The Antarctic contribution to Holocene global sea level rise

The Holocene glacial and climatic development in Antarctica differed considerably from that in the Northern Hemisphere. Initial deglaciation of inner shelf and adjacent land areas in Antarctica dates back to between 10-8 Kya, when most Northern Hemisphere ice sheets had already disappeared or diminished considerably. The continued deglaciation of currently ice-free land in Antarctica occurred gradually between ca. 8-5 Kya. A large southern portion of the marine-based Ross Ice Sheet disintegrated during this late deglaciation phase. Some currently ice-free areas were deglaciated as late as 3 Kya. Between 8-5 Kya, global glacio-eustatically driven sea level rose by 10-17m, with 4-8 m of this increase occurring after 7 Kya. Since the Northern Hemisphere ice sheets had practically disappeared by 8-7 Kya, we suggest that Antarctic deglaciation caused a considerable part of the global sea level rise between 8-7 Kya, and most of it between 7-5 Kya. The global mid-Holocene sea level high stand, broadly dated to between 8-4 Kya, and the Littorina-Tapes transgressions in Scandinavia and simultaneous transgressions recorded from sites e.g. in Svalbard and Greenland, dated to 7-5 Kya, probably reflect input of meltwater from the Antarctic deglaciation.     

Mountain-derived versus shelf-based glaciations on the western Taymyr Peninsula, Siberia

The early Russian researchers working in central Siberia seem to have preferred scenarios in which glaciations, in accordance with the classical glaciological concept, originated in the mountains. However, during the last 30 years or so the interest in the glacial history of the region has concentrated on ice sheets spreading from the Kara Sea shelf. There, they could have originated from ice caps formed on areas that, for eustatic reasons, became dry land during global glacial maximum periods, or from grounded ice shelves. Such ice sheets have been shown to repeatedly inundate much of the Taymyr Peninsula from the north-west. However, work on westernmost Taymyr has now also documented glaciations coming from inland. On at least two occasions, with the latest one dated to the Saale glaciation (marine isotope stage 6 [MIS 6]), warm-based, bedrock-sculpturing glaciers originating in the Byrranga Mountains, and in the hills west of the range, expanded westwards, and at least once did such glaciers, after moving 50–60 km or more over the present land areas, cross today's Kara Sea coastline. The last major glaciation affecting south-western Taymyr did, however, come from the Kara Sea shelf. According to optically stimulated luminescence dates, this was during the Early or Middle Weichselian (MIS 5 or 4), and was most probably not later than 70 Kya. South-western Taymyr was not extensively glaciated during the last global glacial maximum ca. 20 Kya, although local cold-based ice caps may have existed.     

A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland

Evidence from north-west Iceland's shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to Ø4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.     

Ice-free conditions in Novaya Zemlya 35 000–30 000 cal years B.P., as indicated by radiocarbon ages and amino acid racemization evidence from marine molluscs

Novaya Zemlya was covered by the eastern part of the Barents–Kara ice sheet during the glacial maximum of marine isotope stage 2 (MIS 2). We obtained ¹⁴C ages on 37 samples of mollusc shells from various sites on the islands. Most samples yielded ages in the range of 48–26 ¹⁴C Ky. Such old samples are sensitive to contamination by young ¹⁴C, and therefore their reliability was assessed using replicate analyses and amino acid geochronology. The extent of aspartic acid racemization (Asp D/L) indicates that many of the ¹⁴C ages are correct, whereas some are minimum ages only. The results indicate that a substantial part of Novaya Zemlya was ice-free about 35–27 ¹⁴C Kya, and probably even earlier. Corresponding shorelines up to >140 m a.s.l. indicate a large Barents–Kara ice sheet during early MIS 3. These results are consistent with findings from Svalbard and northern Russia: in both places a large MIS 4/3 Barents–Kara ice sheet is postulated to have retreated about 50 Kya, followed by an ice-free interstadial that lasted until up to ca. 25 Kya. The duration of the MIS 2 glaciation in Novaya Zemlya was calculated by applying the D/L values to a kinetic equation for Asp racemization. This indicates that the islands were ice covered for less than 3000 years if the basal temperature was 0^oC, and for less than 10 000 years if it was −5^oC.     

