Late Quaternary paleoenvironments of an ephemeral wetland in North Dakota,USA: relative interactions of ground-water hydrology and climate change

This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 ¹⁴C yr BP to 11,100 ¹⁴C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 ¹⁴C yr BP and finally, at 8,500 ¹⁴C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500 ¹⁴C yr BP) to deciduous parkland, followed by grassland at 9,300 ¹⁴C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused the lower-elevation Wendel site to capture the ground water of up-gradient lakes.     

Geomorphology, Stratigraphy, and Radiocarbon Chronology of LlanquihueDrift in the Area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile

Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in the region of the southern Chilean Lake District, Seno Reloncav', Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363–29,385 ¹⁴C yr BP , 26,797 ¹⁴C yr BP , 22,295–22,570 ¹⁴C yr BP , and 14,805–14,869 ¹⁴C yr BP . Advances may also have culminated at close to 21,000 ¹⁴C yr BP , shortly before 17,800 ¹⁴C yr BP , and shortly before 15,730 ¹⁴C yr BP . The maximum at 22,295–22,567 ¹⁴C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805–14,869 ¹⁴C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 ¹⁴C yr BP and more than 39,660 ¹⁴C yr BP . In the southern part of the field area, the Golfo de Ancud lobe, as well as the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 ¹⁴C yr BP . Pollen analysis of the Taiquemmire, which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that the Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 ¹⁴C yr BP for the Lago Llanquihue piedmont glacier.     

Ephemeral rollover points and clinothem evolution in the modern Po Delta based on repeated bathymetric surveys

Reconstructions of ancient delta systems rely typically on a two-dimensional (2D) view of prograding clinothems but may miss their three-dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high-resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high-resolution multi-beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth-bar) and muddy seaward-dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high-resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one-year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut-and-fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.     

Relict Shorelines and Ice Flow Patterns of the Northern Puget Lowland From Lidar Data and Digital Terrain Modelling

Airborne lidar data from the northern Puget Lowland provide information on the spatial variability and amplitude of raised postglacial shorelines, marine deltaic features and glaciomarine sediments deposited between approximately c. 12 920 and 11 050 ¹⁴C yr BP (15 960‐12 364 cal yr BP). Relict shorelines preserved in embayments on Whidbey and Camano islands (between 47°54′N and 48°24′N) are found up to an altitude of c. 90 m and record glacio‐isostatic movements attributed to postglacial rebound. The tilt of the regional minimum highstand sea level surface to the north of 0.80 m km^?1, with local variability from 0.25 m km^?1 to 0.77 m km^?1, is consistent with previous studies (Thorson 1989; Dethier et al. 1995). The local variability is related to the uncertainty in the depth of the water column above these features at the time of deposition and probable tectonic deformation. The information generated by these lidar data is most valuable in posing new research questions, generating alternative research hypotheses to those already formulated in the northern Puget Lowland.     

Glacial Deposition Along an Ice-Contact Slope: An Example from the Southern Lake District, Chile

Glacially derived sediments and structures vary systematically with topographic position along ice-contact slopes near the margin of former temperate piedmont glaciers in the southern Lake District, Chile. Features along lower positions of the slopes include glaciolacustrine sediments and large-scalethrusting. Middle slope features include low-angle subglacial thrusting of thin slices of lacustrine sediment. Upper slope features include high-angle thrusting, and lodgement and flow till. Locally, the same ordering of these features occurs in vertical sequence.
A common theme that unifies the processes represented by these features is the distribution of hydrostatic pressure. Thicker ice (producing higher glacier overburden pressures) and aquicludes of fine-grained sediments toward the center of the basin result in high hydrostatic pressures, whereas thin ice and porous out-wash reduce these pressures at the top of the ice-contact slopes.However, such a distribution cannot completely explain the observed vertical sequences; hence, local variations in hydrostatic pressure because of lithologic contrasts probably play a role. Nowhere along the ice-contact slope are pervasively deformed glacial sediments observed; therefore they cannot be invoked to explain the low glacier profiles of these piedmont glacier lobes.     

