Late Quaternary palaeoenvironment of northern Guatemala: evidence from deep drill cores and seismic stratigraphy of Lake Petén Itzá

Long sediment cores were collected in spring 2006 from Lake Petén Itzá, northern Guatemala, in water depths ranging from 30 to 150 m, as part of an International Continental Scientific Drilling Program project. The sediment records from deep water consist mainly of alternating clay, gypsum and carbonate units and, in at least two drill sites, extend back >200 kyr. Most of the lithostratigraphic units are traceable throughout the basin along seismic reflections that serve as seismic stratigraphic boundaries and suggest that the lithostratigraphy can be used to infer regional palaeoenvironmental changes. A revised seismic stratigraphy was established on the basis of integrated lithological and seismic reflection data from the basin. From ca 200 to ca 85 ka, sediments are dominated by carbonate‐clay silt, often interbedded with sandy turbidites, indicating a sediment regime dominated by detrital sedimentation in a relatively humid climate. At ca 85 ka, an exposure horizon consisting of gravels, coarse sand and terrestrial gastropods marks a lake lowstand or partial basin desiccation, indicating dry climate conditions. From ca 85 to ca 48 ka, transgressive carbonate‐clay sediments, overlain by deep‐water clays, suggest a lake level rise and subsequent stabilization at high stage. From ca 48 ka to present, the lithology is characterized by alternating clay and gypsum units. Gypsum deposition correlates with Heinrich Events (i.e. dry climate), whereas clay units coincide with more humid interstadials.     

Seismic evidence of up to 200 m lake‐level change in Southern Patagonia since Marine Isotope Stage 4

Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south‐eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo‐shorelines indicating a stepwise water‐level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake‐level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present‐day lake level point towards a long period of lake‐level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp . Dryer conditions during the Mid‐Holocene are evidenced by a dropping lake level, resulting in a basin‐wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo‐shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.     

Identification of arid phases during the last 50 cal. ka BP from the Fuentillejo maar‐lacustrine record (Campo de Calatrava Volcanic Field,Spain)

Geochemical (element analysis, molecular analysis of organic compounds), physical, palynological, mineralogical and sedimentary facies analysis were performed to characterise the sedimentary record in Fuentillejo maar‐lake in the Central Spanish Volcanic Field of Campo de Calatrava, in order to reconstruct the palaeoenvironmental and palaeoclimatic processes which controlled vegetation patterns and deposition of different sedimentary facies. The upper 20 m of core FUENT‐1 show variations in clastic input, water chemistry, vegetation and organic fraction sources in the lake throughout the Late Pleistocene and Holocene. The temporal framework provided by ¹⁴C accelerator mass spectrometry dating allows assigning the sequence to the last 50 cal. ka BP. Arid phases identified in the FUENT‐1 sequence are correlated to Heinrich events (HE) and to stadials of the Dansgaard/Oeschger (D/O) cycles. Siliciclastic facies with high magnetic susceptibility values, high Juniperus pollen content, a low Paq index (aquatic macrophysics proxy index), a decrease in the relative percentage of the n‐C₂₇ and an increase in the n‐C₃₁ alkanes are indicative of arid and colder climatic events related to HE 2, HE 1 and the Younger Dryas (YD). Similar short cold and arid phases during the Holocene were identified at 9.2–8.6, 7.5–7 and 5.5–5 cal. ka BP. In dolomite–mud facies, the pollen data show an increase in the herbs component, mainly – Chenopodiaceae, Artemisia and Ephedra – steppe taxa; a low Paq index, a decrease in the relative percentage of the n‐C₂₇ alkane and an increase in the n‐C₃₁ alkane are also observed. This facies was probably the result of lower lake levels and more saline–alkaline conditions, which can be interpreted as linked to arid–warm periods. These warm and arid phases were more frequent during Marine Isotope Stage (MIS) 3 and the interstadials of MIS 2. HE 4, HE 2, HE 1 and the YD in core FUENT‐1 were immediately followed by increases of warm steppe pollen assemblages that document rapid warming similar to the D/O cycles but do not imply increasing humidity in the area. Fuentillejo hydrology is controlled by changes in the atmospheric and oceanic systems that operated on the North Atlantic region at millennial scale during the last 50 cal. ka BP. Copyright © 2009 John Wiley & Sons, Ltd.     

Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

The Baltic Sea (~393 000 km²) is the largest brackish sea in the world and its hydrographic and environmental conditions are strongly dependent on the frequency of saline water inflows from the North Sea. To improve our understanding of the natural variability of the Baltic Sea ecosystem detailed reconstructions of past saline water inflow changes based on palaeoecological archives are needed. Here we present a high‐resolution study of benthic foraminiferal assemblages accompanied by sediment geochemistry (loss on ignition, total organic carbon) and other microfossil data (ostracods and cladocerans) from a well‐dated 8‐m‐long gravity core taken in the Bornholm Basin. The foraminiferal diversity in the core is low and dominated by species of Elphidium. The benthic foraminiferal faunas in the central Baltic require oxic bottom water conditions and salinities >11–12 PSU. Consequently, shell abundance peaks in the record reflect frequent saline water inflow phases. The first appearance of foraminiferal tests and ostracods in the investigated sediment core is dated to c. 6.9 cal. ka BP and attributed to the first inflows of saline and oxygenated bottom waters into the Bornholm Basin during the Littorina Sea transgression. The transgression terminated the Ancylus Lake phase, reflected in the studied record by abundant cladocerans. High absolute foraminiferal abundances are found within two time intervals: (i) c. 5.5–4.0 cal. ka BP (Holocene Thermal Maximum) and (ii) c. 1.3–0.75 cal. ka BP (Medieval Climate Anomaly). Our data also show three intervals of absent or low saline water inflows: (i) c. 6.5–6.0 cal. ka BP, (ii) c. 3.0–2.3 cal. ka BP and (iii) c. 0.5–0.1 cal. ka BP (Little Ice Age). Our study demonstrates a strong effect of saline and well‐oxygenated water inflows from the Atlantic Ocean on the Baltic Sea ecosystem over millennial time scales, which is linked to the major climate transitions over the last 7 ka.     

Paleohydrological Changes in Highland Desert Rivers and Human Occupation, 7000‐3000 Cal. Yr B.P., South‐Central Andes,Argentina

This article focuses on local paleohydrological changes experienced by the Las Pitas and Miriguaca Rivers in the south‐central Andes of Argentina and their impacts on hunter‐gatherers as they transitioned to food‐producing communities 7000–3000 cal. yr B.P. Paleoenvironmental reconstruction based on geomorphology, alluvial sedimentology, and diatom evidence indicates a dry phase of reduced streamflow between ca. 6700 and 4800 cal. yr B.P. for the Las Pitas River, and 6600 and 3000 cal. yr B.P. for the Miriguaca River. A phase of more humid environmental conditions commenced after ca. 4900 cal. yr B.P. along the Las Pitas River, and after 3000 cal. yr B.P. along the Miriguaca River. Differences in the chronology and magnitude of hydrological changes along both rivers are related to topographic and hydrological characteristics of their respective watersheds. Higher catchment elevation and enhanced orographic precipitation favored greater sensitivity for the Las Pitas River to short humid events during the middle‐to‐late Holocene. The archaeological evidence suggests that the paleohydrological changes within these catchments played a significant role in human occupational dynamics such that the Las Pitas River offered better environmental conditions for human occupation relative to the Miriguaca River as foragers increasingly relied on plant and animal domestication.     

Vegetation and climate changes in northwestern Russia during the Lateglacial and Holocene inferred from the Lake Ladoga pollen record

