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Long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa
Global and Planetary Change 26 Ž2000. 159–171 www.elsevier.comrlocatergloplacha
Long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa D.O. Olago a,) , F.A. Street-Perrott b, R.A. Perrott b, M. Ivanovich c , D.D. Harkness d , E.O. Odada a a Department of Geology, UniÕersity of Nairobi, P.O. Box 30197, Nairobi, Kenya Department of Geography, UniÕersity of Wales Swansea, Swansea SA2 8PP, Wales, UK Analytical Sciences Centre, Business DeÕelopment Office, Harwell Laboratory B7, Oxfordshire OX11 0RA, UK d NERC Radiocarbon Laboratory, NEL Technology Park, East Kilbride, Glasgow G75 0QU, UK b
c
Abstract In this paper we examine the long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa from a continuous lacustrine sequence retrieved from Sacred Lake, Mount Kenya Ž0803X N, 37832X E, 2350 m a.s.l.., covering the last interglacial–glacial transition to the present. The trends in mineral magnetics and stable carbon isotopes are proxy indicators of changes in precipitation on the mountain over the last glacial–interglacial cycle. Spectral analysis by a fast fourier transform method revealed that the stable carbon isotope trend Ž d13C. has strong signals at the 23,000 and 11,500 year frequencies. The mineral magnetic signature does not register the 23,000 year cycle observed in the d13C signature. It has, however, a strong signal at an 11,500 year frequency, and sharp but relatively weak peaks at ca. 7500 and 5000 year frequencies are recorded. The dominant 23,000 year frequency recorded in the d13C signature reflects the strong effect of the precessional cycle on tropical climate and ecosystems, and is most probably effected via global atmospheric pCO 2 and temperature changes. The shorter cycles at 11,500 year Žindicated by both mineral magnetics and d13 C trends., and 7500 and 5000 years BP Žapparent in the mineral magnetic record. are attributed to precipitation variations, whose temporal cycles are dominated by the higher precessional harmonics. q 2000 Elsevier Science B.V. All rights reserved. Keywords: paleomoonsoon; mineral magnetics; stable carbon isotopes; Milankovitch forcing
1. Introduction The glacial–interglacial sequences of climatic change are largely due to changes in solar insolation, driven by variations in the earth’s orbital parameters Že.g. Croll, 1864; Milankovitch, 1920; Berger, 1977, )
Corresponding author. Tel.: q254-2-447-740; fax: q254-2449-539. E-mail addresses: [email protected], [email protected] ŽD.O. Olago..
1979, 1980.. These include the variation in the earth’s orbit around the sun Žeccentricity., variations in the tilt Žobliquity. of the earth’s axis Žwhich varies from 21.8 to 24.48., and the precession of the equinoxes ŽImbrie and Imbrie, 1979.. These variations affect earth surface temperatures by altering latitudinal radiation receipts, and have strong cyclic signals at periods of ca. 95,800 years Žeccentricity., 41,000 years Žobliquity., and 23,000 years and 19,000 years Žprecession. ŽBerger, 1977; Imbrie and Imbrie, 1979.. Solar radiation receipts at low latitudes are mainly
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affected by variations in eccentricity and precession of the equinoxes, whereas higher latitudes are mainly affected by variations in obliquity. Spectral analysis of the polar ice core and deep sea records show cycles at orbital ŽMilankovitch. frequencies reflect-
ing the significant forcing effect of orbital variations on the earth’s climate Že.g. Pisias and Leinen, 1984; Imbrie et al., 1984; Pokras and Mix, 1987; Barnola et al., 1987; Chappellaz et al., 1990.. Changes in solar radiation inputs resulting from variations in the
Fig. 1. Location of Sacred Lake on the northeastern flank of Mount Kenya.
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earth’s orbital parameters ŽMilankovitch forcing. are not, however, sufficient to account fully for the strength of the observed climatic signal ŽBerger, 1979, 1992.. The strong orbital signals thus largely reflect the importance of earth-intrinsic climatic feedback mechanisms which amplify the external forcing. The amplification may be linked, for example, to the waxing and waning of polar ice sheets Že.g. Denton and Hughes, 1983; Denton et al., 1986., or to changes in atmospheric CO 2 concentration Že.g. Genthon et al., 1987.. Over glacial–interglacial cycles, there have occurred changes in the intensity of atmospheric circulation, the displacement of wind belts, and changes in moisture flux to the continents ŽFlohn and Nicholson, 1980; Bradley, 1985.. These changes have been inferred from numerous geological records, both land and sea based, and are generally corroborated by numerical simulations using global circulation models ŽGCMs.. Spectral analysis of faunal and lithogenic aeolian indicators, and numerical simulations have revealed that atmospheric circulation is influenced by Milankovitch cycles Že.g. Pokras and Mix, 1987; Short et al., 1991.. In addition, a high correlation of the Indian Ocean monsoons and the orbital precession parameter from the faunal and lithogenic records in deep sea cores has been observed ŽPrell, 1984; Clemens and Prell, 1990.. These data indicate that circulation in the tropics is driven by the precessional cycle, albeit indirectly, as solar insolation changes are translated first into surface temperature changes Žwith direct CO 2 and CH 4 feedbacks., which in turn result in circulation changes Ždue to differential heating of land and sea depending on locality, and to differences in the heat capacities of land and sea, respectively. Že.g. Kutzbach and Guetter, 1986.. In this paper, the mineral magnetic and stable carbon isotope trends in a sediment core from a high altitude lake on Mount Kenya are examined to determine the long-term trend of paleomonsoon dynamics in equatorial Africa.
2. Location of the study site Mt. Kenya is an extinct, heavily denuded volcano that lies on the equator at ca. 378 longitude. It lies within the East African climatic zone, and exhibits
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vegetation zonation of a similar type to the other highland regions of East Africa. Sacred Lake, a closed crater lake occupying a basaltic explosion crater ca. 1 km across, is located at 0803X N, 37832X E ŽFig. 1., at an altitude of 2350 m a.s.l. in the humid montane rain forest of Mt. Kenya, where the mean annual rainfall is ca. 1780 mm. Characteristic tree taxa within the catchment include Pygeum africanum, Macaranga kilimandscharica, Neoboutonia macrocalyx, Apodytes dimidiata, Afrocrania Õolkensii and Syzygium guineese. The lake, which is surrounded by a very diverse belt of floating swamp, is dystrophic and rainwater-fed, and had a maximum depth of 5 m at the time of coring.