Sclerochronological oxygen and carbon isotope ratios in Radix (Gastropoda) shells indicate changes of glacial meltwater flux and temperature since 4,200 cal yr BP at Lake Karakul,eastern Pamirs (Tajikistan)

We report δ¹⁸O and δ¹³C values of 21 fossil shells from the aquatic gastropod Radix from a sediment core taken in the eastern basin of Lake Karakul, Tajikistan (38.86–39.16°N, 73.26–73.56°E, 3,928 m above sea level) and covering the last 4,200 cal yr BP. The lake is surrounded by many palaeoshorelines evidencing former lake-level changes, most likely triggered by changes in meltwater flux. This hypothesis was tested by interpreting the isotope ratios of Radix shells together with δ¹⁸O values of Ostracoda and of authigenic aragonite. The mean δ¹⁸O values of Radix and Ostracoda fall along the same long-term trend indicating a change in the isotopic composition of precipitation, which contributed to the glaciers in the catchment as snow and finally as melt water to the lake. The sclerochronological δ¹⁸O and δ¹³C patterns in Radix shells provide seasonal weather information, which is discussed in context with previously proposed climatic changes during the last 4,200 cal yr BP. The period between ~4,200 and 3,000 cal yr BP was characterized by stepwise glacier advance in the catchment most likely due to a precipitation surplus. Subsequently the climate remained relatively cold but the lake level fluctuated, as indicated by ostracod shell isotope data. From ~1,800 cal yr BP the sclerochronological patterns provide evidence for increasing melt water flux and transport of allochthonous carbon into the lake, most likely due to an accelerated glacier retreat. The period around 1,500 cal yr BP was characterized by strong warming, increasing meltwater flux, glacier retreat and an increasing lake level. Warm conditions continued until ~500 cal yr ΒP probably representing the end of the Medieval Warm Period. A short relatively cold (dry?) period and a lower lake level are assumed for ~350 cal yr BP, possibly an analogue to the Maunder Minimum cooling in the North Atlantic region. Our results show that the lake system is complex, and that changes were triggered by external forcing and feedbacks. The similarity of δ¹⁸O values in Radix and ostracod shells demonstrates that both archives provide complementary information.     

黄河口不同粒度泥沙沉积与扩散分析

本文采集和收集大量黄河三角洲沉积物剖面和钻孔泥沙粒度资料以及黄河口来沙粒度组成数据,定量研究了黄河口泥沙的沉积与扩散特征.结果 显示黄河口来沙以粉砂为主,黏土次之,砂最少,年均中值粒径无长期变化趋势.黄河三角洲平原相泥沙与来沙的黏土、粉砂和砂含量无显著差异;前缘相泥沙比来沙的黏土含量较低,砂含量较高;前三角洲相泥沙比来...     

MASS WASTING IN THE SWAUK WATERSHED,WASHINGTON

Mass wasting evidence is common along the margins of the Columbia River Basalts. I identified, mapped, dated, and assessed the environment of nearly 160 discrete slope failures (excluding rockfall) along the margins of the Columbia River Basalts in the Swauk watershed of central Washington. Rotational slides, translational slides, flows, and complex slide-flows were identified via topographic map, airphoto, and field analysis. Geographic information systems analysis revealed that these features cover 38% of the watershed. Translational slides are the most numerous of the slope failures, whereas complex slide-flows cover the most area. I placed each slope failure into a relative age category (active, inactive-young, inactive mature, and inactive-old) based on the characteristics of the main scarp, lateral flanks, internal morphology, vegetation cover, and toe relationships. Most Swauk watershed slope failures are inactive-mature. Organic sediments from an inactive-mature sag pond formed ～6880 ¹⁴C yr BP, whereas inactive-young sediments dated at ～5930 ¹⁴C yr BP. Inactive slope failures are often associated with steep slopes, inclined beds, incompetent geologic units, or streamcuts. Streamcuts, roadcuts, or clearcuts typically accompany active slope failures. Rain-on-snow events and associated mass wasting in winter 1996 provide a plausible trigger analog for inactive mass wasting. Rockfall deposits cover ～29% of the watershed, range from inactive to active in age, and occur atop pre-existing slope failures in well-jointed Columbia River Basalts. Mass wasting has played a key role in shaping the topographic and hydrologic patterns of the watershed. [Key words: mass wasting, watershed, Washington state, Columbia River Basalts, rain-onsnow.]     

