Stable carbon isotopes (δ<Superscript>13</Superscript>C) of total organic carbon and long-chain <Emphasis Type="Italic">n</Emphasis>-alkanes as proxies for climate and environmental change in a sediment core from Lake Petén-Itzá, Guatemala

Sediment core PI-6 from Lake Petén Itzá, Guatemala, possesses an ~85-ka record of climate and environmental change from lowland Central America. Variations in sediment lithology suggest large and abrupt changes in precipitation during the last glacial and deglacial periods, and into the early Holocene. We measured stable carbon isotope ratios of total organic carbon and long-chain n-alkanes from the core, the latter representing a largely allochthonous (terrestrial) source of organic matter, to reveal past shifts in the relative proportion of C₃–C₄ terrestrial biomass. We sought to test whether stable carbon isotope results were consistent with other paleoclimate proxies measured in the PI-6 core, and if extraction and isotope analysis of n-alkanes is warranted. The largest δ¹³C variations are associated with Heinrich Events. Carbon isotope values in sediments deposited during the last glacial maximum indicate moderate precipitation with little fluctuation. The deglacial was a period of pronounced climate variability, e.g. a relatively warm and moist Bølling–Allerød, but a cool and dry Younger Dryas. Arid periods of the deglacial were inferred from samples with high δ¹³C values in total organic carbon, which reflect times of greater proportions of C₄ plants. These inferences are supported by stable isotope measurements on ostracod shells and relative abundance of grass pollen from the same depths in core PI-6. Similar trends in carbon stable isotopes measured on bulk organic carbon and n-alkanes suggest that carbon isotope measures on bulk organic carbon in sediments from this lake are sufficient to infer past climate-driven shifts in local vegetation.     

Carbon isotopes of marl and lake sediment organic matter reflect terrestrial landscape change during the late Glacial and early Holocene (16,800 to 5,540 cal yr B.P.): a multiproxy study of lacustrine sediments at Lough Inchiquin,western Ireland

A 7.6-m lake sediment core from a marl lake, Lough Inchiquin, records variation in landscape evolution from 16,800 cal yrs B.P. to 5,540 cal yrs B.P. We observe significant variations (up to 12‰) in δ ¹³C_org and δ ¹³C_calcite values that are interpreted to reflect secular changes in lake water δ ¹³C_DIC values that result from a regional landscape transition from barren limestone bedrock to a forested ecosystem. Lake water δ ¹³C_DIC values are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (−27.1 to −31.2‰VPDB) via oxidized soil organic matter and weathered limestone bedrock (+3.4‰VPDB). Isotope excursions in lacustrine sediment records are forced not only by changes in productivity but also by changes in the terrestrial environment. This has profound implications for the interpretation of paleoclimate records derived from lacustrine sediment and suggests that selection of appropriate lakes can provide records of terrestrial change where other related records are not available.     

Biomarker and stable carbon isotope analyses of sedimentary organic matter from Lake Tswaing: evidence for deglacial wetness and early Holocene drought from South Africa

Comparing the organic matter (OM) composition of modern and past lake sediments contributes to the understanding of changes in lacustrine environments over time. We investigate modern plant and lake-water samples as well as modern and ancient sediment samples from the Tswaing Crater in South Africa using biomarker and stable carbon isotope analyses on bulk OM and specific biomarker compounds. The characteristic molecular markers for higher land plants (predominantly C3-type deciduous angiosperms) in Lake Tswaing are long-chain n-alkanes (n-C₂₇₋₃₃), n-alkanols (n-C₂₈₊₃₀), stigmasterol, β-sitosterol, β-amyrin, α-amyrin and lupeol. The C₁₇ n-alkane, tetrahymanol, gammaceran-3-one and C₂₉ sterols dominate the lipid fraction of autochthonously produced OM. By comparing stable carbon isotope analyses on bulk OM and the characteristic biomarkers, we follow the modern carbon cycle in the crater environment and find indications for methanotrophic activity in the lake from isotopically depleted moretene. A comparative study of core sediments reveals changes in the terrestrial (C3 versus C4) and aquatic bioproductivity and allows insights into the variability of the carbon cycle under the influence of changing climatic conditions for the time from the end of the last glacial (Termination I) to the late Holocene, ca. 14,000–2,000 calibrated years before present (years BP). The most pronounced changes occur in the aquatic realm after ca. 10,000 years BP when our results imply climate swings from more humid to more arid and after 7,500 years BP to gradually more humid conditions again, which can be related to a shift in the position of the Inter-Tropical Convergence Zone or to changes in the tropical atmosphere–ocean interaction. Long-chain alkenones (LCAs) have been identified in ancient lake sediments from Africa for the first time. They occur in samples older than 7,500 years BP and their distribution (dominance of C₃₈ and of tri- over tetra-unsaturated LCAs) is distinctly different from other published records suggesting a to date unknown source organism.     

