Benthic foraminifera and their stable isotope composition in sediment cores from Lake Qarun,Egypt: changes in water salinity during the past ~500 years

We studied the sedimentology, benthic foraminifera, molluscs, and δ¹⁸O and δ¹³C of Ammonia tepida tests in two late Holocene sediment cores from Lake Qarun (Egypt). The cores, QARU2 (upper section, 8.2 m) and QARU4 (1.4 m), span approximately the past 500 years of sedimentation. Benthic foraminifera first appeared in the upper part of QARU2 at 314 cm depth, ca. AD 1550. This depth marks the beginning of colonization of the lake by foraminifera and indicates a change in lake water salinity, as foraminifera cannot tolerate fresh water. Initially, three species of benthic foraminifera colonized the lake, Ammonia tepida, Cribroelphidium excavatum and Cribrononion incertum. Relative abundance of these species fluctuated throughout cores QARU2 and QARU4 and highest overall faunal diversity occurred at the beginning of the twentieth century. High relative abundances of C. incertum and deformed tests are attributed to periods of greater lakewater salinity. Peaks in both δ¹⁸O and δ¹³C indicate times of higher evaporation and reduced fresh water inflow. Inferred salinity was high around AD 1700 and after AD 1990. Rapid response of climate proxy variables indicates the high sensitivity of Lake Qarun to environmental changes over the past several 100 years. Increases in lakewater Mg concentration during past evaporative events, associated with less fresh water inflow, probably provided conditions suitable for C. incertum to build its white or transparent tests. Gradual decrease of C. incertum, until its disappearance at 100 cm depth ca. AD 1890, indicates a more persistent trend in lake water chemistry. Higher concentrations of dissolved sulphates were the likely cause of this species disappearance. Recent, twentieth-century sediments were deposited under optimal salinity (37‰) for benthic fauna, but further environmental changes are indicated by the decrease or disappearance of several benthic foraminifera and mollusc species. Intermittent hypoxia in the lake’s bottom waters, caused by cultural eutrophication, may account for these most recent changes.     

Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300?years

Stratigraphic variations in diatom composition and phytolith abundance in a sediment core from a small, hydrologically isolated waterbody, Lake Nhauhache, Mozambique, provide evidence of water-level fluctuations over the past ~2,300?years. Ten AMS radiocarbon dates on bulk sediment samples show that the lake came into existence about 2,300?years ago and that it has dried out since then, but only for brief time periods. Changes in the diatom assemblage composition indicate that lake level fluctuated in response to shifting humidity conditions. The changes reflect wetter conditions ca. 300 BC?CAD 800, more variable conditions between AD 800 and 1150, a distinct dry phase within the time span AD 1150?C1700 and a return thereafter to more humid conditions until present. There is general agreement between the Lake Nhauhache record and other records from the Southern Hemisphere summer rainfall region. This suggests that sediments from small interdunal lakes, which are abundant along the coast of southern Africa, provide reliable, regional paleoenvironmental information about an area from which more such data are needed.     

A paleolimnological record of rainfall and drought from East Java, Indonesia during the last 1,400?years

Variations in the location and strength of convection in the Western Pacific Warm Pool (WPWP) have a profound impact on the distribution and amount of global rainfall. Much of the variability in WPWP convection is attributed to variations in the El Ni?o-Southern Oscillation, for which the long-term trends and forcing mechanisms remain poorly understood. Despite the importance of WPWP convection to global climate change, we have very few paleohydrological reconstructions from the region. Here we present a new paleolimnologic and paleohydrologic record spanning the past 1,400 years using a multi-proxy dataset from Lake Logung, located in East Java, Indonesia that provides insights into centennial-scale trends in warm pool hydrology. Organic matter δ¹³C data indicate that East Java became wetter over the last millennium until ca. 1800 Common Era (CE), consistent with evidence for the southward migration of the Intertropical Convergence Zone (ITCZ) during this time. Superimposed on this long-term trend are four decade- to century-scale droughts, inferred from organic matter δ¹³C and calcite abundance data. They are centered at 1030, 1550, 1830, and 1996 CE. The three more recent droughts correlate with hydrologic anomalies documented in other proxy records from the WPWP region on both sides of the equator, and the two most recent droughts correlate in time with historically documented periods of multiple, intense El Ni?o events. Thus, our record provides strong evidence that century-scale hydrologic variability in this region relates to changes in the Walker Circulation. Human activity within the lake catchment is apparent since 1860 CE.     

Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka

We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO₂ (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC _Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ¹⁸O _Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.     

Flood response to rainfall variability during the last 2000 years inferred from the Taravilla Lake record (Central Iberian Range, Spain)

A sedimentological, geochemical and palynological study of the Taravilla Lake sequence (Central Iberian Range, NE Spain) provides a detailed record of allochthonous terrigenous layers that intercalate within the lacustrine sediments over the last 2000 years. These terrigenous layers are interpreted as the result of extreme hydrological events that caused higher clastic input to the basin. Anthropogenic influence caused by fires or deforestation is rejected as the main factor generating these layers because human impact, inferred from the pollen reconstruction, was minimal when the terrigenous layers reached their greatest frequency. The reconstructed occurrence of these events in the Taravilla Lake record is coherent with the paleoflood history of the Tagus River, characterized by a notable increase of extreme events at the beginning of the Little Ice Age. The Taravilla record suggests a relationship between the occurrence of extreme hydrological events, solar variability, and the North Atlantic Oscillation for the NE Iberian Peninsula.     

Distinguishing climatic from direct anthropogenic influences during the past 400 years in varved sediments from Lake Holzmaar (Eifel,Germany)

A 336-year floating varve chronology from Lake Holzmaar (Eifel, Western-Germany) covering the recent period has been established by microfacies analysis of thin sections. This sequence terminates 23 cm below the core top. In the top 23 cm, the varves are disturbed. By means of linear regression, the varve sequence was dated to the period AD 1607–1942. The influences of climatic variability and anthropogenic activities in the lakes catchment (e.g., forestry, agriculture) on lithology, fabric, and microfossil content of the varve sublaminae could be discriminated by applying statistical analyses (ordination and clustering) to the combination of the sublaminae in the varves and their thickness. Four clusters are obtained. Cluster 1 indicates cold springs, and shorter, cooler summers reflected primarily in below-average varve thickness (VT) for two stable phases: from AD 1650–1700 (during the Maunder Minimum) and from AD 1750–1785. Cluster 2 indicates years with conditions transitional to that indicated by cluster 1, characterized by vigorous and prolonged spring circulation with massive blooms of the nordic-alpine Aulacoseira subarctica. The samples assigned to Cluster 3 and Cluster 4 show the imprint of anthropogenic influences. Cluster 3 (AD 1795–1815 and AD 1825–1885) is characterized by above-average VT due to high detritus input throughout the year. The increased soil erosion can be linked to anthropogenic deforestation as a consequence of the production increase of the Eifelian iron industry at the end of the 18th century. This input dampens the climatic signal of a colder Dalton Minimum, which is reflected in a short drop in VT centered around AD 1810. At about AD 1885, Cluster 4 conditions, characterized by increased nutrient concentrations, low detritus input, and longer periods of stable summer stratification, become the stable state in Lake Holzmaar. They indicate the response of the lake to natural reforestation and the use of artificial fertilizers in the catchment, which began, according to historical records, in the 1850s in the Eifel region. The prolonged, stable summer stratification periods may be the first indication of the modern warming trend. A drop in VT centered around AD 1890 and recurring cluster-1 conditions may indicate the Damon Minimum.     

Lake evolution and hydroclimate variation at Lake Qinghai (China) over the past 32 ka inferred from ostracods and their stable isotope composition

We used ostracod species assemblages and their δ¹⁸O values in a 32-m sediment core from Lake Qinghai, China, along with information from cores collected at other sites in the lake, to infer lake evolution and hydroclimate changes since the last glacial. Dominant ostracod species Ilyocypris bradyi and its low δ¹⁸O values showed that Lake Qinghai was small in size or even consisted of several playa lakes, and the 1F core site could have even been in a wetland setting, under cold and dry climate conditions before 15.0 ka. Presence of Limnocythere inopinata with low δ¹⁸O values, and absence of I. bradyi after 15.0 ka, indicate the lake area increased or that the playas merged. The decrease or disappearance of ostracods with high δ¹⁸O values showed that the lake shrunk under dry climate from 12.0 to 11.6 ka. After 11.6 ka, hydroclimate shifts inferred from ostracod species changes (Eucypris mareotica and L. inopinata) and their δ¹⁸O values were as follows: (1) 11.6–7.4 ka—larger, but still small lake area with greater moisture availability under primarily dry climate conditions, (2) 7.4 to 3.2 ka—increasing lake level under a warmer and wetter climate, and (3) 3.2 ka to present—stable, large, brackish lake. The low ratio of lake water volume to runoff, and close proximity of the core site to freshwater input from the river mouth would have resulted in relatively lower ostracod δ¹⁸O values when Lake Qinghai was small in area during the interval from 32.0 to 15.0 ka. Lower ostracod δ¹⁸O values during interstadials and throughout the entire Last Glacial Maximum and early deglacial (ca. 24.0–16.0 ka) were caused by a greater contribution of seasonal meltwater from ice or snow and low incoming precipitation δ¹⁸O values related to cold climate conditions in the region at that time.     

