Late Quaternary stream sedimentation in the humid tropics: a review with new data from NE Queensland, Australia

Geomorphology 39 Ž2001. 53–68 www.elsevier.nlrlocatergeomorph

Late Quaternary stream sedimentation in the humid tropics: a review with new data from NE Queensland, Australia Michael F. Thomas a,) , Jonathan Nott b, David M. Price c a

Department of EnÕironmental Science, UniÕersity of Stirling, Stirling FK9 4LA, UK b James Cook UniÕersity of N Queensland, Cairns, Australia c UniÕersity of Wollongong, NSW, Wollongong, Australia

Received 2 November 1999; received in revised form 28 April 2000; accepted 17 October 2000

Abstract There is now a wide agreement that temperature depression in the humid tropics during the Last Glacial Maximum ŽLGM. was at least 58C. Most estimates of precipitation reduction at the LGM range from 25–30% to 50–65%, based on proxy data, but the recent CCM1 model envisages only around 12%. Dates obtained from river sediments indicate major changes to fluvial activity in the late Quaternary. Isotope Zone 3 sediments Ž58–28 ka BP. are widespread and possibly indicate cooler conditions. Post-28 ka BP, and certainly post-21 ka BP, river regimes altered radically towards fan building, braiding or major reduction in all activity. This paper reports on fan formation in NE Queensland between 26 and 14 ka BP and reviews evidence for comparable changes in humid tropical areas of S America, W Africa and SE Asia, including records of Holocene sedimentation. Within a global rhythm of major changes to river regimes in the humid tropics during the late Quaternary, it is now possible to detect regional variations in stream response to climatic change. At the LGM, reductions in stream power may have led to fan formation in NE Queensland, while vegetation changes may have contributed to increased sediment loads and braiding in some forest marginal areas. But, in W Africa, greater aridity may have been responsible for enfeebled streams leaving few records. Channel cutting, then deposition of coarse sediment in braided rivers marked the transition to the early Holocene in W Africa, and fans became entrenched in NE Queensland. This regime persisted until forest recovery was complete by 9.5–8.5 ka BP, when widespread overbank deposition occurred and a change towards meandering channels took place widely across the humid tropical zone, followed by several cut-and-fill episodes in the middle and late Holocene. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Alluvial fans; Australia; Climatic change; Humid tropics; Late Quaternary

1. Introduction There is now a wide agreement that temperature depression in the humid tropics during the LGM was ) Corresponding author. Tel.: q44-1786-467840; fax: q441786-467843. E-mail address: [email protected] ŽM.F. Thomas..

at least 58C, following fresh interpretations of ice core and ocean-drilling records ŽGuilderson et al., 1994; Thompson et al., 1995, 1998; Broecker, 1995; Stute et al., 1995., and this accords with the recent Community Climate Model, Version 1 ŽCCM1. output ŽKutzbach et al., 1998.. Previous disagreement persisted for nearly two decades, following the acceptance of the CLIMAP Project Members Ž1976,

0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 0 1 . 0 0 0 5 1 - 4

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1981. predictions of ice age SSTs in tropical latitudes that limited change to 28C. This view produced many conflicts with interpretations of empirical data ŽRind and Peteet, 1985; Lautenschlager, 1991; Guilderson et al., 1994.. It is less easy to point to an agreed position concerning the fluctuation of precipitation amount, intensity and seasonality during this period. The recent modelling by Kutzbach et al. Ž1998. indicates precipitation over much of the tropics remained at 88% of present-day means at the LGM, a reduction of only 12%, although they note that the weakened summer monsoon may have led to a greater decline in SE Asia. Simulation for 6000 BP predicts rainfall 5% above present. Neither of these figures matches closely interpretations of palaeo-lake level data or interpretations of stream sedimentation in Africa ŽThomas and Thorp, 1980; Thorp and Thomas, 1992; Thomas, 1999.. However, the Universities Global Atmospheric Modelling Programme ŽUGAMP. GCM of Dong et al. Ž1996. indicates a regionally averaged decline in precipitation at 21 000 BP Žcal. of 52.6% for N Africa Ž10–308N. and 65.3% for S Asia Ž20–358N., with precipitation minus evaporation ŽP y E. decreases of around 80% for both regions. By 16 000 BP Žcal . or 13 500 BP Ž14 C., the CCM1 model shows that orbital forcing would have enhanced precipitation considerably in the low latitudes, but predicts that the greatest change to monsoon summer precipitation in N Africa and Asia came at 11 000 BP Žcal. or 9000 BP Ž14 C.. Predictions for 6000 BP Žcal. are similar to 11 000 BP Žcal.. The UGAMP model shows an increase in rainfall above present-day means of 14.4% at 6000 BP Žcal. in N Africa, while southern Asia increases by 10.4%. When compared with predictions for 21 000 BP Žcal., these figures suggest major changes Ž) 60%. to rainfall totals and, while these figures do not apply to the inner tropics, it is difficult to envisage that changes would have been negligible in neighbouring areas. Other estimates of precipitation reduction at the LGM, based on interpretations of proxy data, range from Ž25. 30% to 50 Ž65.% ŽPeters and Tetzlaff, 1990; Heaney, 1991; Van der Hammen and Absy, 1994; Van der Hammen and Hooghiemstra, 2000; Verstappen, 1994, 1997.. A 200-fold increase in dust at the LGM found in an ice core from the Peruvian Andes by Thompson

et al. Ž1995. can be interpreted as an indication of Amazonian dryness. However, similar increases in dust content within Atlantic ocean cores was suggested by Ruddiman Ž1997. to be more of an indication of increased wind strengths. This is not necessarily an alternative explanation, because drying out of the climate in Africa was probably associated with increased strength of the NE Trades. It is, therefore, more a question of the degree of aridity that can be inferred from proxy data of this kind. A similar argument surrounds the interpretation of palaeo lake levels. Lake catchments respond to changes in the hydrologic conditions, arising from P–E variations, rainfall intensity and ground cover, which all influence the runoff coefficient. But very low lake levels at the LGM, such as those recorded in E Africa for Lake Albert Žy54 m. ŽBeuning et al., 1997., and Lake Tanganyika Žy250 to y) 300 m. ŽGasse et al., 1989; Baltzer, 1991; Vincens et al., 1993., appear to require more than reduced cloudiness and increased wind strengths ŽSee Fig. 1 for location of sites.. Pollen spectra have been the most widely used proxy data source for palaeoclimatic interpretation, and inferences regarding shifts of regional rainfall means based on cores from lake and mire sites converge to show a decline of the rainforests over wide areas of the tropics. Servant et al. Ž1993. drew comparisons between Carajas ´ in N Brazil, Salitre in SE Brazil, and Barombi Mbo in S Cameroun, and found that all three sites were forested until after 30 000 BP Ž14 C.. Regression or fragmentation of the rainforest appears to have commenced shortly after this at Carajas ´ and Salitre, and also at Lake Bosumtwi ŽMaley, 1987., but was delayed until after 23 000 BP Ž14 C. at Barombi Mbo. At these and many other sites in the humid tropics, the rainforest was not re-established until after 9500 BP Ž14 C.. Although disagreement continues on the question of rainforest regression and refugia Žsee Jolly et al., 1998; Colinvaux et al., 2000; Gasse, 2000., it has been suggested for both Africa and S America that rainforests will have survived in those areas where rainfall today exceeds 2500–3000 mm ay1 ŽVan der Hammen and Absy, 1994; Van der Hammen and Hooghiemstra, 2000; Maley, 1987, 1996.. Between SE Asia and N Australia, low sea levels at the LGM produced an extended landmass, only interrupted by

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Fig. 1. Location maps for field sites mentioned in the text. Shaded areas represent forest refuges at the last glacial maximum as proposed by Adams and Faure Ž1995..

