The Response of Aboriginal Burning Practices to Population Levels and El Niño–Southern Oscillation Events during the mid- to late-Holocene: a case study from the Sydney Basin using charcoal and pollen analysis

Pollen and macroscopic charcoal have been analysed from a sedimentary sequence representing approximately 6100 years from a site within Wollemi National Park. This is located to the north-west of Sydney and forms a part of the Greater Blue Mountains World Heritage Area. The overall aim of the work was to examine the prehistoric interrelationships between vegetation, fire and human activity. There were relatively minor changes in the vegetation over the last ～6000 years, perhaps reflecting the climate- and fire-resilient nature of the sclerophyllous vegetation found on Hawkesbury Sandstone throughout the Sydney Basin. Casuarinaceae declined in the late Holocene, a trend that has been detected in numerous palaeoecological studies throughout south-eastern Australia. This decline was unrelated to fire, which has been a persistent feature at the site over the entire analysed sequence. The fire regime at the site changed from 5.7 ka, which is interpreted as reflecting the onset of increased climatic variability associated with El Niño-Southern Oscillation (ENSO) events. Another dramatic change in the fire regime occurred at 3 ka, which was coeval with archaeological changes in the region. It is possible that the change in fire activity from 3 ka represented an alteration to Aboriginal management strategies associated with an increasing population and/or the increased risk of conflagrations in an ENSO-dominated climate.     

Paleolimnological evidence for increased landslide activity due to forest clearing and land-use since 3600 cal BP in the western Swiss Alps

Schwarzsee is located in the western Swiss Alps, in a region that has been affected by numerous landslides during the Holocene, as evidenced by geological surveys. Lacustrine sediments were cored to a depth of 13 m. The vegetation history of the lake's catchment was reconstructed and investigated to identify possible impacts on slope stability. The pollen analyses record development of forest cover during the middle and late Holocene, and provide strong evidence for regional anthropogenic influence such as forest clearing and agricultural activity. Vegetation change is characterized by continuous landscape denudation that begins at ca. 4300 cal. yrs BP, with five distinct pulses of increased deforestation, at 3650, 2700, 1500, 900, and 450 cal. yrs BP. Each pulse can be attributed to increased human impact, recorded by the appearance or increase of specific anthropogenic indicator plant taxa. These periods of intensified deforestation also appear to be correlated with increased landslide activity in the lake's catchment and increased turbidite frequency in the sediment record. Therefore, this study gives new evidence for a strong influence of vegetation changes on slope stability during the middle and late Holocene in the western Swiss Alps, and may be used as a case study for anthropogenically induced landslide activity.     

Anthropogenic origin of a change in the fire-climate relationship in northern China after ~2000 yr BP: Evidence from a 15,500-year black carbon record from Dali Lake

There are debates regarding whether a wet and warm climate or a dry and cold climate dominated Holocene fire activity in northern China on the millennial timescale, and when human activities overtook climate change as the dominant control on fire occurrence in the region. Here we present a high-resolution fire history for the past ~15,500 years from a sediment core in Dali Lake, located in the foothills of the Greater Hinggan Mountains, one of the areas of highest fire risk in China. The results demonstrate that fire activity was rare during the last deglaciation (~15,500-11,700 yr BP), gradually increased at the beginning of the Holocene, and reached its highest level during ~9000-5000 yr BP, after which there was a decreasing trend. However, after ~2000 yr BP this decreasing trend ended, and the most prominent feature is a peak in fire activity during the Medieval Warm Period (MWP). Overall, fire activity corresponded well to changes in the East Asian summer monsoon (EASM) precipitation on the millennial timescale during ~15,500-2000 yr BP, but this relationship changed after ~2000 yr BP. We propose that fire activity in northern China on the millennial timescale during ~15,500-2000 yr BP was dominated by the biofuels reserve under the control of the EASM precipitation. In contrast, with the intensification of human activities after ~2000 yr BP, human activity caused a ~62%-73% increase in fire activity, which altered the fire-climate relationship that had previously prevailed in northern China. Our results indicate that a wet-warm climate (increased EASM intensity), rather than a dry-cold climate, was the dominant control on fire activity in northern China during 15,500-2000 yr BP on the millennial timescale, but that human activities played an important role in fire occurrence after ~2000 yr BP.     