Ostracode carbonate δ18O- and δ13C-signature of hydrological and climatic changes affecting Lake Neuchâtel,Switzerland, since the latest Pleistocene

Floating and grounded peat plateaus were studied in fens in the Yukon Territory (Canada). The peat deposit may be over 4 m thick and consists of a lower bed of aquatic peat overlain by humic fen peat, mesic fen peat and woody peat. Permafrost in the grounded peat plateaus is older than the 1200 year old White River Ash, whereas permafrost in the floating peat plateau is younger.Peat accumulation rates since 1200 years B.P. were greater in the fens (85–100 cm) than on the surface of the peat plateaus (25–55 cm). Where the peat plateau is free-floating, it will persist until the climate changes, causing the icy core to thaw. Where the peat plateau is frozen to the mineral substrate, it slowly drowns since the fen peat accumulates faster than the woody peat. This drowning results in degradation of the landform independently of the climate. Only degradation of floating peat plateaus can be used to identify climatic changes.This publication is the first paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr. P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.     

Vegetation changes in the southern Pyrenean flank during the last millennium in relation to climate and human activities: the Montcortès lacustrine record

We report vegetation changes of the last millennium inferred from palynological analysis of a sediment core from Lake Montcortès, situated at ~1,000 m elevation in the southern pre-Pyrenean flank. The record begins in the Middle Ages (~AD 800) and ends around AD1920, with an average resolution of ~30 years. The reconstructed vegetation sequence is complex and shows the influence of both climate and humans in shaping the landscape. Pre-feudal times were characterized by the presence of well-developed conifer forests, which were intensely burned at the beginning of feudal times (AD 1000) and were replaced by cereal (rye) and hemp cultivation, as well as meadows and pastures. In the thirteenth century, a relatively short period of warming, likely corresponding to the Medieval Warm Period, was inferred from the presence of a low Mediterranean scrub community that is today restricted to <800 m elevation. This community disappeared during Little Ice Age cooling in the fifteenth century, coinciding with a decline in human activities around the lake. Forest recovery began around AD 1500, at the beginning of the Modern period, coinciding with wetter climate. Forests, however, declined again during the seventeenth century, coinciding with maximum olive and hemp cultivation. This situation was reversed in post-Modern times (nineteenth century), characterized by an intense agricultural crisis and a significant decline in population that favored forest re-expansion. Correlations with nearby Estanya Lake, situated about 350 m below, provide a regional picture of environmental change. Besides some climate forcing evident in both sequences, human activities seem to have been the main drivers of landscape and vegetation change in the southern Pyrenean flank, in agreement with conclusions from other studies in high-mountain environments.     

Climate and human impact on a meromictic lake during the last 6,000 years (Montcortès Lake,Central Pyrenees,Spain)

Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km²; z _max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS ¹⁴C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.     

Sediment lithology and stable isotope composition of organic matter in a core from a cirque in the Krkonoše Mountains,Czech Republic

This study presents detailed lithostratigraphy and stable carbon and nitrogen isotopic variations in a 520-cm-long sediment core from a cirque basin in the Labsky důl Valley, Krkonoše Mountains, Czech Republic. Detailed study of the core reveals five major periods of sedimentation during the last 7600 years: silt and sand deposition during ~7.6–5.1 ka cal BP, Sphagnum peat accumulation during ~5.1–4.0 ka cal BP, sandy silt and sand during ~4.0–2.8 ka cal BP, raised peat bog during ~2.8–2.0 ka cal BP (Sphagnum peat), and sedimentation of sandy silt since ~2.0 ka cal BP. The δ¹³C values of the organic matter in the core vary in the range typical for C3 plants, from −24.35 to −27.68‰, whereas the δ¹⁵N values vary from −2.65 to +4.35‰. Core sections having ash contents ≥70% have δ¹⁵N > 1‰ and δ¹³C < −26‰, whereas those having ≤70% ash content have δ¹⁵N < 1‰ and δ¹³C > −26‰. Strong linear correlations are observed between δ¹³C and δ¹⁵N values as well as between C:N ratios and δ¹⁵N values in the horizons with ash content >10%, primarily for sand and silt horizons. On the other hand, poor correlations between δ¹³C and C:N ratio, as well as δ¹⁵N and C:N ratio, were observed in Sphagnum peat layers (45–125 and 185–265 cm). We conclude that the primary stable isotope variations are not preserved in the layers where significant correlation between δ¹⁵N and C:N ratio is observed. The relatively small δ¹³C variation in the uppermost Sphagnum peat layer suggests stable temperature during ~2.8–2.0 ka cal BP.     

