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.     

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.     

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.     

Relationships between southern Chilean varved lake sediments,precipitation and ENSO for the last 600 years

In this paper, the relationships between paleo-precipitation and the regional influence of El Nino Southern Oscillation (ENSO) in South America are assessed from a high-resolution calendar varve-thickness record. Two short laminated sediment cores (53 and 61 cm length) from Lago Puyehue (40° S) are analysed by continuous varve measurements through the last 600 years. The calendar varve years are determined by the occurrence of graded planktonic-rich layers. The annual sediment accumulation rates are reconstructed by using the standard varve-counting methods on thin sections. The 1980–2000 varve-thickness record is interpreted in terms of climate through correlation with limnological and local monthly instrumental climate databases. The comparison between the standardized varve thickness with the instrumental records reveals a strong correlation (r = 0.75, р = 0.07) between the total varve thickness and the austral autumn/winter precipitation. We argue that strong austral winter winds and precipitation are the forcing factors for the seasonal turn-over and phytoplankton increase in the lake sediments. During strong El Nino events the precipitation and the winds decrease abnormally, hence reducing the thickness of the biogenic sediments deposited after the winter turn-over. Our results show one significant regional maximum peak of winter precipitation (>900 mm) in the mid 20th century and a significant period with lower winter precipitation (<400 mm) before the 15th century, i.e., the late Medieval Warm Period. The first peak in the mid 20th century is confirmed by the regional precipitation database. The influence of ENSO cycles over the last 600 years is assessed by spectral analysis in Fagel et al. (2007). The possible influence of the regional volcanism and/or the seismic activity on the local climate record is also discussed. This is the sixth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M. -F. Loutre and E. Chapron.     

Variations of monsoonal precipitation over the last 16,000 years in the eastern Nanling Mountains,South China

A 350-cm-long sediment core was recovered from Dahu Swamp in the eastern Nanling Mountains in south China for paleoclimatic investigation. Twelve ¹⁴C dates determined on organic-rich bulk samples establish a chronological sequence for this core and yield a bottom age of ~16,000 cal years BP. Multiproxies including dry bulk density, magnetic susceptibility, organic carbon isotope, median grain size, silt-size fraction and total organic matter content were used to study variations of precipitation in relation to the East Asian monsoon. The core sediments are characterized by shifts between the lacustrine sediments and marshy sediments, implying hydrological variations between expansion and shrinkage of the water body in the swamp, and suggesting relatively wetter and drier conditions, respectively. Two relatively wetter episodes lasting from ~15,000 to 14,000 cal years BP and from ~13,500 to 12,800 cal years BP were revealed, possibly corresponding to the Bølling and the Allerød warming events, and three relatively drier phases occurred between ~16,000 and 15,000 cal years BP, between ~14,000 and 13,500 cal years BP and between ~12,800 and 11,500 cal years BP. The three events synchronize with the Oldest Dryas, the Older Dryas and the Younger Dryas cooling events, respectively. A distinctly humid period lasted from ~10,000 to 6,000 cal years BP in the early- and mid- Holocene was interpreted as the Holocene Optimum period. Several short dry events were revealed, including the most pronounced one at ~8,100 cal years BP that coincides with the “8,200 year” cooling event. Multiproxies indicate an evidently dry climate prevailing in the mid-Holocene (from ~6,000 to 3,000 cal years BP), reflecting a weakening of the East Asian summer monsoon. The general trend of Holocene climate shows agreement with the 25°N summer solar insolation, suggesting that the orbitally induced insolation have played a key role in the Holocene climate in south China.     

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.     

Mining pollution triggered a regime shift in the cladoceran community of Lake Kirkkojärvi,southern Finland

Mining is one of the key industries in the world and mine water pollution is a serious threat to aquatic ecosystems. Historical monitoring data on the pollution history and impacts in aquatic ecosystems, however, are rarely available, so paleolimnological methods are required to explore the consequences of past pollution. We studied the history of cladoceran community dynamics in Lake Kirkkojärvi, southern Finland, including the periods before, during and after mining. We analyzed the geochemical composition and cladoceran subfossil remains in a ²¹⁰Pb-dated sediment core to evaluate the magnitude, rate, and direction of cladoceran community changes through time. The cladoceran community was altered significantly by mining activity that occurred during the mid-twentieth century. During more recent times, however, eutrophication effects have overridden the impacts of mining. After mining ceased, the cladoceran community underwent an abrupt regime shift towards taxa that reflect more eutrophic conditions. This change was caused by intensive farming activity and fertilizer use over the past few decades. The recent history of Lake Kirkkojärvi is a textbook example of a regime shift triggered by multiple human-caused stressors. Our findings also highlight the utility of cladocerans as bio-indicators in pollution research and illustrate the sensitivity of aquatic ecosystems to anthropogenic modification.     