Radiocarbon Age Determinations of Peat and Wood Samples from Mount Fulufjället, Dalarna, Sweden

During the investigation of the erosional effects of the rainstorm on 30-31 August 1997 on Mount Fulufjället, two samples have been radiocarbon age determined. The results give a maximum age of events occurring earlier. One sample of an exposed Picea trunk in the sandur of the stream St Göljån was radiocarbon dated to 1320±120 ¹⁴C years BP. The lowermost part of a peat sequence in a soligenous mire in the valley of the stream Tangån was radiocarbon dated to 1455±70 ¹⁴C years.     

A record of vegetation dynamics and lake level changes from Lake Emakat,northern Tanzania,during the last c. 1200 years

Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (¹⁴C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS ¹⁴C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.     

Late glacial and Holocene marine records from the Independence Fjord and Wandel Sea regions, North Greenland

The first marine sediment cores from the unexplored Independence Fjord system and the Wandel Sea, North Greenland, have been investigated to reveal the glacial marine history of the region. Two key sites in the Independence Fjord system, and an earlier analysed site from the Wandel Sea continental slope, off the mouth of Independence Fjord, are presented. The Independence Fjord sites reveal an early Holocene record (10.0–8.9 Kya) of fine-grained reddish muds with calcareous microfossils, dominated by the benthic foraminifera Cassidulina neoteretis . We suggest that a semi-permanent fast ice cover characterized the region in the early Holocene, and that the deeper troughs in the mouth region of the Independence Fjord system were intruded by subsurface Atlantic water. A stiff diamicton, at least 1.3 m thick, with coal and sandstone clasts of mainly local origin, and a 0.5-m-thick Holocene cover, are found in one of the sites. The diamicton is assumed to represent a subglacial till predating the early Holocene sediments (>10 Kya). Shallow seismic records off the mouth of Independence Fjord reveal kilometre-sized troughs with signs of glacial erosion, till deposition and a Holocene glaciomarine deposition. These features could indicate that glacial ice debouching from the Independence Fjord system at some time during the last glacial period extended to the mid-outer Wandel Sea shelf. Data from a high-resolution sediment core previously retrieved from the adjacent Wandel Sea slope indicate that the maximum ice sheet advance in this area culminated about 25–20 Kya.     

A "critical" climatic evaluation of last interglacial (MIS 5e) records from the Norwegian Sea

Sediment cores from the Norwegian Sea were studied to evaluate interglacial climate conditions of the marine isotope stage 5e (MIS 5e). Using planktic forminiferal assemblages as the core method, a detailed picture of the evolution of surface water conditions was derived. According to our age model, a step-like deglaciation of the Saalian ice sheets is noted between ca. 135 and 124.5 Kya, but the deglaciation shows little response with regard to surface ocean warming. From then on, the rapidly increasing abundance of subpolar forminifers, concomitant with decreasing iceberg indicators, provides evidence for the development of interglacial conditions sensu stricto (5e- ss ), a period that lasted for about 9 Ky. As interpreted from the foraminiferal records, and supported by the other proxies, this interval of 5e- ss was in two parts: showing an early warm phase, but with a fresher, i.e., lower salinity, water mass, and a subsequent cooling phase that lasted until ca. 118.5 Kya. After this time, the climatic optimum with the most intense advection of Atlantic surface water masses occurred until ca. 116 Kya. A rapid transition with two notable climatic perturbations is observed subsequently during the glacial inception. Overall, the peak warmth of the last interglacial period occurred relatively late after deglaciation, and at no time did it reach the high warmth level of the early Holocene. This finding must be considered when using the last interglacial situation as an analogue model for enhanced meridional transfer of ocean heat to the Arctic, with the prospect of a future warmer climate.     

