Lake Record Research Articles

Lake sediment records of Holocene hydroclimate and impacts of the Mount Mazama eruption, north-central Washington, USA

Multi-proxy sediment records from Castor Lake and Scanlon Lake, north-central Washington, provide a late Quaternary perspective on lake/catchment hydrologic and ecosystem responses to climate change and the Mazama volcanic ashfall event. Analyses of authigenic carbonate mineral oxygen and carbon isotope values, organic carbon and nitrogen content, and sedimentological facies were conducted on sediment from Castor Lake in order to reconstruct lake/catchment hydrologic balance, the source and abundance of organic matter, and variations in lake level. Sedimentary facies characterization was conducted on the Scanlon Lake sediment in order to provide supporting evidence for the lake-level shifts inferred from the Castor Lake data. Marked changes in Castor Lake proxy values occur from 12,860 to 11,440 calendar years before present (yr BP), suggesting lake/catchment responses to the Younger Dryas cold reversal including higher lake levels, substantial variability in productivity, and increased catchment erosion. High δ18O values at 9630 yr BP indicate that lake levels were lower than at present and that the early Holocene was dry. Sedimentological analyses and δ18O data demonstrate that subsequent to the Mazama climactic eruption (∼7600 yr BP), a transition to the lowest lake levels of the Holocene occurred over several centuries, with maximum δ18O values at 7290 (90% uncertainty range: 7020–7500) yr BP and low lake levels persisting until 6190 (5960–6410) yr BP. Lithological changes in the Scanlon Lake record support these inferences. The lowest δ18O values of the Holocene occur in sediment from ∼5000 yr BP, indicating high lake levels at this time, after which a secular trend toward higher δ18O values and lower lake levels occurred. The prolonged lowstand in the centuries following the Mazama eruption suggests that catchment hydrologic characteristics were strongly impacted by the ashfall through effects on soil water retention capacity, that similar responses may have occurred in comparable settings within the Mazama ashfall zone, and that such volcanic events have the potential confound the climate signal in lake sediment records. Were a similar event to happen in the future, catchment hydrology, stream/river discharge, and lake/catchment ecosystem dynamics could be affected for centuries.

Glacial and environmental changes over the last 60 000 years in the Polar Ural Mountains, Arctic Russia, inferred from a high‐resolution lake record and other observations from adjacent areas

Our knowledge about the glaciation history in the Russian Arctic has to a large extent been based on geomorphological mapping supplemented by studies of short stratigraphical sequences found in exposed sections. Here we present new geochronological data from the Polar Ural Mountains along with a high‐resolution sediment record from Bolshoye Shchuchye, the largest and deepest lake in the mountain range. Seismic profiles show that the lake contains a 160‐m‐thick sequence of unconsolidated lacustrine sediments. A well‐dated 24‐m‐long core from the southern end of the lake spans the last 24 cal. ka. From downward extrapolation of sedimentation rates we estimate that sedimentation started about 50–60 ka ago, most likely just after a large glacier had eroded older sediments from the basin. Terrestrial cosmogenic nuclide (TCN) exposure dating (10Be) of boulders and Optically Stimulated Luminescence (OSL) dating of sediments indicate that this part of the Ural Mountains was last covered by a coherent ice‐field complex during Marine Isotope Stage (MIS) 4. A regrowth of the glaciers took place during a late stage of MIS 3, but the central valleys remained ice free until the present. The presence of small‐ and medium‐sized glaciers during MIS 2 is reflected by a sequence of glacial varves and a high sedimentation rate in the lake basin and likewise from 10Be dating of glacial boulders. The maximum extent of the mountain glaciers during MIS 2 was attained prior to 24 cal. ka BP. Some small present‐day glaciers, which are now disappearing completely due to climate warming, were only slightly larger during the Last Glacial Maximum (LGM) as compared to AD 1953. A marked decrease in sedimentation rate around 18–17 cal. ka BP indicates that the glaciers then became smaller and probably disappeared altogether around 15–14 cal. ka BP.