3. Materials and methods Two sediment cores measuring 16.34 m Žcore SL1. and 13.40 m Žcore SL2. in length were recovered from the lake using a raft-mounted modified Livingstone piston corer. The longer core SL1 is taken as the type section of the lake sediment stratigraphy and is the core which is discussed below. The principal sedimentary units in the Sacred Lake core SL1 are diamicts, thin, discrete tephra horizons, root mats, organic lake mud and waterlily peat ŽFig. 2.. Of these sedimentary units, the organic lake mud is by far the most dominant ŽFig. 2.. The sediments are carbonate-free, and are diatomaceous except between 806 and 221 cm where diatoms are rare or absent ŽBen Khelifa, pers. comm... Zone III sediments consist of two diamicts at the upper and lower portions sandwiching a smooth, dark, organic lake mud bed which contains one root mat and two tephra units — tephras V and IV ŽFig. 2.. The diamicts are composed of a silty organic mud matrix within which are embedded clasts of sand-sized and gravel-sized weathered pumice fragments and small lenses of almost pure organic matter. Zone II sediments are dominated by a smooth, dark, organic lake mud bed with three tephra units Žtephras III, II and I. in its lower section, and three root mat units in the upper section ŽFig. 2.. Zone I sediments consist of a waterlily-dominated peat ŽFig. 2.. The highly organic lake sediment core was analysed for mineral magnetic trends Žisothermal remanent magnetism, IRM. and profiles of the stable
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Fig. 2. Stratigraphy, radiocarbon and UrTh dates in Sacred Lake core SL1.
carbon isotopes of the bulk organic matter. Core chronology was established by conventional and AMS radiocarbon dating of the organic matter and UrTh dating of tephra horizons and some organic matter horizons. Conventional radiocarbon dating of the cores was carried out at the NERC Radiocarbon Laboratory, Glasgow, Scotland. Accelerator Mass Spectrometry ŽAMS. radiocarbon dating was done at
the Radiocarbon Accelerator Unit, Oxford. Experimental UrTh dating on both the organic sediment and tephra horizons was done at the Analytical Sciences Centre, UK Atomic Energy Authority, Harwell, Oxon. Total organic carbon ŽTOC. and total nitrogen ŽTN. contents were determined using a Carlo–Erba CHN Analyser. Stable carbon isotope analyses were done at the NERC Radiocarbon Labo-
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ratory, East Kilbride, and all excess material was then analysed for TOC and TN contents.
4. Mineral magnetics and stable carbon isotopes as indicators of precipitation variations In mineral magnetic studies, the response of magnetic materials to a range of artificially applied magnetic fields is measured. It depends on the sensitivity of iron compounds to physicochemical changes, and on the tendency in many environments for some of these compounds to persist as diagnostic ‘tracers’ over long periods ŽThompson and Oldfield, 1986.. Mineral magnetic studies have been found to be applicable in, amongst others: soil forming processes ŽMullins, 1977; Longworth et al., 1979; Maher, 1986.; slope evolution ŽDearing et al., 1986. and drainage basin erosion and sedimentation ŽDearing et al., 1981; Appleby et al., 1985.; differentiation of atmospheric dusts and aerosols ŽHunt, 1986; Oldfield et al., 1985. and the historical record of their deposition into peat bogs, lakes, snow fields and ice caps ŽOldfield et al., 1978; Tolonen and Oldfield, 1986.; stratigraphic correlations on a wide range of temporal and spatial scales ŽThompson et al., 1975; Oldfield et al., 1980.; and the interpretation of the palaeoclimatic record in deep-sea sediments ŽOldfield and Robinson, 1985.. If in situ magnetic mineral formation or depletion within a lacustrine sedimentary environment is negligible, the mineral magnetic characteristics of the sediments can be attributed primarily to allochthonous inputs. This will be largely controlled by precipitation-driven sediment inputs due to erosion in, and transportation from the catchment area, and will be modulated primarily by changes in vegetation cover which determines the degree of stability of the unconsolidated sediments and soils in the catchment. The principal mechanism or reaction pathway of photosynthesis in plants leading to the synthesis of sugar through fixation of CO 2 from the atmosphere is known as the Calvin–Benson cycle, or, more commonly, the C 3 cycle. Other pathways are the Hatch–Slack cycle Žor C 4 cycle. and the Crassulacean Acid Metabolism cycle Žor CAM cycle.. The various stages in the process of CO 2 uptake from the atmosphere by plants, through to its subsequent in-
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corporation into the plant structure, are each accompanied by varying degrees of carbon isotope discrimination. In atmospheric CO 2 , d13 C is near y7‰. Diffusion and the carboxylation reactions Žknown as Rubisco or RuBP Žribulose-1,5-biphospate. carboxylation in C 3 plants and PEP Žphosphoenolpyruvate. carboxylation in C 4 plants. are expected to deplete d13 C by about 4‰ and 19‰, respectively ŽVogel, 1980., and the resulting d13 C values range from y9‰ to y40‰ in upper plants, according to their photosynthetic pathway ŽAndreux et al., 1990.. Data on terrestrial plants collected by Deines Ž1980. show that C 3 and C 4 plants form two well separated populations with average d13 C values of y26‰ and y12‰, respectively. In contrast, more variable d13 C values, with a bimodal distribution corresponding to that of C 3 and C 4 plants, were found in succulents with Crassulacean Acid Metabolism ŽCAM.. Since the changes in forest soils follow major changes in the vegetation ŽBuringh, 1984., upper litter layers, in which non-decomposed residues predominate, are expected to present d13 C values closer to those of the dominant vegetation ŽAndreux et al., 1990; Balesdent, 1991; Boutton et al., 1993., and the mean d13 C isotopic composition of soil organic matter ŽSOM. therefore reflects that of the vegetation or succession of vegetation which produced it ŽDeines, 1980; O’Brien and Stout, 1978; Dzurec et al., 1985; Cerling et al., 1989; Andreux et al., 1990; Balesdent, 1991; Schwartz, 1991; Boutton et al., 1993; McPherson et al., 1993.. This also applies to the d13 C content of lake sediments ŽMeyers, 1994., especially in cases where the catchment area is restricted in size, so that the allochthonous organic component reflects that of the immediate catchment area Žcf. Smith and Epstein, 1971.. Relating d13 C values to C:N ratios can therefore help underpin the organic sediment source with greater precision, and provide information on local vegetational fluxes and aquatic productivity ŽKrishnamurthy et al., 1986.. For example, since C 4 plants can fix CO 2 very efficiently at both much lower and much higher concentrations than C 3 plants Že.g. Klein and Klein, 1988; Ehleringer et al., 1991., are more water-efficient users than C 3 plants ŽOsmond et al., 1982., and CO 2 fixation in many C 4 plants is not impaired by temperature ŽLong et al., 1975; Bjorkman et al., 1976., dynamics ¨ of vegetation change over geological timescales can
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be related to changes in temperature, atmospheric CO 2 and precipitation. This approach has been employed successfully in the reconstruction of palaeoclimatological and palaeoenvironmental change in many parts of the world from palaeosol and lake sediment records Že.g. Krishnamurthy et al., 1986; Cerling et al., 1987..
