Holocene Environmental History Research Articles

A tale of two rivers

The Mississippi River is the largest and economically most important river in North America. It drains 40% of the conterminous United States, and like most of the world’s major rivers, it has been substantially altered by centuries of wholesale land-use conversion, urbanization, and hydrological modification. Those changes are manifest in seriously impaired water quality along many of its reaches and an everwidening zone of bottom-water hypoxia in the northern Gulf of Mexico, where it discharges to the ocean (Rabalais et al. 2002). At the other end of the Mississippi drainage system, some 850 river-kilometers from its headwaters in northern Minnesota, lies the confluence of two contrasting tributaries that in many ways epitomize the changes that the Mississippi has undergone (Fig. 1). One is the main-stem of the Mississippi itself, an often turbid and nutrient-rich waterway draining nearly half of the state of Minnesota (100,000 km) including the large urban centers of Minneapolis and St. Paul, as well as 28,000 km of row-crop agriculture in the southern half of the state. The other is the St. Croix River, a major tributary draining 22,000 km of eastern Minnesota and northwestern Wisconsin, renowned for its high water quality and biological diversity, and one of the eight original US waterways included under the 1968 National Wild and Scenic River Act. While the St. Croix is often cited as a pristine example of what the conjoining Mississippi and other north-temperate rivers may have looked like prior to Euro-American settlement, the truth is that water quality in both rivers is impaired, though the Mississippi clearly more so than the St. Croix. Both rivers suffer from an apparent excess of phosphorus and the Mississippi from high levels of suspended solids (turbidity). Major efforts are now underway to identify the various sources of impairment and to create longterm plans for reducing nutrient and sediment loads to the rivers. Such plans are technically known in the US as TMDLs (Total Maximum Daily Load). While such reduction goals are based largely on eco-region water-quality standards (as well as achievability), they require at their core an understanding of natural (background) loads such as might have occurred prior to Euro-American settlement. For most major rivers, an assessment of background conditions would be based on some type of watershed-runoff model or comparison to so-called ‘‘reference’’ systems thought to be minimally altered by human activity (e.g. Smith et al. 2003)––with all the uncertainty associated with such approaches. However, by fortunate ‘‘accident’’ This editorial is the introduction to a series of eight papers dedicated to the ‘‘Recent Environmental History of the Upper Mississippi River’’ published in this special issue of the Journal of Paleolimnology. D.R. Engstrom served as guest editor of the special issue.

Palaeoenvironment of mid- to late Holocene loess deposit of the southern margin of the Tarim Basin, NW China

Holocene environmental history in the Tarim Basin and the Taklimakan Desert is known mainly from isolated eolian and lacustrine deposits and remain puzzling. Here we present an adequately preserved loess section, covering the past 5000 years, at a highland (2,850 m a.s.l) on the northern slope of Kunlun Mountains. Pollen preserved in the section reveal a drying trend with significant moisture fluctuations around 3000–2600 cal yr BP and 1800 cal yr BP at the study site. Comparing the pollen, grain size from the same section provides a different scene occurred in the Tarim basin and the Taklimakan desert. Comparison of grain size to A/C ration of pollen suggests that active sand southward shifting in south margin of the desert is coincident with increasing moisture condition at the section locality, implying a casual link. This moisture pattern occurred at the upper and lower elevation of the slope is best explained by the vertical variation of local precipitation along the slope.

Late Weichselian and Holocene environmental history of Iceland

Late Weichselian and Holocene environmental history of Iceland

Holocene Environmental History and Evolution of a Tidal Salt Marsh in San Francisco Bay, California

Analyses of three sediment cores collected from a tidal salt marsh located on the western edge of San Pablo Bay in the San Francisco Bay estuary have produced long-term records of late Holocene marsh development. The records from these cores include a suite of elements, organic carbon content, fossil seeds, pollen, and stable carbon isotopes. The stratigraphy indicates fresher water conditions than present between 3400 and 2000 cal YBP. A tidal marsh became established at China Camp after about 2000 cal YBP; between 2000 cal YBP and approximately 700 cal YBP, conditions in the estuary were apparently more saline and variable. This interval was terminated by at least three or possibly five flood events, as evidenced by coarse clastic materials most likely washed down from the surrounding uplands. These floods represent high rainfall events, possibly El Niño years, and occurred during the late Medieval Climatic Anomaly. Greater plant diversity and pollen from some species with low salt tolerance are found in a core collected near the upland edge of the marsh and date to about 200 years ago, suggesting fresher conditions than today. Over the past 50 years, the diversity of marsh vegetation has decreased, and salt tolerant plants (especially Salicornia virginica) have become the dominant species. These changes are likely a result of the impacts of water diversions and upstream dams in the San Francisco Bay watershed.

