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Reconstruction of a subalpine grass-dominated ecosystem, Lake Rutundu, Mount Kenya: a novel multi-proxy approach
Palaeogeography, Palaeoclimatology, Palaeoecology 177 (2002) 137^149 www.elsevier.com/locate/palaeo
Reconstruction of a subalpine grass-dominated ecosystem, Lake Rutundu, Mount Kenya: a novel multi-proxy approach K.J. Ficken a;b; *, M.J. Wooller a;1 , D.L. Swain a;2 , F.A. Street-Perrott a , G. Eglinton b a
b
Tropical Palaeoenvironments Research Group, Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK Biogeochemistry Research Centre, Department of Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Accepted 16 August 2001
Abstract Palaeoecological reconstructions based on a single proxy are limited, but by combining pollen, biogeochemistry and grass cuticle analysis, ecosystem structure and function can be better understood. Lake Rutundu is a small, subalpine lake on the northeast flank of Mt Kenya. During the last glacial, pollen evidence suggests a shrub grassland dominated by Afroalpine taxa and Poaceae, representing a dry, cold, open environment. The N13 C values of terrestrial biomarkers imply a high proportion of C4 plants. Grass cuticle analysis allows resolution of the different C4 subtypes and shows that the vegetation was dominated by tall C4 panicoid grasses, prone to frequent fires. During the Holocene, Poaceae pollen declined while subalpine shrubs increased. The N13 C values of terrestrial biomarkers imply a C3 dominated vegetation. Together with an expansion of rainforest at lower altitudes, this suggests wetter conditions more favourable to C3 plants. Increased percentages of C3 pooid grass cuticles confirm a reduction in moisture stress. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: pollen; compound-speci¢c carbon isotopes; grass cuticles; Mt Kenya; Lake Rutundu; palaeoecology
1. Introduction The family Poaceae (grasses) contains a diverse array of species ( s 10 000 species and 655 genera)
1 Present address: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW Washington, DC 20015, USA. 2 Present address: Scottish Agricultural College, Crichton Royal Farm, Mid Park, Bankend Road, Dumfries DG1 4SZ, UK. * Corresponding author. Fax: +44^1792^295-955. E-mail address: gg¢[email protected] (K.J. Ficken).
(Watson and Dallwitz, 1994) whose distribution ranges from the tropics to the Arctic. This diversity includes important morphological, phenological and physiological variability. Biomes dominated by grasses cover a substantial part of the Earth's surface and as highly productive ecosystems, play a vital role in modern biogeochemical cycles (Chaloner and Creber, 1990). Grasses serve as particularly good indicators of past climates as they generally have short life cycles (relative to woody perennial trees and shrubs) and can adapt quickly to environmental change. It has been hypothesised (Jolly and Haxeltine,
0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 3 5 6 - X
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1997; Street-Perrott et al., 1997; Cerling et al., 1998) that the climate of the last glacial favoured C4 grasses in the tropics as the climate of many areas was signi¢cantly drier than today (Wright et al., 1993; Hostetler and Clark, 2000) and atmospheric pCO2 was reduced to about 190^200 Watm (Barnola et al., 1987). At the ecosystem level, the variations in temperature, rainfall and ambient CO2 /O2 ratio would have altered the competitive balance between plants using the C3 and C4 photosynthetic pathways. C4 and CAM plants are more CO2 - and water-e¤cient than C3 plants (Ehleringer et al., 1997) and hence would have had a selective advantage with lower pCO2 and widespread aridity. The distribution of C3 and C4 grasses provides an important climatic indicator (Teeri and Stowe, 1976; Livingstone and Clayton, 1980; Teeri et al., 1980; Hattersley, 1992). Therefore a fuller understanding of the contribution of C3 and C4 grasses to terrestrial biomes in the past will enhance palaeoclimatic reconstructions. Lake sediments receive allochthonous material from a variety of plant sources, including pollen, leaf waxes and plant cuticles, which are often suf¢ciently well preserved to act as indicators of past vegetation. Any palaeoenvironmental reconstruction using a single method, such as pollen analysis, is restricted by the limitations of that proxy. Palaeoenvironmental reconstructions using pollen provide a broad understanding of changes in vegetation communities. Distinctions can be made between trees, shrubs and herbs as well as between individual taxa within these groups. However, in the reconstruction of palaeograsslands, pollen analysis is limited in its use as Poaceae and most Cyperaceae (sedges) cannot be identi¢ed below family level (Moore et al., 1991). N13 C measurements on bulk organic matter (TOC) are also regularly used in palaeoenvironmental reconstructions along with C/N ratios (Talbot and Johannessen, 1992; Meyers and Lallier-Verge©s, 1999). However, in lake sediments, organic matter is derived from multiple sources; by using N13 CTOC values alone it is impossible to say whether algae, aquatic macrophytes or terrestrial plants were driving the isotopic signal. In lakes, the C/N ratio must also be used with caution as cells of the green alga Botryococcus braunii have very high
C/N ratios (Huang et al., 1995) and can be misinterpreted as originating from terrestrial plant detritus. Lipid analysis, however, can address the problems of interpreting these bulk parameters. Individual, or groups of, organic compounds can be assigned to speci¢c sources and are termed biomarkers or chemical fossils (Eglinton and Calvin, 1967). Although the data base of distributions of individual homologues and of classes of compounds within contributing plant species is presently very limited, the long-chain n-alkyl lipids ( v C26 ) have been shown to be derived from terrestrial higher plant leaf waxes (Eglinton and Calvin, 1967; Tulloch, 1976; Kolattukudy et al., 1976). The recent development of compound-speci¢c carbon isotope analysis enables the N13 C values of individual biomarkers such as terrestrial plant lipids or algal lipids to be measured. Analysis of grass cuticles enhances the palaeoenvironmental reconstruction further as the di¡erent C4 grass subtypes can be identi¢ed. The assumption that C4 grasses always indicate arid conditions was challenged by Hattersley (1992) when the C4 photosynthetic pathway was classi¢ed into subtypes. The C4 subtype NADP-ME is associated with more mesic conditions and has the ability to function at cooler temperatures than the NAD-ME C4 subtype. Therefore, by combining pollen, compound-speci¢c N13 C and grass cuticle analyses much more information can be obtained about the terrestrial plant community. In this paper, we combine these three independent but complementary methods to reconstruct the terrestrial environment surrounding Lake Rutundu, Mt Kenya over the last 38 300 cal. yr BP. This is, to the best of our knowledge, the ¢rst lake where these three methods have been employed together. 1.1. Modern vegetation On Mt Kenya, the present-day vegetation shows marked altitudinal zonation (Fig. 1). The basal plateau zone (disturbed/savanna, Fig. 1) is dominated by Themeda triandra (C4 ) and other panicoid grasses (mainly C4 ). This grassland grades up into the Montane Rainforest Belt (wet and dry montane forest; C3 ), which forms a horseshoe occupying the wettest areas and con-
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Fig. 1. Location map of Mount Kenya showing Lake Rutundu and the main vegetation belts.
tains sparse panicoid (C3 /C4 ), arundinoid (C3 ) and bambusoid (C3 ) grasses, together with sedges (C3 /C4 ). A gap in the Montane Rainforest Belt occurs on the dry northern £ank of Mt Kenya and is dominated by grasses found in the basal plateau zone (Fig. 1). Above treeline (2900^3400m asl) lies the Ericaceous Belt, characterised by tall,
woody, microphyllous, subalpine shrubs (C3 ) and pooid (C3 ) grasses. This grades up into the Afroalpine Belt, which is occupied by C3 pooid tussock grasses and low shrubs with some C3 sedges. The Nival Zone consists mainly of bare rock and ice, but isolated C3 pooid grasses are present in very sheltered areas.
