Periods Of Low Sea Level Research Articles

Climate, African and Beringian subaerial continental shelves, and migration of early peoples

The impacts of rapid climate change likely influenced human behaviour during the last glacial cycle, potentially stimulating habitation of, and migration along, continental shelves exposed during periods of lowered sea level. Last glacial maximum (LGM) climate model simulation shows cooler surface air temperatures in Eastern Beringia than Western and Central Beringia. Central Beringia was slightly wetter than present day (PD), whereas Eastern and Western Beringia were drier than PD. However Eastern Beringia was wetter than either Central or Western Beringia. Increased C3 grass coverage is shown in Eastern and Western Beringia, but increased C3 grass net primary productivity (NPP) appears only in Western Beringia; needleleaf trees and many shrubs disappear, replaced by barren soil and C3 grass. Such change would have squeezed species dependant on needleleaf trees out of Western and Eastern Beringia and potentially initiated their migration to more southerly parts of East Asia where a diminishing needleleaf tree environment remained. Shrub-reliant species in Eastern and Western Beringia and C3 grass-dependant species in East Asia and Eastern Beringia would have found additional and productive habitat on the subaerial continental shelf. The LGM simulation identifies cooler LGM temperatures and increased aridity in South and particularly Central Africa, and associated large reductions in LGM broadleaf tree NPP and coverage in Africa, reduced C3 grass NPP particularly in Central Africa, decreased needleleaf tree NPP in South Africa, reduced shrub coverage in North and Central Africa, and greater barren ground coverage in Central and South Africa. Reduced productivity and shifting vegetation during the LGM may have squeezed flora, fauna, and human populations onto the potentially more productive and proximally located exposed continental shelf.

Thorium‐230 normalized particle flux and sediment focusing in the Panama Basin region during the last 30,000 years

Application of the 230Th normalization method to estimate sediment burial fluxes in six cores from the eastern equatorial Pacific (EEP) reveals that bulk sediment and organic carbon fluxes display a coherent regional pattern during the Holocene that is consistent with modern oceanographic conditions, in contrast with estimates of bulk mass accumulation rates (MARs) derived from core chronologies. Two nearby sites (less than 10 km apart), which have different MARs, show nearly identical 230Th‐normalized bulk fluxes. Focusing factors derived from the 230Th data at the foot of the Carnegie Ridge in the Panama Basin are >2 in the Holocene, implying that lateral sediment addition is significant in this part of the basin. New geochemical data and existing literature provide evidence for a hydrothermal source of sediment in the southern part of the Panama Basin and for downslope transport from the top of the Carnegie Ridge. The compilation of core records suggests that sediment focusing is spatially and temporally variable in the EEP. During oxygen isotope stage 2 (OIS 2, from 13–27 ka BP), focusing appears even higher compared to the Holocene at most sites, similar to earlier findings in the eastern and central equatorial Pacific. The magnitude of the glacial increase in focusing factors, however, is strongly dependent on the accuracy of age models. We offer two possible explanations for the increase in glacial focusing compared to the Holocene. The first one is that the apparent increase in lateral sediment redistribution is partly or even largely an artifact of insufficient age control in the EEP, while the second explanation, which assumes that the observed increase is real, involves enhanced deep sea tidal current flow during periods of low sea level stand.

Pleistocene isolation in the Northwestern Pacific marginal seas and limited dispersal in a marine fish, Chelon haematocheilus (Temminck & Schlegel, 1845)

The Northwestern Pacific has a unique tectonic and geographical history with several marginal seas separating Asia from the Pacific Ocean. During low sea level periods of Pleistocene glaciations, populations might have been isolated in three marginal seas: the Sea of Japan, East China Sea and South China Sea. Following postglacial sea level rise, we would expect the populations isolated in the three regions to have been homogenized by high dispersal potential. To assess these hypotheses, we explore the intraspecific phylogeographical patterns in redlip mullet, Chelon haematocheilus. Fragments of 435 bp at the 5' end of mitochondrial DNA control region were sequenced for 272 individuals from nine localities over most of the species' range. Three distinct lineages were detected, which might have diverged in the three marginal seas during Pleistocene low sea levels. Contrary to homogenization expectation, there were strong differences in the geographical distribution of the three lineages. Analyses of molecular variance and the population statistic Phi(ST) also revealed significant genetic structure among populations of the three marginal seas. These results indicate that gene flow in Chelon haematocheilus is far more restricted spatially than predicted by the potential dispersal capabilities of this species. The lack of phylogeographical structure in East China Sea may reflect a recent range expansion after the last glacial maximum and insufficient time to attain migration-drift equilibrium.

