Environmental change over the last millennium recorded in two contrasting crater lakes in western Uganda,eastern Africa (Lakes Kasenda and Wandakara)

The last millennium is a key period for understanding environmental change in eastern Africa, as there is clear evidence of marked fluctuations in climate (effective moisture) that place modern concern with future climate change in a proper context, both in terms of environmental and societal impacts and responses. Here, we compare sediment records from two small, nearby, closed crater lakes in western Uganda (Lake Kasenda and Lake Wandakara), spanning the last 700 (Wandakara) and 1200 years (Kasenda) respectively. Multiproxy analyses of chemical sedimentary parameters (including C/N ratios, δ¹³C of bulk organic matter and δ¹³C and δ¹⁸O of authigenic carbonates) and biotic remains (diatoms, aquatic macrofossils, chironomids) suggest that Kasenda has been sensitive to climate over much of this period, and has shown substantial fluctuations in conductivity, while Wandakara has a more muted response, likely due to the increasing dominance of human activity as a driver of change within the lake and catchment over the length of our record. Evidence from both records, however, supports the idea that lake levels were low from ～AD 700–1000 AD, with increasing aridity from AD 1100–1600, and brief wet phases around AD 1000 and 1400. Wetter conditions are recorded in the 1700s, but drought returned by the end of the century and into the early 1800s, becoming wetter again from the mid-1800s. Comparison with other records across eastern Africa suggests that while some events are widespread (e.g. aridity beginning ～ AD 1100), at other times there is a more complex spatial signature (e.g. in the 1200s to 1300s, and from the 1400s to 1600s). This study highlights the important role of catchment-specific factors (e.g. lakemorphometry, catchment size, and human impact) in modulating the sensitivity of proxies, and lake records, as indicators of environmental change, and potential hazards when regional inference is based on a single site or proxy.     

Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

The carbon isotope composition (δ¹³C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C₃ and C₄ photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ¹³C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C₂₇ and C₂₉) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C₂₉) similar to that of terrestrial C₄ plants. Thus, the use of δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C₄ plants. For sedimentary C₃₁, however, the contribution from submerged plants was minor, so that the δ¹³C values for C₃₁ n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C₄ vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ¹³C values of sedimentary long chain n-alkanes (C₂₇, C₂₉ and C₃₁) may carry different environmental signals. While the δ¹³C values of C₃₁ were a reliable proxy for C₄/C₃ terrestrial vegetation composition, the δ¹³C values of C₂₇ and C₂₉ n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.     

Evidence of climatic change from oxygen and carbon isotope variations in sediments of a small arctic lake,Canada

Radiocarbon dating of the organic-rich sediments of Lake Illisarvik in the outer Mackenzie Delta indicates that formation of the lake occurred approximately 9500yr BP, with maximum expansion around 6000 yr BP. Sedimentation rates have remained relatively constant at an average of 0.3mm/yr. ¹³C results on biogenic and inorganic carbonates and organics indicate a change from dominantly terrestrial organics (?27 to ?28%₀) to submerged aquatic vegetation or plankton (?18 to ?23%₀) upon formation of the lake (9500yr BP), and a dramatic return to dominantly terrestrial organics at 5800yr BP (δ¹³C = ?27 to ?30%₀). This latter shift is accompanied by a drastic reduction in the macroflora and fauna populations. ¹⁸O results suggest that a warmer climate than today existed prior to the shift at 5800yr BP.     

Tepexpan Palaeoindian site, Basin of Mexico: multi-proxy evidence for environmental change during the late Pleistocene–late Holocene

The Tepexpan Palaeoindian skeleton was discovered in 1947 close to the former Lake Texcoco margin, in the Basin of Mexico. The find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation. Here we report new AMS radiocarbon dates associated with the sedimentary succession at Tepexpan with ages between 19,110 ± 90 and 612 ± 22 ¹⁴C years BP and a new uranium-series date for the skeleton with an age of 4700 ± 200 years BP that indicates a mid Holocene age. The sedimentary succession was studied in detail using: stable isotopes, diatoms, organic geochemistry and tephrochronology. The multi-proxy evidence suggests large changes around the margins of Lake Texcoco in terms of the balance between aquatic and terrestrial plants, C₃ and C₄ plants, saline, alkaline and freshwater conditions, volcanic activity, marginal reworking of lake sediments and input from the drainage basin through the late Pleistocene–late Holocene. These changes had large impacts on the prehistoric human populations living by the lake shores since the late Pleistocene in the Basin of Mexico.     