Holocene lake carbon sequestration,hydrological status and vegetation change,China

Lakes have received considerable attention as long-term sinks for organic carbon(C) at regional and global scales. Previous studies have focused on assessment and quantification of carbon sinks, and few have worked on the relationship between millennial-scale lake C sequestration, hydrological status and vegetation, which has important scientific significance in improving our understanding of lake C stocks and storage mechanisms. Here, we present a comprehensive study of pollen records, organic geochemical proxies, lake-level records, sediment accumulation rate(SAR) and organic C accumulation rate(CAR) in China since the Holocene. We also include numerical climate classification and lake-level simulations, to investigate variations of lake C sequestration, hydrological status and vegetation during the Holocene. Results indicate that the evolution of lake C accumulation showed an out-of-phase relationship with hydrological status and vegetation in China. Lake C accumulation exhibited an overall trend of increasing from the early to late Holocene in response to gradually increasing terrestrial organic matter input. However, China as a whole experienced the densest vegetation cover in the middle Holocene, corresponding to the mid-Holocene optimum of a milder and wetter climate. Optimal hydrological conditions were asynchronous in China; for example, early Holocene in Asian monsoon dominated areas, and middle Holocene in westerlies controlled regions. Our synthesis indicated that climate change was the main factor controlling the long-term variability in lake C accumulation, hydrologic conditions, as well as vegetation, and human influences were usually superimposed on the natural trends.     

Temporal patterns in lacustrine stable isotopes as evidence for climate change during the late glacial in the Southern European Alps

We investigated oxygen and carbon isotopes of bulk carbonate and of benthic freshwater ostracods (Candona candida) in a sediment core of Lago Piccolo di Avigliana that was previously analyzed for pollen and loss-on-ignition, in order to reconstruct environmental changes during the late glacial and early Holocene. The depth–age relationship of the sediment core was established using 14 AMS ¹⁴C dates and the Laacher See Tephra. While stable isotopes of bulk carbonates may have been affected by detrital input and, therefore, only indirectly reflect climatic changes, isotopes measured on ostracod shells provide unambiguous evidence for major environmental changes. Oxygen isotope ratios of ostracod shells (δ¹⁸O_C) increased by ～6‰ at the onset of the Bølling (～14,650 cal BP) and were ～2‰ lower during the Younger Dryas (～12,850 to 11,650 cal BP), indicating a temporal pattern of climate changes similar to the North Atlantic region. However, in contrast to records in that region, δ¹⁸O_C gradually decreased during the early Holocene, suggesting that compared to the Younger Dryas more humid conditions occurred and that the lake received gradually increasing input of ¹⁸O-depleted groundwater or river water.     

An overview of late Holocene climate and environmental change inferred from Arctic lake sediment

The 14 papers in this Special Issue of the Journal of Paleolimnology report new records of Holocene climate and environmental change from Arctic lakes, with emphasis on the last 2000 years. The study sites span the high latitudes of North America and extend into northwestern Europe. The studies rely on multiple proxy indicators to reconstruct past climate, including: varve thicknesses, chironomid, diatom, and pollen assemblages, biogenic-silica and organic-matter content, oxygen-isotope ratios in diatoms, and the frequency of lake-ice-rafted aggregates. These proxies primarily document changes in past summer temperatures, the main control on physical and biological processes in lakes at high latitudes. The records will be integrated into a larger network of paleoclimate sites to investigate the spatial and temporal variability of climate change and to compare the paleoclimate inferences with the output of general circulation models. This is the Introduction to a series of fourteen papers published as a special issue dedicated to reconstructing late Holocene climate change from Arctic lake sediments. The special issue is a contribution to the International Polar Year and was edited by Darrell Kaufman.     