14C测定年代报告(二)

本报告公布本实验室1984年9月至1986年2月测定的¹⁴C年代数据48个。     

Periglacial landscape evolution and environmental changes of Arctic lowland areas for the last 60 000 years (western Laptev Sea coast, Cape Mamontov Klyk)

Non-glaciated Arctic lowlands in north-east Siberia were subjected to extensive landscape and environmental changes during the Late Quaternary. Coastal cliffs along the Arctic shelf seas expose terrestrial archives containing numerous palaeoenvironmental indicators (e.g., pollen, plant macro-fossils and mammal fossils) preserved in the permafrost. The presented sedimentological (grain size, magnetic susceptibility and biogeochemical parameters), cryolithological, geochronological (radiocarbon, accelerator mass spectrometry and infrared-stimulated luminescence), heavy mineral and palaeoecological records from Cape Mamontov Klyk record the environmental dynamics of an Arctic shelf lowland east of the Taymyr Peninsula, and thus, near the eastern edge of the Eurasian ice sheet, over the last 60 Ky. This region is also considered to be the westernmost part of Beringia, the non-glaciated landmass that lay between the Eurasian and the Laurentian ice caps during the Late Pleistocene. Several units and subunits of sand deposits, peat–sand alternations, ice-rich palaeocryosol sequences (Ice Complex) and peaty fillings of thermokarst depressions and valleys were presented. The recorded proxy data sets reflect cold stadial climate conditions between 60 and 50 Kya, moderate inderstadial conditions between 50 and 25 Kya and cold stadial conditions from 25 to 15 Kya. The Late Pleistocene to Holocene transition, including the Allerød warm period, the early to middle Holocene thermal optimum and the late Holocene cooling, are also recorded. Three phases of landscape dynamic (fluvial/alluvial, irregular slope run-off and thermokarst) were presented in a schematic model, and were subsequently correlated with the supraregional environmental history between the Early Weichselian and the Holocene.     

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.]     

Paleoenvironmental reconstruction of central-western Argentina from analysis of Late-Pleistocene mammal droppings

Plant macrofossils studied in coprolites from small-sized, medium-sized and large mammals, dated by ¹⁴C to ca. 30.8–24 and 9 kybp, allowed to record changes in local vegetation and its relation with geomorphological units. Paleobotanical samples were determined using microhistological analysis. We found that plant associations and the vegetation–geomorphology relation were similar in the two periods analyzed. According to this study, the Monte plant formation would reflect certain stability during the analyzed period in arid central-western Argentina, although this does not imply the absence of variations in plant structure.     

Paleoenvironmental reconstruction of Holocene coastal depositsfrom the Southeastern Buenos Aires Province, Argentina

An estuarine sequence outcropping at La Ballenera Creek (BuenosAires Province), dated between 6,800 and 4,100 ¹⁴C years BP, wasinvestigated for diatoms and molluscs. The sea level history along the BuenosAires coastline has been reconstructed from discrete beach ridge sequencesalong low-lying plains. The La Ballenera profile is located on a cliffcoast where a sequence recorded environmental changes. Fifty-eightdiatom species were grouped based on their salinity and life formcharacteristics. Cluster analysis allowed the division of the sequence intothree diatom zones. Mollusc tolerances were also used to discern theenvironmental changes induced by sea-level fluctuations. The base of thesequence recorded the initiation of the marine influence about6,790–6,200 years ago. Diatom assemblages consist ofbrackish-freshwater tychoplankton accompanied by brackish benthicdiatoms. Among the molluscs, the freshwater-brackish species Heleobiaparchappii dominates. Between 6,200 and ca. 4,800 ¹⁴C years BP, anestuarine lagoon environment is indicated by benthic and epiphyticmarine-brackish diatoms, as well as by the estuarine snail Heleobiaaustralis. After 4,800 ¹⁴C years BP, the diatom assemblages and therelative abundance of freshwater molluscs indicate a marshy environment withlower salinity content. The La Ballenera records the salinity changes thataffect an estuary that infilled during the 2,700 years after the maximum sealevel reached in mid-latitudes of South America.     

Wind-determined sediment distribution and Holocene sediment yield in a small,Danish, kettle lake

Sediment distribution was mapped by multiple corings in a small oligotrophic lake in northwestern Denmark. Sediment cores along a representative West-East transect were dated by ¹⁴C and correlated using pollen, mineral magnetics and general lithology. Estimates of whole-lake Holocene sediment accumulation were used to calculate sediment yield (terrestrial erosion). Results indicate that: 1) sediment yield was low 10000–5000 BP and increased strongly 5000–2500 BP and again 2500–1200 BP, 2) sediment focusing and waves and currents induced by strong winds were the major processes controlling sediment distribution throughout the Holocene; 3) the dominant wind direction of strong winds has been westerly throughout the Holocene; and 4) the lake was probably more productive in the last 5000 years than in the period from 10000 to 5000 BP.     