Fire and vegetation history during the last 3800 years in northwestern Montana

Foy Lake in northwestern Montana provides a record of annual-to-decadal-scale landscape change. Sedimentary charcoal and pollen analyses were used to document fire and vegetation changes over the last 3800 years, which were then compared to similar records from AD 1880 to 2000. The long-term record at Foy Lake suggests shifts between forest and steppe as well as changes in fire regime that are likely the result of climate change. Fire activity (inferred from the frequency of charcoal peaks) averaged 18 fire episodes/1000 years from 3800 to 2125 cal year BP, and increased from 16 fire episodes/1000 years at 2125 cal year BP to 22 episodes/1000 years at 750 cal year BP, a period when the pollen data suggest that steppe vegetation yielded to increasing patches of forest cover. Between 2125 and 750 cal year BP, increased forest cover produced more background charcoal than before and after this period, when vegetation was dominated by steppe. Between 750 and 75 cal year BP steppe has expanded and fire episode frequency averaged 33 episodes/1000 years, increasing to a maximum of 40 episodes/1000 years at ca. 300 cal year BP and then decreasing to present levels. Since AD 1880, the pollen record indicates an increase in shrubs and grasses from AD 1895 to 1960 as a result of vegetation changes associated with timber harvesting and livestock grazing. No fires have been documented in the Foy Lake watershed since AD 1880. Charcoal from the extralocal fires of AD 1910, burning over 4,111,249 ha in Idaho, Montana, and Wyoming, however, is present in Foy Lake. Between AD 1970 and 2000, increased arboreal pollen in the record is consistent with observations that the forest has become more closed. The activities of Euro–Americans have led to a decline in forest cover between AD 1880 and 1970, followed by a recent increase as trees are now growing in areas previously occupied by steppe. Euro–Americans are likely the cause of a reduction in fire activity in watershed since AD 1880.     

Hillslope gullying in the Solway Firth — Morecambe Bay region, Great Britain: Responses to human impact and/or climatic deterioration?

In the Solway Firth — Morecambe Bay region of Great Britain there is evidence for heightened hillslope instability during the late Holocene (after 3000 cal. BP). Little or no hillslope geomorphic activity has been identified occurring during the early Holocene, but there is abundant evidence for late Holocene hillslope erosion (gullying) and associated alluvial fan and valley floor deposition. Interpretation of the regional radiocarbon chronology available from organic matter buried beneath alluvial fan units suggests much of this geomorphic activity can be attributed to four phases of more extensive gullying identified after 2500–2200, 1300–1000, 1000–800 and 500 cal. BP. Both climate and human impact models can be evoked to explain the crossing of geomorphic thresholds: and palaeoecological data on climatic change (bog surface wetness) and human impact (pollen), together with archaeological and documentary evidence of landscape history, provide a context for addressing the causes of late Holocene geomorphic instability. High magnitude storm events are the primary agent responsible for gully incision, but neither such events nor cooler/wetter climatic episodes appear to have produced gully systems in the region before 3000 cal. BP. Increased gullying after 2500–2200 cal. BP coincides with population expansion during Iron Age and Romano-British times. The widespread and extensive gullying after 1300–1000 cal. BP and after 1000–800 cal. BP coincides with periods of population expansion and a growing rural economy identified during Norse times, 9–10th centuries AD, and during the Medieval Period, 12–13th centuries AD. These periods were separated by a downturn associated with the ‘harrying of the north’ AD 1069 to 1070. The gullying episode after 500 cal. BP also coincides with increased anthropogenic pressure on the uplands, with population growth and agricultural expansion after AD 1500 following 150 years of malaise caused by livestock and human (the Black Death) plagues, poor harvests and conflicts on the Scottish/English border. The increased susceptibility to erosion of gullies is a response to increased anthropogenic pressure on upland hillslopes during the late Holocene, and the role of this pressure appears crucial in priming hillslopes before subsequent major storm events. In particular, the cycles of expansion and contraction in both population and agriculture appear to have affected the susceptibility of the upland landscape to erosion, and the hillslope gullying record in the region, therefore, contributes to understanding of the timing and spatial pattern of human exploitation of the upland landscape.     