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1.2. Modern climate Mt Kenya receives most of its rainfall from the southeast monsoon during March to June (long rains) and October to November (short rains), and as a consequence, its southeastern £ank receives the most rainfall ( s 2500 mm/yr at 2000^ 3000 m) and its northern £ank is the driest ( 6 1500 mm/yr; Thompson, 1966). The mountain does not experience marked seasonal variations in temperature due to its location on the Equator, but does exhibit a large diurnal temperature range (V14³C at treeline) (Coe, 1967). 1.3. Lake Rutundu Lake Rutundu is a volcanic crater lake located at 3078 m asl in the Ericaceous Belt on the northeast £ank of Mt Kenya (Fig. 1). The lake is small (0.4 km2 ) and oligotrophic, with a maximum depth of 11 m, and is situated V90 m above treeline in vegetation dominated by C3 woody, subalpine shrubs and grasses, almost all of which use the C3 photosynthetic pathway. Lake Rutundu was produced by a single event eruption and exhibits no hot spring or fumarolic activity. The lake was ¢rst cored by Coetzee and Van Zinderen Bakker (Coetzee, 1967). 2. Methodology 2.1. Bulk sediment parameters A 7.55 m long core of carbonate-free, diatomaceous lake sediment was collected from the central, deepest part of Lake Rutundu in 1996 using a modi¢ed Livingstone piston corer. The cores were wrapped in plasticiser-free cling¢lm, aluminium foil and plastic sheeting and returned to the UK in sealed plastic tubes. The core was scanned for volume magnetic susceptibility, described and sectioned at 1 cm intervals and stored in sealed Sterilin petri dishes at 4³C before analysis. The TOC and nitrogen contents were measured on replicate subsamples using an elemental analyser (EA). N13 C was measured on TOC by EA^IRMS (elemental analyser^isotope ratio mass spectrom-
Fig. 2. Age^depth relationship (squares represent rejected dates) and accumulation rate of Lake Rutundu sediments.
etry) at approximately 30-cm intervals. Sixteen AMS 14 C dates were measured on the TOC, although three dates were rejected due to suspected contamination by more recent carbon (Fig. 2). The remaining 13 dates were calibrated to give calendar ages using CALIB3.0 (Stuiver and Reimer, 1993) for those dates in the range 0^18 000 14 C yr BP and from the equation of Bard et al. (1997) for those older than 18 000 14 C yr BP. The dates show that the core provides a continuous record covering the last glacial/interglacial transition and the whole of the Holocene. The extrapolated basal age is 38 300 cal. yr BP (Fig. 2). The calculated sediment in£ux in mg/ cm2 /yr is the product of sediment accumulation rate (Fig. 2) and the dry bulk density. TOC in£uxes in mg/cm2 /yr were calculated from the TOC content, bulk density and the sediment accumulation rate.
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Fig. 3. Lithology and bulk sediment properties of the Lake Rutundu core.
2.2. Pollen Pollen grains were separated from the sediment, identi¢ed and counted following the standard technique of Faegri and Iversen (1989). The samples were counted in a random, non-stratigraphical order to avoid any bias due to preconceived changes in the pollen spectra. All pollen, spores and green algae were identi¢ed to the lowest possible taxonomic level using reference material; the degree of reliability is indicated by standard conventions (Birks, 1973; Berglund and Ralska-Jasiewiczowa, 1986). Pollen in£ux was calculated by adding a known quantity of an exotic marker (Eucalyptus pollen grains) to a known volume of sediment and then counting the numbers of exotic markers against the total number of pollen grains (300 counted). Pollen concentrations were combined with the age^depth curve to derive the pollen in£ux in grains/cm2 /yr. The local pollen sum was calculated using Poaceae, Cyperaceae, Ericacaeous Belt (Ericaceae type, Protea, Cli¡ortia nitidula, Stoebe kilimandscharica, Alchemilla and Anthospermum cf. usambarense) and Afroalpine
Belt (Compositae, Artemisia afra and Myosotis) pollen only. Percentages of the local pollen sum were calculated for Poaceae, Cyperaceae, Ericacaeous Belt and Afroalpine Belt pollen. 2.3. Biogeochemistry The methods used are described in detail by Ficken et al. (1998). Twenty-one 1 cm-long core segments (the outside of the core segment was removed to prevent contamination from smearing), representing major shifts in the N13 CTOC curve, were selected and the powdered, freezedried sediment extracted by sonication and centrifugation using a solvent system of sequentially decreasing polarity (3U100% MeOH, 2U1:1 MeOH:DCM and 5U100% DCM). The total extracts were each split into an acid and a neutral fraction by solid phase extraction (Aminopropyl Bond Elute). The neutral fraction was fractionated further into a hydrocarbon fraction, an alcohol fraction and a polar fraction by thin layer chromatography (silica gel 60, 0.25 mm thick). The hydrocarbon fraction was further fractionated
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into adduct and non-adduct fractions by urea adduction. The acid fraction was methylated with methanolic HCl and the alcohol fraction derivatised by bis(trimethylsilyl)-tri£uoroacetamide prior to analysis by gas chromatography (GC). The compounds were quanti¢ed using GC and identi¢ed by gas chromatography^mass spectrometry (GC^MS). In£uxes of the terrestrial biomarker lipids in Wg/cm2 /yr were calculated in a similar way to that of the TOC. 2.3.1. Lipid analyses GC analyses of all fractions were carried out on a Varian 3400 GC ¢tted with an on-column injector and a £ame ionisation detector. A CPsil5CB (Chrompack) fused silica capillary column (50 mU0.32 mm; 0.17 Wm ¢lm thickness) was used. The oven temperature was held at 60³C for 1 min, ramped at 10³C/min to 180³C and then ramped at 4³C/min to 300³C, where it was held for 25 min. The carrier gas was hydrogen. GC^MS analyses of representative samples were performed on a Carlo Erba Mega gas chromatograph (on-column injection, 70 eV EI) interfaced directly with a Finnigan 4500 mass spectrometer in order to identify the lipid components. The column and temperature programme were the same as for the GC analyses. Helium was used as the carrier gas. 2.3.2. Compound-speci¢c carbon isotope analyses The carbon isotope measurements of the individual n-alkanes (adducted fraction), n-alkanols (alcohol fraction) and n-alkanoic acids (acid fraction) were performed using a Varian 3400 GC attached to a Finnigan MAT Delta-S isotope ratio mass spectrometer via a combustion interface consisting of an alumina reactor (0.5 mm i.d.) containing copper and platinum wires (0.1 mm o.d.). Again, the column and the temperature programme were the same as for the GC analyses. All carbon isotopic ratios are expressed in x relative to the Pee Dee Belemnite standard. 2.4. Grass cuticles The methods used to extract grass cuticles are outlined by Wooller et al. (2000) and Wooller
(2002). Brie£y, a standard palynological method was applied to remove humic compounds (Moore et al., 1991); the wet sediment was then sieved through a 180-Wm sieve. The s 180 Wm fraction was mounted onto a microscope slide. Entire slides were rapidly scanned in systematic traverses at 100U magni¢cation. Morphological features were examined at 600^1000U magni¢cation. Cuticle fragments were grouped into morphotypes according to common micromorphological features described by Watson and Dallwitz (1988, 1994). Features recorded for each morphotype were entered into the African subset of the `Grass Genera of the World' data base (Watson and Dallwitz, 1988, 1994). The resulting list of possible genera was pruned to include only those recorded for upland Kenya (Ibrahim and Kabuye, 1987; Agnew, unpublished data). The subfossil cuticles were compared with type slides prepared from modern herbarium specimens and with scanning electron photomicrographs (Palmer and Tucker, 1981, 1983; Palmer et al., 1985, 1986; Palmer and Gerbeth-Jones, 1988). 3. Results 3.1. Bulk parameters The sediments consist of silty, diatomaceous lake mud below 400 cm, with a layer of ¢nely bedded sand at 509^517 cm (Fig. 3). Between 408 and 318 cm the sediments consist of organic diatomaceous lake mud with layers of ¢ne sand and plant detritus. Above 318 cm, the sediment is an organic, diatomaceous mud with another sand layer at 308^310 cm. A signi¢cant shift in bulk sediment properties occurs at about 318 cm, which is dated at approximately 14 300 cal. yr BP. As a result, this paper will refer to the sediments below 318 cm as being of glacial age and those above as late glacial/Holocene in age. TOC contents are low in the glacial sediments ( 6 5%) and increase in the late-glacial/Holocene muds to 15^20% (Fig. 3). The calculated in£ux of TOC was also much greater during the late glacial/Holocene than during glacial times (Fig. 4). C/N ratios range between 3 and 8 in the glacial-
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Fig. 4. Lithology, chronology and in£uxes of bulk sedimentary properties, terrestrial biomarkers, grass cuticles and pollen in the Lake Rutundu core.
age sediments and then rise sharply in the late glacial/Holocene muds to values between 10 and 20 (Fig. 3). The Lake Rutundu sequence shows variations in N13 CTOC with an amplitude of approximately 10x. The heaviest N13 CTOC values are found in the glacial-age sediments (e.g. 318.5x at V27 400 cal. yr BP) and the lightest values in the late-glacial/Holocene sediments (e.g. 328.2x at V8100 cal. yr BP) (Fig. 3). 3.2. Pollen Pollen deposition at Rutundu re£ects a combination of local and long-range transport (Swain, 1999). Although the sediments contain a signi¢cant amount of forest-derived pollen, most of this forest pollen was was dominated by highly transportable types, such as Podocarpus (Swain, 1999). Hence, it is unlikely that there has ever been any signi¢cant forest vegetation around the lake.
