BULK SEDIMENT ANALYSES 139 o

2 %

B

Hydrogen/% dry sed.

Figure 4.9 Hydrogen wj C/H ratio, core UACT6.

o 2 %

B

TOC/% dry sed.

CHAPTER 4 BULK SEDIMENT ANALYSES 140

hydrogen content. This is seen in Figure 4.9. The negative correlation between C/H ratio and hydrogen content is significant at the 99% level (R^=0.18, N=231). A slight positive correlation exists between C/H ratio and carbon content, although this is only significant at the 95% and not the 99% level (R^=0.13, N=231). A much stronger positive relationship between C/H ratio and carbon content is seen by Mackereth (1966, Figure 3, page 174) in his study of three lakes in the English Lake District. Mackereth attributes this relationship to the release of inorganic hydrogen from water bound to the mineral matrix of the sediment. The same effect is responsible for the inverse relationship between TOC content and the LOI/TOC ratio discussed previously (Figure 4.3). Interestingly, although release of bound water only affected the LOI/TOC ratio at TOC concentrations below c. 4%, it appears to affect the C/H ratio at all carbon contents up to 16%. We would thus expect a much stronger positive correlation between C/H and carbon content in UACT6 than that seen in Figure 4.10. Mackereth (1966, page 173) does point out that the presence of this inorganic hydrogen source has implications for the interpretation of C/H ratio profiles; “The large variation in C/H ratio brought about [by the release of tightly bound water] masks any small variation in this ratio which may exist in the organic material of different age or origin”.

4.4 Chlorins

Chlorins are early degradation products of chlorophyll (Harris et al., 1996). They are found in practically all sediments, both marine and lacustrine (Harradine et a l,

1996b). Chlorins may be present as free compounds, or bound to other molecules such as steroids or hopanoids to form chlorin esters. O f the former, Keely and Maxwell (1991) identified two chlorin structures, phaeophytin a and pyrophaeophytin

a, in sediments from Priest Pot, English Lake District. O f the latter, Pearce et al.

(1993) and Harradine et al. (1996a) identified various steryl, hopanyl and tetrahymanyl chlorin esters in sediments of Lake Valencia, Venezuela. Although individual chlorins and chlorin esters were not identified in the sediment of Lochan Uaine, techniques for doing so are described by Eckardt et al. (1991), Keely and Maxwell (1991), King and Répéta (1991, 1994), Prowse and Maxwell (1991), Pearce

CHAPTER 4 BULK SEDIMENT AN ALYSES 141

et al. (1993), Harradine et al. (1996a,b), Goericke et al. (1999), Talbot et al.

(1999a,b), and Sachs and Répéta (2000). Identification of chlorins at a similar lake to Lochan Uaine, Lochnagar in the eastern Cairngorms, is currently in progress (Dalton

et al., 2000).

4.4.1 Origin of chlorins

There is general agreement in the literature that chlorins derive from phytoplankton chlorophyll a (Harradine et al., 1996a; Harris et a l, 1996) and chlorophyll b (Talbot

et al., 1999b). Presumably, therefore, chlorins also derive from the chlorophyll of benthic organisms, although this has yet to be confirmed. The transformation of chlorophyll to chlorins and chlorin esters is thought to proceed via several pathways. Eckardt et al. (1992) suggest that the transformation may occur as a response to algal senescence at the conclusion of a bloom, although Harradine et al. (1996b) report that studies designed to recreate this process have been unsuccessful in producing steryl chlorin esters. Another pathway for chlorin formation appears to be through zooplankton herbivory of algae, and chlorin esters have been found in zooplankton faecal pellets (Harradine et al., 1996b; Goericke et al., 1999; Talbot et al., 1999a,b). The significance of this pathway in Lochan Uaine is not known as no studies have yet been carried out on the Lochan Uaine zooplankton community. However, the predominance of benthic diatoms in the sediment record, and the almost complete absence of planktonic taxa, suggests that zooplankton herbivory may be relatively unimportant at Lochan Uaine. Grazing of the epilithic flora by benthic invertebrates may be a more significant route for chlorin formation. Identification of the chlorins present may indicate the method of formation. In particular, the steroid component of steryl chlorin esters is thought to represent the sterol content of the source (Harradine

et a l , 1996b), although Talbot et al. (1999a,b) report slight changes in distribution associated with zooplankton herbivory. The derivation of chlorins from higher plant chlorophyll, rather than algal chlorophyll, is not thought to occur at Lochan Uaine. Neither chlorophyll nor chlorins are likely to survive in catchment soils due to the rapid degradation that occurs in these environments. Confirmation of the absence of a higher plant source for chlorins in montane lake sediments awaits further analysis, including the identification of sedimentary chlorins (Dalton et al., 2000).

CHAPTER 4 BULK SEDIMENT ANALYSES 142

As chlorins are produced from the chlorophyll of lake biota, their sedimentary concentration is thought to provide a direct proxy for lake productivity. Harris et al.

(1996) demonstrate how chlorin concentration in a marine core from off the coast of northwest Africa provides a potentially more reliable measure of palaeoproductivity than other proxies, such as biogenic silica which derives mainly from marine diatoms and as such does not represent the entire phytoplankton community. They also note the strong positive correlation between chlorin content and organic matter content of the sediment. They conclude that chlorin content reflects changes in total primary productivity, although they point out that the response may not be linear. Without evidence to the contrary, it seems reasonable to apply this chlorin content-primary productivity interpretation to the analysis of chlorin content in UACT6.

4.4.2 Chlorin content of UACT6

The downcore variation in total sedimentary solvent-extractable chlorins is shown in Figure 4.11. Regions of the core containing low chlorin concentrations are extremely well defined, and occur at depths of approximately 1-5, 17-20 and 32-35 cm. These correspond very well with the periods of low TOC in the core (also shown on Figure 4.11). The main difference between the chlorin and TOC profiles is in the core section from 5-15 cm depth. The relative chlorin concentration compared to the rest of the core is much greater than that seen in the TOC curve. The differences across this section account for the comparatively low R^-value (0.226) seen for the correlation between chlorin and TOC content (Figure 4.12) compared to that between, for example, TOC and LOI (Figure 4.2). Nonetheless, the correlation is still significant at above the 99% level.

As the chlorin concentrations in Figure 4.11 are expressed as a proportion of TOC, the observed downcore variations should be independent of variations in LOI. This is important, as it was not known whether the LOI variations were caused by changes in the organic input to the sediment, changes in the mineral input, or a combination of both. A variation in mineral input to Lochan Uaine, for example due to catchment erosion and the subsequent inwash of pulses of clastic material, would not have affected the concentration of chlorins relative to TOC mentioned above. The chlorin and TOC data thus provide strong evidence that at least part of the LOI profile of

CHAPTER 4 BULK SEDIMENT ANALYSES 143