5. Core chronology The Sacred Lake core chronology was established by a combination of a third order polynomial curve incorporating both the radiocarbon and experimental UrTh dates Ž1634 to 600 cm. and an interpolated curve Ž600 to 0 cm. using interleaved dates from the parallel cores SL1 Ž1634 cm. and SL2 Ž1340 cm. ŽFig. 3. ŽOlago, 1995.. The internal consistency of the radiocarbon dates Žas a function of depth., a low
probability of groundwater contamination in the lake whose water budget is strongly dominated by direct precipitation inputs, and the young age at 242 cm Ž1760 years BP. which excludes juvenile CO 2 contamination, suggest that the ages are very reliable ŽFig. 3.. The UrTh date of 215,000 years BP derived from a disturbed organic lake mud unit was rejected on the basis of :230 Th contamination by reworked volcaniclastic sediments Žcf. Rowe et al., 1989., and an extremely poor linear best fit line obtained by the pseudo-isochron dating method. The Ash IV date Ž142,000 years BP. was also rejected on the basis of 230 Th contamination by reworked sediments. Inconsistencies between, and the large error margins characteristic of, the remainder of the UrTh dates are due to inherent biases such as may arise due to: the difficulties of sample preparation Žcf. Barrett et al., 1992.; experimental error, in situ diagenetic processes ŽIvanovich, 1982a.; preferential
Fig. 3. Sacred Lake geochronology.
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leaching of U and Th isotopes as the deposit experiences interstitial fluids ŽIvanovich, 1982b; Szabo and Rosholt, 1982; Osmond and Cowart, 1982.; and contamination of the primary ashes by basement material during the eruption phase. Although the dates older than 40,000 years BP are few in number, perhaps the large number, consistency and reliability of the younger dates place a strong constraint on the trend of the regression curve estimating age in the older sections of the core. The logarithmic age-depth function ŽFig. 3., which was used as a guide to help determine age in the older sections of the core by taking into account the effect of sediment autocompaction with increasing depth Žassuming a constant sedimentation rate., has a high regression coefficient Žca. 0.9. and suggests that the oldest part of the core may be roughly 90,000 years. It appears to compensate well for sediment autocompaction, but fails to account for the variable sedimentation rates which are apparent in the core profile ŽFig. 3.. The cubic regression curve fits the data quite well and has low residuals in the range where the ages are younger than 36,000 years BP, but may slightly be overestimating age in the lower part of the core, from 1200 cm to the base. A pointer that strongly suggests that the age-depth function probably well estimates the actual age of the core is, paradoxically, the spectral analysis of the stable carbon isotope Ž d13 C. and mineral magnetic trends within the core, which coincide not only with the major Milankovitch forcing frequencies, but also discriminate between the main factors driving the changes in the two sediment parameters Žsee below.. The dates cannot be extended below the top of the basal diamict in core SL1 as the diamicts were deposited relatively rapidly and are well-homogenised, and therefore a basal age limit of 115,000 years BP Žthe end of the last interglacial period. is set for the core. 6. Mineral magnetic trends The relatively high susceptibility Ž x . concentrations Žwhich reflect the changing concentrations of ferrimagnetic Ži.e. magnetite and maghaemite. minerals in a sample ŽOldfield, 1988.. and likewise the saturation isothermal remanent magnetic ŽSIRM. concentrations Žwhich, although including a contribu-
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tion from all remanence-carrying magnetic minerals in a sample, is contributed to mainly by ferrimagnetic crystals ŽOldfield, 1988.. indicate that magnetitermaghaemite are important in the diamict sections in Zone III, part of Zone II where the ashes occur, and the top ca. 80 cm of sediment ŽFig. 4.. In the sections dominated by organic lake mud or peat, with the exception of the near-surface sediments, low susceptibility and acquisition IRM signals are recorded ŽFig. 4.. The two parameters SIRM and x are highly correlated, and the SIRMrx ratio for the sediments is 10 kA my1 . This ratio corresponds exactly to that observed for 1000 natural samples resulting from magnetite having an effective grain size of 5 mm, and from both susceptibility and saturation remanence dominantly reflecting magnetite concentration ŽThompson and Oldfield, 1986.. The similarity in the x v Žvolume susceptibility., x Žmass specific susceptibility., IRM and ‘hard’ IRM ŽHIRM, a measure of the haematite concentration. trends in Sacred Lake sediments suggests that post-diagenetic mineral transformations were not significant despite the fact that these parameters were measured after a time interval of a few months on moist Ž x v . and dry samples Ž x and IRM., respectively Žcf. Hilton and Lishman, 1985; Hilton et al., 1986; Snowball and Thompson, 1988; Oldfield et al., 1992.. An inference of negligible post-depositionalrpost extrusion and storage transformation is supported by the fact that the variations in HIRM are positively correlated to the other mineral magnetic parameters Žcf. Snowball, 1993.. However, in terms of absolute concentrations a reduction probably occurred. The principal component scores of the mineral magnetic parameters Ž x and acquisition IRM. show that the first principal component ŽPC1. explains the total variance in the data set in core SL1. This curve is similar to the SIRM curve and thus PC1 reflects the total concentration of magnetic minerals present. The above evidence supports a dominantly allochthonous origin of the magnetic mineral assemblage in the Sacred Lake sediments, and indicates that the processes governing dilution and concentration of the magnetic minerals have not affected the interparametric ratios. These data suggest that the dominant factor governing the magnetic mineral concentration in the Sacred Lake sediments is the degree to which allochthonous clas-
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Fig. 4. Mineral magnetic trends in Sacred Lake core SL1.
tic magnetic minerals are diluted by autochthonous organic matter. 7. Stable carbon isotope trends The TOC concentrations from the base of the core to ca. 930 cm are below 30% ŽFig. 5.. Above 880 cm, the TOC concentration is generally between 50% and 60%. The section 930 to 880 cm is transitional, with TOC concentrations increasing from 30% to 51%, respectively. TN concentrations are between 1% and 2% from the base of the core to ca. 900 cm. From 900 cm to the surface, TN concentrations increase from 2% to 4.5%, respectively. C:N ratios are between 10 and 20 from 1634 to 930 cm ŽFig. 5.. The C:N ratio increases to ) 27 by 880 cm, and high C:N ratios Ž) 27. are maintained to 780 cm.