The new pollen record from the upper 12.75 m of a sediment core obtained in Lake Ladoga documents regional vegetation and climate changes in northwestern Russia over the last 13.9 cal. ka. The Lateglacial chronostratigraphy is based on varve chronology, while the Holocene stratigraphy is based on AMS ¹⁴C and OSL dates, supported by comparison with regional pollen records. During the Lateglacial (c. 13.9–11.2 cal. ka BP), the Lake Ladoga region experienced several climatic fluctuations as reflected in vegetation changes. Shrub and grass communities dominated between c. 13.9 and 13.2 cal. ka BP. The increase in Picea pollen at c. 13.2 cal. ka BP probably reflects the appearance of spruce in the southern Ladoga region at the beginning of the Allerød interstadial. After c. 12.6 cal. ka BP, the Younger Dryas cooling caused a significant decrease in spruce and increase in Artemisia with other herbs, indicative of tundra‐ and steppe‐like vegetation. A sharp transition from tundra‐steppe habitats to sparse birch forests characterizes the onset of Holocene warming c. 11.2 cal. ka BP. Pine forests dominated in the region from c. 9.0 to 8.1 cal. ka BP. The most favourable climatic conditions for deciduous broad‐leaved taxa existed between c. 8.1 and 5.5 cal. ka BP. Alder experiences an abrupt increase in the local vegetation c. 7.8 cal. ka BP. The decrease in tree pollen taxa (especially Picea) and the increase in herbs (mainly Poaceae) probably reflect human activity during the last 2.2 cal. ka. Pine forests have dominated the region since that time. Secale and other Cerealia pollen as well as ruderal herbs are permanently recorded since c. 0.8 cal. ka BP.     

Pliocene‐Quaternary history of Futuna Island,south Vanuatu,southwest Pacific

U‐series ages from thermal ionisation mass spectrometry are reported here for the raised coral reefs of Futuna Island, which lies adjacent to the eastern margin of the backarc Futuna Trough in south Vanuatu, southwest Pacific. U‐series ages from coral from the lowest raised reef indicate that its upper part is most likely to be ca 210 ka, whereas the most elevated raised reef has a likely age of ca 520 ka (range 600–440 ka). The inferred Pliocene‐Quaternary history for Futuna Island and the adjacent Futuna Trough is: (i) formation of the Pliocene—Early Quaternary basaltic‐andesite cone in a southeast part of the Vanuatu Island Arc; (ii) inception of the Futuna Trough (adjacent to the west margin of Futuna Island) since 1.8 Ma; (iii) subsequent uplift of the volcanic cone above sea‐level caused ～500 m of its upper part to be removed by marine erosion; (iv) the island then subsided and at least 160 m of limestone was deposited on the truncated cone; and (v) during the period 520 ka to ca 210 ka seven fringing reefs formed at the margin of the cone as the island was uplifted. Since ca 210 ka Futuna further subsided and, as a result, the post ca 210 ka history of the island is obscure.     

The palaeoenvironmental and palaeohydrological evolution of Padul Peat Bog (Granada, Spain) over one million years, from elemental, isotopic and molecular organic geochemical proxies

The elemental (concentration of organic carbon, atomic H/C and C/N ratios), isotopic (δ¹³C values of organic matter) and molecular (predominant n-alkane chain length and carbon preference index (CPI)) organic components were measured for 600 samples taken from a 107-m long core from the Padul Basin (Andalusia, Spain). The record runs from the Lower Pleistocene (ca. 1 Ma B.P.) to the mid-Holocene (ca. 4.5 ka B.P.) with, in general, little diagenesis (removal of components). Two markedly different hydrogeological scenarios were interpreted: (1) From ca. 1 Ma to ca. 400 ka B.P. run-off recharge was significant and water depths were greater (lacustrine scenario). From ca. 400 to 4.5 ka B.P., the Padul Basin became a peat bog s.s. with the major water input coming from groundwater inflow. From ca. 400 to ca. 180 ka B.P. alternating episodes with either predominant grasses, trees or aquatic macrophytes which were linked to wet/dry phases, took place. An important deglaciation episode has been interpreted to occur between ca. 180 and 170 ka B.P. The global climatic changes occurring from ca. 170 to 25 ka B.P. were not recorded in the proxies, though they do show important variations linked to the Last Glacial Maximum and the beginning of the Holocene (ca. 25–10 ka B.P.): (2) Cold phases coexisting with dry periods produced the recession of forests and the development of grasses. After these periods, as both temperature and precipitation increased, forests expanded and the water level, linked to thaw, rose, especially at ca. 20 ka B.P. Few changes occurred during the Holocene, although there were short alternations between wet and dry episodes. Overall, the techniques applied proved to be excellent palaeoenvironmental proxies for studying the basin’s palaeoclimatological and palaeohydrological evolution.     