The misrepresentation of the planktonic diatom assemblage in traps and sediments: southern Lake Malaŵi,Africa

Sediment trap collections near Cape Maclear, Lake Malai, were compared to phytoplankton and surface sediment diatoms to assess taphonomic variations. The sedimenting diatom community became progressively different from the diatom plankton with increasing depth: long Nitzschia species were strongly under-represented in the traps (annually, 53% among planktonic diatoms vs. 14% in the offshore 29 m trap; p0.005 by Kruskal-Wallis test), while Melosira was greatly over-represented in traps (32% vs. 57%; p<0.005). The abundances of the minor taxa (Rhopalodia, Fragilaria, Cymbella, and Surirella) were greatly enhanced in traps relative to the plankton, but they were still relatively uncommon (<3% of all diatoms each). Differences in grazing, dissolution, and sinking rates alone are insufficient to account for these distortions; a combination of these, plus perhaps unknown factors, strongly influence the deposited assemblage.These misrepresentations were also present at the sediment surface. The greatest discrepancy was noted for Melosira (32% of plankton vs. 53% of sediment surface diatoms; p<0.005) and for elongate Nitzschia species (53% of plankton vs. 0.8% in sediments; p<0.005). In Lake Malai, at least, paleolimnologists must not assume a straightforward correlation of modern and fossil assemblages.     

Climate oscillations evidenced by spectral analysis of Southern Chilean lacustrine sediments: the assessment of ENSO over the last 600 years

The Chilean Lake District (38–42°S) is strongly influenced by Southern westerlies-driven precipitations. At 40°S Lago Puyehue provides high resolution sedimentation rates (∼1–2 mm/yr) suitable for annual climate reconstruction. Several short and long sediment cores were collected in this lake. Their analysis aim at a better understanding of climate mechanisms related to ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis of two short varved cores collected from underflow and interflow key sites. According to varve chronology controlled by ¹³⁷Cs and ²¹⁰Pb profiles and chronostratigraphical markers, the short core from underflow site (PU-I) spans 294 ± 18 years and the core in the interflow site (PU-II) covers 592 ± 9 years. Several methods of spectral analysis were applied on the total varve thickness to identify potential periodicities in the signal. Blackman–Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular spectrum analysis were applied on the whole record. In addition, evolutive MTM and wavelet analyses allow to identify temporal influence of some periodicities. In the PU-I studied interval (AD 1700–2000), a period at ∼3.0 years appears in a large part of the interval, mostly in the recent part. Periods at ∼5.2 and ∼23 years also show up. PU-II record (AD 1400–2000) displays the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the sub-decadal periods identified by Dean and Kemp (2004) and linked to the El Nino Southern Oscillation and the Pacific Decadal Oscillation. Differences in the recorded periodicities between PU-I and PU-II sites are consistent with different sedimentation processes in the lake. According to climate instrumental data for the last 20 years, varves in PU-I site are mostly related to fluvial dynamics and regional climate factors, i.e., precipitation, temperature and wind. In PU-II site, varves increment is related to both regional and global climate forcing factors, i.e., El Nino Southern Oscillation. The evolutive MTM analysis and the wavelet analysis suggest a striking break in the periodicities at around AD 1820. Finally relationships between El Nino and longer term climate phase like the Little Ice Age (LIA) are also assessed. This is the seventh in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.     

Untangling the influence of in-lake productivity and terrestrial organic matter flux on 4,250 years of mercury accumulation in Lake Hambre, Southern Chile

There is ongoing debate about the relative influence of aquatic production, flux, and sedimentation of aquatic and terrestrial organic matter on mercury accumulation in lake sediments. In this study, lake sediments spanning the past 4,250 years, were collected from remote, organic-rich Lake Hambre, Patagonia (53° S) and investigated for changes in the accumulation of pre-anthropogenic mercury and organic matter of aquatic and terrestrial origin. Natural mercury accumulation varied by up to a factor of four, comparable to the recent anthropogenic forcing of the mercury cycle (factor 3–5). Hydrogen and Oxygen indices (HI and OI, Rock–Eval©) and nitrogen/carbon ratios of the organic matter, combined with multi-element sediment data, reveal intense changes in aquatic productivity as well as influx of terrestrial organic matter into the lake. Evaluation of the multi-element dataset using Principal Component Analysis shows clear covariation of mercury with other soil-derived elements such as copper and yttrium. This covariance reflects a common transport mechanism, i.e. leaching of trace-element-bearing organic matter complexes from catchment soils. Correlation between changes in aquatic productivity and mercury concentrations occurs in some sections of the record, but we do not suggest they are linked by a direct causal relationship. Mass balance approaches suggest that mercury scavenging and accumulation in this organic-rich lake is controlled by the supply of mercury from catchment soils rather than the amount of organic material produced within the water column. A common controlling mechanism, i.e. changing climate, however, is thought to independently drive variations in both the flux of terrestrial organic matter mercury complexes and aquatic productivity.     