Of the pollen types with local sources, Poaceae and Cyperaceae dominated the glacial-age sediments and their in£ux into Lake Rutundu remained relatively high until approximately 8300 cal. yr BP (Fig. 4). Afroalpine pollen types also exhibited higher in£uxes during glacial times (Fig. 4). In contrast, the total in£ux of Ericaceous Belt taxa, including undi¡erentiated Ericaceae, Cli¡ortia and Proteaceae, increased substantially during the late glacial and Holocene (Fig. 4). 3.3. Biogeochemistry The n-alkyl lipid (n-alkane, n-alkanol and n-alkanoic acid) concentrations are greater in the lateglacial/Holocene sediments than in those of glacial age by an order of magnitude (Fig. 5). The nalkanes are the least abundant, being an order of magnitude lower in concentration than the n-alkanols and n-alkanoic acids throughout the core.
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The n-alkyl lipids are mostly dominated by longchain components (vC26 ) (Ficken, unpublished data), which represent terrestrial higher plant leaf waxes (Eglinton and Calvin, 1967) (Fig. 5). The chain length of the n-alkanes ranges from C19 to C35 with chain length maxima varying between C25 and C31 . There is a strong odd/even predominance (odd-numbered carbon chain lengths are found in greater abundance than even-numbered carbon chain lengths) in the majority of the samples, as expected for terrestrial higher plants. With a few exceptions, the n-alkanols range from C20 to C32 with chain length maxima at C26 or C28 . They show a strong even/odd predominance (evennumbered chain lengths in greater abundance than the odd-numbered carbon chain lengths), compatible with a higher plant origin. The distributions of the n-alkanoic acids range from C16 to C32 with chain length maxima at C18 or C28 . Like the n-alkanols, the n-alkanoic acids display a strong even/odd predominance. The short-chain n-alkanoic acids (C16 , C18 ) are found in relatively greater concentrations in the surface muds and in the glacial-age sediments (Fig. 5). The in£ux of terrestrial biomarkers (C27 ^C33 odd carbon numbered n-alkanes; C26 ^C32 even carbon numbered n-alkanols and n-alkanoic acids) is greater in the late-glacial/Holocene muds than in the glacial-age sediments (Fig. 4). The weighted average N13 C values (calculated using the concentration of each compound so that those compounds contributing more to the organic carbon contribute more to the average isotopic signature) of the long-chain n-alkanes range from 324.8x in the glacial-age muds to 331.1x in the late glacial/Holocene. However, the n-alkanols and n-alkanoic acids exhibit a much larger range of weighted average N13 C values for the long-chain homologues, ranging from 316.2x and 319.3x in the glacial-age sediments to 332.6x and 332.4x in the late glacial/Holocene, respectively (Fig. 6). In the glacialage sediments below 318 cm (V14 300 cal. yr BP) the mean weighted average isotopic values of the terrestrial biomarkers are 327.5x, 320.8x and 323.5x for the n-alkanes, n-alkanols and n-alkanoic acids respectively and are indicative of C4 plants, whereas in the late glacial/Holocene the
averages are lighter (more negative), indicative of C3 plants (Fig. 5). 3.4. Grass cuticle analysis The in£ux of graminoid cuticle was relatively high in the glacial, but reached a maximum around 8900 cal. yr BP (Fig. 4). Charred cuticle fragments of tussock-forming panicoid C4 grasses (notably Themeda triandra and Brachiaria sp.) are most frequent in the glacial-age sediments whereas in the late-glacial/Holocene muds, there is an increase in C3 pooid grass cuticles (Wooller, 2000). 4. Discussion 4.1. Glacial environment The TOC in£ux was low during the last glacial suggesting a reduced input of carbon into the lake at this time (Fig. 4). C/N ratios were also low, indicating that the main carbon source was algae (Fig. 3). In the glacial age sediments Poaceae pollen constitutes about 60% of the local pollen sum (Fig. 6), remaining high until about 8300 cal. yr BP. Cyperaceae pollen is much lower in abundance (V10% of the local pollen sum) while Ericaceous Belt and Afroalpine pollen form about 15% each. This suggests that the vegetation around Lake Rutundu during this time was dominated by grasses and was structurally similar to that of the modern Afroalpine Belt. However, as mentioned above, pollen analysis cannot distinguish between the C3 and C4 photosynthetic pathways. Terrestrial higher plant leaf waxes contain abundant long-chain n-alkyl lipids with high odd/even or even/odd predominance (Eglinton and Calvin, 1967; Kolattukudy et al., 1976). In Lake Rutundu, the lipid ¢ngerprints are dominated by the long-chain n-alkyl lipids, especially the n-alkanols and n-alkanoic acids, thus implying a terrestrial higher plant source. While no speci¢c biomarker has yet been identi¢ed in grasses, the C26 n-alkanol has been found to be particularly abundant in several species (Tulloch, 1976; Bull et
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Fig. 5. Histograms showing the distribution patterns of n-alkanes, n-alkanols and n-alkanoic acids in ¢ve representative samples. Concentrations (y-axis) are in Wg/g dry weight of sediment.
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al., 2000). The C26 n-alkanol is found in high concentrations in many of the alcohol fractions from Lake Rutundu and so probably re£ects a large contribution of grass leaf wax to the lake. However, other terrestrial plants also produce this n-alkanol and more work needs to be undertaken on the lipid geochemistry of grasses. n-Alkanols with chain lengths in the C22 ^C26 range have also been identi¢ed in some freshwater algae (Eustigmatos and Vischeria) and cyanobacteria (Anabaena) (Volkman et al., 1999 and references therein); hence, an algal source of the C26 n-alkanol cannot be ruled out. However, the freshwater eustigmatophytes mentioned above showed a strong predominance of the C22 n-alkanol with minor amounts of the C24 and C26 n-alkanols, although the C26:1 n-alkenol was a major component. Given the high concentrations of the C26 and C28 n-alkanols (Fig. 5) in the Lake Rutundu sediments it seems unlikely that algae were the source of these compounds. Moreover, although Anabaena is fairly ubiquitous and its presence in Lake Rutundu cannot be ruled out it tends to prefer more eutrophic waters. The unique co-occurring C26 diols and triols found in Anabaena (Volkman et al., 1999) were not present in Lake Rutundu, con¢rming that this cyanobacterium was not a major source of the C26 n-alkanol. The short-chain nalkanoic acids (C16 and C18 ) are proportionally more common than the long-chain components (Fig. 5) in the glacial-age sediments. These short-chain n-alkanoic acids are found in abundance in algae and thus support the inference from the C/N ratios that the TOC was mostly derived from algae. The relatively low in£ux of long-chain length n-alkyl lipids throughout the glacial suggests reduced terrestrial plant productivity during this period (Fig. 4). Compound-speci¢c isotope values of the terrestrial biomarkers from the glacial-age sediments exhibit much heavier N13 C values than those of the late-glacial/Holocene muds (Fig. 6). This is especially seen in the n-alkanols of glacial age that exhibit values of up to approximately 315x. This implies that the vegetation surrounding the lake was mostly composed of C4 plants. Since the pollen record indicates an abundance of grass pollen during this time we infer
that the vegetation surrounding Lake Rutundu consisted largely of C4 grasses. Grass cuticle analysis enables the di¡erent C4 grass subtypes to be identi¢ed, thereby enhancing the palaeoenvironmental reconstruction. During the glacial, charred cuticle fragments of tussockforming panicoid C4 grasses, notably Themeda triandra and Brachiaria sp., predominated. This con¢rms the inference made from the pollen and compound-speci¢c isotope analyses that C4 grasses were present around Lake Rutundu. Fig. 6 shows that of the C4 grasses, the typical arid-zone taxa (the NAD-ME subtype) are rare throughout the core. However, mesic grasses using the NADP-ME C4 subpathway are important throughout (Fig. 6). The prevalence of the NADP-ME grasses suggests that signi¢cant precipitation occurred during a warm growing season (Hattersley, 1992), which implies that although the glacial environment was generally drier than today, moisture was still available. 4.2. The late glacial/Holocene TOC increases rapidly in the late-glacial/Holocene sediments (Fig. 2) indicating a greater input of carbon into the lake than during glacial times. The C/N ratios are also much higher in the lateglacial/Holocene muds than in the glacial-age sediments, suggesting a greater input of terrestrial organic matter. The high C/N ratios in this case cannot be attributed to Botryococcus braunii as this alga was not found in the pollen preparations. During the late glacial/Holocene, both the Ericaceous Belt pollen in£ux and percentage increased, implying that the vegetation consisted of tall, subalpine shrubs with a grassy understorey (Figs. 4 and 6). Poaceae pollen remained relatively high until 8300 cal. yr BP, when it decreased to about 30% of the local pollen sum (Fig. 6). Both Cyperaceae and Afroalpine pollen percentages also decreased during the late glacial/Holocene. As mentioned earlier, lipid geochemistry cannot readily distinguish between di¡erent terrestrial higher plant sources. The n-alkyl lipids are dominated by long-chain components throughout the core, suggesting a terrestrial input to the lake
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Fig. 6. Stratigraphical changes in the percentage composition of macroscopic ( s 180 Wm) C3 and C4 grass cuticles and major pollen groups, and in the weighted average isotopic values of the terrestrial biomarkers, in Lake Rutundu sediments.
