Late Pleistocene snowline fluctuations at Nevado Coropuna (15°S), southern Peruvian Andes

Deposits preserved on peaks in the southern Peruvian Andes are evidence for past glacial fluctuations and, therefore, serve as a record of both the timing and magnitude of past climate change. Moraines corresponding to the last major expansion of ice on Nevado Coropuna date to 20‐25 ka, during the last glacial maximum. We reconstructed the snowline at Coropuna for this period using a combined geomorphic‐numeric approach to provide a first‐order estimate of the magnitude of late‐Pleistocene climate change. Our reconstructions show that snowline was approximately 550‐770 m lower during the last glacial maximum than during the late Holocene maximum, which ended in the 19^th century, and ～750 m lower than today. While these values are similar to data from nearby Nevado Solimana, reconstructions from the neighbouring peak of Nevado Firura reveal a smaller snowline depression, suggesting the glacial response to climate forcing in the tropics is strongly influenced by non‐climatic factors. These data constitute some of the first directly dated palaeo‐snowline data from the arid tropics and suggest that the magnitude of the last glaciation in at least parts of the tropical Andes was similar to late‐Pleistocene events at higher latitudes. Copyright © 2011 John Wiley & Sons, Ltd.     

Late Pleistocene–Holocene environmental and climatic history of a freshwater paramo ecosystem in the northern Andes

This study presents the first multi-proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD.     

Last glacial maximum equilibrium-line altitude and paleo-temperature reconstructions for the Cordillera de Mérida, Venezuelan Andes

The pattern and magnitude of glacier equilibrium-line altitude (ELA) lowerings in the tropics during the last glacial maximum (LGM) are topics of current debate. In the northern tropics, paleo-ELA data are particularly limited, inhibiting the ability to make regional and large-scale paleoclimatic inferences. To improve these records, nine paleo-glaciers in the Venezuelan Andes were reconstructed based on field observations, aerial photographs, satellite imagery and high-resolution digital topographic data. Paleo-glacier equilibrium-line altitudes (ELAs) were estimated using the accumulation-area ratio (AAR) and the area-altitude balance ratio (AABR) methods. During the local LGM in Venezuela (∼ 22,750 to 19,960 cal yr BP), ELAs were ∼ 850 to 1420 m lower than present. Local LGM temperatures were are at least 8.8 ± 2°C cooler than today based on a combined energy and mass-balance equation to account for an ELA lowering. This is greater than estimates using an atmospheric lapse rate calculation, which yields a value of 6.4 ± 1°C cooler. The paleo-glacial data from the Venezuelan Andes support other published records that indicate the northern tropics experienced a greater ELA lowering and possibly a greater cooling than the Southern Hemisphere tropics during the LGM.     

Late Pleistocene Mérida Glaciation, Venezuelan Andes

The name Mérida Glaciation is proposed to designate the alpine glaciation which affected the central Veneruelan Andes; during the Late Pleistrocenc. Two main morainie levels have been recognired: one between 2600 and 2700 m, and another between 3000 and 3500 m elevation. The snow line during the last glacial advance was lowered approximately 1200 m below the present snow line (3700 m). Rodiocarbon dating indicates that the moraines are older than 10,000 years B.P., and probahly older than 13,000 years B.P. The lower morainie level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).     

Late Pleistocene glaciers in Darhad Basin, northern Mongolia

Late Pleistocene glaciers around Darhad Basin advanced to near their maximum positions at least three times, twice during the Zyrianka glaciation (at ∼ 17-19 ka and ∼ 35-53 ka), and at least once earlier. The Zyrianka glaciers were smaller than their predecessors, but the equilibrium-line altitude (ELA) difference was < 75 m. End moraines of the Zyrianka glaciers were ∼ 1600 m asl; ELAs were 2100-2400 m asl. ¹⁴C and luminescence dating of lake sediments confirm the existence of paleolake highstands in Darhad Basin before ∼ 35 ka. Geologic evidence and ¹⁰Be cosmic-ray exposure dating of drift suggests that at ∼ 17-19 ka the basin was filled at least briefly by a glacier-dammed lake ∼ 140 m deep. However, lake sediments from that time have not yet been recognized in the region. A shallower paleolake briefly occupied the basin at ∼ 11 ka, but between ∼ 11 and 17 ka and after ∼ 10 ka the basin was probably largely dry. The timing of maximum glacier advances in Darhad appears to be approximately synchronous across northern Mongolia, but different from Siberia and western Central Asia, supporting the inference that paleoclimate in Central Asia differed among regions.     