Vegetation and geochemical responses to Holocene rapid climate change in the Sierra Nevada (southeastern Iberia): the Laguna Hondera record

Abstract. High-altitude peat bogs and lacustrine records are very sensitive to climate changes and atmospheric dust input. Recent studies have shown a close relationship between regional climate aridity and enhanced eolian input to lake sediments. However, changes in regional-scale dust fluxes due to climate variability at short scales and how alpine environments were impacted by climatic- and human-induced environmental changes are not completely understood. Here we present a multi-proxy (palynological, geochemical and magnetic susceptibility) lake sediment record of climate variability in the Sierra Nevada (southeastern Iberian Peninsula) over the Holocene. Magnetic susceptibility and geochemical proxies obtained from the high mountain lake record of Laguna Hondera evidence humid conditions during the early Holocene, while a trend towards more arid conditions is recognized since ∼7000 cal yr BP, with enhanced Saharan eolian dust deposition until the present. This trend towards enhanced arid conditions was modulated by millennial-scale climate variability. Relative humid conditions occurred during the Iberian Roman Humid Period (2600–1450 cal yr BP) and predominantly arid conditions occurred during the Dark Ages and the Medieval Climate Anomaly (1450–650 cal yr BP). The Little Ice Age (650–150 cal yr BP) is characterized in the Laguna Hondera record by an increase in runoff and a minimum in eolian input. In addition, we further suggest that human impact in the area is noticed through the record of Olea cultivation, Pinus reforestation and Pb pollution during the Industrial Period (150 cal yr BP–present). Furthermore, we estimated that the correlation between Zr and Ca concentrations stands for Saharan dust input to the Sierra Nevada lake records. These assumptions support that present-day biochemical observations, pointing to eolian input as the main inorganic nutrient source for oligotrophic mountain lakes, are comparable to the past record of eolian supply to these high-altitude lakes.

A 14 ka high-resolution δ18O lake record reveals a paradigm shift for the process-based reconstruction of hydroclimate on the northern Tibetan Plateau

The influence of the mid-latitude westerlies (MLW) competing with the Asian summer monsoons (ASM) over the Tibetan Plateau (TP) remains a matter of discussion on how and to which extent both atmospheric systems have been controlling hydro-climate during the Holocene. Depleted oxygen isotopes in lake deposits were commonly interpreted in terms of enhanced summer monsoon moisture supply, implying a migration of the ASM deep into the interior of the plateau during Holocene periods. In order to test this relationship we used a high resolution oxygen isotope record (mean 20 yr resolution) in combination with carbonates and mineral phases, titanium flux, grain size and ostracod abundances derived from a 6.84 m long sediment core in the endorheic Kuhai Lake basin, north-eastern TP. The results confirm 1) continuous positive co-variance between enriched δ18Ocarb and total carbonates during the last 14 ka, indicative of dominant seasonal influence on multi-decadal to centennial scale isotopic signatures in lake water and respective carbonate precipitation, 2) negative co-variance between allochthonous sediment flux and δ18Ocarb (and carbonates) attributed to relative increase of flux rates during non-summer seasons, 3) correspondence of lake level variations with carbonate mineral phases and the occurrence/disappearance of ostracod assemblages, and 4) inverse relationships between isotopic signatures in ASM-dominated and MLW-controlled lake records across the TP. Enriched δ18Ocarb in Kuhai Lake sediments was primarily a result of high evaporation during the summer seasons, while ASM-related rainfall amount did not play an important role, likely counterbalanced by isotopic signatures from different water sources. Conversely, depleted δ18Ocarb was mainly attributed to water supply during non-summer seasons of colder temperatures and generally light isotopic signatures from MLW-derived sources. This finding may lead to a paradigm shift in such way that depleted δ18O in carbonates is primarily not the result from ASM-related rainfall as previously assumed. The reconstructed hydro-climatic history of Kuhai Lake indicates the dominance of westerly-derived climate during the Younger Dryas interval (12.8–11.5 ka) under very shallow pond-like conditions. Despite climate amelioration during the early Holocene (11.5–7.5 ka) hydrological conditions remained unstable with frequent alternations between dominance of summer and winter seasons. During the middle Holocene (7.5–5.5 ka) the lake experienced highest lake levels dominated by summer monsoon-related water supply, assigned to the Holocene hydro-climatic optimum. Frequent high-amplitude fluctuations afterwards (5.1–2.9 ka) refer to cooling/drying events under enhanced MLW influence accompanied by a strong lake level decline. The late Holocene (2.9 ka- Present) period experienced moderate isotopic variations and fluctuating lake levels in response to variable influence of summer- or winter-related hydro-climatic conditions.This seesaw-like pattern with amplitudes of >10‰ in δ18Ocarb resembles fluctuations in cave records and variations between air and seawater (Dole effect). High correspondence with cooling events derived from North Atlantic drift ice and meltwater discharge indicate close ties to northern hemispheric climate transmitted by the MLW across the TP.