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a narrow seaway E of Timor, and Adams and Faure Ž1995, 1997. have indicated extension of savannas across this zone. In the context of the Amazon basin, the sites along the Andean foothills of western Amazonia may have been relatively cold but also remained moist at the LGM ŽColinvaux et al., 1997.. A similar argument might apply to sites in southern Cameroun. But sites in plateau areas surrounding major river basins such as the Amazon and Congo should have broad regional significance. In addition to the record at Carajas, ´ a site at Katira in Rondonia Ž98S, rainfall 2000–2500 mm ay1 . appears to show replacement of forest by savanna after 44 000 BP Ž14 C., and at 18 500 BP Ž14 C.. Savanna conditions also prevailed along the Guiana coastlands as far north as Georgetown, according to Van de Hammen and Absy Ž1994.. On the southern Bateke ´ ´ Plateau in Congo Ž1–48S, with rainfall 1300–1900 mm ay1 ., Elenga et al. Ž1994. concluded from analysis of swamp pollen that hydromorphous forest gave way to hygrophytic grassland after 24 000 BP Ž14 C. until around 13 000 BP Ž14 C.. The paucity of available pollen data for the Late Quaternary in humid tropical Africa is reflected in the data set presented by Jolly et al. Ž1998, Table 4., which contains no sites within the present-day tropical rainforests and only four within the tropical seasonal forests, from a total of 68 pollen records. The significance of gaps in the stratigraphic record may be missed, especially those indicated by pollen sequences. This has been stressed by Ledru et al. Ž1998., who re-analysed sediments from seven pollen sites in Brazil and concluded that in none of them were sediments from the LGM represented, the drier climates after 24 000 BP Ž14 C. resulting in a hiatus of several thousand years, terminating around 17 000 BP Ž14 C.. Such a hiatus has long been known from the alluvial record in West Africa ŽThomas and Thorp, 1980; Hall et al., 1985; Thomas et al., 1985; Thorp and Thomas, 1992.. A different question surrounds the issue of rainfall reduction and its significance to geomorphic systems in the tropics. Research by Knox Ž1984, 1993. on the Mississippi, indicates that overbank flooding was significantly affected during the Holocene by small shifts in annual precipitation in the order of 10% and temperature changes of - 18C. If similar reasoning applies to the humid tropics, then it might not be

necessary to invoke large changes to climatic means in order to bring about significant alterations to stream behaviour. This might also imply that major vegetation changes are not a necessary precursor to altered patterns of erosion and sedimentation in rivers. It is, therefore, necessary to adduce specific evidence from the sedimentary record, to support the conclusion that major shifts in rainfall amounts have been involved. Published evidence of this kind ŽThomas, 1994; Thomas and Thorp, 1995; Thorp and Thomas, 1992. tends to support three major conclusions. Ž1. There was a long period of significant and widespread sedimentation during Isotope Zone 3, from ca. 58 to 27 ka BP. This appears to have been a period intermediate between interglacial and full glacial conditions, perhaps with reduced temperatures and rainfall in equatorial lowlands. Oscillations between humid and sub-humid conditions continued in W Africa until around 21 ka BP Ž14 C.. Ž2. A major change to fluvial activity took place during a variable period of 4–8 ka between 21 and 13 ka BP Ž14 C.. This is marked by a hiatus in the sedimentary record of many rivers, and a change of regime in others. Ž3. A rapid change in climate following the last glacial termination led to channel cutting and flood deposition in both small and large rivers from 12.7 until ca. 10 ka BP Ž14 C.. Sediment sequences can be interpreted to imply reduced flows during the Younger Dryas, and midHolocene ‘dry’ phases, but their resolution is currently insufficient to confirm these oscillations ŽThomas and Thorp, 1995.. The importance of many other indications of aridity reported in the literature on the humid tropics is often overlooked, mainly because features remain undated. Severe aridity in the forested areas of S Ghana ŽBirim River area. is indicated by deep desiccation cracks with sand infilling, found in weathered phyllite below the margins of shallow tributary valleys excavated for diamonds, and first recognised by Junner Ž1943.. The diamonds come from poorly sorted sediments thought to be mudflow lobes. Stoneline deposits are controversial in origin ŽThomas, 1994.. In the outer forest and savanna zones of Africa and SE Brazil, they commonly exhibit thick accumulations Žtypically 0.2–0.5 m. of

M.F. Thomas et al.r Geomorphology 39 (2001) 53–68

weathered and patinated clasts together with duricrust fragments. These sediments probably have extended histories and are polygenetic, but many can be interpreted as having passed through a period of surface exposure, and concentration as lag deposits, under an open vegetation cover.

2. Late Quaternary stream sedimentation in the lowland humid tropics With the knowledge that quite small shifts of climate means may imply important changes to patterns of storm events and flooding in rivers ŽKnox, 1993., together with the clear implication from other proxy data that rainfall shifts in many parts of the tropics may have exceeded 50%, major changes in catchment behaviour during the later Quaternary will have occurred. Here, new data from NE Queensland, Australia will be compared with the partial chronologies available for sites in Africa, Brazil and SE Asia. 2.1. AlluÕial fan formation in NE Queensland, Australia The humid tropical areas of NE Queensland and Northern Australia have been neglected by geomorphologists working in Australia ŽTooth and Nanson, 1995., despite the Lynch’s Crater pollen record, which is widely quoted in relation to the Quaternary in the tropics ŽKershaw, 1976, 1978, 1992.. However, Nanson and Price Ž1998. have presented a large data set, based on TL dates for rivers across Australia, and these data indicate that the interstadials of the marine oxygen isotope Stages 7, 5 and 3 were wet, while the intervening stages were relatively dry. A dry LGM in the south and east was flanked by very wet conditions before and after, and the LGM itself may have been wet in parts of the far north. The Atherton Tableland Ž17810X Lat.., where Lynch’s Crater is located ŽFigs. 1 and 2., is bordered to the east by an escarpment, where rainfall can range from 3000 to ) 8000 mm ay1 . A reduction of 50% in these values would result in a highly seasonal climate, still subject to heavy rains from 1500 to ) 4000 mm ay1 . Forest would probably survive in the wettest areas, sustained by high moisture