Holocene environmental and climatic changes inferred from Wulungu Lake in northern Xinjiang, China

Sedimentological, geochemical and palynological data from Wulungu Lake in northern Xinjiang, China, are used to reconstruct environmental and climate changes since 9550 cal yr BP. High abundance of Sparganium and Poaceae, low Md (median diameter) and δ¹³C_organic values indicate aridity between 9550 and 6730 cal yr BP. High Md and δ¹³C_organic values, and the prevalence of desert-steppe and steppe vegetation between 4200 and 560 cal yr BP, indicate that effective moisture increased after 6730 cal yr BP, peaking at 4200 and 560 cal yr BP. Low Md values, a negative excursion of δ¹³C_org, and the transition from steppe to desert vegetation since 560 cal yr BP reflect a decrease in effective moisture during the latest Holocene. Late Holocene human activities were indicated by sharp increase in the abundance of Pediastrum then. Variations in carbonate contents indicate that temperature was generally high between 9550 and 7740 cal yr BP, low between 7740 and 6730 cal yr BP, intermediate between 6730 and 560 cal yr BP, and low during the last 560 yr. Regional comparison indicates that the Asian monsoon did not extend to Wulungu Lake and westerlies were the main factor in determining the moisture availability during the Holocene.     

Influence of the biological carbon pump effect on the sources and deposition of organic matter in Fuxian Lake,a deep oligotrophic lake in southwest China

Biological carbon pumping (BCP) is a key process in which dissolved inorganic carbon in terrestrial aquatic ecosystems is utilized by aquatic autotrophs for photosynthesis and transformed into autochthonous organic matter (AOC). However, the mechanisms underlying BCP and the amount of generated AOC deposited effectively, are still poorly understood. Therefore, we conducted a systematic study combining modern hydrochemical monitoring and a sediment trap experiment in Fuxian Lake (Yunnan, SW China), the second-deepest plateau, oligotrophic freshwater lake in China. Temperature, pH, EC (electrical conductivity), DO (dissolved O₂), [HCO₃⁻], [Ca²⁺], SI_c, partial CO₂ (pCO₂) pressure, and carbon isotopic compositions of HCO₃⁻ (δ¹³C_DIC) in water from Fuxian Lake all displayed distinct seasonal and vertical variations. This was especially apparent in an inverse correlation between pCO₂ and DO, indicating that variations of hydrochemistry in the lake water were mainly controlled by the metabolism of the aquatic phototrophs. Furthermore, the lowest C/N ratios and highest δ¹³C_org were recorded in the trap sediments. Analyses of the C/N ratio demonstrated that the proportions of AOC ranged from 30% to 100% of all OC, indicating that AOC was an important contributor to sedimentary organic matter (OC). It was calculated that the AOC flux in Fuxian Lake was 20.43 t C km⁻² in 2017. Therefore, AOC produced by carbonate weathering and aquatic photosynthesis could potentially be a significant carbon sink and may have an important contribution to solving the lack of carbon sinks in the global carbon cycle.

     

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America

Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C₂₁-C₃₁) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C₂₁n-alkane, and the C₂₅ and C₂₇ alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C₃₀ 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U₃₇^K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U₃₇^K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.     

Complex trajectories of aquatic and terrestrial ecosystem shifts caused by multiple human-induced environmental stresses

