Abrupt resumption of the African Monsoon at the Younger Dryas—Holocene climatic transition

ARTICLE IN PRESS

Quaternary Science Reviews 26 (2007) 690–704

Abrupt resumption of the African Monsoon at the Younger Dryas—Holocene climatic transition Yannick Garcin, Annie Vincens, David Williamson, Guillaume Buchet, Joe¨l Guiot CEREGE-CNRS, UMR 6635, Universite´ Paul Ce´zanne, B.P. 80, F-13545 Aix-en-Provence Cedex 04, France Received 1 March 2006; accepted 31 October 2006

Abstract A high-resolution sedimentary record from Lake Masoko (Tanzania), based on pollen assemblages and magnetic susceptibility, shows that the most prominent environmental change of the last 45 000 years occurred ca 11.7 cal. ka BP, near the end of the Younger Dryas event. During this climatic transition, the Masoko catchment vegetation changed from being intolerant to a long/severe dry season to being tolerant, while the inferred lake-dynamics indicates strengthened seasonal fluctuations and/or lower levels than before. Comparison of the Masoko record with other regional palaeoclimatic data shows that evidence of this climatic transition is widespread in tropical Africa. The proposed failure of the African Monsoon during the Younger Dryas, associated with a southward position/ migration of the meteorological equator in East Africa, was followed by an abrupt and lasting resumption of monsoon activity, and more pronounced migration of the Intertropical Convergence Zone (ITCZ) over the African continent. Such a reorganisation of the atmospheric circulation, equally observed across the whole tropical region (South America, East and West Asia, and Africa), could have been a strong amplifier of northern high latitude changes in temperature and precipitation across this major climatic transition. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction Today, the tropics play a key role in global climate: they are the main source of heat and water vapour for earth’s atmospheric convection and they drive climatic changes of global impact, such as El Nin˜o-Southern Oscillation phenomenon (ENSO) (Zebiak and Cane, 1987). However, during previous periods, particularly the Last Glacial Period, they have often been interpreted as passive areas in comparison to the North Atlantic region. The latter has been generally assumed to be the active trigger of both abrupt and long-term climatic changes. Recent palaeoclimatic data (e.g., Peterson et al., 2000; Hendy and Kennett, 2003; Lea et al., 2003; Visser et al., 2003; Ivanochko et al., 2005) and climatic models (e.g., Clement et al., 1999, 2000, 2001) have, however, highlighted the importance of the tropics in past global climatic changes (see also Cane, 1998; Seager and Battisti, 2006).

Corresponding author. Fax: +33 04 42 97 15 95.

E-mail address: [email protected] (Y. Garcin). 0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.10.014

Previous detailed studies carried out on the sediments of Lake Masoko, located in tropical Southern Africa [pollen (Vincens et al., 2003), phenols (Merdaci, 1998), diatoms (Barker et al., 2000, 2003), charcoal (Thevenon et al., 2003) and rock-magnetism (Williamson et al., 1999; Garcin et al., 2006c)], have shown that this lake is climatically sensitive, and its deposits are, therefore, excellent records of environmental/climatic changes. Palaeoclimatic reconstructions based on changes in lake hydrology observed in the Masoko sequence have revealed several wet and dry phases over the last 45 000 years. These suggest that the strength of the African Monsoon and variations of the mean Intertropical Convergence Zone (ITCZ) position over tropical Africa (see Fig. 1A) were mainly driven by the amount of solar radiation received at low latitudes, modulated by the Earth’s precessional cycle (Fig. 2) (Garcin et al., 2006a, c) as previously described by Kutzbach and Street-Perrott (1985), Pokras and Mix (1987), Finney et al. (1996), Partridge et al. (1997), deMenocal et al. (2000), Thevenon et al. (2002) and Trauth et al. (2003). Similar changes in regional hydrology have been observed over the whole tropical region at equivalent

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

0°

20°E

14°N-23°N 33 35 34 32 36 37

31

8°S-14°N

20 17

18 0

500 1000 Km

GISP2

21

14 13

12 11 10

N

6-7-8 9

4 15°S-8°S

Cariaco

3 Lake Masoko ?

Masoko

Aa

20°N

15

19

16

Dongge

ITCZ Augus t ITC Z Ja nua ry 0°

40°E

26 27 28 25 24 22 23

29

30

691

?

2

0°

5 1

?

20°S

B

Fig. 1. (A) Present ITCZ seasonal migration at a global scale (dotted lines). The stars represent the location of Lake Masoko (Tanzania), Cariaco site (Venezuela), Dongge Cave site (China) and GISP2 site (Greenland) discussed in the text. (B) Location map of the African palaeoclimatic sites cited in the text. The star represents the location of Lake Masoko. The triangle indicates Mount Kilimanjaro. The circles are the location of the 41 lakes used to reconstruct the past lake status of the African tropics during the period 16–7 cal. ka BP (Fig. 5B). This compilation of palaeolake records was made using the same method as Hoelzmann et al. (2004). Radiocarbon dates from each sites were calibrated using CALIB 5.0.1 (Stuiver and Reimer, 1993). We defined three latitudinal patterns based on different responses during climatic changes. Pattern 15–81S characterizes the lakes (1–3) located south and close to Lake Masoko. Pattern 81S–141N characterizes the lakes (4–21) north of Lake Masoko and south of the Sahara. Pattern 14–231N characterizes the lakes located in the Sahara (22–37). Question marks (southern areas) indicate absence of detailed lake studies spanning the studied period. 1, Lake Malawi (Finney et al., 1996; Gasse et al., 2002); 2, Lake Rukwa (Barker et al., 2002; Thevenon et al., 2002); 3, Lake Cheshi (Stager, 1988); 4, Lake Tanganyika (Haberyan and Hecky, 1987; Gasse et al., 1989); 5, Lake Magadi (Roberts et al., 1993; Williamson et al., 1993); 6–7–8, Lakes Naivasha, Elmenteita, Nakuru (Richardson and Dussinger, 1986); 9, Lake Victoria (Johnson et al., 1996; Stager et al., 2002); 10, Lake Kivu (Haberyan and Hecky, 1987); 11, Lake Edward (Russell et al., 2003); 12, Lake Albert (Beuning et al., 1997); 13, Lake Turkana (Johnson, 1996); 14, Lake Ziway-Shala (Gillespie et al., 1983; Chalie´ and Gasse, 2002); 15, Lake Abhe´ (Gasse and Street, 1978); 16, Lake Barombi Mbo (Maley and Brenac, 1998); 17, Lake Bamili (Stager and AnfangSutter, 1999); 18, Lake Bosumtwi (Talbot and Johannessen, 1992; Peck et al., 2004); 19, Lake Tilla (Salzmann et al., 2002); 20, Lake Bougdouma (Gasse et al., 1990); 21, Lac Chad (Servant and Servant-Vildary, 1980); 22, Meidob Hills (Pachur and Hoelzmann, 2000); 23, Erg of Nagashush: Lake III and Lake Gureinat (Pachur and Hoelzmann, 2000); 24, Wadi Howar (Pachur and Kropelin, 1987); 25, El Atrun Oasis (Pachur and Kropelin, 1987; Ritchie, 1987); 26, West Nubian Lake Bassin, Wadi Fesh-Fesh (Pachur and Hoelzmann, 1991, 2000; Hoelzmann et al., 2001); 27, Oyo (Ritchie et al., 1985); 28, Dry Selima and Selima Oasis (Pachur and Kropelin, 1987; Haynes et al., 1989); 29, Kawar-Bilma (Servant, 1983; Baumhauer, 1991); 30, Fachi-Dogonboulo (Servant, 1983; Baumhauer, 1991); 31, Tin Ouffadene (Fontes and Gasse, 1991); 32, Ine Kousamene (Hillaire-Marcel et al., 1983; Petit-Maire and Riser, 1983); 33, Tagnout-Chaggaret (Petit-Maire and Riser, 1981, 1983; Hillaire-Marcel et al., 1983); 34, Wadi Haijad (Petit-Maire et al., 1987); 35, TaoudenniAgorgott (Petit-Maire et al., 1987; Fabre and Petit-Maire, 1988; Petit-Maire, 1991); 36, Chemchane (Le´zine et al., 1990; Le´zine, 1993); 37, Erg Akchar (Deynoux et al., 1993).

timescales (e.g., Peterson et al., 2000; Baker et al., 2001; Haug et al., 2001; Wang et al., 2001; Yuan et al., 2004) and have also been attributed to shifts in the mean position of the ITCZ controlled by insolation forcing. Here, we report in detail the abrupt climatic transition between the Younger Dryas to Holocene periods, recorded in tropical Southern Africa ca 11 700 years ago, based on a high-resolution 14C-dated sedimentary sequence from Lake Masoko. Climatic proxies extracted from this sequence between 16 and 0 calibrated calendar thousand years before present (cal. ka BP) and comparisons with more regional data demonstrate that this transition is associated with a large modification of the atmospheric circulation over tropical Africa. These observations are concordant with the major reorganisation of the wind patterns in

Africa at this time shown by Filippi and Talbot (2005) and Talbot et al. (2005). The timing of observed climatic changes in Africa are synchronous with South America and East and West Asia, and suggest that the tropics played a major role during this abrupt climatic transition. 2. Modern regional setting The crater-lake Masoko (9120.00 S, 33145.30 E; 840 m above sea level) is a circular maar-volcano (ca 900 m in diameter, Fig. 3A) formed ca 50 000 years ago (Gibert et al., 2002). The Masoko crater belongs to the Rungwe volcanic area, located in the western branch of the East African Rift, north of Lake Malawi (Fig. 1B). The

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

692

gonitised tuff, and basalt or phonolite boulders). The oligotrophic freshwater lake (diameter 700 m; maximum depth 38 m) is closed, without surface outlet.