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surpluses from the wet season, but would give way to more open communities below ca. 2000 mm ay1 in a climate with an extended dry season. The pollen record from Lynch’s Crater shows a generally drier climate after ca. 79 000 BP Žestimated reduction from ) 2500 to ca. 1500 mm ay1 ., and intensified aridity after 38 000 BP Ž14 C. with a possible minimum rainfall of - 800 mm ay1 around the LGM ŽKershaw, 1978, 1992.. If these estimates are correct, then the rainfall reduction at the LGM would have been 68%. Interestingly, however, Nott and Price Ž1999. have adduced evidence for major palaeofloods from TL dated plunge-pool deposits at the Wangi Falls, 150 km S of Darwin ŽFig. 1., dating to the period culminating at the LGM Ž30 000 BP–ca. 22 000 BP-TL., and to the 9000–4000 BP ŽTL. period. The occurrence of such extreme events may imply enhanced seasonal precipitation from the NW monsoon, across the N Australian coastlands during the period up to the LGM, while rainfall decreased sharply along the NE coast of Queensland, as a result of the weakening of the SE Trades. Such regional differences in the direction of climatic changes during the Quaternary emphasise the need to distinguish the synchrony of climatic oscillation from the nature and direction of climatic change. The absence of any clear climate signal for Stage 3 in the tropical N of Australia was noted by Kershaw and Nanson Ž1993., but the significance of this observation, based mainly on the evidence from Lynch’s Crater, is not clear on present information. We report here first results of a study of alluvial fans, which extend from the E facing escarpment to below sea level in the area around Cairns in NE Queensland ŽFig. 2.. Some information on Quaternary sedimentation in the area has been provided in the course of geological mapping ŽWillmott and Stephenson, 1989., and the hydroclimatology has also received some emphasis ŽBonell, 1991., but hitherto there have been no dated sequences from these sediments. Quartz sand samples from the fans have been dated using thermoluminescence ŽTL, see Note 1., and radiocarbon dates have been obtained for one section using soil carbon Žno charcoal. from five palaeosols. The preliminary results appear to show that the main phase of fan building took place during a period of 10 000 years, bridging the LGM from ) 25 000 to - 15 000 BP ŽTL. ŽTable 1..

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Fig. 2. NE Queensland, showing sampling sites.

The samples came from several different catchments ŽFig. 2.. Farthest north is the Kewarra fan,

which is situated in an embayment within the escarpment and has received colluvium as well as channel

M.F. Thomas et al.r Geomorphology 39 (2001) 53–68 Table 1 TL dates for sands from alluvial fans in the cairns area, NE Queensland, Australia Mulgrave River fan

9.4"0.7 ka 17.9"1.5 ka 25.9"3.2 ka Behana Creek fan 17.4"2.0 ka Freshwater fan 15.2"1.8 ka 18.5"3.1 ka Kewarra fan 5.5 ka"0.4 ka Žproximal. 14.4 ka"1.3 ka Ždistal. 25.7 ka"4.2 ka Miriwinni fan palaeosols Ž14 C dates from a single profile. a M5 P-S 3010"170 BP M4 P-S 14 220"550 BP M3 P-S 14 760"670 BP b M2 P-S 8969"64 BP b M1 P-S 16 970"140 BP a TL analyses for these sediments did not produce reproducible results, possibly because of the presence of feldspar from granite in the catchment. b These samples were dated using AMS.

sediments. The oldest sediments are older than 25 000 BP, based on TL determinations, and are found in stream deposits 3 km from the source catchments and close to sea level. The distal end of the fan is graded to below present sea level. Overlying these sediments and higher up the fan surface is a later suite of ochreous sands dated to ) 14 000 BP ŽTL.. Fine colluvial sand near the apex gave a date of 5000–6000 BP ŽTL.. Freshwater Creek drains from the Lamb Range above the escarpment ŽFig. 2., via a series of waterfalls, and then flows N in a well-defined valley flanked by hills. Extensive terrace units are found in this valley and there are colluvial sediment lobes descending from the hills, which appear to overlie the terrace but a clear stratigraphic relationship has not been established. In one section exceeding 200-m long, cobble gravels are overlain by thick, poorly stratified fine sandy gravel. Two samples from this material returned dates of 18 500 and 15 200 BP ŽTL.. Farther south, the Mulgrave River drains a catchment extending above the main escarpment, then flows NE to the town of Gordonvale, before swinging through 908 and flowing SE to the ocean ŽFigs. 2 and 3.. Extensive terrace units are underlain by at

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least 8 m of sediment in the vicinity of Gordonvale, where they comprise palaeofans cut by the presentday river. The oldest sediments, sampled at 7.5 m depth, date from around 26 000 BP ŽTL. and sedimentation continued until at least 17 000–18 000 BP ŽTL. at Mulgrave Terrace. Shallow sediments at 2.0-m depth from a nearby site at Behana Creek, which drains a separate catchment, also gave a date of ca. 17 500 BP ŽTL.. Near Gordonvale at the apex of the Mulgrave Fan, one sample at 3.0 m produced a date of 9000–10 000 BP ŽTL.. The most southerly site is found on the Miriwinni River, and forms a high terrace above the present stream ŽFig. 2.. The catchment for this river is mostly granite and the sediments are very coarse sand or fine gravel, with little clay and silt present. A series of 5 A h horizons, marking buried soils developed on the sands, were sampled for radiocarbon dating. It is assumed that the soils developed on abandoned alluvial surfaces and were subsequently buried passively by later flood deposits. No charcoal was included in the analysis. The results ŽTable 1. show that the sediments were laid down between 17 000 and 14 000 BP Ž14 C.. The much younger, near surface sample, is probably contaminated with recent carbon, but there is also one age reversal that is not explained. The coastal plains N and S of Cairns are thus comprised mainly of coalescing or adjacent alluvial fans, at least some of which appear to continue below sea level. They mark a period of copious sedimentation, and are now dissected by the modern drainage. The sedimentation appears to have begun before 26 000 BP ŽTL., but how long before is not known. Deposition appears to have been more or less continuous until well after 17 000 BP ŽTL.. In Freshwater Creek, the most recent sediments of the terrace have an age between 17 000 and 13 400 BP ŽTL., while at Miriwinni, the buried carbon has an age between 14 772 and 13 672 BP Ž14 C.. The soil carbon is likely to be younger than the sediments in which it is found. This factor, plus the calibration error suggests that the Miriwinni sediments were deposited between ca. 19 000 and - 16 000 BP ŽCal... The only Holocene dates come from near surface samples towards the apexes of the Kewarra and Mulgrave fans and we feel unable to say much about these here. Some very coarse cobble and boulder units

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Fig. 3. Detail of field sites near Gordonvale, related to the Mulgrave River. Geological detail after Willmott and Stephenson Ž1989..

found below the sandy materials could not be sampled but are presumably older. An interpretation of these dates is that the copious sedimentation in the scarpfoot zone was due to loss of stream power consequent on reduction in rainfall, combined with an increased sediment yield resulting from a more open vegetation canopy over much of the catchment area. If the pollen record at Lynch’s Crater, which is within 10–20 km of the upper catchment of the Mulgrave River ŽFig. 2., is used as a guide, aridification intensified after 38 000 BP and became even more severe around the LGM, with a reduction in rainfall of ) 60% ŽKershaw, 1978, 1992.. Such a reduction in precipitation could have been accompanied by a decrease in heavy rain from typhoons, a factor leading to copious sedimentation

in Japan as shown by Sugai Ž1993.. Thus, in this humid tropical location, a major reduction in rainfall probably led to fan formation along the escarpment, and where major streams exit on to the coastal plain.

3. Discussion This area of NE Australia probably experienced much greater fluctuations in rainfall amount and intensity during the Quaternary than many inner tropical areas, as suggested by Knox Ž1995.. It is, therefore, interesting to compare the Queensland record with published results from other tropical locations.