From 780 cm to the surface sediment, the C:N ratios decline progressively from 20 to 10, respectively. The stable carbon isotopes Ž d13 C. have a spectacular range: the lowest value is y31.5‰ Ž576 cm., while the highest value is y14.1‰ Ž1417 cm., giving a range of 17.4‰ ŽFig. 5.. The more positive Žor heavier. values occur below 850 cm, while the more negative Žor lighter. values occur above 800 cm. The level 850 cm to 800 cm is transitional. Much larger variability and range of d13 C values occurs below 850 cm as compared to above 800 cm ŽFig. 5.. Stable carbon isotope analysis of the sediments indicate that most of the late Quaternary period on Mount Kenya Ž110,000 to 14,000 years BP Ž1634 to 905 cm. – glacial period. was characterised by terrestrial C 4 vegetation types Žgrassland. at higher altitudes and mixed C 3 –C 4 Žgrassland with scattered
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Fig. 5. Trends in total organic carbon, total nitrogen, C:N ratios and d13 C in Sacred Lake core SL1.
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trees and shrubs. vegetation types at lower altitudes, while low, productivity-related 13 C discrimination occurred in the aquatic environment. The period 14,000 to 9000 years BP Ž905 to 570 cm. is transitional to Holocene climatic conditions, with progressive expansion of terrestrial C 3 vegetation, and increased 13 C discrimination in the aquatic environment. These changes occurred in step with atmospheric CO 2 changes recorded in polar ice cores over the late Quaternary period. Biogeochemical analysis of the organic fraction in Sacred Lake shows strong algal contributions, particularly during periods of
low 13 C discrimination, and that the trends in carbon isotope fractionation by plankton are similar to those followed by terrestrial vegetation, suggesting a stronger response to atmospheric CO 2 changes rather than temperature, particularly during the glacial period when atmospheric CO 2 concentrations of 200 to 240 ppmv, ranged, at the altitude of Sacred Lake, between 140 and 200 ppmv, a potent stress factor particularly to C 3 plants, which are inefficient CO 2 utilisers compared to C 4 plants. In addition to this direct effect, CO 2 is believed to have been responsible Žby feedback mechanisms. for about 50% of the
Fig. 6. Spectral characteristics of the mineral magnetic and d13 C trends in Sacred Lake core SL1.
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temperature signal over this time period — this further negates the importance of temperature, which has to date been considered to be the primary climatic factor effecting environmental change in the tropics. Arid periods enhanced the low CO 2 stress factor and favoured further grassland expansion, save for the late glacial–early Holocene transition period, where increases in atmospheric CO 2 and temperature reduced the competitive advantage conferred to C 4 plants over C 3 plants by moisture stress.
8. Spectral trends Spectral analysis by a fast fourier transform method Žprogramme SPECTRA in SAS. was performed on the mineral magnetic and stable carbon isotope trends of the Sacred Lake sediments, core SL1. The stable carbon isotope trends have strong signals at the 38,000, 23,000 and 12,000 year frequencies ŽFig. 6.. The 38,000 year frequency signal is based only a few points due to the relatively short temporal span of the record, and is thus probably an artefact of constraints due to the temporal span of the record. The mineral magnetic signature has a strong signal at an 11,500 year frequency; sharp but relatively weak peaks at ca. 7500 and 5000 year frequencies are recorded ŽFig. 6.. These cycles at orbital ŽMilankovitch. frequencies reflect the significant forcing effect of orbital variations on climate and environment at the altitude of Sacred Lake, Mount Kenya. The dominant 23,000 year frequency recorded in the d13 C signature reflects the strong effect of the precession cycle on the climate and ecosystems of Sacred Lake and its catchment area, and is most probably effected via global atmospheric pCO 2 and temperature changes, as reflected in polar ice cores and deep sea sediment cores. The shorter cycles at 11,500 years Žindicated by both mineral magnetics and d13 C trends., and 7500 and 5000 years BP Žapparent in the mineral magnetic record. reflect the second through to the fifth precessional harmonics Ž p 2 s 11,500; p 3 s 7300; p 4 s 5500; p 5 s 4400 years.; these higher precessional harmonics have been observed in an equatorial Atlantic sediment core ŽPokras and Mix, 1987. and simulated orbital data ŽShort et al., 1991.. The second precessional har-
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monic Ž p 2 s 11,500 years. is related to the twice yearly passage of the sun across equatorial sites ŽShort et al., 1991., and thus reflects the effect of the seasonality of rainfall Žtwice yearly. on Mount Kenya. Its relatively smaller peak Žin comparison to the 23,000 year cycle. in the d13 C signature indicates that precipitation played a relatively minor but significant role in the modulation of climate and environment on Mount Kenya. The higher precessional harmonics Ž p 3 to p 5 . also observed in the mineral magnetic signature are most probably related to orbitally driven precipitation indices as well, as they do not occur in the d13 C record that dominantly reflects pCO 2 and temperature effects. The dominance of the second precessional cycle that reflects the strongest temoral cycle of precipitation on Mount Kenya contrasts with outer tropical and sub-tropical records such as the Pretoria Salt Pan Record ŽPatridge, 1995., where the precipitation cycle is dominated by the 19,000 to 23,000 year precessional frequency, reflecting the characteristic once yearly rainy season. 9. Conclusion Long-term temporal trends of palaeomonsoon dynamics within the equatorial region is dominated by the second precessional harmonic cycle of 11,500 years, which reflects the regions dual seasonality of rainfall. This equatorial continental record of palaeomonsoon dynamics correlates well with equatorial deep sea sediment records and numerical simulations of the characteristics of palaeomonsoon dynamics in the equatorial region. The strong linkages with orbital parameters and earth-intrinsic climatic factors on global scales reflects the underlying dependency of regional climates on earth-extrinsic and earth-intrinsic global climate parameters. References Andreux, F., Cerri, C., Vose, P.B., Vitorello, V.A., 1990. In: Harrison, A.F., Ineson, P., Heal, O.W. ŽEds.., Nutrient Cycling in Terrestrial Ecosystems: Field methods, Application and Interpretation. Elsevier Applied Science, London, New York, pp. 259–275. Appleby, P.G., Dearing, J.A., Oldfield, F., 1985. Magnetic studies of erosion in a Scottish lake-catchment. I. Core chronology and correlation. Limnol. Oceanogr. 30, 144–153. Balesdent, J., 1991. Estimation des renouvellement du carbone des