A ∼6100 14C yr record of El Niño activity from the Galápagos Islands

Lithostratigrahic and mineralogic analyses of sediments from hypersaline Bainbridge Crater Lake, Galápagos Islands, provide evidence of past El Niño frequency and intensity. Laminated sediments indicate that at least 435 moderate to very strong El Niño events have occurred since 6100 ¹⁴C yr BP (7130 cal yr BP), and that frequency and intensity of events increased at about 3000 ¹⁴C yr BP (3100 cal yr BP). El Niño activity was present between 6100 and 4000 ¹⁴C yr BP (4600 cal yr BP) but infrequent. The Bainbridge record indicates that there has been considerable millennial-scale variability in El Niño since the mid-Holocene.     

Interhemispheric Linkage of Paleoclimate During the Last Glaciation

Combined glacial geologic and palynologic data from the southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé in middle latitudes (40°35’–42°25’S) of the Southern Hemisphere Andes suggest (1) that full-glacial or near-full-glacial climate conditions persisted from about 29,400 to 14,550 ¹⁴C yr BP in late Llanquihue time, (2) that within this late Llanquihue interval mean summer temperature was depressed 6°–8°C compared to modern values during major glacier advances into the outer moraine belt at 29,400, 26,760, 22,295–22,570, and 14,550–14,805 ¹⁴C yr BP , (3) that summer temperature depression was as great during early Llanquihue as during late Llanquihue time, (4) that climate deteriorated from warmer conditions during the early part to colder conditions during the later part of middle Llanquihue time, (5) that superimposed on long-term climate deterioration are Gramineae peaks on Isla Grande de Chiloé that represent cooling at 44,520–47,110 ¹⁴C yr BP (T-11), 32,105–35,764 ¹⁴C yr BP (T-9), 24,895–26,019 ¹⁴C yr BP (T-7), 21,430–22,774 ¹⁴C yr BP (T-5), and 13,040–15,200 ¹⁴C yr BP (T-3), (6) that the initial phase of the glacial/interglacial transition of the last termination involved at least two major steps, one beginning at 14,600 ¹⁴C yr BP and another at 12,700–13,000 ¹⁴ C yr BP , and (7) that a late-glacial climate reversal of ≥2–3° C set in close to 12,200 ¹⁴C yr BP , after an interval of near-interglacial warmth, and continued into Younger Dryas time. The late-glacial climate signal from the southern Chilean Lake District ties into that from proglacial Lago Mascardi in the nearby Argentine Andes, which shows rapid ice recession peaking at 12,400 ¹⁴C yr BP , followed by a reversal of trend that culminated in Younger-Dryas-age glacier readvance at 11,400–10,200 ¹⁴C yr BP . Many full- and late-glacial climate shifts in the southern Lake District match those from New Zealand at nearly the same Southern Hemisphere middle latitudes. At the last glacial maximum (LGM), snowline lowering relative to present-day values was nearly the same in the Southern Alps (875 m) and the Chilean Andes (1000 m). Particularly noteworthy are the new Younger-Dryas-age exposure dates of the Lake Misery moraines in Arthur's Pass in the Southern Alps. Moreover, pollen records from the Waikato lowlands on North Island show that a major vegetation shift at close to 14,700 ¹⁴C yr BP marked the beginning of the last glacial/interglacial transition (Newnham et al. 1989). The synchronous and nearly uniform lowering of snowlines in Southern Hemisphere middle-latitude mountains compared with Northern Hemisphere values suggests global cooling of about the same magnitude in both hemispheres at the LGM. When compared with paleoclimate records from the North Atlantic region, the middle-latitude Southern Hemisphere terrestrial data imply interhemispheric symmetry of the structure and timing of the last glacial/interglacial transition. In both regions atmospheric warming pulses are implicated near the beginning of Oldest Dryas time (～14,600 ¹⁴C yr BP) and near the Oldest Dryas/Bölling transition (～12,700–13,000 ¹⁴ C yr BP ). The second of these warming pulses was coincident with resumption of North Atlantic thermohaline circulation similar to that of the modern mode, with strong formation of Lower North Atlantic Deep Water in the Nordic Seas. In both regions, the maximum Bölling-age warmth was achieved at 12,200–12,500 ¹⁴ C yr BP , and was followed by a reversal in climate trend. In the North Atlantic region, and possibly in middle latitudes of the Southern Hemisphere, this reversal culminated in a Younger-Dryas-age cold pulse. Although changes in ocean circulation can redistribute heat between the hemispheres, they cannot alone account either for the synchronous planetary cooling of the LGM or for the synchronous interhemispheric warming steps of the abrupt glacial-to-interglacial transition. Instead, the dominant interhemispheric climate linkage must feature a global atmospheric signal. The most likely source of this signal is a change in the greenhouse content of the atmosphere. We speculate that the Oldest Dryas warming pulse originated from an increase in atmospheric water-vapor production by half-precession forcing in the tropics. The major thermohaline switch near the Oldest Dryas/Bölling transition then couldhave triggered another increase in tropical water-vapor production to near-interglacial values.     