Large shifts in the isotopic compositions of organic matter (OM) in lake sediments, over the last few hundred years, are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial OM due to human disturbances. Based on multiple-proxy data (C:N ratio, δ¹³C and δ¹⁵N of OM, δ¹³C of calcite, lithology and fossil pollen) from a 700-year sediment core at White Lake, New Jersey (USA), we propose a new explanation that relates these large shifts in OM δ¹³C and δ¹⁵N to human-induced changes in aquatic OM producers. Combined records of geochronology, fossil pollen and lithology from White Lake reveal that the upland forest was cleared by European settlers for farmland beginning around 1745 A.D. and has gradually reforested since 1930 after the abandonment of the farmlands. For the pre-agricultural period, OM had relatively constant but extremely low δ¹³C_VPDB (−35.8 to −34.5‰) and δ¹⁵N_Air (−3.5 to −2.5‰) and high atomic C:N ratios (13.7 to 16.7), indicating a stable anoxic lake environment with prominent microbial producers. Following the human disturbance (since 1745), high OM mass accumulation rates and abundances of the green alga Pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands. However, carbonate δ¹³C remains constant or even decreases during this period, implying that increasing productivity did not elevate the δ¹³C of dissolved inorganic carbon and thus cannot explain the observed large increase in OM δ¹³C (7.4‰) and δ¹⁵N (5.8‰) over this period. Instead, δ¹³C, δ¹⁵N and C:N ratios of OM and differences in δ¹³C between calcite and OM suggest that the large increase in OM δ¹³C and δ¹⁵N can be attributed to a human-induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton. During the post-agricultural period, mass accumulation rates of OM, carbonate and silicate, and the δ¹³C of OM and calcite all decreased significantly, corresponding to stabilization of the uplands. However, over the last 70 years, an intensifying aquatic stress from the deposition of ¹⁵N-enriched industrial pollutants has resulted in a steady increase of 1.9‰ in δ¹⁵N. Proxy records for lake (δ¹³C and δ¹⁵N of OM) and upland conditions (pollen and silicates) at White Lake show complex trajectories of the aquatic and terrestrial ecosystems in response to past human disturbances.     

Distributions of n-alkanes and their hydrogen isotopic composition in plants from Lake Qinghai (China) and the surrounding area

Various aquatic plants from Lake Qinghai, the largest inland saline lake in China, and terrestrial plants from the surrounding area were investigated for the distribution of n-alkanes and their δD values. The n-alkanes in the samples range from C₁₅ to C₃₃ with C preference index (CPI) values of 4.0–29.7. The n-C₂₃ or n-C₂₅ alkane is the dominant compound in the aquatic submerged plants. The aquatic emergent and terrestrial plants have an abundance maximum at n-C₂₇, n-C₂₉ or n-C₃₁. The average chain length (ACL) values, ranging from 26.0 to 29.6, are closely related to the plant species. The n-alkanes from the aquatic plants have mean δD values of −169‰ to −121‰ and those from the terrestrial plants values of −173‰ to −109‰. The H isotopic composition (δD) and fractionation differ significantly among the plants studied. Comparison shows that additional evaporative enrichment of the lake water associated with saline lakes and humidity influence the δD values of the n-alkanes in aquatic and terrestrial plants, respectively. The mean δD values of n-alkanes in the plants decrease with increasing ACL value. The n-alkanes from the different types of plants are more depleted in D relative to environmental water and those from aquatic plants (with a mean value of −143‰) have a greater isotopic fractionation than terrestrial plants (mean value −113‰).     

Late Quaternary vegetation changes around Lake Rutundu,Mount Kenya,East Africa: evidence from grass cuticles,pollen and stable carbon isotopes

Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C₄ photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C₄ grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC₄ subpathway. Copyright © 2003 John Wiley & Sons, Ltd.     

湖芯沉积物揭示的末次冰消开始时期 普莫雍错湖区环境变化*

文章讨论了末次冰消开始时期青藏高原南部普莫雍错湖芯沉积反映的环境暖湿化以及原因。通过对现代湖泊和周边河流水文状况调查,发现流域内发育众多冰川,冰川融水构成维持湖面稳定的重要来源;湖面以及湖泊沉积环境的变化与冰川融水以及相应的温度改变具有密切的联系。利用湖泊沉积岩芯PM-1孔,通过加速器¹⁴ C测年和粒度、元素、碳酸盐含量、总有机碳以及有机碳同位素、总氮、分子标志化合物、孢粉等环境指标的分析,发现在16.4~15.4cal.kaB.P.有大量流水进入湖泊,使湖面扩大,湖水加深;流域地表产生大量有机碎屑,并被流水带入湖泊进行沉积;流域内的喜湿植被得以发展。该时段湖泊扩张、陆源植被发展的原因一方面得益于冰期过后气候向温暖湿润方向转化,另一方面温度上升带来大量冰川融水则可能具有更加重要的影响,对于深入理解当前青藏高原冰川普遍退缩和一些湖泊的水面上涨具有重要的参考价值。     