Hydrogen isotope ratios of individual lipids in lake sediments as novel tracers of climatic and environmental change: a surface sediment test

We determined hydrogen isotope ratios of modern lake-waters and individual lipids from surface sediments of 36 lakes in the eastern North America. The lakes selected lie on two transects (south–north transect from Florida to Ontario and east–west transect from Wisconsin to South Dakota) and encompass large temperature and moisture gradients, and a wide range of lake water δD values (>100‰). The study allows a rigorous test of the applicability of using δD values of sedimentary lipids as paleoclimatic and paleoenvironmental proxies. We examined a range of lipids including C₁₇ n-alkane, straight chain fatty acids, phytol and sterols in both free extracts and ester-bound fractions in the solvent extracted sediments. Useful isotopic indicators are expected to show a linear correlation and constant fractionation factor between their δD values in surface sediments and modern lake water. Our results demonstrate that several lipid compounds, free and ester-bound palmitic acid (16:0), C₁₇ n-alkane, and phytol are useful candidates for paleoclimate reconstructions, in addition to two sterols that have been suggested previously (. Compound-specific D/H ratios of lipid biomarkers from sediments as a proxy for environmental and climatic conditons. Geochim. Cosmochim. Acta 65: 213–222). Authigenic or biogenic carbonate in sediments is conventional material for paleoclimatic study using ocean and lake sediments. However, because majority of lake sediments do not contain suitable carbonate materials for isotopic study, hydrogen isotope ratios of these lipids provide invaluable new sources of paleoclimatic and paleoenvironmental information.     

Climatic and environmental change in Yanchi Lake, Northwest China since the Late Glacial: A comprehensive analysis of lake sediments

Modern climate research has shown that the Asian summer monsoon water vapor transport is limited to the eastern part of the Qilian Mountains. On the Holocene millennial-scale, whether the northwest boundary of the summer monsoon varies according to climate change is a key scientific issue. Yanchi Lake is located in the northern Qilian Mountains and the middle of the Hexi Corridor, where the modern climate is less affected by the Asian summer monsoon. It is a key research area for examining the long-term variations of the Asian summer monsoon. Paleoclimatic data, including AMS ^14C dates of pollen concentrates and bulk organic carbon, lithology, grain-size, mineral composition and geochemical proxies were acquired from sediments of Yanchi Lake. The chronological results show that the lower part of the lacustrine section is formed mainly in the Late Glacial and early Holocene period, while the proxies＇ data indicate the lake expansion is associated with high content of mineral salts. The middle part of this section is formed during the transitional period of the early and middle Holocene. Affected by the reworking effect, the pollen concentrates AMS^14C dates from the middle part of the section are generally older than those from the lower part. Since the mid-Holocene, Yanchi Lake retreated significantly and the deposition rate dropped obvi- ously. The Yanchi Lake record is consistent with the Late Glacial and Holocene lake records in the Qinghai-Tibet Plateau and the climatic records in typical monsoon domain, which indicate the lake expansion and the strong Asian summer monsoon during the Late Glacial and early Holocene. The long-term monsoonal pattern is different from the lake evolution in Central Asia on the Holocene millennial-scale. This study proves the monsoon impacts on the northwestern margin of the summer monsoon, and also proves the fact that the northern boundary of the summer monsoon moves according to millennial-scale climate change.     

Biomarker-based paleo-record of environmental change for an eutrophic,tropical freshwater lake,Lake Valencia,Venezuela