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3.1. Western and equatorial Africa In W Africa ŽThomas and Thorp, 1980., sediments from the headwater streams of the Rivers Moa and Sewa, in forest marginal areas of Sierra Leone Ž98N Lat., rainfall 2000 q mm ay1 ., yielded no radiocarbon dates Žfrom ) 30 samples. between around 20 500 and 12 700 BP Ž14 C.. This finding was confirmed by subsequent work on the River Birim in Ghana, within 50 km of Lake Bosumtwi Ž68N Lat., rainfall 1750 q mm ay1 . ŽHall et al., 1985.. This lake became low and saline after 27 500 BP Ž14 C., then fluctuated in composition and level until a high stand Žq45 m. was achieved after 12 500 BP Ž14 C. ŽTalbot et al., 1984; Talbot and Johannessen, 1992.. Prior to about 21 000 BP Ž14 C., the rivers of the W African region had broad floodplains, represented today by a low terrace, and there is evidence of thick, fine overbank deposits and widespread swamps. There are also indications of cut and fill episodes, with post-27 000 BP Ž14 C. clays inset into older clay deposits. The long hiatus that followed, of around 8 ka, is attributed to a major decline in runoff and absence of significant sedimentation. After 12 700 BP Ž14 C., periods of channel incision and coarse sedimentation mark a different regime, the earliest phases of which are represented by episodes of strong scour and deposition of coarse gravels, presumably responding to high peak flows during the Pleistocene–Holocene transition. But the climate was possibly highly variable with an arid Younger Dryas period intervening, before the main pluvial phase of the early Holocene began after 10 000 BP Ž14 C. and continued for around 3 ka, marked by abundant sedimentation. The earliest ‘post-glacial’ sediments at both field sites are commonly found in buried bedrock channels cut 5 m below the low terraces of 20 000 q BP Ž14 C.. In the Congo Basin, Preuss Ž1990. has recorded deposition by braided, sand-bed streams from 23 000 to 17 200 BP Ž14 C., followed by progressively higher humidity and a return to meandering rivers between 17 200 and 11 500 BP Ž14 C.. But this apparently very early return to humid conditions is not found in the E African lakes, neither is it apparent from the marine record of terrigenous sedimentation from the Congo River ŽJansen et al., 1984., where arid conditions lasted until 14 500 BP Ž14 C.. Indications of strong

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morphodynamic activity in the eastern part of the Congo Basin were interpreted as indicators of more open, savanna conditions by Runge Ž1992, 1996., who obtained dates of 17 000–18 000 BP Ž14 C. for unidentified trees buried in allochthonous sediments. Kadomura Ž1995. summarised the data for humid equatorial Africa, and suggested that prolonged semi-arid conditions with braided rivers prevailed from 70 000 to 40 000 BP. Humid conditions, accompanied by podzolisation prevailed from 40 000 to 30 000 BP Ž14 C.; cooling and semi-aridity occurred between 30 000 and 12 000 BP Ž14 C., and warming with reforestation and meandering rivers developed post-12 000 BP Ž14 C., lasting until around 3000 BP Ž14 C.. 3.2. Amazonia and SE Brazil Major environmental changes took place during the late Quaternary in the forested areas of both Amazonia and SE Brazil ŽIriondo and Latrubesse, 1994; Latrubesse and Franzinelli, 1998; Latrubesse and Rancy, 1998; Ledru, 1993; Ledru et al., 1996; Van der Hammen and Absy, 1994; Van der Hammen and Hooghiemstra, 2000; Stevaux, 1994; Stevaux and dos Santos, 1998.. The earliest dated cold and arid period was prior to 40 000 BP Ž14 C., while a long cool moist episode from ) 40 000 to 27 000 BP Ž14 C. has parallels with findings from other areas. This period of sedimentation, dating to Isotope Zone 3 Ž58–27 ka BP., was recorded from the Caqueta´ River in Colombian Amazonia ŽVan der Hammen et al., 1992a,b; Van der Hammen and Hooghiemstra, 2000. and scattered indications of cold arid conditions around 50 000 BP have been recorded for Andean tributaries in Venezuela ŽSchubert, 1988.. Ledru et al. Ž1996. also suggested an arid phase in southern Brazil between ca. 50 000 and 40 000 BP. Other instances have been recorded in SW Amazonia ŽLatrubesse and Ramonell, 1994. and SE Brazil ŽRiccomini et al., 1989.. Latrubesse and Franzinelli Ž1998. have recently described terraces in the Upper Rio Negro basin dating to the same period Ž) 40 000–27 000 BP.. They call these sediments the Tiquie´ Formation and they are typically planar and trough cross-bedded sands, with subsidiary silts and clays, impregnated with organic matter and iron. The upper 5 m of sediment has been transformed by

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podzolisation into residual white sand, and in almost all respects these deposits resemble those of possibly similar age in Kalimantan Žbelow.. Deposition of sediments in a lower terrace commenced some time before 13 240 BP Ž14 C. and appears to have continued until around 4000 BP Ž14 C.. These deposits are generally silts and clays, containing black organic layers. Latrubesse and Rancy Ž1998. indicate further evidence for this period of sedimentation from the Jurua´ valley in southwestern Amazonia and from the Ucayali River in the same region. In Colombia, Van der Hammen et al. Ž1992a,b. found no datable organic sediments between 30 000 and 12 600 BP Ž14 C. in the Caqueta´ River, and concluded that the period was marked by erosion. Iriondo and Latrubesse Ž1994. have offered evidence for a dry Late Glacial climate in central Amazonia and commented on the reduced discharges of the rivers in this area, with the formation of a ‘deflation corridor’ under low water, and highly seasonal climatic conditions with enhanced Trade Winds. These observations have to be weighed against conflicting views, based on interpretation of the pollen record, expressed by Colinvaux et al. Ž2000..

The sediments forming podzolised fan-terraces were probably deposited before 27 000 BP Ž14 C., older than the LGM, but all the radiocarbon assays returned ages ) 40 000 BP Ž14 C., and no dates from near-surface sediments were obtained. The terraces dip seawards at inclinations of ( 1 in 300, to pass beneath Holocene beach barriers, and were deposited when sea levels were lower than y35 m ŽThorp et al., 1990.. The fan terraces were subsequently incised as sea level fell to ) y100 m at the LGM, and also possibly in response to tectonic uplift inland and a reduction in sediment yield. This incision may have occurred between ca. 18 000 and 12 000 BP Ž14 C.. Two dates of close to 10 000 BP Ž14 C. were obtained from basal sands beneath the Holocene floodplain. Some undated fan-like sediments derive from very small catchments, which are little more than embayments in the hilly relief, indicating the influence of colluvial processes rather than the remobilisation of sediment stores in floodplains. This reasoning lends support to the idea that there was a strong modification of the hillslope vegetation cover at some time in the late Quaternary, when runoff was insufficient to transfer all eroded sediment into nearby stream channels Žsee also Verstappen, 1997..