Holocene mountain glacier history in the Sukkertoppen Iskappe area, southwest Greenland

Mountain glaciers and ice caps (GIC) independent of the Greenland Ice Sheet respond rapidly to climate variations and records of their past extent provide information on the natural envelope of climate variability. Here, we use a multi-proxy approach that combines proglacial lake sediment analysis, cosmogenic nuclide surface-exposure dating (in situ10Be and 14C), and radiocarbon dating of recently ice-entombed moss to generate a centennial-scale record of Holocene GIC fluctuations in southwestern Greenland. Following local deglaciation ∼10-9 ka, sediments from proglacial Crash Lake record a glacier advance at ∼9 ka that is indistinguishable from nearby ice sheet moraines, implying a synchronous response of GIC and the Greenland Ice Sheet to a centennial-scale climate event. Following this local glacier advance, GIC experienced net recession until ∼4.6 ka. Radiocarbon ages of in situ moss (n = 29) and Crash Lake sediments reveal intervals of glacier expansion at ∼1.8, 1.2 and 0.7 ka that are superimposed on an overall trend of net glacier expansion throughout the late Holocene. In situ14C concentrations from bedrock adjacent to radiocarbon-dated moss samples further constrain the duration of ice cover through the Holocene in this region. We find that our glacier-size proxy records during the past ∼4 ka are broadly consistent with relatively lower temperatures recorded in GISP2 and occur during, or following, intervals of volcanic perturbations. Thus, we speculate that volcanic activity, although less frequent and intense than in the early Holocene and during the Little Ice Age, may have led to centennial-scale variability imprinted on net glacier expansion due to decreasing summer insolation through the late Holocene.

The reconstruction of past forest dynamics over the last 13,500 years in SW Sweden

Evidence for unbroken continuity of tree taxa over the last c. 13,500 years is presented from a biodiversity ‘hotspot’ nature reserve in south-west Sweden. Forest composition, continuity, fire and disturbance events are reconstructed using palaeoecological methods. A lake record reveals that Pinus sylvestris, Betula spp., Salix spp., Populus tremula and Hippophae rhamnoides were the initial trees scattered in a semi-open, steppe environment. This developed into forest with Pinus, Betula, Corylus, Alnus, Ulmus and Populus with evidence for frequent fires. Deciduous trees became more significant as fires became less frequent and Quercus, Fraxinus and Tilia expanded. Fire frequencies increased again in the Bronze Age probably associated with anthropogenic use of the forest, and the first Fagus sylvatica pollen was recorded. Burning continued through the Iron Age, but charcoal is briefly absent for a period often referred to as the ‘Late Iron Age Lull’. The forest re-expanded with successions involving Juniperus, but with an altered composition from the earlier mixed deciduous community, to one dominated by Fagus. This is coincident with the first pollen records for Picea abies. The early Holocene mixed forest with frequent low-intensity fires is potentially associated with the greatest diversity of red-listed insect species. Forest continuity and the fragmented reservoir populations of old deciduous trees in the Fagus-dominated forest today are likely to have been critical in preserving the present-day, species-rich, rare epiphytic flora, wood-inhabiting fungi and invertebrate communities. As many of these forest fragments may become more vulnerable with future climate change, tree diversity with some disturbance may become essential for survival of the endangered saproxylic species.