25

20

A

15

YD

Jan. insolation (W m-2)

10

χ (10-6 m3 kg-1) lf

3. Materials and methods

5

460

9°S

B

450 440

0

430 420

0

10

20 30 Age (cal. ka BP)

40

Fig. 2. (A) Magnetic susceptibility composite profile spanning the last 45 cal. ka BP (50-year averages, black line). (B) January insolation at 91S (Berger, 1978) (grey surface). The susceptibility profile displays two wellmarked ca 23 ka cycles roughly in phase with the summertime insolation, suggesting that orbital precession is a main driver of the African climate (Kutzbach and Street-Perrott, 1985; Pokras and Mix, 1987; Finney et al., 1996; Partridge et al., 1997; Thevenon et al., 2002; Trauth et al., 2003; Garcin et al., 2006a). Triangles show 14C age control points (Gibert et al., 2002). The rectangle indicates the time interval of this study, including the Younger Dryas event (YD, grey band).

monsoon-type climate of Masoko shows a strong seasonal variability, controlled by the annual migration of the ITCZ over Africa (Nicholson, 1996). Rains occur when ITCZ is farther south and brings more than 90% of the total annual rainfall at Masoko (annual rainfall ca 2400 mm), i.e., during the austral summer (December–March) and persists for 2 months (April–May). The six remaining months (June–November), when the ITCZ moves northward, are dry (minimum 2–3 months with less than 50 mm/month rainfall and often without rainfall) and are dominated by strong trade winds. This contrasted distribution of rainfall results today in strong hydrological responses of Lake Masoko with seasonal lake-level fluctuations between 1 and 2 m and the occurrence of Brachystegia/Uapaca deciduous/semi-deciduous ‘Miombo’ woodlands with Zambezian affinities in the lake catchment (White, 1983; Vincens et al., 2003). The presence of this vegetation type rather than a forest type, as would be expected under the present-day high annual rainfall amount, highlights the importance of the seasonal distribution of rainfall, rather than its total amount, on the structure and floristic composition of tropical lowland vegetations. The catchment (tuff ring) has short (50–70 m length) and steep (30–501) slopes (Fig. 3A), and is covered by young ash soils (Fig. 3C) overlying unconsolidated volcanoclastics (pala-

Sediment cores retrieved in the deepest central part of the lake (Fig. 3A) provide a 32 m composite sequence spanning the last 45 cal. ka BP (Garcin et al., 2006c). The sediments mainly consist of organic silty muds interbedded with tephra and turbidite event-like layers. They represent a continuous deposition of lacustrine biogenic elements associated with catchment inputs (clastic and organic), only interrupted by event deposits. The improved corechronology is based on an event-layers-free sequence (with tephras and turbidites removed) (Garcin et al., 2006c) and the age-depth model has been established on the basis of 25 accelerator mass spectrometer (AMS) radiocarbon dates (Gibert et al., 2002). Fifteen radiocarbon dates have been measured for the time-period discussed in this study (16–0 cal. ka BP) (Fig. 4A and Table 1). Pollen samples were chemically treated following the standard method (Faegri and Iversen, 1975). For each sample, at least 500 pollen grains and spores were counted. The identifications were based on the reference collection of some 7000 specimens at CEREGE, Aix-en-Provence, and on specialised publications relevant to East African pollen morphology (Bonnefille, 1971a, b; Bonnefille and Riollet, 1980). Magnetic measurements were carried out on wet samples from lake sediments. The low-field magnetic susceptibility (wlf) was measured on a Kappabridge (KLY-2) and normalised to the dry-mass of the sample. Susceptibilities from lake sediments were then compared with susceptibilities from catchment materials, providing indications on the magnetic particle sources and the associated erosion/ sedimentation processes on the crater. Additional detailed magnetic, sedimentological and geochemical measurements performed both on the catchment and the lake materials that support our interpretations are described by Williamson et al. (1999) and Garcin et al. (2006b, c). 4. Results To help the interpretation of the abrupt Younger Dryas to Holocene climatic transition, a larger time-window is considered here, extending from 16 to 0 cal. ka BP, and documenting a millennial-scale pacing of climatic changes through the last deglaciation to the Holocene including the Bølling–Allerød–Younger Dryas climatic oscillations. 4.1. Hydrological and climatic proxies from the Masoko sediments To infer regional hydrologic and climatic changes recorded in the Masoko sediments we have used two proxies: pollen assemblages (mean sampling resolution

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

A

B

693

C

Lakeshore Lake depth (m) 35 30 25

0

Soil profile -6

3

-1

χ (10 m kg ) lf 2 6 10 14

0 W

E

Depth (m)

5

10-4

χlf (m3 kg-1)

N Lakeshore (840 m a.s.l.) 0 100 200 m Contour intervals = 5 m

Sediment cores

Altitude (m)

W 880 860 840 820 800 780

1 1.5

Tuff 2 2.5

E

Lake

Soil

Modern lacustrine sediments

10-5

Catchment materials

Lacustrine deposits 0

100 200 300 400 500 600 700 800 900 Distance (m)

0

100 200 300 400 Distance to the lake centre (m)

Fig. 3. (A) Morphometric properties of the Masoko crater. Top: map of the crater. Bottom: E–W synthetic profile of crater topography. (B) Distribution of the surface magnetic susceptibilities for the whole crater (log scale), plotted according to the lake depth and to the distance to the lake centre. Highest susceptibilities are found at the lakeshore (the shore is main source of titanomagnetite for the lake) and lowest susceptibilities are found at the lake bottom. Changes in susceptibility from the deep lake are strongly dependent on the shore position, driven by lake-level changes. Inset (scanning electron microscopy image) shows titanomagnetite grains from the lakeshore area. (C) Soil profile from the crater-crest. Magnetic susceptibility indicates a twotime enrichment in titanomagnetite from the soil, relatively to the underlying tuff (bedrock), due to pedogenic processes (i.e., chemical weathering).

ca 300 years, increased to less than 100 years at the Younger Dryas–Holocene boundary) and magnetic susceptibility (sampling resolution ca 30 years). Pollen assemblages, based on a good knowledge of the modern habitat, ecology and distribution in East Africa of the identified pollen taxa, provide indications on regional climatic conditions and provide notably robust information on the duration of the dry season. As a simple concentration proxy for the input of the denser terrigenous minerals derived from the catchment, magnetic susceptibility provides a direct measure of changes in the lake hydrology. At Masoko, low-field magnetic susceptibility of the sediments (wlf) is primarily dependent on the concentration of the coarse detrital multidomain titanomagnetite (Fe3xTixO4) contained in terrigenous inputs (Williamson et al., 1999; Garcin et al., 2006c). This heavy mineral (volumetric mass: ca 5000 kg m3) originates from the crater rocks (tuff ring) and is concentrated on the lakeshore (Fig. 3B). It is subsequently transported to the deep lake by changes in lake-level and wave turbulence at the lakeshore. Today, runoff erosion processes mostly occur in the area of the lakeshore (an unvegetated and saturated zone). In the remaining part of the basin (only 35% of the whole crater

surface), such processes are relatively restricted, due to the high hydraulic conductivity of catchment materials (soil and tuff 41 m h1). Titanomagnetite transportation to the deep lake is mostly constrained by lake processes. Modern observations show that strong lake oscillations and strengthened wind-driven turbulence as well as runoff erosion at the lakeshore allow the transport of the titanomagnetite to the deep lake, particularly during lake lowstands, when the lakeshore progrades basinward. Therefore, sediment magnetic susceptibility can be used as an index of hydrological regime: high susceptibility values indicate stronger seasonal lake-level fluctuations and/or lake lowstands, while low susceptibility values indicate lesser seasonal lake-level fluctuations and/or lake highstands. In addition to lake shoreline processes, changes in catchment vegetation can likely also control the supply of titanomagnetite to the lake. The establishment of a forest in the catchment during wetter periods would increase the tree cover and root cohesion, preventing soil erosion by intercepting the rainfall and promoting the stability of the soil (Dhakal and Sidle, 2003; Keim and Skaugset, 2003; Gyssels et al., 2005). Forest development would also increase pedogenic processes (Lucas, 2001), favouring the

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

694 1

2

3

4

5

6

7

8

9

10

11

HOLOCENE

100

12

13

YD

14

15

B/A

GL

A

200

16

14C

ages

1-σ error 2-σ error

300 400 500 600

B

Long/severe dry season tolerant trees Uapaca nitida Uapaca kirkiana 20 10

Pollen (%)

Composite depth (cm)

0 0

0

30 20

Pollen (%)

40 Long/severe dry season intolerant trees Ulmaceae Moraceae Macaranga

10

60 50 40 30 20 10 0

Herbs

Local trees

14

Lake supplied with (Ti-) magnetite

C

12

40 30 20 10 0

Pollen (%)

20 10 0

Aquatics

Pollen (%)

0

Pollen (%)

0 Shade-demanding plants (Urticaceae)

-6 3 -1 χ lf (10 m kg )

10 8 6 (Ti-) magnetite stored on the catchment

4 2 0 0

1

2

3

4

5

6

7 8 9 10 Age (cal. ka BP)

11

12

13

14

15

16

Fig. 4. Climate proxy data from the Lake Masoko for the period 16–0 cal. ka BP [data are based on a turbidite- and tephra-layers-free sequence (Garcin et al., 2006c)]. (A) Radiocarbon ages (calendar years) measured at various sample depths, shown with 1-s and 2-s confidence intervals (Table 1) (Gibert et al., 2002) and the age model constructed from all the set of dates (grey line) (Garcin et al., 2006c). (B) Pollen taxa distribution. Top pannels: Main tree taxa, providing indications on the length of the dry season, and shade-demanding plants frequencies indicating periods of relatively closed habitat. Bottom pannels: aquatics, herbs and local trees. (C) Masoko low-field magnetic susceptibility (wlf, black line). Higher susceptibility mostly reflects greater magnetite-rich clastic input from lakeshore area and is interpreted to indicate enhanced seasonal lake-level fluctuations and/or lake lowstands. The pollen assemblages and magnetic susceptibility records show the abruptness of the termination of the Younger Dryas-like event (grey band) at Lake Masoko. Labels at the top are Glacial (GL), Bølling/Allerød (B/A), Younger Dryas (YD) and Holocene climatic periods.

