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Maps of Pleistocene Sea Levels in Southeast Asia: Shorelines, River Systems and Time Durations Harold K

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Maps of Pleistocene Sea Levels in Southeast Asia: Shorelines, River Systems and Time Durations Harold K ^ Journal of Biogeography, 27, I 153-l I67 , Maps of Pleistocene sea levels in Southeast Asia: shorelines, river systems and time durations Harold K. Voris Field Museum of Natural History, Chicago, Illinois, USA Abstract Aim Glaciation and deglaciation and the accompanying lowering and rising of sealevels during the late Pleistoceneare known to have greatly affected land massconfigurations in SoutheastAsia. The objective of this report is to provide a seriesof mapsthat estimatethe areasof exposed land in the Indo-Australian region during periods of the Pleistocenewhen sealevels were below present day levels. Location The maps presentedhere cover tropical SoutheastAsia and Austral-Asia. The east-westcoverage extends 8000 km from Australia to Sri Lanka. The north-south coverage extends 5000 km from Taiwan to Australia. Methods Present-day bathymetric depth contours were used to estimate past shore lines and the locations of the major drowned river systemsof the Sunda and Sahul shelves. The timing of sea level changesassociated with glaciation over the past 250,000 years was taken from multiple sourcesthat, in some cases,account for tectonic uplift and sub- sidenceduring the period in question. Results This report provides a seriesof maps that estimate the areasof exposed land in the Indo-Australian region during periods of 17,000, 150,000 and 250,000 years before present. The ancient shorelinesare basedon present day depth contours of 10, 20, 30, 40,50,75, 100 and 120 m. On the maps depicting shorelinesat 75,100 and 120 m below present levels the major Pleistoceneriver systemsof the Sunda and Sahul shelves are depicted. Estimatesof the number of major sealevel fluctuation events and the duration of time that sealevels were at or below the illustrated level are provided. Main conclusions Previous reconstructionsof sea-levelchange during the Pleistocenehave emphasizedthe maximum lows. The perspectiveprovided here emphasizesthat sea levels were at their maximum lows for relatively short periods of time but were at or below intermediate levels (e.g. at or below 40 m below present-day levels) for more than half of each of the time periods considered. Keywords Sealevels, Pleistocene,Southeast Asia, maps, biogeography, palaeogeography. (e.g. Inger & Chin, 1962; Heaney, 1985, 1991; Morley & iNTRODUCTlON Flenley, 1987; Rainboth, 1991; Dodsonet al., 1995; How & The importance of changing sealevels over geologicaltime Kitchener, 1997). Thesestudies cite a variety of sourcesthat has long been considered essentialto our understanding leadback to Molengraaff & Weber (1919, X921), Molengraaff the distribution of both aquatic and terrestrial organisms (1921),van Bemmelen(1949), Kuenen (1950), Beaufort (1951), (Molengraaff & Weber, 1919; Darlington, 1957; Wallace, Wyrtki (1961), Emery et al. (1972); Biswas(1973), Verstappen 1881). Many of the zoogeographicworks that considerthe (1975), Batchelor (1979), Potts (1983) or Gibbons & Clunie Indo-Chinese and Indo-Australian regions recognize the (1986). Theseauthors’ maps vary but mostare limited to estim- importance of the Sundaand Sahul shelvesin forming land ating the extent of exposedcontinental shelf at a particular bridgesand connectingriver systemsduring the Pleistocene past sealevel, usually 100 m below presentlevels (BPL) and, in somecases, illustrating palaeo river systemson the Sunda and Sahulshelves. Their mapsprimarily serveto illustratehow Correspondence: Department of Zoology, Field’Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 606052496 USA. E-mail: the SundaShelf connectsthe major islandsof the Indonesian [email protected] archipelago(Sumatra, Java and Borneo) with Indochina and I 8 2000 Blackwell Science Ltd I I54 H. K.Voris how the Sahul Shelf forms a broad connection between _ - Hydrographic and Navigation maps, a National Geographic Australia and New Guinea. The documentation for many of hydrographic map and WORLDBATH, based on more than these maps was limited and the authors did ,not relate the seven million soundings taken world-wide. Appendix I map contours to particular time periods or rime durations. provides details on these sources and the specific charts that Sea-level changes over the past 30 Ma have been well were consulted. documented in association with tectonic changes (Hall & Holloway, 1998),&d glaciation over the past 250,000 years River systems (Ollier, 1985; Matthews, 1990; Dawson, 1992). Papers that focus on sea-level changes during the Pleistocene generally The estimated locations of the major river systems of the Sunda infer sea levels from either geological mapping of retreating and Sahul shelves were based on present day bathymetric shoreline margins e.g. the use of coral reef terraces ‘on trop- depth contours (see Appendix I) as well as the original maps ical