Wallace’s line revisited: has vicariance or dispersal shaped the distribution of Malesian hawkmoths (Lepidoptera: Sphingidae)?

Large-scale biogeographical patterns of Sphingidae in the Malesian archipelago are investigated, using Geographic Information System-supported estimates of species ranges determined from a compilation of specimen-label databases. Distribution maps for all species and checklists for 114 islands are presented at http://www.sphingidae- sea.biozentrum.uni-wuerzburg.de. Phenetic analyses of island faunas reveal 'textbook'-like patterns of biogeograph- ical associations in the region, with Wallace's line emerging as the main faunal discontinuity in the region. Further analyses, using partial Mantel statistics, indicate that historical features of geography (such as land bridges formed during periods of lower sea levels) are an important determinant of faunal similarity, but recent dispersal can also explain a significant portion of present day hawkmoth distributions. Faunal turnover in relation to distance between islands is steeper for Smerinthinae than for other subfamilies, indicating different dispersal abilities among higher taxonomic groups. These differences are discussed in the light of life-history differences between the subfamilies. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 455-468.

Late Quaternary sediments on the outer shelf of the Korea Strait and their paleoceanographic implications

Sedimentological and micropaleontological characteristics of core sediments from the outer shelf of the Korea Strait, which connects the northern East China Sea and the East Sea (Sea of Japan), were investigated to elucidate the paleoceanographic environment, especially the timing of the Kuroshio inflow, since the last glacial maximum. The core sediments, containing continuous records of the last 15,000 years, are characterized by a relatively high mud content (more than 50%, on average) and well-developed tide-influenced sedimentary structures. Their mineralogy suggests that the material originated from the paleo-Nakdong River system, which extended across the shelf of the Korea Strait during low sea-level periods. Planktonic foraminifers reveal a series of well-defined changes in paleoceanographic conditions during the late Pleistocene–Holocene. Down-core variations in the abundance of four foraminiferal assemblages, i.e., cold, coastal, tropical–subtropical, and Kuroshio water groups comprising characteristic planktonic species, suggest the occurrence of a distinct paleoenvironmental change in the surface water at 7,000 years b.p., i.e., from 15,000 to 7,000 years b.p., the area was influenced by coastal waters whereas since ca. 7,000 years b.p., it has been under the influence of open-sea water related to the Kuroshio Current flow, associated with both higher temperature and higher salinity. In particular, Pulleniatina obliquiloculata increased markedly in abundance at this time, documenting the inflow of the Kuroshio into the study area. These data indicate that the coastal water stage terminated at ca. 7,000 years b.p. when the warm Kuroshio and its major branch, the Tsushima Current, began to flow into the East Sea, as is the case today. The intrusion of the Tsushima Current through the Korea Strait after ca. 7,000 years b.p. resulted in abrupt changes in sedimentation rates and a dramatic increase in abundance of the Kuroshio indicator species, P. obliquiloculata.

Annually laminated lake sediments and environmental changes in Bashang Plateau, North China

Angulinuo Lake is the biggest lake on the Bashang Plateau, North China, and is 47.6 km 2 in area and 2–6 m in depth. A core from the inner part of Angulinuo Lake was sliced and the sediment was observed by Scanning Electronic Microscope (SEM). Annual laminations characterized by variable color and grain size were found and interpreted as recording the cyclic deposition of lacustrine clay and aeolian dust. The results of chemical analysis of coarse grains conducted by SEM-EDAX, and grain size analysis of modern aeolian dust in the ice on Angulinuo Lake, support an aeolian origin for the light coarse layers. Image analysis technique was used to calculate the size and number of coarse grains in each layer. The coarse grains were fractionated into four classes: > 42 μm, 14–42 μm, 14–4.2 μm and 1.4–4.2 μm. In general, the abundance of the four classes shows similar temporal variation patterns. Around Angulinuo Lake, the winter monsoon is strong and transports aeolian dust into the lake. When the winter monsoon is strong, the size and amount of coarse grains are expected to increase. We infer that the winter monsoon was weaker during 8430–5440 year BP, and was unstable in the later part of this period. From 5440 year BP, the winter monsoon became stronger, and then weaker from 3250 to 2490 year BP. During 2490–1170 year BP, the winter monsoon was slightly stronger, but since 1170 year BP, it has become weaker again. The changes of the winter monsoon intensity recorded in the annual laminations in Angulinuo Lake sediments correspond well to environmental changes in North China and to changes in sea level during the same period. Periods of weaker winter monsoon correspond to times of higher sea levels while the periods of stronger winter monsoon correspond to the Neoglaciation stage in China and the periods of lower sea levels.

