Selection of sediment analyses

This thesis aims to explore further the LOI fluctuations evident in the sediment of Lochan Uaine through analysis of the sediment organic matter, and attempts to link the observed variations to potential climatic influences. The decision to focus analyses

CHAPTER 1 INTRODUCTION 39

on the sediment organic fraction rather than the inorganic fraction was based on several factors. The previous analyses at Lochan Uaine described above suggest that productivity may be important in driving the fluctuations in LOI. They also show that the sediment inorganic fraction may not be particularly responsive to these cycles. The diatom concentration is a potential proxy for lake productivity, yet shows no obvious fluctuations to match those in the LOI profile. This apparent lack of response may be due to changes in sediment accumulation rate varying in phase with the LOI fluctuations (Battarbee et al., in press). There is also the added complication that diatoms contribute both inorganic and organic m atter to the sediment, and it is not known whether higher diatom productivity would be reflected by an increase or decrease in LOI (Willemse and Tomqvist, 1999).

The high diatom content of the sediment inorganic matter suggests that other mineral fractions are relatively unimportant as contributors to the sediment. Clastic material from the catchment is identified from visual inspection, and from measurements of magnetic properties and metal concentrations. None of these is observed to vary synchronously with LOI. It is possible that clastic inwash pulses may account for some of the small scale LOI variations in cores from Lochan Uaine (Battarbee et al.,

in press), but it seems unlikely that they can explain the larger LOI variations.

This evidence suggests that measurements of properties of sediment inorganic matter are of relatively little use in analysing past productivity variations in Lochan Uaine. The organic fraction appears to be of greater use. In particular, the coherent variations in LOI, bulk organic and chironomid head capsule concentration provide a strong indication that the LOI cycles are associated with changes in lake primary productivity. For this reason, it was decided to concentrate further analysis of Lochan Uaine sediment on the study of the sedimentary organic fraction. This study focuses on the determination of the sources of input of organic matter to the sediment, with a view to identifying a reliable palaeoproductivity signal.

CHAPTER 1 INTRODUCTION 40

Various sediment properties are analysed to examine organic matter sources. These properties, and the reasons for choosing to analyse them, are summarised in the following section.

1.2.1 Carbon, hydrogen, nitrogen and carbonate

Carbon and hydrogen are ubiquitous elements in organic matter, and nitrogen is also very common. The total organic carbon content of the sediment is determined by subtracting the carbonate carbon content from the total carbon content. Similarities between the LOI profile and the carbon, hydrogen and nitrogen profiles would show that organic matter is the principal form in which these elements are present in the sediment. However, concentrations of carbon, hydrogen and nitrogen alone do not provide an indicator of organic matter source or total productivity, as these concentrations may be affected by variations in the sediment inorganic content. Numerous authors have used the C/N ratio as an indicator of organic matter source

{e.g. Hakanson and Jansson, 1983; Meyers and Ishiwatari, 1993; Tyson, 1995;

Bianchi et al., 1999). Aquatic micro-organisms such as algae and bacteria generally have high protein contents, and the high nitrogen content of this protein gives aquatic organisms lower C/N ratios than higher plants (Goodell, 1972; Meyers et al., 1984b). The C/N ratio may thus indicate whether the organic matter in a sediment is from an algal/bacterial source, a higher plant source, or a mixture of the two. Variations in the C/N ratio are used to reconstruct the changing importance of inputs from these different sources. The C/H ratio is a less well-established indicator of organic source, particularly in young sediments, but there is a suggestion that it may be used to broadly indicate the origin of organic material (Talbot and Livingstone, 1989).

1.2.2 Bulk organic stable carbon isotope values (ô^^C)

Given the strong correlation observed between LOI and bulk organic Ô'^C in a section of core UACT4 (Battarbee et al., 1996; Battarbee et al., in press) it is important to try and replicate these results in core UACT6, the subject of this study. C3 plants as found in the catchment of Lochan Uaine exhibit a well-known range of ô'^C values

{e.g. Descolas-Gros and Fontugne, 1990; Proctor et al., 1992; Killops and Killops, 1993; Tyson, 1995; Meyers and Lallier-Vergès, 1999). Fractionation in aquatic

CHAPTER 1 INTRODUCTION 41

macrophytes and micro-organisms is less well-defined, and may exhibit a wider range of values {e.g. Nakai, 1972; Schidlowski, 1988; Meyers and Benson, 1988; Michel et al., 1989). Nonetheless, variations in bulk organic potentially indicate variations in the importance of inputs from higher plant and algal/bacterial sources.