throughout its history. However, the lipid concentrations are much greater in the late-glacial/Holocene sediments than in those of glacial age (Fig. 5); the in£ux of terrestrial plant biomarkers also rises signi¢cantly (Fig. 4). The increase in the in£uxes of TOC and terrestrial biomarkers (Fig. 4) was probably due to a greater input of terrestrial plant detritus, suggesting that the terrestrial primary productivity increased during the late glacial/Holocene. The weighted average N13 C values of the terrestrial biomarkers became much lighter during the late glacial/Holocene, averaging around 327x in contrast to values of 315x during glacial times (Fig. 6). This implies that there was a higher representation of C3 plants in the late glacial/Ho-
locene. The pollen data support this inference as Ericaceous Belt shrubs, whose pollen becomes abundant during this period, use the C3 photosynthetic pathway (Swain, 1999). In addition, the C26 n-alkanol, a major constituent of grass leaf waxes, became isotopically lighter during the late glacial/Holocene, suggesting that the grasses mostly used the C3 photosynthetic pathway. Grass cuticle analysis con¢rms that the grass £ora became dominated by pooid taxa, using the C3 photosynthetic pathway, during the late glacial/Holocene (Wooller, 2000). However, some C4 grasses belonging to the NADP-ME subtype were still present in the lake catchment throughout the Holocene (Fig. 6).
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This shift from C4 grasses that possess a CO2 concentrating mechanism to C3 subalpine shrubs and pooid grasses during the late glacial/Holocene can be attributed to increasing atmospheric pCO2 and precipitation, which overwhelmed the e¡ects of a modest climate warming. Hostetler and Clark (2000) inferred from glacier mass-balance modelling that air temperatures were 3.5^6.6³C lower than today and precipitation 25% lower on Mt Kenya at the last glacial maximum. As the reduced concentration of CO2 was global in extent (Barnola et al., 1987), we attribute the dominance of C4 grasses on Mt Kenya during the last glacial to a combination of lower precipitation and lower pCO2 , although the relative importance of these factors cannot be assessed without detailed modelling studies. 5. Conclusions By combining three independent methods (pollen analysis, lipid and isotope geochemistry and grass cuticle analysis) we have provided supportive, complementary information about the terrestrial palaeoenvironment. During the last glacial, terrestrial primary productivity was lower than today. C4 mesic grasses and sedges outcompeted C3 plants due to lower atmospheric pCO2 and precipitation. During the late glacial and Holocene, higher pCO2 and rainfall led to increased terrestrial primary productivity and the spread of C3 plants, especially tall subalpine shrubs. Acknowledgements We thank the Office of the President, Nairobi, for research permission to work on Mt Kenya. Financial support was provided by the NERC (GR3/9523) and a NERC studentship (M.J.W.). AMS 14 C dates were funded by the NERC Radiocarbon Dating Committee (708/0997). Mr R.A. Perrott is thanked for his logistical support. Dr A. Agnew was very generous with his expertise on the identification of Mt Kenyan grasses and sedges. Dr R. Evershed, Mr J. Carter and Mr A. Gledhill are gratefully acknowledged for access to
the NERC GC-MS and GC-IRMS facilities (GR3/ 2951, GR3/3758, GR3/7731) at the University of Bristol. The authors would like to thank the three reviewers for their constructive comments.
References Bard, E., Rostek, F., Sonzogni, C., 1997. Interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry. Nature 385, 707^710. Barnola, J.M., Raynaud, D., Korotkevich, Y.S., Lorius, C., 1987. Vostok ice core provides 160,000 year record of atmospheric CO2 . Nature 329, 408^414. Berglund, B.A., Ralska-Jasiewiczowa, M., 1986. Pollen and pollen diagrams. In: Berglund, B.E. (Ed.), Handbook of Holocene Palaeoecology and Palaeohydrology. John Wiley and Sons, Chichester, pp. 455^484. Birks, H.H., 1973. Modern macrofossil assemblages in lake sediments in Minnesota. In: Birks, H.H., West, R.G. (Eds.), Quaternary Plant Ecology. Blackwell Scienti¢c Publications, Oxford, pp. 173^189. Bull, I.D., van Bergen, P.F., Nott, C.J., Poulton, P.R., Evershed, R.P., 2000. Organic geochemical studies of soils from the Rothamsted classical experiments ^ V. The fate of lipids in di¡erent long term experiments.. Org. Geochem. 31, 389^408. Cerling, T.E., Ehleringer, J.R., Harris, J.M., 1998. Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution. Phil. Trans. R. Soc. London B 353, 159^171. Chaloner, W.G., Creber, G.T., 1990. Do fossil plants give a climatic signal? J. Geol. Soc. London 147, 343^350. Coe, M.J., 1967. The Ecology of the Alpine Zone of Mount Kenya. Dr W. Junk, The Hague. Coetzee, J.A., 1967. Pollen analytical studies in East Africa. Palaeoecol. Afr. 3, 1^146. Eglinton, G., Calvin, M., 1967. Chemical fossils. Sci. Am. 216, 32^43. Ehleringer, J.R., Cerling, T.E., Helliker, B.R., 1997. C4 photosynthesis, atmospheric CO2 and climate. Oecologia 112, 285^299. Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis, 4th edn. John Wiley and Sons, Chichester. Ficken, K.J., Street-Perrott, F.A., Perrott, R.A., Swain, D.L., Olago, D.O., Eglinton, G., 1998. Glacial/interglacial variations in carbon cycling revealed by molecular and isotope stratigraphy of Lake Nkunga, Mt Kenya, East Africa. Org. Geochem. 29, 1701^1719. Hattersley, P.W., 1992. C4 photosynthetic pathway variation in grasses (Poaceae): its signi¢cance for arid and semi-arid lands. In: Chapman, G.P. (Ed.), Deserti¢ed Grasslands: Their Biology and Management. Academic Press, London, pp. 181^212. Hostetler, S.W., Clark, P.U., 2000. Tropical climate at the last
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K.J. Ficken et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 177 (2002) 137^149 glacial maximum inferred from glacier mass-balance modeling. Science 290, 1747^1750. Huang, Y., Street-Perrott, F.A., Perrott, R.A., Harkness, D.D., Olago, D., Eglinton, G., 1995. Molecular and carbon isotopic stratigraphy of sediments from the last glacial/interglacial sequence of a tropical freshwater lake, Sacred Lake, Mt Kenya. In: Grimalt, J.O., Doronsoro, C. (Eds.), Organic Geochemistry: Developments and Applications to Energy, Climate, Environment and Human History. Pergamon, Oxford, pp. 826^829. Ibrahim, K.M., Kabuye, C.H.S., 1987. An Illustrated Manual of Kenya Grasses. Food and Agriculture Organisation of the United Nations, Rome. Jolly, D., Haxeltine, A., 1997. E¡ect of low glacial atmospheric CO2 on tropical African montane vegetation. Science 276, 786^788. Kolattukudy, P.E., Croteau, R., Buckner, J.S., 1976. Biochemistry of plant waxes. In: Kolattukudy, P.E. (Ed.), Chemistry and Biochemistry of Plant Waxes. Elsevier, Amsterdam, pp. 289^347. Livingstone, D.A., Clayton, W.D., 1980. An altitudinal cline in tropical African grass £oras and its paleoecological signi¢cance. Quat. Res. 13, 392^402. Meyers, P.A., Lallier-Verge©s, E., 1999. Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates. J. Paleolimnol. 21, 345^372. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. An Illustrated Guide to Pollen Analysis. Blackwell Scienti¢c Publications, Oxford. Palmer, P.G., Gerbeth-Jones, S., 1988. A Scanning Electron Microscope Survey of the Epidermis of East African Grasses, V, and West African Supplement. Smithsonian Institution Press, Washington, DC. Palmer, P.G., Tucker, A.E., 1981. A Scanning Electron Microscope Survey of the Epidermis of East African Grasses, I. Smithsonian Institution Press, Washington, DC. Palmer, P.G., Tucker, A.E., 1983. A Scanning Electron Microscope Survey of the Epidermis of East African Grasses, II. Smithsonian Institution Press, Washington, DC. Palmer, P.G., Gerbeth-Jones, S., Hutchison, S., 1985. A Scanning Electron Microscope Survey of the Epidermis of East African Grasses, III. Smithsonian Institution Press, Washington, DC. Palmer, P.G., Gerbeth-Jones, S., Hutchison, S., 1986. A Scanning Electron Microscope Survey of the Epidermis of East African Grasses, IV. Smithsonian Institution Press, Washington, DC. Street-Perrott, F.A., Huang, Y., Perrott, R.A., Eglinton, G., Barker, P., Khelifa, L.B., Harkness, D.D., Ivanovich, M., Olago, D.O., 1997. Impact of lower atmospheric CO2 on tropical mountain ecosystems: carbon-isotope evidence. Science 278, 1422^1426.