玉龙雪山冰川稳定同位素分馏冬夏对比

利用玉龙雪山白水1号冰川区冬季和夏季表面积雪、雪坑、融水以及白水河河水中δ¹⁸O资料,对比分析了冬季和夏季我国典型季风温冰川系统内稳定同位素分馏行为的差异。分析结果表明,夏季冰川系统内各水体相变过程中稳定同位素分馏程度均比冬季强烈,指示出夏季季风海洋型冰川强烈消融的特点。另外,不论是冬季还是夏季,从表面积雪到融水再到由融水补给的河流,δ¹⁸O垂直变化梯度依次增大,反映了从固态降雪向冰川融水补给的河流河水转换过程中,稳定同位素分馏程度逐渐增强,体现了沉积后过程对海洋型冰川区同位素记录的影响具有空间差异性。     

Role of initial depth at basin margins in sequence architecture: field examples and computer models

A delay in the onset of sedimentation during fault‐related subsidence at a basin margin can occur in both extensional settings, where footwall tilting may cause a diversion of drainage patterns, and in strike‐slip basins, where a source area may be translated along the basin margin. The ‘initial depth’ created by this delay acts as pre‐depositional accommodation and is a partly independent variable. It controls the geometry of the first stratal units deposited at the basin margin and thus modifies the response of the depositional system to subsequent, syndepositional changes in accommodation. In systems with a sharp break in the depositional profile, such as the topset edge in coarse‐grained deltas, the initial depth controls the foreset height and therefore the progradational distance of the topset edge. The topset length, in turn, influences topset accommodation during cyclical base level variations and therefore is reflected in the resulting stacking patterns at both long‐ and short‐term time scales. In the simplified cases modelled in this study, it is the relationship between the initial depth and the net increase in depth over the interval of a relative sea‐level cycle (ΔH) that governs long‐ and short‐term stacking patterns. In situations where the initial depth is significantly larger than ΔH, the topset accommodation of the first delta is insufficient to contain the volume of sediment of younger sequences formed during subsequent relative sea‐level cycles. Therefore, the depositional system tends to prograde over a number of relative sea‐level cycles before the topset area increases so that the long‐term stacking pattern changes to aggradation. Stacking patterns of high‐frequency sequences are influenced by a combination of topset accommodation available and position of the short‐term relative sea‐level cycles on the rising or falling limb of a long‐term sea‐level curve. This determines whether deposits of short‐term cycles are accommodated in delta topsets or foresets, or in both. Variations in stacking pattern caused by different initial depths may be misinterpreted as due to relative sea level or sediment supply changes and it is necessary to consider initial bathymetry in modelling and interpretation of stacking patterns, especially in fault‐bounded basins.     

黄河三角洲岸线及现行河口区水下地形演变

根据实测的岸线和水深数据,利用Surfer 和Mapinfo 等软件进行数据处理,结合不同阶段利津站输沙量,分析了黄河三角洲岸线及现行河口区水下地形演变。结果表明:1953-2000年,68%左右的入海泥沙淤积在口门和滨海区。由于入海流路变迁,不同岸段的岸线变化具有各自的特征。刁口河流路以西岸线基本稳定;刁口河流路以东—孤东油田以北岸线经历先淤后冲,属于强侵蚀岸段,但在防潮大堤的保护下得到人为控制下的稳定;清水沟流路形成的岸线整体向海淤进,但清8 出汊后,清水沟老河口沙嘴南侧出现侵蚀。1976-1996 年,现行河口(清水沟流路) 水下地形总体上表现为淤积,顶坡段变缓,前坡段变陡。1996-2005 年,清水沟老河口水下地形顶坡段和前坡段发生侵蚀,底坡段呈现淤积;出汊新河口水下地形继续淤积,但程度和范围都比1976-1996 年的小。孤东油田近岸侵蚀加剧。     