History of a presently slightly acidic lake in northeastern Alberta,Canada as determined through analysis of the diatom record

The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.     

A 7000-Year Record of Environmental Change,Bear Lake,Rocky Mountain National Park,USA

Few long-term records of the fire history of Rocky Mountain National Park exist. Data from a lake sediment core was used to reconstruct changes in vegetation and fire frequencies over the last 7000 cal yr. Bear Lake is a high-elevation lake surrounded by subalpine vegetation in Rocky Mountain National Park, Colorado. Pollen data indicate that a warm and dry climate prevailed between ca. 7000 and 5000 cal yr BP. Temperatures increased until shortly before ca. 3500 cal yr BP when evidence for a marked decline is seen. Cooler-than-present conditions were maintained until ca. 1700 cal yr BP, when conditions transitioned to more like those of the present-day climate. Based on macroscopic charcoal analyses, fire frequency had varied between two and five episodes per 1000 years. The largest peak in charcoal was at ca. 590 cal yr BP. The fire return interval has varied with climate over time; however, we calculate a fire return interval of 325 years over the past 7000 years. Given these results, fire activity is likely to increase under current Intergovernmental Panel on Climate Change climate projections of an increase in annual temperatures.     

Using multiple archives to understand past and present climate–human–environment interactions: the lake Erhai catchment,Yunnan Province,China

A 6.48 m sediment core sequence from Erhai lake, Yunnan Province, provides a multi-proxy record of Holocene environmental evolution and human activity in southwest China. These sedimentary records provide proxy time series for catchment vegetation, flooding, soil erosion, sediment sources and metal workings. They are complemented by independent regional climate time-series from speleothems, archaeological records of human habitation, and a detailed documented environmental history. The article attempts to integrate these data sources to provide a Holocene scale record of environmental change and human–environment interactions. These interactions are analysed in order to identify the roles of climate and social drivers on environmental change, and the lessons that may be learned about the future sustainability of the landscape. The main conclusions are: lake sediment evidence for human impacts from at least 7,500 cal year BP is supported by a terrestrial record of cultural horizons that may extend back to ∼9,000 cal year BP. A major shift in the pollen assemblage, defined by detrended correspondence analysis, at ∼4,800 cal year BP marks the transition from a ‘nature-dominated’ to a ‘human-dominated’ landscape. From 4,300 cal year BP, a change in river discharge responses may signal the beginning of hydraulic modification through drainage and irrigation. Major increases in disturbed land taxa and loss of forest taxa from 2,200 cal year BP onward, also associated with the start of significant topsoil erosion, register the expansion of agriculture by Han peoples. It is also the start of silver smelting linked to trade along the SW Silk Road with Dali becoming a regional centre. Peak levels of disturbed land taxa, topsoil and gully erosion are associated with the rise and fall of the Nanzhao (CE 738–902) and Dali (CE 937–1253) Kingdoms, and the documented environmental crisis that occurred in the late Ming and Qing dynasties (CE 1644–1911). The crisis coincides with a stronger summer monsoon, but exploitation of marginal agricultural land is the main driver. These historical perspectives provide insight into the resilience and sustainability of the modern agricultural system. The largest threat comes from high magnitude-low frequency flooding of lower dry farmed terraces and irrigated valley plains. A sustainable future depends on reducing the use of high altitude and steep slopes for grazing and cultivation, maintaining engineered flood defences and terraces, and anticipating the behaviour of the summer monsoon. This article is based on a keynote address delivered by John Dearing at the 10th International Paleolimnology Symposium, June 2006, Duluth, Minnesota, USA.     