3.3. NW Kalimantan, Indonesia Studies of small catchments draining into the Sunda Sea in NW Kalimantan Ž0–18N Lat., rainfall ) 3000 mm ay1 . ŽThorp et al., 1990; Thorp and Thomas, 1992, 1993. drew attention to broad fan-like spreads of sands fringing the bedrock relief. Finite radiocarbon dates of 54 200 q 3400ry2400 BP Ž14 C. and 51 000 q 2100ry1700 BP Ž14 C. were obtained from buried logs, suggesting the onset of sedimentation could have been during fluctuations of climate within Isotope Zone 3. However, the entrainment of old wood into more recent sediments is a possibility here ŽKalicki and Krapiec, 1995.. However, other records from W Malaysia ŽKamaludin et al., 1993., indicate a period or periods of copious sedimentation between ca. 60 000 and 30 000 BP Ž14 C., and that the climate became more moist after the latter date until ca. 27 000 BP Ž14 C.. Increased seasonality and more open vegetation, combined with rainfall reduced by 30–50% from modern means ) 3000 mm ay1 , could have produced such a result, as argued for the LGM in NE Queensland.

3.4. SaÕanna areas with high seasonality and rainfalls - ca. 1500 mm a y 1 Studies from the cerrado Žsavanna. areas of S Brazil contribute to a wider picture. Braided river sediments dating to before 40 000 BP Ž14 C. in the upper Parana´ River, in a lower rainfall area of S Brazil Ž22843X S, rainfall 1200 mm ay1 . have been described by Stevaux Ž1994. and Stevaux and dos Santos Ž1998., who attributed their characteristics to a tropical or sub-tropical semi-arid climate. Also in the cerrado area Ž1300 mm ay1 . of SE Brazil, Turcq et al. Ž1997. examined sediments deposited by the Tamandua´ river, a small stream with a catchment of only 150 km2 and a local relief of 100 m. They found that extensive organic-rich deposits marking high watertables, gave way to braided channel sands and alluvial fans dating to a dry interval between 17 000 and 10 000 BP Ž14 C.. In Malawi, central Africa, Meadows Ž1983, 1985. recorded rapid stream sedimentation within shallow, dambo type valleys on the Nyika Plateau Žca. 108S

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Lat.., beginning around 12 000 BP Ž14 C. and continuing until ca. 5000 BP Ž14 C.. Although lake level studies from this part of Africa suggest that the climate at the time of the LGM may have been relatively moist ŽStager, 1988; Owen et al., 1990; Scholtz and Finney, 1994; De Busk, 1998., the data are compatible with the sedimentation spanning a period of enhanced rainfall, and commencing at a time of climatic transition and instability.

4. Regional and global rhythms The cooling of tropical climates by more than 58C at the LGM is now widely accepted. The extent and amount of associated dryness are more disputed, but most views converge around rainfall reductions of Ž25. 30–50 Ž65.%. Experience will not have been uniform throughout the tropics and it is likely that changes to precipitation will have been greater in areas of steep climatic gradient such as W Africa and NE Queensland and less in regions of uniform and widespread humidity such as the Amazon and Congo basins. Following previous attempts to offer a general picture of late Quaternary stream response to climate change ŽThomas and Thorp, 1995; Thorp and Thomas, 1992., it now looks increasingly possible to partition the humid tropics according to regional patterns of present-day rainfall. Local variations in rainfall and edaphic patterns must also have been important, and Kadomura Ž1995. has suggested that savannisation may have affected higher slopes and interfluves, while the forest survived in many lowland and riverine sites. This pattern could arise in landscapes of moderate relief, where no orographic effect is present, and can partly explain anomalies in pollen spectra from mostly wet sites, where climatic conditions became marginal for forest survival and regeneration. This fragmentation of the forest would also explain important changes to hillslope dynamics and stream behaviour within areas where forests survived in part throughout the LGS. Catchment response to altered rainfall patterns and amounts, and also ground cover, should offer a much clearer expression of regional environmental changes than many pollen sites reflecting conditions

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at lake and swamp margins ŽSugita et al., 1999.. Quaternary scientists have often failed to consider the geomorphological and sedimentological data as evidence for major palaeoenvironmental change, and this has compounded skepticism concerning the magnitude of the changes that have affected the humid tropics during the late Pleistocene and the early Holocene. However, the evidence derived from fluvial deposition requires careful evaluation and some pitfalls can be indicated. Ž1. Sediments deposited by migrating river channels can be re-mobilised, entraining large logs and other organic debris, which can produce misleading estimates for the age of sediments in which they are subsequently found. Confidence increases where there are ‘right way up’ sequences of dates, large numbers of dated samples and clear stratigraphic relationships between a deposit and prior and subsequent sedimentary phases. Ž2. There is a problem of ‘missing evidence’, where sampling of floodplain sediments from a single reach or from scattered outcrops, fails to intersect each major sedimentary phase. This problem is obviously overcome by appropriate sampling strategies. However, very large river systems such as the Amazon or Congo can present severe practical problems in this context. Ž3. Catchment characteristics vary and those with mountain headwaters, such as the Andean tributaries of the Amazon, will respond to the rhythm of glacier extension and melting. Very large catchments not only integrate the effects of extreme events affecting one part of the basin; they may also contain areas experiencing different long-term responses to climatic change. Ž4. Many catchments have been affected by tectonic disturbance. In NW Kalimantan, known subsidence offshore was probably accompanied by uplift inland, while the western Amazon has experienced elevation of the Andes and complex tectonics in the foreland basin. Ž5. The effects of extreme storm events may be over-represented in the sedimentary record and it is important to accommodate such ‘event stratigraphies’ within longer-term temporal patterns of sediment deposition by the use Žinter alia. of bracketing dates for sedimentary units. We know little about the distribution of extreme events in time, and they may

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be more frequent during rapid climatic change, than in periods of more stable climate—wet or dry. Ž6. When all geologic, topographic and environmental circumstances have been accounted for, the river system will still undergo changes understood as ‘complex response’ ŽSchumm, 1977. to external forcing factors, and the operation of internal thresholds. As a result, not all changes in sediment calibre or sedimentation rate will reflect long-term climate changes. But, notwithstanding all of these difficulties of interpretation, river floodplains offer an unrivalled insight into the behaviour of landscapes within a nested hierarchy of basins: at local, regional and sub-continental scales of enquiry. 5. Evolutionary aspects of sedimentation Using the data presented here, the available records appear to show one of two sequences ŽTable 2..

During Isotope Zone 3 ŽMiddle Pleniglacial., there is evidence for extensive floodplains, and rapid aggradation, but also indications of important fluctuations in regime. Diachronous change after 27 ka BP led to divergent changes, with incision taking place in equatorial Kalimantan and W Amazonia, braiding and fan formation in more marginal areas of the rainforest, such as NE Queensland, and a cessation of activity in many W African streams after 21 ka BP until 12.7 Ka BP. In the Holocene, floodplains evolved by vertical and lateral accretion, constrained by earlier terraces and by bedrock. In many cases, rivers filled incised channels by vertical accretion in a series of episodes Žas in Ghana.. There is also evidence, although it is poorly dated, of widespread overbank flooding in the post-12 700 BP Ž14 C. period, with flood silts and clays deposited over older terraces and floodplain units ŽThorp and Thomas, 1992.. Subsequent cut-and-fill episodes have in general taken place within the inner floodplains, indicating declining discharges since the early Holocene.