Hydrogeomorphic controls on Holocene lacustrine loss-on-ignition records

Holocene loss-on-ignition (LOI) records were developed for radiocarbon-dated sediment cores from 20 lakes in the Uinta Mountains of northeastern Utah, USA. The hydrogeomorphic setting of each lake was characterized, permitting an analysis of connections between LOI values and lake setting. Median LOI values exhibit a significant negative correlation with lake elevation, with the range of elevations and the maximum slope of the watershed, with the extent of ice/snow and exposed rock in the watershed, and with the volume of through-flowing water. In contrast, LOI is positively correlated with the extent of forest cover and wetlands in the watershed. In addition, forward stepwise linear modeling demonstrates that lake area/lake depth ratio is a significant negative predictor for median LOI, that shoreline complexity is a significant positive predictor for maximum LOI, and the standard deviation of LOI values is greater in lakes surrounded by watersheds with more wetlands and surface water. Some LOI time series contain excursions at finer temporal scales that are recorded synchronously in adjacent lakes, however many excursions appear in only a single record. Similarly, step changes in LOI that are clear in some records are not preserved in the sediments of nearby lakes. Over longer timescales, some LOI records are essentially constant during the Holocene. In contrast, some lakes record steadily rising LOI values, and approximately half of the lakes have complicated LOI records with millennial-scale variability. Lakes with large surface area, large area/depth ratios, large watershed area/lake area ratios, rugged watersheds, and extensive ice/snow cover are more likely to record steady LOI values over time. The degree to which LOI records from neighboring lakes vary emphasizes that lacustrine proxy records reflect a strong filtering effect, whereby the signal of an external forcing is recorded to varying degrees depending on the nature of the lake and surrounding watershed.

Late Quaternary Environments of the Teton Mountains, Wyoming: A Pollen Record from Green Lake

Late Quaternary forest succession in Wyoming’s Rocky Mountains, occurred in random patterns, because it reflects differences between the Glacial vegetation at lower elevations on the east vs. the west of the Rockies, as well as along the mountain crest to the south. Differential melting of mountain glaciers resulted in differences in the timing of recolonization. Significant variations in the composition of plant assemblages occurred due to delays in species’ arrival, and even in the exclusion of species. Holocene climate variability, especially ongoing global warming, added to the complex dynamics of plant assemblages with warm climate species replacing early Holocene, cooler climate species. The pollen record from Green Lake (located in a glacial cirque on the west side of the Teton Mountains in Teton County, Wyoming) addresses the local vegetation response from just before the fall of Mazama ash to the end of the middle Holocene warm period. Although the earlier portion of the pollen sequence records some of the last adjustments as some plant species were still arriving after de-glaciation, by the time Mazama ash fell it was climate variation that determined most of the dynamics observed in the Green Lake record. The results reveal a sequence of wetter and drier periods based upon the presence of diagnostic tree species. A moist late early Holocene was followed by a dry middle Holocene, which ended about 6,400 cal. B.P., and was followed between 5,000 to 2,800 cal. B.P. by a sequence of drier and moister climate episodes.

Late Holocene climatic variability in Subarctic Canada: Insights from a high-resolution lake record from the central Northwest Territories.

We examined late Holocene (ca. 3300 yr BP to present-day) climate variability in the central Northwest Territories (Canadian Subarctic) using a diatom and sedimentological record from Danny’s Lake (63.48ºN, 112.54ºW), located 40 km southwest of the modern-day treeline. High-resolution sampling paired with a robust age model (25 radiocarbon dates) allowed for the examination of both lake hydroecological conditions (30-year intervals; diatoms) and sedimentological changes in the watershed (12-year intervals; grain size records) over the late Holocene. Time series analysis of key lake ecological indicators (diatom species Aulacoseira alpigena, Pseudostaurosira brevistriata and Achnanthidium minutissimum) and sedimentological parameters, reflective of catchment processes (coarse silt fraction), suggests significant intermittent variations in turbidity, pH and light penetration within the lake basin. In the diatom record, we observed discontinuous periodicities in the range of ca. 69, 88–100, 115–132, 141–188, 562, 750 and 900 years (>90% and >95% confidence intervals), whereas the coarse silt fraction was characterized by periodicities in the >901 and <61-year range (>95% confidence interval). Periodicities in the proxy data from the Danny’s Lake sediment core align with changes in total solar irradiance over the past ca. 3300 yr BP and we hypothesize a link to the Suess Cycle, Gleissberg Cycle and Pacific Decadal Oscillation via occasional inland propagation of shifting air masses over the Pacific Ocean. This research represents an important baseline study of the underlying causes of climate variability in the Canadian Subarctic and provides details on the long-term climate variability that has persisted in this region through the past three thousand years.

Climate, Fire, and Vegetation Mediate Mercury Delivery to Midlatitude Lakes over the Holocene.