Stone Age settlement and Holocene shore displacement in the Narva‐Luga Klint Bay area,eastern Gulf of Finland

Based on geological and archaeological proxies from NW Russia and NE Estonia and on GIS‐based modelling, shore displacement during the Stone Age in the Narva‐Luga Klint Bay area in the eastern Gulf of Finland was reconstructed. The reconstructed shore displacement curve displays three regressive phases in the Baltic Sea history, interrupted by the rapid Ancylus Lake and Litorina Sea transgressions c. 10.9–10.2 cal. ka BP and c. 8.5–7.3 cal. ka BP, respectively. During the Ancylus transgression the lake level rose 9 m at an average rate of about 13 mm per year, while during the Litorina transgression the sea level rose 8 m at an average rate of about 7 mm per year. The results show that the highest shoreline of Ancylus Lake at an altitude of 8–17 m a.s.l. was formed c. 10.2 cal. ka BP and that of the Litorina Sea at an altitude of 6–14 m a.s.l., c. 7.3 cal. ka BP. The oldest traces of human activity dated to 8.5–7.9 cal. ka BP are associated with the palaeo‐Narva River in the period of low water level in the Baltic basin at the beginning of the Litorina Sea transgression. The coastal settlement associated with the Litorina Sea lagoon, presently represented by 33 Stone Age sites, developed in the area c. 7.1 cal. ka BP and existed there for more than 2000 years. Transformation from the coastal settlement back to the river settlement indicates a change from a fishing‐and‐hunting economy to farming and animal husbandry c. 4.4 cal. ka BP, coinciding with the time of the overgrowing of the lagoon in the Narva‐Luga Klint Bay area.     

Carbon isotope evidence of last glacial climate variations in the tropical NW Leizhou Peninsula,South China

Zhong, W., Cao, J., Xue, J., Ouyang, J., Wang, H., Tang, X. & Gao, X. 2012 (January): Carbon isotope evidence of last glacial climate variations in the tropical NW Leizhou Peninsula, South China. Boreas, Vol. 41, pp. 102–112. 10.1111/j.1502‐3885.2011.00224.x. ISSN 0300‐9483. Bulk organic δ¹³C values of sediment sequences are commonly used to indicate past climatic changes, although the causes and mechanisms leading to the observed organic carbon isotope responses are presently not fully understood. Based on two sedimentary sequences, namely a peat profile and a fluvial and swamp sequence from the tropical NW Leizhou Peninsula in South China, we interpret more negative bulk organic δ¹³C values to suggest wetter and warmer conditions, implying a stronger East Asian (EA) summer monsoon, and less negative δ¹³C values to indicate drier and cooler conditions, reflecting a weakening of the EA summer monsoon. A warm and wet period occurred between c. 48.0 and 28.0 cal. ka BP. In the study region, a climate shift occurred at c. 22.0 (or 20.0) cal. ka BP, and the driest and coldest period occurred between c. 19.0 and 16.0 cal. ka BP. After c. 12.0 cal. ka BP, the climate changed towards wetter and warmer conditions. Several possible millennial‐scale dry and cold oscillations characterized possibly by C4 plants, or by more positive organic δ¹³C values during the period when C3 plants dominated, are cautiously and tentatively interpreted as showing a similarity with Heinrich events and other cold millennial‐scale oscillations evidenced from a Greenland ice core and Chinese stalagmite. We demonstrate that bulk organic δ¹³C records of sedimentary sequences in the study region have potential for indicating the changes in vegetation composition that were closely associated with climate variation during the last glacial period.     

