RECOLECCIÓN DE LEYES Y NORMAS PROMULGADAS POR EL ESTADO

40 80 120 Chlorins/mg g^oc 0.3 0.6 0.9 20 E o Q. (U Q

Figure 4.11 TOC and chlorin concentrations, core UACT6.

100 R '= 0.226 ++ - - i h ÛÜ 0Û

g

Figure 4.12 Chlorin vs TOC concentrations, core UACT6. The regression line is shown, and is significant at above the 99% level.

CHAPTER 4 BULK SEDIMENT ANALYSES 144

UACT6 may be explained by variations in the input of organic matter, and specifically chlorins, to the lake sediment record. Further evidence to corroborate this hypothesis is discussed with reference to lipid concentrations in Chapter 5. The nature of the chlorin curve is also interesting, in light of the conclusions of Harris et al. (1996) who note that it is not known whether the response of chlorin content to productivity changes is linear. The chlorin minima appear to be much better defined than the corresponding minima in LOI and TOC. The onset of each minimum occurs rapidly, and the terminations occur even more rapidly. During the minima, chlorin concentrations are only c. 25% of those seen throughout the rest of the core, a much greater difference than is seen in LOI and TOC. These responses are similar to those expected of a system in which a threshold value is being crossed, rather than one in which the response is linear. Without further data it is difficult to speculate as to the nature of such a threshold. The chlorin signal seen in Figure 4.11 could also reflect differential preservation of chlorins during periods of differing sediment organic content. The mechanisms by which this could occur are not known.

4.5 Bulk organic stable carbon isotope analysis

While the ratio of *^C to *^C (Ô^^C: Equation 2.1) is one of the most frequently used bulk measurements in marine and lacustrine sediments, the factors influencing 5’^C are complicated. The c. 8% mass difference between and means that reactions, be they physical, chemical or biological, tend to discriminate between the two isotopes. In particular, photosynthesis discriminates quite strongly against the heavier isotope. Photosynthesis is the pathway by which most organisms assimilate carbon (with the exception of e.g. chemosynthetic bacteria, or heterotrophs which feed on other organisms), hence organisms exhibit depletion and have lower, more negative

values. A corresponding enrichment in with higher, less negative values, will be seen in the carbon source. All values in the following review are quoted relative to the PDB standard.

CHAPTER 4 BULK SEDIMENT ANALYSES 145

4.5.1 Carbon sources in terrestrial and lacustrine environments

The value of any organic carbon deposited in a lake sediment will reflect a variety of sources and influences. Allochthonous organic sources mainly take the form of terrestrial plant remains and eroded soils. Terrestrial plants and many soil micro­ organisms obtain CO2 directly from the atmosphere, hence the isotopic fractionation occurring in these organisms will be determined in part by the value of atmospheric CO2. This value can vary over a variety of different timescales, from short (daily to annual) variations caused by changing biomass productivity (Farquhar

et al., 1989), to longer (decadal to centennial) variations caused by factors such as volcanism (Kump and Arthur, 1999) and fossil fuel burning (Keeling et al., 1979; see also Section 4.5.2). Ultimately, variations occur on geological timescales, associated with ice ages, global vegetation changes, and so on (Hayes et al., 1999). Nonetheless, for the puposes of the study of the late Holocene, the value of atmospheric CO2 is generally taken to be about -7%o ± c.l%o. This range is small and can probably be disregarded as a factor influencing values observed in lake sediments, especially given the large isotopic fractionations associated with CO2 assimilation and utilisation by organisms. Isotopic fractionation in higher plants is discussed in greater detail in Section 4.5.3.

In soils there can be a significant utilisation of CO2 released by decomposition of organic detritus. This CO2 will have an isotopic composition similar to that of the decaying organic matter, and hence can be substantially more depleted in than ‘normal’ atmospheric CO2 (Schleser and Jayasekera, 1985; Farquhar et al., 1989). However, it is assumed that reassimilation of this CO2 is unhkely to have a major influence on terrestrial plants at Lochan Uaine, due to the low biomass present in the catchment. This low biomass prevents the retention of depleted CO2 beneath a vegetation canopy, and limits the subsequent utilisation of that CO2 in photosynthesis.

It is worth mentioning that methane may sometimes be used as a carbon source by soil micro-organisms. This process requires anaerobic conditions for methane production by methanogens, but also aerobic conditions for methane utilisation by methylotrophs. Such conditions may be found in waterlogged soils, such as peat bogs, with the

CHAPTER 4 BULK SEDIMENT ANALYSES 146

methane moving upwards from an anaerobic environment into an aerobic environment. The extent of this process is not known at Lochan Uaine, but given the skeletal nature of the catchment soils, and the absence of well-developed peat bogs, it is assumed to be minimal.