长江三峡考古遗址文化断层研究

长江三峡考古遗址的地理位置、14C 测年数据及考古学文化年代、地面高程、文化层的连续性、文化层的埋深和厚度、文化断层的埋深和厚度等统计数据综合分析的结果表明长江三峡地区新石器时代到历史时期考古遗址沉积地层中有5 期文化断层: (1) 城背溪文化末期到大溪文化初期(7200~6000a BP); (2) 大溪文化关庙山类型三期(5800~5500a BP); (3) 夏代 (4000~3500a BP); (4) 春秋晚期至战国时期(2500~2200a BP); (5) 宋代以来(960A.D.以来)。探讨文化断层与灾变事件发生之间的关系, 从而揭示三峡地区史前和历史时期环境演变过程及其与人类活动的影响。     

Fine Resolution Palaeoecology Confirms Anthropogenic Impact During the Late Holocene in the Lower Hawkesbury Valley,NSW

This study reconstructs environmental conditions at Mill Creek, within the lower Hawkesbury-Nepean Valley, by the use of fine resolution palaeoecological studies of sediments. Archaeological surveys and historical accounts are used to investigate ecosystem response to known human activities. Research found that during prehistoric times (between 820 BP and the 1790s), the study area was well vegetated with dry sclerophyll communities on the valley sides, and a highly productive wetland community occupied the moist valley floor. Valley sides were generally stable, and little fire was evident in the landscape. Sediment was mainly deposited on the valley floor from the Hawkesbury-Nepean River during flood. After European settlement in the 1790s, sedimentation rates and charcoal preserved within sediments increased significantly, probably due to agricultural and clearing practices, both locally and upstream. Consequently, vegetation communities were altered. In 1967, the study area became part of Dharug National Park. Forestry and agricultural activities were excluded from the catchment, and the fire regime was reduced in frequency. Present-day vegetation communities appear to be becoming more like those that prevailed during prehistoric times. However, sedimentation rates remain elevated due to continued disturbance to the greater Hawkesbury River catchment. Recent poor water quality, coupled with a succession of floods on the Hawkesbury River, has increased wetland productivity levels, and their spatial extent, at Mill Creek.     

全新世巢湖沉积物炭屑记录的火环境变化

通过对巢湖湖泊沉积岩芯不同粒级炭屑浓度和磁化率等指标变化的研究,揭示了全新世以来的区域火环境变化及人类活动影响。结果表明：① 全新世早期,气候由寒冷干旱向温暖湿润过渡,但仍较为干旱,炭屑浓度也相对略高,火活动比较频繁,地方性和区域性火时有发生;② 全新世中期是最温暖湿润的适宜期,炭屑浓度出现全新世以来的低值,火活动微弱,但在文化兴盛时期炭屑浓度出现峰值,则归因于人类活动的影响;③ 全新世晚期,在气候趋于凉干的环境背景下,人类活动的增强更加剧了火活动的频率和强度,炭屑浓度大幅增加;④ 近200 a炭屑浓度的降低可能与湖区附近已没有足够生物量引起火灾有关。     

基于孢粉的中全新世以来共和盆地人类活动探讨

采用1310个中国北方表土孢粉,共和盆地达连海化石孢粉数据,利用现代类比法,重建7.0cal kaBP共和盆地的人类活动指数(HI),结果显示:7~5.4 cal kaBP,人类活动非常微弱,盆地内主要为细石器狩猎采集活动,对自然环境影响非常有限;5.4~4.6 cal kaBP人类活动开始显现,宗日文化的定居、农业活动,已经开始对自然环境产生影响;进入3.8~2.0 cal kaBP的青铜时代,HI指示人类活动显著增强,这与当时社会及自然环境背景紧密相关,当时卡约文化遗址数量多达260余处,人口增长,畜牧业确立与普及,风沙活动较弱;1.8~1.2 cal kaBP HI达到峰值,此时汉—唐时期盆地内出现了城址,聚落建制和规模大大提高,人口数量较为稳定,畜牧业发展达到鼎盛,风沙活动较为微弱,人类活动强度达到顶峰;1.0 cal kaBP之后,人类活动相对减弱,这与盆地内风沙活动开始增强,聚落和人口减少,及其畜牧业衰落有关。重建的HI阶段性变化特征与盆地内人类活动的考古和历史记录较为吻合,揭示共和盆地人类活动强度变化,主要受聚落、人口、经济活动形式和沙地变化等自然—社会因素共同驱动,也说明采用孢粉记录来指示过去人类活动变化是可行的。     