Sedimentary evolution of a Late Pleistocene temperate red algal reef (Coralligène) on Rhodes,Greece: correlation with global sea‐level fluctuations

Autochthonous red algal structures known as coralligène de plateau occur in the modern warm‐temperate Mediterranean Sea at water depths from 20 to 120 m, but fossil counterparts are not so well‐known. This study describes, from an uplifted coastal section at Plimiri on the island of Rhodes, a 450 m long by 10 m thick Late Pleistocene red algal reef (Coralligène Facies), interpreted as being a coralligène de plateau, and its associated deposits. The Coralligène Facies, constructed mainly by Lithophyllum and Titanoderma, sits unconformably upon the Plio‐Pleistocene Rhodes Formation and is overlain by a Maerl Facies (2 m), a Mixed Siliciclastic‐Carbonate Facies (0·2 m) and an Aeolian Sand Facies (2·5 m). The three calcareous facies, of Heterozoan character, are correlated with established members in the Lindos Acropolis Formation in the north of the island, while the aeolian facies is assigned to the new Plimiri Aeolianite Formation. The palaeoenvironmental and genetic‐stratigraphic interpretations of these mixed siliciclastic‐carbonate temperate water deposits involved consideration of certain characteristics associated with siliciclastic shelf and tropical carbonate shelf models, such as vertical grain‐size trends and the stratigraphic position of zooxanthellate coral growths. Integration of these results with electron spin resonance dates of bivalve shells indicates that the Coralligène Facies was deposited during Marine Isotope Stage 6 to 5e transgressive event (ca 135 to 120 ka), in water depths of 20 to 50 m, and the overlying Maerl Facies was deposited during regression from Marine Isotope Stage 5e to 5d (ca 120 to 110 ka), at water depths of 25 to 40 m. The capping Aeolian Sand Facies, involving dual terrestrial subunits, is interpreted as having formed during each of the glacial intervals Marine Isotope Stages 4 (71 to 59 ka) and 2 (24 to 12 ka), with soil formation during the subsequent interglacial periods of Marine Isotope Stages 3 and 1, respectively. Accumulation rates of about 0·7 mm year^?1 are estimated for the Coralligène Facies and minimum accumulation rates of 0·2 mm year^?1 are estimated for the Maerl Facies. The existence of older red algal reefs in the Plimiri region during at least Marine Isotope Stages 7 (245 to 186 ka) and 9 (339 to 303 ka) is inferred from the occurrence of reworked coralligène‐type lithoclasts in the basal part of the section and from the electron spin resonance ages of transported bivalve shells.     

Zoophycos in deep‐sea sediments indicates high and seasonal primary productivity: Ichnology as a proxy in palaeoceanography during glacial–interglacial variations

Trace fossils provide valuable palaeoenvironmental information in hemipelagic settings. This is particularly true in the case of Zoophycos, an easily recognizable trace fossil in core. At IODP site U1385, Zoophycos was found throughout an interval representing 1.5 Ma, covering 45 glacial–interglacial cycles mediated by obliquity (41 ka) and short‐term eccentricity (100 ka). Zoophycos is most common in sediments deposited during glacial times and when the sedimentation rate was intermediate and primary production was high and seasonal. Occurrences of Zoophycos elsewhere support a similar relationship with seasonal organic‐matter deposition. This is particularly significant considering that seasonality of organic‐matter deposition is difficult to decipher from the sediment record. Zoophycos appears to represent a useful proxy to characterize high and seasonal organic‐matter deposition and primary productivity in Neogene hemipelagic deposits.     

Holocene palaeoceanographic changes in Barrow Strait,Canadian Arctic: foraminiferal evidence

Holocene changes in the benthic and planktic foraminiferal fauna (>63 µm) from a marine sediment core (ARC‐3 Canadian Arctic Archipelago, 74° 16.050′ N, 91° 06.380′ W, water depth 347 m) show that significant environmental and palaeoceanographic variations occurred during the last 10 ka. Foraminiferal assemblages are restricted to the ca. 4.5–10 ka interval as younger samples are mostly barren of foraminifera due to intense carbonate dissolution after ca. 4.5 ka. Foraminiferal assemblages in the ca. 4.5–10 ka interval are dominated by the benthic species Islandiella helenae and Cassidulina reniforme (57% of total), with Elphidium clavatum, Cibicides lobatulus and Buccella frigida also being common in this interval. The dominance of these species indicates a seasonal sea ice regime which is consistent with the occurrence of the sea ice diatom‐derived organic geochemical biomarker IP₂₅ throughout the core. The abundances of C. reniforme and E. clavatum decline upcore; consistent with more frequent mixing of the Barrow Strait water column during the early Holocene. It is likely that the influence of CO₂‐rich Arctic surface water masses have caused an increase in bottom water corrosivity after ca. 8.5 ka, and dissolution has been further enhanced by sea ice‐related processes after ca. 6 ka, concomitant with increased IP₂₅ fluxes. Dissolution is strongest when IP₂₅ fluxes are highest, suggesting a link between the sea ice and benthic systems. Copyright © 2010 John Wiley & Sons, Ltd.     

