Mid-holocene climate change in Lake Bosumtwi, Ghana

Lake Bosumtwi is one of the most widely studied palaeoclimate archives in West Africa. Results from numerous AMS ¹⁴C dates of samples from four piston cores from Lake Bosumtwi show that an abrupt sedimentary transition from a mid-Holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr B.P. High-resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature, and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial Africa from Ghana to the Congo basin. Northern and Eastern Africa experienced a similar abrupt shift toward aridity during the late Holocene, but at about 5000 cal yr B.P., a difference in timing that illustrates the regional nature of climate changes during the Holocene and the importance of feedback mechanisms in regulating Holocene climate variability. Furthermore, an abrupt change at about 3000 cal yr B.P. occurs at several sites adjacent to the tropical and subtropical Atlantic, which may hint at major changes in the surface temperatures of the tropical Atlantic and/or Pacific at this time.     

Sediment and hydro biogeochemistry of Lake Nainital, Kumaun Himalaya, India

The picturesque Nainital Lake, in the Uttarakhand state of India, is one of the major tourist attractions in the northern part of India. The increasing tourism and population around these lakes are a major concern for the ecology and good sustenance of the lakes. The present study is aimed to understand the behaviour of nutrients and metals in the sediment and their association with chemical forms in the lake. The study was accomplished by studying the water, interstitial water and sediments for major oxides, nutrients and metals in the lake. The different chemical forms of phosphorus and metals in the sediments were done using sequential extraction procedures. The water chemistry (Ca + Mg:Na + K) and the sediment chemistry (CIA and Al₂O₃/K₂O) show that the rocks in the catchment area play an important role in the geochemistry of the lake. The metals in the water also show that the Tallital basin is more polluted than the Mallital basin, may be due to the influence of Bus station. The high concentration of chloride, NH₄, SO₄ and metals in the sediment water interface and the interstitial water shows denitrification, sulfidisation and sulfide oxidation in the anoxic bottom water. The sediment composition shows that the phosphorus in the water is sequestered as carbonate flour apatite, and the metals precipitate as carbonate. The geo- accumulation index shows that the metals zinc, cobalt and nickel show moderate polluted nature than other metals. In general, the lake is less affected by anthropogenic activities. The chemical processes undergoing within the lake, like sulfidisation and sulfide oxidation, oxide dissolution and denitrification and organic matter degradation play an important role in the remobilization of the metals from the lake sediments.     

Late Quaternary environmental change and human impact around Lake Bolac,western Victoria,Australia

The late Quaternary history of the Lake Bolac region is reconstructed from sedimentary and pollen analysis of lake and swamp sequences and is supported by archaeological remains from lunette sediments. Freshwater deposits recovered from lake Turangmoroke appear to date from the last glacial period, when the vegetation was composed of herbfield and grassland. Subsequent dry conditions at the end of the Pleistocene caused the deflation of some accumulated sediments. On refilling, the lake had become saline. As moisture availability increased, casuarina woodland, dominated by Allocasuarina verticillata, became the regional vegetation cover. Domination by casuarina ended ca. 8000–7000 yr BP, probably as a result of rising saline ground-water tables. Casuarinas were replaced to some degree by eucalypts, augmented by the development of a riverine Eucalyptus camaldulensis woodland. Swamp deposition commenced in the channel of neighbouring Fiery Creek at ca. 4000 yr BP, most likely as the result of a reduction in stream flow with decreased precipitation. Despite suggestions of significant impact on the vegetation by Aboriginal people from other sites in southeastern Australia, there is no evidence of any alteration here from periods of occupation dated to around 13000 yr BP and after 3500 yr BP. The period of European occupation, though, is marked by significant changes in fire patterns, reduced tree cover, loss of floristic diversity, increased erosion rates and salinity levels, and decreased water levels.     

Assessment of the response of subaqueous methane hydrate deposits to possible climate change in the twenty-first century

Doklady Earth Sciences -     

Hydrological and nitrogen distributed catchment modeling to assess the impact of future climate change at Trichonis Lake, western Greece

According to regional climatic models, climate change may affect Mediterranean lakes significantly in terms of water availability and quality. Trichonis Lake catchment covers a semi-mountainous area of 403 km² including the largest Greek lake by volume (2.6?×?10⁹?m³), located in western Greece. The impact of climate change on the hydrology and water quality of the lake, in terms of lake water level and nutrient concentrations, has been assessed. Water balance estimates and geographical information system tools were then used to set up a physically based, spatially distributed model. The calibrated model was simulated for two future scenarios specified by the Intergovernmental Panel on Climate Change: A2 (pessimistic) and B2 (more optimistic), which involved temperature/evaporation/evapotranspiration increase and small precipitation decrease. The model was calibrated efficiently for the 1990–1992 period. The two basic climatic scenarios illustrated that the responses of the lake water levels will show a decrease of 24.2 and 12 cm, respectively, and an increase of total nitrogen concentrations by 3.4 and 10%, in relation to the early 1990s values. These important findings suggest that mitigation and optimum management plans should be developed to eliminate the aforementioned climate change impacts and further research should follow.     