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

695

Table 1 Lake Masoko radiocarbon chronology for the period 16–0 cal. ka BP (Gibert et al., 2002) Corrected depth (cm)

Analytical number of AMS-14C ages

Material

14

C age (yr BP)

Calibrated 14C age (cal. yr BP)

Calibrated age 1-s error bounds

Calibrated age 2-s error bounds

17.5 32.6 51.7 121.6 184.3 207.6 245.3 276.6 295.1 316.7 443.2 457.5 486.5 512.4 600.9

Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay Orsay

TOM TOM TOM Charcoal Wood Wood mCHARC mCHARC mCHARC TOM TOM TOM mCHARC mCHARC TOM

270750 400750 965770 1610760 2940770 3750770 4480780 6050760 7070790 6790790 9410780 9660790 11 9707120 11 6707120 13 8307150

305 500 920 1530 3130 4120 5150 6890 7880 7650 10 620 11 140 13 840 13 500 16 450

(429/284) (510/332) (932/794) (1550/1413) (3211/2996) (4232/3986) (5288/4980) (6978/6798) (7974/7795) (7715/7568) (10 740/10 519) (11 197/10 797) (13 967/13 721) (13 660/13 397) (16 734/16 207)

(479/148) (521/316) (1051/730) (1690/1367) (3328/2888) (4401/3903) (5313/4874) (7156/6741) (8039/7686) (7826/7486) (11 070/10 441) (11 225/10 744) (14 104/13 562) (13 761/13 289) (16 990/15 982)

H-1832 H-1836 H-1655 H-992 H-1172 H-1173 H-1774 H-1940 H-1707 H-1699 H-1939 H-1920 H-1731 H-1705 H-1841

Depth scale is based on a turbidite- and tephra-layers-free composite sequence (Garcin et al., 2006c). Abbreviations: TOM, Total Organic Matter; mCHARC, micro-charcoal (hand-picked). 14C calibration method: Program CALIB 5.0.1, Stuiver and Reimer (1993). Error bars correspond to 1-s and 2s deviations.

enrichment of soil in immobile/refractory titanomagnetite (Fig. 3C). Thus, during such wet periods, most of the titanomagnetite remains stored in the catchment where it concentrates. In contrast, during drier periods, the establishment of sparser vegetation (i.e., woodland or grassland) in the catchment would (i) reduce the protection against erosion and (ii) slow down pedogenic processes. This could in turn strengthen erosion by rainfall and increase soil erosion, leading to a destabilization of the soil accumulated during the previous wet phase. During such dry periods, a larger amount of titanomagnetite would be transported to the lake. 4.2. Masoko hydrological changes between 16 and 11.7 cal. ka BP From 15 cal. ka BP, pollen assemblages indicate that an arboreal cover developed progressively in the Masoko catchment during the Bølling–Allerød, reaching a maximum extension between 13.4 and 12 cal. ka BP, when the herbs, mainly grasses, were at minimum frequencies (Fig. 4B). The main tree taxa identified during this period are: Macaranga-type (including Mallotus oppositifolius and Mareya brevipes), Ulmaceae (Celtis and Trema orientalis) and Moraceae [Myrianthus-type (including Musanga), Trilepisium madagascariensis, Milicia-type excelsa (including Antiaris toxicaria and Morus mesozygia) and Ficus]. Today, these taxa mainly occur in semi-deciduous forested environments (occasionally as reforestation pioneers, e.g., Macaranga and Trema), under climatic conditions with an annual precipitation of ca 1000–2000 mm yr1 and a short dry season of 3 months maximum, during which the rainfall is not less than 20 mm (Polhill, 1966; White, 1983; Radcliffe-Smith, 1987; Berg and Hijman, 1989; Vincens

et al., 2000). Some of these tree taxa can also be found locally in gallery forests from surrounding drier areas such as the Zambezian region. However, it is unlikely that these gallery forests existed in the past around Lake Masoko, due to the steep morphology of the catchment (slopes ca 30–501). Based on modern conditions, the best interpretation of the occurrence of these taxa and the development of semi-deciduous forest is as the result of a shorter and less severe dry season. From 15 to 11.8 cal. ka BP, maximum frequencies of Urticaceae are observed (Fig. 4B), indicating shady conditions related to a relatively closed canopy (Friis, 1989), supporting our interpretation of the change in vegetation and the occurrence of humid conditions. From 16 to 12.9 cal. ka BP, the magnetic susceptibility curve shows intermediate values (ca 6  106 m3 kg1) and high variability (Fig. 4C), indicating episodic high susceptibility clastic inputs, probably associated with active lakeshore transportation processes. At 12.8 cal. ka BP, the magnetic susceptibility strongly decreases to its lowest values and remains low until 11.7 cal. ka BP. These changes are roughly synchronous with the Younger Dryas cold episode observed in the Northern Hemisphere ca 12.8–11.6 cal. ka BP (Bard and Kromer, 1995). During the ‘Younger Dryas’ period, volcanic ash deposition at Masoko makes difficult to use diatoms as a clear climate signal (Barker et al., 2003). Instead, the low susceptibility values of the tephra-free record may be interpreted as representing reduced seasonal lake-level fluctuations and/ or a lake highstand. Further, the beginning of this period of low susceptibility coincides with the maximum development of forest in the catchment. This coincidence could indicate, in addition to lakeshore processes, that the catchment vegetation controlled the delivery of titanomagnetite to the lake. Increased tree cover, together with

ARTICLE IN PRESS 696

Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

strengthened root cohesion, may have acted as a buffer and limited the transportation of this heavy mineral from the catchment, thus amplifying the decrease of sediment magnetic susceptibility. 4.3. Masoko hydrological changes between 11.7 and 0 cal. ka BP At ca 11.7 cal. ka BP, there is an abrupt change in the vegetation and magnetic susceptibility at Masoko, which is synchronous with the onset of the Holocene (Fig. 4). The previous forest taxa become scarce and Uapaca develops, indicating that the vegetation responded to a major climatic change. Today, Uapaca, and particularly Uapaca nitida and Uapaca kirkiana, two types identified in the pollen assemblages, forms an important component of deciduous/semi-deciduous Zambezian woodlands (White, 1983; Carter and Radcliffe-Smith, 1988). These woodlands dominate under climatic regime with more than 5 months of low rainfall, including some months without rainfall. The change in the Masoko vegetation therefore indicates the establishment of a more marked seasonal climate ca 11.7 cal. ka BP that persisted during most of the Holocene. The existing semi-deciduous forest, which cannot be sustained under a long and severe dry season, would have disappeared under the increased precipitation seasonality. These results are supported by the magnetic susceptibility, which increases sharply at ca 11.7 cal. ka BP, then remains strong and relatively stable during the early Holocene, with values analogous to the modern values, reflecting primarily the lake response to stronger seasonal contrasts. It should be noted, however, that the more open vegetation cover at this time has probably modified the erosion and transportation processes on the catchment, increasing the supply of titanomagnetite to the lake. 5. Discussion 5.1. Deglacial events in Masoko area compared to the other regions The Masoko magnetic record confirms the global extension of climatic oscillations at the end of the Last Glacial Period. The susceptibility curve clearly shows hydrological changes in tropical Southern Africa, which are in phase with the Bølling–Allerød–Younger Dryas climatic oscillations, identified in Greenland (GISP2 record, Stuiver et al., 1995), the tropical Atlantic (Cariaco record, Hughen et al., 1996) and East Asia (Dongge Cave record, Dykoski et al., 2005) (Figs. 5A, C–F). However, given the dating uncertainty of the current radiocarbon chronology (7200 years), it is impossible to establish an exact synchronicity of these events at Masoko. As shown in Fig. 4, changes in the vegetation at Masoko were not always in phase with changes in the titanomagnetite supply prior to ca 11.7 cal. ka BP. The disassociation between the two signals may be explained by a progressive

increase of moisture from ca 15 cal. ka BP leading to a progressive development of forest in the catchment, which did not significantly affect erosion and transportation processes before ca 12.8 cal. ka BP. Subsequently, and coincident with the maximum expansion of the forest, a hydrological threshold was passed: the titanomagnetite was preferentially stored on the catchment, in probable association with reduced seasonal lake-fluctuations leading to the accumulation of sands on the lakeshore. The development of a semi-deciduous forested environment in the Masoko catchment ca 15 cal. ka BP is consistent with the onset of the ‘African Humid Period’, described in other African sites (Johnson et al., 1996; Williams et al., 2006) including north tropical sites (deMenocal et al., 2000; Le´zine and Cazet, 2005). These hydrological changes at low-latitudes are nearly synchronous with the onset of the Bølling period in the Northern Hemisphere high-latitudes (i.e., Greenland), characterized by an abrupt warming (Severinghaus and Brook, 1999) associated with a strong increase in atmospheric methane levels (Brook et al., 1996). During the Younger Dryas, low lake-levels have been observed in a number of African sites located in the northern tropics and in the northern part of the southern tropics (Fig. 5B). These have been interpreted as resulting from dry climatic conditions with increased northerly tradewinds (Street-Perrott and Perrott, 1990; Talbot and Johannessen, 1992; Roberts et al., 1993; Williamson et al., 1993; deMenocal et al., 2000; Gasse, 2000; Stager et al., 2002; Peck et al., 2004). The general decrease in precipitation in East Africa during this event is not recorded in the Masoko region, where pollen evidence shows a forested environment under humid conditions without a wellmarked dry season, while low magnetic susceptibility values indicate reduced seasonal lake-level fluctuations and/or a lake highstand. This anticorrelation in climatic patterns between areas located north and south of 81S suggests that the Younger Dryas event, at least, was associated with a southward shift of the rainfall belt over Eastern Africa. As previously proposed by Nicholson (1982) and Harrison et al. (1985), the equatorial climatic belt was probably located south of the geographical equator at this time, due to the reversed interhemispheric thermal gradient. The Northern Hemisphere was colder than the Southern Hemisphere, in contrast to the modern pattern. The more humid conditions and weaker seasonal contrasts at Masoko before and during the Younger Dryas indicate the existence of a longer period of rain south of 81S, with possibly two wet seasons, as observed today near the geographical equator (Walter and Lieth, 1960–1967). These changes are supported by the results of recent climate modelling experiments in which high-latitude ice cover variations and the resulting interhemispheric thermal gradients invoke a migration of the ITCZ (Chiang et al., 2003; Chiang and Bitz, 2005; Broccoli et al., 2006). During the Younger Dryas, precession-driven summer insolation (Berger, 1978) was close to a maximum in the