1.2.3 Chlorins

Few studies of chlorin concentrations in lakes have been made, and their use to date has been restricted principally to marine sediments (Harris et al., 1996). Chlorins are early degradation products of chlorophyll, principally chlorophyll a (Harradine et al.,

1996a) but also chlorophyll b (Talbot et al., 1999b). Several mechanisms have been proposed for chlorin formation in aquatic environments, for example via the grazing of zooplankton on algae (Goericke et al., 1999; Talbot et a l, 1999a,b). As such, chlorin concentrations may represent a direct proxy for aquatic productivity. The high resolution stratigraphie analysis of chlorin concentrations in Lochan Uaine represents the first study of this kind, and aims to determine whether chlorins can be used to infer past primary productivity from lake sediments.

1.2.4 Lipids

The sediment properties described above - CHN, carbonate, bulk Ô^^C, and chlorin content - are all bulk measurements. As such it is often hard to interpret the data. For instance, an increase in bulk values could indicate an increased contribution from algal and bacterial organic sources, a decreased contribution from C3 terrestrial plant sources, or a combination of both. Analysis of sedimentary lipids is undertaken to assess the contributions from these various sources.

Numerous studies show that it is possible to attribute certain lipids to particular source organisms or groups of organisms. M ost other organic components such as carbohydrates and proteins cannot be used for this purpose, with the exception of some porphyrins (Hayes, 1993). Among the many thousands of hpid components potentially found in sediments, those which appear to be useful indicators of organic source, and are frequently used as such, include the /i-alkanes (Ho and Meyers, 1994; Farrimond and Flanagan, 1996; N ott et al., 2000), the w-alkanoic acids (Wiinsche et

CHAPTER 1 INTRODUCTION 42

a l, 1988; Farr et al., 1990; Rieley et al., 1991b; Wilkes et al., 1999), the n-alkanols (Cranwell and Volkman, 1981; Kawamura and Ishiwatari, 1985; Robinson et al.,

1986; Ficken et al., 1998), and the sterols (Gaskell and Eglinton, 1975; Huang and Meinschein, 1979; Ishiwatari et al., 1980; Meyers et al., 1984b). All of these groups are analysed in the Lochan Uaine sediment, along with various other components of interest.

Although there is a vast literature on the subject of hpid distributions in modem and fossil environments, it was felt important to analyse potential contributors of lipids to the sediment first hand. To this end, specimens of modem vegetation were collected from Lochan Uaine and its catchment. This allows a direct comparison to be made between the hpid signal in the catchment and that in the lake sediment. In particular, the comparison is used to assess degradation of lipids before, during or after deposition. All hpids are susceptible to degradation, but some are more susceptible than others (Giger et al., 1980; Robinson et ah, 1984a; Meyers and Benson, 1988; Cranwell, 1981, 1984b). It is important to recognise which sedimentary hpids represent a true signal of changes in vegetation source or productivity, and which represent a signal altered by degradation processes.

1.2.5 Compound-specific ô^^C values

Compound-specific ô^^C values may be measured for the major hpids present in the sediment record. This analysis serves two purposes. Firstly, it can be used to indicate the ô^^C values of different organic sources, in particular the suggestion given previously that C3 plants from the catchment display lighter ô^^C values than algae and bacteria from the lake. This is important in the interpretation of the bulk organic ô^^C record. Secondly, downcore changes in compound-specific ô^^C values may be compared to the bulk organic Ô’^C curve. It is possible that variations in bulk organic Ô’^C reflect not only changing contributions from different organic sources, but also a response to other forcings, including chmate. Temperature is known to affect values in a variety of ways (DeNiro and Epstein, 1977; Schleser, 1995; Mayer and Schwark, 1999; Jahnke et al., 1999). Downcore variations in within hpids from a

CHAPTER 1 INTRODUCTION 43

particular source are unlikely to be caused by the changes in source which influence the bulk record, and an alternative explanation must be sought.

1.2.6 Dating

In addition to the analysis of sediment properties, it is essential to date the core if it is to be compared to other records of late Holocene climate variability. Core UACT4 has been dated using ^^^b and thirty-six AMS radiocarbon dates on bulk sediment samples (Battarbee et a l, 1996, Battarbee et a l, in press). It was decided to use correlation of LOI profiles to transfer the chronology from UACT4 to UACT6. Additionally, an attempt was made to use microtephra layers from Icelandic volcanoes to verify the transferred chronology (Dugmore et al., 1995a; Pilcher et al., 1996; Rose et al., 1996).