内蒙古岱海地区小冰期气候演化特征的地球化学记录

通过对地处气候敏感带的岱海湖泊岩芯介形类壳体丰度、Sr/Ca和Mg/Ca摩尔比值，以及自生碳酸盐氧同位素组成等地球化学指标的综合分析，结合岩芯的岩性特征和^210Pb、^14C测年，重建了岱海小冰期以来气候环境演化过程，揭示小冰期前期本区以冷湿气候为主，在中后期，气候寒冷干旱，间有气候转温和的冷湿、湿干气候。本区小冰期气温变化与华北其他地区具有一致性，但降水与气温组合状况复杂，反映了明显的区域特征，其原因可能是气候转型期经向环流加强，气旋活动加强所致。     

Changing climate and human response: The impact of recent events on climatology

Over the last decade climate has become an increasingly significant factor in world affairs because of its effect on food supplies, energy consumption and environmental quality. At the same time the scientific community has had to re-appraise the nature and scope of climatology, increasing political relevance, leading to a growth in interdisciplinary activity involving both natural and social scientists. The 1980s promise to continue this broadening of scope as the two-way interaction of human societies and the atmospheric system are further explored and evaluated. This paper examines these trends, paying particular reference to the main climate-based environmental concerns currently being debated: the effect of halocarbons on the ozone shield, climate and food production, desertification, the CO₂ ‘greenhouse’ effect, acid rain and the significance of teleconnections.     

Influences of climate change and human activities on Tarim River runoffs in China over the past half century

The changes in annual runoff of the three original rivers and the mainstream of Tarim River were analyzed by the non-parametric tests based on the hydrologic data during the period of 50 years. Using hydrologic data, meteorological data and the fitted equation, the impacts of climate change and human activities on annual runoff of the mainstream were assessed. Based on the analysis, the following conclusions can be drawn: (1) headstream runoff has increased in the past 50 years, and has sharply jumped after 1990; (2) mainstream runoff decreased progressively in the past 50 years, which indicated that interference from human activities was the main cause for the decreasing runoff. This had greater negative influence than positive influence, which caused the mainstream average runoff to decrease by 5.4 × 10⁸ m³ from 1990 to 2008 as compared to 1957–2008; (3) if human activities remained at pre-1990 levels, climate change alone would have caused the runoff of mainstream of Tarim River to increase by 5.4 × 10⁸ m³ annually in the past 20 years; (4) if the climate had remained at pre-1990 conditions, human activities alone would have caused the runoff of mainstream of Tarim River to increase by 5.4 × 10⁸ m³ annually over the past 20 years. However, mainstream average runoff was 42.6 × 10⁸ m³ from 1990 to 2008 with the negative effects of human activities masked by the larger, positive effect of climate changes. The results in this paper provide a scientific basis for conservation strategies, sustainable management, and ecological restoration of the Tarim River Basin.     

Past occurrences of hypoxia in the Baltic Sea and the role of climate variability,environmental change and human impact

The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively.     

The 8.2-ka abrupt climate change event in Brown's Lake, northeast Ohio

Many Northern Hemisphere paleoclimatic records, including ice cores, speleothems, lake sediments, ocean cores and glacier chronologies, indicate an abrupt cooling event about 8200 cal yr BP. A new well-dated series of sediment cores taken from Brown's Lake, a kettle in Northeast Ohio, shows two closely spaced intervals of loess deposition during this time period. The source of loess is uncertain; however, it is likely from an abandoned drainage and former glacial lake basin located to the north of the stagnant ice topography that gave rise to the kettle lake. Strong visual stratigraphy, loss on ignition data and sediment grain size analyses dated with 3 AMS radiocarbon dates place the two intervals of loess deposition between 8950 and 8005 cal yr BP. The possibility of a two-phase abrupt climate change at this time is a finding that has been suggested in other research. This record adds detail to the spatial extent and timing as well as possible structure of the 8.2-ka abrupt climate change event.     

Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed

Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia.     

Human impacts, climate change, and aquatic ecosystem response during the past 2000 yr at Lake Wandakara, Uganda

Analyses of carbon and hydrogen isotope ratios of terrestrial leaf waxes and the carbon and nitrogen abundance, ratio, and isotopic composition of bulk sediments from Lake Wandakara, a crater lake in western Uganda, East Africa, document human and climatic controls on the aquatic system and on the surrounding terrestrial vegetation during the past two millennia. Our data indicate that Wandakara was a relatively stable, productive lake surrounded by C₃ vegetation from AD 70 to 1000. Abrupt changes in the δ¹³C of terrestrial leaf waxes indicate a series of abrupt shifts in the relative abundance of C₃ and C₄ vegetation caused by a combination of climate change and human activities around Wandakara beginning at AD 1000. Abrupt shifts in bulk sediment organic geochemistry, particularly C/N ratios and δ¹⁵N, indicate that human activities at this time caused permanent changes in the limnology of Lake Wandakara, including eutrophication. Our results suggest that the biogeochemistry of Lake Wandakara was more sensitive to shifting human impacts than to climate variations during the past millennium, highlighting the importance of understanding the intensity of pre-colonial human impacts on Africa's aquatic ecosystems.     