Late Pleistocene stratigraphy at Boliden, northern Sweden

Till-covered sand and silt sediments with organic material from Boliden, northern Sweden, have been resampled and reinvestigated. Texture and petrographic analyses demonstrated one till bed overlying, and one underlying the sandy sediments. Low frequencies of diatoms were noted, representing freshwater, halophilous and brackish water taxa. The diatoms may be allochthonous or autochthonous, or redeposited from older deposits. Pollen analysis revealed a new picture of the composition of the vegetation, as compared with earlier investigations. The flora consisted of an arctic-subarctic treeless shrub and herb vegetation dominated by Salix, Betula nana , Gramineae and Cyperaceae. Artemisia and several arctic-alpine herbs were important elements. The identification of Salix wood as well as fruits and fruit scales of Betula nana and Betula cf. humilis is in agreement with the pollen-analytical results, as is the occurrence of Nordicboreal species of Coleoptera. Two radiocarbon datings resulted in an age of c. 45,000 B. P. and >40,000 B.P. The Boliden till-covered sediments were most probably deposited during an Early or Middle Weichselian interstadial, but the initial or final stage of an interglacial (Eemian) is also possible.     

Towards a chronology of the Late Pleistocene in the northern Alpine Foreland

Dating results from terrestrial records in the northern foreland of the Alps have been compiled in order to establish an independent chronostratigraphy for the climate history of this region. U/Th dates of peat deposited during the final phase of the Last Interglacial indicate that it lasted until at least c. 115 000 yr ago. The Early Würmian started with a period of severe cold climate causing a substitution of forest by tundra-like vegetation. It is assumed that during this period glaciers advanced to the margin of the foreland of at least the Western Alps. Sediments attributed to this glaciation are dated to about 103 000 yr. Three subsequent interstadials, all characterized by coniferous forest, were interrupted by cold stadials with steppe to tundra-like vegetation. The first interstadial is dated to about 95 000 yr. There is evidence for an interstadial with open coniferous woodland and three phases of steppe vegetation during the Middle Würmian, between c. 50000 and 30 000 yr ago. The last glaciation of the Alpine Foreland reached its maximum extension between 24 000 and 21 500 yr and glaciers rapidly collapsed before ˜17 500 yr ago. A series of minor re-advances during the Lateglacial is reported from within the Alps, but the glaciers barely reached the main Alpine valleys during this time. The last of these advances formed the Egesen moraine and occurred at about 11 800 yr ago during the Younger Dryas.     

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark

Several well-preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present-day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant-scale cross-bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar-trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit-platform growth based on scaled wave tank experiments. The giant-scale cross-bedded unit corresponds to prograding of a coarse-grained subaqueous spit-platform while the topset, bar-trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse-grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant-scale cross-bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit-platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit-platform sequences allows precise determination of sea-level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant-scale cross-bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas.     

Late Miocene freshwater mussels from the intermontane Chota Basin,northern Ecuadorean Andes

Prior to the development of the modern Amazonian drainage network during the Late Miocene-Early Pliocene, large areas of western Amazonia may have been occupied by an extensive lacustrine and wetland environment known as the Pebas and Acre systems. These depositional systems are thought to have formed in response to foreland subsidence east of the uplifting Andes. Based on the occurrence at a present-day elevation of 1600 m of fossil pearly freshwater mussels of the genus Anodontites (indet. species) in intermontane Chota Basin of northern Ecuador (the westernmost South American fossil record for this genus), we discuss their potential implications for understanding of westernmost limit of the Miocene wetland ecosystem and consequently a later timing for the regional uplift of the Eastern Cordillera.     

