi values and the microscopic detection of planktonic diatoms. Moreover, Moro et al. (2004) confirm the high concentration of K for a depth of 20 cm, which they relate to NPK fertilizers applied in the catchment area, results that also agree with our study.The pollen record from Serra do Tabuleiro (28 ?S) provides a nicely dated and densely analyzed Early Holocene record (Behling and Oliveira, 2017). After the warm and wet second half of the YD, Campos vegetation (grassland) with a warm and dry climate dominated the Early Holocene until 8000 cal. BP. Compiling eleven pollen records covering the latitudes between 15 and 35 ?S, Ledru et al. (1998) document warm and dry Early Holocene climatic conditions for the time period 11,400?7700 cal. BP. In a comparable synthesis, Behling (2002) uses 14 pollen records between 18 and 28 ?S and suggests ?6300 cal. BP as the end of warm and dry Early Holocene conditions. This data supports our interpretation of environmental conditions at Lagoa Dourada, where grassland vegetation under dry and warm climate conditions is proposed to explain erosion of fine-grained material in the catchment area via slope-wash (no or much less trees to protect against soil erosion), river transport and flooding until 7800 cal. BP (Fig. 8). This is also supported by data from NE Brazil, where the Middle Holocene shift from forest to grassland vegetation (Caatinga), i.e. from wet to dry conditions, caused an increase in soil-erosion rates due to less dense vegetation exposing soils to erosion during precipitation events (Jaqueto et al., 2016; Utida et al., 2020). In SE Brazil, this process not only caused intense colluvial deposition but also the formation of high fluvial terraces in the drainage systems of rivers dated to 11,400?9500 cal. BP (Suguio et al., 1989).We are grateful to Vivian Luciana Jeske-Pieruschka (Universidade Federal do Cear?, Fortaleza, Brazil) for assistance in the field and organizing permits for the Vila Velha State Park. Support for sample preparation and analyses in the GEOPOLAR lab (University of Bremen, Germany) was provided by Carsten Smidt, Sabine Stahl and Rafael Stiens. Thanks also go to Hermann Behling (University of G?ttingen, Germany) for support with field work and for critical comments on the draft as well as to Lujan Garcia (University of Bremen, Germany) for improving an earlier version of the manuscript. Finally, we acknowledge the constructive comments contributed by two anonymous reviewers. This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) as project GI 732/8?1. Funding Information: We are grateful to Vivian Luciana Jeske-Pieruschka (Universidade Federal do Ceará, Fortaleza, Brazil) for assistance in the field and organizing permits for the Vila Velha State Park. Support for sample preparation and analyses in the GEOPOLAR lab ( University of Bremen, Germany ) was provided by Carsten Smidt, Sabine Stahl and Rafael Stiens. Thanks also go to Hermann Behling (University of Göttingen, Germany) for support with field work and for critical comments on the draft as well as to Lujan Garcia ( University of Bremen, Germany ) for improving an earlier version of the manuscript. Finally, we acknowledge the constructive comments contributed by two anonymous reviewers. This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) as project GI 732/8–1. Publisher Copyright: © 2021 Elsevier Ltd
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