Bronze Age at Lake Bourget (NW Alps, France): Vegetation, human impact and climatic change

Pollen analyses of three sediment cores performed on the archaeological lake dwellings of Chindrieux and Tresserve (Savoie, France), on the eastern shore of Lake Bourget, enable the reconstruction of vegetation history and human impact during the Bronze Age. Results show a good correlation between lake-level fluctuations and the evolution of anthropogenic indicators. The end of the Neolithic period and the early Bronze Age are characterised by a discreet occupation of the Lake Bourget surroundings, in spite of a warmer climatic condition. The middle Bronze Age, contemporaneous with wetter and cooler conditions, is related to a decrease in pollen of cultivated plants and weed. Farming activities reappear in the late Bronze Age and human impact increases suddenly at the very end of the late Bronze Age. Three anthropogenic phases are clearly visible: a short decrease in anthropogenic indicators and a high lake-level phase occur in the middle of this period of intense farming development. A new decrease in human impact appears at the beginning of the Iron Age.     

社会-生态弹性视角下的历史气候变化影响社会发展机制

过去全球变化（PAGES）研究高度关注的热点领域之一是各个时间尺度上的气候变化如何影响人类社会发展,其目的不仅是为了尽最大可能地还历史本真,更重要地是为了揭示具有一般意义的过程与机理并以史为鉴。社会-生态弹性是人类社会的内在属性,可以理解为人类社会忍受各种环境变化、社会、经济与政治动荡冲击的能力,弹性理论能够很好地阐释历史气候变化影响的过程与机制。本文尝试以此为视角,结合中外历史案例,对"历史气候变化如何影响社会发展"问题进行探讨,主要结论如下：1）气候变化对人类社会的影响与社会-生态弹性状态有关,如果气候变化影响仅突破人类社会某一子系统的弹性阈值,整个人类社会仍可通过其他子系统的适应性调整继续维持稳定;2）历史气候变化影响的5种表现形式与社会-生态弹性状态的关系是,人类社会对气候变化敏感但未突破弹性阈值时表现为周期波动或脉冲式变化,适宜性转型或崩溃发生在气候变化影响突破人类社会的弹性阈值的情况下;而迁徙-替代不论气候变化影响是否突破弹性阈值均可发生;3）人类社会可通过适应气候变化过程中的社会学习和创新增强甚至在更高水平上重构社会-生态弹性。     

Response of geochemical records in lacustrine sediments to climate change and human impact during middle Holocene in Mengjin, Henan Province, China

In this paper, we examined the response of geochemical records in lacustrine sediments to climate and human activity in Mengjin, Henan Province of China during the middle Holocene. Indices used in this study include K, Na, Mn, Ba, Sr, Al, Ti, cation ratios, and Gramineae pollen in lacustrine sediments. Results indicate a drought event in this area around 5660 cal a BP, which weakened soil leaching and changed the element composition of surface soil runoff entering the paelo-lake and lacustrine sediments. Human activity appearing from 4695 cal a BP to 4000 cal a BP also affected the chemical composition of lake sediments. Human activities led to enhanced soil erosion and destroyed soil leaching horizons. Therefore, the substance of soil illuvial layer exposed to the surface and entering the paleo-lake were substantially altered by human activities.     

Late Glacial-Holocene sequence of Lake Saint-Point (Jura Mountains, France): Detrital inputs as records of climate change and anthropic impact

A sediment sequence (SP05, 12.5 m long) was taken from the deep zone of Lake Saint-Point (850 m a.s.l.). Sedimentological analyses highlight two main contrasted periods of sedimentation: the Last Glacial Maximum (LGM)/Late Glacial characterized by high silicates and quartz contents the Holocene dominated by the carbonated fraction. At the beginning of the Holocene (11 400 years cal. BP), silicates fraction flux abruptly decreased. The shift between the Late Glacial and the Holocene periods may be explained by forest development in the catchment. From 10 200 to 6800 years cal. BP, silicates and detrital carbonate fractions remained stable before they progressively increased steady till 5000 years cal. BP. Both increases cannot be totally attributed to an anthropic impact since pollen data indicate continuous anthropic activities only dated back from 3000 years cal. BP. They thus resulted from a dominant climatic control. From 5000 years cal. BP, silicates content still increased while detrital carbonates input became steady due to a change in pedogenetic processes affecting the catchment. During the last millennium, silicates and detrital carbonate decreased, probably due to pastureland development.