The lipids in a sediment core from Lake Valencia, a hypereutrophic freshwater lake in Venezuela, are examined to understand environmental changes over the last ∼13,000 years. From the latest Pleistocene to the earliest Holocene, total organic carbon (TOC) substantially increased from 2.2 to 10%, while total organic carbon over total nitrogen (TOC/TN) decreased from as high as 34 to as low as 10. Correspondingly, the concentration of terrestrially derived triterpenoids markedly decreased, and the dominant n-alkane shifted from C₃₁ to C₂₃ or C₂₅. During the same period, algal biomarkers such as botryococcenes, dinosterol, isoarborinol, C₂₀ HBIs and 1,15C₃₂ keto-ol markedly increased in abundance. These changes suggested a greater contribution of algal organic matter at the onset of the Holocene, which was concurrent with increasing rainfall and the formation of a permanent lake (Lake Valencia) in the Aragua Valley, Venezuela. The age profile of Paq, a n-alkane based proxy, showed large oscillations (0.20–0.81), reflecting historical variations in source strength of submerged/floating vs. terrestrial/emergent OM inputs. An abrupt increase in tetrahymanol abundance at ∼7,260 cal years BP suggests the establishment of an oxic–anoxic boundary in the lake’s water column. After reaching its maximum abundance at ∼2,100 cal year BP, botryococcenes, a biomarker of Botryococcus braunii, gradually decreased to below the detection limit in the uppermost sediments, while different algal/microbial biomarkers such as diploptene, dinosterol and isoarborinol substantially increased. These different historical profiles of algal/microbial biomarkers reflect different responses of source organisms to environmental changes throughout this period. The δ¹³C determinations presented exceptionally enriched values for botryococcene isomers (−7.7 to −15.1‰), indicating the utilization of bicarbonate as carbon sources in an extremely productive ecosystem.     

Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (δ15N, δ13C) of Lake Pupuke

Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165?cal.?year BP. The origin of organic matter was established using total-organic?Ccarbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (??¹³C_OM) and nitrogen (??¹⁵N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165?years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600?cal.?year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the ??¹⁵N and ??¹³C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600?cal.?year BP.     

Paleoclimate changes inferred from stable isotopes and magnetic properties of organic-rich lake sediments in Arctic Norway

Stable isotope measures in organic matter are frequently used as indicators of past climate change. Although such analyses can provide valuable information, there is considerable uncertainty associated with studies of organic-rich sediments, especially those from Arctic lakes and bogs. We studied stable isotopes of carbon and nitrogen, and magnetic properties in a sediment core from a small alkaline lake with a high sedimentation rate, Lake Nattmålsvatn, Norway. There is good correspondence among the different sediment variables during the late glacial, and they seemingly reflect major climate variations such as the Allerød Interstade and the Younger Dryas, as well as the transition into the current interglacial. During the early Holocene, however, these relationships are more complex and δ¹³C and δ¹⁵N values do not stabilize until ~7,500 cal year BP. A significant excursion in all variables occurs between 6,850 and 6,500 cal year BP and is interpreted to represent climate deterioration. Holocene δ¹³C values vary little and indicate that isotopically-depleted dissolved inorganic carbon (DIC) in the lake, possibly influenced by methanotrophy and high pCO₂, dominated the lake’s carbon cycle. Holocene δ¹⁵N is similarly muted, likely due to the availability of abundant dissolved nitrogen. Bulk organic matter is probably dominated by phytoplankton remains produced beneath the ice cover in late spring and during ice breakup when isotopically-depleted DIC, pCO₂ and ammonium availability were maximal. Thus, use of δ¹³C and δ¹⁵N as indicators of Holocene paleoclimate and paleoproductivity variation can be challenging in a lake such as Nattmålsvatn, where ice cover isolates the basin for large parts of the year, allowing dissolved respiratory gases to accumulate in the water column. In contrast, magnetic variables appear to better track climate variations. In particular, runoff-driven influx of minerogenic sediments shows high variability that can be attributed to regional changes in Holocene winter precipitation. The most striking shifts occur between 4,000 and 2,300 cal year BP.     

Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake,middle reaches of the Yangtze River,China

Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ¹³C_Org, δ¹⁵N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ¹³C_Org, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ¹⁵N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.     

