A common feature of meromictic lakes is that there is a lack of water circulation and the lake waters are permanently stratified into two layers which do not interfere with each other (Hakala, 2004). Geographically, Lake Suigetsu is surrounded by lakes and hills (Figure 1.2) (Kossler et al., 2011) and a permanent chemocline is formed in Lake Suigetsu, between 3 m to 8 m, separating the aerobic freshwater mixolimnion of the upper water layers from the anaerobic water below which is a saline sulfidogenic monimolimnion (Kondo et al., 2000, 2006; Matsuyama, 1973a, 1973b; Matsuyama and Saijo, 1971). Hutchinson (1937) indicated that under certain circumstances, meromictic condition of a lake can result from an external event (i.e. the incursion of saline water), continuous supply of denser mineralised water into the lake or the increase of electrolyte concentration due to highly decomposed organic material at the lake bottom. In addition, the development of meromictic lakes can also be attributable to its location, such as surrounded by thick forest, sheltering landscape or lake morphology which yielded minimal wind action. After an extended period of time, the lake waters will become permanently stratified into different transition zones. Hutchinson (1937) termed the layer separating the upper and lower strata as chemocline and the layer above the
21 chemocline as mixolimnion while the water layer below the chemocline is characterised as monimolimnion (Hakala, 2004).
The location of Lake Suigetsu has also given rise to the formation of laminated sediment at the bottom of the lake. It has connections to several lakes in close proximity including the freshwater Lake Mikata and the polyhaline Lake Hiruga (Matsuyama and Saijo, 1971). The main water inflow originates from the Hasu River, which first reaches Lake Mikata before Lake Suigetsu. Lake Mikata in this case, serves as a filter, indirectly helping to strain off most of the coarse materials conveyed from the Hasu River (Figure 1.2) (Kawakami et al., 1996). Likewise, the hills and steep slopes surrounding Lake Suigetsu which are sheltered by dense forest vegetation also contribute to the fine lamination deposited in Lake Suigetsu as it helps to minimise landslide and soil erosion activity in spite of weather circumstances (Kossler et al., 2011).
22 Due to the lack of the mixing of the lake waters and the deposition of fine materials into the lake, the sediment is less disturbed and a steady depositional environment was established in the lake basin for approximately 100,000 years which ultimately produced a well-defined laminated sediment (varves) (Figure 1.3) (Nagayoshi et al., 2007; Kitagawa and van der Plicht, 1998a). The regional climate around Lake Suigetsu can be characterised by both summer and winter monsoons. During summer, Japan receives mostly south-easterly winds and humidity from the Pacific Ocean, while in winter the dominant north-westerly winds come from Siberia. The wind over the relatively warm surface water of the Sea of Japan picks up much moisture and eventually provides heavy winter precipitation to Japan, particularly along the western side of the country, including the Lake Suigetsu region (Nakagawa et al., 2012). Therefore, the surrounding temperatures of Lake Suigetsu can represent proxies for the cold of Siberian air mass in winter and the warmth of Pacific air mass during summer monsoon intensity, with related winter and summer precipitations (Nakagawa et al., 2006). In addition, due to this strong seasonal shift, varves with various thickness and texture are formed (Mackay et al., 2003), which are composed of rich diatoms layers and aeolian dust and humus layers (Nakagawa et al., 2005).
Figure 1.3: Fine annually laminated varves of SG06 core of Lake Suigetsu (Nakagawa et al., 2012)
23 1.4.1.2. Internationally recognised importance of the Lake Suigetsu
In order to reconstruct past climate chronology, several records can be used to develop the 14C radiocarbon calibration curves. These include the partially varved sedimentary records from the Cariaco Basin (Hughen et al., 2006), uranium series-dated corals (Fairbanks et al., 2005) and marine sediment (Reimer and Reimer, 2001). Tree- ring records are one of the archives known to provide nearly accurate calendar age year based on to their annual growth bands (Staff et al., 2011). However, there is a limitation associated with this calibration model in that it only extends up to 12,593 calendar years BP (Schaub et al., 2008).
The occurrence of varves sediment in Japan was first discovered when four short sediment cores (SG1-4) were obtained from Lake Suigetsu in 1991 and 1993 (Kitagawa and van der Plicht 1998b). Subsequently in year 1998 and 2000, Kitagawa and van der Plicht had generated a radiocarbon profile by combining more than 300 radiocarbon ages measured from the terrestrial leaf macrofossils based on the varved sediment of Lake Suigetsu (Kitagawa and van der Plicht, 1998a). This study revealed the extension back to the radiocarbon detection limit (ca. 50,000 year cal BP) and it was an early attempt to produce a radiocarbon dating model beyond the tree-ring limit (11,400 cal BP at that period of time) (Kromer and Becker, 1993).
Lake Suigetsu became internationally recognised when this model was published. However, there were several issues allied with core SG93. It was later recognised as ‘young varves’ that had been underestimated due to the undercounting of the varves and over-counting of the annual bandings in other sites (van der Plicht et al., 2004). In summer 2006, another research team funded by the UK Natural Environment Research Council (NERC) conducted a new sediment coring exercise at the lake’s depocenter of 34 m,reaching the base of the sedimentary profile (73.19 m below the lake bottom) (Staff et al., 2011). Cores were recovered from four parallel boreholes (A, B, C and D), all within 20 m horizontal distance from each other, with fully overlapping core segments (Figure 1.4) and no chronological gaps in between, unlike core SG93 (Nakagawa et al., 2012). Fukusawa (1995) indicated that the annually laminated sedimentary record of Lake Suigetsu is a ‘natural timekeeper’ and reliable recorder of environmental change, therefore, is capable of providing an independent whilst high
24 resolution and precise age control of event stratigraphy for palaeoclimatic reconstruction (Nakagawa et al., 2003; 2005; 2006).