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Stuiver, M., Reimer, P.J., 1993. Extended C-14 data-base and revised Calib 3.0 C-14 age calibration program. Radiocarbon 35, 215^230. Swain, D.L., 1999. Late Quaternary Palaeoecology of Mount Kenya, East Africa: Investigating the Potential Impact of Sub-ambient CO2 Concentration on the Distribution of Montane Vegetation. Ph.D. Thesis, University of Wales, Swansea. Talbot, M.R., Johannessen, T., 1992. A high resolution palaeoclimate record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of organic matter. Earth Planet. Sci. Lett. 110, 23^37. Teeri, J.A., Stowe, L.G., 1976. Climatic patterns and the distribution of C4 grasses in North America. Oecologia 23, 1^ 12. Teeri, J.A., Stowe, L.G., Livingstone, D.A., 1980. The distribution of C4 species of the Cyperaceae in North America in relation to climate. Oecologia 47, 307^310. Thompson, B.W., 1966. The mean annual rainfall of Mt Kenya. Weather 21, 48^49. Tulloch, A.P., 1976. Chemistry of waxes of higher plants. In: Kolattukudy, P.E. (Ed.), Chemistry and Biochemistry of Plant Waxes. Elsevier, Amsterdam, pp. 236^287. Volkman, J.K., Barrett, S.M., Blackburn, S.I., 1999. Eustigmatophyte microalgae are potential sources of C29 sterols, C22 C28 n-alcohols and C28 -C32 n-alkyl diols in freshwater environments. Organic Geochemistry 30, 307^318. Watson, L., Dallwitz, M.J., 1988. Grass Genera of the World: Illustrations of Characters, Descriptions, Classi¢cation, Interactive Identi¢cation, Information Retrieval. Australian National University, Research School of Biological Sciences, Canberra. Watson, L., Dallwitz, M.J., 1994. The Grass Genera of the World. CAB International, Cambridge. Wooller M.J., 2002. Fossil grass cuticles from lacustrine sediments: a review of methods applicable to the analysis of tropical African lake cores. Holocene 12, 107^115. Wooller, M.J., 2000. The Palaeoecology of Mount Kenya: Evidence from Grass Cuticle Analysis. Ph.D. Thesis, University of Wales, Swansea. Wooller, M.J., Street-Perrott, F.A., Agnew, A.D.Q., 2000. Late Quaternary ¢res and palaeoecology of Mount Kenya, East Africa: evidence from charred grass cuticles in lake sediments. Palaeogeogr. Palaeoclimatol. Palaeoecol. 164, 207^230. Wright, H.E. Jr., Kutzbach, J.E., Webb, T. III, Ruddiman, W.F., Street-Perrott, F.A., Bartlein, P.J., 1993. Global Climates since the Last Glacial Maximum. University of Minnesota Press, Minneapolis, MN.
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Reconstruction of a subalpine grass-dominated ecosystem, Lake Rutundu, Mount Kenya: a novel multi-proxy approach K.J. Ficken a;b; *, M.J. Wooller a;1 , D.L. Swain a;2 , F.A. Street-Perrott a , G. Eglinton b a
b
Tropical Palaeoenvironments Research Group, Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK Biogeochemistry Research Centre, Department of Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Accepted 16 August 2001
Abstract Palaeoecological reconstructions based on a single proxy are limited, but by combining pollen, biogeochemistry and grass cuticle analysis, ecosystem structure and function can be better understood. Lake Rutundu is a small, subalpine lake on the northeast flank of Mt Kenya. During the last glacial, pollen evidence suggests a shrub grassland dominated by Afroalpine taxa and Poaceae, representing a dry, cold, open environment. The N13 C values of terrestrial biomarkers imply a high proportion of C4 plants. Grass cuticle analysis allows resolution of the different C4 subtypes and shows that the vegetation was dominated by tall C4 panicoid grasses, prone to frequent fires. During the Holocene, Poaceae pollen declined while subalpine shrubs increased. The N13 C values of terrestrial biomarkers imply a C3 dominated vegetation. Together with an expansion of rainforest at lower altitudes, this suggests wetter conditions more favourable to C3 plants. Increased percentages of C3 pooid grass cuticles confirm a reduction in moisture stress. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: pollen; compound-speci¢c carbon isotopes; grass cuticles; Mt Kenya; Lake Rutundu; palaeoecology
1. Introduction The family Poaceae (grasses) contains a diverse array of species ( s 10 000 species and 655 genera)
1 Present address: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW Washington, DC 20015, USA. 2 Present address: Scottish Agricultural College, Crichton Royal Farm, Mid Park, Bankend Road, Dumfries DG1 4SZ, UK. * Corresponding author. Fax: +44^1792^295-955. E-mail address: gg¢[email protected] (K.J. Ficken).