Landscape and Coast Development of A Lowland Fjord Margin Following Deglaciation, East Greenland

The landscapes of western Jameson Land bordering Hall Bredning fjord comprise upper river basins, glacial landscapes, lower river basins and a near‐shore zone. The upper river basins are incised into bedrock and display no cover of young sediments whilst the glacial landscapes, located closer to the coast, are dominated by Pleistocene deposits and an irregular topography with hills and ridges. The lower river basins, dissecting the glacial landscapes, are connected to the upper river basins and contain well‐defined Holocene delta terraces. The near‐shore zone, which includes the present coast, displays a few raised shorelines. Geomorphological observations combined with stratigraphic work and ¹⁴C dates provide a chronological framework for the development of landscape and shoreline, as presented by a four‐stage reconstruction. The first stage covers the deglaciation of western Jameson Land at the Weichselian‐Holocene transition after a collapse of the main fjord glacier in Hall Bredning. The sea inundated the low‐lying areas on Jameson Land forming small side‐entry fjord basins that possibly follow the track of older valleys. This was followed by a second stage, the paraglacial period, when large meltwater production and sediment transport resulted in a fast infilling of the side‐entry fjord basins by deltas. These are now exposed in terraces in the lower river basins at 70–80 m a.s.l. During a third stage, the relaxation period, fluvial activity decreased and the land surface was increasingly occupied by a cover of tundra vegetation. A glacio‐isostatic rebound resulted in a relative sea level fall and fluvial incision. During stages two and three the coast was exposed to shallow marine processes that aided the alignment of the coast. Stages one to three presumably lasted for less than 2000 years. During stage four, the stable period, lasting for several thousand years till the present, there were minor adjustments of shoreline and landscape. The four‐step reconstruction describes the sedimentary response of a lowland fjord margin to dramatic changes in climate and sea level. The distribution of erosion and sedimentation during this development was mainly controlled by topography. The reconstruction of the latest environmental development of Jameson Land puts new light on Jameson Land's long and complex Quaternary stratigraphic record. The reconstruction may also be used as a model for the interpretation of deposits in similar areas elsewhere.     

Abrupt Vegetation and Climate Changes During the Last Glacial Maximumand Last Termination in The Chilean Lake District: A Case Study from Canal De La Puntilla (41°S)

ABSTRACT. Multiple overlapping and replicate pollen stratigraphies from Canal de la Puntilla (40°57’09”S, 72°54’18”W, 120 m elevation) reveal that a Nothofagus dombeyi-type parkland occupied the Valle Central of the Chilean Lake District during the portion of the Last Glacial Maximum between 20,200 and about 14,600 ¹⁴C yr BP . Dominating this landscape was Nothofagus dombeyi-type and Gramineae, accompanied by taxa commonly found today in Subantarctic environments and above the Andean tree-line in the Lake District (Perezia-type, Valeriana, and Huperzia selago), along with cushion bog taxa characteristic of Magellanic Moorlands (Donatia fascicularis and Astelia pumila). Within this open landscape Nothofagus dombeyi-type expanded between 20,200 and 15,800 ¹⁴C yr BP , interrupted by a brief reversal between 19,200 and 18,800 ¹⁴ C yr BP and followed by a prominent increase in Gramineae pollen between 15,800 and about 14,600 ¹⁴C yr BP . A major rise of Nothofagus dombeyi-type began at about 14,600 ¹⁴C yr BP , followed by decline in non-arboreal taxa and a remarkable expansion of North Patagonian Rain Forest taxa in pulses centered at 14,200 and 13,000 ¹⁴C yr BP . Podocarpus nubigena expanded between 12,200 and 9800 ¹⁴C yr BP , along with increases in Misodendrum and Maytenus disticha-type between 11,000 and 9800 ¹⁴C yr BP. Paleovegetation records suggest that mean annual temperature was 6–7°C colder than at present during the coldest episodes between 20,200 and about 14,600 ¹⁴C yr BP , with twice the modern annual precipitation between 20,200 and 13,000 ¹⁴C yr BP, suggesting a northward shift and intensification of westerly stormtracks. Slight climate warming occurred between 20,200 and 15,800 ¹⁴C yr BP , interrupted by cooling events at 19,200 and 15,800 ¹⁴C yr BP . The initial warming of the last termination started at 14,600 ¹⁴C yr BP , followed by warming pulses at 14,200 and 13,000 ¹⁴C yr BP. These events brought glacial conditions to a cool-temperate climate, slightly cooler and wetter than modern climate, accounting for a total temperature recovery of ≥5°C by about 13,000 ¹⁴C yr BP . A general reversal in trend is inferred with cooling events at 12,200 and 11,000 ¹⁴C yr BP .     

