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Wet and Arid Phases in the Southeast African Tropics Since the Last Glacial Maximum

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Wet and Arid Phases in the Southeast African Tropics Since the Last Glacial Maximum Wet and arid phases in the southeast African tropics since the Last Glacial Maximum Isla S. Castañeda Large Lakes Observatory, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Josef P. Werne Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, USA Thomas C. Johnson Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, Minnesota 55812, USA ABSTRACT Plant leaf wax carbon isotopes provide a record of C3 versus C4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and an antiphase relation- ship prevailed. The abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the Intertropical Convergence Zone (ITCZ) during the last glacial. Southward displacements of the ITCZ are also linked to arid conditions in southeast Africa during the Younger Dryas and the Little Ice Age. Keywords: Africa, C4 vegetation, Younger Dryas, Intertropical Convergence Zone. INTRODUCTION In tropical Africa, changes in the hydrological cycle have a far greater impact on human welfare than does the relatively small range of 34 E 35 E temperature variability. Thus it is important to understand the timing of M98-2PG past wet and arid phases in low-latitude regions. Tropical vegetation is a 10 S 363 m sensitive indicator of aridity because its distribution is mainly controlled M98-1P by precipitation, which, in tropical East Africa, is infl uenced by the con- Sahara desert 403 m vective intensity and seasonal migrations of the Intertropical Conver- July ITCZ 11 S C gence Zone (ITCZ) and the Inter-Oceanic Confl uence (IOC) (Leroux, 2001). The C3 (Calvin-Benson) and C4 (Hatch-Slack) cycles are the two Sahel-grass savanna main pathways of carbon fi xation utilized by plants (O’Leary, 1981). C4 plants are characterized by higher water-use effi ciency and are common Afro- today in tropical savannas, temperate grasslands, and semiarid regions N tree savanna Mont (Raven et al., 1999). Aridity is the dominant control on the large-scale Jan. ITCZ rain distribution of C3 versus C4 vegetation in tropical Africa (Schefuß et al., 2003), and thus changes in aridity can be examined by determining the forest Victoria past distribution of C versus C vegetation. 3 4 July IOC Tanganyika Lake Malawi (Fig. 1) sediments have provided one of the few 50 km B continuous and high-resolution climate records since the Last Glacial 8 S Maximum (LGM) on the African continent. Aridity in Lake Malawi Lake Malawi has been inferred from lake-level histories based on seismic refl ection Rukwa data (Johnson and Ng’ang’a, 1990), benthic diatom assemblages (Gasse 793 m Lake Masoko Jan. IOC et al., 2002), and geochemical analyses of endogenic calcite (Ricketts 840 m 9 S and Johnson, 1996). While the evidence for a major lake-level lowstand Lake Malawi ass during the LGM is indisputable, the history of aridity in the region dur- 475 m gr ing the Holocene, particularly regarding conditions in the early Holo- 32 E 33 E 34 E savanna A cene, has not been resolved. Seismic refl ectors are not well dated, the relative abundance of benthic diatoms can be infl uenced by preservation Figure 1. A: Vegetation zones of East Africa (Schefuß et al., 2003) and sediment redeposition, and the presence and/or absence of endo- and summer and winter positions of Intertropical Convergence Zone genic carbonates in sediments is not a particularly sensitive indicator of (ITCZ) and Inter-Oceanic Confl uence (IOC) (Leroux, 2001). The IOC hydrological conditions. separates Atlantic and Indian Ocean moisture. B: Location of Lake Malawi (LM) in relation to Lakes Masoko and Rukwa. C: Location In comparison, tropical vegetation is a sensitive indicator of arid- of sediment cores M98–1P (10°15.99S′, 34°19.19′E) and M98–2PG ity, and the carbon isotopic composition of plant leaf waxes, used to dis- (9°58.6′S, 34°13.8′E) in the northern basin of LM. tinguish between inputs of C3 and C4 vegetation (Collister et al., 1994), provides an additional independent method of examining past aridity. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2007; 2007 v. 35; no. 9; p. 823–826; doi: 10.1130/G23916A.1; 2 fi gures; Data Repository item 2007200. 