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

northern tropics and close to a minimum in the southern tropics (Fig. 2B) and might normally have been expected to strengthen the summer African Monsoon in the northern tropics (Kutzbach et al., 1993). The reduction in the monsoon circulation during the Younger Dryas must then be explained by another mechanism, in association with the 8

9

10 11

HOLOCENE

12 13 14 YD

A Lake Masoko (9°S)

B/A

15 16 GL

Long/severe dry season tolerant trees

20 10

Pollen (%)

7

40 30

Long/severe dry season intolerant trees

20 10

14

0

(10-6 m3 kg-1)

12

χlf

10 8 6 4

African tropics: lake status High

'Green Sahara'

14°N-23°N 8°S-14°N M. 15°S-8°S

n Intermediate

30 20

Low

10 0

Dry

O

The most abrupt and prominent environmental change recorded in the vegetation and supply of titanomagnetite at Lake Masoko occurs at 11.7 cal. ka BP, synchronously with the Younger Dryas–Holocene transition. However, the observed environmental transition may post-date the true age of the climatic transition due to a possible delayed response of the vegetation to climatic change. Such vegetation lags have already been reported in tropical Africa (e.g., Maley, 1991; Beuning et al., 2003). Concerning the whole tropical vegetation (low latitude), proposed lags are variable, ranging from several decades (Hughen et al.,

-36

-40

D GISP2 CH

800 700 600 500 400

4

E

220 210 200 190 180 170 160

Cariaco grey scale

F

-10 -9 -8

Dongge Cave δ18O

-7 -6 -5

7

8

(ppbV)

-38

-42

(0-255)

? 18

9

10 11 12 13 14 Age (cal. ka BP)

15 16

(‰, V-SMOW)

(‰, V-SMOW)

-34

?

?

C GISP2 δ

thermohaline circulation (Broecker et al., 1989; Stocker, 2000) or perhaps in association with the millennial-scale ENSO-like (Beaufort et al., 2001) and ‘super ENSO’ (Stott et al., 2002) variabilities. Recent studies of the palaeoENSO activity, based on both climate modelling and model-data comparisons, have shown that the alteration of tropical Pacific Ocean sea surface temperatures by gradual orbital forcing could induce abrupt climatic changes of unexpected magnitude, similar to the Younger Dryas event (Clement et al., 2001). More generally, the abrupt onset of the Younger Dryas in Northern Europe and North America is attributed to a shutdown of the thermohaline circulation, triggered by a catastrophic release of ice sheet meltwater (Broecker et al., 1989). The consequence was a rapid drop in northward ocean heat transports leading to an abrupt cooling, particularly in the Northern Hemisphere. The most pronounced of the Northern Hemisphere temperature changes would have reversed the interhemispheric thermal gradient, displacing the meteorological equator south of the geographical equator in Africa, thus inhibiting the monsoon system and depriving most of the tropical Northern Africa of moisture. 5.2. Regional extent and possible large impact of the Younger Dryas–Holocene climatic transition as recorded in tropical Africa

B

2

697

Fig. 5. Comparison of Lake Masoko-sediment record with regional and global climate proxy data for the period 16–7 cal. ka BP. (A) Lake Masoko pollen-inferred length/severity of the dry season (top) and magnetic susceptibility (wlf, bottom). (B) Lake-level status for the African tropics (vertical scale ¼ number of lakes)—compilation based on 41 lakes (see Fig. 1B). Green boxes show the lakes in the far north, i.e., in the Sahara region (14–231N), blue boxes show the lakes north of Masoko and south of the Sahara (81S–141N), and red boxes show the lakes close to and south of Masoko (8–151S). Question marks indicate periods at which lake deposits in the Sahara region may have been partly removed by posterior phases of eolian-erosion activity. (C) Oxygen isotope (Stuiver et al., 1995) and (D) atmospheric methane concentrations (Brook et al., 1996) from the GISP2 ice core, Greenland. (E) Sediment reflectance (grey scale) of the Cariaco marine core (Hughen et al., 1996). (F) Oxygen isotope of the Dongge Cave stalagmite (Dykoski et al., 2005). From ca 14.7 cal. ka BP (dotted grey line), Masoko susceptibility profile correlates well and shows synchronous millennial-scale oscillations with GISP2, Cariaco and Dongge records. Climatic periods and transitions are defined as in Fig. 4.

ARTICLE IN PRESS 698

Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

2004) up to 2000 years (Jennerjahn et al., 2004), probably associated to differential ecological thresholds (Maslin, 2004). Conversely, mid-high latitude vegetation shows lags of o100–200 years during climatic changes and often decadal responses (Peteet, 2000; Williams et al., 2002). A precise definition of any lead or lag of the timing of the Younger Dryas termination in tropical Africa, relative to other regions, is limited by the lack of continuous varved sequences covering this period. Using the appearance of nearly synchronous shifts in tropical Africa, this climatic transition has been dated within the range of ca 12–11.3 cal. ka BP (Talbot and Johannessen, 1992; Roberts et al., 1993; Williamson et al., 1993; Beuning et al., 1998; deMenocal et al., 2000; Gasse, 2000; Stager et al., 2002; Filippi and Talbot, 2005; Schefuss et al., 2005). The return to a more contrasted seasonality in the regional rainfall during this climatic transition, suggests that monsoon activity restarted and that migrations of the ITCZ became more pronounced. A comparison of the Masoko record with palaeoclimatic data from tropical Africa supports widespread recovery of the monsoon circulation as early as the Younger Dryas–Holocene transition (see the reviews of Butzer et al., 1972; Livingstone, 1975; Street and Grove, 1976 for the first evidences of this major climatic change in tropical Africa). The latest preserved snow accumulation recorded on top Mount Kilimanjaro, highest peak of Africa, was estimated at ca 11.7 cal. ka BP (Thompson et al., 2002) and indicates that the African Monsoon was newly active, promoting ice cap growth that continued during the Holocene until the present day. The regional extent of this event is also recorded by the rise, then overflow, of several lakes located to the north of Masoko (Fig. 5B, see also Street and Grove, 1979; Roberts et al., 1993; Williamson et al., 1993; Hoelzmann et al., 1998, 2004; Damnati, 2000). In contrast, the few lakes located close to Masoko (i.e., Cheshi and Malawi) show an opposing signal with generally decreasing water levels after the Younger Dryas (Stager, 1988; Finney et al., 1996; Gasse et al., 2002; Filippi and Talbot, 2005). At the scale of the African continent, the monsoon front reached 141N after ca 11.5 cal. ka BP, then 231N (Sahara region) after ca 10.8 cal. ka BP (Fig. 5B). During the climatic transition to the present-interglacial climate, progressive coupling mechanisms, involving the ocean– atmosphere system, probably mediated the stepwise nature of the monsoon recovery observed across the northern part of tropical Africa. This climatic transition has also been recorded in changes in the tropical African vegetation. As shown in Fig. 6, a latitudinal gradient of rainfall change may have been a significant factor in promoting the northward expansion of forest vegetation. Tree forest taxa such as Ulmaceae, Moraceae and Macaranga, developed from 15 cal. ka BP in the southern Lake Masoko, then disappeared at ca 11.7 cal. ka BP, while in the equatorial Lake Victoria basin (0–21S), they expand only at ca 13.5 cal. ka BP, but remain well established until the Late Holocene