18.5kaB. P.以来东北四海龙湾玛珥湖全岩有机碳同位素记录及其古气候环境意义

结合含水量、TOC含量和TOC/TN比值变化曲线,18·5kaB·P·以来的四海龙湾玛珥湖沉积物全岩有机碳同位素组成（^δ13C_TOC）记录可划分为3个阶段:1)末次冰期晚期（18·5～14·7kaB·P·）,^δ13C_TOC值偏正,变化范围为-29·50‰～-26·18‰,平均值约为-28·10‰;2)末次冰消期（14·7～11·7kaB·P·）,^δ13C_TOC值显著偏负,变化范围为-33·92‰～-28·40‰,平均值约为-31·75‰,在^δ13C_TOC值变化曲线上表现为一个低谷,但在类似YoungerDryas的冷干事件期间（12·7～11·7kaB·P·）,^δ13C_TOC值再次显著偏正,最高可达-28·4‰;3)全新世以来（11·7kaB·P.至今）,^δ13C_TOC值变化幅度不大（-30·85‰～-27·37‰）,基本上都在平均值-29·1‰左右。研究表明,大气CO₂浓度变化是影响18·5kaB·P·以来四海龙湾玛珥湖^δ13C_TOC值变化的主导因素。     

n-Alkane evidence for the onset of wetter conditions in the Sierra Nevada,California (USA) at the mid-late Holocene transition, ~ 3.0 ka

n-Alkane biomarker distributions in sediments from Swamp Lake (SL), in the central Sierra Nevada of California (USA), provide evidence for an increase in mean lake level ~ 3000 yr ago, in conjunction with widespread climatic change inferred from marine and continental records in the eastern North Pacific region. Length distributions of n-alkane chains in modern plants growing at SL were determined and compared to sedimentary distributions in a core spanning the last 13 ka. As a group, submerged and floating aquatic plants contained high proportions of short chain lengths (< nC₂₅) compared to emergent, riparian and upland terrestrial species, for which chain lengths > nC₂₇ were dominant. Changes in the sedimentary n-alkane distribution over time were driven by variable inputs from plant sources in response to changing lake level, sedimentation and plant community composition. A shift toward shorter chain lengths (nC_21, nC₂₃) occurred between 3.1 and 2.9 ka and is best explained by an increase in the abundance of aquatic plants and the availability of shallow-water habitat in response to rising lake level. The late Holocene expansion of SL following a dry mid-Holocene is consistent with previous evidence for increased effective moisture and the onset of wetter conditions in the Sierra Nevada between 4.0 and 3.0 ka.     

Stable C and N isotope record of short term changes in water level in lakes of different morphometry: Lake Anastazewo and Lake Skulskie,central Poland

We present the stable C isotope record of the changes within the past 20 yr in water level of two morphologically different lakes in central Poland. The aim was to explain the relationship between lake water level and the δ¹³C signature of bulk sedimentary organic matter (δ¹³C_TOC) and to assess the potential of δ¹³C_TOC as a paleolimnological proxy of lake level change. This was done by comparison of the fossil δ¹³C_TOC record with instrumental data for lake level change in a shallow and small lake as well as in one large and deep basin. In both lakes the water table varied greatly between 1980 and 2000 AD. The δ¹³C_TOC data were supplemented with δ¹⁵N and bulk geochemistry data, as well as paleoecological data. We show that δ¹³C_TOC reacted to short term and low amplitude fluctuation in water level, but the response was highly dependent on the morphometry of the lake. In the shallow and small basin, δ¹³C_TOC decreased along with lake level drop due to oxidation and greater input of organic matter from macrophytes colonizing the lake bottom. On the contrary, in the deep/large lake δ¹³C_TOC decreased with increasing water level due to enhanced delivery of soil-derived OM to the lake during highstands. Our results have broad paleolimnological implications as they show that δ¹³C_TOC cannot act as a universal paleohydrological proxy. Its interpretation for a particular lake can be ambiguous and must be supported with additional geochemical and paleocological information.     