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Long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa D.O. Olago a,) , F.A. Street-Perrott b, R.A. Perrott b, M. Ivanovich c , D.D. Harkness d , E.O. Odada a a Department of Geology, UniÕersity of Nairobi, P.O. Box 30197, Nairobi, Kenya Department of Geography, UniÕersity of Wales Swansea, Swansea SA2 8PP, Wales, UK Analytical Sciences Centre, Business DeÕelopment Office, Harwell Laboratory B7, Oxfordshire OX11 0RA, UK d NERC Radiocarbon Laboratory, NEL Technology Park, East Kilbride, Glasgow G75 0QU, UK b
c
Abstract In this paper we examine the long-term temporal characteristics of palaeomonsoon dynamics in equatorial Africa from a continuous lacustrine sequence retrieved from Sacred Lake, Mount Kenya Ž0803X N, 37832X E, 2350 m a.s.l.., covering the last interglacial–glacial transition to the present. The trends in mineral magnetics and stable carbon isotopes are proxy indicators of changes in precipitation on the mountain over the last glacial–interglacial cycle. Spectral analysis by a fast fourier transform method revealed that the stable carbon isotope trend Ž d13C. has strong signals at the 23,000 and 11,500 year frequencies. The mineral magnetic signature does not register the 23,000 year cycle observed in the d13C signature. It has, however, a strong signal at an 11,500 year frequency, and sharp but relatively weak peaks at ca. 7500 and 5000 year frequencies are recorded. The dominant 23,000 year frequency recorded in the d13C signature reflects the strong effect of the precessional cycle on tropical climate and ecosystems, and is most probably effected via global atmospheric pCO 2 and temperature changes. The shorter cycles at 11,500 year Žindicated by both mineral magnetics and d13 C trends., and 7500 and 5000 years BP Žapparent in the mineral magnetic record. are attributed to precipitation variations, whose temporal cycles are dominated by the higher precessional harmonics. q 2000 Elsevier Science B.V. All rights reserved. Keywords: paleomoonsoon; mineral magnetics; stable carbon isotopes; Milankovitch forcing
1. Introduction The glacial–interglacial sequences of climatic change are largely due to changes in solar insolation, driven by variations in the earth’s orbital parameters Že.g. Croll, 1864; Milankovitch, 1920; Berger, 1977, )
Corresponding author. Tel.: q254-2-447-740; fax: q254-2449-539. E-mail addresses: [email protected], [email protected] ŽD.O. Olago..
1979, 1980.. These include the variation in the earth’s orbit around the sun Žeccentricity., variations in the tilt Žobliquity. of the earth’s axis Žwhich varies from 21.8 to 24.48., and the precession of the equinoxes ŽImbrie and Imbrie, 1979.. These variations affect earth surface temperatures by altering latitudinal radiation receipts, and have strong cyclic signals at periods of ca. 95,800 years Žeccentricity., 41,000 years Žobliquity., and 23,000 years and 19,000 years Žprecession. ŽBerger, 1977; Imbrie and Imbrie, 1979.. Solar radiation receipts at low latitudes are mainly
0921-8181r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 0 0 . 0 0 0 4 1 - 2
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affected by variations in eccentricity and precession of the equinoxes, whereas higher latitudes are mainly affected by variations in obliquity. Spectral analysis of the polar ice core and deep sea records show cycles at orbital ŽMilankovitch. frequencies reflect-
ing the significant forcing effect of orbital variations on the earth’s climate Že.g. Pisias and Leinen, 1984; Imbrie et al., 1984; Pokras and Mix, 1987; Barnola et al., 1987; Chappellaz et al., 1990.. Changes in solar radiation inputs resulting from variations in the
Fig. 1. Location of Sacred Lake on the northeastern flank of Mount Kenya.
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earth’s orbital parameters ŽMilankovitch forcing. are not, however, sufficient to account fully for the strength of the observed climatic signal ŽBerger, 1979, 1992.. The strong orbital signals thus largely reflect the importance of earth-intrinsic climatic feedback mechanisms which amplify the external forcing. The amplification may be linked, for example, to the waxing and waning of polar ice sheets Že.g. Denton and Hughes, 1983; Denton et al., 1986., or to changes in atmospheric CO 2 concentration Že.g. Genthon et al., 1987.. Over glacial–interglacial cycles, there have occurred changes in the intensity of atmospheric circulation, the displacement of wind belts, and changes in moisture flux to the continents ŽFlohn and Nicholson, 1980; Bradley, 1985.. These changes have been inferred from numerous geological records, both land and sea based, and are generally corroborated by numerical simulations using global circulation models ŽGCMs.. Spectral analysis of faunal and lithogenic aeolian indicators, and numerical simulations have revealed that atmospheric circulation is influenced by Milankovitch cycles Že.g. Pokras and Mix, 1987; Short et al., 1991.. In addition, a high correlation of the Indian Ocean monsoons and the orbital precession parameter from the faunal and lithogenic records in deep sea cores has been observed ŽPrell, 1984; Clemens and Prell, 1990.. These data indicate that circulation in the tropics is driven by the precessional cycle, albeit indirectly, as solar insolation changes are translated first into surface temperature changes Žwith direct CO 2 and CH 4 feedbacks., which in turn result in circulation changes Ždue to differential heating of land and sea depending on locality, and to differences in the heat capacities of land and sea, respectively. Že.g. Kutzbach and Guetter, 1986.. In this paper, the mineral magnetic and stable carbon isotope trends in a sediment core from a high altitude lake on Mount Kenya are examined to determine the long-term trend of paleomonsoon dynamics in equatorial Africa.
2. Location of the study site Mt. Kenya is an extinct, heavily denuded volcano that lies on the equator at ca. 378 longitude. It lies within the East African climatic zone, and exhibits
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vegetation zonation of a similar type to the other highland regions of East Africa. Sacred Lake, a closed crater lake occupying a basaltic explosion crater ca. 1 km across, is located at 0803X N, 37832X E ŽFig. 1., at an altitude of 2350 m a.s.l. in the humid montane rain forest of Mt. Kenya, where the mean annual rainfall is ca. 1780 mm. Characteristic tree taxa within the catchment include Pygeum africanum, Macaranga kilimandscharica, Neoboutonia macrocalyx, Apodytes dimidiata, Afrocrania Õolkensii and Syzygium guineese. The lake, which is surrounded by a very diverse belt of floating swamp, is dystrophic and rainwater-fed, and had a maximum depth of 5 m at the time of coring.
