Autotrophic response to lake age,conductivity and temperature in two West Greenland lakes

Predicted changes in future climate necessitate a better understanding of climate impacts on lake biota, and the role of within-lake processes in modifying biotic response. Therefore we examined two climate-related variables (lake-water conductivity and GRIP temperatures) and lake ontogeny (lake age), to determine their influence on lake autotrophic communities in two neighbouring closed-basin lakes from West Greenland spanning the past 8,000 years. Using sedimentary pigments as proxies for lake autotrophic communities, we used synchrony and variance partitioning analyses (VPA) to test three specific hypotheses (a) that lake primary production would increase with lake age, (b) that climate would be the dominant process controlling autotrophic communities in these pristine lakes and (c) that the response of autotrophs to conductivity and temperature would vary depending upon the age of the lake. The results supported our first hypothesis, showing that lakes changed significantly with age, exhibiting an increase and decline in production in the first millennium of their existence, followed by a steady increase in production and increasingly frequent abrupt switches between mixed and meromictic states. The highly synchronous detrended response (r = 0.769) of lake autotrophs in the two study lakes, supported our hypothesis that climate was the dominant factor controlling lake autotrophs. However, VPA revealed that our climate-related variables (temperature and conductivity) explained only small amounts of variance alone (<12.9%) because covariance among them hindered efficient partitioning. In support of our third hypothesis, autotrophs responded significantly to temperature and conductivity in interaction with lake age (>50% variance explained) and with each other (>28% variance explained), such that autotrophic response changed as lakes aged. In spite of this, lakes sometimes responded independently, as a result of differences in the relative proportion of benthic to pelagic production and because of differences in lake morphometry. Together these results show that long-term control of lake autotrophs by climate and lake age is modified on shorter timescales by non-linear responses related to within-lake processes, and by the interaction of different climate variables with each other and with lake age.     

Holocene climate and environmental history of East Greenland inferred from lake sediments

Prediction of future Arctic climate and environmental changes, as well as associated ice-sheet behavior, requires placing present-day warming and reduced ice extent into a long-term context. Here we present a record of Holocene climate and glacier fluctuations inferred from the paleolimnology of small lakes near Istorvet ice cap in East Greenland. Calibrated radiocarbon dates of organic remains indicate deglaciation of the region before ~10,500 years BP, after which time the ice cap receded rapidly to a position similar to or less extensive than present, and lake sediments shifted from glacio-lacustrine clay to relatively organic-rich gyttja. The lack of glacio-lacustrine sediments throughout most of the record suggests that the ice cap was similar to or smaller than present throughout most of the Holocene. This restricted ice extent suggests that climate was similar to or warmer than present, in keeping with other records from Greenland that indicate a warm early and middle Holocene. Middle Holocene magnetic susceptibility oscillations, with a ~200-year frequency in one of the lakes, may relate to solar influence on local catchment processes. Following thousands of years of restricted extent, Istorvet ice cap advanced to within 365 m of its late Holocene limit at ~AD 1150. Variability in the timing of glacial and climate fluctuations, as well as of sediment organic content changes among East Greenland lacustrine records, may be a consequence of local factors, such as elevation, continentality, water depth, turbidity, and seabirds, and highlights the need for a detailed spatial array of datasets to address questions about Holocene climate change.     

Holocene palaeoecology of southwest Greenland inferred from macrofossils in sediments of an oligosaline lake

We retrieved a lake sediment record from an oligosaline, meromictic lake in southwest Greenland. The record spans the last 8,200 cal. years and was radiocarbon dated and analysed for macroscopic remains of plants and animals. The record extends the known history of several invertebrate species back in time, and provides minimum ages for their immigration to Greenland after the last deglaciation. Shells of the ostracod Ilyocypris bradyi were found in sediments dated to the time interval c. 7,000–6,500 cal. years BP. Shells of this species were found previously in a nearby oligosaline lake, where its occurrence was dated to about the same short time interval. The species is a thermophilous, non-arctic taxon that is absent from the present day Greenland fauna, and we suggest that its former occurrence in west Greenland marks the peak of the Holocene thermal maximum. This is in agreement with other records from Greenland.     