西藏玛旁雍错沉积揭示的晚冰期以来环境变化

玛旁雍错位于青藏高原西南部,是研究印度季风演化的理想地区之一。利用活塞采样器在玛旁雍错75 m水深处获取了一根4.69 m长岩芯,利用^210Pb和AMS^14C测年确定了岩芯的年代,根据元素含量(XRF扫描)、总有机碳(TOC)、总无机碳(TIC)和总氮(TN)等代用指标,重建了该地区14 cal.ka B.P.以来气候环境变化。结果显示,冰消期(14~12 cal.ka B.P.),TOC和TIC含量变化显示了湖区环境整体上较为暖湿;12.0~11.4 cal.ka B.P.,径流明显减弱,可能是新仙女木事件的反映;早中全新世(11.4~3.4 cal.ka B.P.),湖区环境整体上以暖湿为主,气候有所波动,出现了两次冷事件(10.2~9.8 cal.ka B.P.和8.2 cal.ka B.P.左右),7.4~6.6 cal.ka B.P.,出现了一个暖时期;晚全新世(3.4~0 cal.ka B.P.),湖区环境整体上趋向于干旱化,并伴有两次相对湿润时期(2.6~2.0 cal.ka B.P.和1.4~1.0 cal.ka B.P.)。玛旁雍错环境变化表明,晚冰期以来(14.0~3.4 cal.ka B.P.),受到太阳辐射的影响,印度季风带来的降水以及冰川融水较多,湖区环境较为湿润,整体上有利于内源生物的生存;晚全新世(3.4~0 cal.ka B.P.),北半球夏季太阳辐射量减少,印度季风减弱,径流减弱,湖区环境朝干旱化方向发展。     

A millennial‐scale Late Pleistocene–Holocene palaeoclimatic record from the western Chihuahua Desert,Mexico

We present elemental concentrations and magnetic susceptibility data from a new 270‐cm‐long sediment core collected from the western part of palaeolake Babicora and infer millennial‐scale hydrological variations over the last 27 cal. ka in the western Chihuahua Desert. Variations in the available water content at the sediment–air interface of the watershed, lake salinity and lake productivity are inferred from values of the chemical index of alteration (CIA), CaCO ₃ and C_org, respectively. An abrupt increase in runoff at c. 24 cal. ka BP appears correlative with the Heinrich 2 (H2) event. Except for this event, diminished runoff between c. 27 and 19 cal. ka BP indicates lower annual precipitation (weak summer rainfall) during the Last Glacial Maximum. The deposition of chemically altered sediments between c. 25 and 22 cal. ka BP results from the higher sediment–water interaction in the watershed owing to lower evaporation, cooler conditions and higher precipitation during the H2 event. Since 19 cal. ka BP the runoff has been characterized by high‐amplitude fluctuations with intervals of reduced precipitation identified at c. 19, 18, 17.5, 13–14, 11.5, 10, 7.5 and 3 cal. ka BP.     