(Watson and Dallwitz, 1994) whose distribution ranges from the tropics to the Arctic. This diversity includes important morphological, phenological and physiological variability. Biomes dominated by grasses cover a substantial part of the Earth's surface and as highly productive ecosystems, play a vital role in modern biogeochemical cycles (Chaloner and Creber, 1990). Grasses serve as particularly good indicators of past climates as they generally have short life cycles (relative to woody perennial trees and shrubs) and can adapt quickly to environmental change. It has been hypothesised (Jolly and Haxeltine,
0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 3 5 6 - X
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1997; Street-Perrott et al., 1997; Cerling et al., 1998) that the climate of the last glacial favoured C4 grasses in the tropics as the climate of many areas was signi¢cantly drier than today (Wright et al., 1993; Hostetler and Clark, 2000) and atmospheric pCO2 was reduced to about 190^200 Watm (Barnola et al., 1987). At the ecosystem level, the variations in temperature, rainfall and ambient CO2 /O2 ratio would have altered the competitive balance between plants using the C3 and C4 photosynthetic pathways. C4 and CAM plants are more CO2 - and water-e¤cient than C3 plants (Ehleringer et al., 1997) and hence would have had a selective advantage with lower pCO2 and widespread aridity. The distribution of C3 and C4 grasses provides an important climatic indicator (Teeri and Stowe, 1976; Livingstone and Clayton, 1980; Teeri et al., 1980; Hattersley, 1992). Therefore a fuller understanding of the contribution of C3 and C4 grasses to terrestrial biomes in the past will enhance palaeoclimatic reconstructions. Lake sediments receive allochthonous material from a variety of plant sources, including pollen, leaf waxes and plant cuticles, which are often suf¢ciently well preserved to act as indicators of past vegetation. Any palaeoenvironmental reconstruction using a single method, such as pollen analysis, is restricted by the limitations of that proxy. Palaeoenvironmental reconstructions using pollen provide a broad understanding of changes in vegetation communities. Distinctions can be made between trees, shrubs and herbs as well as between individual taxa within these groups. However, in the reconstruction of palaeograsslands, pollen analysis is limited in its use as Poaceae and most Cyperaceae (sedges) cannot be identi¢ed below family level (Moore et al., 1991). N13 C measurements on bulk organic matter (TOC) are also regularly used in palaeoenvironmental reconstructions along with C/N ratios (Talbot and Johannessen, 1992; Meyers and Lallier-Verge©s, 1999). However, in lake sediments, organic matter is derived from multiple sources; by using N13 CTOC values alone it is impossible to say whether algae, aquatic macrophytes or terrestrial plants were driving the isotopic signal. In lakes, the C/N ratio must also be used with caution as cells of the green alga Botryococcus braunii have very high
C/N ratios (Huang et al., 1995) and can be misinterpreted as originating from terrestrial plant detritus. Lipid analysis, however, can address the problems of interpreting these bulk parameters. Individual, or groups of, organic compounds can be assigned to speci¢c sources and are termed biomarkers or chemical fossils (Eglinton and Calvin, 1967). Although the data base of distributions of individual homologues and of classes of compounds within contributing plant species is presently very limited, the long-chain n-alkyl lipids ( v C26 ) have been shown to be derived from terrestrial higher plant leaf waxes (Eglinton and Calvin, 1967; Tulloch, 1976; Kolattukudy et al., 1976). The recent development of compound-speci¢c carbon isotope analysis enables the N13 C values of individual biomarkers such as terrestrial plant lipids or algal lipids to be measured. Analysis of grass cuticles enhances the palaeoenvironmental reconstruction further as the di¡erent C4 grass subtypes can be identi¢ed. The assumption that C4 grasses always indicate arid conditions was challenged by Hattersley (1992) when the C4 photosynthetic pathway was classi¢ed into subtypes. The C4 subtype NADP-ME is associated with more mesic conditions and has the ability to function at cooler temperatures than the NAD-ME C4 subtype. Therefore, by combining pollen, compound-speci¢c N13 C and grass cuticle analyses much more information can be obtained about the terrestrial plant community. In this paper, we combine these three independent but complementary methods to reconstruct the terrestrial environment surrounding Lake Rutundu, Mt Kenya over the last 38 300 cal. yr BP. This is, to the best of our knowledge, the ¢rst lake where these three methods have been employed together. 1.1. Modern vegetation On Mt Kenya, the present-day vegetation shows marked altitudinal zonation (Fig. 1). The basal plateau zone (disturbed/savanna, Fig. 1) is dominated by Themeda triandra (C4 ) and other panicoid grasses (mainly C4 ). This grassland grades up into the Montane Rainforest Belt (wet and dry montane forest; C3 ), which forms a horseshoe occupying the wettest areas and con-
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Fig. 1. Location map of Mount Kenya showing Lake Rutundu and the main vegetation belts.
tains sparse panicoid (C3 /C4 ), arundinoid (C3 ) and bambusoid (C3 ) grasses, together with sedges (C3 /C4 ). A gap in the Montane Rainforest Belt occurs on the dry northern £ank of Mt Kenya and is dominated by grasses found in the basal plateau zone (Fig. 1). Above treeline (2900^3400m asl) lies the Ericaceous Belt, characterised by tall,
woody, microphyllous, subalpine shrubs (C3 ) and pooid (C3 ) grasses. This grades up into the Afroalpine Belt, which is occupied by C3 pooid tussock grasses and low shrubs with some C3 sedges. The Nival Zone consists mainly of bare rock and ice, but isolated C3 pooid grasses are present in very sheltered areas.
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1.2. Modern climate Mt Kenya receives most of its rainfall from the southeast monsoon during March to June (long rains) and October to November (short rains), and as a consequence, its southeastern £ank receives the most rainfall ( s 2500 mm/yr at 2000^ 3000 m) and its northern £ank is the driest ( 6 1500 mm/yr; Thompson, 1966). The mountain does not experience marked seasonal variations in temperature due to its location on the Equator, but does exhibit a large diurnal temperature range (V14³C at treeline) (Coe, 1967). 1.3. Lake Rutundu Lake Rutundu is a volcanic crater lake located at 3078 m asl in the Ericaceous Belt on the northeast £ank of Mt Kenya (Fig. 1). The lake is small (0.4 km2 ) and oligotrophic, with a maximum depth of 11 m, and is situated V90 m above treeline in vegetation dominated by C3 woody, subalpine shrubs and grasses, almost all of which use the C3 photosynthetic pathway. Lake Rutundu was produced by a single event eruption and exhibits no hot spring or fumarolic activity. The lake was ¢rst cored by Coetzee and Van Zinderen Bakker (Coetzee, 1967). 2. Methodology 2.1. Bulk sediment parameters A 7.55 m long core of carbonate-free, diatomaceous lake sediment was collected from the central, deepest part of Lake Rutundu in 1996 using a modi¢ed Livingstone piston corer. The cores were wrapped in plasticiser-free cling¢lm, aluminium foil and plastic sheeting and returned to the UK in sealed plastic tubes. The core was scanned for volume magnetic susceptibility, described and sectioned at 1 cm intervals and stored in sealed Sterilin petri dishes at 4³C before analysis. The TOC and nitrogen contents were measured on replicate subsamples using an elemental analyser (EA). N13 C was measured on TOC by EA^IRMS (elemental analyser^isotope ratio mass spectrom-
Fig. 2. Age^depth relationship (squares represent rejected dates) and accumulation rate of Lake Rutundu sediments.
etry) at approximately 30-cm intervals. Sixteen AMS 14 C dates were measured on the TOC, although three dates were rejected due to suspected contamination by more recent carbon (Fig. 2). The remaining 13 dates were calibrated to give calendar ages using CALIB3.0 (Stuiver and Reimer, 1993) for those dates in the range 0^18 000 14 C yr BP and from the equation of Bard et al. (1997) for those older than 18 000 14 C yr BP. The dates show that the core provides a continuous record covering the last glacial/interglacial transition and the whole of the Holocene. The extrapolated basal age is 38 300 cal. yr BP (Fig. 2). The calculated sediment in£ux in mg/ cm2 /yr is the product of sediment accumulation rate (Fig. 2) and the dry bulk density. TOC in£uxes in mg/cm2 /yr were calculated from the TOC content, bulk density and the sediment accumulation rate.
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Fig. 3. Lithology and bulk sediment properties of the Lake Rutundu core.