Ancient Eskimo dwelling sites and Holocene relative sea-level changes in southern Disko Bugt, central West Greenland

A coordinated geological-archaeological investigation has been carried out in southern Disko Bugt with the primary purpose of elucidating Holocene relative sea-level (RSL) changes. Two RSL curves representing the Early-Middle Holocene emergence of respectively southeastern and southwestern Disko Bugt have been constructed. Elevations of paleo-Eskimo sites of different ages have been surveyed and supplemented with similar elevations compiled from the literature. Detailed investigations have been carried out at two partly submerged Dorset I sites. At both sites, the stratigraphy of the foreshore has been recorded in terrain profiles.
It is concluded that the RSL history of southern Disko Bugt was one of steady emergence during Early-Middle Holocene followed by submergence in Late Holocene. The stratigraphy of the foreshore at the two Dorset I sites indicates that RSL has been at least 2-2.5 m below sea-level, and that the Transgression to present sea-level started after ca 1 ka B.P.     

The relative influences of climate and catchment processes on Holocene lake development in glaciated regions

Following deglaciation, the long-term pattern of change in diatom communities and the inferred history of the aquatic environment are affected by a hierarchy of environmental controls. These include direct climate impacts on a lake’s thermal and hydrologic budgets, as well as the indirect affects of climate on catchment processes, such as weathering, soil development, microbial activity, fire, and vegetation composition and productivity, which affect the transfer of solutes and particulates from the terrestrial ecosystem into the lake. Some of these catchment influences on lacustrine systems operate as time-dependent patterns of primary succession that are set in motion by glacier retreat. This paper provides a conceptual model of some dominant pathways of catchment influence on long-term lake development in glaciated regions and uses a series of paleolimnological examples from arctic, boreal, and temperate regions to evaluate the relative role of direct climate influences and of catchment processes in affecting the trajectory of aquatic ecosystems during the Holocene in different environmental contexts.     

Late Holocene lake-level and lake development signals in Lower Herring Lake, Michigan

Paleolimnological investigations of a marginal lake in the Lake Michigan basin revealed signals of long-term lake-level changes primarily controlled by climatic forces. Multiple analyses identified concurrent signals in sediment chemistry, grain size, and the microfossil record. Coarse-grained sediments, benthic diatoms, and nutrient response species increased as lake levels rose or fell. Finer sediments and higher percentages of taxa associated with stable thermocline conditions occurred during high-lake periods. Sedimentary evidence revealed corresponding strong high-lake signals c. 2500–2200, 1800–1500, 1170–730, and 500–280 BP. Low-lake periods occurred c. 1500–1170 and 700–500 B.P. An additional signal of lake-level decline was apparent beginning c. 280 BP but was interrupted by anthropogenic effects. Evidence of extreme low-lake levels (c. 1400–1300 BP), and signals for a medieval warming period (1030–910 BP) and the Maunder minimum (370–325 BP) indicate occurrence of short-lived dry climatic conditions.     

Coastal morpho-stratigraphy and Holocene relative sea level changes at Tuapaat, southeastern Disko Island, central West Greenland

The geomorphology of a coarse clastic coastal landscape at Tuapaat (69°24'N 52°36'W), southeastern Disko Island, central West Greenland, is described, and a coastal morpho-stratigraphy of the landscape is constructed. ¹⁴C ages on marine shells, whale bones, peat and gyttja are used to construct relative sea level changes throughout the Holocene. The emergence of SE Disko Island occurred in the early part of the Holocene. The Holocene marine limit is situated ca. 80m a.s.l. Between 4.7 and 1.0ka BP, the relative sea level approached the present sea level and it has probably been below present sea level between 4.7 ka BP and the present. The morpho-stratigraphy in the lowest part of the coastal landscape at Tuapaat suggests a complex late Holocene relative sea level history which includes at least 3-4 transgressions during the last ca. 2.5 ka.     