Vegetation and sedimentation in the Lake Edward Basin, Uganda–Congo during the late Pleistocene and early Holocene

New palynological and sedimentological data obtained from the basal 3 m of core E96-2P from Lake Edward, Uganda–Congo record conditions wetter than present in the Edward basin from 11 000 to 6700 yr BP, in phase with other climate and vegetation records of northern hemispheric East Africa. Dominant pollen taxa include Celtis spp., Alchornea spp., Olea spp., and Moraceae indicating a moist semi-deciduous tropical forest. More xeric indicators such as Amaranthaceae and Asteraceae together with Poaceae comprise less than 5% of the pollen sum throughout this interval as compared with between 44 and 50% during a lake lowstand at 2000 cal yr BP and at the core top (near modern). The differences between these two assemblages suggest a 25 to 60% increase in annual precipitation during the early- to mid-Holocene as compared to modern (1500–2000 vs. 1200 mm/yr today). Early Holocene sediments in E96-2P are composed of laminated diatom oozes with moderately high total sulfur concentrations (2.8–4.7%) and no authigenic calcite, also indicative of conditions wetter than present. Between 9000 and 6700 yr BP, palynological and sedimentary proxies indicate sub-millennial-scale events related to changes in riverine discharge and runoff in the Edward basin. We attribute the variability in runoff, and hence precipitation, to Holocene variability in Indian or Atlantic Ocean SSTs or to shifts in the relative contribution of Indian and Atlantic moisture sources to the western Rift of equatorial Africa.     

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

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

Environmental changes in an alpine lake (Gossenköllesee, austria) over the last two centuries – the influence of air temperature on biological parameters

Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.     

Aquatic macrophyte vegetation development in Kråkenes Lake, western Norway, during the late-glacial and early-Holocene

Macrofossil analyses were carried out on the late-glacial and early-Holocene sediments of the radiocarbon-dated master core at Kråkenes Lake, western Norway, to investigate the aquatic vegetation changes. Ranunculus sect. Batrachium and Nitella were the earliest pioneers after deglaciation ca. 12,300 ¹⁴C yr BP. The Allerød vegetational succession was very slow during ca. 1000 ¹⁴C yrs in a cool climate and conditions that were similar to those above tree-line in Norway today. The rapid cooling at the start of the Younger Dryas stadial caused extensive disturbance, and with the development of an active cirque glacier in the catchment, plants and animals were almost exterminated from the lake by inflow of permanently cold and turbid water. Rising temperatures caused the glacier to melt at the end of the Younger Dryas. The biotic response to the rapid warming was immediate, with pioneer Ranunculus sect. Batrachium and Nitella expanding within 1-3 decades, closely followed by other elodeids. The lake witnessed a remarkable isoetid succession, with phases dominated by Limosella aquatica, Subularia aquatica, Elatine hydropiper, Isoetes echinospora, and, later, I. lacustris. About 800 yrs into the Holocene most of the macrophytes declined. The short-lived isoetids became extinct, but other taxa probably survived vegetatively. The reasons for this decline are unknown, but are probably related to nutrient depletion in combination with other factors. About 550 yrs later, I. lacustris and Nymphaea colonised, and a stable flora and vegetation developed. This study illustrates the large and rapid changes that occurred over the first 1400 yrs of the Holocene in the macrophyte flora and vegetation in Kråkenes Lake before stability was attained, pointing to the value of a palaeoecological study in tracing aquatic successions over time, and highlighting our lack of knowledge of the underlying ecological factors responsible for such rapid and marked changes.     

