BULK SEDIMENT AN ALYSES

^^14. If the form of ratio was not specified, it was assumed that the C/N weight ratio was used.

4.3.4.2 The C/N ratio in lake sediments

Measured values of C/N in lake surface sediments include: 6 in Lake Biwa, Japan (Koyama, 1972; Meyers and Horie, 1993); 9.19 in Lake M otuso, 8.56 in Lake Haruna, and 8.36 in Lake Suwa, Japan (Kawamura and Ishiwatari, 1985); 16 in Lake Vattem and 18 in Lake Vanem, Sweden (Hakanson and Jansson, 1983); 8 in Walker Lake, Nevada (Meyers and Benson, 1988); 9 in Pyramid Lake, Nevada (Tenzer et al.,

1997); 8 in Lake Michigan (Rea et ah, 1980; Meyers et a l, 1984b; Meyers and Eadie, 1993); 12 in Cobum Mountain Pond, Maine (Ho and Meyers, 1994); 11 in Lake Baikal, Siberia (Qiu et a l, 1993); and 14 in Lake Bosumtwi, Ghana (Talbot and Johannessen, 1992). These C/N ratios are mostly low and indicate either an algal input, or a mixture of algal and terrestrial inputs. Studies of 51 lakes by Hecky et al.

(1993) show arctic and subarctic lakes to have lower C/N ratios (c. 8-10) than lakes in temperate and tropical regions (c. 10-20). Also, small lakes are more deficient in nitrogen than large lakes and have correspondingly higher C/N ratios.

Downcore changes in the C/N ratio are used to reconstruct past changes in lake sediment inputs. Such studies have been undertaken at Upton Broad, England (Cranwell, 1984b), Lake Steisslingen, Germany (Mayer and Schwark, 1999), Lago Paione Superiore, Italy (Guilizzoni et al., 1996), Lake Michigan (Meyers and Eadie, 1993), Cobum Mountain Pond (Ho and Meyers, 1994), Swan Lake, Nebraska (Hassan et al., 1997), Lake Pleasant, Massachusetts (Kaushal and Binford, 1999), M ono Lake, California (Jelhson et al., 1996), Austin Lake, Michigan (Krishnamurthy

et al., 1995), Lakes Parker, Hollingsworth and Griffin, and Clear Lake, Florida

(Brenner et al., 1999), Lake Baikal (Qiu et al., 1993), Lake Biwa (Ishiwatari and Uzaki, 1987; Meyers and Takemura, 1997), Lake H am na (Ishiwatari et al., 1980), Karewa lake sediments, India (Krishnamurthy et al., 1986), Lake Nkunga, Kenya (Ficken et al., 1998), Sacred Lake, Kenya (Huang et al., 1999), Lake Bosumtwi, Ghana (Talbot and Johannessen, 1992), and Lake Carajas, Brazil (Sifeddine et al.,

CHAPTER 4 BULK SEDIMENT ANALYSES 134

sources, high ratios as representing allochthonous (terrestrial plant) sources, and intermediate values as representing a mixture of sources.

4.3.4.3 C/N ratio of UACT6

The C/N profile of UACT6 was given in Figure 4.4. Unlike many of the studies mentioned above, no major changes in C/N are visible in the core, with the exception of the upper 5 cm. From 5 cm depth to the core base the C/N values lie almost entirely within the range 10-13, and the extreme values recorded for this section are 8.6 and 14.3. These values indicate inputs from both autochthonous and allochthonous sources. Slightly elevated values are found from 32-34 cm depth, coinciding with a period of low TOC and low nitrogen in the sediment. This would be interpreted as an increase in the input of terrestrial plant material to the sediment relative to the input from aquatic organisms. Significantly, no such increase in C/N is seen between 15 and 20 cm depth where a similar decrease in sedimentary TOC and nitrogen is found.

The section from 0-5 cm depth differs from the rest of the core in that much higher C/N values are seen, with some values exceeding 20. The conventional interpretation of these values indicates a significant increase in the proportion of terrestrial plant material reaching the sediment. There are, however, several indications that this interpretation may not be correct. Firstly, the high C/N values from 0-5 cm depth coincide with a period of low TOC and nitrogen in the sedimentary record. The other two such periods seen in UACT6 were discussed above; that from 32-34 cm shows shghtly elevated C/N ratios, while no change in the C/N profile is seen from 15-20 cm. If these three events are the result of identical processes, such as a reduction of within-lake primary productivity as suggested by the chloiin and lipid records (discussed in later sections), identical changes in the C/N profile would be expected. The lack of consistent changes in C/N would thus suggest that the three major periods of low TOC and low N are caused by different sets of environmental processes, rather than a recurrence of a single set of environmental processes.

CHAPTER 4 BULK SEDIMENT ANALYSES 135

The second possibility is that the C/N profile reflects processes occurring in the active sediment near the mud-water interface. Specifically, nitrogen is known to be mobile in sediments through both biological and non-biological diagenesis (Meyers and Benson, 1988; Tyson, 1995). Non-aromatic carbon-to-nitrogen bonds are weaker than carbon- to-carbon bonds, hence nitrogen will be released more rapidly than carbon during diagenesis of organic compounds (Toth and Lerman, 1977). Prahl et al. (1980) note that this nitrogen may be remineralized more rapidly than organic carbon, resulting in higher C/N ratios in the top few centimetres of sediment. Burial within sediments has the effect of stabilising C/N ratios (Meyers and Ishiwatari, 1993), thus the high C/N ratios in the top of UACT6, and the lower values through the rest of the core, may be explained in terms of nitrogen mobility.

One problem with the above explanation is that the C/N values from 0-5 cm in UACT6 are not consistently high, as would be expected, but show large variability between contiguous samples. The topmost sample (0.0-0.2 cm) has a C/N ratio of 12, comparable to that seen in the rest of the core below 5 cm depth. Hence, the third possibility in explaining the observed C/N profile is that of measurement error. A comparison of nitrogen concentration with C/N ratio (Figure 4.7) shows that the five highest C/N values recorded occur in the five core samples with the lowest nitrogen content. It may be that periods with a low sedimentary nitrogen content reflect a decrease in lake productivity, a higher relative input of terrestrial organic matter, and a corresponding increase in C/N ratio, as discussed above. However, the strong inverse relationship between nitrogen content and C/N ratio in these five samples is not seen in the rest of the core. The possibility of measurement errors at low concentrations of nitrogen cannot be ruled out. Notably, the five samples with the lowest nitrogen and highest C/N values do not also have the lowest TOC contents (Figure 4.8). The importance of this is not known. Mackereth (1966) also finds no correlation between C/N and TOC content across the range of values seen in UACT6, although TOC values below 4% are associated with lower C/N ratios. This is attributed to the release of NH^"^ ions from glacial clays by the Kjeldahl digestion method used by Mackereth. This effect is unlikely to be apparent in UACT6 due to the higher TOC contents and the different nitrogen analysis method employed. The

CHAPTER 4 BULK SEDIMENT ANALYSES 136