Local Holocene environmental indicators in Upper Mesopotamia: Pedogenic carbonate record vs. archaeobotanical data and archaeoclimatological models

It has been suggested that pedogenic carbonate coatings on stones in archaeological sites can be used to reconstruct late Quaternary environmental changes in the area of the Fertile Crescent. This research compares a pedogenic carbonate record from southeastern Turkey with three other local or regional sources of palaeoenvironmental information in order to evaluate the correlation of this new proxy with other independent data from the area: (1) ancient plant macroremains from archaeological sites as ecological indicators, (2) stable carbon isotope composition of ancient plant macroremains available in the literature and from our own investigations in six mid-Holocene archaeological sites, and (3) modelled Holocene precipitation and temperature changes in the northern Fertile Crescent. Comparison involved selected palynological and isotopic data from lacustrine sediments as well as speleothem isotopic records in Near Eastern locations available in the literature. Although the data show a certain amount of variation of proxy records on the regional scale, for the northern Fertile Crescent (Upper Mesopotamia) they suggest three pronounced features of the Holocene environmental history: (1) a trend toward warmer temperatures may have affected the area during the early Holocene (between ca. 10,000–6000 cal BP), (2) increased moisture availability and relatively high temperatures in the mid-Holocene (ca. between 5000 and 4000 cal BP), and (3) a significant decrease in moisture availability around 4000 cal BP. Further research is needed to enhance the chronological resolution of both pedogenic carbonate and plant macrofossil records. This study stresses the complexity of the Holocene palaeogeography of the Near East and underlines the need for careful analysis of local sources of palaeonvironmental information in the region.

Holocene climate inferred from biological (Diptera: Chironomidae) analyses in a Southampton Island (Nunavut, Canada) lake

Concerns about the effects of global warming on Arctic environments have stimulated multidisciplinary research into the history of their long-term climatic and environmental variability to improve future predictions of climate in these remote areas. Here we present the first palaeolimnological study for Southampton Island using analyses of chironomids supported by sedimentological analyses, carried out on a 1 m long core retrieved from a lake located in the northeastern part of the island. This core was made up of marine sediments underneath 65 cm of freshwater lake sediments. A marine shell, humic-acids and chironomid head capsules were used to date this sequence. The Holocene environmental history of the lake consisted of two major contrasting periods. The first one, between about 5570 and 4360 cal. yr BP, was climatically unstable, with common postglacial chironomid taxa such as Corynocera oliveri-type, Paracladius and Microspectra radialis-type. This period also corresponded to the highest chironomid-inferred August air temperature (10°C) for the whole record and to significant increases in major chemical elements as detected by x-ray fluorescence. During the second period, which lasted from about 3570 cal. yr BP until the present, limnological conditions seemed to stabilize after a change to cold oligotrophic chironomid taxa, such as Heterotrissocladius subpilosus-group, with no major variations in the abundance of chemical elements. Inferred August air temperatures ranged between 8 and 9°C. This study provided unique information on the timing of the Holocene Thermal Maximum in the Foxe Basin area, a region with very little information available on long-term climate change. This region showed, so far, relatively few signs of recent climatic change, as opposed to other regions in the High Arctic.

Nationalized Nature on Picture Postcards: Subtexts of Tourism from an Environmental Perspective