Quaternary depositional patterns and sea-level fluctuations, northeastern North Carolina

A detailed record of late Quaternary sea-level oscillations is preserved within the upper 45 m of deposits along an eight km transect across Croatan Sound, a drowned tributary of the Roanoke/Albemarle drainage system, northeastern North Carolina. Drill-hole and seismic data reveal nine relatively complete sequences filling an antecedent valley comprised of discontinuous middle and early Pleistocene deposits. On interfluves, lithologically similar marine deposits of different sequences occur stacked in vertical succession and separated by ravinement surfaces. Within the paleo-drainage, marine deposits are separated by fluvial and/or estuarine sediments deposited during periods of lowered sea level. Foraminiferal and molluscan fossil assemblages indicate that marine facies were deposited in a shallow-marine embayment with open connection to shelf waters. Each sequence modifies or truncates portions of the preceding sequence or sequences. Sequence boundaries are the product of a combination of fluvial, estuarine, and marine erosional processes. Stratigraphic and age analyses constrain the ages of sequences to late Marine Isotope Stage (MIS) 6 and younger (∼ 140 ka to present), indicating multiple sea-level oscillations during this interval. Elevations of highstand deposits associated with late MIS 5 and MIS 3 imply that sea level was either similar to present during those times, or that the region may have been influenced by glacio-isostatic uplift and subsidence.

Variability in the uranium isotopic composition of the oceans over glacial–interglacial timescales

Uranium-series mass spectrometric analyses of corals from the uplifted last glacial terraces at Huon Peninsula, Papua New Guinea, that grew from 50,000 years ago to 30,000 years ago show systematically low values of 234U/ 238U, at the time of coral growth, compared with modern corals. When combined with coral data from other studies a systematic trend emerges indicating shifts in the 234U/ 238U ratio at times of major glacial–interglacial transitions that involve large variations in sea-levels. From last glacial to Holocene, the rate of change in δ 234U is approximately 1‰ per thousand years. The variations in the U budget of the oceans appear to be due to accumulation of excess 234U in near shore areas in anoxic and suboxic sediments, in salt marshes and mangroves, in estuaries, and in continental margins during periods of warm climate and high sea-levels. These near-shore areas are exposed during periods of low sea level resulting in rapid oxidation of U into highly soluble phases. The subsequent release of 234U-enriched uranium into the oceans occurs over a sustained period, in step with rising sea-levels.

The Upper Jurassic (Kimmeridgian) fluvial–aeolian systems of the southern Neuquén Basin, Argentina

The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuquén Basin in Argentina represents a succession up to 40 m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial–aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level.