Last deglaciation and Holocene environmental change at high altitude in the Pyrenees: the geochemical and paleomagnetic record from Marboré Lake (N Spain)

Sedimentological, geochemical and magnetic data in a ~ 7-m sequence from Marboré Lake (2612 m asl, central Pyrenees) provide information about environmental variability since the last glacier retreat (14.6 cal ka BP) in high-altitude Pyrenean environments. The sediment sequence is composed of millimeter- to centimeter-thick rhythmites made of finer greyish laminae and coarser-grain, carbonate-bearing laminae arranged in varied patterns throughout the sequence. Finer laminae are interpreted as deposition during periods of predominantly ice-covered conditions, whereas coarser, carbonate-bearing sediments reflect periods of higher runoff. The age model, based on 13 ¹⁴C dates and a reservoir effect assessed with ²¹⁰Pb and ¹³⁷Cs, is coherent with known synchronous vegetation changes across the Pyrenees. Warmer intervals such as GI-1 (14.6–12.8 cal ka BP, Bølling/Allerød period), 10.4–8.2 cal ka BP in the Early Holocene, 7.5–5.2 cal ka BP in the Mid Holocene and the Medieval Climate Anomaly (AD 900–1300), are characterized by peaks in productivity and higher carbonate preservation. Deposition during colder periods such as GS-1 (12.8–11.7 cal ka BP), the Neoglacial (ca. 5.2–3.5 cal ka BP) and the Little Ice Age (last 400 years) show an increase in finer laminae. The presence of magnetite throughout the whole section suggests that Marboré Lake maintained predominantly oxic conditions since its formation. Changes in magnetic properties and the increase in magnetite from 3.5 cal ka BP to present, however, indicate a more oxic environment at the lake bottom during the last few millennia. The occurrence of Pb concentration peaks in sediments of Roman and modern age demonstrates the global distribution of heavy metal deposition, even into high-mountain lakes.     

Trace metal and geochemical variability during 5,500 years in the sediment of Lake Lehmilampi,Finland

A high-resolution geochemical profile from a 5,500-year-old sediment core of Lake Lehmilampi in eastern Finland was analyzed to study long-term trends and variability in element concentrations and accumulation rates. The accumulation rates of all studied elements followed the same trend, responding to changes in the total sedimentation rate. Concentration profiles differed among elements and showed considerable variation over time. Principal components analysis (PCA) was used on the concentration data to identify groups of elements that have similar geochemical controls. The first principal component was influenced by changes in mineral matter accumulation, and it incorporated elements that are associated with stable allochthonous minerals (such as Mg, K, Cs, Rb, Li, Ti and Ga), as well as elements in forms that become diluted when mineral matter increases (e.g., S, Fe and Mn). The second and third principal components showed that a large proportion of the variance was accounted for by elements with continuously increasing or decreasing concentrations related to pedogenetical development of the catchment soil. In the case of Hg, Pb and Cd, however, accumulation rates increased faster at the surface than is simply accounted for by changes in total sedimentation rates. For Cu, Cr, Ni and Zn, concentrations increased over the past 150 years, but there were no indications of a significant addition due to atmospheric deposition. These elements had more variable concentrations before the mid nineteenth century than after, as did elements that are often used for normalization. These findings suggest that lake sediments may not properly reflect the history of atmospheric metal deposition in remote areas.     

The recording of floods and earthquakes in Lake Chichój,Guatemala during the twentieth century

Laguna Chichój (Lake Chichój) is the only deep permanent lake in the central highlands of Guatemala. The lake is located in the boundary zone between the North American and Caribbean plates. The lake has been struck by devastating earthquakes and tropical cyclones in historical times. We investigated the imprint of twentieth century extreme events on the sedimentary record of this tropical lake using a bathymetric survey of the lake, coring the lake floor, and providing a chronology of sediment accumulation. The lake occupies a series of circular depressions likely formed by the rapid dissolution of a buried body of gypsum. ²¹⁰Pb and ¹³⁷Cs inventories and varve counting indicate high rates of sedimentation (1–2 cm year^?1). The annually layered sediment is interrupted by turbidites of two types: a darker-colored turbidite, enriched in lake-derived biogenic constituents, and interpreted as a seismite, and a lighter-colored type, enriched in catchment-derived constituents, interpreted as a flood layer. Comparison of our ¹³⁷Cs-determined layer ages with a catalog of twentieth century earthquakes shows that an earthquake on the Motagua fault in 1976 generated a conspicuous darker-colored turbidite and slumped deposits in separate parts of the lake. The entire earthquake inventory further reveals that mass movements in the lake are triggered at Modified Mercalli Intensities higher than V. Tropical cyclonic depressions known to have affected the lake area had limited effect on the lake, including Hurricane Mitch in 1998. One storm however produced a significantly thicker flood layer in the 1940s. This storm is reportedly the only event to have generated widespread slope failures in the lake catchment. It is thus inferred that abundant landsliding provided large amounts of concentrated sediment to the lake, through hyperpycnal flows.     