Glacial history, Holocene shoreline displacement and palaeoclimate based on radiocarbon ages in the area of Bockfjorden, north-western Spitsbergen, Svalbard

Age determinations of bivalve shells indicate that Bockfjorden, a fjord in north-western Spitsbergen, Svalbard, was deglaciated shortly before 10 Kya, and that the upper marine limit in this area, with an altitude of about 50 m a.s.l., has the same age. During most of the Holocene, the glaciers in Bockfjorden were less extensive than they are today. Their maximum Holocene extension occurred during the Little Ice Age. The initial shoreline emergence after the deglaciation was rapid, and former shorelines younger than 8.5 Ky are below the present sea level. A mid-Holocene transgression of the sea is traced as well as a transgression during the last thousand years.     

Siberian river run-off and Late Quaternary glaciation in the southern Kara Sea, Arctic Ocean: preliminary results

The extent of the Barents-Kara Ice Sheet during the eastern Last Glacial Maximum (LGM) is not yet fully known. A detailed echo-sounding survey performed during the Boris Petrov Expedition 2001 permitted the detailed mapping of part of it. Based on the profiling results, a southern connection between the LGM Barents-Kara Ice Sheet and a local ice sheet on Taymyr Peninsula appears to be unlikely. Based on sediment core data and profiling results, most of the terrigenous river-derived material accumulated in the estuaries during late Holocene times, whereas during early Holocene times of lowered sea level major amounts were transported further offshore and accumulated on the shelf. During the post-glacial sea level rise, the main depocentre migrated southward, reaching its present position no earlier than about 6 cal. Ky BP (or 5.2 Kya). Future studies of accelerator mass spectrometry (AMS) ¹⁴C-dated sediment cores will allow a detailed reconstruction of the variability of fluvial sediment discharge and the history of glaciation in the Kara Sea during late Quaternary times.     

Environmental history of the Colombian savannas of the Llanos Orientales since the Last Glacial Maximum from lake records El Pinal and Carimagua

Late Quaternary environments have been studied by pollen analysis of lake sediments from the savannas of the Colombian Llanos Orientales at 180 m elevation. The pollen record form Laguna El Pinal (4°08N, 70°23W), dated by 6 AMS radiocarbon dates, starts at 18,290 ¹⁴C yr B.P. The record from Laguna Carimagua (4°04N, 70°14W), also dated by 6 AMS dates, starts at 8270 ¹⁴C yr B.P. Both records show a landscape dominated by grassland savanna with only few woody savanna taxa, such as Curatella and Byrsonima, frequent fires, and little occurrence of forest and/or gallery forest along the rivers. The savanna ecosystem at the studied sites was relatively stable during the last 18,000 yrs, but minor changes in floral composition, and in the proportion of savanna/forest, have been recorded. Very little gallery forest and the non permanent lake conditions of Laguna El Pinal reflect the driest period, interpreted to reflect low rainfall rates and long dry seasons during the Last Glacial Maximum until 10,690 ¹⁴ C yr B.P. During the Late Glacial, Laguna El Pinal was a permanent shallow lake, and changed into a lake with higher water levels during the Holocene, indicating wetter conditions. Expansion of regional gallery forest also started at around 10,690 ¹⁴C yr B.P. Little vegetational change observed in Laguna Carimagua at 5570 ¹⁴ C yr B.P., in combination with a simultaneous decrease of savanna observed in previously studied lakes, suggest a change to regional wetter conditions. Thus, the Holocene before 5500 ¹⁴ C yr B.P. was somewhat drier than the following period until about 3850 ¹⁴C yr B.P. In both records, Late Holocene lake deposits are incomplete. Shore vegetation of Laguna Carimagua always included a minor contribution of the palms Mauritia and Mauritiella. The marked increase of palms during the last c. 3800 yrs points to increased human impact on the vegetation under the wettest Holocene climate regime.     