晚冰期月亮湖炭屑记录反映的古气候演化

位于大兴安岭中段的月亮湖地处季风/非季风影响过渡地带,其沉积岩心下部886～546 cm的炭屑记录揭示了末次冰期晚期到全新世早期(20.9～10.8 cal.ka B.P.)的古气候演化历史,反映了东亚季风对研究区气候的影响。研究区炭屑浓度的变化主要由可供燃烧的生物量决定,生长在气候温暖时期的森林草原能够提供更多可供燃烧的生物量。在同一植被类型的条件下,气候寒冷湿润时炭屑浓度低,气候温暖干旱时炭屑浓度高。20.9～18.0 cal.ka B.P.炭屑浓度较低, 气候寒冷偏干,18.0～15.3 cal.ka B.P.炭屑浓度最低,气候寒冷湿润,15.3～14.4 cal.ka B.P.炭屑浓度增高,气候开始向温暖的方向发展,14.4～11.8 cal.ka B.P.炭屑浓度快速变化,气候也经历了一系列的快速变化,11.8～10.8 cal.ka B.P.炭屑浓度总体较高,气候温暖湿润。<50 μm的炭屑浓度指示了区域火演化的历史, >50 μm的炭屑则反映了当地野火发生的状况。     

我国季风边缘区湖泊沉积记录的全新世亚洲夏季风衰退事件

亚洲夏季风是全球季风系统的重要组成部分,亚洲夏季风的变化对其控制区域自然生态系统的多样性和生态平衡,以及社会经济发展有重要的影响。本文选择位于现代亚洲夏季风边缘区对季风变化响应敏感的湖泊达连海为研究对象,基于陆生植物残体和全有机质的AMS14C定年建立了钻孔顶部24.6 m沉积物的年代框架,利用粒度指标重建了全新世研究区水文变化过程以及亚洲夏季风衰退事件序列。结果显示,沉积物中存在数层砂层,代表了湖泊低水位时期,进而指示了亚洲夏季风衰退事件。这些事件处在11.6~11.3 cal.ka B.P.、10.4~9.5 cal.ka B.P.、6.4~6.0 cal.ka B.P.、4.6~4.4 cal.ka B.P.、3.7~3.4 cal.ka B.P.、3.1~2.9 cal.ka B.P.以及2.0~0.9 cal.ka B.P.,可以发现中晚全新世以来亚洲夏季风衰退事件发生的频率显著增加。进一步与北半球高纬地区与低纬地区的气候突变事件记录对比显示,全新世百年-千年时间尺度上亚洲夏季风强度的变化与低纬ENSO活动存在密切的联系。     

Late Younger Dryas and early Holocene palaeoenvironments in the Skagerrak,eastern North Atlantic: a multiproxy study

Erbs‐Hansen, D. R., Knudsen, K. L., Gary, A. C., Jansen, E., Gyllencreutz, R., Scao, V. & Lambeck, K. 2011: Late Younger Dryas and early Holocene palaeoenvironments in the Skagerrak, eastern North Atlantic: a multiproxy study. Boreas, 10.1111/j.1502‐3885.2011.00205.x. ISSN 0300‐9843 A high‐resolution study of palaeoenvironmental changes through the late Younger Dryas and early Holocene in the Skagerrak, the eastern North Atlantic, is based on multiproxy analyses of core MD99‐2286 combined with palaeowater depth modelling for the area. The late Younger Dryas was characterized by a cold ice‐distal benthic foraminiferal fauna. After the transition to the Preboreal (c. 11 650 cal. a BP) this fauna was replaced by a Cassidulina neoteretis‐dominated fauna, indicating the influence of chilled Atlantic Water at the sea floor. Persisting relatively cold bottom‐water conditions until c. 10 300 cal. a BP are presumably a result of an outflow of glacial meltwater from the Baltic area across south‐central Sweden, which led to a strong stratification of the water column at MD99‐2286, as also indicated by C. neoteretis. A short‐term peak in the C/N ratio at c. 10 200 cal. a BP is suggested to indicate input of terrestrial material, which may represent the drainage of an ice‐dammed lake in southern Norway, the Glomma event. After the last drainage route across south‐central Sweden closed, c. 10 300 cal. a BP, the meltwater influence diminished, and the Skagerrak resembled a fjord with a stable inflow of waters from the North Atlantic through the Norwegian Trench and a gradual increase in boreal species. Full interglacial conditions were established at the sea floor from c. 9250 cal. a BP. Subsequent warm stable conditions were interrupted by a short‐term cooling around 8300–8200 cal. a BP, representing the 8.2 ka event.     