Response of Sandy Lake in Schirmacher Oasis,East Antarctica to the glacial-interglacial climate shift

Freshwater lakes in Antarctica fluctuate from ice-free state (during austral summer) to ice-cover state (during austral winter). Hence the lakes respond instantly to the seasonal climate of the region. The Antarctic seasons respond sharply to the glacial and interglacial climates and these signatures are archived in the lake sediments. A sediment core from Sandy Lake, a periglacial lake located in Schirmacher Oasis of East Antarctica records distinct changes in grain-size, C, N, C/N ratios (atomic), δ¹³C_OM and δ¹⁵N_OM contents during the last 36 ky. The contents of the sedimentary organic matter (OM) proxies (C_org ~ 0.3 ± 0.2%, C/N ratios ~9 ± 5 and δ¹³C_OM ~?18 ± 6‰) indicate that the OM in this lake sediment is a product of mixing of terrestrial and lacustrine biomass. Distinctly lower contents of C_org (~0.2%) and sand (~50%), low C/N ratios (~8) and depleted δ¹³C_OM (~?20‰) during the Last Glacial Maximum (LGM: 32–17 ky BP based on Vostok Temperatures) suggest greater internal (autochthonous) provenance of organic matter and limited terrestrial (allochthonous) inputs probably due to long and intense winters in the Antarctic. Such intense winters might have resulted the lake surface to be ice-covered for most part of the year when the temperatures remained consistently colder than the Holocene temperatures. The denitrification within the lake evident by enriched δ¹⁵N_OM (>10‰) during Antarctic LGM might have resulted from oxygen-limitation within the lake environment caused by insulated lake surface. The gradual increases in δ¹³C_OM, C/N and sand content starting at ~11 ky BP and attaining high values (~?11‰, ~10 and ~80% respectively) at ~6 ky BP together suggest a subtle change in the balance of sources of organic matter between algal and macrophyte/bryophyte nearly 8–9 ky later to the beginning of the deglaciation. Thus the seasonal opening-up of the Sandy Lake similar to the modern pattern started with the establishment of the optimum temperature conditions (i.e., 0 °C anomaly) in the Antarctic, prior to which the lake environment might have remained mostly insulated or closed.     

Mid- to- late Holocene hydroclimatic changes on the Chinese Loess Plateau: evidence from <Emphasis Type="Italic">n</Emphasis>-alkanes from the sediments of Tianchi Lake

We have reconstructed the history of mid-late Holocene paleohydrological changes in the Chinese Loess Plateau using n-alkane data from a sediment core in Tianchi Lake. We used P_aq (the proportion of aquatic macrophytes to the total plant community) to reflect changes in lake water level, with a higher abundance of submerged macrophytes indicating a lower water level and vice versa. The P_aq-based hydrological reconstruction agrees with various other lines of evidence, including ACL (average chain length), CPI (carbon preference index), C/N ratio and the n-alkane molecular distribution of the sediments in Tianchi Lake. The results reveal that the lake water level was relatively high during 5.7–3.2 ka BP, and decreased gradually thereafter. Our paleohydrological reconstruction is consistent with existing paleoclimate reconstructions from the Loess Plateau, which suggest a humid mid-Holocene, but is asynchronous with paleoclimatic records from central China which indicate an arid mid-Holocene. Overall, our results confirm that the intensity of the rainfall delivered by the EASM (East Asian summer monsoon) is an important factor in affecting paleohydrological changes in the region and can be considered as further evidence for the development of a spatially asynchronous “northern China drought and southern China flood” precipitation pattern during the Holocene.     

Persistence of Artemisia steppe in the Tangra Yumco Basin,west-central Tibet,China: despite or in consequence of Holocene lake-level changes?

The closed Tangra Yumco Basin underwent the strongest Quaternary lake-level changes so far recorded on the Tibetan Plateau. It was hitherto unknown what effect this had on local Holocene vegetation development. A 3.6-m sediment core from a recessional lake terrace at 4,700 m a.s.l., 160 m above the present lake level of Tangra Yumco, was studied to reconstruct Holocene flooding phases (sedimentology and ostracod analyses), vegetation dynamics and human influence (palynology, charcoal and coprophilous fungi analyses). Peat at the base of the profile proves lake level was below 4,700 m a.s.l. during the Pleistocene/Holocene transition. A deep-lake phase started after 11 cal ka BP, but the ostracod record indicates the level was not higher than ~4,720 m a.s.l. (180 m above present) and decreased gradually after the early Holocene maximum. Additional sediment ages from the basin suggest recession of Tangra Yumco from the coring site after 2.6 cal ka BP, with a shallow local lake persisting at the site until ~1 cal ka BP. The final peat formation indicates drier conditions thereafter. Persistence of Artemisia steppe during the Holocene lake high-stand resembles palynological records from west Tibet that indicate early Holocene aridity, in spite of high lake levels that may have resulted from meltwater input. Yet pollen assemblages indicate humidity closer to that of present potential forest areas near Lhasa, with 500–600 mm annual precipitation. Thus, the early mid-Holocene humidity was sufficient to sustain at least juniper forest, but Artemisia dominance persisted as a consequence of a combination of environmental disturbances such as (1) strong early Holocene climate fluctuations, (2) inundation of habitats suitable for forest, (3) extensive water surfaces that served as barriers to terrestrial diaspore transport from refuge areas, (4) strong erosion that denuded the non-flooded upper slopes and (5) increasing human influence since the late glacial.     