Palaeoenyironments and human impact at Burraga Swamp in Montane rainforest,Barrington tops National Park,New South Wales,Australia

Palaeoecological studies have identified the broad patterns of environmental and climate change in highland south‐eastern Australia, but the detail of human impact on a variety of parameters and their interlinkages is largely missing. This study compares the erosion, productivity, fire and vegetation history in prehistoric and historical times at Burraga Swamp in montane rainforest in New South Wales. The known human impact is meagre; the major presently sustained impacts involve forestry in the surrounding sclerophyll forests and a low level of visits to the swamp by day‐walkers. While no significant changes in the largely oligotrophic conditions or in fire frequency were detected, changes in erosion rates and some vegetation change can be attributed to impacts since European settlement. There has been a small decline in eucalypts and a loss of fern cover, while grasses, Urtica and exotic species have expanded. It is clear that upland sites are sensitive to environmental change including low‐level human impact.     

Long-Term Fire Trends in Hispaniola and Puerto Rico from Sedimentary Charcoal: A Comparison of Three Records

Charcoal preserved in lake sediments is commonly used to reconstruct past trends in fire occurrence. However, interpretation of the charcoal record is often complicated, as changes in charcoal influx could represent natural shifts in fire regimes associated with changes in climate, changes in vegetation, or changes in patterns of anthropogenic burning. Here we examine sedimentary charcoal records from three lakes on the Caribbean islands of Hispaniola and Puerto Rico: Laguna Saladilla in the Dominican Republic, Lake Miragoane in Haiti, and Laguna Tortuguero in Puerto Rico. All records are based on microscopic charcoal fragments quantified from pollen slides and cover the last 7,000 or more years of the Holocene. We compare charcoal influx values to archeological and palynological evidence of human activity and explore the role of increasing winter insolation over the Holocene in driving increased charcoal deposition beginning ca. 6,000–5,000 cal yr BP. An increase in charcoal influx at Laguna Tortuguero at ca. 5,200 cal yr BP, previously interpreted as a signal of human settlement predating archeological evidence, may instead reflect insolation-driven shifts in winter drying that led to more frequent and possibly more intense natural fires. Decreased charcoal influx around 3,200 cal yr BP may signal human modification of the environment that altered fire frequency and/or intensity. Comparing the records from these three lowland Caribbean sites highlights possible intervals of synchronous, climate-driven burning as distinct from more localized anthropogenic burning.     

Fan beheading and drainage diversion as evidence of a 3200-2800 BP earthquake event in the Esmeraldas-Tumaco seismic zone: A case study for the effects of great subduction earthquakes

The San Lorenzo area belongs to the Esmeraldas–Tumaco seismic zone where some of the strongest earthquakes of South America occurred during the 20th century. This paper provides evidence for a succession of geomorphic changes characterized by the disruption of the Quaternary drainage network and the reshaping of the Cayapas–Santiago estuary. The rise of the La Boca uplift bordered by the La Boca and San Lorenzo faults is responsible for the southward diversion of the Palabi, Tululbi, Bogotá and Carolina rivers toward the Santiago and Cayapas rivers. The increase of the discharge directed to the Cayapas River generated the change of the channel pattern downstream from the confluence, and the avulsion of a new estuary through the coastal plain. According to the dating of beach ridges the avulsion occurred in the period 3200–2800 BP. This period corresponds also to a faster accretion of the beach ridge margin, interpreted as a response to a small uplift of the shore. The coherency of the three morphologic evidences—diversion of drainage network, avulsion and increase of coastal accretion—suggest a unique morphotectonic event, in relation with the activity of the Esmaraldas–Tumaco seismic zone. The opening of a direct communication through the mangrove margin may have brought favorable conditions for the development of the La Tolita archaeological site after 3000 BP.     