Abstract Tourism is an important part of many national economies in Europe and beyond. Tourism imagery is a reflection of national environmental values and national claims upon nature. Postcards are a good example for the use of images in environmental history, and an important medium of tourist discourse. They are designed to sell tourist dreams about landscapes. In some ways, they are very uniform. They depict blue skies, green meadows, mountains, snow, whatever is to be sold as typical to the paying user. This uniformity is paradoxical, as what tourist destinations are striving to sell is their uniqueness. Upon close study, differences are revealed within the uniformity. Cards are full of national symbols and make claims about national natures. National styles of depicting nature have developed alongside increasingly targeted destination marketing. At a recent environmental history conference, a panel asked if nations had a place in environmental history. This paper argues, as did Ted Steinberg in his book on the history of the lawn in the U.S.A., that, for some topics, national boundaries do matter in environmental history. National styles are part of the socio-cultural legacies of different countries. Humans play a special role in picture postcards, being either sold as decorative “natives” to enhance the card’s picturesque appeal, or depicted as model tourists showing the proper use of the landscape portrayed, be it eating shrimp, mountaineering, or boating and fishing. The present paper uses postcards from several European countries to discuss national natures in comparison. The development of statements about national natures over time is interesting, as picture postcards do echo environmental values in different countries. The paper employs picture postcards to show how the environment was and is constructed for tourist use and for purposes of national interest. Postcards transmit environmental values. Since these are also national values, looking at cards reveals what may be a hidden agenda of difference within a politically but not culturally unified Europe.

Devensian Late-glacial environmental change in the Gordano Valley, North Somerset, England: a rare archive for southwest Britain

The Late-glacial and Holocene environmental history of the Gordano Valley, North Somerset, UK has been reconstructed using pollen, sediment particle size and mineralogical analyses and radiocarbon dating. A Devensian sediment ridge across the valley confined the waters of a small lake, within which the initial sedimentation was minerogenic. Radiocarbon dating of overlying organic-rich deposits suggests that this began late in the Dimlington Stadial c. 18,000–15,000 Cal. BP. Petrographic analyses indicate the minerogenic sediments were partly wind-blown in origin. Climatic amelioration during the Windermere Interstadial c. 15,000 Cal. BP encouraged a shift from minerogenic to biogenic sedimentation. A brief return to minerogenic sedimentation between c. 10,400 and c. 9,520 Cal. BP was followed by uninterrupted fen peat accumulation throughout the Holocene. The later minerogenic horizon appears to represent the Loch Lomond Stadial. Few stratigraphic sequences preserving the complete Devensian Late-glacial–Holocene transition exist in southwest Britain, making the sedimentary archive of the Gordano Valley valuable regionally for reconstructing Late-glacial climate change.

Protracted drought during the late Holocene in the Lacandon rain forest, Mexico

A 3.4 m core was obtained for paleoecological analysis from Naja Lake in the Lacandon region of southeastern Mexico. This is the first study of the Lacandon area aimed at reconstructing late Holocene environmental history. The basal section of the core yielded an AMS date of 2020 14C years b.p. The Naja pollen record shows that the lower montane rain forest characterized by Moraceae, Mimosoideae, Leguminosae and Combretaceae/Melastomataceae, coexisted with a pine-oak forest throughout the late Holocene. No Zea mays pollen was found during routine pollen counts, but the presence of both secondary pollen taxa and abundant charcoal particles suggest some degree of regional human impact. A marked increase in Pinus pollen, together with a reduction in lower montane rain forest taxa, is interpreted as evidence for a strong, protracted drought from 1260 to 730 14C years b.p.

Monitoring contrasting land management in the savanna landscapes of northern Australia.

We compared measures of ecosystem state across six adjacent land-tenure groups in the intact tropical savanna landscapes of northern Australia. Tenure groups include two managed by Aboriginal owners, two national parks, a cluster of pastoral leases, and a military training area. This information is of relevance to the debate about the role of indigenous lands in the Australian conservation estate. The timing and frequency of fire was determined by satellite imagery; the biomass and composition of the herb-layer and the abundance of large feral herbivores by field surveys; and weediness by analysis of a Herbarium database. European tenures varied greatly in fire frequencies but were consistently burnt earlier in the dry season than the two Aboriginal tenures, the latter having intermediate fire frequencies. Weeds were more frequent in the European tenures, whilst feral animals were most abundant in the Aboriginal tenures. This variation strongly implies a signature of current management and/or recent environmental history. We identify indices suitable for monitoring of management outcomes in an extensive and sparsely populated landscape. Aboriginal land offers a unique opportunity for the conservation of biodiversity through the maintenance of traditional fire regimes. However, without financial support, traditional practices may prove unsustainable both economically and because exotic weeds and feral animals will alter fire regimes. An additional return on investment in Aboriginal land management is likely to be improved livelihoods and health outcomes for these disadvantaged communities.