(Kendall, 1969). Tree taxa indicative of a long/severe dry season, such as Uapaca nitida and Uapaca kirkiana, are not found throughout the Lake Victoria pollen record. In the northern tropics, evidence from Lake Tilla (Nigeria, 101N), which today is located in Sudanian savanna, indicates dry conditions before 13 cal. ka BP (Salzmann et al., 2002). At this site, humid tree taxa with guineo-sudanian affinities do not appear until ca 10.4 cal. ka BP, and suggest that the rainfall belt reached the northernmost tropics later than the southern and equatorial regions. These tree taxa include Uapaca species (White, 1983) (excluding U. nitida and U. kirkiana that only occur south of the equator, Hutchinson and Dalziel, 1958), found today at the forest–savanna boundary 350 km south of the site. More recently, pollen data from northern highlands of Tanzania (Lake Emakat, Empakaai Crater, 2.91S, 2300 m a.s.l.) show that the montane forest environments that dominated prior to 10.5 cal. ka BP were replaced by drier and more open formations, implying more humid climatic conditions during the Younger Dryas than during the Early Holocene (Ryner et al., 2006). High-resolution studies of organic sedimentation from Lakes Malawi, Tanganyika and Bosumtwi have highlighted an abrupt change in the wind-driven circulation of these lakes during this climatic transition (Filippi and Talbot, 2005; Talbot et al., 2005). During this time, Lake Malawi changed from a mixing regime determined by mainly northerly winds to one dominated by southerly winds (Filippi and Talbot, 2005). These results support the scenario of a large modification of the atmospheric circulation over the African continent during the end of the Younger Dryas. The trigger of the Younger Dryas abrupt termination remains elusive. In Greenland, this cold event terminated with an abrupt warming of 101C (7 41C) in less than a decade (Grachev and Severinghaus, 2005). Snow accumulation increased twofold in three years (Taylor et al., 1997) and the concentrations of greenhouse gases (CH4, N2O) strongly increased within decades (Brook et al., 1996; Goldstein et al., 2003) (Figs. 5C and D). The extreme rapidity of this high amplitude climatic change, which immediately preceded the moister and warmer presentinterglacial climate, suggests that the atmospheric circulation was the primary conveyor involved, allowing rapid responses, associated with changes in oceanic circulation dynamics (see the review of Vidal and Arz, 2004 for the ocean–atmosphere system coupling). In the absence of any clear evidence concerning the factors influencing the Younger Dryas termination, we speculate that when the influence of the global cold surge weakened, a climatic threshold was rapidly crossed in the tropical areas, probably influenced by the particular orbital configuration affecting the Northern Hemisphere. Indeed, summertime insolation was close to a maximum in the northern tropics. This enhanced solar heating may have allowed a recovery of monsoon activity (Kutzbach and Street-Perrott, 1985) and probably have caused a rapid

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

7

8

9

10

11

HOLOCENE

12

13

YD

14

B/A

15

699

16

GL

Lake Tilla; 690 m asl; 10°N (Salzmann et al., 2002) Pollen (%)

20 10 Uapaca Ulmaceae

0 0

Moraceae Macaranga

0 0

DESERT

No pollen L. Tilla

L. Victoria

Lake Victoria; 1135 m asl; 0-2°S (Kendall, 1969) Equator Uapaca

0 20

L. Masoko

Ulmaceae

0 50 40

DE SE

30

RT

Pollen (%)

10

20

Present-day expansion of the Zambezian phytogeographical region (White, 1983)

Moraceae

10 0

Macaranga

0

Length of the rain season

Lake Masoko; 840 m asl; 9°S (This study) Tree taxa

30 20 Pollen (%)

10

U. nitida and U. kirkiana

0 Ulmaceae

0 10

10

Dry 2 to 3 months

Uapaca

4 to 6 months

Ulmaceae

7 to 9 months

Moraceae Macaranga

Moraceae

0 20

Climatic affinities

10 to 12 months

Macaranga

0 7

8

9

10 11 12 13 Age (cal. ka BP)

14

15

16

Fig. 6. Left: pollen-diagrams of selected tree taxa (Uapaca, Ulmaceae, Moraceae and Macaranga) along a North-South transect across tropical Africa, for the period 16–7 cal. ka BP. Northern tropics: Lake Tilla (Salzmann et al., 2002); equator: Lake Victoria (Kendall, 1969); southern tropics: Lake Masoko (this study). Radiocarbon dates from Lake Tilla and Lake Victoria sites were calibrated using CALIB 5.0.1 (Stuiver and Reimer, 1993). Error bars (1-s) define the age range. Climatic periods and transitions are defined as in Fig. 4. Right: length of the present-day rain season over Africa, a characteristic of rainfall seasonality, modified from Nicholson (2000). The geographic limits of the Zambezian phytogeographical region (thick dotted line) according to White (1983) are also shown. This region, where Uapaca woodlands develop in tropical Southern Africa today, coincides with the climatic region where the length of the rain season is between 4 and 6 months.

movement of the ITCZ to the North. Climate modelling experiments have shown the importance of feedback processes, such as vegetation versus albedo and surface

ocean temperature versus moisture transport feedbacks (Kutzbach et al., 1996; Kutzbach and Liu, 1997; Braconnot et al., 1999; Claussen et al., 1999; deMenocal et al., 2000),

ARTICLE IN PRESS 700

Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

and these feedbacks may have amplified the abruptness of such a transition. For example, the expansion of vegetation will decrease the albedo of the continental masses, in turn enhancing the land–sea thermal contrast, a vector of moisture advection and monsoon circulation (Su and Neelin, 2005). Pronounced latitudinal migration of the ITCZ would have redistributed moisture over the African continent, the broadest landmass of the tropics, increasing the wetland areas, a source of methane gas (Da¨llenbach et al., 2000). The latter is possibly a trigger of climatic changes, due to the key role of atmospheric greenhouse gas (mainly water vapour, carbon dioxide, methane and nitrous dioxide) in the global climatic system (Flu¨ckiger et al., 2004). The major reorganisation of the atmospheric circulation over tropical Africa is also supported by similar changes in South America (Hughen et al., 1996; Lea et al., 2003) (Fig. 5E), East Asia (Wang et al., 2001; Dykoski et al., 2005) (Fig. 5F) and West Asia (Sirocko et al., 1996). The synchronous reactivations of tropical monsoonal systems at this time would have promoted the transfer of moisture and heat from equator to pole, and could explain in part the marked increase of temperature, precipitation and greenhouse gases recorded in the Greenland ice cores (Dansgaard et al., 1989; Taylor et al., 1997; Goldstein et al., 2003; Grachev and Severinghaus, 2005). Finally, the tropics and their specific atmospheric circulation could have acted as amplifiers of the high-latitude climatic changes recorded during the Younger Dryas termination, as formerly suggested from the records of the Asian Monsoon (Sirocko et al., 1996). 6. Conclusion The Lake Masoko sedimentary record offers evidence for drastic climatic changes in southeastern Africa during the last glacial–interglacial transition. High-resolution environmental/climatic proxy indicators, i.e., pollen assemblages and magnetic susceptibility, indicate that during this climatic transition, the local vegetation changed from a type intolerant of a long/severe dry season (dominated by Macaranga, Ulmaceae and Moraceae trees) to a type tolerant to such a climate (dominated by Uapaca trees), while the lake-level was characterized by strong seasonal fluctuations and/or lowstands. Comparisons of the Masoko record with other regional palaeoclimatic data, especially with a compilation of African lake-level reconstructions, indicate that the effects of this climatic transition were widespread in Africa. The proposed African Monsoon failure during the Younger Dryas, associated with a southward position of the meteorological equator in East Africa, was replaced by an abrupt and lasting resumption of monsoon activity and increased latitudinal migrations of the ITCZ over the African continent. However, further high-resolution records covering the last deglaciation in tropical Southern Africa are firmly needed to better constrain the timing of this climatic scenario.

At a global scale, synchronous reorganisations of the atmospheric circulation are observed in tropical regions (i.e., in South America, East and West Asia, and Africa) and could support a role of strengthened monsoon activity in the tropics as a climatic-amplifier (Ivanochko et al., 2005) during this major climatic boundary, which is generally associated with northern high-latitudes temperature and precipitation changes. Acknowledgements We thank M. Taieb and the ECC-RUKWA project team, who collected the cores in 1994 and 1996; P.E. Mathe´, L. Bergonzini, A. Majule, S. Kajula, M. Decobert for technical help and S. Brewer for constructive comments. We also thank M.R. Talbot, D. Taylor and N. Roberts for many valuable comments on this manuscript. We acknowledge the support of the ECLIPSE CLEHA program of the Institut National des Sciences de l’Univers, the ACI Ecologie Quantitative of the French Ministry of Research (Project RESOLVE), the PNEDC/INSU Project ECHO, the EU Project MOTIF (EVK2-CT-2002-00153), the Institute of Resource Assessment (University of Dar es Salaam) and the Tanzania Commission for Science and Technology (COSTECH). References Baker, P.A., Rigsby, C.A., Seltzer, G.O., Fritz, S.C., Lowenstein, T.K., Bacher, N.P., Veliz, C., 2001. Tropical climate changes at millennial and orbital timescales on the Bolivian Altiplano. Nature 409, 698–701. Bard, E., Kromer, B., 1995. The Younger Dryas: absolute and radiocarbon chronology. In: Troelstra, S.R., van Hinte, J.E., Ganssen, G.M. (Eds.), The Younger Dryas, Proceedings of the Royal Dutch Academy of Science, pp. 161–166. Barker, P., Telford, R., Merdaci, O., Williamson, D., Taieb, M., Vincens, A., Gibert, E., 2000. The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change. Holocene 10, 303–310. Barker, P., Telford, R., Gasse, F., Thevenon, F., 2002. Late Pleistocene and Holocene palaeohydrology of Lake Rukwa, Tanzania, inferred from diatom analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 187, 295–305. Barker, P., Williamson, D., Gasse, F., Gibert, E., 2003. Climatic and volcanic forcing revealed in a 50,000-year diatom record from Lake Massoko, Tanzania. Quaternary Research 60, 368–376. Baumhauer, R., 1991. Palaeolakes of the South Central Sahara— problems of Paleoclimatological interpretation. Hydrobiologia 214, 347–357. Beaufort, L., de Garidel-Thoron, T., Mix, A.C., Pisias, N.G., 2001. ENSO-like forcing on oceanic primary production during the Late Pleistocene. Science 293, 2440–2444. Berg, C.C., Hijman, M.E.E., 1989. Moraceae. In: Polhill, R.M. (Ed.), Flora of Tropical East Africa. Balkema, Rotterdam, pp. 1–95. Berger, A., 1978. Long-term variations of caloric insolation resulting from the earth’s orbital elements. Quaternary Research 9, 139–167. Beuning, K., Kelts, K., Stager, C.J., 1998. Abrupt climatic changes associated with the arid Younger Dryas interval in Africa. In: Lehman, J.T. (Ed.), Environmental Change and Response in East African Lakes. Kluwer Academic Publishers, Dordrecht, pp. 147–156.