3. Materials and methods Two sediment cores measuring 16.34 m Žcore SL1. and 13.40 m Žcore SL2. in length were recovered from the lake using a raft-mounted modified Livingstone piston corer. The longer core SL1 is taken as the type section of the lake sediment stratigraphy and is the core which is discussed below. The principal sedimentary units in the Sacred Lake core SL1 are diamicts, thin, discrete tephra horizons, root mats, organic lake mud and waterlily peat ŽFig. 2.. Of these sedimentary units, the organic lake mud is by far the most dominant ŽFig. 2.. The sediments are carbonate-free, and are diatomaceous except between 806 and 221 cm where diatoms are rare or absent ŽBen Khelifa, pers. comm... Zone III sediments consist of two diamicts at the upper and lower portions sandwiching a smooth, dark, organic lake mud bed which contains one root mat and two tephra units — tephras V and IV ŽFig. 2.. The diamicts are composed of a silty organic mud matrix within which are embedded clasts of sand-sized and gravel-sized weathered pumice fragments and small lenses of almost pure organic matter. Zone II sediments are dominated by a smooth, dark, organic lake mud bed with three tephra units Žtephras III, II and I. in its lower section, and three root mat units in the upper section ŽFig. 2.. Zone I sediments consist of a waterlily-dominated peat ŽFig. 2.. The highly organic lake sediment core was analysed for mineral magnetic trends Žisothermal remanent magnetism, IRM. and profiles of the stable
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Fig. 2. Stratigraphy, radiocarbon and UrTh dates in Sacred Lake core SL1.
carbon isotopes of the bulk organic matter. Core chronology was established by conventional and AMS radiocarbon dating of the organic matter and UrTh dating of tephra horizons and some organic matter horizons. Conventional radiocarbon dating of the cores was carried out at the NERC Radiocarbon Laboratory, Glasgow, Scotland. Accelerator Mass Spectrometry ŽAMS. radiocarbon dating was done at
the Radiocarbon Accelerator Unit, Oxford. Experimental UrTh dating on both the organic sediment and tephra horizons was done at the Analytical Sciences Centre, UK Atomic Energy Authority, Harwell, Oxon. Total organic carbon ŽTOC. and total nitrogen ŽTN. contents were determined using a Carlo–Erba CHN Analyser. Stable carbon isotope analyses were done at the NERC Radiocarbon Labo-
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ratory, East Kilbride, and all excess material was then analysed for TOC and TN contents.
4. Mineral magnetics and stable carbon isotopes as indicators of precipitation variations In mineral magnetic studies, the response of magnetic materials to a range of artificially applied magnetic fields is measured. It depends on the sensitivity of iron compounds to physicochemical changes, and on the tendency in many environments for some of these compounds to persist as diagnostic ‘tracers’ over long periods ŽThompson and Oldfield, 1986.. Mineral magnetic studies have been found to be applicable in, amongst others: soil forming processes ŽMullins, 1977; Longworth et al., 1979; Maher, 1986.; slope evolution ŽDearing et al., 1986. and drainage basin erosion and sedimentation ŽDearing et al., 1981; Appleby et al., 1985.; differentiation of atmospheric dusts and aerosols ŽHunt, 1986; Oldfield et al., 1985. and the historical record of their deposition into peat bogs, lakes, snow fields and ice caps ŽOldfield et al., 1978; Tolonen and Oldfield, 1986.; stratigraphic correlations on a wide range of temporal and spatial scales ŽThompson et al., 1975; Oldfield et al., 1980.; and the interpretation of the palaeoclimatic record in deep-sea sediments ŽOldfield and Robinson, 1985.. If in situ magnetic mineral formation or depletion within a lacustrine sedimentary environment is negligible, the mineral magnetic characteristics of the sediments can be attributed primarily to allochthonous inputs. This will be largely controlled by precipitation-driven sediment inputs due to erosion in, and transportation from the catchment area, and will be modulated primarily by changes in vegetation cover which determines the degree of stability of the unconsolidated sediments and soils in the catchment. The principal mechanism or reaction pathway of photosynthesis in plants leading to the synthesis of sugar through fixation of CO 2 from the atmosphere is known as the Calvin–Benson cycle, or, more commonly, the C 3 cycle. Other pathways are the Hatch–Slack cycle Žor C 4 cycle. and the Crassulacean Acid Metabolism cycle Žor CAM cycle.. The various stages in the process of CO 2 uptake from the atmosphere by plants, through to its subsequent in-
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corporation into the plant structure, are each accompanied by varying degrees of carbon isotope discrimination. In atmospheric CO 2 , d13 C is near y7‰. Diffusion and the carboxylation reactions Žknown as Rubisco or RuBP Žribulose-1,5-biphospate. carboxylation in C 3 plants and PEP Žphosphoenolpyruvate. carboxylation in C 4 plants. are expected to deplete d13 C by about 4‰ and 19‰, respectively ŽVogel, 1980., and the resulting d13 C values range from y9‰ to y40‰ in upper plants, according to their photosynthetic pathway ŽAndreux et al., 1990.. Data on terrestrial plants collected by Deines Ž1980. show that C 3 and C 4 plants form two well separated populations with average d13 C values of y26‰ and y12‰, respectively. In contrast, more variable d13 C values, with a bimodal distribution corresponding to that of C 3 and C 4 plants, were found in succulents with Crassulacean Acid Metabolism ŽCAM.. Since the changes in forest soils follow major changes in the vegetation ŽBuringh, 1984., upper litter layers, in which non-decomposed residues predominate, are expected to present d13 C values closer to those of the dominant vegetation ŽAndreux et al., 1990; Balesdent, 1991; Boutton et al., 1993., and the mean d13 C isotopic composition of soil organic matter ŽSOM. therefore reflects that of the vegetation or succession of vegetation which produced it ŽDeines, 1980; O’Brien and Stout, 1978; Dzurec et al., 1985; Cerling et al., 1989; Andreux et al., 1990; Balesdent, 1991; Schwartz, 1991; Boutton et al., 1993; McPherson et al., 1993.. This also applies to the d13 C content of lake sediments ŽMeyers, 1994., especially in cases where the catchment area is restricted in size, so that the allochthonous organic component reflects that of the immediate catchment area Žcf. Smith and Epstein, 1971.. Relating d13 C values to C:N ratios can therefore help underpin the organic sediment source with greater precision, and provide information on local vegetational fluxes and aquatic productivity ŽKrishnamurthy et al., 1986.. For example, since C 4 plants can fix CO 2 very efficiently at both much lower and much higher concentrations than C 3 plants Že.g. Klein and Klein, 1988; Ehleringer et al., 1991., are more water-efficient users than C 3 plants ŽOsmond et al., 1982., and CO 2 fixation in many C 4 plants is not impaired by temperature ŽLong et al., 1975; Bjorkman et al., 1976., dynamics ¨ of vegetation change over geological timescales can
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be related to changes in temperature, atmospheric CO 2 and precipitation. This approach has been employed successfully in the reconstruction of palaeoclimatological and palaeoenvironmental change in many parts of the world from palaeosol and lake sediment records Že.g. Krishnamurthy et al., 1986; Cerling et al., 1987..