Table 2 Observed fluvial sequences during the LGS Ž1. 58–28 ka BP ŽMiddle Pleniglacial. Ø prolonged depositional phase during post-58 ka cooling marked by extensive floodplains Žnow low terraces. and by fan deposits; some cut-and-fill episodes as at ca. 36 ka probably mark humidrarid fluctuations Ž2. Post-28 ka BP ŽLate Pleniglacial. Response A Žreduced rainfall—rainforest persisting. Ø incision at the end of the Middle Pleniglacial in Amazonia Žafter 26 ka BP. ™ Ø renewed sedimentation in the early Holocene Response B Žreduced rainfall with high seasonality—open forests and grassland. Ø fan deposition andror braided river channels during LGS Žafter 28 ka BP. ™ Ø incision of fans towards the end of the LGS Žpost 14 ka BP. ™ Ø infill of incised channels with localised floodplain formation starting in the early Holocene Ždambos formed in headwater valleys in plateau areas such as central Africa.. Response C Žreduced rainfall post 22 ka BP—forest replaced by savanna. Ø continuing moist conditions maintain wide floodplains with backswamps and fine overbank sediments in W Africa Ž) 30 to ) 22 ka BP. ™ Ø reduced sedimentation with few or no recorded units after 21 ka BP, but possible incision towards the end of the stage ™ Ø channel incision at the Pleistocene–Holocene transition Žpost-12.7 ka BP., with deposition of coarse gravels and sands as braid bars in wide open channels with little overbank sedimentation Ž3. Holocene fluctuations Žpost 13r12.5 ka. Ø cut-and-fill sequences during the Holocene, leading to both lateral and vertical accretion; evolving from braided to mainly meandering channel-floodplain systems Žgenerally declining flood peaks and mean discharges with small channels inset into former floodplains. Ø too few detailed records to offer any general model. In W Africa, the following units were recognised: ŽA. 12.7 ™ 10.5 ka Žchannel scour and coarse flood gravels with timbers. ŽB. 9.5 ™ 7.5 ka Žwidespread coarse flood gravels with timbers.; ŽC. 2.6 ka ™ present Žabundant sedimentation, including overbank fines.. Some gravel bars were deposited during the mid-Holocene dry period, post-4.3 ka.

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For the Holocene sequence in W Africa, and possibly elsewhere, a concept of ‘delayed response’ can be used, in the sense that the parameters of the environmental systems do not change in a synchronous manner. Thus, climatic change may come first, involving warming and the re-establishment of monsoon circulation and the ITCZ migratory pattern. It seems likely that this was an unsteady process involving changed patterns and frequencies of large storms and a gradual establishment of post-glacial rainfall patterns. From the late glacial until the Younger Dryas change was probably rapid but erratic, and lakes did not overflow in the manner of the early Holocene pluvial period from 10 000 to 8000 BP Ž14 C.. Vegetation changes seem to have been delayed by the erratic nature of the climate and probably by the YD interruption, so that rainforest patterns were not fully developed until some time between 9500 and 8500 BP Ž14 C., when rainfalls may have been ) 10% higher than today. Some time after 8000 BP Ž14 C., rainfall seems to have declined though evidence from plunge pool deposits in N Australia shows at least occasional major floods until the mid-Holocene. The question of human agency in vegetation change and its impact on stream regimes has not been addressed here, but it seems probable that it was effective mainly after the mid-Holocene in most areas. It is a subject for a further discussion, but we do not think that anthropogenic impacts were the cause of the major changes in stream sedimentation discussed in this paper. Note 1. The samples for TL analysis all displayed a broad temperature plateau Ž300–5008C., indicating that they were well bleached. A combination of additive and regenerative techniques, using 90–125 mm grains, was employed. Samples were cleaned in HCL and etched briefly in HF, then serially irradiated using a 90 Sr plaque source. Other samples were bleached, using a sunlamp. These bleached samples were then irradiated, and all the samples were used to produce regenerated curves.

Acknowledgements M.F. Thomas was funded by The Carnegie Trust for the Universities of Scotland to undertake field

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research in NE Queensland. Thermoluminescence dating was carried out at Wollongong University by D.M. Price, and radiocarbon analyses by A. Hogg at the Waikato University radiocarbon laboratory, New Zealand. Thanks are due to M.B. Thorp, University College, Dublin, for helpful comments on an earlier draft.

References Adams, J.M., Faure, H. ŽEds.., 1995. Preliminary Land Ecosystem Map of the World since the Last Glacial Maximum Žwww.soton.ac.ukr ; tjmsradams1.html.. Quaternary Environmental Network. Adams, J.M., Faure, H., 1997. Palaeovegetation maps of the Earth during the Last Glacial Maximum, and the early and mid Holocene: an aid to archaeological research. Journal of Archaeological Science 24, 623–647. Baltzer, F., 1991. Late Pleistocene and Recent detrital sedimentation in the deep parts of northern Lake Tanganyika ŽEast African Rift.. Special Publication International Association of Sedimentologists 13, 147–173. Beuning, K.R.M., Talbot, M.R., Kelts, K., 1997. A revised 30 000 palaeoclimatic and palaeohydrologic history of Lake Albert, East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 136, 259–279. Bonell, M., 1991. Progress and future research needs in water catchment conservation within the wet tropical coast of NE Queensland. Proceedings Institute of Tropical Rainforest Studies, Townsville, Australia, pp. 59–85. Broecker, W.S., 1995. Cooling the tropics. Nature 376, 212–213. CLIMAP Project Members, 1976. The surface of the ice-age earth. Science 191, 1131–1138. CLIMAP Project Members, 1981. Seasonal reconstruction of the earth’s surface at the last glacial maximum. Geological Society of America, Map and Chart Series, 36. Colinvaux, P.A., Bush, M.A., Steinitz-Kannan, M., Miller, M.C., 1997. Glacial and postglacial pollen records from the Ecuadorian Andes and Amazon. Quaternary Research 48, 69–78. Colinvaux, P.A., De Oliveira, P.E., Bush, M.A., 2000. Amazonian and neotropical plant communities on glacial time-scales: the failure of the aridity and refuge hypotheses. Quaternary Science Reviews 19, 141–169. De Busk, G.H., 1998. A 37 500-year pollen record from Lake Malawi and implications for the biogeography of afromontane forests. Journal of Biogeography 25, 479–500. Dong, B., Valdes, P.J., Hall, N.M.J., 1996. The changes of monsoonal climates due to Earth’s orbital perturbations and Ice Age boundary conditions. Palaeoclimates 1, 203–240. Elenga, H., Schwartz, D., Vincens, A., 1994. Pollen evidence of late Quaternary vegetation and inferred climate changes in Congo. Palaeogeography, Palaeoclimatology, Palaeoecology 109, 345–356.

