Diatom-based Late Quaternary precipitation record for lowland tropical South America

Abstract

The late Quaternary palaeoclimatic history of the lowland Southern Hemisphere Tropics of South America (SHTSA) has been little studied and analysis of key climatic events, such as the Last Glacial Maximum (centred ~ 21,000 years ago (21 cal. ka BP)) and the glacial-Holocene transition is limited. Studies from the SH tropical Andes and the Atlantic seaboard demonstrate a strengthening of the South American summer monsoon during the LGM, in tune with the ~ 20 kyr precession orbital cycle. However, palynological studies from SHTSA suggest a drier LGM. There are difficulties in interpreting different palaeoenvironmental proxy records and the extent to which they reflect changes in temperature, precipitation, and/or atmospheric CO2 concentrations. In particular, the palaeoenvironmental significance of palynological data is often unclear. Also, high frequency, millennial-scale events have not been captured in records from the lowland SHTSA due to a lack of high resolution temporal records. Diatoms have been used widely in other parts of the world to reconstruct lake level change and therefore provide an independent proxy for precipitation, and an understanding of the modern diatom ecology is essential for accurate palaeoreconstruction. The main rationale of this thesis is to address the uncertainty of the glacial-Holocene climate in South America. To this end, this thesis aims to: (a) investigate the distribution, ecology, and flora of diatom taxa at Laguna La Gaiba (17°45’S, 57°40’W) (LLG) in the heart of lowland tropical South America, where very few modern diatom studies exist; (b) determine whether modern diatom assemblages at LLG will provide a useful analogue for palaeoenvironmental reconstructions, in particular, lake depth reconstruction; (c) provide a detailed late Quaternary lake level reconstruction for the lowland interior of SHTSA, based upon fossil diatom analysis of a sedimentary core in LLG. Descriptive, quantitative and multivariate analyses were applied to modern diatom assemblages and environmental variables to ascertain the modern diatom environment of LLG. Diatom, pollen, and geochemical analyses, chronologically constrained by 18 AMS 14C dates, were performed on a sediment core extracted from LLG. Key findings indicate: (1) Aulacoseira ambigua, A. ambigua var. robusta. A. distans and A. granulata var. angustissima were the most abundant species. Shallowwater species, such as Staurosira and Eunotia spp., dominated the shallows and littoral zone, whilst deep-water species, such as Aulacoseira sp., dominated in open water; (2) The highest percent variance in the diatom data was explained by depth and pH; (3) Analysis of fossil diatom assemblages from the LLG core demonstrated that the Last Glacial Maximum (LGM) and late glacial period (prior to 12.5 kyr BP) was drier than present. This corroborates and significantly strengthens pollen-based palaeo-hydrological reconstructions from the same core; (4) An abrupt shift from 12.5 kyr BP from shallow water to deep water diatoms signals major flooding of LLG associated with the transition from relatively drier glacial conditions to wetter Holocene conditions and also highlights an anomalously wet period centred over 12.2 kyr BP that falls within the Younger Dryas chronozone; (5) Deep-water diatoms remain high throughout the Holocene, which means that the mid-Holocene aridity inferred from the pollen data (expansion of seasonally-dry tropical forest) is not captured by the diatom data. These results not only present the modern diatom ecology of a little studied area in lowland Bolivia, but also highlight the potential of diatoms as a proxy for past lake level fluctuations, improving the understanding of late Quaternary palaeoclimate of tropical South America. Used as part of a multiproxy reconstruction, this record has provided a more complete picture of the variation between regions of late Quaternary climate change in South America, as evidence of a dry LGM climate contrasts with the robust, well-dated climate archives of the central Andes and E Brazil. This suggests the climate in the continental interior of SHTSA was not driven by the precesionally-forced monsoon cycle but is in step with changes in glacialinterglacial cycle boundary conditions.