青藏高原海拔4000 m区域人类活动的新证据

青藏高原早期人类活动对于研究人类对极端环境响应与适应至关重要,4000 m海拔区域是人类向高原腹地迁移与扩散的关键。下大武1号遗址（XDW1）发现了11290±69 cal. a BP人类活动遗留的灰烬层,及其年代主要集中在11.2 cal. ka BP的细石叶、石片等石制品,是目前青藏高原4000 m海拔区域发现的最早的人类活动证据之一,说明在全新世伊始,人类就已经登上了海拔4000 m的青藏高原主体。据此推测人类在高原东北部扩张的时空演变过程,末次冰消期人类在海拔3000~4000 m高原东北缘青海湖盆地、共和盆地等区域活动,全新世初期扩张至4000 m海拔高原主体,全新世大暖期向高原腹地深入;上述3个人类活动阶段与末次冰消期环境的改善、全新世伊始的气候迅速转暖和全新世暖期盛期的到来等重大气候变化阶段密切相关。     

Diatom succession related to land use during the last 6000 years: a study of a small eutrophic lake in southern Sweden

Diatoms, pollen, physical and magnetic analyses of the sediments have been used to reconstruct the development over the last 6000 years of Lake Bussjösjön, a small lake in southern Sweden. Stratigraphic variations in a core of more than 15 m reveal changes in diatom assemblages, which correspond closely to changes in pollen, loss-on-ignition, and magnetic measurements that are related to land use and vegetation changes in the catchment. From ca 6000 BP to 2700 BP, a forest surrounded what was then a slightly eutrophic lake. The sudden appearance of Cyclostephanos dubius (Fricke) Round and several epiphytic/epipsammic diatoms at 2700 BP coincides with deforestation of the catchment (2700 BP to 2500 BP). A change in land use from predominantly pasture to arable land from 1300 BP to 1100 BP caused a high level of soil erosion with a decrease of C. dubius and the increase of Stephanodiscus species. An increase of epiphytic/epipsammic species coincides with increased arable farming and the change from a field-rotation to a crop-rotation system, and shows not only an increase in eutrophication but also changes in water depth. The influence of the catchment through time resulted in a smaller, shallower and eutrophic to hypertrophic lake.     

湖光岩玛珥湖晚全新世人类活动的叶蜡烷烃及其碳同位素沉积记录

通过对广东省湛江市湖光岩玛珥湖晚全新世沉积物中叶蜡烷烃（n-C₂₉,n-C₃₁,n-C₃₃）及其碳同位素（δ^{13C）进行分析,探讨该湖区约3.5 ka BP以来的植被变化历史,重点讨论了人类活动对该区域植被变化可能带来的影响。叶蜡烷烃δ13区C₃植物占绝对优势,表现为典型的C在1.7 ka BP以前偏轻,例如长链烷烃碳同位素在1.78 ka BP为-33.27 ‰,表明湖C₃型生态系统;在1.7 ka BP之后碳同位素明显快速偏重,在1.63 ka BP为-28.26‰,指示了C₄植物出现和C₄比例上升的过程。C₄植物的出现通常指示气候的干旱化,但是烷烃P_aq和ACL指数（平均链长）则显示湖区气候可能在1.7 ka BP后变湿润。通过与研究区甘蔗的同位素和ACL指数的比较分析,推测这一矛盾现象可能与湖区开始有一定规模的甘蔗种植活动有关。}     

9000 years of chironomid assemblage dynamics in an Alpine lake: long-term trends, sensitivity to disturbance, and resilience of the fauna