66

M.F. Thomas et al.r Geomorphology 39 (2001) 53–68

Gasse, F., 2000. Hydrological changes in the African tropics since the Last Glacial Maximum. Quaternary Science Reviews 19, 189–211. Gasse, F., Ledee, ´ ´ 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. Guilderson, T.P., Fairbanks, R.G., Rubenstone, J.L., 1994. Tropical temperature variations since 20 000 years ago: modulating interhemispheric climate change. Science 263, 663–665. Hall, A.M., Thomas, M.F., Thorp, M.B., 1985. Late Quaternary alluvial placer development in the humid tropics: the case of the Birim Diamond Placer, Ghana. Journal of the Geological Society 142, 777–787. Heaney, L.R., 1991. A synopsis of climatic and vegetation change in South East Asia. Climatic Change 19, 53–61. Iriondo, M., Latrubesse, E.M., 1994. A probable scenario for a dry climate in central Amazonia during the late Quaternary. Quaternary International 21, 121–128. Jansen, J.H.F., van Weering, T.C.E., Gieles, R., van Iperen, J., 1984. Late Quaternary oceanography and climatology of the Zaire-Congo fan and the adjacent eastern Angola basin. Netherlands Journal of Sea Research 17, 201–249. Jolly, D., Harrison, S.P., Damnati, B., Bonnefille, R., 1998. Simulated climate and biomes of Africa during the late Quaternary: comparison with pollen and lake status data. Quaternary Science Reviews 17, 629–657. Junner, N.R., 1943. The diamond deposits of the Gold Coast. Gold Coast, Geological Survey, Bulletin 12, 55 Reprinted, 1958, Governement Printer, Accra, Ghana. Kadomura, H., 1995. Palaeoecological and palaeohydrological changes in the humid tropics during the last 20 000 years, with reference to equatorial Africa. In: Gregory, K.J., Starkel, L., Baker, V.R. ŽEds.., Global Continental Palaeohydrology. Wiley, Chichester, pp. 177–202. Kalicki, T., Krapiec, M., 1995. Problems of dating alluvium using buried subfossil tree trunks: lessons from the ‘black oaks’ of the Vistula Valley, Central Europe. Holocene 5, 243–250. Kamaludin, H., Nakamura, T., Price, D., Woodroffe, C.D., Fujii, S., 1993. Radiocarbon and thermoluminescence dating of the Old Alluvium from a coastal site in Perak, Malaysia. Sedimentary Geology 83, 1–12. Kershaw, A.P., 1976. A Late Pleistocene and Holocene pollen diagram from Lynch’s Crater, northeastern Queensland, Australia. New Phytologist 77, 469–498. Kershaw, A.P., 1978. Record of last interglacial–glacial cycle from northeastern Queensland. Nature 272, 159–161. Kershaw, A.P., 1992. The development of rainforest–savanna boundaries in tropical Australia. In: Furley, P.A., Proctor, J., Ratter, J.A. ŽEds.., Nature and Dynamics of Forest–Savanna Boundaries. Chapman & Hall, London, pp. 255–271. Kershaw, A.P., Nanson, G.C., 1993. The last full glacial cycle in the Australian region. Global and Planetary Change 7, 1–9. Knox, J.C., 1984. Fluvial responses to small scale climate changes. In: Costa, J.E., Fleischer, P.J. ŽEds.., Developments and Applications of Geomorphology. Springer, Berlin, pp. 318–342. Knox, J.C., 1993. Large increases in flood magnitude in response to modest changes in climate. Nature 361, 430–432.

Knox, J.C., 1995. Fluvial systems since 200 000 years BP. In: Gregory, K.J., Starkel, L., Baker, V.R. ŽEds.., Global Continental Palaeohydrology. Wiley, Chichester, pp. 87–108. Kutzbach, J., Gallimore, R., Harrison, S., Behling, P., Selin, R., Laarif, F., 1998. Climate and biome simulations for the past 21 000 years. Quaternary Science Reviews 17, 473–506. Latrubesse, E.M., Franzinelli, E., 1998. Late Quaternary alluvial sedimentation in the upper Rio Negro Basin, Amazonia, Brazil: palaeohydrological implications. In: Benito, G., Baker, V.R., Gregory, K.J. ŽEds.., Palaeohydrology and Environmental Change. Wiley, Chichester, pp. 259–271. Latrubesse, E.M., Ramonell, C.G., 1994. A climatic model for southwestern Amazonia in last glacial times. Quaternary International 21, 163–169. Latrubesse, E.M., Rancy, A., 1998. The late Quaternary of the Jurua River, southwestern Amazonia: geology and vertebrate palaeontology. Quaternary of South America and the Antarctic Peninsula 11, Balkema, Rotterdam. Lautenschlager, M., 1991. Simulation of the ice age atmosphere—January and July means. Geologische Rundschau 80 Ž3., 513–534. Ledru, M.-P., 1993. Late Quaternary environmental and climatic changes in central Brazil. Quaternary Research 39, 90–98. Ledru, M.-P., SoaresBraga, P.I., Soubies, ` F., Fournier, M., Martin, L., Suguio, K., Turcq, B., 1996. The last 50 000 years in the Neotropics ŽSouthern Brazil.: evolution of vegetation and climate. Palaeogeography, Palaeoclimatology, Palaeoecology 123, 239–257. Ledru, M.-P., Bertaux, J., Sifeddine, A., Suguio, K., 1998. Absence of Last Glacial Maximum records in lowland tropical forests. Quaternary Research 49, 233–237. Maley, J., 1987. Fragmentation de la foret ˆ dense humide Africaine et extension des biotopes montagnards au Quaternaire recent: ´ nouvelles donnees ´ polliniques et chronologiques. Implications paleoclimatiques et biogeographiques. Palaeoecology of Africa ´ ´ 18, 307–334. Maley, J., 1996. The African rain forest—main characteristics and changes in vegetation and climate from the Upper Cretaceous to the Quaternary. In: Alexander, I.J., Swaine, M.D., Watling, R. ŽEds.., Essays on the Ecology of the Guinea-Congo Rain Forest. Proceedings of the Royal Society of Edinburgh, Section B, vol. 104, pp. 31–73. Meadows, M.E., 1983. Past and present environments of the Nyika Plateau, Malawi. Palaeoecology of Africa 16, 353–390 Balkema, Rotterdam. Meadows, M.E., 1985. Dambos and environmental change in Malawi, central Africa. In: Thomas, M.F., Goudie, A.S. ŽEds.., Dambos: Small Channelless Valleys in the Tropics. Zeitschrift fur ¨ Geomorphologie, Supplementband, vol. 52, pp. 147–169. Nanson, G.C., Price, D.M., 1998. Australian flow regime and climate over the past 300 ka. Third International Meeting on Global Continental Palaeohydrology, Rissho University Japan. Abstracts 28, Kumagaya, Japan. Nott, J., Price, D.M., 1999. Waterfalls, floods and climate change: evidence from tropical Australia. Earth and Planetary Science Letters 171, 267–276. Owen, R.B., Crossley, R., Johnson, T.C., Tweddle, D., Kornfield, I., Davison, S., Eccles, D.H., Engstrom, D.E., 1990. Major