Isotopic indicators of deformation in the Red Sea

Palaeostress analysis of spreading oceanic ridges requires data on extension over time and at different locations along strike so that mechanical models can be formulated and tested. The seafloor palaeomagnetic record provides average spreading rates but as the last (normal) chron measures 790,000 years an alternative method is required for identifying episodes of spreading at time scales of <10 3 yr which can be fused with seismic and geodetic sequences. On the assumption that the level of hydrothermal circulation on the sea floor is linked with tectonic activity, palaeotemperature and geochemical changes in the seawater determined from cores of the sea floor sediments should serve as proxies for episodes of extension provided they can be dated and alternative explanations for them ruled out. The Red Sea provides a study area for evaluating this proposal because it is a marginal sea with a shallow threshold which limits mixing with the open ocean especially during periods of low sea level, it receives little runoff from the arid lands that surround it, and its median ridge is hydrothermally active. Chemical evidence has been reported for two broadly synchronous late Quaternary hydrothermal episodes in the Atlantis II and Thetis deeps; 87Sr/ 86Sr ratios in the Atlantis II deep are thought to reflect variations in hydrothermal activity. Published δ 13C and δ 18O data from cores both within brine pools and outside them indicate fluctuations during the Holocene for which no climatic counterparts on bordering land areas or in the Arabian Sea are documented; moreover our study has revealed a major (positive) δ 18O excursion older than 40,000 yr in Globigerinoides ruber in Core CH10003-3 PC at a depth of 1374 m which is not associated with hardgrounds or a δ 13C fluctuation and which may have resulted from a hydrothermal pulse. There is no such signal in the core for events of magnitude up to 8.3 that are recorded in the palaeoseismic record of the last 50,000 yr in the Lisan Marls, which suggests that the δ 18O pulse from Core CH10003-3 corresponded with an event of M>8.3 in the RedSea-Dead Sea rift system Other isotopes may prove to be more sensitive and therefore responsive to smaller deformation.

Extending the chronology of deposits at Blombos Cave, South Africa, back to 140 ka using optical dating of single and multiple grains of quartz

Optically stimulated luminescence (OSL) measurements are reported for both single aliquots (of two different sizes) and single grains of quartz from deposits within Blombos Cave. Ages have been obtained for six sediments from the Middle Stone Age (MSA) occupation levels and for two sterile sands, one underlying the archaeological sediment and one overlying the Later Stone Age occupation levels. The ages for the archaeological sediments were obtained from single-grain measurements that enabled unrepresentative grains to be rejected. The MSA occupation levels have ages that, within error limits, are in stratigraphic order and fall between the OSL age for the oldest dune sand (143.2 ± 5.5 ka) and a previously published OSL age for the sterile sand (∼70 ka) that separates the Middle and Later Stone Age deposits. The earliest MSA archaeological phase, M3, from where fragments of ochre were found as well as human teeth, is dated to 98.9 ± 4.5 ka, coinciding with the sea-level high of oxygen isotope substage 5c. The cave then appears to be unoccupied until oxygen isotope substage 5a on the basis of four OSL ages for archaeological phase M2, ranging from 84.6 ± 5.8 to 76.8 ± 3.1 ka; these levels contained large hearths and bone tools. An age of 72.7 ± 3.1 ka was obtained for the final MSA archaeological phase, M1, from which deliberately engraved ochre and shell beads were recovered along with bifacial stone points. We conclude that the periods of occupation were determined by changes in sea level, with abundant sources of seafood available in times of high sea level and with the cave being closed by the accumulation of large dunes during periods of low sea level, such as during oxygen isotope stages 4 and 6.

Extreme levels of intra-specific divergence among Cape Peninsula populations of the Cape galaxias, Galaxias zebratus Castelnau 1861, reveals a possible species complex

The Cape galaxias, Galaxias zebratus, is part of the paleao-endemic fauna characteristic of the south-western Cape, South Africa, and is the only galaxiid found in continental Africa. A 284-bp fragment of the cytochrome b region of the mtDNA was sequenced from 48 individual galaxiids, representing 10 populations from the Cape Peninsula. Five sequences, for four additional populations sampled at the extremes of the species range, were obtained from the literature. Analysis of cyt b mtDNA from these 14 populations of G. zebratus revealed five distinct and highly divergent lineages with low levels of intra-population mtDNA haplotype diversity. A new and distinct genetic lineage is described from the southern part of the Cape Peninsula. Estimates of genetic divergence between populations ranged from <1% to >17%. The observed level of sequence divergence represents the largest yet reported for any single fish species. The distribution of these lineages and their degree of sequence divergence refutes a model of isolation by distance. Results suggest that periods of low sea level may have been important in creating opportunities and alternative routes for dispersal and migration for Cape Peninsula populations.