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704 Beuning, K.R.M., Talbot, M.R., Kelts, K., 1997. A revised 30,000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 259–279. Beuning, K.R.M., Talbot, M.R., Livingstone, D.A., Schmukler, G., 2003. Sensitivity of carbon isotopic proxies to paleoclimatic forcing: a case study from Lake Bosumtwi, Ghana, over the last 32,000 years. Global Biogeochemical Cycles 17, 1–11. Bonnefille, R., 1971a. Atlas des pollens d’Ethiopie: Principales espe`ces des foreˆts de montagne. Pollen et Spores 13, 15–72. Bonnefille, R., 1971b. Atlas des pollens d’Ethiopie: pollens actuels de la basse valle´e de l’Omo (Ethiopie). Re´coltes botaniques 1968. Adansonia 2, 463–518. Bonnefille, R., Riollet, G., 1980. Pollens des savanes d’Afrique orientale. CNRS, Paris, 140pp. Braconnot, P., Joussaume, S., Marti, O., de Noblet, N., 1999. Synergistic feedbacks from ocean and vegetation on the African monsoon response to mid-Holocene insolation. Geophysical Research Letters 26, 2481–2484. Broccoli, A.J., Dahl, K.A., Stouffer, R.J., 2006. Response of the ITCZ to Northern Hemisphere cooling. Geophysical Research Letters 33, L01702, doi:10.1029/2005GL024546. Broecker, W.S., Kennett, J.P., Flower, B.P., Teller, J.T., Trumbore, S., Bonani, G., Wolfli, W., 1989. Routing of meltwater from the Laurentide ice-sheet during the Younger Dryas cold episode. Nature 341, 318–321. Brook, E.J., Sowers, T., Orchardo, J., 1996. Rapid variations in atmospheric methane concentration during the past 110,000 years. Science 273, 1087–1091. Butzer, K.W., Isaac, G.L., Richardson, J.L., Washbourn-Kamau, C., 1972. Radiocarbon dating of East African lake levels. Science 175, 1069–1076. Cane, M.A., 1998. Climate change—a role for the tropical Pacific. Science 282, 59–61. Carter, S., Radcliffe-Smith, A.R., 1988. Euphorbiaceae (part 2). In: Polhill, R.M. (Ed.), Flora of Tropical East Africa. Balkema, Rotterdam, pp. 409–597. Chalie´, F., Gasse, F., 2002. Late Glacial–Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia). Palaeogeography, Palaeoclimatology, Palaeoecology 187, 259–283. Chiang, J.C.H., Bitz, C.M., 2005. Influence of high latitude ice cover on the marine Intertropical Convergence Zone. Climate Dynamics 25, 477–496. Chiang, J.C.H., Biasutti, M., Battisti, D.S., 2003. Sensitivity of the Atlantic Intertropical Convergence Zone to Last Glacial Maximum boundary conditions. Paleoceanography 18, 1094, doi:10.1029/ 2003PA000916. Claussen, M., Kubatzki, C., Brovkin, V., Ganopolski, A., Hoelzmann, P., Pachur, H.J., 1999. Simulation of an abrupt change in Saharan vegetation in the mid-Holocene. Geophysical Research Letters 26, 2037–2040. Clement, A.C., Seager, R., Cane, M.A., 1999. Orbital controls on the ElNin˜o/Southern Oscillation and the tropical climate. Paleoceanography 14, 441–456. Clement, A.C., Seager, R., Cane, M.A., 2000. Suppression of El-Nin˜o during the mid-Holocene by changes in the Earth’s orbit. Paleoceanography 15, 731–737. Clement, A.C., Cane, M.A., Seager, R., 2001. An orbitally driven tropical source for abrupt climate change. Journal of Climate 14, 2369–2375. Da¨llenbach, A., Blunier, T., Flu¨ckiger, J., Stauffer, B., Chappellaz, J., Raynaud, D., 2000. Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Last Glacial and the transition to the Holocene. Geophysical Research Letters 27, 1005–1008. Damnati, B., 2000. Holocene lake records in the Northern Hemisphere of Africa. Journal of African Earth Sciences 31, 253–262. Dansgaard, W., White, J.W.C., Johnsen, S.J., 1989. The abrupt termination of the Younger Dryas climate event. Nature 339, 532–534.

701

deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of the African humid period: rapid climate responses to gradual insolation forcing. Quaternary Science Reviews 19, 347–361. Deynoux, M., Kocurek, G., Benan, C.A.A., Crabaugh, M., Havholm, K., Pion, J.C., 1993. Sequence stratigraphy in a desert environment— example from the Akchar Erg in Western Mauritania (West-Africa). Comptes Rendus de l’Acade´mie des Sciences Paris Se´rie II 317, 1199–1205. Dhakal, A.S., Sidle, R.C., 2003. Long-term modelling of landslides for different forest management practices. Earth Surface Processes and Landforms 28, 853–868. Dykoski, C.A., Edwards, R.L., Cheng, H., Yuan, D.X., Cai, Y.J., Zhang, M.L., Lin, Y.S., Qing, J.M., An, Z.S., Revenaugh, J., 2005. A highresolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave, China. Earth and Planetary Science Letters 233, 71–86. Fabre, J., Petit-Maire, N., 1988. Holocene climatic evolution at 22–231N from two palaeolakes in the Taoudenni area (Northern Mali). Palaeogeography, Palaeoclimatology, Palaeoecology 65, 133–148. Faegri, K., Iversen, J., 1975. Textbook of Pollen Analysis. Blackwell, Oxford, 295pp. Filippi, M.L., Talbot, M.R., 2005. The palaeolimnology of northern Lake Malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter. Quaternary Science Reviews 24, 1303–1328. Finney, B.P., Scholz, C.A., Johnson, T.C., Trumbore, S., 1996. Late Quaternary lake-level changes of Lake Malawi. In: Johnson, T.C., Odada, E.O. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach Publishers, Amsterdam, pp. 495–508. Flu¨ckiger, J., Blunier, T., Stauffer, B., Chappellaz, M., Spahni, R., Kawamura, K., Schwander, J., Stocker, T.F., DahlJensen, D., 2004. N2O and CH4 variations during the last glacial epoch: insight into global processes. Global Biogeochemical Cycles 18, GB1020, doi:10.1029/2003GB002122. Fontes, J.C., Gasse, F., 1991. PALHYDAF (Palaeohydrology in Africa) program: objectives, methods, major results. Palaeogeography, Palaeoclimatology, Palaeoecology 84, 191–215. Friis, I., 1989. Urticaceae. In: Polhill, R.M. (Ed.), Flora of Tropical East Africa. Balkema, Rotterdam, pp. 1–64. Garcin, Y., Vincens, A., Williamson, D., Guiot, J., Buchet, G., 2006a. Wet phases in tropical Southern Africa during the last glacial period. Geophysical Research Letters 33, L07703, doi:10.1029/2005GL025531. Garcin, Y., Williamson, D., Bergonzini, L., Radakovitch, O., Vincens, A., Buchet, G., Guiot, J., Brewer, S., Mathe´, P.E., Majule, A., 2006b. Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years. Journal of Paleolimnology, in press, doi:10.1007/S10933-006-9033-6. Garcin, Y., Williamson, D., Taieb, M., Vincens, A., Mathe´, P.E., Majule, A., 2006c. Centennial to millennial changes in maar-lake deposition during the last 45,000 years in Tropical Southern Africa (Lake Masoko, Tanzania). Palaeogeography, Palaeoclimatology, Palaeoecology 239, 334–354. Gasse, F., Street, F.A., 1978. Late Quaternary lake-level fluctuations and environments of the northern rift valley and Afar region (Ethiopia and Djibouti). Palaeogeography, Palaeoclimatology, Palaeoecology 25, 145–150. Gasse, F., Le´de´e, V., Massault, M., Fontes, J.C., 1989. Water-level fluctuations of Lake Tanganyika in phase with oceanic changes during the last glaciation and deglaciation. Nature 342, 57–59. Gasse, F., Te´het, R., Durand, A., Gibert, E., Fontes, J.C., 1990. The arid humid transition in the Sahara and the Sahel during the last deglaciation. Nature 346, 141–146. Gasse, F., 2000. Hydrological changes in the African tropics since the Last Glacial Maximum. Quaternary Science Reviews 19, 189–211. Gasse, F., Barker, P., Johnson, T., 2002. A 24,000 yr diatom record from the northern basin of Lake Malawi. In: Odada, E.O., Olago, D.O.

