Palaeoecological Potential of Phytoliths from Lake Sediment Records from the Tropical Lowlands of Bolivia
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Palaeoecological potential of phytoliths from lake sediment records from the tropical lowlands of Bolivia Article Accepted Version Creative Commons: Attribution-Noncommercial-No Derivative Works 4.0 Plumpton, H. J., Mayle, F. E. and Whitney, B. S. (2020) Palaeoecological potential of phytoliths from lake sediment records from the tropical lowlands of Bolivia. Review of Palaeobotany and Palynology, 275. 104113. ISSN 0034-6667 doi: https://doi.org/10.1016/j.revpalbo.2019.104113 Available at http://centaur.reading.ac.uk/87043/ It is advisable to refer to the publisher’s version if you intend to cite from the work. See Guidance on citing . To link to this article DOI: http://dx.doi.org/10.1016/j.revpalbo.2019.104113 Publisher: Elsevier All outputs in CentAUR are protected by Intellectual Property Rights law, including copyright law. Copyright and IPR is retained by the creators or other copyright holders. Terms and conditions for use of this material are defined in the End User Agreement . www.reading.ac.uk/centaur CentAUR Central Archive at the University of Reading Reading’s research outputs online 1 Palaeoecological potential of phytoliths from lake sediment records from the tropical 2 lowlands of Bolivia 3 Authors 4 Heather J. Plumptona*, Francis M. Maylea, Bronwen S. Whitneyb 5 Author affiliations 6 aSchool of Archaeology, Geography and Environmental Science, University of Reading, UK 7 bDepartment of Geography and Environmental Sciences, Northumbria University, UK 8 *Corresponding author. Email address: [email protected]. Postal address: Russell Building, 9 School of Archaeology, Geography and Environmental Science, University of Reading, Whiteknights, 10 P.O. Box 227, Reading RG6 6DW, Berkshire, UK. 11 Abstract 12 Phytolith analysis is conventionally an archaeo-botanical tool used to study past human activity using 13 material from excavations or soil pits. However, phytolith analysis also has potential as a 14 palaeoecological tool, to reconstruct vegetation changes through periods of climatic change and 15 human influence. To study phytoliths from lake sediment alongside pollen requires an understanding 16 of phytolith taphonomy in lakes. Theoretical models suggest phytoliths represent more local 17 vegetation at smaller spatial scales than pollen from lake sediments, but this has not been tested 18 empirically in the Neotropics. This paper compares pollen and phytolith assemblages from the same 19 lake sediment surface sample, from a suite of lakes of different sizes across different vegetation 20 types of lowland tropical Bolivia. We find three factors driving phytolith composition in lakes: 21 taphonomy, lake size and phytolith productivity. By comparing phytolith assemblages with pollen 22 assemblages, we find that they provide different taxonomic information and generally complement 23 each other as palaeo-vegetation proxies. We also demonstrate empirically that pollen assemblages 24 in lake samples represent a larger catchment area than phytolith assemblages. Our findings suggest 1 25 that phytoliths can be particularly useful in providing local-scale vegetation histories from large 26 lakes, to complement the regional-scale vegetation histories provided by pollen data. 27 28 Keywords 29 Phytoliths, pollen, taphonomy, lake records, tropical, South America 30 1. Introduction 31 Phytolith analysis is conventionally an archaeo-botanical tool to study past human activity, used by 32 archaeologists studying material from excavations and/or soil pits. However, phytolith analysis also 33 has the potential, as a palaeoecological tool, to reconstruct vegetation changes through periods of 34 climatic change and/or human influence. There are two key areas where phytoliths can provide 35 additional information beyond the conventional vegetation reconstruction proxy of fossilised pollen: 36 taxonomic information and spatial information. While the taxonomic benefits of phytolith analysis 37 are reasonably well studied, the spatial scale phytolith records represent is less certain, particularly 38 when analyses are conducted on typical palaeoecological samples taken from lake sediment. 39 There are many taxonomic benefits of phytolith analysis for palaeoecologists. For example, phytolith 40 analysis can differentiate sub-families of Poaceae and genera of Cyperaceae, neither of which is 41 currently possible from pollen analysis, and which can be particularly helpful in identifying different 42 herbaceous habitats such as forest understorey, savannahs, and semi-aquatic lacustrine vegetation. 