Sur les premiers peuplements du Pacifique sud

About the first human settlements in South Pacific. South Pacific includes Australia and the Pacific Islands: Melanesia, Micronesia, Polynesia. Which human populations settled them? When, why and how? Where did they come from? Did environment influence these migrations? An exhaustive discussion of these vast questions could not be undertaken rigorously within the framework of this short article. We thus propose to discuss here very precise points that could bring a new lighting on the first settlements of South Pacific. According to the oldest archaeological sites of Australia, first arrivals of Homo sapiens in the area occurred at least 40 000 years ago, and possibly as early as 50 to 60 ka BP. From a palaeoanthropological point of view, the question of the origin of these anatomically modern H. sapiens is under debate for several years: did they come from a relatively recent ‘out of Africa’ migration (Out-of-Africa hypothesis) or did they evolve locally from the last Indonesian H. erectus (multiregional hypothesis)? Partisans of these two models disagree on several fundamental points, and particularly on the interpretation of certain morphometric affinities between the most recent Indonesian H. erectus and the ‘robust’ Australian fossil H. sapiens from Kow Swamp and Cohuna. The application of 3D geometric morphometrics (Procrustes analysis) makes it possible to approach this question under a new angle. The shapes of these two sets of fossil hominids are clearly distinct, questioning seriously the assumption of a local direct evolution. For these oldest human settlements as well as for later migrations, multidisciplinary studies (archaeology, palaeoenvironment, palaeoanthropology, genetic, and linguistic) allow us to reconstruct the outlines of the first human settlements of these areas. Climate and environment interacted with these migrations of populations. The very first example is the formation of land bridges between Asian mainland and the Indonesian archipelago during Quaternary glaciations allowing the passage of humans and fauna. And periods of lower sea levels possibly also favoured dispersion from islands to islands. Winds and sea currents, directly related with climate and its variations, also intervened in the settlement of the Pacific. Natural environment, which is impoverished eastward, has been enriched by animals and plants transported by humans. In relation to their way of life as well as to available resources, peoples initially settled littoral zones (Lapita sites), then moved inland very quickly, as demonstrated by excavations carried out in the North of Grande Terre, New Caledonia. To cite this article: A.-M. Sémah, C. R. Palevol 5 (2006) .

Évolution géomorphologique de l'avant-pays du Sud-Ouest de la Nouvelle-Calédonie durant les derniers cycles glaciaires

We present the morphological analysis of a combined digital elevation model (DEM) obtained by merging data from terrestrial and coastal regions of southern New-Caledonia. Our aim is to describe the Quaternary morphotectonic evolution of the southwestern lagoon of New Caledonia, a region with a shallow average bathymetry (20 m), recently submerged in response to both tectonic and eustatic fluctuations. The marine DEM was obtained by interpolating 131 000 depth soundings from MOP–SHOM (‘Mission oceanographique du Pacifique’–‘Service hydrographique et oceanographique de la Marine’, France). We confirm that morphologies such as meandering channels and deeply incised canyons are the result of a sub-aerial fluviatile evolution during periods of low sea levels. The marine sedimentation in this region is very recent, probably corresponding to no more than two glacial–interglacial cycles. Geometrical relationships between submerged paleo-rivers and streams, including captures and incisions, strongly suggest that Quaternary active tectonics affected river development. We identify two hydrographic palaeo-networks suggesting outflow changes, from parallel to the present-day island axis to perpendicular. To cite this article: V. Chevillotte et al., C. R. Geoscience 337 (2005).

Nature and origin of the submarine Albany canyons off southwest Australia

The Albany canyons complex off southwest Australia extends 700 km from Cape Leeuwin to east of Esperance. The submarine canyons head on the uppermost continental slope and extend from there up to 90 km offshore, to the lowermost slope and onto the abyssal plain. Distributaries have transported shelf carbonate grains at least 150 km onto the abyssal plain. The largest canyons have cut down 1500 – 2000 m in places. In general, on the upper slope they have cut down into harder, older rocks: canyon walls are steep, canyon axes slope at up to 20°, and ancient structures control their orientation. On the lower slope the canyons generally have not eroded down into harder rocks: canyon walls are less steep, canyon axis slopes are lower, and the canyons are generally oriented downslope. The canyons have exposed Jurassic and younger sedimentary rocks: their nature, canyon morphology and information from seismic reflection profiles have helped us build an understanding of canyon history. Floodplain deposition rather than erosion occurred during Australia – Antarctic rifting in the Late Jurassic, so river canyons (possible precursors of marine canyons) were unlikely to have been cut. A transition from non-marine to shallow-marine sedimentation characterised Early Cretaceous deposition in the slowly developing rift of the Australo-Antarctic Gulf. Gradients were low and canyon cutting unlikely. Deep river canyons were probably cut during uplift and erosion immediately before the Santonian breakup from Antarctica and their paths probably controlled later marine canyons. Only with the onset of rapid sea-floor spreading and subsidence in the Middle Eocene (ca 43 Ma) did gradients steepen and major marine canyon cutting become possible. The major sea-level fall at the Middle/Late Eocene boundary (ca 40 Ma) may perhaps have accelerated canyon formation. Carbonate sedimentation started to replace siliciclastic sedimentation in the late Middle Eocene, and became completely dominant in the Oligocene as the Antarctic Circumpolar Current started to scour the outer shelf of fines. However, carbonate grains displaced from the outer shelf could continue to cut the canyons, largely during periods of low sea level.