Constraining Holocene lake levels and coastal dune activity in the Lake Michigan basin

Sediment cores collected from embayed lakes along the east-central coast of Lake Michigan are used to construct aeolian sand records of past coastal dune mobility, and to constrain former lake levels in the Lake Michigan basin. Time series analysis of sand cycles based on the weight-percent aeolian sand within lacustrine sediment, reveals statistically significant spectral peaks that coincide with established lake level cycles in Lake Michigan and the Gleissberg sunspot cycle of minima. Longer cycles of ~ 800 and ~ 2200 years were also identified that correspond to solar cycles. Shorter cycles between 80 and 220 years suggest a link between coastal dune mobility, climate, and lake levels in the Lake Michigan basin. Radiocarbon-dated sedimentary contacts of lacustrine sediment overlying wetland sediment record the Nipissing transgression in the Lake Michigan basin. Lake level rise closely mimics the predicted uplift of the North Bay outlet, with lake level rise slowing when outflow was transferred to the Port Huron/Sarnia outlet. The Nipissing highstand was reached after 5000 cal (4.4 ka) BP.     

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.     

The role of pre‐existing structures during rifting,continental breakup and transform system development,offshore West Greenland

Continental breakup between Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, which are connected via the Davis Strait, a region mostly comprised of continental crust. This study contributes to the debate regarding the role of pre‐existing structures on rift development in this region using seismic reflection data from the Davis Strait data to produce a series of seismic surfaces, isochrons and a new offshore fault map from which three normal fault sets were identified as (i) NE‐SW, (ii) NNW‐SSE and (iii) NW‐SE. These results were then integrated with plate reconstructions and onshore structural data allowing us to build a two‐stage conceptual model for the offshore fault evolution in which basin formation was primarily controlled by rejuvenation of various types of pre‐existing structures. During the first phase of rifting between at least Chron 27 (ca. 62 Ma; Palaeocene), but potentially earlier, and Chron 24 (ca. 54 Ma; Eocene) faulting was primarily controlled by pre‐existing structures with oblique normal reactivation of both the NE‐SW and NW‐SE structural sets in addition to possible normal reactivation of the NNW‐SSE structural set. In the second rifting stage between Chron 24 (ca. 54 Ma; Eocene) and Chron 13 (ca. 35 Ma; Oligocene), the sinistral Ungava transform fault system developed due to the lateral offset between the Labrador Sea and Baffin Bay. This lateral offset was established in the first rift stage possibly due to the presence of the Nagssugtoqidian and Torngat terranes being less susceptible to rift propagation. Without the influence of pre‐existing structures the manifestation of deformation cannot be easily explained during either of the rifting phases. Although basement control diminished into the post‐rift, the syn‐rift basins from both rift stages continued to influence the location of sedimentation possibly due to differential compaction effects. Variable lithospheric strength through the rifting cycle may provide an explanation for the observed diminishing role of basement structures through time.     

A Compilation of Radiocarbon Dates from Disko Bugt,Central West Greenland/Meteorological Observations in 1996 at the Arctic Station,Qeqertarsuaq (Godhavn), Central West Greenland/A Discussion on Pingos in Mellemfjord,Disko, Central West Greenland/Open System Pingos in Mellemfjord,Disko, Central West Greenland: A Reply to Gurney and Worsley

188 new and previously published radiocarbon dates on Holocene material from Disko Bugt, central West Greenland, are presented together with relevant informations (laboratory number, place name, dated material, geografic coordinates, altitude and δ¹³C-value) about the dates.

In October 1990 an automatic weather station was established at the Arctic station (65 °15′N,53 °31′W), Qeqertarsuaq (Godhavn), Central West Greenland. The Station register parameters each 20 min., and the parameters have been described in an earlier paper in this journal by Nielsen et al. (1995). The present paper summarises main points of the climate during 1996.