Midge-inferred temperature reconstructions and vegetation change over the last ~15,000?years from Trout Lake, northern Yukon Territory, eastern Beringia

Two cores from Trout Lake, northern Yukon, yielded quantitative estimates of summer air temperatures using fossil midge larvae. Warming began around 14,400?cal?yr BP, with inferred mean July air temperatures reaching values warmer than present by 12,800?cal?yr BP. A 1?°C cooling from 12,200 to 11,200?cal?yr BP closely corresponds with the Younger Dryas chronozone. A broad temperature maximum occurred between 10,800 and 9,800?cal?yr BP, with mean July air temperature about 2.2?°C warmer than present. This represents an early Holocene thermal maximum and coincides with increased organic content of the sediment. Both the shallow- and deep-water cores show similar temperature trends for their overlapping periods. The inferred rise in mean July air temperature at 14,200?cal?yr BP coincides with a shift in vegetation from an herb- to shrub-dominated landscape. In contrast, the increase in Alnus pollen at 6,400?cal?yr BP does not coincide with a change in temperature, but may be a response to a rise in precipitation.     

Phototrophic activity and redox condition in Lake Hamana, Japan, indicated by sedimentary photosynthetic pigments and molybdenum over the last ∼250 years

We analyzed photosynthetic pigments, total organic carbon (TOC), biogenic silica, and Mo, a redox-sensitive element, in ²¹⁰Pb-dated sediment cores to reconstruct the historical changes in primary productivity and anoxia in the central basin of Lake Hamana, a brackish lake in Shizuoka Prefecture, Japan, over the last 250 years. The algal photosynthetic pigments we analyzed included chlorophyll a (and its derivatives), chlorophyll b (and its derivatives), and carotenoids such as -carotene, lutein, zeaxanthin, diatoxanthin, fucoxanthin, alloxanthin, and -carotene. Marker pigments for phototrophic sulfur bacteria were also recorded, including okenone and bacteriopheophytin a, originating from Chromatium (a genus of purple sulfur bacteria), and isorenieratene and bacteriochlorophylls e ₁, e ₂ and e ₃ (and corresponding bacteriopheophytins) from brown Chlorobium (a brown-colored group of green sulfur bacteria). The occurrence of these pigments throughout the length of all cores indicates that the anoxia in Lake Hamana has existed over at least the last 250 years. The indicators related to primary productivity – TOC and pigments of aerobic and anaerobic phototrophs – and an indicator of anoxia, Mo, increased after 1860, indicating that productivity had increased in both the oxic and anoxic (sulfidic) zones. The depth profiles of the indicators in the sediment cores showed that among phototrophic sulfur bacteria, Chromatium preferentially increased relative to brown Chlorobium when the lake productivity was high, and hence high anoxia existed in the lake. This can be explained by a shallowing of the oxic/anoxic boundary zone due to changes in temporal and/or spatial extents of seasonal anoxia, which made the light intensity in the upper anoxic zone high enough for Chromatium to grow. The upper Chromatium layer may absorb the wavelengths of light that favor the growth of brown Chlorobium in the water column, resulting in a relative decrease in brown Chlorobium. During the 1950s, the trends among the indicators changed significantly. This change is attributed to the construction of training walls, built to direct tidal currents into the lake, on the Imagire-guchi Channel, the sole inlet of seawater to the lake, during 1954–1956, and the resultant increases of seawater intrusion and lake salinity. A decrease in okenone and bacteriopheophytin a, or in okenone/isorenieratene ratio, after 1960 accompanying a decrease in Mo, is attributed to a deepening of the anoxic zone, where the light intensity became too low for growth of Chromatium, more light-demanding than the brown Chlorobium. A decrease of zeaxanthin (cyanobacteria) after 1960 relative to lutein (green algae) and diatoxanthin (diatoms and dinoflagellates) indicated a change in algal assemblage, presumably due to the increased salinity. Principal component analysis with a data set of total algal carotenoids, okenone, isorenieratene, and Mo also suggested that a major change occurred around the 1950s.