The roles of geological history and colonization abilities in genetic differentiation between mammalian populations in the Philippine archipelago

AbstractAim To test hypotheses that: (1) late Pleistocene low sea‐level shorelines (rather than current shorelines) define patterns of genetic variation among mammals on oceanic Philippine islands; (2) species‐specific ecological attributes, especially forest fidelity and vagility, determine the extent to which common genetic patterns are exhibited among a set of species; (3) populations show reduced within‐population variation on small, isolated oceanic islands; (4) populations tend to be most highly differentiated on small, isolated islands; and (5) to assess the extent to which patterns of genetic differentiation among multiple species are determined by interactions of ecological traits and geological/geographic conditions.Location The Philippine Islands, a large group of oceanic islands in Southeast (SE) Asia with unusually high levels of endemism among mammals.Methods Starch‐gel electrophoresis of protein allozymes of six species of small fruit bats (Chiroptera, Pteropodidae) and one rodent (Rodentia, Muridae).Results Genetic distances between populations within all species are not correlated with distances between present‐day shorelines, but are positively correlated with distances between shorelines during the last Pleistocene period of low sea level; relatively little intraspecific variation was found within these ‘Pleistocene islands’. Island area and isolation of oceanic populations have only slight effects on standing genetic variation within populations, but populations on some isolated islands have heightened levels of genetic differentiation, and reduced levels of gene flow, relative to other islands. Species associated with disturbed habitat (all of which fly readily across open habitats) show more genetic variation within populations than species associated with primary rain forest (all of which avoid flying out from beneath forest canopy). Species associated with disturbed habitats, which tend to be widely distributed in SE Asia, also show higher rates of gene flow and less differentiation between populations than species associated with rain forest, which tend to be Philippine endemic species. One rain forest bat has levels of gene flow and heterozygosity similar to the forest‐living rodent in our study.Main conclusions The maximum limits of Philippine islands that were reached during Pleistocene periods of low sea level define areas of relative genetic homogeneity, whereas even narrow sea channels between adjacent but permanently isolated oceanic islands are associated with most genetic variation within the species. Moreover, the distance between ‘Pleistocene islands’ is correlated with the extent of genetic distances within species. The structure of genetic variation is strongly influenced by the ecology of the species, predominantly as a result of their varying levels of vagility and ability to tolerate open (non‐forested) habitat. Readily available information on ecology (habitat association and vagility) and geological circumstances (presence or absence of Pleistocene land‐bridges between islands, and distance between oceanic islands during periods of low sea level) are combined to produce a simple predictive model of likely patterns of genetic differentiation (and hence speciation) among these mammals, and probably among other organisms, in oceanic archipelagos.

Local endemism within the Western Ghats-sri Lanka biodiversity hotspot.

The apparent biotic affinities between the mainland and the island in the Western Ghats-Sri Lanka biodiversity hotspot have been interpreted as the result of frequent migrations during recent periods of low sea level. We show, using molecular phylogenies of two invertebrate and four vertebrate groups, that biotic interchange between these areas has been much more limited than hitherto assumed. Despite several extended periods of land connection during the past 500,000 years, Sri Lanka has maintained a fauna that is largely distinct from that of the Indian mainland. Future conservation programs for the subcontinent should take into account such patterns of local endemism at the finest scale at which they may occur.