通过现代观测来检验湖面蒸发对中国干旱、半干旱区湖泊水位变化的影响(英文)

Qinghai Lake and Zhuye Lake, ~400 km apart, are located in the northwest margin of the Asian summer monsoon. Water of these two lakes mostly comes from the middle and eastern parts of the Qilian Mountains. Previous studies show that the Holocene climate changes of the two lakes implied from lake records are different. Whether lake evaporation plays a role in asynchronous Holocene climate changes is important to understand the lake records. In this paper, we used modern observations beside Qinghai Lake and Zhuye Lake to test the impact factors for lake evaporation. Pan evaporation near the two lakes is mainly related to relative humidity, temperature, vapor pressure and sunshine duration. But tem-perature has different impacts to lake evaporation of the two lakes, which can affect Holocene millennial-scale lake level changes. In addition, differences in relative humidity on the millen-nial-scale would be more significant, which also can contribute to asynchronous lake records.     

过去陆地生态系统碳储量估算研究

准确估算陆地生态系统碳储量并认识其空间分布和时间演变规律是碳循环研究的关键 问题。本文回顾了全球与中国陆地生态系统在碳储量估算研究方面的若干进展, 包括基于各种方 法和资料的主要估算结果及其尚存在的不确定性。重点评述了末次盛冰期和中全新世两个时期 陆地生态系统碳储量的变化及其影响因素, 对8.2kaB.P.以来全球大气CO₂ 浓度呈现升高的现象 及其可能原因进行了讨论。全新世中晚期全球大气CO₂ 浓度逐渐升高与旧大陆地区陆地生态系 统碳储量减少的事实是一致的, 新石器时期特别是农业文明开始以后人类活动对陆地植被的持 续干预可能是造成陆地生态系统碳储量减少的原因之一。     

Carbon and nitrogen isotopic composition of different strains ofArtemia sp

Stable carbon and nitrogen isotope ratios have been determined on 41 strains ofArtemia sp. from different geographic regions around the world. The δ¹³C and δ¹⁵N values ranged between −13.7 to−25.0 per mil and −0.7 to 21.2 per mil respectively.Artemia δ¹³C values from coastal environments are consistent with a marine origin for the food sourceArtemia from inland salt lakes have a range of carbon isotope values suggesting C₃, C₄ and CAM based organic matter could form the base of theArtemia food chain. These data indicate thatArtemia having a wide range of carbon and nitrogen isotope values are available for tropho-dynamic research studies that quantify the effect of respired CO₂ on tissue and CaCO₃ shell¹³C/¹²C ratios. Such stable isotope variation also suggests that stable isotope fingerprinting remains a viable technique for identifying specificArtemia collection sites.     

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g⁻¹), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm⁻¹, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g⁻¹, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm⁻¹, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g⁻¹, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.     

我国干旱区湖泊演化研究综述

根据湖泊沉积记录研究环境演变是古气候变化研究的重要内容之一,在古环境演化研究以及预测未来气候等方面具有重要意义。随着中国湖泊科学的不断发展,我国在干旱与半干旱地区湖泊演化方面取得了大量的研究成果,与此同时也存在着争论和分歧。针对晚更新世高湖面存在时间、中全新世气候干湿状况以及近50a基于遥感技术研究湖面波动等方面的研究进展进行了综述和分析,并对今后的研究重点提出了一些建议。