The rise in mercury concentrations in lake sediment deposited over the last ∼150 years is widely recognized to have resulted from human activity. However, few studies in the Great Lakes region have used lake sediment to reconstruct atmospheric mercury deposition on millennial time scales. Here we present a 9000-year mercury record from sediment in Copper Falls; a small closed-basin lake on the Keweenaw Peninsula. Prior to abrupt increases in the 19th and 20th centuries, mercury remains at relatively low concentrations for the last 9000 years. Higher mercury fluxes in the early Holocene (3.4 ± 1.1 μg m-2 yr-1) are attributed to drier conditions and greater forest fire occurrence. The gradual decline in mercury flux over the middle to late Holocene (1.9 ± 0.2 μg m-2 yr-1) is interpreted to reflect a transition to wetter conditions, which reduced forest fires, and promoted the development of soil organic matter and deciduous forests that sequestered natural sources of mercury. The Copper Falls Lake record highlights the sensitivity of watersheds to changes in mercury inputs from both human and natural forcings, and provides millennial-scale context for recent mercury contamination that will aid in establishing baseline values for restoration efforts.

Dynamics of the wetland vegetation in large lakes of the Yangtze Plain in response to both fertilizer consumption and climatic changes

Using moderate-resolution imaging spectroradiometer (MODIS) data that cover the 15-year period from 2000 to 2014 and a phenology-based classification method, the long-term changes in the wetland vegetation of 25 large lakes on the Yangtze Plain were obtained. The classification method was developed based on the phenological information extracted from time series of MODIS observations, which demonstrated mean user’s/producer’s accuracies of 76.17% and 84.58%, respectively. The first comprehensive record of the spatial distribution and temporal dynamics of wetland vegetation in the large lakes on the Yangtze Plain was created. Of the 25 lakes examined, 17 showed a decreasing trend of vegetation area percentages (VAPs) during the study period, and 7 were statistically significant (p < 0.05). The same number of lakes was found to display decreasing trends in vegetation greenness over this 15-year period, and these decreasing trends were statistically significant (p < 0.05) for 11 of the lakes. Substantially fewer lakes showed increases in either their VAPs or their vegetation greenness values. Analysis using a multiple general linear model revealed that the amounts of chemical fertilizer used for farmlands surrounding the lakes, precipitation, daily sunshine hours, temperature and water turbidity played the most important roles in regulating the interannual changes in vegetation greenness in 40% (10/25), 12% (3/25), 4% (1/25), 20% (5/25) and 12% (3/25) of the lake wetlands, respectively. On average, the combined effects of these five driving factors above explained 89.08 ± 7.89% of the variation in greenness over this 15-year period for the 25 lakes. This wetland vegetation environmental data record (EDR) of large lakes in Yangtze Plain demonstration will provide a crucial baseline information for the wetland environment conservation and restoration.

Grazing activities in the southern central Pyrenees during the last millennium as deduced from the non-pollen palynomorphs (NPP) record of Lake Montcortès

Human activities during the last millennium have shaped most of the present-day landscapes. During this time, in the southern central Pyrenees several climate periods and phases of variable human disturbance have driven varied landscape responses. In previous studies of the sediment deposits in Lake Montcortès (southern central Pyrenees) several climatic shifts (including the Medieval Climate Anomaly and the Little Ice Age), as well as forest clearance by fire, agriculture and cattle raising as the main human impacts were recorded since Medieval times. In this work we use non-pollen palynomorphs (NPP) on the same sequence to reconstruct the different human activities, with a special focus on grazing, at an average resolution of around 30 years per sample. Independent NPP proxies for fire and forest clearance were found, supporting former studies. Moreover, the NPP record shows four periods of high abundance of coprophilous fungi: 1) 830–865 CE; 2) 1120–1290 CE; 3) 1530–1795 CE; and 4) 1865–1880 CE. These four periods correspond to phases of increased livestock farming, as reported in historical documents. Comparison of pollen, historical documents, and NPP records show that the impact on landscape dynamics during the last millennium have been mostly related to variable human population density controlled by historical, political and cultural changes in the Pyrenean mountains. Moreover, the use and comparison of several independent proxies have highlighted the strength of the indicator value obtained.

