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Diatom Distribution in the Lower Save River, Mozambique

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Diatom Distribution in the Lower Save River, Mozambique Department of Physical Geography Diatom distribution in the lower Save River, Mozambique Taxonomy, salinity gradient and taphonomy Marie Christiansson Master’s thesis NKA 156 Physical Geography and Quaternary Geology, 60 Credits 2016 Preface This Master’s thesis is Marie Christiansson’s degree project in Physical Geography and Quaternary Geology at the Department of Physical Geography, Stockholm University. The Master’s thesis comprises 60 credits (two terms of full-time studies). Supervisor has been Jan Risberg at the Department of Physical Geography, Stockholm University. Examiner has been Stefan Wastegård at the Department of Physical Geography, Stockholm University. The author is responsible for the contents of this thesis. Stockholm, 11 September 2016 Steffen Holzkämper Director of studies Abstract In this study diatom distribution within the lower Save River, Mozambique, has been identified from surface sediments, surface water, mangrove cortex and buried sediments. Sandy units, bracketing a geographically extensive clay layer, have been dated with optical stimulated luminescence (OSL). Diatom analysis has been used to interpret the spatial salinity gradient and to discuss taphonomic processes within the delta. Previously, one study has been performed in the investigated area and it is of great importance to continue to identify diatom distributions since siliceous microfossils are widely used for paleoenvironmental research. Two diatom taxa, which were not possible to classify to species level have been identified; Cyclotella sp. and Diploneis sp. It is suggested that these represent species not earlier described; however they are assigned a brackish water affinity. Diatom analysis from surface water, surface sediments and mangrove cortex indicate a transition from ocean water to a dominance of freshwater taxa c. 10 km upstream the delta front. Further, ratios between marine/brackish taxa for samples from surface water and surface sediments do not correspond. It is therefore suggested that diatoms in surface sediments underestimate prevailing salinity conditions in water. In the investigated area extensive taphonomic processes seem to have large impact on diatom frustules in sediments and may bias interpretations. Therefore it is recommended to carefully investigate geology, geomorphology and vegetation before diatom analysis is applied in studies of delta paleoenvironments. 1 Table of contents Abstract ................................................................................................................................................... 1 Table of contents ..................................................................................................................................... 2 1. Introduction ........................................................................................................................................ 3 1.1 Objectives ...................................................................................................................................... 6 1.2 Background ................................................................................................................................... 6 1.2.1 Salinity stratification .............................................................................................................. 6 1.2.2 Diatoms .................................................................................................................................. 7 1.2.3 SEM/ESEM ............................................................................................................................. 9 1.2.4 OSL ......................................................................................................................................... 9 2. Description of the investigated area ................................................................................................ 10 3. Methodology..................................................................................................................................... 13 3.1 Siliceous microfossils................................................................................................................... 15 3.2 ESEM ........................................................................................................................................... 19 3.3 OSL .............................................................................................................................................. 19 4. Result and interpretation ................................................................................................................. 19 4.1 Surface water samples ................................................................................................................ 20 4.2 Surface sediment samples .......................................................................................................... 21 4.3 Mangrove cortex samples ........................................................................................................... 21 4.4 Buried sediments ........................................................................................................................ 28 4.5 Taxonomic remarks ..................................................................................................................... 38 5. Discussion.......................................................................................................................................... 41 5.1 Taxonomy .................................................................................................................................... 41 5.2 Salinity gradient .......................................................................................................................... 42 5.3 Taphonomy ................................................................................................................................. 44 5.4 OSL and radiocarbon dating ........................................................................................................ 49 5.5 Future studies ............................................................................................................................. 50 6. Conclusions ....................................................................................................................................... 51 7. Acknowledgement ............................................................................................................................ 52 8. References ........................................................................................................................................ 53 9. Appendix 1 ........................................................................................................................................ 69 10. Appendix 2 ...................................................................................................................................... 71 11. Appendix 3…………………………………………………………………………………………………………………………………76 12. Appendix 4…………………………………………………………………………………………………………………………………80 13. Appendix 5…………………………………………………………………………………………………………………………………81 14. Appendix 6………………………………………………………………………………………………………………….................82 2 1. Introduction Research on past climate is required to forecast future global climate changes (e.g. Palmer & Abbott, 1986; Shennan et al., 1993; Denys and de Wolf 1999; Zong & Horton, 1999; Church et al., 2013). One aspect implies changes of the global mean sea level, which can involve either a rise or a lowering affecting coastal environments, e.g. deltas (Tamura et al., 2012; Church et al., 2013). A rising sea-level involves effects on wave-induced erosion, tides and currents influencing shorelines and mangrove forests holding complex ecosystems (Nicholls & Cazenave, 2010). A lowering could imply an increased erosion by fluvial and terrestrial processes combined with effects on floras and faunas. As the global sea-level has fluctuated over time, especially during the Quaternary (Lowe & Walker, 1997), changes can be traced in coastal environments (Woodroffe, 1990). To increase knowledge of the effects from approaching eustatic sea-level rise, past fluctuations should be elaborated. Paleoenvironmental research is often hampered by chronological drawbacks (cf. Andrews et al., 1999; Wang et al., 2013; Bala et al., 2016). Deltaic sediments have an advantage since they may be dated by optically stimulated luminescence (OSL) as a complement to radiocarbon dating (cf. Bishop et al., 2004; Zhao et al., 2008; Erginal et al., 2009). One potential approach to study paleo-climate and paleo-environment is analysis of siliceous microfossils, mainly diatoms (e.g. Simonsen, 1969; Palmer & Abbott, 1986; Vos & de Wolf, 1993:1; Espinosa, 1994; Denys, 1999; Denys and de Wolf, 1999). As living diatoms are sensitive to changes of salinity, pH and nutrition (Cooper, 1999; Jiang, et al., 2001; Hassan, et al., 2006) and fossil frustules (shells) in sediments normally are well preserved (Ferguson Wood, 1967), they may indicate environmental changes such as tidal currents, flooding events and sea level fluctuations (Swan, 1983; Vos & de Wolf, 1988). Previous studies have shown benefits of using diatom analyses to reconstruct these changes in delta environments (cf. e.g. John, 1987; Zalat, 1995; Zong et al., 2009; Ellison, 2008; França et al., 2015). To improve the interpretation of fossil diatoms, further investigations on recent assemblages are beneficial as they are
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