Late Pleistocene glaciation of Páramo de La Culata,north-central Venezuelan Andes

Late Pleistocene glacial features in the Páramo de La Culata region, north-central Venezuelan Andes, include: 1. depositional features: morainic till and fluvio-glacial deposits (terrace gravels); 2. sculptured features: glaciated valleys, cirques, horns, and arêtes; and 3. erosional features: striation and grooving, polished rock, roches moutonnées and whaleback forms, and erratic boulders. Two main levels of moraines were found, an older one at 2600 m elevation, and a younger one between 3000 and 3500 m. The difference in age is reflected by the higher degree of weathering, erosion, and vegetation cover of the lower level, as compared with the higher level. Radiocarbon dating, and a comparison and correlation of these glacial features with those of adjacent regions, indicates that the lower morainic level (2600 m) is probably the result of the main glacial advance of the Late Wisconsin Glaciation. The main morainic level (3000 to 3500 m) was probably formed by the latest Wisconsin glacial advance. The Late Pleistocene snow-line depression reached approximately 1200 m below the present snow-line (i. e., down to approximately 3500 m).     

Late Pleistocene stratigraphy, sedimentology and paleoenvironmental evolution of the Tarija-Padcaya basin (Bolivian Andes)

We illustrate the results of geomorphological, stratigraphical and sedimentological analysis of the Tarija-Padcaya basin, a wide depression in the eastern side of the Bolivian Cordillera. The basin is well known for the rich mammal fauna discovered since the beginning of the 19th century. The sedimentary infilling belongs to the Tolomosa Formation, corresponding to a major synthem subdivided into three main sub-synthems, mostly made of fluvial and alluvial fan sediments locally weathered by paleoalfisols (Ancon Grande sub-synthem), glacial and fluvio-glacial sediments (Puente Phayo sub-synthem) and finally alluvial fan and alluvial plain sediments (San Jacinto sub-synthem). Radiocarbon dating provides a chronology for the last sub-synthem and testifies that the sequence encompasses the Last Interglacial-Glacial cycle and constitutes a good proxy record for Late Pleistocene climatic changes. The occurrence of glacial deposits in the deeper part of the sedimentary filling suggests a major ice advance during MIS 4 and, together with glacial geomorphological evidence, points to further glacial erosion during the Last Glacial Maximum (LGM). The importance of glacial deposition and erosion opens the question of correlation with the events reported in the nearby Altiplano, where glacial deposits have been recognized only along the slopes of the higher volcanoes. In the Altiplano the LGM has been claimed to be characterized by an absence of deposition or deep erosion, due to extreme dryness, but the Tarija record suggests an erosional event of a scale that would imply the occurrence of a large ice cap.     

A 24,700-yr paleolimnological history from the Peruvian Andes

A new paleolimnological dataset from Lake Pacucha (13 °S, 3095 m elevation) in the Peruvian Andes provides evidence of changes in lake level over the past 24,700 yr. A late-glacial highstand in lake level gave way to an early-Holocene lowstand. This transition appears to have paralleled precessional changes that would have reduced insolation during the wet-season. The occurrence of benthic/salt-tolerant diatoms and CaCO₃ deposition suggest that the lake had lost much of its volume by c. 10,000 cal yr BP. Pronounced Holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c. 8300 to 5000 cal yr BP. While a polymictic lake formed at c. 5000 cal yr BP. These relatively wet conditions were interrupted by a series of drier events, the most pronounced of which occurred at c. 750 cal yr BP. Paleolimnological changes in the Holocene were more rapid than those of either the last glacial maximum or the deglacial period.     

藏北高原环纳木错首次发现晚更新世湖滩岩

200-2001年夏,在开展藏北高原"纳木错第四纪环境演变调查研究"与"1:25万当雄幅"区域地质填图工作中,笔者首次在环纳木错湖岸阶地上发现了一种与湖泊演化有关的湖相沉积岩石--湖滩岩.目前,国际上只有海滩岩一说,湖滩岩尚罕见正式报道.     