The Recent Environmental History of Tiger Leaping Gorge: environmental degradation and local land development in northern Yunnan

Over the past 150 years few Western and Chinese scholars have examined the nature of the environment in Hutiao xia (Tiger Leaping Gorge) and its various inhabitants. However, as a unique geographical and geological feature, one of Asia's deepest gorges, part of the headwaters of the Yangtze River, and a common destination for tourists, the gorge has taken on greater and greater significance. Americans, Chinese literati, Party Cadres, and others have commented on Tiger Leaping Gorge. This essay will examine both the local environment and local environmental practices related to the gorge. Considering historical and contemporary land use practices, the gorge and its changing status can act as a microcosm of western China's various kinds of land development strategies, their implications for local and larger concerns, and land use ideologies of various sorts. Through discussing soil composition, water management, natural vegetation patterns, population growth, and recent developments in tourism, this essay will discuss the human impact on the natural environment of Tiger Leaping Gorge between the 1870s and 2002. It will not only demonstrate patterns in environmental degradation based on both natural and human processes, it will also show that depending on the form of institutional authority and local management practices, the environment in the gorge has been most effected by the political and ideological changes of the last half-century in comparison to earlier times. However, despite increasing challenges in the face of various human factors, some recent environmental practices have succeeded in helping to minimize elements of local soil and vegetation degradation.

Late Pleistocene and Holocene environmental history of the Iguala Valley, Central Balsas Watershed of Mexico

The origin of agriculture was a signal development in human affairs and as such has occupied the attention of scholars from the natural and social sciences for well over a century. Historical studies of climate and vegetation are closely associated with crop plant evolution because they can reveal the ecological contexts of plant domestication together with the antiquity and effects of agricultural practices on the environment. In this article, we present paleoecological evidence from three lakes and a swamp located in the Central Balsas watershed of tropical southwestern Mexico that date from 14,000 B.P. to the modern era. [Dates expressed in B.P. years are radiocarbon ages. Calibrated (calendar) ages, expressed as cal B.P., are provided for dates in the text.] Previous molecular studies suggest that maize (Zea mays L.) and other important crops such as squashes (Cucurbita spp.) were domesticated in the region. Our combined pollen, phytolith, charcoal, and sedimentary studies indicate that during the late glacial period (14,000-10,000 B.P.), lake beds were dry, the climate was cooler and drier, and open vegetational communities were more widespread than after the Pleistocene ended. Zea was a continuous part of the vegetation since at least the terminal Pleistocene. During the Holocene, lakes became important foci of human activity, and cultural interference with a species-diverse tropical forest is indicated. Maize and squash were grown at lake edges starting between 10,000 and 5,000 B.P., most likely sometime during the first half of that period. Significant episodes of climatic drying evidenced between 1,800 B.P. and 900 B.P. appear to be coeval with those documented in the Classic Maya region and elsewhere, showing widespread instability in the late Holocene climate.

Holocene environmental change in a montane region of southern Europe with a long history of human settlement

This paper uses a palynological sequence to examine the Holocene (8390–160 cal yr BP) environmental history of the Sierra de Baza (Granada, southeastern Spain) with the goal of establishing the mechanisms exerting control over vegetation change. During the period ca 8390–6320 cal yr BP, Pinus dominated the pollen spectra, indicating a forested landscape over the high-elevation areas of the Sierra. From ca 6320–3800 cal yr BP, an expansion of deciduous oaks and other broad-leaf trees took place. After an optimum around 5800–5600 cal yr BP, mesophytes decreased in the 3800–2560 cal yr BP interval while a fire-prone scrub became established. The main loss of forest accompanied the spread of thorny matorral after ca 2560 cal yr BP. Overall, this mountain region has shown itself to be sensitive to a range of influences, among which a continental climate that has become increasingly arid over the last 5000 years, the scarcity of soils suitable for cultivation, a geology that includes sources of copper and other metals and, especially, the incidence of grazing as well as the repeated appearance of fires during the last 4000 years, are highlighted. The history of the vegetation of the Sierra de Baza seems clearly influenced by changes in local economy. Here we discuss how ecological transitions have interacted with cultural changes, with emphasis on the locally highly populated Chalcolithic (5700–4400 cal yr BP) and Argaric (4400–3550 cal yr BP) periods, as well as the Iberian period (3200–2220 cal yr BP). The sierra was abandoned during the Iberian Period which was, paradoxically, when the highest human impact on mountain vegetation is noticeable.