The recovered overlapping sediments contain finely laminated varves and occasionally thick layers are also found in the sediment cores. These layers are suggested to be the event layers which are categorised into two empirical types; i) the light-coloured massive clay layers and ii) a slightly coarser clay layers, but generally much thicker and with dark coloured layer underlying. It is hypothesised that the light- coloured clay layers were formed by large flood events while thicker layers with coarser material are interpreted as small-scale turbidites. Such turbidites can either be formed by surface runoff of the slopes around the lake or caused by earthquakes (Nakagawa et al., 2012).
Figure 1.4: Example of overlapping lamina patterns from parallel borehole A and B (Nakagawa et al., 2012)
25 1.4.2. Climate record from Lake Suigetsu
The majority of macrofossil samples in the sediment cores are tree leaves and small twigs, bark, seeds, and a few segments of insects are also found (Staff et al., 2011). Sampling undertaken for 14C calibration was throughout the last ~12,000 cal yr, almost entirely representing the Holocene (Nakagawa et al., 2003). In addition, based on the pollen analysis, Nakagawa et al. (2005) have reconstructed the past climate of the Late Quaternary in central Japan.
Figure 1.5: Biome reconstructed based on pollen records of Lake Mikata, spanning over the last 45,000 BP (Nakagawa et al., 2002).
Based on the SG06 varve chronology, the vegetation around Lake Suigetsu before 15,000 BP was identified as cool mixed forest (COMX) that comprised of coniferous trees and deciduous broad-leaved trees (an example shown in Figure 1.5). During this interval, the mean annual temperature increased from ca. 5 to 10°C, suggesting a continuation of deglacial warming which started at about 20,000 BP (ca. 3°C) in the region of Lake Suigetsu (Nakagawa et al., 2002). Changes in vegetation from COMX to typical temperate deciduous (TEDE) forest of Japan were identified at the boundary of 15,000 BP as coniferous pollen disappeared. Although the change at this transition corresponds to the onset of the GI-1e/Bølling period in the North Atlantic, it is not possible to conclude that the change was as abrupt as the onset of the GI-1e/Bølling phase in the North Atlantic. Furthermore, the climate around Lake
26 Suigetsu became colder between 12,300 and 11,250 BP. The vegetation showed the increase in Fagus crenata and a total absence of the coniferous trees, indicating cooler condition that is comparable to the winter season (Nakagawa et al., 2005). During this period, the climate was unstable with pollen productivity frequently changed, and the abrupt increase of Pinus subgen. (typical pioneer species in Japanese temperate forest; Nakagawa et al., 2005), indicating an abrupt climate change. At the end of the cold phase, the vegetation around Lake Suigetsu was TEDE forest with temperature approximately 5°C lower than the present. The temperature continues to rise to the present level and warm mixed forest (WAMX) around Lake Suigetsu was established sometime after 10,000 BP which is likely to suggest the beginning of the Holocene period.
The reconstruction of a biome based on Lake Suigetsu is in agreement with Lake Mikata’s (Gotanda et al., 2002), however the study was mainly focused on the period during the Last Termination. Based on the biome reconstructed from the pollen records of Lake Mikata, the period from > 40,000 to 25,000 BP was represented by TEDE forest. During the period from 25,000 to 22,000 BP, the vegetation was classified by both TEDE and COMX forests as the affinity scores of both biomes are close. From the period of 22,000 to 18,000 BP, COMX forest was dominant whereas period between 18,000 to 15,000 BP, the vegetation began interchanging between COMX and TEDE forests.
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Chapter 2
Aims and Objectives
The primary aim of this research was first to identify possible molecular biomarkers by elucidating and identifying the diversity of both bacterial and archaeal communities in the sedimentary records of Lake Suigetsu, spanning the past c. 150,000 years, by comparing to the biome reconstructed based on Lake Mikata. Specific objectives addressed during the course of this study are outlined as follow;
Chapter 4: Molecular fingerprinting analysis of fossil DNA throughout the 73 m sediment core of Lake Suigetsu (c. 150,000 years) to elucidate bacterial community structure and diversity patterns to search for potential biomarkers for past climate reconstruction.
Chapter 5: Lake Suigetsu sediments have records of a seawater invasion event in 1664 AD. So, molecular analyses of the effects of environmental changes on bacterial community structure and diversity, particularly the salinity effects were targeted. Investigations were focused on bacterial community structure and diversity before and after seawater invasion. The possible timeline and depth at which the salinity influx event was the initial stroke was identified and the taxa between the anoxic saline lake waters and brackish sediments were compared.
Chapter 6: Lake Suigetsu sediments also recorded information on the glacial to postglacial climate condition between 15,000 to 14,500 BP, as indicated by pollen records. Thereby, an exploration of the effects of climate change on bacterial diversity and community structure in sediments from a transition from cooler to warmer climate condition was performed. Identification of whether the bacterial taxa demonstrated a measurable change during these periods as well as if there is strong correlation between specific taxa and climatic conditions was also investigated.
Chapter 7: Culture-dependent analysis to distinguish bacterial diversity in brackish sediment (335 BP) from freshwaters (6,860 BP and 10,911 BP), to allow the determination of whether ancient bacteria from fossil sediments are ‘culturable’. The microbial data between this study and molecular techniques were also compared.
28 Chapter 8: Polyphasic approach to the characterisation of potentially new Actinobacteria strains recovered from the freshwater sediment records and description of novel species of Dermacoccus, Dietzia, Leifsonia and Rhodococcus.
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