M.F. Thomas et al.r Geomorphology 39 (2001) 53–68 low levels of Lake Malawi and their implications for speciation of cichlid fishes. Proceeding of the Royal Society of London 240, 519–553. Peters, M., Tetzlaff, G., 1990. West African palaeosynoptic patterns at the last glacial maximum. Theoretical and Applied Climatology 42, 67–79. Preuss, J., 1990. L’evolution des paysages du bassin interieur du ´ ´ Zaire In: Lan¨ pendent les quarantes derniers millenaires. ´ franchi, R., Schwartz, D. ŽEds.., Paysages Quaternaires de lAfrique centrale Atlantique. ORSTOM, Paris, pp. 260–270. Riccomini, C., Peloggia, A.U.G., Saloni, J.C.L., Kohnke, M.W., Figueira, R.M., 1989. Neotectonic acticivity in the Serra do Mar rift system Žsoutheastern Brazil.. Journal of South American Earth Sciences 2, 191–197. Rind, D., Peteet, D., 1985. Terrestrial conditions at the last glacial maximum and CLIMAP sea-surface temperature estimates: are they consistent? Quaternary Research 24, 1–22. Ruddiman, W.F., 1997. Tropical Atlantic terrigenous fluxes since 25 000 yrs BP. Marine Geology 136, 189–207. Runge, J., 1992. Geomorphological observations concerning palaeoenvironmental conditions in eastern Zaire. Zeitschrift fuer Geomorphologie, N.F., Supplementband 91, 109–122. Runge, J., 1996. Palaeoenvironmental interpretation of geomorphological and pedological studies in the rain forest Acore areasB of eastern Zaire Žcentral Africa.. South African Geographical Journal 78, 91–97. Scholtz, C.A., Finney, B.P., 1994. Late Quaternary sequence stratigraphy of Lake Malawi ŽNyasa., Africa. Sedimentology 41, 163–179. Schubert, C., 1988. Climatic changes during the last glacial maximum in northern South America and the Caribbean: a review. Interciencia 13, 128–137. Schumm, S.A., 1977. The Fluvial System. Wiley, Chichester, 336 pp. Servant, M., Maley, J., Turcq, B., Absy, M.-L., Brenac, P., Fournier, J., Ledru, M.-P., 1993. Tropical forest changes during the Late Quaternary in African and South American lowlands. Global and Planetary Change 7, 25–40. Stager, J.C., 1988. Environmental changes at Lake Cheshi, Zambia since 40 000 years BP. Quaternary Research 29, 54–65. Stevaux, J.C., 1994. The upper Parana´ River ŽBrazil.: geomorphology, sedimentology and paleoclimatology. Quaternary International 21, 143–161. Stevaux, J.C., dos Santos, M.L., 1998. Palaeohydrological changes in the Upper Parana´ River, Brazil, during the late Quaternary: a facies approach. In: Benito, G., Baker, V.R., Gregory, K.J. ŽEds.., Palaeohydrology and Environmental Change. Wiley, Chichester, pp. 273–285. Stute, M., Forster, M., Frischkorn, H., Serejo, A., Clark, J.F., Schlosser, P., Broecker, W.S., Bonani, G., 1995. Cooling of tropical Brazil Ž58C. during the Last Glacial Maximum. Science 269, 379–383. Sugai, T., 1993. River terrace development by concurrent fluvial processes and climate changes. Geomorphology 6, 243–252. Sugita, S., Gaillard, M.-J., Brostrom, ¨ A., 1999. Landscape openness and pollen records: a simulation approach. Holocene 9, 409–421.

67

Talbot, M.R., Johannessen, T., 1992. A high resolution palaeoclimatic record of the last 27 500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustribne organic matter. Earth and Planetary Science Letters 110, 23–37. Talbot, M.R., Livingstone, D.A., Palmer, P.G., Maley, J., Melack, J.M., Delibrias, G., Gulliksen, S., 1984. Preliminary results from sediment cores from lake Bosumtwi, Ghana. Palaeoecology of Africa 16, 173–192. Thomas, M.F., 1994. Geomorphology in the Tropics. Wiley, Chichester. Thomas, M.F., 1999. Evidence for high energy landforming events on the central African plateau: eastern province, Zambia. Zeitschrift fuer Geomorphologie N.F. 43, 273–297. Thomas, M.F., Thorp, M.B., 1980. Some aspects of the geomorphological interpretation of Quaternary alluvial sediments in Sierra Leone. Zeitschrift fuer Geomorphologie, N.F., Supplementband 36, 140–161. Thomas, M.F., Thorp, M.B., 1995. Geomorphic response to rapid climatic and hydrologic change during the Late Pleistocene and Early Holocene in the humid and sub-humid tropics. Quaternary Science Reviews 14, 193–207. Thomas, M.F., Thorp, M.B., Teeuw, R.M., 1985. Palaeogeomorphology and the occurrence of diamondiferous placer deposits in Koidu, Sierra Leone. Journal of the Geological Society of London 142, 789–802. Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Henderson, K.A., Cole-Dai, J., Bolzan, J.F., Liu, K.-B., 1995. Late glacial stage and Holocene tropical ice core records from Huascaran, ´ Peru. Science 269, 46–50. Thompson, L.G., Davis, M.E., Mosley-Thompson, E., Sowers, T.A., Henderson, K.A., Zagorodnov, V.S., Lin, P.N., Mikhalenko, V.N., Campen, R.K., Bolzan, J.F., Cole-Dai, J., Francou, B., 1998. A 25 000-year tropical climate history from Bolivian ice cores. Science 282, 1858–1864. Thorp, M.B., Thomas, M.F., 1992. The timing of alluvial sedimentation and floodplain formation in the lowland humid tropics of Ghana, Sierra Leone, and western Kalimantan ŽIndonesian Borneo.. Geomorphology 4, 409–422. Thorp, M.B., Thomas, M.F., 1993. Discussion: Late Pleistocene sedimentation and landform development in western Kalimantan ŽIndonesian Borneo.. Reply by the Authors. Geologie en Mijnbouw 71, 363–368. Thorp, M.B., Thomas, M.F., Martin, T., Whalley, W.B., 1990. Late Pleistocene sedimentation and landform development in western Kalimantan ŽIndonesian Borneo.. Geologie en Mijnbouw 69, 133–150. Tooth, S., Nanson, G.C., 1995. The geomorphology of Australia’s fluvial systems: retrospect, perspect and prospect. Progress in Physical Geography 19, 35–60. Turcq, B., Pressinotti, M.M.N., Martin, L., 1997. Palaeohydrology and palaeoclimate of the past 33 000 years at the Tamandua´ River, central Brazil. Quaternary Research 47, 284–294. Van der Hammen, T., Absy, M.C., 1994. Amazonia during the last glacial. Palaeogeography, Palaeoclimatology, Palaeoecology 109, 247–261. Van der Hammen, T., Hooghiemstra, H., 2000. Neogene and

68

M.F. Thomas et al.r Geomorphology 39 (2001) 53–68

Quaternary history of vegetation, climate, and plant diversity in Amazonia. Quaternary Science Reviews 19, 725–742. Van der Hammen, T., Duivenvoorden, J.F., Lips, J.M., Urrego, L.E., Espejo, N., 1992a. Late Quaternary of the middle Caqueta´ River area ŽColombian Amazonia.. Journal of Quaternary Science 7, 45–55. Van der Hammen, T., Urrego, L.E., Espejo, N., Duivenvoorden, J.F., Lips, J.M., 1992b. Late-glacial and Holocene sedimentation and fluctuations of river water level in the Caqueta´ River area ŽColombian Amazonia.. Journal of Quaternary Science 7, 57–67. Verstappen, H.Th., 1994. Climatic change and geomorphology in south and south-east Asia. Geo-Eco-Trop 16, 101–147.

Verstappen, H.Th., 1997. The effect of climate change on southeast Asian geomorphology. Journal of Quaternary Science 12, 413–418. Vincens, A., Chalie, ´ F., Bonnefille, R., 1993. Pollen-derived rainfall and temperature estimates from Lake Tanganyika and their implication for late Pleistocene water levels. Quaternary Research 40, 343–350. Willmott, W.F., Stephenson, P.J., 1989. Rocks and Landscapes of the Cairns District. Queensland Department of Mines, Brisbane.