Late Glacial and Holocene environmental history of a buried ice-wedge system, Barrow, Alaska

The DFG funded project (RI 809/17-1) aims to reconstruct environmental changes in the Eurasian Arctic during the Late Quaternary interstadial and interglacial periods basing on microfossils records from permafrost sequences of NE-Siberia. We present the testate amoebae records from the Cape Mamontovy Klyk (Laptev Sea coast) covering the last 50 14C ka.Testate amoebae (Protozoa: Rhizopoda, Testacea) are one cell microorganisms with a distinct organic shell (testa). They are common in wet environments such as wetlands but they are also frequent in soils, especially in high organic content soils. Their well-defined ecological preferences for habitats and good preservation of fossil shells provide a basis for the rhizopod analysis as a method for reconstruction of environmental changes and climate in the past. The analysis show that differences of Pleistocene and the Holocene climatic conditions are well reflected in the testate amoebae records. The main characteristic of Mamontovy Klyk fossil rhizopod complexes is the dominance of eurybiotic soil genera (e.g. Centropyxis, Cyclopyxis, Plagiopyxis and Phryganella). Fluctuations in species abundances and diversity well reflect alternating of warm-wet and cold-dry periods during the Late Pleistocene, in particular the Karginsky interstadial ca 50-30 14C ka BP and Sartan stadial 30-10 ka 14C BP. It is obvious that climate changes were more drastic during the Pleistocene. These changes were at least partly responsible for the disappearance of some rare rhizopod species (e.g. Argynnia sp., Paraquadrulla irregularis). Considering the species diversity as a result of ecological and climatic conditions, we assume that habitat and climatic conditions during the periods with higher species diversity (e.g. Karginsky interstadial), were more favourable than during the Sartan stadial.

Holocene environmental history on the eastern slope of the Polar Ural Mountains, Russia

The Holocene environmental history of the eastern slope of the Polar Ural Mountains has been reconstructed using pollen, spores, algae and other microfossils from the Chernaya Gorka palsa section (67°05'N, 65°21'E, 170 m a.s.l.). An initial oligotrophic lake was formed at the study site c. 9800–9500 14C yr BP Although tundra communities dominated the vegetation in the area, birch and larch trees might have grown at lower elevations. Dry and disturbed soil habitats also occurred around the lake. Algae (mostly Pediastrum and Botryococcus) started to expand in the lake as climate gradually improved after c. 9500 14C yr BP. However, the role of mosses (mostly Calliergon and Drepanocladus) was most important for the infilling of the lake basin. Increased temperatures and subsequent improvement of hydrological conditions resulted in vegetation changes: stands of willows developed rapidly and the role of tree birch in the local vegetation increased. The lake was completely filled at c. 8600 14C yr BP. Peat accumulation started with Bryales mosses and, later, Sphagnum became dominant. Stands ofLarix, Picea and Betula became well developed during the Boreal climate optimum. Tree birch began to spread into the tundra. Different Bryales mosses formed peat c. 8000–6500 14C yr BP. Cyperaceae later became the main peat‐forming element. Dense spruce canopies with Larix sibirica and Betula pubescens surrounded the study site during the Atlantic period, pointing to the warmest climate during the Holocene. Summer temperatures might have been up to 3–4°C higher than today. However, a decline of spruce and an increase of birch around 6700–6300 14C yr BP may reflect some climate deterioration. There are no dated deposits younger than 6000 14C yr BP. It is assumed that Subboreal climate deterioration resulted in the development of permafrost and formation of the palsa at the site. The deposits, now protruding above the surrounding terrain, were eroded by wind, water and cryogenic processes.

Holocene environmental history on the eastern slope of the Polar Ural Mountains, Russia

Holocene environmental history on the eastern slope of the Polar Ural Mountains, Russia

Holocene Environmental and Climate History of Trettetjørn, a Low-alpine Lake in Western Norway, Based on Subfossil Pollen, Diatoms, Oribatid Mites, and Plant Macrofossils