Joint vegetation and mammalian records at the early Pleistocene sequence of Bòvila Ordis (Banyoles-Besalú Basin, NE Spain) and their bearing on early hominin occupation in Europe

The early Pleistocene site at Bovila Ordis (Banyoles-Besalu Basin in NE Iberia) delivered in the past a significant pollen succession which can be assigned to the upper Matuyama geomagnetic chron owing to the identification of the Cobb Mountain magnetic subchron (1.2 Ma ago). It has been previously shown by palynology that the succession of lakes recorded at Bovila Ordis corresponds to a succession of late early Pleistocene glacial–interglacial phases, between 1.2 and 1 Ma, which can be correlated with MIS 35, 33 and 31. The present investigation focuses on lake 1 (borehole BOIV) and lake 3 (boreholes BOI, BOII and BOIII and outcrops) from this site, where both fossil pollen and rodent teeth were recovered together in the same sediment, and offers the potential to provide a vegetation and climatic context to sites that are dated by rodent teeth, such as some hominin sites in Western Europe. Here we analyse the rodent content and their biostratigraphical implications in the context of early hominin dispersal in Western Europe. In lake 1, pollen data had revealed in previous analysis a complete glacial–interglacial cycle and parts of a second one. The uppermost part of BOIV sequence yielded rodent remains that can be assigned to the arvicolid species Allophaiomys chalinei and Mimomys cf. savini. Previous palynological investigations from lake 3 had revealed a temperate deciduous forest, indicative of warm and humid conditions. At the top of the section of lake 3, a regression of the forest and the spread of open landscape elements indicated drier, more steppic conditions. The palaeobiological record of lake 3 includes pollen and large (Cervus philisi, Hippopotamus antiquus and Equus stenonis) and small mammal remains (Mimomys savini and Allophaiomys lavocati). The rodent material from lake 1 clearly indicates an early Pleistocene age somewhat older than that of lake 3. According to these new biostratigraphic data in the context of previous palynological and chronological data, it is clear that Allophaiomys chalinei entered Iberia before 1 Ma ago and A. lavocati remained until 1 Ma ago. The fact that Allophaiomys lavocati and Allophaiomys chalinei, two arvicolids currently associated with the earliest evidence of hominin presence in Western Europe, appear in Bovila Ordis at a time of mild, temperate conditions reinforce the idea that the first hominin dispersal in this region was strongly dependent on favourable climatic conditions.

Millennial-scale variability in Holocene aquatic productivity from Burial Lake, Arctic Alaska

Holocene records of lacustrine primary production are commonly used to reconstruct past changes in environmental and climatic conditions. While several methods exist to infer paleoproductivity trends, few studies to date have applied multiple geochemical indices in the same core sequence from Arctic lakes to evaluate their fidelity and sensitivity to specific climate variables over long (Holocene length) timescales. In this study, we evaluate sub-century to millennial-scale fluctuations in paleoproductivity over the Holocene using geochemical (biogenic opal and sedimentary chlorin) analyses of sediments from Burial Lake in the western Brooks Range, Alaska. Large fluctuations in opal and related proxies occur at millennial timescales over the last 10,000 years. We interpret the changes in opal to result from variability in diatom productivity, which is indirectly mediated by climate primarily through changes in the duration of the ice-free growing season and the availability of limiting nutrients at this oligotrophic, tundra lake. Comparison of the opal and sedimentary chlorin record, which is correlated with TOC, shows contrasting patterns on both short (century to multi-century) and relatively long (millennial) time scales. The concentration of opal far exceeds that of TOC and variations in sediment dry bulk density, driven by changes in the accumulation of opal, are likely responsible in part for the variations in sedimentary chlorin. Further, C/N ratio values indicate a mixed algal-terrestrial source of sedimentary organic matter. This result highlights the complexity in the climatic interpretation of sedimentary chlorin as an index of whole lake production, because the signal is prone to dilution/concentration from opal and also reflects a combination of aquatic and terrestrial production. Time series analysis of the productivity records indicates the presence of a significant ∼1500-yr oscillation in opal concentration, which has been found in North Atlantic Ocean proxy records and numerous other marine and terrestrial paleorecords. Comparison of diatom productivity against a sea-ice inferred reconstruction of the Arctic Oscillation (AO) from the Beaufort Sea (Darby et al., 2012) shows that periods of reduced productivity at Burial Lake coincide with inferred positive phases of the AO (AO+). Combined with modern observations of sea ice extent and meteorological data, we hypothesize that AO + conditions and a strengthened polar jet correspond with a shortened ice-free growing season, a decrease in the availability of limiting nutrients, and lower levels of diatom production at Burial Lake. Comparison of the spectral properties between opal and the AO reconstruction reveal similar millennial scale variations with ∼1500-yr variability during the middle Holocene that transition to ∼1000-yr variability during the late Holocene. In light of these findings, we suggest the possibility that millennial variations in diatom productivity observed in the Burial Lake record are related to millennial variability in high-latitude atmospheric circulation similar to the AO. These results shed light on the sensitivity of aquatic ecosystems in northern Alaska to changes in the duration of the ice-free growing season, the availability of limiting nutrients for phytoplankton growth, and Arctic-wide atmospheric circulation dynamics over the Holocene on millennial timescales.