RADIOCARBON AGES ON ORGANICS FROM PIEDMONT ALLUVIUM,AJO MOUNTAINS,ARIZONA

Alluvium in dry lands is considered difficult to date by radiocarbon methods because of the paucity of organic matter. Although organic materials of sufficient size for conventional ¹⁴C dating are rare, wet sieving of alluvium in the Sonoran Desert yields sufficient organics for ¹⁴C measurements by accelerator mass spectrometry (AMS). Detrital charcoal from two Quaternary fluvial fill terraces on the western side of the Ajo Mountains yielded ¹⁴C ages of 14,880 ± 70 yr B.P. (CAMS-12408) for the Qt 1 terrace and 2490 ± 60 yr B.P. (CAMS-12414) and 2510 ± 60 yr B.P. (CAMS-12415) for the smaller inset Qt 2 terrace. These ¹⁴C ages are consistent with what is known about rates of soil development in the region. The earlier aggradation event appears to be supported by regional and possibly global climate change at about 14,000 ¹⁴C yr B.P. The more recent aggradation event does not appear to be synchronous with periods of frequent paleofloods in the southwest. The offset between ¹⁴C and ³⁶Cl ages for the same terraces provides a general indication of the time taken for the clasts to be transported to their current positions on the terraces. [Key words: soils, organic matter, 14C, 36Cl, Quaternary dating methods, piedmont, geomorphology, Sonoran Desert.]     

Considerations on the chronological and causal relationships between talus flatirons and palaeoclimatic changes in central and northeastern Spain

Talus flatirons are relatively common landforms in arid and semiarid areas. They are frequent in the Duero, Tajo and Ebro Tertiary Basins in Spain. The generation of a flatiron requires the alternation of periods with a prevalence of accumulation processes followed by periods with a preponderance of erosion processes. In the three Tertiary basins considered here talus flatirons have been found in numerous places and up to five stages of slope evolution have been recognised in some locations. The second youngest stage (S₂) has been dated in several places in the Ebro and Duero Basins through ¹⁴C. The age of this stage ranges from 2529 ± 52 to 3590 ± 40 ¹⁴C yBP. The third youngest stage (S₃) has yielded dates of 27,862 ± 444 radiocarbon years BP in the Ebro Basin, and 28,550 ± 130 ¹⁴C yBP in the Duero Basin. S₄ has been dated in the Ebro Basin to 35,570 ± 490 ¹⁴C yBP. The S₂ slope facets correspond to the Iron and Bronze Age Cold Stages. The S₃ and S₄ flatirons may be correlated with the Heinrich events H₃ and H₄. These dates may indicate that the accumulation periods on the slopes correspond to cold global events. The dates obtained for the stages S₃ and S₄ in the central and northeastern sectors of Spain and their good correlation with Heinrich events suggest that flatirons could be related to climatic sequences in the Upper Pleistocene and Holocene.     

Carbonate-associated sulfate as a proxy for lake level fluctuations: a proof of concept for Walker Lake, Nevada

Closed-basin alkaline lakes record climate change particularly well because they generally contain a sedimentary record that is high in carbonate mineral content from which climate proxies can be determined. Various approaches are used to estimate paleo-lake level and volume (δ¹⁸O, dating of “shoreline” tufas, biotic proxies, etc.), yet all carry certain caveats that limit their usefulness. Ultimately, the relationship between the chemistry of the lake, the volume of the lake, and the response of the proxy will determine how well a proxy serves a paleolimnologic purpose. Here, we discuss the use of carbonate-associated sulfate (CAS), the sulfate contained within the lattice of carbonate minerals that precipitate in lake water, as a proxy for lake water chemistry and by extension, lake volume. Walker Lake, an alkaline closed-basin lake in western Nevada, has experienced a well-documented lake-level decline since 1880 and provides a test case for CAS as a lake-level proxy. By extracting the CAS from sedimentary carbonate and tufas that have been age dated, we can relate these values to lake sulfate content based on historical or other proxy data. We confirm that CAS tracks lake sulfate. Our study of sedimentary carbonates demonstrates that CAS is a linear function of lake sulfate through a range of 10–25 mM, which corresponds to a change in lake level of 30 m. As confirmation of the CAS technique, we analyzed a stromatolitic tufa dated using AMS ¹⁴C. The CAS trend in the stromatolite suggested that it grew during a lake-level decline, a result consistent with other proxy data. Finally, laboratory experiments were conducted that demonstrate CAS is monotonically correlated with sulfate concentration and that precipitation kinetics are not likely a major control on CAS in alkaline lakes, but that ionic strength of the solution exerts a strong control on CAS.