Open system, or more correctly, hydraulic pingos, are genetically poorly understood. A continuing problem concerns their need for a perennial groundwater supply (intra- or sub-permafrost). This has to be maintained despite the existence of continuous permafrost in many areas where they are located. Recent work on Disko Island has suggested a new type of hydraulic pingo developing only in a “marsh environment”. It is argued that the marsh setting is not relevant to the formation of these features and that they are simply hydraulic pingos.

Abstract

A group of marsh initiated open system pingo remnants from the Iterdlagssûp kûgssua valley mouth, in Mellemfjord, Disko Island, Central West Greenland was described in Christiansen (1995). Gurney and Worsley (1997) state that the location of this group of pingo remnants in the Iterdlagssûp kûgssua valley mouth is of no relevance to their genesis, and that they presumably were the result of an assumed late Holocene sea level regression, causing permafrost to be established in the valley bottom. In this reply the arguments by Gurney and Worsley (1997) on the Iterdlagssûp kûgssua valley pingos are commented, and it is argued by way of sea level information, frost penetration and water supply that the special setting must indeed have caused pingo initiation and growth. Furthermore, the area has experienced a relative sea level rise during the late Holocene.     

The influence of Holocene tree-line advance and retreat on an arctic lake ecosystem: a multi-proxy study from Kharinei Lake,North Eastern European Russia

A consequence of predicted climate warming will be tree-line advance over large areas of the Russian tundra. Palaeolimnological techniques can be used to provide analogues of how such changes in tree-line advance and subsequent retreat affected lake ecosystems in the past. A Holocene sediment core taken from Kharinei Lake (Russia) was dated radiometrically and used for multi-proxy analyses with the aim of determining how climate and tree-line dynamics affected the productivity, community structure, carbon cycling and light regime in the lake. Pollen and macrofossil analyses were used to determine the dates of the arrival and retreat of birch and spruce forest. C:N ratios and percent loss-on-ignition were used to infer past changes in sediment organic matter. Visible-near-infrared spectroscopy and diatom analysis were used to infer past changes in lake-water carbon. Algal pigments and aquatic macrophytes were used to determine changes in lake productivity and light. Chironomids together with remains of the aquatic flora and fauna were used to provide information on past July temperature and continentality. Lake sedimentation was initiated shortly before 11,000 cal. years BP, when both chironomid- and pollen-inferred temperature reconstructions suggest higher summer temperatures than present, between 1 and 2°C warmer, and lake productivity was relatively high. A few trees were already present at this time. The spruce forest expanded at 8,000 cal. year BP remaining in the vicinity of the lake until 3,500 cal. year BP. This period coincided with a high concentration of organic material in the water column, and relatively high benthic productivity, as indicated by a high benthic: planktonic diatom ratio. After tree-line retreat, the optical transparency of the lake increased, and it became more open and exposed, and was thus subject to greater water-column mixing resulting in a higher abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.     

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.     

Holocene development and anthropogenic disturbance of a shallow lake system in Central Ireland recorded by diatoms

Three cores from two connected lakes in Central Ireland (Lough Kinale and Derragh Lough) were investigated using diatom analysis to establish the Holocene development of the lacustrine system, any local variations within the lakes and any anthropogenic influences. The study area was situated in a lowland location and the lakes were shallow, unstratified and interconnected. Litho-and bio-stratigraphical analyses of the lake cores and deposits beneath a mire separating the two lakes showed the changing spatial configuration of the lake system in the early Holocene and the separation of the initial lake into three basins (cf. lacustrine cells) and finally into two interlinked lakes. The evolution of the lake system is conceptualised as the development of distinct lacustrine cells, and its sediments have recorded changes in the physical (geography, depth and sedimentation) and chemical (water chemistry) properties of the lakes inferred through diatom analyses. The longest sequence, from the early Holocene, records fluctuating lake levels and these are correlated with geomorphological mapping and surveying of palaeoshorelines. The diatom assemblages of the upper 2 m of the three cores, covering approximately the last 2000–3000 radiocarbon years show considerable difference in trophic status and life-form categories. This is related to the location of the cores in the lake and also the distance from human settlement with particular reference to proximity to crannog (artificial island) construction and use. The most central core from the deepest part of Lough Kinale has the least representation of the human settlement and agricultural activity in the catchment and on the fringes of the lake, whereas the core taken from the edge of a crannog is able to identify when construction and use of the crannog occurred. The local nature of the palaeoecological response to human activity due to incomplete water mixing has the advantage of allowing the lake sediment cores to be used to determine spatially discrete settlement patterns.     