ARTICLE IN PRESS 702

Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

(Eds.), The East African Great Lakes: Limnology, Palaeolimnology and Biodiversity. Kluwer Academic Publishers, Dordrecht, Netherlands, pp. 393–414. Gibert, E., Bergonzini, L., Massault, M., Williamson, D., 2002. AMS-C14 chronology of 40.0 cal ka BP continuous deposits from a crater lake (Lake Massoko, Tanzania)—modern water balance and environmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology 187, 307–322. Gillespie, R., Street-Perrott, F.A., Switsur, R., 1983. Post-glacial arid episodes in Ethiopia have implications for climate prediction. Nature 306, 680–683. Goldstein, B., Joos, F., Stocker, T.F., 2003. A modeling study of oceanic nitrous oxide during the Younger Dryas cold period. Geophysical Research Letters 30, 1092, doi:10.1029/2002GL016418. Grachev, A.M., Severinghaus, J.P., 2005. A revised +10 +/ 4 degrees C magnitude of the abrupt change in Greenland temperature at the Younger Dryas termination using published GISP2 gas isotope data and air thermal diffusion constants. Quaternary Science Reviews 24, 513–519. Gyssels, G., Poesen, J., Bochet, E., Li, Y., 2005. Impact of plant roots on the resistance of soils to erosion by water: a review. Progress in Physical Geography 29, 189–217. Haberyan, K.A., Hecky, R.E., 1987. The late Pleistocene and Holocene stratigraphy and paleolimnology of Lakes Kivu and Tanganyika. Palaeogeography, Palaeoclimatology, Palaeoecology 61, 169–197. Harrison, S.P., Metcalfe, S.E., Pittock, A.B., Salinger, M.J., StreetPerrott, F.A., Roberts, N., 1985. A climatic model of the last glacial/ interglacial transition based on palaeotemperature and palaeohydrological evidence. In: Vogel, J.C. (Ed.), Late Cainozoic Palaeoclimates of the Southern Hemisphere. A.A. Balkema, Rotterdam, pp. 21–34. Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C., Rohl, U., 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293, 1304–1308. Haynes, J.C.V., Eyles, C.H., Pavlish, L.A., Ritchie, J.C., Rybak, M., 1989. Holocene palaeoecology of the eastern Sahara; Selima Oasis. Quaternary Science Reviews 8, 109–136. Hendy, I.L., Kennett, J.P., 2003. Tropical forcing of North Pacific intermediate water distribution during Late Quaternary rapid climate change? Quaternary Science Reviews 22, 673–689. Hillaire-Marcel, C., Riser, J., Rognon, P., Petit-Maire, N., Rosso, J.C., Soulie-Marche, I., 1983. Radiocarbon chronology of Holocene hydrologic changes in northeastern Mali. Quaternary Research 20, 145–164. Hoelzmann, P., Jolly, D., Harrison, S.P., Laarif, F., Bonnefille, R., Pachur, H.J., 1998. Mid-Holocene land-surface conditions in northern Africa and the Arabian Peninsula: a data set for the analysis of biogeophysical feedbacks in the climate system. Global Biogeochemical Cycles 12, 35–51. Hoelzmann, P., Keding, B., Berke, H., Kropelin, S., Kruse, H.-J., 2001. Environmental change and archaeology: lake evolution and human occupation in the Eastern Sahara during the Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 169, 193–217. Hoelzmann, P., Gasse, F., Dupont, L.M., Salzmann, U., Staubwasser, M., Leuschner, D.C., Sirocko, F., 2004. Paleoenvironmental changes in the Arid and Subarid Belt (Sahara–Sahel–Arabian Peninsula) from 150 kyr to present. In: Battarbee, R.W., Gasse, F., Stickley, C.E. (Eds.), Past Climate Variability through Europe and Africa. Springer, Dordrecht, The Netherlands, pp. 219–256. Hughen, K.A., Overpeck, J.T., Peterson, L.C., Trumbore, S., 1996. Rapid climate changes in the tropical Atlantic region during the last deglaciation. Nature 380, 51–54. Hughen, K.A., Eglinton, T.I., Xu, L., Makou, M., 2004. Abrupt tropical vegetation response to rapid climate changes. Science 304, 1955–1959. Hutchinson, J., Dalziel, J.M., 1958. Flora of West Tropical Africa, vol I, part two. Oversea Governments and Administrations, London, pp. 297–828 (Revised by Keay, R.W.J. Crown Agents). Ivanochko, T.S., Ganeshram, R.S., Brummer, G.-J.A., Ganssen, G., Jung, S.J.A., Moreton, S.G., Kroon, D., 2005. Variations in tropical

convection as an amplifier of global climate change at the millennial scale. Earth and Planetary Science Letters 235, 302–314. Jennerjahn, T.C., Ittekkot, V., Arz, H.W., Behling, H., Patzold, J., Wefer, G., 2004. Asynchronous terrestrial and marine signals of climate change during Heinrich events. Science 306, 2236–2239. Johnson, T.C., 1996. Sedimentary processes and signals of past climatic change in the large lakes of the East African rift valley. In: Johnson, T.C., Odada, E.O. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach Publishers, Amsterdam, pp. 367–412. Johnson, T.C., Scholz, C.A., Talbot, M.R., Kelts, K., Ricketts, R.D., Ngobi, G., Beuning, K., Ssemmanda, I., McGill, J.W., 1996. Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes. Science 273, 1091–1093. Keim, R.F., Skaugset, A.E., 2003. Modelling effects of forest canopies on slope stability. Hydrological Processes 17, 1457–1467. Kendall, R.L., 1969. An Ecological History of the Lake Victoria Basin. Ecological Monographs 39, 121–176. Kutzbach, J.E., Liu, Z., 1997. Response of the African Monsoon to orbital forcing and ocean feedbacks in the Middle Holocene. Science 278, 440–443. Kutzbach, J.E., Street-Perrott, F.A., 1985. Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP. Nature 317, 130–134. Kutzbach, J.E., Guetter, P.J., Behling, P.J., Selin, R., 1993. Simulated climatic changes: results of the COHMAP climate-model experiments. In: Wright, H.E., Kutzbach, J.E., Webb III, T., Ruddiman, W.F., Street-Perrott, F.A., Bartlein, P.J. (Eds.), Global Climates Since the Last Glacial Maximum. University of Minnesota Press, Minnesota, pp. 24–93. Kutzbach, J.E., Bonan, G., Foley, J., Harrison, S.P., 1996. Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene. Nature 384, 623–626. Lea, D.W., Pak, D.K., Peterson, L.C., Hughen, K.A., 2003. Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination. Science 301, 1361–1364. Le´zine, A.M., 1993. Chemchane, histoire d’une Sebkha. Se´cheresse 4, 25–30. Le´zine, A.M., Cazet, J.P., 2005. High-resolution pollen record from core KW31, Gulf of Guinea, documents the history of the lowland forests of West Equatorial Africa since 40,000 yr ago. Quaternary Research 64, 432–443. Le´zine, A.M., Casanova, J., Hillaire-Marcel, C., 1990. Across an Early Holocene humid phase in western Sahara—pollen and isotope stratigraphy. Geology 18, 264–267. Livingstone, D.A., 1975. Late Quaternary climatic change in Africa. Annual Review of Ecology and Systematics 6, 249–280. Lucas, Y., 2001. The role of plants in controlling rates and products of weathering: importance of biological pumping. Annual Review of Earth and Planetary Sciences 29, 135–163. Maley, J., 1991. The African Rain-Forest vegetation and paleoenvironments during Late Quaternary. Climatic Change 19, 79–98. Maley, J., Brenac, P., 1998. Vegetation dynamics, palaeoenvironments and climatic changes in the forests of western Cameroon during the last 28,000 years BP. Review of Palaeobotany and Palynology 99, 157–187. Maslin, M., 2004. Ecological versus climatic thresholds. Science 306, 2197–2198. Merdaci, O., 1998. Changements climatiques au cours des derniers 30 000 ans en Afrique Sud-Equatoriale (Tanzanie) par l’e´tude des pigments et phe´nols lacustres. Unpublished Thesis, University Aix-Marseille III, 208pp. Nicholson, S.E., 1982. Pleistocene and Holocene climates in Africa. Nature 296, 779. Nicholson, S.E., 1996. A review of climate dynamics and climate variability in eastern Africa. In: Johnson, T.C., Odada, E.O. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach Publishers, Amsterdam, pp. 25–56.

ARTICLE IN PRESS Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704 Nicholson, S.E., 2000. The nature of rainfall variability over Africa on time scales of decades to millenia. Global and Planetary Change 26, 137–158. Pachur, H.J., Kropelin, S., 1987. Wadi Howar—paleoclimatic evidence from an extinct river system in the Southeastern Sahara. Science 237, 298–300. Pachur, H.J., Hoelzmann, P., 1991. Paleoclimatic implications of late Quaternary lacustrine sediments in Western Nubia, Sudan. Quaternary Research 36, 257–276. Pachur, H.J., Hoelzmann, P., 2000. Late Quaternary palaeoecology and palaeoclimates of the eastern Sahara. Journal of African Earth Sciences 30, 929–939. Partridge, T.C., deMenocal, P.B., Lorentz, S.A., Paiker, M.J., Vogel, J.C., 1997. Orbital forcing of climate over South Africa: a 200,000-year rainfall record from the Pretoria Saltpan. Quaternary Science Reviews 16, 1–9. Peck, J.A., Green, R.R., Shanahan, T., King, J.W., Overpeck, J.T., Scholz, C.A., 2004. A magnetic mineral record of Late Quaternary tropical climate variability from Lake Bosumtwi, Ghana. Palaeogeography, Palaeoclimatology, Palaeoecology 215, 37–57. Peteet, D., 2000. Sensitivity and rapidity of vegetational response to abrupt climate change. Proceedings of the National Academy of Sciences of the United States of America 97, 1359–1361. Peterson, L.C., Haug, G.H., Hughen, K.A., Rohl, U., 2000. Rapid changes in the hydrologic cycle of the tropical Atlantic during the last glacial. Science 290, 1947–1951. Petit-Maire, N., 1991. Pale´oenvironnements du Sahara. Lacs holoce`nes a` Taoudenni (Mali). CNRS, Marseille/Paris, 239pp. Petit-Maire, N., Riser, J., 1981. Holocene lake deposits and palaeoenvironments in Central Sahara, Northeastern Mali. Palaeogeography, Palaeoclimatology, Palaeoecology 35, 45–61. Petit-Maire, N., Riser, J., 1983. Sahara ou Sahel? Quaternaire re´cent du Bassin de Taoudenni (Mali). CNRS, Marseille, 473pp. Petit-Maire, N., Fabre, J., Carbonel, P., Schulz, E., Aucour, A.M., 1987. La de´pression de Taoudenni (Sahara malien) a` l’Holoce`ne. Ge´odynamique 2, 127–160. Pokras, E.M., Mix, A.C., 1987. Earths precession cycle and Quaternary climatic-change in tropical Africa. Nature 326, 486–487. Polhill, R.M., 1966. Ulmaceae. In: Hubbard, C.E., Milne-Redhead, E. (Eds.), Flora of Tropical East Africa. Crown Agents for Oversea Governments and Administrations, London, pp. 1–14. Radcliffe-Smith, A.R., 1987. Euphorbiaceae (part 1). In: Polhill, R.M. (Ed.), Flora of Tropical East Africa. Balkema, Rotterdam, pp. 1–407. Richardson, J.L., Dussinger, R.A., 1986. Paleolimnology of mid-elevation lakes in the Kenya Rift Valley. Hydrobiologia 143, 167–174. Ritchie, J.C., 1987. A Holocene pollen record from Bir-Atrun, Northeast Sudan. Pollen et Spores 29, 391–410. Ritchie, J.C., Eyles, C.H., Haynes, C.V., 1985. Sediment and pollen evidence for an Early to Mid-Holocene humid period in the Eastern Sahara. Nature 314, 352–355. Roberts, N., Taieb, M., Barker, P., Damnati, B., Icole, M., Williamson, D., 1993. Timing of the Younger Dryas event in East-Africa from lakelevel changes. Nature 366, 146–148. Russell, J.M., Johnson, T.C., Kelts, K.R., Laerdal, T., Talbot, M.R., 2003. An 11 000-year lithostratigraphic and paleohydrologic record from Equatorial Africa: Lake Edward, Uganda–Congo. Palaeogeography, Palaeoclimatology, Palaeoecology 193, 25–49. Ryner, M.A., Bonnefille, R., Holmgren, K., Muzuka, A., 2006. Vegetation changes in Empakaai Crater, northern Tanzania, at 14,800–9300 cal yr BP. Review of Palaeobotany and Palynology 140, 163–174. Salzmann, U., Hoelzmann, P., Morczinek, I., 2002. Late Quaternary climate and vegetation of the Sudanian Zone of Northeast Nigeria. Quaternary Research 58, 73–83. Schefuss, E., Schouten, S., Schneider, R.R., 2005. Climatic controls on central African hydrology during the past 20,000 years. Nature 437, 1003–1006. Seager, R., Battisti, D.S., 2006. Challenges to our understanding of the general circulation: abrupt climate change. In: Schneider, T., Sobel,