Flood frequencies reveal Holocene rapid climate changes (Lower Moulouya River,northeastern Morocco)

Current high‐resolution palaeoenvironmental records reveal short‐term Holocene coolings. One of these major Holocene rapid climate changes occurred between 3.2 and 2.7 cal. ka BP. The sensitivity of river systems vis‐à‐vis slight and short‐term Holocene climatic variations is a subject of controversy in the scientific community. In this paper, we present a 4.0 to 1.4 cal ka BP palaeoflood record from the Lower Moulouya River (northeastern Morocco) to demonstrate the high sensitivity of semiarid rivers in the southwestern Mediterranean towards Holocene environmental changes. The Lower Moulouya flood deposits are characterised by thick, well‐stratified, predominantly clayey to silty overbank fine sediments. These cohesive sediments show evidence of excellent preservation conditions against fluvial erosion and contain a continuous record of mid to late Holocene flood sequences. The Moulouya palaeoflood record can be interpreted in the context of regional and global high‐resolution proxy data, revealing a strong coupling with Holocene rapid climate changes. The centennial‐scale Moulouya palaeohydrological history will be discussed with palaeoenvironmental data from the same record (palaeomagnetics, sedimentary charcoal record, anthracological analyses, snail analyses) to generate new ideas about the mid to late Holocene hydrological cycle in the southwestern Mediterranean. The deduced features of pronounced Lower Moulouya flooding and the decreased fire recurrence during Holocene cooling remain somewhat inconsistent with the interpretation of other palaeohydrological and paleaoecological records from the southwestern Mediterranean. However, enhanced Lower Moulouya flood frequencies between 3.2 and 2.7 cal. ka BP agree with increased floodplain aggradation in other major river systems of Mediterranean North Africa. Copyright © 2009 John Wiley & Sons, Ltd.     

Sedimentological and palaeohydrological characterization of Late Pleistocene and Holocene tufa mound palaeolakes using trenching methods in the Spanish Pyrenees

Lakes developed in the inner depressions of tufa mounds are rare geomorphic features and still poorly understood. Sedimentation in this unusual type of endorheic lake with a very restricted catchment area is highly sensitive to environmental and hydrological changes. The Isona tufa mound complex, north‐eastern Iberian Peninsula, is associated with the discharge zone of a confined artesian aquifer and comprises 11 tufa mounds consisting of an annular rimstone enclosing a central depression filled with lake deposits. Data gathered from trenches excavated in four palaeolakes located within three different morphostratigraphic units permitted a precise analysis of the geometrical characteristics and stratigraphic relationships of the deposits and provided a sedimentation model for the Late Quaternary infilling of the spring‐fed lakes. The work illustrates that trenches allow a precise characterization of the stratigraphic arrangements, lateral facies changes and deformation structures, which are not apparent in studies relying solely on borehole records, and facilitate sampling for dating and geochemical analyses. The five sedimentary facies described represent different evolutionary stages of the lakes, including: (i) carbonate‐rich palustrine deposits probably related to periods with strong hydrological seasonality; (ii) massive highly bioturbated organic ooze; (iii) banded organic carbonate‐rich facies associated with an increase in the regional effective moisture; (iv) fine‐grained quartz‐rich aeolian/slope‐wash sediments; and (v) colluvial facies deposited following the desiccation of the lakes located at higher altitudes. Geochemical and sedimentological analyses of the lacustrine sequences provided information on the palaeohydrological evolution of the Isona tufa mound complex and the palaeoenvironmental conditions of the area over the last 28 ka. Radiometric dating suggests that deposition occurred simultaneously at ca 22 ka in palaeolakes situated at different elevations. A drop in the piezometric level prompted by the opening of springs at lower altitudes probably caused the deactivation of the upper springs and the desiccation of the lakes. Arid conditions prevailed in the area during the Late Glacial and the early Holocene (28·0 to 8·5 ka bp ). More humid conditions recorded from 8·5 to 4·2 ka and again since 1·7 ka are in accordance with palaeoenvironmental reconstructions available in the Western Mediterranean since the Last Glacial Maximum.