823 Long-chain, odd-numbered n-alkanes (C25-C35) are a major component of terrestrial plant leaf waxes (Eglinton and Hamilton, 1967) and are gener- ally well preserved in sediments. Lake Malawi is located near the pres- ent southernmost extent of the ITCZ and the lake’s vast drainage basin is cal ka 0 2 4 6 8 10121416182022 dominated by tree savanna (mainly C3 plants), but grass savanna (mainly -27 C4 grasses) is present to both the north and south (Fig. 1). The proximity C 13 A of Lake Malawi to this ecosystem boundary makes it a sensitive location δ -28 C29 alkane to examine past aridity-driven vegetation shifts. -29 -25 C -26 13 METHODS alkane -30 δ We examined sediment cores M98–1P and M98–2PG from the 29 C31 alkane -27 C north basin of Lake Malawi (Fig. 1). The age models of both cores were -31 -28 previously published (Johnson et al., 2002, 2001) and are not discussed C -24 alkane 13 -29 δ further here. Ages are reported as thousands of calibrated years (cal ka) 31 before present. Compound-specifi c carbon isotope analysis of n-alkanes -25 C33 alkane -30 C was performed using gas chromatography–isotope ratio monitoring– -26 mass spectrometry. Each n-alkane sample was run in duplicate and the alkane 33 YD standard deviation of all samples is <± 0.5‰. Method details are avail- C -27 able in the GSA Data Repository.1 5 B Δ SST RESULTS AND DISCUSSION 4 strong winds Lake Malawi samples are characterized by a moderate to high odd 3 C) SST cooler 24 o versus even carbon number predominance (average carbon preference Δ 2 ToC index of 3.9) indicating terrestrial plant inputs (Eglinton and Hamilton, 26 Angola coast 1 13 weak winds 1967). We use the weighted mean δ C values of the C –C n-alkanes to 28 29 33 0 ( Temp δ13 drier examine past vegetation shifts and hereafter refer to this record as Calk. -26 86 warmer 30 60% Lake Malawi The percentage of C4 grasses is estimated from a binary mixing model 13 -27 TEX δ − − based on Calk, assuming end-member values of 36‰ and 21.5‰ for 55% δ13 32 C 300 alk C3 and C4 vegetation, respectively (Collister et al., 1994; Fig. 2). C -28 50% δ13 400 Lake Malawi Calk values were enriched during the LGM, cor- 13 δ 45% CH4 responding to an estimated vegetation assemblage of 61% C4 grasses -29 500 TD (Fig. 2B). Following the LGM, a gradual transition to increased C abun- weighted mean 3 600 dances occurred until 13.6 ka. The onset of the Younger Dryas (YD) is -30 GISP2 700 Atmospheric marked by an abrupt return to increased C inputs, and represents a shift methane (ppb) 4 wetter from 45% to 60% C grasses. Fluctuating but generally heavier δ13C 800 4 alk 480 values during the early Holocene (11–7.7 ka) indicate a vegetation assem- 280 ) blage of 49%–55% C grasses. C inputs increased after 7.7 ka, and C 2 4 3 3 460 insolation vegetation reached maximum abundance at 4.9 ka. Subsequently, a return 240 (ppm) 2 to increased C4 abundances occurred; 56% C4 grasses are noted at 0.4 ka. S (W/m 440 o Temperature, aridity, and the concentration of atmospheric carbon CO Taylor Dome 10 CO2 200 dioxide (pCO2) can infl uence C3 versus C4 variability (Schefuß et al., Jan. insolation C δ13 420 2003). Comparing the Lake Malawi Calk record to the Taylor Dome CO record (Monnin et al., 2001), we note that increased abundances of 0 2 4 6 8 10121416182022 2 cal ka C4 vegetation occur at times of low (LGM), intermediate (YD), and high (Holocene) pCO2 (Fig. 2C), suggesting, as previously reported (e.g., Figure 2. African paleoclimate records. Younger Dryas (YD) cold Huang et al., 2001), that pCO2 changes alone are insuffi cient to drive period is highlighted in all graphs. A: Carbon isotope composition of δ13 the C -C n-alkanes. Solid data points indicate samples from core vegetation change. Comparing the Calk record to the Lake Malawi 29 33 temperature record (Powers et al., 2005), a correlation is noted between M98–1P and open data points indicate samples from core M98–2PG. Error bars indicate the standard deviation of duplicate runs. B: δ13 cooler temperatures and heavier Calk values (Fig. 2B). While it may Atmospheric methane (CH4) records from the Greenland Ice Sheet appear that temperature is the main control on vegetation type, we note Project (GISP2) and Taylor Dome (TD, Antarctica) ice cores (Brook δ13 that strong links between temperature and aridity exist in southeast et al., 2000) compared with Lake Malawi Calk (open squares) and Africa, with warm and wet and cool and dry conditions being associated temperature (gray triangles; Powers et al., 2005) records, and Atlan- tic meridional sea surface temperature gradient (ΔSST) from the (Powers et al., 2005; Brown and Johnson, 2005; Johnson et al., 2002).
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