Reconstruction of the mid-Holocene paleoclimate of the Ecuadorian Andean páramo at Tres Lagunas, Ecuador

Previous paleoclimate studies have suggested a warm/dry event during the mid-Holocene in páramo vegetation of the tropical Andes of South America. However, the timing of the mid-Holocene warm/dry event in Tres Lagunas, Ecuador, remains uncertain, since a previously reported bog core record characterized the warm/dry event during a hiatus in sediment deposition. In order to understand the timing of the warm/dry event in Tres Lagunas in relation to regional records, a lake sediment core was collected. Subsamples from the core were analyzed for radiocarbon dates, pollen, magnetic susceptibility, and charcoal to reconstruct the depositional, vegetational, and fire history of the area. A near-constant sedimentation rate in the lake core indicated that the lake did not dry, in contrast to the adjacent previously reported bog core. Increases in lower elevation pollen types suggest a warm period between 2700 and 2200 cal. yr BP, with the peak of warming at 2200 cal. yr BP co-occurring with the end of hiatus in the bog core record. Statistically significant increases in charcoal influx and magnetic susceptibility from 3900 to 800 cal. yr BP also suggest a dry climate during this period. While the lake record corroborates the presence of a warm/dry period in Tres Lagunas during the mid-Holocene, this record suggests a shorter period of warm/dry climate where the intensity was not sufficient to cause the lake to dry. However, anthropogenic alteration of the landscape, either vegetation or burning, must also be considered when interpreting Holocene records from this region.

Potential forcings of summer temperature variability of the southeastern Tibetan Plateau in the past 12 ka

Investigating potential forcing mechanisms of terrestrial summer temperature changes from the Asian summer monsoon influenced area is of importance to better understand the climate variability in these densely populated regions. The results of spectral and wavelet analyses of the published chironomid reconstructed mean July temperature data from Tiancai Lake on the SE Tibetan Plateau are presented. The evidence of solar forcing of the summer temperature variability from the site on centennial timescales where key solar periodicities (at 855 ± 40, 465 ± 40, 315 ± 40 and 165 ± 40 year) are revealed. By using a band-pass filter, coherent fluctuations were found in the strength of Asian summer monsoon, Northern Hemisphere high latitude climate and high elevation mid-latitude (26°N) terrestrial temperatures with solar sunspot cycles since about 7.6 ka. The two abrupt cooling events detected from the Tiancai Lake record, centered at ∼9.7 and 3.5 ka were examined respectively. Coupled with the paleoclimate modeling results, the early Holocene event (9.7 ka) is possibly linked to an ocean-atmospheric feedback mechanism whereas the latter event (3.5 ka) may be more directly related to external forcing.

A multi-proxy approach to understanding complex responses of salt-lake catchments to climate variability and human pressure: A Late Quaternary case study from south-eastern, Spain

