n-Alkane biomarkers as a proxy for source of OM

In addition to the C/N ratios, n-alkanes can also provide valuable information on the contribution of terrestrially- and aquatic-derived OM based on the fact that aquatic algae and terrestrial higher plants produce distinctly different chain lengths. Aquatic algae produce short chain homologues (C17–C21 n-alkanes) (Giger et al., 1980, Cranwell et al., 1987), whereas terrestrial organisms produce long-chain n-alkanes (C25–C33 n-alkanes). In between are the mid-chain homologues (C23, C24 and/or C25 n-alkanes), which are produced by submerged aquatic macrophytes (Ficken et al., 2000). The difference between short-chain and long chain n-alkanes therefore reflects the source organism. When conditions in the lake change (e.g. nutrient levels, mixing extent, ice-cover), the contribution of

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short-chain n-alkanes will change accordingly. For example, when environmental conditions change and there is a corresponding increase in aquatic productivity, more short-chained n-alkanes will be produced and vice versa. The changes in the molecular composition of n-alkanes in the down-core sedimentary record therefore provides valuable information on changes in aquatic versus terrestrial productivity over a given time period.

n-Alkanes derived from terrestrial higher plants are transported into Lake Toyoni by fluvial and/or aeolian processes. Due to the dense vegetation in the catchment of Lake Toyon it is assumed that the majority of n-alkanes in the Lake Toyoni sedimentary record are sourced from the catchment area and washed into the lake during run-off during the spring melt and summer rainfall and also transported via aeolian processes. n-Alkanes derived from aquatic algae (short chained) and submerged plants (mid chains) (Ficken et al., 2000) increase when aquatic productivity increases. The molecular compositions of n-alkanes therefore provide information on variations on terrestrial- and aquatic input in the lake.

The molecular compositions of n-alkanes provide a number of n-alkane based indices; for example, the carbon preference index (CPI index), Average Chain Length (ACL) and the proportion aquatic (Paq). The CPI index gives information of the extent of odd over even carbon number predominance (Bray and Evans, 1961). n-Alkanes produced by higher plants have a strong odd over even carbon number predominance (Eglinton and Hamilton, 1967). In contrast, algal n-alkanes do not contain the strong odd over even carbon number predominance.

Biomarkers from different biological origins have different CPI values; therefore, the n-alkane CPI in sediment is an indicator of the sources of biological origin (e.g. Simoneit et al., 1979). High CPI values (>3) results from a strong odd/even predominance which is a characteristics of higher plant wax n-alkanes. In contrast, n-alkanes from bacteria and algae show a weak odd/even predominance and give low CPI values (~1) (Cranwell et al., 1987). In addition, n-alkane distributions in sediments altered by diagenesis or bacteria may lack the strong odd over even preference of primary plant-wax n-alkanes (Meyers and Ishiwatari, 1993a); therefore, low CPI values may also indicate increased microbial activity during the time of deposition.

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Variations in the CPI values offer insights into the source of n-alkanes. Xie et al.

(2004) have shown that CPI values from loess/paleosol sequences of western Chinese Loess Plateau demonstrated that low CPI values of paleosol layers represent warmer and wetter climatic conditions; whereas, high CPI values represent colder and drier climatic conditions (Xie et al., 2004). Similarly, this trend was also found in two Japan Sea marine sediment cores which indicated that the variation of CPI values was in consistent with glacial/interglacial cycles, with lower CPI values occurring in warmer and wetter interglacial periods (Ishiwatari et al., 1994, Yamada and Ishiwatari, 1999). The variability of the CPI values in the Japan Sea was attributed to species variations of terrestrial higher plants due to changes in the climate; CPI values were lower in warmer climates (Ishiwatari et al., 1994). In the case of Lake Toyoni, an increase in temperature would promote aquatic productivity in the lake and a decrease in CPI values.

Lake Toyoni will also be strongly influenced by the dense vegetation in the catchment area of the lake, and hence high CPI values.

Another n-alkane molecular composition proxy is the ACL proxy. Vegetation types are the main influence on chain length of terrigenous leaf lipids. For example, n-alkanes derived from grasslands have longer chain lengths than leaf lipids from plants in forests (Cranwell, 1973). In the case of Lake Toyoni, the understory in the modern day environment is bamboo, which will have a significant influence on the ACL values. Changes in the vegetation over time are reflected in the ACL values. In addition to ACL value and vegetation type, a relationship between ACL and plant stresses (e.g. temperature and/or aridity) have been previously suggested (Gagosian and Peltzer, 1986, Kawamura et al., 2003). The humidity in Hokkaido during the growing season (June-September;

Seki et al., 2010) is >70%; therefore, aridity will not influence the ACL values at this site. Temperature on the other hand may influence ACL values. Increasing the ACL raises the melting point of protective leaf surface waxes and thus plants growing at higher temperatures may synthesize longer chain n-alkanes in order to maintain the protective waxy coating on their leaves (Rommerskirchen et al., 2003). Hence, plants produce longer chain compounds; therefore, higher ACL values, in warmer climates (Poynter et al., 1989).

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The Paq index was developed to reflect the relative contribution of aquatic macrophytes and emergent aquatic and terrestrial plants based on a survey of African Lakes, which found that the distribution of n-alkanes from floating/submerged marophytes maximise at C23 and C25 (Ficken et al., 2000).

Emergent aquatic plants, on the other hand, had n-alkane distributions similar to those of the terrestrial vegetation, typically dominated by the long-chain length homologues (>C29). According to Ficken et al. (2000), Paq values greater than 0.4 indicate the dominance of submerged and floating macrophytes (Ficken et al., 2000). For the modern plants, this proxy gives average values of 0.09 for terrestrial (range 0.01–0.23), 0.25 for emergent (range 0.07–0.61) and 0.69 for submerged/floating species (range 0.48–0.94). However, mid-chained n-alkanes are not exclusively produced by emergent and floating/submerged macrophytes.

Mid-chained n-alkanes are also produced by higher plants (Eglinton and Hamilton, 1963), which is particularly important with respect to Lake Toyoni, because higher plants from the catchment of Lake Toyoni will have a strong influence on the n-alkane distributions in the down-core sedimentary record.

Mid-chained n-alkanes in Lake Toyoni are unlikely to be exclusively derived from emergent and floating/submerged macrophytes, however, the Paq index will provide an approximate measure of variations in the sedimentary contribution from submerged/floating aquatic macrophytes in Lake Toyoni however will not reflect the dominance of floating/submerged macrophytes.