Late Quaternary glacial chronology on Nevado Illimani, Bolivia, and the implications for paleoclimatic reconstructions across the Andes

¹⁰Be terrestrial cosmogenic nuclide surface exposure ages from moraines on Nevado Illimani, Cordillera Real, Bolivia suggest that glaciers retreated from moraines during the periods 15.5-13.0 ka, 10.0-8.5 ka, and 3.5-2.0 ka. Late glacial moraines at Illimani are associated with an ELA depression of 400-600 m, which is consistent with other local reconstructions of late glacial ELAs in the Eastern Cordillera of the central Andes. A comparison of late glacial ELAs between the Eastern Cordillera and Western Cordillera indicates a marked change toward flattening of the east-to-west regional ELA gradient. This flattening is consistent with increased precipitation from the Pacific during the late glacial period.     

大兴安岭北段雅鲁河断裂晚更新世活动记录

大兴安岭北段雅鲁河断裂为典型的活动断层,对区域晚更新世地质和地貌演化及现代地震具有控制意义。在扎兰屯市成吉思汗镇北雅鲁河东侧发现雅鲁河断裂活动记录,以野外资料为基础,通过相关松散堆积物的光释光测年,厘定断层活动特征和活动时限,并结合相关经验公式,估测该记录反映的古地震特征。研究结果显示,雅鲁河断裂晚更新世在扎兰屯南部地区存在断层活动,断层走向近SN向、倾向265°、倾角约55°,性质为逆断层;断层活动错断了中生代流纹岩和晚更新世早-中期洪冲积物,并被晚更新世晚期残积和冲积物覆盖,断层活动发生在距今53.4±6.7~39.1±3.6ka之间;活动记录所反映的古地震矩震级(M0)和面波震级(Ms)为6.7级,地震发生时地表破裂长度(SRL)约为30.2~37.8km。     

The last occurrence of Pleistocene megafauna in the Ecuadorian Andes

The latest Pleistocene—Holocene megafauna extinction is a global event, particularly dramatic in the Americas. In a previous paper the authors hypothesised a scenario for this extinction event in South America, where mastodonts first suffered from the changing climate environment, followed by the mylodonts and equids. These different latest Pleistocene—Holocene megafauna extinction “waves” in Ecuadorian Andes have been dated using ¹⁴C methods on material from selected sites in north and central Ecuadorian Interandean Depression. An outline of the physiographic evolution of the Interandean Depression in Ecuador is offered and the stratigraphic setting of the fossiliferous sites is discussed. The present results confirm the author's hypothesis on the megafauna extinction pattern, previously published in terms of relative age. The importance of climatic changes during Last Glacial Maximum at low latitudes is discussed.     

An optical age chronology of Late Pleistocene fluvial deposits in the northern lower Mississippi valley

The lower Mississippi valley (LMV) contains many large braided channel belts that are preserved west of the Holocene floodplain. Previous efforts to establish geochronologic control on channel-belt construction have been hindered by the lack of organic material for radiocarbon dating. Luminescence techniques provide a burial date for the sediment itself and may prove useful in this context. Samples from three channel belts in the northern LMV were analyzed using the single aliquot-regenerative technique on 90–125 μm quartz. Optical ages (19.7–17.8, 16.1–15.0 and 12.5–12.1 ka) are consistent with geomorphic relationships and indicate that channel belts were formed in the late Pleistocene under glacial conditions. These optical ages provide the first detailed chronology of LMV channel-belt formation and are the first step towards developing a chronology for the entire LMV.     

Aeolian activity in northern Amazonia: optical dating of Late Pleistocene and Holocene palaeodunes

Palaeodunes were examined on the eastern margin of the Rio Branco–Rupununi savanna, northeast Amazonia. Optical dating suggests that the onset of aeolian activity was between 17 000 and 15 000 yr ago, just after the Last Glacial Maximum. Both the palaeodune axes and modern dominant wind directions have northeast to east‐northeast directions, implying no significant shift in atmospheric circulation patterns over northeast Amazonia during the Late Pleistocene to Holocene. Major regional climate change events, such as the Younger Dryas, do not appear to have had any effect on the rates of aeolian deposition at the study site. Aeolian activity appears to have continued to the present day, showing a remarkably constant deposition rate of around 0.13 m kyr^?1 initially, increasing smoothly to the present. Until more palaeodunes in northern Amazonia are dated, it is impossible to determine if this record of gradual aeolian deposition is a reliable regional palaeoclimate indicator, rather than being the result of local bioclimatic and geomorphological effects. Copyright © 2004 John Wiley & Sons, Ltd.