Subfossil chironomid analysis was applied to a sediment core from Sägistalsee, a small lake at present-day tree-line elevation in the Swiss Alps. During the whole 9000-year stratigraphy the chironomid fauna was dominated by taxa typical of alpine lakes. Major faunistic trends were caused by changes in accumulation rates of three taxa, namely Procladius, Stictochironomus, and Tanytarsus lugens-type. In the early Holocene Procladius was the dominant taxon. In younger samples, Stictochironomus tended to have as high or higher abundances and both taxa showed an increase in accumulation rates. A possible cause of this succession is the decrease of lake-water depth due to infilling of the lake basin and changes in associated limnological parameters. The immigration of Picea (spruce) at ca. 6500 cal. ¹⁴C yrs BP and the resulting denser woodlands in the lake's catchment may have promoted this trend. During three phases, from ca. 70–1450, 1900–2350, and 3500–3950 cal. BP, remains of Procladius, Stictochironomus, and Tanytarsus lugens-type are absent from the lake sediment, whereas other typical lake taxa and stream chironomids show no change in accumulation rate. Together with sediment chemistry data, this suggests that increased oxygen deficits in the lake's bottom water during these intervals caused the elimination of chironomids living in the deepest part of the lake. All three periods coincide with increased human activity in the catchment, as deduced from palaeobotanical evidence. Therefore, enhanced nutrient loading of the lake due to the presence of humans and their livestock in the catchment is the most likely cause of the increased anoxia. The chironomid fauna reacted the same way to intensive pasturing during the last ca. 1500 years as to Bronze Age clear-cutting and more moderate pasturing during the Bronze, Iron, and Roman Ages, suggesting that alpine lake ecosystems can be extremely sensitive to human activity in the catchment. On the other hand, the chironomid assemblages show a considerable amount of resilience to human disturbance, as the chironomid fauna reverted to the pre-impact stage after the first two periods of human activity. In recent years, even though pasturing decreased again, the chironomid fauna has only partly recovered. This is possibly due to other human-induced changes in the lake ecosystem, e.g., the stocking of the lake with fish. The chironomid stratigraphy is difficult to interpret climatologically as the strongest changes in chironomid-inferred temperatures coincide with periods of intensive human activity in the catchment.     

A multi-proxy holocene record of environmental change fromthe sediments of Skinny Lake, Iskut region, northern British Columbia,Canada

A stratigraphic record from a lake in the Central Plateau Regionof northern British Columbia reveals changes in environment and inferredclimate during the Holocene. Upon deglaciation (ca. 11500 BP), Skinny Lakebecame an embayment of an ice-dammed lake. High clastic sedimentationrates, an unstable landscape, and cool, possibly wet conditions likelypersisted until the early Holocene (ca. 9000 BP). From ca. 9000–8300 BPdeclining lake levels coupled with warm and dry conditions resulted in theformation of a prominent marl bed. A colonizing shrub and herb assemblagepersisted from 9000 BP until about 8300 BP when it was replaced by a spruce(Picea) and subalpine fir (Abieslasiocarpa) forest under slightly cooler and moister conditions. Themiddle Holocene was warmer-than-present, however, decreasingtemperature and increasing precipitation trends characterize the period fromca. 6000 BP–3000 BP. The transition to modern climate at 3000 BP isevident primarily in the lithostratigraphic record and corresponds with theinitiation of the Tiedemann glacial advance (ca. 3300 BP) in thesouth-coastal mountains of British Columbia. A significant change infossil pollen occurs at ca. 2400 BP and is characterised by an increase in pinepollen accompanied by decreases in alder (Alnus), spruceand fir. This also coincides with an increase in west-sourced exoticwestern hemlock (Tsuga heterophylla) and cedar type(Cupressaceae) pollen possibly transported by regional changes in air masscirculation patterns associated with Aleutian Low dynamics. This studydemonstrates that both lithostratigraphic and biotic proxies are helpful inreconstructing the timing and nature of climate change and that each may havevarying sensitivities to a particular type of change.