Sources of carbon incorporated by aquatic organisms are potentially rather more complex than for terrestrial organisms. With the exception of some emergent macrophytes, which are not present at Lochan Uaine, aquatic organisms are unable to use atmospheric CO2 directly. Several carbon sources may be utilised for photosynthesis by aquatic organisms. CO2 may be present in water either as a gaseous (CO2) or dissolved (H2CO3) species. Pearson and Coplen (1978) suggest that dissolved CO2 is around \%o lighter than gaseous CO2 across the normal temperature range of lakes. By contrast, where bicarbonate (HCO3 ) is present and in isotopic equilibrium with the atmosphere, values for bicarbonate are generally around 4-l%o, compared to -l%o for atmospheric CO2 (Pearson and Coplen, 1978). Thus, bicarbonate may be 8-9%o less depleted in than dissolved CO2. These differences in the isotopic compositions of the carbon sources will influence the corresponding

values of organisms which assimilate carbon from the different sources.

Other possible carbon sources for aquatic organisms include dissolved and mineral carbonate, carbon monoxide, and methane. Carbonate is most important as a carbon source in high-pH lake waters, as discussed in Section 4.5.2, but is generally not significant in acidic lakes. Carbon monoxide is not considered an important source of carbon in any lacustrine environments, due to the ease with which it is either reduced to methane or oxidised to CO2 (Wetzel, 1983). However, where anaerobic conditions exist, methane may be produced by methanogenic bacteria. This was discussed above with respect to soils. Methane produced within a lake may be significantly depleted in ^^C by comparison with dissolved atmospheric CO2 (Hâkansson, 1985). Anaerobic conditions are most likely to be found within the sediment column, or at depth within the water column, especially in productive lakes or during periods of high aquatic productivity. The importance of methane as a carbon source will be greatly reduced in lakes with limited anaerobic conditions, such as well mixed, oligotrophic lakes.

CHAPTER 4 BULK SEDIMENT ANALYSES 147

Finally, carbon may be present in lake waters in the form of dissolved CO2 released by respiration. As this carbon has previously been assimilated by the organism via the usual photosynthetic route, it will have a value similar to that of the plant as a whole. O ’Leary (1981) gives examples of values for respired CO2 from plants. These values are from 2%oto I2%cless depleted in than the whole plant, with the difference appearing greatest for CO2 released during respiration in the light. O ’Leary also notes that the source of CO2 for respiration is important, as lipids are more depleted in than carbohydrates (see Chapter 6). As with methane, the importance of respired CO2 in assimilation by aquatic organisms is likely to be lowest in well mixed, oligotrophic lakes.

4.5.2 Sources of carbon in Lochan Uaine

Bulk measurements in sediments have traditionally focused on two carbon types: organic carbon and inorganic carbon (carbonate). Lochan Uaine is an acidic, ultra- oligotrophic lake lying on an entirely granitic catchment, and as a result there is likely to be only a minimal contribution of bicarbonate to the lake system from dissolution of catchment bedrock (Hâkansson, 1985). Almost all carbon in Lochan Uaine and its catchment derives ultimately from the atmosphere. Catchment vegetation will assimilate CO2 directly from the atmosphere, while aquatic organisms will mainly utilise carbon dissolved in lake water. The present day acidity of the water (pH 5.8) means that most inorganic carbon will be in the form of CO2 or H2CO3 (known collectively as H2CO3 ) with only a small contribution possible from HCO3 (Figure 4.13). A diatom-inferred pH reconstruction of a core from Lochan Uaine confirms that the lake has been no less acidic than at present for the last c. 2 0 0 0 yr, which is the approximate age of core UACT6 (Battarbee et aL, 1996). No carbonate precipitation occurs in the lake, or is likely to have occurred at any stage during the lake’s history. This is due in part to the low pH and to the lack of carbonate input from catchment erosion. Additionally, the low nutrient status of Lochan Uaine makes depletion of CO2 in surface waters during algal blooms less likely, thus preventing calcite supersaturation and the subsequent precipitation of carbonates (Kelts and Hsu, 1978). Consequently, the carbon in the lake sediment exists almost entirely as organic carbon

CHAPTER 4 BULK SEDIMENT ANALYSES 148 % 100 0 6 4 5 7 8 9 10 11 12 13 pH

F ig u re 4.13 Relative proportions of dissolved carbonate species at different pH values (from Kelts and Hsii, 1978, page 299).

(Figure 4.1). Bulk measurements in core UACT6 were made only on the organic

carbon fraction.

Studies of more specific carbon fractions in the sediment have been undertaken by various authors. Analysis of in carbonate tests of aquatic organisms is common, but these are not found in acidic lakes such as Lochan Uaine. In other cases, parts of the organic fraction have been separated for individual analysis, such as humic compounds or the lignin and cellulose components. Most recently, stable carbon isotope analysis of individual organic compounds has become possible. Such measurements were undertaken on sediment from Lochan Uaine and are discussed in Chapter 6.

Interpretation of the bulk organic carbon isotope record contained in any sedimentary sequence requires an understanding of the pathways by which that carbon became incorporated into the sediment, and of the processes affecting that incorporation. Specifically, the isotope fractionation encountered at each stage in the process must be examined.

Given the lack of carbonate inputs from the Lochan Uaine catchment outlined in the previous section, it is fairly safe to assume that the majority of the carbon in Lochan