5. Core chronology The Sacred Lake core chronology was established by a combination of a third order polynomial curve incorporating both the radiocarbon and experimental UrTh dates Ž1634 to 600 cm. and an interpolated curve Ž600 to 0 cm. using interleaved dates from the parallel cores SL1 Ž1634 cm. and SL2 Ž1340 cm. ŽFig. 3. ŽOlago, 1995.. The internal consistency of the radiocarbon dates Žas a function of depth., a low
probability of groundwater contamination in the lake whose water budget is strongly dominated by direct precipitation inputs, and the young age at 242 cm Ž1760 years BP. which excludes juvenile CO 2 contamination, suggest that the ages are very reliable ŽFig. 3.. The UrTh date of 215,000 years BP derived from a disturbed organic lake mud unit was rejected on the basis of :230 Th contamination by reworked volcaniclastic sediments Žcf. Rowe et al., 1989., and an extremely poor linear best fit line obtained by the pseudo-isochron dating method. The Ash IV date Ž142,000 years BP. was also rejected on the basis of 230 Th contamination by reworked sediments. Inconsistencies between, and the large error margins characteristic of, the remainder of the UrTh dates are due to inherent biases such as may arise due to: the difficulties of sample preparation Žcf. Barrett et al., 1992.; experimental error, in situ diagenetic processes ŽIvanovich, 1982a.; preferential
Fig. 3. Sacred Lake geochronology.
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leaching of U and Th isotopes as the deposit experiences interstitial fluids ŽIvanovich, 1982b; Szabo and Rosholt, 1982; Osmond and Cowart, 1982.; and contamination of the primary ashes by basement material during the eruption phase. Although the dates older than 40,000 years BP are few in number, perhaps the large number, consistency and reliability of the younger dates place a strong constraint on the trend of the regression curve estimating age in the older sections of the core. The logarithmic age-depth function ŽFig. 3., which was used as a guide to help determine age in the older sections of the core by taking into account the effect of sediment autocompaction with increasing depth Žassuming a constant sedimentation rate., has a high regression coefficient Žca. 0.9. and suggests that the oldest part of the core may be roughly 90,000 years. It appears to compensate well for sediment autocompaction, but fails to account for the variable sedimentation rates which are apparent in the core profile ŽFig. 3.. The cubic regression curve fits the data quite well and has low residuals in the range where the ages are younger than 36,000 years BP, but may slightly be overestimating age in the lower part of the core, from 1200 cm to the base. A pointer that strongly suggests that the age-depth function probably well estimates the actual age of the core is, paradoxically, the spectral analysis of the stable carbon isotope Ž d13 C. and mineral magnetic trends within the core, which coincide not only with the major Milankovitch forcing frequencies, but also discriminate between the main factors driving the changes in the two sediment parameters Žsee below.. The dates cannot be extended below the top of the basal diamict in core SL1 as the diamicts were deposited relatively rapidly and are well-homogenised, and therefore a basal age limit of 115,000 years BP Žthe end of the last interglacial period. is set for the core. 6. Mineral magnetic trends The relatively high susceptibility Ž x . concentrations Žwhich reflect the changing concentrations of ferrimagnetic Ži.e. magnetite and maghaemite. minerals in a sample ŽOldfield, 1988.. and likewise the saturation isothermal remanent magnetic ŽSIRM. concentrations Žwhich, although including a contribu-
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tion from all remanence-carrying magnetic minerals in a sample, is contributed to mainly by ferrimagnetic crystals ŽOldfield, 1988.. indicate that magnetitermaghaemite are important in the diamict sections in Zone III, part of Zone II where the ashes occur, and the top ca. 80 cm of sediment ŽFig. 4.. In the sections dominated by organic lake mud or peat, with the exception of the near-surface sediments, low susceptibility and acquisition IRM signals are recorded ŽFig. 4.. The two parameters SIRM and x are highly correlated, and the SIRMrx ratio for the sediments is 10 kA my1 . This ratio corresponds exactly to that observed for 1000 natural samples resulting from magnetite having an effective grain size of 5 mm, and from both susceptibility and saturation remanence dominantly reflecting magnetite concentration ŽThompson and Oldfield, 1986.. The similarity in the x v Žvolume susceptibility., x Žmass specific susceptibility., IRM and ‘hard’ IRM ŽHIRM, a measure of the haematite concentration. trends in Sacred Lake sediments suggests that post-diagenetic mineral transformations were not significant despite the fact that these parameters were measured after a time interval of a few months on moist Ž x v . and dry samples Ž x and IRM., respectively Žcf. Hilton and Lishman, 1985; Hilton et al., 1986; Snowball and Thompson, 1988; Oldfield et al., 1992.. An inference of negligible post-depositionalrpost extrusion and storage transformation is supported by the fact that the variations in HIRM are positively correlated to the other mineral magnetic parameters Žcf. Snowball, 1993.. However, in terms of absolute concentrations a reduction probably occurred. The principal component scores of the mineral magnetic parameters Ž x and acquisition IRM. show that the first principal component ŽPC1. explains the total variance in the data set in core SL1. This curve is similar to the SIRM curve and thus PC1 reflects the total concentration of magnetic minerals present. The above evidence supports a dominantly allochthonous origin of the magnetic mineral assemblage in the Sacred Lake sediments, and indicates that the processes governing dilution and concentration of the magnetic minerals have not affected the interparametric ratios. These data suggest that the dominant factor governing the magnetic mineral concentration in the Sacred Lake sediments is the degree to which allochthonous clas-
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Fig. 4. Mineral magnetic trends in Sacred Lake core SL1.
tic magnetic minerals are diluted by autochthonous organic matter. 7. Stable carbon isotope trends The TOC concentrations from the base of the core to ca. 930 cm are below 30% ŽFig. 5.. Above 880 cm, the TOC concentration is generally between 50% and 60%. The section 930 to 880 cm is transitional, with TOC concentrations increasing from 30% to 51%, respectively. TN concentrations are between 1% and 2% from the base of the core to ca. 900 cm. From 900 cm to the surface, TN concentrations increase from 2% to 4.5%, respectively. C:N ratios are between 10 and 20 from 1634 to 930 cm ŽFig. 5.. The C:N ratio increases to ) 27 by 880 cm, and high C:N ratios Ž) 27. are maintained to 780 cm.