ABSTRACTHolocene lake and catchment environmental history and regional climate are reconstructed from lake sediments at Trettetjørn, a small lake situated close to the present-day treeline in western Norway. Sediments began to accumulate in the lake ca. 8575 (± 115) cal yr BP. Pollen-inferred mean July temperatures (Tjul) fluctuated below 12°C with two cooler phases ca. 8400 and 8200 cal yr BP. The pollen-inferred annual precipitation (Pann) was lowest during the early Holocene and varied around 1600 mm yr−1. At the same time the highest diatom-inferred pH values (6.8) were reconstructed, probably due to input of base cations from the immature catchment soils. Betula pubescens became established around the lake ca. 8270 cal BP, soon followed by Pinus sylvestris. Maximum Tjul of ca. 12.5°C occurred from 7760 to 5200 cal yr BP. The oribatid mite assemblages confirm the vegetation development from semiopen grassland to forest. Inferred Tjul was variable after 5200 cal yr BP and declined markedly around 4175 cal yr BP. At the same time more oceanic conditions are inferred with changes in the vegetation and mite assemblages suggesting the expansion of mires. After ca. 5000 cal yr BP and towards the present, the diatom concentration in the core becomes low and variable, thickness of the valves within the same taxon varies, and sometimes diatom frustules are completely absent from the sediment column. The diatom valve dissolution has affected both the diatom assemblages and the diatom-inferred pH up to the present-day. All proxies suggest that human impact affected the catchment after ca. 2000 cal yr BP followed by more intensive impact after ca. 1555 cal yr BP. The enlargement of the settlement at Upsete during the construction of the Bergen–Oslo railway (1894–1909 AD) is reflected by a charcoal peak and the presence of spheroidal carbonaceous particles coincides with a change in the diatom assemblages.

Holocene environmental history in northwest Finnish Lapland reflected in the multi-proxy record of a small subarctic lake

A 2.5-m-long sediment core was retrieved from Lake Somaslampi, a small lake located in a kame field on the north slope of the Scandes Mountains in Finnish Lapland. Holocene environmental changes were inferred from the lithological, geochemical, pollen, diatom and Cladocera records stored in the lake sediment. The chronology was based on six radiocarbon AMS dates supported by a palynological control chronology. The sediment profile consists of a glacial sedimentary sequence truncated by a lacustrine one. A hiatus, tentatively correlated with climate cooling and advances of glaciers during the 8.2 ka yrs BP “Finse cooling Event”, occurs between these sequences. The glacial sequence was composed of fluvioglacial clastics, smoothly changing into glacio-lacustrine diatomaceous ooze deposited in a meromictic proglacial lake that covered the kame field. The meromixis was probably caused by the greater depth of the lake, the extended ice-cover, and the microbial mats covering large areas of the lake bottom. A distinct change in the biota of the glacio-lacustrine sediments indicates higher trophic conditions than during deposition of the fluvioglacial clastics. The late-Pleistocene vegetation was characterised by subarctic birch tundra vegetation (Betula–Salix–Ericaceae) with low biodiversity gradually changing to Betula–Pinus dominance in the early Holocene. The lake was deep and had a diatom inferred pH ~ 7 indicated also by the dominance of planktonic Cladocera. The base of the lacustrine sediment sequence (6,650–6,300 cal. BP) consisted of loess-rich sediment indicating an increase in eolian activity. This is also supported by the pollen record, which is dominated by more long-distant taxa such as Alnus and Pinus, and by the increased C/N ratio of the sediment. After the initial meromictic phase of the lake, an abrupt lowering of the water level occurred. Lake Somaslampi was isolated from the larger Pre-Lake Somas basin and became holomictic, shallow, much warmer and more productive, until the deterioration of climate around 3,000 yr BP and the increased input of clastics from the tundra soils. The vegetation followed the general climatic trend by gradually changing from the dominance of Betula and Pinus to the dominance of more tundra-related vegetation like Poaceae and Cyperaceae. However, the higher frequencies of planktonic Cladocera and centric diatoms in the most recent sediments indicates higher trophic conditions, increased turbulence and a prolonged ice-free period, which can possibly be linked to the recent climate warming especially in areas of higher altitude and latitude.

Holocene environmental history of Lake Vuolep Njakajaure (Abisko National Park, northern Sweden) reconstructed using biological proxy indicators

Abstract Holocene environmental and climatic changes are reconstructed using analyses of biological proxies in lake sediments from Vuolep Njakajaure, a lake located near the altitudinal treeline i ...

Environmental Changes at the Desert Margin: An Assessment of Recent Paleolimnological Records in Lake Qarun, Middle Egypt

Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Z max 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to 137Cs and 210Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year−1 during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water