Peat record of climate change for the last 3000 years in Yangmu, Mishan region of Sanjiang Plain

The study of global climate change for the last 2000 years is very important for predicting climate evolution in the future. In order to explore the evidence of climate change for that period, the Chinese scientists made convincing statements using high-resolution substitution data such as tree-ring, coral and ice core. Continuous accumulated peat sediment is the better substitution data to provide climate information. Selecting the peatlands with a certain area and less human interference, th…     

Chironomid assemblages in cores from multiple water depths reflect oxygen-driven changes in a deep French lake over the last 150 years

We sampled modern chironomids at multiple water depths in Lake Annecy, France, before reconstructing changes in chironomid assemblages at sub-decadal resolution in sediment cores spanning the last 150 years. The lake is a large, deep (z_max = 65 m), subalpine waterbody that has recently returned to an oligotrophic state. Comparison between the water-depth distributions of living chironomid larvae and subfossil head capsules (HC) along three surface-sediment transects indicated spatial differences in the influence of external forcings on HC deposition (e.g. tributary effects). The transect with the lowest littoral influence and the best-preserved, depth-specific chironomid community characteristics was used for paleolimnological reconstructions at various water depths. At the beginning of the twentieth century, oxygen-rich conditions prevailed in the lake, as inferred from M. contracta-type and Procladius sp. at deep-water sites (i.e. cores from 56 to 65 m) and Paracladius sp. and H. grimshawi-type in the core from 30 m depth. Over time, chironomid assemblages in cores from all three water depths converged toward the dominance of S. coracina-type, indicating enhanced hypoxia. The initial change in chironomid assemblages from the deep-water cores occurred in the 1930s, at the same time that an increase in lake trophic state is inferred from an increase in total organic carbon (TOC) concentration in the sediment. In the 1950s, an assemblage change in the core from 30 m water depth reflects the rapid expansion of the hypoxic layer into the shallower region of the lake. Lake Annecy recovered its oligotrophic state in the 1990s. Chironomid assemblages, however, still indicate hypoxic conditions, suggesting that modern chironomid assemblages in Lake Annecy are decoupled from the lake trophic state. Recent increases in both TOC and the hydrogen index indicate that changes in pelagic functioning have had a strong indirect influence on the composition of the chironomid assemblage. Finally, the dramatic decrease in HC accumulation rate over time suggests that hypoxic conditions are maintained through a feedback loop, wherein the accumulation of (un-consumed) organic matter and subsequent bacterial respiration prevent chironomid re-colonization. We recommend study of sediment cores from multiple water depths, as opposed to investigation of only a single core from the deepest part of the lake, to assess the details of past ecological changes in large deep lakes.     

Depositional rates and dating techniques of modern deposits in the Brno reservoir (Czech Republic) during the last 70 years

Facies analysis, magnetic susceptibility, and analysis of grain size, TOC content and isotopes (¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, ²²⁸Ra, and ²³⁸U concentrations) were used to determine the history of the modern deposits of the Brno reservoir. The sedimentary succession can be subdivided into two main units. The lower unit is formed predominantly by medium- to coarse-grained silty sands and is interpreted as a fluvial succession deposited before the Svratka River was dammed. The upper unit consists of brownish planar laminated silts and rarely of clayey or sandy silts and is interpreted as a product of the reservoir deposition. The concentrations of ²³⁸U reflect the history of uranium mining in the upper part of the Svratka River catchment. As a consequence, ²¹⁰Pb radionuclide concentrations cannot be used for establishing a sediment chronology. Concentrations of ¹³⁷Cs show two marked peaks, the upper of which is attributed to the Chernobyl reactor accident in 1986, and the lower one is attributed to the maximum rate of atomic weapons testing in 1963. From these peaks, mean depositional rates of 3.2 cm year⁻¹ for the time period of 1986–2007 and of 3.4 cm year⁻¹ between 1963 and 1986 are calculated. Based on the known age of the reservoir, which was constructed in 1939, we can also calculate mean depositional rate for the time period of 1939–1963, which is 3.1 cm year⁻¹.     

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.     

Variations in levels of deposition of atmosphere-borne industrial pollutants at three oligotrophic lakes in Ireland over the last 50–150 years: sediment-based archives of sources,levels and ecological sensitivity

Atmosphere-borne acidic pollutants constitute an important, potentially critical, risk to aquatic ecosystems. Long-time series datasets are a requirement in the assessment of the biological impacts of both increased and decreased loadings of acidic pollutants and allow for the anticipation of future pollution loadings and their effects that can be acted upon with some confidence. Little is known concerning levels of deposition of atmospheric pollutants and their effects on acid-sensitive aquatic ecosystems in Ireland. Moreover, unequivocal evidence of acidification by atmospheric deposition is limited. The objective of this study was to provide a preliminary assessment of pH response in three lake sites that are situated in acid-sensitive areas of Ireland, and to determine whether pH response tends to be uniform or differs due to variations in atmospheric loading and catchment properties. Levels of spheroidal carbonaceous particle (SCP) deposition have been used as a general indicator of the timing and extent of atmospheric pollutant loading and ecosystem response has been assessed by estimating diatom-inferred pH with a weighted-averaging calibration model (72 lakes) developed specifically for the Irish ecoregion. Levels of diatom-inferred pH (DI-pH) appear to respond to increased and decreased levels of atmospheric pollution loads—as represented by SCPs—at two of the three sites examined in the east and west of Ireland. In contrast, the site situated in the southwest of Ireland showed a varying and less directional DI-pH response, possibly due to lower deposition pressures, large lake size, and more buffered edaphic geology.