EVIDENCE FOR LATE-GLACIAL ICE DAMMED LAKES IN THE CENTRAL STRAIT OF MAGELLAN AND BAHÍA INÚTIL, SOUTHERNMOST SOUTH AMERICA

ABSTRACT. This paper critically appraises the evidence for a succession of ice-dammed lakes in the central Strait of Magellan ( c. 53°S) c. 17 000–12 250 cal. yr BP. The topographic configuration of islands and channels in the southern Strait of Magellan means that the presence of lakes provides compelling constraints on the position of former ice margins. Lake shorelines and glacio-lacustrine sediments have been dated by their association with a key tephra layer from Volcan Reclús (c. 15 510–14 350 cal. years bp ) and by ¹⁴C-dated peats. The timing of glacial lake formation and associated glacier readvances is at odds with the rapid and widespread glacier retreat of the Patagonian ice fields further north after c. 17 000 cal. yr bp , suggesting rather that the lakes were coeval with the Antarctic Cold Reversal and persisted to the Late-glacial/Holocene transition. This apparent asymmetrical latitudinal response in glacier behaviour may reflect overlapping spheres of northern hemisphere and Antarctic climatic influence in the Magellan region.     

东南极拉斯曼丘陵湖泊沉积生物标志物记录及其环境气候意义

东南极拉斯曼丘陵地区莫愁湖（69°22.3’ S,76°22.0’ E）沉积柱中的有机生物标志物记录了全新世中晚期该地区气候演变过程。不饱和长链烯酮在沉积柱111-76 cm （6450-5100 cal. yr. BP）和36-30 cm（3700-3500 cal. yr. BP）深度有检出,76 cm深度以上基本消失,表明该地区在5100 cal. yr. BP前后气候开始由冷转暖,冰川消融,陆壳抬升,相对海平面下降,同时大量的冰融水使湖泊逐渐淡化。沉积柱底部长链烯酮的检出阶段与东南极相对海平面较高时期相一致,而沉积柱36-30 cm（3700-3500 cal. yr. BP）深度不饱和长链烯酮的痕量检出则揭示了一个短暂的气候干冷,湖泊盐度升高的时期。沉积物中正构烷烃反映的当地气候变化所控制的湖生植物群落演变过程与上述过程基本一致。     

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS ¹⁴C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 ¹⁴C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 ¹⁴C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 ¹⁴C yr BP), pollen data suggest moist conditions, but high ¹⁸O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in ¹⁸O values and associated change in ostracod species at 6800 ¹⁴C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 ¹⁴C yr BP may indicate early human disturbance. By 2800 ¹⁴C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 ¹⁴C yr BP, following the collapse of Classic Maya civilization.     

Relationships between Trophic State, Paleosalinity and Climatic Changes during the First Holocene Marine Transgression in Rocha Lagoon, Southern Uruguay