703

A.S. (Eds.), The Global Circulation of the Atmosphere, Princeton University Press, in press. Servant, M., 1983. Se´quences continentales et variations climatiques: e´volution du bassin du Tchad au Ce´nozoique supe´rieur, ORSTOM, Paris, 573pp. Servant, M., Servant-Vildary, S., 1980. L’environnement quaternaire du bassin du Tchad. In: Williams, M.A., Faure, H. (Eds.), The Sahara and the Nile. Balkema, Rotterdam, pp. 132–162. Severinghaus, J.P., Brook, E.J., 1999. Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice. Science 286, 930–934. Sirocko, F., Garbe-Schonberg, D., McIntyre, A., Molfino, B., 1996. Teleconnections between the subtropical monsoons and high-latitude climates during the last deglaciation. Science 272, 526–529. Stager, J.C., 1988. Environmental changes at Lake Cheshi, Zambia since 40,000 years B.P. Quaternary Research 29, 54–65. Stager, J.C., Anfang-Sutter, R., 1999. Preliminary evidence of environmental changes at Lake Bambili (Cameroon, West Africa) since 24,000 BP. Journal of Paleolimnology 22, 319–330. Stager, J.C., Mayewski, P.A., Meeker, L.D., 2002. Cooling cycles, Heinrich event 1, and the desiccation of Lake Victoria. Palaeogeography, Palaeoclimatology, Palaeoecology 183, 169–178. Stocker, T.F., 2000. Past and future reorganizations in the climate system. Quaternary Science Reviews 19, 301–319. Stott, L., Poulsen, C., Lund, S., Thunell, R., 2002. Super ENSO and global climate oscillations at millennial time scales. Science 297, 222–226. Street, F.A., Grove, A.T., 1976. Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa. Nature 261, 385–390. Street, F.A., Grove, A.T., 1979. Global maps of lake-level fluctuations since 30,000 yr B.P. Quaternary Research 12, 83–118. Street-Perrott, F.A., Perrott, R.A., 1990. Abrupt climate fluctuations in the tropics—the influence of Atlantic-Ocean circulation. Nature 343, 607–612. 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. Stuiver, M., Grootes, P.M., Braziunas, T.F., 1995. The GISP2 delta18O climate record of the past 16,500 years and the role of the sun, ocean, and volcanoes. Quaternary Research 44, 341–354. Su, H., Neelin, J.D., 2005. Dynamical mechanisms for African monsoon changes during the mid-Holocene. Journal of Geophysical Research— Atmospheres 110, D19105, doi:10.1029/2005JD005806. Talbot, M.R., Johannessen, T., 1992. A high-resolution paleoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic-matter. Earth and Planetary Science Letters 110, 23–37. Talbot, M.R., Filippi, M.L., Jensen, N.B., Tiercelin, J.-J., 2005. An abrupt change in the African monsoon at the end of the Younger Dryas? Eos Trans. AGU 86(52), Fall Meet. Suppl., Abstract PP21A-1545. Taylor, K.C., Mayewski, P.A., Alley, R.B., Brook, E.J., Gow, A.J., Grootes, P.M., Meese, D.A., Saltzman, E.S., Severinghaus, J.P., Twickler, M.S., White, J.W.C., Whitlow, S., Zielinski, G.A., 1997. The Holocene Younger Dryas transition recorded at Summit, Greenland. Science 278, 825–827. Thevenon, F., Williamson, D., Taieb, M., 2002. A 22 kyr BP sedimentological record of Lake Rukwa (81S, SW Tanzania): environmental, chronostratigraphic and climatic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 187, 285–294. Thevenon, F., Williamson, D., Vincens, A., Taieb, M., Merdaci, O., Decobert, M., Buchet, G., 2003. A late-Holocene charcoal record from Lake Masoko, SW Tanzania: climatic and anthropologic implications. Holocene 13, 785–792. Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Henderson, K.A., Brecher, H.H., Zagorodnov, V.S., Mashiotta, T.A., Lin, P.N., Mikhalenko, V.N., Hardy, D.R., Beer, J., 2002. Kilimanjaro ice core records: evidence of Holocene climate change in tropical Africa. Science 298, 589–593.

ARTICLE IN PRESS 704

Y. Garcin et al. / Quaternary Science Reviews 26 (2007) 690–704

Trauth, M.H., Deino, A.L., Bergner, A.G.N., Strecker, M.R., 2003. East African climate change and orbital forcing during the last 175 kyr BP. Earth and Planetary Science Letters 206, 297–313. Vidal, L., Arz, H.W., 2004. Oceanic climate variability at millennial timescales: models of climate connections. In: Battarbee, R.W., Gasse, F., Stickley, C.E. (Eds.), Past Climate Variability through Europe and Africa. Springer, Dordrecht, The Netherlands, pp. 31–44. Vincens, A., Dubois, M.A., Guillet, B., Achoundong, G., Buchet, G., Kamgang Kabeyene Beyala, V., de Namur, C., Riera, B., 2000. Pollen–rain–vegetation relationships along a forest–savanna transect in southeastern Cameroon. Review of Palaeobotany and Palynology 110, 191–208. Vincens, A., Williamson, D., Thevenon, F., Taieb, M., Buchet, G., Decobert, M., Thouveny, N., 2003. Pollen-based vegetation changes in southern Tanzania during the last 4200 years: climate change and/or human impact. Palaeogeography, Palaeoclimatology, Palaeoecology 198, 321–334. Visser, K., Thunell, R., Stott, l., 2003. Magnitude and timing of temperature change in the Indo-Pacific warm pool during deglaciation. Nature 421, 152–155. Walter, H., Lieth, H., 1960–1967. Klimadiagramm-Weltatlas. FischerVerlag, Jena. Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.C., Dorale, J.A., 2001. A high-resolution absolute-dated Late Pleistocene monsoon record from Hulu Cave, China. Science 294, 2345–2348.

White, F., 1983. The Vegetation of Africa. A Descriptive Memoir to Accompany the UNESCO/AETFAT/UNSO Vegetation Map of Africa, Paris, 356pp. Williams, J.W., Post, D.M., Cwynar, L.C., Lotter, A.F., Levesque, A.J., 2002. Rapid and widespread vegetation responses to past climate change in the North Atlantic region. Geology 30, 971–974. Williams, M.A.J., Talbot, M.R., Aharon, P., Salaam, Y.A., Williams, F., Brendeland, K.-I., 2006. Abrupt return of the summer monsoon 15,000 years ago: new supporting evidence from the lower White Nile valley and Lake Albert. Quaternary Science Reviews 25, 2651–2665. Williamson, D., Taieb, M., Damnati, B., Icole, M., Thouveny, N., 1993. Equatorial extension of the Younger Dryas event—rock magnetic evidence from Lake Magadi (Kenya). Global and Planetary Change 7, 235–242. Williamson, D., Jackson, M.J., Banerjee, S.K., Marvin, J., Merdaci, O., Thouveny, N., Decobert, M., Gibert-Massault, E., Massault, M., Mazaudier, D., Taieb, M., 1999. Magnetic signatures of hydrological change in a tropical Maar-lake (Lake Massoko, Tanzania): preliminary results. Physics and Chemistry of the Earth Part a—Solid Earth and Geodesy 24, 799–803. Yuan, D.X., Cheng, H., Edwards, R.L., Dykoski, C.A., Kelly, M.J., Zhang, M.L., Qing, J.M., Lin, Y.S., Wang, Y.J., Wu, J.Y., Dorale, J.A., An, Z.S., Cai, Y.J., 2004. Timing, duration, and transitions of the Last Interglacial Asian Monsoon. Science 304, 575–578. Zebiak, S.E., Cane, M.A., 1987. A Model El-Nin˜o Southern Oscillation. Monthly Weather Review 115, 2262–2278.