43 Further taxonomic advantages of phytoliths include the identification of Heliconiaceae, a key 44 disturbance indicator (Piperno, 2006), as well as other economically useful taxa unidentifiable by 45 their pollen, such as squash (Cucurbita) (Bozarth, 1987) and rice (Oryzoideae) (Hilbert et al., 2017), 46 which can provide important insights into past human land-use and human-environment 47 interactions. Furthermore, soil surface sample work by Dickau et al. (2013) and Watling et al. (2016) 48 have demonstrated that several neotropical ecosystems (humid evergreen forest, palm forest, semi- 2 49 deciduous dry forest, seasonally inundated savannah and terra firme savannah) can be 50 differentiated based solely on the phytolith assemblage from soil samples. 51 The combination of pollen and phytolith analyses should therefore provide additional, 52 complementary palaeoecological information. As pollen does not preserve well in soils, this can be 53 achieved by sampling a combination of soil samples (for phytolith analysis) and lake records (for 54 pollen analysis). However, this combination is complicated by the different spatial scales and 55 temporal resolution provided by soil versus lake sediment records. The temporal resolution of 56 palaeo-records from the soil column is typically significantly lower than that of lake sediment 57 records due to bioturbation of soil via plant roots, insects and other animal disturbances which move 58 material through the soil profile (Butler, 1995; Gabet et al., 2003). The spatial scale represented by 59 soil samples is generally much smaller than that of lake records, as lakes act as sinks for microscopic 60 particles from the surrounding vegetation transported via wind or water (Bennett and Willis, 2002). 61 To avoid these complications, as phytoliths preserve well in lake sediments as well as soils, both 62 analyses can be conducted on lake sediment which would enable the same temporal resolution to 63 be achieved for both proxies, enabling direct comparison of pollen and phytolith assemblages and 64 providing truly complementary taxonomical and spatial information. This approach has been applied 65 to several Late Quaternary neotropical lake records, including La Yaguada, Panama, where Late 66 Pleistocene cooling was identified by phytoliths from montane forest taxa such as Magnolia and 67 Chrysobalanaceae (Bush et al., 1992; Piperno et al., 1990) and Monte Oscuro, Panama, where a 68 wetter Holocene climate and human disturbance were identified based on pollen and phytoliths 69 records (Piperno and Jones, 2003). At Lagunas Granja and San José in lowland Bolivia, pollen and 70 phytolith analyses revealed late Holocene Pre-columbian land use (Carson et al., 2015; Whitney et 71 al., 2013). In these studies phytoliths are interpreted as representing a more local spatial scale than 72 pollen, based on a theoretical understanding of phytolith deposition and transport, backed up in the 73 latter two studies by comparisons with shoreline vegetation inventories. 3 74 Integration of pollen and phytolith records from lake sediments requires an understanding of their 75 respective taphonomy in lakes. Piperno proposed a theoretical model of phytolith representation in 76 lakes (Piperno, 2006, 1990) whereby phytolith source area depends largely on the lake site 77 characteristics. For example, in stream-fed lakes in areas with high precipitation phytoliths have the 78 potential to be transported long distances via soil erosion and runoff over land and into streams. In 79 open, frequently burnt environments they can be transported up to 2000 km by wind. However, in 80 closed basins surrounded by dense forest, Piperno (2006) proposes that phytoliths are likely to 81 represent only shoreline vegetation. 82 The only published empirical study on phytolith input to lakes (Aleman et al., 2014) was conducted 83 on three small lakes in ecosystems of central Africa: savanna, forest–savanna mosaic, and forest 84 (0.03, 0.36, 0.14 km2 respectively). Aleman found that the proportion of forest cover surrounding 85 the lake and the number of large fires (producing ash clouds) were the main factors influencing 86 phytolith catchment area. However, there is uncertainty over the extent to which the findings from 87 these small lakes are representative of much larger lakes, several km in diameter, which are 88 common throughout the tropics. In addition, these phytolith records were not compared with other 89 vegetation proxies with an estimable source area, such as pollen, and only one sample was taken 90 from each lake. Furthermore, the role of fire, wind and water in phytolith taphonomy in the drier, 91 frequently burnt environments of Aleman’s study area is likely to differ from the wet environments 92 of the Neotropics. 93 This paper therefore aims to explore the potential value of phytoliths as a complementary proxy to 94 pollen for enhancing the palaeoecological information that can be obtained from lake sediments 95 in