Climate and environment in southwest Sweden 15.5–11.3 cal. ka BP

Lake sedimentary records that allow documentation of the distinct climatic and environmental shifts during the early part of the Last Termination are scarce for northern Europe. This multi‐proxy study of the sediments of Atteköpsmosse, southwest Sweden, therefore fills an important gap and provides detailed information regarding past hydroclimatic conditions and local environmental responses to climatic shifts. Lake infilling started c. 15.5 cal. ka BP, but low aquatic productivity, cold summer lake water temperatures, unstable catchments, and scarce herb and shrub vegetation prevailed until c. 14.7–14.5 cal. ka BP. Inflow of warmer air masses and higher July air temperatures favoured a rise in aquatic productivity and lake water summer temperatures, and the establishment of a diverse herb, shrub and dwarf shrub vegetation, which also included tree birch c. 14.5 cal. ka BP. Freshening of the moisture source region c. 13.7–13.6 cal. ka BP does not seem to have had a large impact on the ancient lake and its catchment, as lake aquatic productivity increased further and lake water summer temperatures and minimum mean July air temperatures remained around 12–14 °C. In contrast, further freshening of the moisture source region c. 13 cal. ka BP triggered a decrease in lake productivity, drier conditions and lower lake water summer temperatures. Macroscopic finds of tree Betula and Pinus sylvestris at 13–12.8 cal. ka BP demonstrate the presence of these trees in the lake's catchment. The transition into the Holocene (11.6–11.5 cal. ka BP) is marked by a change in chironomid assemblages and by a rise in lake water summer temperatures and aquatic productivity. These changes were followed by the re‐establishment of a diverse aquatic and terrestrial vegetation, including tree birch and Pinus sylvestris at 11.4 cal. ka BP.     

Carbon chain length distribution in n-alkyl lipids: A process for evaluating source inputs to Lake Qinghai

Lake sediments generally contain a mixture of terrestrial and aquatic source inputs, and determining the major inputs is important for understanding geological records in paleoenvironment and paleoclimate research. In this study we describe the distribution of n-alkanes and n-fatty acids (FAs) in representative modern plants from around Lake Qinghai. We found a significant difference in the average length of n-FA carbon chains (ACL Fa_16–32) in terrestrial (23.3) and aquatic plants (18.6). The results reveal that ACL Fa_16–32 may essentially serve as a proxy for evaluating the major source inputs to lake sediments. Assessment of surface sediments from the lake showed that the FAs originated from a mixture of inputs, with the aquatic source input predominant at most sites. Additionally, the δD values of sediment mid-chain n-acids (C₂₂) showed a relationship with the ACL Fa_16–32 proxy: an increased Fa ACL corresponded to more negative hydrogen isotope ratio values. We suggest that different sources should be considered and ACL Fa_16–32 could be a potential calibration proxy before using δD values to extract reliable isotopic information from lake water. More attention should be paid to source inputs and their relationship to other geochemical proxies in future studies of lake sediments.     

Paleohydrological changes during the last deglaciation in Northern Brazil

We here report a reconstruction of hydrological balance variations in Northern Brazil for the last 20 ka deduced from the δD values of aquatic and land plant molecules extracted from the sediment infill of Lake Caçó. Our reconstructed precipitation, lake water isotope ratio and evaporation–evapotranspiration isotope effect allows us to obtain an estimate of moisture balance, and, to a lesser extent, precipitation amount and seasonality changes. During the end of the Last Glacial Maximum (LGM, between ca 20 and 17.3 ka), high δD values and smaller fractionation of leaf waxes indicate an arid to semi-arid climate with a long lasting dry season. An abrupt change towards much wetter conditions occurred within ca 500 years from 17.3 to 16.8 ka, as shown by a 50‰ decrease in D/H ratios and a marked increase in H isotopic fractionation of leaf waxes. This abrupt isotopic change coincides with a major transformation from savanna-dominated vegetation to humid rain forest around the lake, based on pollen data. Comparisons with other paleo-precipitation records from South American sites indicate that Lateglacial humid conditions were controlled by intensification of the ITCZ and/or a southward shift of its mean position across our study site. Our isotope data show only a small rise in aridity during Younger Dryas event (13–11.5 ka). Although the Holocene was not screened in details, D/H ratios of terrestrial and aquatic compounds show near constant offsets, suggesting stable and relatively humid climate conditions during this period.     