coasts (Bloom & Yonekura, 1990) or the marine oxygen that appear in Molengraaff & Weber (1919), Molengraaff isotope record e.g. deviations from the standard of the 1e0/160 (1921), van BeXmelen (1949), Kuenen (1950), Wyrtki (1961), ratios in foraminiferal tests from deep sea cores (Fairbanks, Emery et al. (1972), Ollier (1985) and Rainboth (1991). The 1989). Typically, these papers present graphical representa- largest of these rivers, the North ,Sunda River, is well defined tions of estimated sea levels plotted over time. by ‘.., a wonderful set of long winding valleys, starting off The objective of this report is to provide a series of maps the mouths of many important rivers and joining in the centre that estimate the areas of exposed land in the Indo-Australian to form a huge north-running channel’ (Kuenen, 1950). In region during periods of the Pleistocene when sea levels were other areas of low relief in the Pleistocene, such as the Java below present levels. The major river systems on the Sunda Sea and the Gulf of Thailand, sedimentation has been extens- and Sahul shelves are depicted on three of the maps. In ive (Emery & Nino, 1963; Emery et&, 1972; Tjia, 1980) addition, estimates of the number of major sea-level fluctuation and the exact location of many of the rivers is largely hypo- events and the duration of time that sea levels were at or below thetical and certainly varied over time due to meandering the illustrated level are provided for a range of sea levels. (Kuenen, 1950; Rainboth, 1991; Bird, 1993). For example, echo sounding profiles south of the Sampit River off the southern coast of Borneo provide visualizations of river valleys MATERIALS AND METHODS 400, 1200 and 5400 m in width (Wyrtki, 1961). The Gulf of Carpenteria River system on the Sahul Shelf is less well Depth contours as past shore lines documented but the fluctuating salinities of Lake Carpentaria In this report present day bathymetric depth contours are during the Pleistocene suggest a complex estuarine embay- used as proxies for previous shore lines at particular times ment (Torgersen et al., 1985). In addition, the shallow and in the past 250,000 years before present (yr BP). Sources narrow straits of the Aru Islands support the presence of used to estimate these contours included the General Pleistocene rivers draining west from New Guinea to the Bathymetric Chart of the Oceans (GEBCO) Series, US edge of the Sahul Shelf (Umbgrove, 1949). _-_ Table I The approximate amount of time during past Pleistocene intervals that sea levels were at or below present levels (BPL)is estimated and the percentage of the period is given. The sources were Fairbanks (1989), Bloom & Yonekura (1990) and Chappell & Shackleton (1986) for 17,000,150~000 and 250,000 yr BP, respectively. Past 17,000 years Past 150,000 years Past 250,000 years Sea level BPL (m) Years’ % of time’ Events3 Years % of time Events Years % of time Events 120 1,000 6 1 3,000 2 ‘1 15,000 6 2 100 4,000 24 1 7,000 5 1 29,000 12 2 75 5,500 32 1 14,000 9 1 42,000 17 2 50 7,000 41 1 40,000 27 5 99,000 40 5 40 7,500 44 1 65,000 43 7 136,000 54 6 30 8,400 49 1 93,000 62 5 167,000 67 6 20 9,200 54 1 107,000 71 4 201,000 80 6 10 11,000 65 1 134,000 89 3 227,000 91 3 1 Approximate number of years in each time period (i.e. past 17,000,150,000 or 250,000 years), that the sea level was at or below the level shown in column 1. ’ Approximate percentage of years in each time period (i.e. past 17,000,150,000 or 250,000 years) that the sea level was at or below the level shown in column 1. 3 The estimated number of times within each time period (i.e. past 17,000, 150,000 br 250,000 years) that the sea level fell below the level shown in column 1. 0 Blackwell Science Ltd 2000,~oomal ofBiogeography, 27, I 153-l 167 Pleistocene sea-level maps of Southeast Asia I I55 stood (Bloom & Yonekura, 1990, p. 104). Usingpresent day Time estimates and events of sea level fluctuakon depth contours as proxies for past sea levels assumesthat’ Sea-levelchanges associated with glaciation over the past within the 250,000-year time frame and a 120-m sea-level 250,000 yr BP have beenestimated by severalresearch teams change,tectonic uplift and subsidence,tidal scouringand the focusingon this entire period or a subsetof this period (e.g. accumulationof sedimentswere either relatively minor factors Milliman & Emery, 1968; Shackleton,1987; Pirazzoli, 1991). affecting present day depth contours or that they are well The time estimatesfor the duration of particular past sea understood and can be factored into estimatesof sea level. levelspresented here for 17,000, 125,000 and 250,000 yr BP The papersthat provide estimatesof sealevels over the past are basedon relatively recent works that attempt to take into 17,000 years considertectonic uplift or subsidenceto be a account multiple sourcesof data and, in somecases, account minor factor (Tjia et al., 1984), but estimatesspanning the for tectonic uplift and subsidenceduring the period in question past 150,000-250,000 years recognizethat estimateduplift (e.g.
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