From 780 cm to the surface sediment, the C:N ratios decline progressively from 20 to 10, respectively. The stable carbon isotopes Ž d13 C. have a spectacular range: the lowest value is y31.5‰ Ž576 cm., while the highest value is y14.1‰ Ž1417 cm., giving a range of 17.4‰ ŽFig. 5.. The more positive Žor heavier. values occur below 850 cm, while the more negative Žor lighter. values occur above 800 cm. The level 850 cm to 800 cm is transitional. Much larger variability and range of d13 C values occurs below 850 cm as compared to above 800 cm ŽFig. 5.. Stable carbon isotope analysis of the sediments indicate that most of the late Quaternary period on Mount Kenya Ž110,000 to 14,000 years BP Ž1634 to 905 cm. – glacial period. was characterised by terrestrial C 4 vegetation types Žgrassland. at higher altitudes and mixed C 3 –C 4 Žgrassland with scattered
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Fig. 5. Trends in total organic carbon, total nitrogen, C:N ratios and d13 C in Sacred Lake core SL1.
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trees and shrubs. vegetation types at lower altitudes, while low, productivity-related 13 C discrimination occurred in the aquatic environment. The period 14,000 to 9000 years BP Ž905 to 570 cm. is transitional to Holocene climatic conditions, with progressive expansion of terrestrial C 3 vegetation, and increased 13 C discrimination in the aquatic environment. These changes occurred in step with atmospheric CO 2 changes recorded in polar ice cores over the late Quaternary period. Biogeochemical analysis of the organic fraction in Sacred Lake shows strong algal contributions, particularly during periods of
low 13 C discrimination, and that the trends in carbon isotope fractionation by plankton are similar to those followed by terrestrial vegetation, suggesting a stronger response to atmospheric CO 2 changes rather than temperature, particularly during the glacial period when atmospheric CO 2 concentrations of 200 to 240 ppmv, ranged, at the altitude of Sacred Lake, between 140 and 200 ppmv, a potent stress factor particularly to C 3 plants, which are inefficient CO 2 utilisers compared to C 4 plants. In addition to this direct effect, CO 2 is believed to have been responsible Žby feedback mechanisms. for about 50% of the
Fig. 6. Spectral characteristics of the mineral magnetic and d13 C trends in Sacred Lake core SL1.
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temperature signal over this time period — this further negates the importance of temperature, which has to date been considered to be the primary climatic factor effecting environmental change in the tropics. Arid periods enhanced the low CO 2 stress factor and favoured further grassland expansion, save for the late glacial–early Holocene transition period, where increases in atmospheric CO 2 and temperature reduced the competitive advantage conferred to C 4 plants over C 3 plants by moisture stress.
8. Spectral trends Spectral analysis by a fast fourier transform method Žprogramme SPECTRA in SAS. was performed on the mineral magnetic and stable carbon isotope trends of the Sacred Lake sediments, core SL1. The stable carbon isotope trends have strong signals at the 38,000, 23,000 and 12,000 year frequencies ŽFig. 6.. The 38,000 year frequency signal is based only a few points due to the relatively short temporal span of the record, and is thus probably an artefact of constraints due to the temporal span of the record. The mineral magnetic signature has a strong signal at an 11,500 year frequency; sharp but relatively weak peaks at ca. 7500 and 5000 year frequencies are recorded ŽFig. 6.. These cycles at orbital ŽMilankovitch. frequencies reflect the significant forcing effect of orbital variations on climate and environment at the altitude of Sacred Lake, Mount Kenya. The dominant 23,000 year frequency recorded in the d13 C signature reflects the strong effect of the precession cycle on the climate and ecosystems of Sacred Lake and its catchment area, and is most probably effected via global atmospheric pCO 2 and temperature changes, as reflected in polar ice cores and deep sea sediment cores. The shorter cycles at 11,500 years Žindicated by both mineral magnetics and d13 C trends., and 7500 and 5000 years BP Žapparent in the mineral magnetic record. reflect the second through to the fifth precessional harmonics Ž p 2 s 11,500; p 3 s 7300; p 4 s 5500; p 5 s 4400 years.; these higher precessional harmonics have been observed in an equatorial Atlantic sediment core ŽPokras and Mix, 1987. and simulated orbital data ŽShort et al., 1991.. The second precessional har-
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monic Ž p 2 s 11,500 years. is related to the twice yearly passage of the sun across equatorial sites ŽShort et al., 1991., and thus reflects the effect of the seasonality of rainfall Žtwice yearly. on Mount Kenya. Its relatively smaller peak Žin comparison to the 23,000 year cycle. in the d13 C signature indicates that precipitation played a relatively minor but significant role in the modulation of climate and environment on Mount Kenya. The higher precessional harmonics Ž p 3 to p 5 . also observed in the mineral magnetic signature are most probably related to orbitally driven precipitation indices as well, as they do not occur in the d13 C record that dominantly reflects pCO 2 and temperature effects. The dominance of the second precessional cycle that reflects the strongest temoral cycle of precipitation on Mount Kenya contrasts with outer tropical and sub-tropical records such as the Pretoria Salt Pan Record ŽPatridge, 1995., where the precipitation cycle is dominated by the 19,000 to 23,000 year precessional frequency, reflecting the characteristic once yearly rainy season. 9. Conclusion Long-term temporal trends of palaeomonsoon dynamics within the equatorial region is dominated by the second precessional harmonic cycle of 11,500 years, which reflects the regions dual seasonality of rainfall. This equatorial continental record of palaeomonsoon dynamics correlates well with equatorial deep sea sediment records and numerical simulations of the characteristics of palaeomonsoon dynamics in the equatorial region. The strong linkages with orbital parameters and earth-intrinsic climatic factors on global scales reflects the underlying dependency of regional climates on earth-extrinsic and earth-intrinsic global climate parameters. References Andreux, F., Cerri, C., Vose, P.B., Vitorello, V.A., 1990. In: Harrison, A.F., Ineson, P., Heal, O.W. ŽEds.., Nutrient Cycling in Terrestrial Ecosystems: Field methods, Application and Interpretation. Elsevier Applied Science, London, New York, pp. 259–275. Appleby, P.G., Dearing, J.A., Oldfield, F., 1985. Magnetic studies of erosion in a Scottish lake-catchment. I. Core chronology and correlation. Limnol. Oceanogr. 30, 144–153. Balesdent, J., 1991. Estimation des renouvellement du carbone des
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