Regional atmospheric circulation change in the North Pacific during the Holocene inferred from lacustrine carbonate oxygen isotopes, Yukon Territory, Canada

Analyses of sediment cores from Jellybean Lake, a small, evaporation-insensitive groundwater-fed lake, provide a record of changes in North Pacific atmospheric circulation for the last ∼7500 yr at 5- to 30-yr resolution. Isotope hydrology data from the southern Yukon indicate that the oxygen isotope composition of water from Jellybean Lake reflects the composition of mean-annual precipitation, δ¹⁸O_p. Recent changes in the δ¹⁸O of Jellybean sedimentary calcite (δ¹⁸O_ca) correspond to changes in the North Pacific Index (NPI), a measure of the intensity and position of the Aleutian Low (AL) pressure system. This suggests that δ¹⁸O_p variability was related to the degree of fractionation during moisture transport from the Gulf of Alaska across the St. Elias Mountains and that Holocene shifts were controlled by the intensity and position of the AL. Following this model, between ∼7500 and 4500 cal yr B.P., long-term trends suggest a predominantly weaker and/or westward AL. Between ∼4500 and 3000 cal yr B.P. the AL shifted eastward or intensified before shifting westward or weakening between ∼3000 and 2000 cal yr B.P. Rapid shifts eastward and/or intensification occurred ∼1200 and 300 cal yr B.P. Holocene changes in North Pacific atmospheric circulation inferred from Jellybean Lake oxygen isotopes correspond with late Holocene glacial advances in the St. Elias Mountains, changes in North Pacific salmon abundance, and shifts in atmospheric circulation over the Beaufort Sea.     

Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa

Pollen and δ¹³C_TOM data obtained from two contrasting lake sequences (Lakes Kamalété and Nguène), located 200 km apart in the lowland rainforest of Gabon, provide complementary local and regional 1500-yr records of high resolution (15–30 yr) vegetation change. A combination of aquatic, semi-aquatic and terrestrial pollen showed in both records that the tropical rainforest increased during periods of high rainfall and decreased during drought intervals. The strong fluctuations of water balance at decadal scale during the “Medieval Warm Period” ( 1100–800 cal yr BP) coincided with a noticeable increase in shade-intolerant taxa, indicating recurring rainforest canopy disturbance. The δ¹³C_TOM signal showed high-amplitude variations in both records, which positively correlates with the rainforest dynamics and local vegetation changes. The similar trends in both the pollen and the δ¹³C_TOM signals between these sites demonstrate the regional broadly synchronous timing of shifting hydrological conditions. The largely positive co-variation between strong fluctuations of hydrological conditions and changes in rainforest structure and composition indicate that regional climatic change is probably the driving force for major rainforest dynamics in Gabon. Any significant anthropogenic impact on vegetation has not been clearly identified, and this issue still needs to be resolved independently by obtaining detailed archeological records across the interval 1400–800 BP, which currently seem to be extremely rare or not easily available.     

Influence of terrestrial vegetation on leaf wax δD of Holocene lake sediments

The hydrogen isotope composition of terrestrial plant leaf wax in sediments is increasingly used as a paleoclimatic indicator. Modern calibration studies suggest that paleoclimatic interpretation of leaf wax δD values requires consideration of the differences in the apparent fractionation of hydrogen isotopes among different groups of plants. However, it is not common that paleoecological data are used to help interpret leaf wax δD profiles. Here we assess the relative importance of factors influencing millennial-scale shifts in δD values of n-alkanoic acids at Steel Lake (Minnesota, USA), an extensively studied site with independent records of vegetation composition, δD of input water to the lake, and evaporation. The δD values of the n-C₂₈ alkanoic acid (δD_C28) vary between −190 and −168‰, and do not correlate with δD of input water or the extent of evaporation. However, δD_C28 is negatively correlated with the δ¹³C values of the n-C₂₈ alkanoic acid (δ¹³C_C28). The correlation, along with pollen assemblage and carbonate δ¹³C records, suggests that Holocene shifts between forest and grassland and/or in the water use efficiency of C₃ plants influenced the stratigraphic variation in leaf wax δD. Thus, paleoecological information, such as that inferred from pollen assemblages and carbon isotopes of plant-derived compounds, may aid paleoclimatic interpretation of leaf wax δD profiles from lake sediments.