The Great Barrier Reef: The Chronological Record from a New Borehole

ABSTRACT A new borehole, 210 mbsf (meters below sea floor) deep, drilled in Ribbon Reef 5 on the Great Barrier Reef off Cooktown, NE Australia, reveals a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces. Nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards. Aminostratigraphy, magnetostratigraphy, radiocarbon dating, uranium series dating, and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation. Carbonate deposition began about 770 ka ago in a relatively deep-water slope environment and is represented by a series of debris flows. Lithoclasts within these rocks, indicate that older limestones already existed in the area. Subsequent accretion involved the downslope accumulation of grainstones and wackestones, sometimes cross-laminated, characterized by intervals with abundant rhodoliths and scattered, probably reworked, corals. Four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka. Unit 5 probably extended to stage 11 (about 400 ka), and unit 6 to stage 9 ( 330 ka). Typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole, units 5-4. The succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence. The lack of evidence of emergence below this surface reflects progressive accretion or progradation or both. Two younger erosion surfaces define further periods of lowered sea level. Unit 7 is attributed to deposition during isotope stage 7, but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones. Unit 8 is correlated with stage 5 (125 ka), and unit 9 is interpreted as Holocene (post 7,700 ka). The limited thicknesses of units 7, 8, and 9 are considered to reflect erosion. The progressive shallowing brought the depositional surface within the zone exposed during lowstands, and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation.

Vicariance or dispersal? Historical biogeography of three Sunda shelf murine rodents (Maxomys surifer, Leopoldamys sabanus and Maxomys whiteheadi)

The Sunda region of south-east Asia comprises the Malay Peninsula and the islands of Java, Sumatra and Borneo, all of which lie on a shallow continental shelf projecting from Indochina. Pleistocene glacial cycles caused sea levels to drop repeatedly, exposing vast areas of the Sunda shelf and creating land bridges among the islands and mainland. These land bridges, the latest of which connected all three of the major Sunda islands to the Malay Peninsula as recently as 9500 years ago, may have enabled mammalian migrations across the Sunda shelf. Pleistocene land bridges on the Sunda shelf have been invoked to explain the current distributions of mammalian taxa occupying ranges corresponding with the Pleistocene limits of land and the appearance of new mammal species in the Pleistocene fossil record. The ability of mammals to move throughout the exposed shelf during periods of low sea level would, however, have been influenced by topographic and ecological features, which have been variously described as savanna-like or as moist tropical rain forest. Using a phylogeographical approach, we test the hypothesis that Pleistocene land bridges enabled widespread movements in three rain-forest-restricted murine rodents of the Sunda shelf: Maxomys surifer, Leopoldamys sabanus and Maxomys whiteheadi. Our results do not support the hypothesis of broad Pleistocene migrations in these taxa, but instead suggest a deep history of vicariant evolution that may correspond with the Pliocene fragmentation of the Sunda block. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81, 91–109.

近畿地方および八ケ岳山麓における過去４３万年間の広域テフラの層序と編年―ＥＤＳ分析による火山ガラス片の主要成分化学組成―

Energy dispersive Xray Spectrometry (EDS) was applied to measure the major element composition of volcanic glass shards. One hundred three analytical tephra samples were collected from the Takashima-oki drilling core sample on Lake Biwa, as well as some representative drilling core samples in the Osaka coastal area and Middle Pleistocene tephra layers at the foot of the Yatsugatake Mountains. The ages of the principal tephra layers were estimated from the correlation between the biostratigraphic horizons from the core samples and the oxygen isotope stratigraphy. The estimated ages of the Kg, K-Ah, U-Oki, AT, Aso-4, K-Tz, Aso-3, BT44, BT51, Ata-Th, Aso-1, Ng-1, Kkt and BT72 tephra layers are 3.1ka, 7.3ka, 10.7ka, 29ka, 87ka, 91ka, 133ka, 203ka, 216ka, 238ka, 249ka, 294ka, 334ka, and 349ka, respectively. These principal tephra ages provide the estimated ages of other tephra layers using the sediment accumulation rates in the Takashima-oki core. Consequently, this study has reconstructed the tephrostratigraphy and chronology during the past 430ky. Furthermore, it is pointed out that caldera forming eruptions have occurred at low sea level periods just before the maximum high sea level.