Paleolimnological data are presented on trophic development, climatic change and sea level variations in Rocha Lagoon, a 72 km² coastal lagoon in southern Uruguay. Using a sediment core that extended from 7000 to about 3700 yr BP, analyses of organic matter, carbonate, diatoms and chrysophyte cysts were used to track the early Holocene paleolimnological conditions of Rocha Lagoon. Opal phytoliths were also counted and identified, both temperature and humidity indices were calculated, and Opal Phytolith Association Zones (OPAZ) were identified by performing Principal Coordinates Analysis (PCO). Diatom Association Zones (DAZ) corresponding to marine/brackish and brackish/freshwater episodes were closely related to changes in trophic state. Those DAZ representing marine/brackish stages exhibited a lower trophic state than those DAZ dominated by brackish and freshwater diatoms. This highlights that during the first Holocene marine transgression, Rocha Lagoon did not continuously exhibit marine/brackish conditions as reported in previous papers. Instead, three brackish/freshwater episodes related to sea level variation and changes in humidity were identified. The first episode, by ~6000 yr BP, was related to sea level change as no significant changes in either temperature or humidity indices were observed. The second episode, between 5000 and 4400 yr BP, was related to both a sea level decrease and an increase in humidity, as a transition from humid to very humid climate was inferred. Concomitant decreases in salinity and increases in trophic state were also observed. The third episode, after ~4000 yr BP, was related to the end of the first Holocene regressive phase when sea level was slightly below present levels. Further decreases in salinity and increases in trophic state were detected. The paleoclimatic trends inferred in this study were in close agreement with other regional studies on climatic change, as cool temperatures were inferred. However, major changes in humidity were also detected. A humid to very humid climate was inferred for ~7000–4500 yr BP, but the occurrence of a semiarid/arid climate was inferred for ~4500–3700 yr BP. Our data suggest that during transgressive and regressive events there might be higher frequency and lower amplitude sea level oscillations that might lead to changes in salinity and trophic state of coastal aquatic systems. Such oscillations could only be tracked by high resolution analyses of sedimentary records and could be best interpreted with complementary data on paleoclimate. In addition, microfossils such as diatoms and opal phytoliths were shown to be very sensitive to such paleoenvironmental changes.     

Paleoecology of The Southern Chilean Lake District-Isla Grande de Chiloé During Middle–late Llanquihue Glaciation and Deglaciation

Subantarctic Parkland and Subantarctic–North Patagonian Evergreen Forest, embracing >40,000 ¹⁴ C years of middle and late Llanquihue glaciation, are reconstructed from pollen contained in multiple interdrift deposits and cores of lake sediments. The subantarctic plant communities at low elevations have since been replaced by temperate Valdivian Evergreen Forest. Data in support of the vegetation reconstruction derive from close-interval sampling (>1400 pollen analysed stratigraphic levels) and high-resolution chronology (>200 AMS and conventional radiocarbon-dated horizons). Pollen sequences are from 15 sites, eight of which are exposures and seven mires, located in relation to lobes of piedmont glaciers that occupied Lago Llanquihue, Seno Reloncav', Golfo de Ancud, and the east-central sector of Isla Grande de Chiloí at the northern limit of the Golfo Corcovado lobe. Recurring episodes of grass maxima representing Subantarctic Parkland, when grass and scrub became widespread among patches of southern beech (Nothofagus), bear a relationship to glacial advances. The implication of the maxima, prominent with advances at 22,400 and 14,800 ¹⁴C yr BP during late Llanquihue glaciation in marine oxygen-isotope Stage 2, is of successive intervals of cold climate with summer temperatures estimated at 6–8°C below the modern mean. The earliest recorded maximum at >50,000 ¹⁴C yr BP is possibly during late Stage 4. At the time of middle Llanquihue glaciation in Stage 3, cool, humid interstades on Isla Grande de Chiloé with Subantarctic Evergreen Forest, which under progressive cooling after 47,000 ¹⁴C yr BP was increasingly replaced by parkland. During stepwise deglaciation, when transitional beech woodland communities supplanting parkland became diversified by formation of thermophilous North Patagonian Evergreen Forest, warming in the order of 5–6°C was abrupt after 14,000 ¹⁴C yr BP . Closed-canopy North Patagonian Evergreen Forest was established by 12,500 ¹⁴C yr BP . Later, after c. 12,000 until 10,000 ¹⁴C yr BP , depending on location, forest at low elevations became modified by expansion of a cold-tolerant element indicative of ≥2–3°C cooler climate. This stepwise climatic sequence is seen at all late-glacial sites. Cool, humid interstadial conditions, punctuated by cold stadial climate, are characteristic of the last ≥40,000 ¹⁴C years of the Pleistocene at midlatitude in the Southern Hemisphere. Pollen sequences from southern South America and terrestrial–marine records from the New Zealand–Tasmania sector express a broad measure of synchrony of vegetational/climatic change for marine oxygen-isotope Stages 2–3. The data, combined with the timing of glacial maxima in the Southern Andes, Southern Alps of New Zealand, and in the Northern Hemisphere, are indicative of synchronous, millennial-scale, midlatitude climatic changes in the polar hemispheres.