Wetland development, permafrost history and nutrient cycling inferred from late Holocene peat and lake sediment records in subarctic Sweden

Permafrost in peatlands of subarctic Sweden is presently thawing at accelerated rates, which raises questions about the destiny of stored carbon and nutrients and impacts on adjacent freshwater ecosystems. In this study we use peat and lake sediment records from the Stordalen palsa mire in northern Sweden to address the late Holocene (5,000 cal BP-present) development of the mire as well as related changes in carbon and nutrient cycling. Formation, sediment accumulation and biogeochemistry of two studied lakes are suggested to be largely controlled by the development of the mire and its permafrost dynamics. Peat inception took place at ca. 4,700 cal BP as a result of terrestrialisation. Onset of organic sedimentation in the adjacent lakes occurred at ca. 3,400 and 2,650 cal BP in response to mire expansion and permafrost aggradation, respectively. Mire erosion, possibly due to permafrost decay, led to re-deposition of peat into one of the lakes after ca. 2,100 cal BP, and stimulated primary productivity in the other lake at ca. 1,900–1,800 cal BP. Carbonate precipitation appears to have been suppressed when acidic poor fen and bog (palsa) communities dominated the catchment mire, and permafrost-induced changes in hydrology may further have affected the inflow of alkaline water from the catchment. Elevated contents of biogenic silica and diatom pigments in lake sediments during periods of poor fen and bog expansion further indicate that terrestrial vegetation influenced the amount of nutrients entering the lake. Increased productivity in the lake likely caused bottom-water anoxia in the downstream lake and led to recycling of sediment phosphorous, bringing the lake into a state of self-sustained eutrophication during two centuries preceding the onset of twentieth century permafrost thaw. Our results give insight into nutrient and permafrost dynamics in a subarctic wetland and imply that continued permafrost decay and related vegetation changes towards minerotrophy may increase carbon and nutrient storage of mire deposits and reduce nutrient fluxes in runoff. Rapid permafrost degradation may on the other hand lead to widespread mire erosion and to relatively short periods of significantly increased nutrient loading in adjacent lakes.     

The Holocene development of Kushu Lake on Rebun Island in Hokkaido, Japan

Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.     

The most extensive Holocene advance in the Stauning Alper, East Greenland, occurred in the Little Ice Age

We present glacial geologic and chronologic data concerning the Holocene ice extent in the Stauning Alper of East Greenland. The retreat of ice from the late-glacial position back into the mountains was accomplished by at least 11 000 cal years B.P. The only recorded advance after this time occurred during the past few centuries (the Little Ice Age). Therefore, we postulate that the Little Ice Age event represents the maximum Holocene ice extent in this part of East Greenland.     

Soil Formation on the Peninsula Tugtuligssuaq,Melville Bay,North West Greenland

Jakobsen, Bjarne Holm: Soil Formation on the Peninsula Tugtuligssuaq, Melville Bay, North West Greenland. Geografisk Tidsskrift 88:86–93 Copenhagen, 1988

The geography of soils has been studied on the peninsula Tugtuligssuaq. Melville Bay. N. W. Greenland. Soil formation and pedogenesis in relation to soil age and paleoclimate are discussed.     

小气候、雪盖及土壤湿度对高山生态系统功能的影响(英文)

1 Introduction High mountain landscapes above the tree-line are often regarded as close to nature, although human impact has obviously changed the environment, partly exceeding its carrying capac- ity (L?ffler, 2000). Due to their fragility regarding expe…