2.2. Pollen Pollen grains were separated from the sediment, identi¢ed and counted following the standard technique of Faegri and Iversen (1989). The samples were counted in a random, non-stratigraphical order to avoid any bias due to preconceived changes in the pollen spectra. All pollen, spores and green algae were identi¢ed to the lowest possible taxonomic level using reference material; the degree of reliability is indicated by standard conventions (Birks, 1973; Berglund and Ralska-Jasiewiczowa, 1986). Pollen in£ux was calculated by adding a known quantity of an exotic marker (Eucalyptus pollen grains) to a known volume of sediment and then counting the numbers of exotic markers against the total number of pollen grains (300 counted). Pollen concentrations were combined with the age^depth curve to derive the pollen in£ux in grains/cm2 /yr. The local pollen sum was calculated using Poaceae, Cyperaceae, Ericacaeous Belt (Ericaceae type, Protea, Cli¡ortia nitidula, Stoebe kilimandscharica, Alchemilla and Anthospermum cf. usambarense) and Afroalpine
Belt (Compositae, Artemisia afra and Myosotis) pollen only. Percentages of the local pollen sum were calculated for Poaceae, Cyperaceae, Ericacaeous Belt and Afroalpine Belt pollen. 2.3. Biogeochemistry The methods used are described in detail by Ficken et al. (1998). Twenty-one 1 cm-long core segments (the outside of the core segment was removed to prevent contamination from smearing), representing major shifts in the N13 CTOC curve, were selected and the powdered, freezedried sediment extracted by sonication and centrifugation using a solvent system of sequentially decreasing polarity (3U100% MeOH, 2U1:1 MeOH:DCM and 5U100% DCM). The total extracts were each split into an acid and a neutral fraction by solid phase extraction (Aminopropyl Bond Elute). The neutral fraction was fractionated further into a hydrocarbon fraction, an alcohol fraction and a polar fraction by thin layer chromatography (silica gel 60, 0.25 mm thick). The hydrocarbon fraction was further fractionated
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into adduct and non-adduct fractions by urea adduction. The acid fraction was methylated with methanolic HCl and the alcohol fraction derivatised by bis(trimethylsilyl)-tri£uoroacetamide prior to analysis by gas chromatography (GC). The compounds were quanti¢ed using GC and identi¢ed by gas chromatography^mass spectrometry (GC^MS). In£uxes of the terrestrial biomarker lipids in Wg/cm2 /yr were calculated in a similar way to that of the TOC. 2.3.1. Lipid analyses GC analyses of all fractions were carried out on a Varian 3400 GC ¢tted with an on-column injector and a £ame ionisation detector. A CPsil5CB (Chrompack) fused silica capillary column (50 mU0.32 mm; 0.17 Wm ¢lm thickness) was used. The oven temperature was held at 60³C for 1 min, ramped at 10³C/min to 180³C and then ramped at 4³C/min to 300³C, where it was held for 25 min. The carrier gas was hydrogen. GC^MS analyses of representative samples were performed on a Carlo Erba Mega gas chromatograph (on-column injection, 70 eV EI) interfaced directly with a Finnigan 4500 mass spectrometer in order to identify the lipid components. The column and temperature programme were the same as for the GC analyses. Helium was used as the carrier gas. 2.3.2. Compound-speci¢c carbon isotope analyses The carbon isotope measurements of the individual n-alkanes (adducted fraction), n-alkanols (alcohol fraction) and n-alkanoic acids (acid fraction) were performed using a Varian 3400 GC attached to a Finnigan MAT Delta-S isotope ratio mass spectrometer via a combustion interface consisting of an alumina reactor (0.5 mm i.d.) containing copper and platinum wires (0.1 mm o.d.). Again, the column and the temperature programme were the same as for the GC analyses. All carbon isotopic ratios are expressed in x relative to the Pee Dee Belemnite standard. 2.4. Grass cuticles The methods used to extract grass cuticles are outlined by Wooller et al. (2000) and Wooller
(2002). Brie£y, a standard palynological method was applied to remove humic compounds (Moore et al., 1991); the wet sediment was then sieved through a 180-Wm sieve. The s 180 Wm fraction was mounted onto a microscope slide. Entire slides were rapidly scanned in systematic traverses at 100U magni¢cation. Morphological features were examined at 600^1000U magni¢cation. Cuticle fragments were grouped into morphotypes according to common micromorphological features described by Watson and Dallwitz (1988, 1994). Features recorded for each morphotype were entered into the African subset of the `Grass Genera of the World' data base (Watson and Dallwitz, 1988, 1994). The resulting list of possible genera was pruned to include only those recorded for upland Kenya (Ibrahim and Kabuye, 1987; Agnew, unpublished data). The subfossil cuticles were compared with type slides prepared from modern herbarium specimens and with scanning electron photomicrographs (Palmer and Tucker, 1981, 1983; Palmer et al., 1985, 1986; Palmer and Gerbeth-Jones, 1988). 3. Results 3.1. Bulk parameters The sediments consist of silty, diatomaceous lake mud below 400 cm, with a layer of ¢nely bedded sand at 509^517 cm (Fig. 3). Between 408 and 318 cm the sediments consist of organic diatomaceous lake mud with layers of ¢ne sand and plant detritus. Above 318 cm, the sediment is an organic, diatomaceous mud with another sand layer at 308^310 cm. A signi¢cant shift in bulk sediment properties occurs at about 318 cm, which is dated at approximately 14 300 cal. yr BP. As a result, this paper will refer to the sediments below 318 cm as being of glacial age and those above as late glacial/Holocene in age. TOC contents are low in the glacial sediments ( 6 5%) and increase in the late-glacial/Holocene muds to 15^20% (Fig. 3). The calculated in£ux of TOC was also much greater during the late glacial/Holocene than during glacial times (Fig. 4). C/N ratios range between 3 and 8 in the glacial-
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Fig. 4. Lithology, chronology and in£uxes of bulk sedimentary properties, terrestrial biomarkers, grass cuticles and pollen in the Lake Rutundu core.
age sediments and then rise sharply in the late glacial/Holocene muds to values between 10 and 20 (Fig. 3). The Lake Rutundu sequence shows variations in N13 CTOC with an amplitude of approximately 10x. The heaviest N13 CTOC values are found in the glacial-age sediments (e.g. 318.5x at V27 400 cal. yr BP) and the lightest values in the late-glacial/Holocene sediments (e.g. 328.2x at V8100 cal. yr BP) (Fig. 3). 3.2. Pollen Pollen deposition at Rutundu re£ects a combination of local and long-range transport (Swain, 1999). Although the sediments contain a signi¢cant amount of forest-derived pollen, most of this forest pollen was was dominated by highly transportable types, such as Podocarpus (Swain, 1999). Hence, it is unlikely that there has ever been any signi¢cant forest vegetation around the lake.
Of the pollen types with local sources, Poaceae and Cyperaceae dominated the glacial-age sediments and their in£ux into Lake Rutundu remained relatively high until approximately 8300 cal. yr BP (Fig. 4). Afroalpine pollen types also exhibited higher in£uxes during glacial times (Fig. 4). In contrast, the total in£ux of Ericaceous Belt taxa, including undi¡erentiated Ericaceae, Cli¡ortia and Proteaceae, increased substantially during the late glacial and Holocene (Fig. 4). 3.3. Biogeochemistry The n-alkyl lipid (n-alkane, n-alkanol and n-alkanoic acid) concentrations are greater in the lateglacial/Holocene sediments than in those of glacial age by an order of magnitude (Fig. 5). The nalkanes are the least abundant, being an order of magnitude lower in concentration than the n-alkanols and n-alkanoic acids throughout the core.
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The n-alkyl lipids are mostly dominated by longchain components (vC26 ) (Ficken, unpublished data), which represent terrestrial higher plant leaf waxes (Eglinton and Calvin, 1967) (Fig. 5). The chain length of the n-alkanes ranges from C19 to C35 with chain length maxima varying between C25 and C31 . There is a strong odd/even predominance (odd-numbered carbon chain lengths are found in greater abundance than even-numbered carbon chain lengths) in the majority of the samples, as expected for terrestrial higher plants. With a few exceptions, the n-alkanols range from C20 to C32 with chain length maxima at C26 or C28 . They show a strong even/odd predominance (evennumbered chain lengths in greater abundance than the odd-numbered carbon chain lengths), compatible with a higher plant origin. The distributions of the n-alkanoic acids range from C16 to C32 with chain length maxima at C18 or C28 . Like the n-alkanols, the n-alkanoic acids display a strong even/odd predominance. The short-chain n-alkanoic acids (C16 , C18 ) are found in relatively greater concentrations in the surface muds and in the glacial-age sediments (Fig. 5). The in£ux of terrestrial biomarkers (C27 ^C33 odd carbon numbered n-alkanes; C26 ^C32 even carbon numbered n-alkanols and n-alkanoic acids) is greater in the late-glacial/Holocene muds than in the glacial-age sediments (Fig. 4). The weighted average N13 C values (calculated using the concentration of each compound so that those compounds contributing more to the organic carbon contribute more to the average isotopic signature) of the long-chain n-alkanes range from 324.8x in the glacial-age muds to 331.1x in the late glacial/Holocene. However, the n-alkanols and n-alkanoic acids exhibit a much larger range of weighted average N13 C values for the long-chain homologues, ranging from 316.2x and 319.3x in the glacial-age sediments to 332.6x and 332.4x in the late glacial/Holocene, respectively (Fig. 6). In the glacialage sediments below 318 cm (V14 300 cal. yr BP) the mean weighted average isotopic values of the terrestrial biomarkers are 327.5x, 320.8x and 323.5x for the n-alkanes, n-alkanols and n-alkanoic acids respectively and are indicative of C4 plants, whereas in the late glacial/Holocene the
averages are lighter (more negative), indicative of C3 plants (Fig. 5). 3.4. Grass cuticle analysis The in£ux of graminoid cuticle was relatively high in the glacial, but reached a maximum around 8900 cal. yr BP (Fig. 4). Charred cuticle fragments of tussock-forming panicoid C4 grasses (notably Themeda triandra and Brachiaria sp.) are most frequent in the glacial-age sediments whereas in the late-glacial/Holocene muds, there is an increase in C3 pooid grass cuticles (Wooller, 2000). 4. Discussion 4.1. Glacial environment The TOC in£ux was low during the last glacial suggesting a reduced input of carbon into the lake at this time (Fig. 4). C/N ratios were also low, indicating that the main carbon source was algae (Fig. 3). In the glacial age sediments Poaceae pollen constitutes about 60% of the local pollen sum (Fig. 6), remaining high until about 8300 cal. yr BP. Cyperaceae pollen is much lower in abundance (V10% of the local pollen sum) while Ericaceous Belt and Afroalpine pollen form about 15% each. This suggests that the vegetation around Lake Rutundu during this time was dominated by grasses and was structurally similar to that of the modern Afroalpine Belt. However, as mentioned above, pollen analysis cannot distinguish between the C3 and C4 photosynthetic pathways. Terrestrial higher plant leaf waxes contain abundant long-chain n-alkyl lipids with high odd/even or even/odd predominance (Eglinton and Calvin, 1967; Kolattukudy et al., 1976). In Lake Rutundu, the lipid ¢ngerprints are dominated by the long-chain n-alkyl lipids, especially the n-alkanols and n-alkanoic acids, thus implying a terrestrial higher plant source. While no speci¢c biomarker has yet been identi¢ed in grasses, the C26 n-alkanol has been found to be particularly abundant in several species (Tulloch, 1976; Bull et
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145
Fig. 5. Histograms showing the distribution patterns of n-alkanes, n-alkanols and n-alkanoic acids in ¢ve representative samples. Concentrations (y-axis) are in Wg/g dry weight of sediment.
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al., 2000). The C26 n-alkanol is found in high concentrations in many of the alcohol fractions from Lake Rutundu and so probably re£ects a large contribution of grass leaf wax to the lake. However, other terrestrial plants also produce this n-alkanol and more work needs to be undertaken on the lipid geochemistry of grasses. n-Alkanols with chain lengths in the C22 ^C26 range have also been identi¢ed in some freshwater algae (Eustigmatos and Vischeria) and cyanobacteria (Anabaena) (Volkman et al., 1999 and references therein); hence, an algal source of the C26 n-alkanol cannot be ruled out. However, the freshwater eustigmatophytes mentioned above showed a strong predominance of the C22 n-alkanol with minor amounts of the C24 and C26 n-alkanols, although the C26:1 n-alkenol was a major component. Given the high concentrations of the C26 and C28 n-alkanols (Fig. 5) in the Lake Rutundu sediments it seems unlikely that algae were the source of these compounds. Moreover, although Anabaena is fairly ubiquitous and its presence in Lake Rutundu cannot be ruled out it tends to prefer more eutrophic waters. The unique co-occurring C26 diols and triols found in Anabaena (Volkman et al., 1999) were not present in Lake Rutundu, con¢rming that this cyanobacterium was not a major source of the C26 n-alkanol. The short-chain nalkanoic acids (C16 and C18 ) are proportionally more common than the long-chain components (Fig. 5) in the glacial-age sediments. These short-chain n-alkanoic acids are found in abundance in algae and thus support the inference from the C/N ratios that the TOC was mostly derived from algae. The relatively low in£ux of long-chain length n-alkyl lipids throughout the glacial suggests reduced terrestrial plant productivity during this period (Fig. 4). Compound-speci¢c isotope values of the terrestrial biomarkers from the glacial-age sediments exhibit much heavier N13 C values than those of the late-glacial/Holocene muds (Fig. 6). This is especially seen in the n-alkanols of glacial age that exhibit values of up to approximately 315x. This implies that the vegetation surrounding the lake was mostly composed of C4 plants. Since the pollen record indicates an abundance of grass pollen during this time we infer
that the vegetation surrounding Lake Rutundu consisted largely of C4 grasses. Grass cuticle analysis enables the di¡erent C4 grass subtypes to be identi¢ed, thereby enhancing the palaeoenvironmental reconstruction. During the glacial, charred cuticle fragments of tussockforming panicoid C4 grasses, notably Themeda triandra and Brachiaria sp., predominated. This con¢rms the inference made from the pollen and compound-speci¢c isotope analyses that C4 grasses were present around Lake Rutundu. Fig. 6 shows that of the C4 grasses, the typical arid-zone taxa (the NAD-ME subtype) are rare throughout the core. However, mesic grasses using the NADP-ME C4 subpathway are important throughout (Fig. 6). The prevalence of the NADP-ME grasses suggests that signi¢cant precipitation occurred during a warm growing season (Hattersley, 1992), which implies that although the glacial environment was generally drier than today, moisture was still available. 4.2. The late glacial/Holocene TOC increases rapidly in the late-glacial/Holocene sediments (Fig. 2) indicating a greater input of carbon into the lake than during glacial times. The C/N ratios are also much higher in the lateglacial/Holocene muds than in the glacial-age sediments, suggesting a greater input of terrestrial organic matter. The high C/N ratios in this case cannot be attributed to Botryococcus braunii as this alga was not found in the pollen preparations. During the late glacial/Holocene, both the Ericaceous Belt pollen in£ux and percentage increased, implying that the vegetation consisted of tall, subalpine shrubs with a grassy understorey (Figs. 4 and 6). Poaceae pollen remained relatively high until 8300 cal. yr BP, when it decreased to about 30% of the local pollen sum (Fig. 6). Both Cyperaceae and Afroalpine pollen percentages also decreased during the late glacial/Holocene. As mentioned earlier, lipid geochemistry cannot readily distinguish between di¡erent terrestrial higher plant sources. The n-alkyl lipids are dominated by long-chain components throughout the core, suggesting a terrestrial input to the lake
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147
Fig. 6. Stratigraphical changes in the percentage composition of macroscopic ( s 180 Wm) C3 and C4 grass cuticles and major pollen groups, and in the weighted average isotopic values of the terrestrial biomarkers, in Lake Rutundu sediments.
throughout its history. However, the lipid concentrations are much greater in the late-glacial/Holocene sediments than in those of glacial age (Fig. 5); the in£ux of terrestrial plant biomarkers also rises signi¢cantly (Fig. 4). The increase in the in£uxes of TOC and terrestrial biomarkers (Fig. 4) was probably due to a greater input of terrestrial plant detritus, suggesting that the terrestrial primary productivity increased during the late glacial/Holocene. The weighted average N13 C values of the terrestrial biomarkers became much lighter during the late glacial/Holocene, averaging around 327x in contrast to values of 315x during glacial times (Fig. 6). This implies that there was a higher representation of C3 plants in the late glacial/Ho-
locene. The pollen data support this inference as Ericaceous Belt shrubs, whose pollen becomes abundant during this period, use the C3 photosynthetic pathway (Swain, 1999). In addition, the C26 n-alkanol, a major constituent of grass leaf waxes, became isotopically lighter during the late glacial/Holocene, suggesting that the grasses mostly used the C3 photosynthetic pathway. Grass cuticle analysis con¢rms that the grass £ora became dominated by pooid taxa, using the C3 photosynthetic pathway, during the late glacial/Holocene (Wooller, 2000). However, some C4 grasses belonging to the NADP-ME subtype were still present in the lake catchment throughout the Holocene (Fig. 6).
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This shift from C4 grasses that possess a CO2 concentrating mechanism to C3 subalpine shrubs and pooid grasses during the late glacial/Holocene can be attributed to increasing atmospheric pCO2 and precipitation, which overwhelmed the e¡ects of a modest climate warming. Hostetler and Clark (2000) inferred from glacier mass-balance modelling that air temperatures were 3.5^6.6³C lower than today and precipitation 25% lower on Mt Kenya at the last glacial maximum. As the reduced concentration of CO2 was global in extent (Barnola et al., 1987), we attribute the dominance of C4 grasses on Mt Kenya during the last glacial to a combination of lower precipitation and lower pCO2 , although the relative importance of these factors cannot be assessed without detailed modelling studies. 5. Conclusions By combining three independent methods (pollen analysis, lipid and isotope geochemistry and grass cuticle analysis) we have provided supportive, complementary information about the terrestrial palaeoenvironment. During the last glacial, terrestrial primary productivity was lower than today. C4 mesic grasses and sedges outcompeted C3 plants due to lower atmospheric pCO2 and precipitation. During the late glacial and Holocene, higher pCO2 and rainfall led to increased terrestrial primary productivity and the spread of C3 plants, especially tall subalpine shrubs. Acknowledgements We thank the Office of the President, Nairobi, for research permission to work on Mt Kenya. Financial support was provided by the NERC (GR3/9523) and a NERC studentship (M.J.W.). AMS 14 C dates were funded by the NERC Radiocarbon Dating Committee (708/0997). Mr R.A. Perrott is thanked for his logistical support. Dr A. Agnew was very generous with his expertise on the identification of Mt Kenyan grasses and sedges. Dr R. Evershed, Mr J. Carter and Mr A. Gledhill are gratefully acknowledged for access to
the NERC GC-MS and GC-IRMS facilities (GR3/ 2951, GR3/3758, GR3/7731) at the University of Bristol. The authors would like to thank the three reviewers for their constructive comments.
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