This article focuses on a former salt lake in the upper Vinalopó Valley in south-eastern Spain. The study spans the Late Pleistocene through to the Late Holocene, although with particular focus on the period between 11 ka cal BP and 3000 ka cal BP (which spans the Mesolithic and part of the Bronze Age). High resolution multi-proxy analysis (including pollen, non pollen palynomorphs, grain size, X-ray fluorescence and X-ray diffraction) was undertaken on the lake sediments. The results show strong sensitivity to both long term and small changes in the evaporation/precipitation ratio, affecting the surrounding vegetation composition, lake-biota and sediment geochemistry.To summarise the key findings the main general trends identified include: 1) Hyper-saline conditions and low lake levels at the end of the Late Glacial 2) Increasing wetness and temperatures which witnessed an expansion of mesophilic woodland taxa, lake infilling and the establishment of a more perennial lake system at the onset of the Holocene 3) An increase in solar insolation after 9 ka cal BP which saw the re-establishment of pine forests 4) A continued trend towards increasing dryness (climatic optimum) at 7 ka cal BP but with continued freshwater input 5) An increase in sclerophyllous open woody vegetation (anthropogenic?), and increasing wetness (climatic?) is represented in the lake record between 5.9 and 3 ka cal BP 6) The Holocene was also punctuated by several aridity pulses, the most prominent corresponding to the 8.2 ka cal BP event. These events, despite a paucity of well dated archaeological sites in the surrounding area, likely altered the carrying capacity of this area both regionally and locally, particularly during the Mesolithic-Neolithic transition, in terms of fresh water supply for human/animal consumption, wild plant food reserves and suitable land for crop growth.

Export of Total, Particulate, and Apatite Phosphorus from Forested and Agricultural Watersheds.

Concentrations of apatite phosphorus (P) have been shown to be greater in the soils of agricultural and disturbed watersheds than in soils of forested watersheds. This observation suggests that changes in the relative concentration of apatite P in the sedimentary record of lakes might be a useful marker for land use changes in the watershed where sediments are dominated by allochthonous loading. To determine if P export in streams is indicative of soil P in the watershed, we measured the export of total, particulate, and apatite P in first-order streams draining four forested and four agricultural watersheds in western Pennsylvania. Streams draining agricultural watersheds exported significantly more total suspended solids (TSS) and more total, particulate, soluble, and apatite P, particularly during periods of high discharge, than did streams draining forested watersheds. Total suspended solids exported from forested watersheds contained similar amounts of apatite P (μg apatite P g TSS), as did agricultural watersheds in spite of lower soil concentrations. However, since agricultural watershed exported almost four times the mass of TSS, the net export of apatite P was greater than from forested streams. These results confirm the increased apatite P export from agricultural watersheds and suggest that inferences drawn from the profiles of apatite P in the sedimentary record of lakes may reliably indicate historical disturbances in watersheds caused by agriculture or other erosional events.

Ice-dammed lateral lake and epishelf lake insights into Holocene dynamics of Marguerite Trough Ice Stream and George VI Ice Shelf, Alexander Island, Antarctic Peninsula

We present new data regarding the past dynamics of Marguerite Trough Ice Stream, George VI Ice Shelf and valley glaciers from Ablation Point Massif on Alexander Island, Antarctic Peninsula. This ice-free oasis preserves a geological record of ice stream lateral moraines, ice-dammed lakes, ice-shelf moraines and valley glacier moraines, which we dated using cosmogenic nuclide ages. We provide one of the first detailed sediment-landform assemblage descriptions of epishelf lake shorelines. Marguerite Trough Ice Stream imprinted lateral moraines against eastern Alexander Island at 120 m at Ablation Point Massif. During deglaciation, lateral lakes formed in the Ablation and Moutonnée valleys, dammed against the ice stream in George VI Sound. Exposure ages from boulders on these shorelines yielded ages of 13.9 to 9.7 ka. Following recession of the ice stream, George VI Ice Shelf formed in George VI Sound. An epishelf lake formed at 15–20 m asl in Ablation and Moutonnée valleys, dated from 9.4 to 4.6 ka, suggesting that the lake was stable and persistent for some 5000 years. Lake-level lowering occurred after this, with the lake level at 12 m at 3.1 ± 0.4 ka and at 5 m asl today. A readvance of the valley glaciers on Alexander Island at 4.4 ± 0.7 ka is recorded by valley glacier moraines overlying epishelf lake sediments. We speculate that the glacier readvance, which occurred during a period of warmth, may have been caused by a dynamic response of the glaciers to a lowering in surface elevation of George VI Ice Shelf.

Fallout isotope chronology of the near-surface sediment record of Lake Bolătău

Fallout isotope (210Pbex,137Cs and 241Am) based dating has been carried out on the near-surface sediment core collected from Lake Bolătău-Feredeu (Bukovina, Romania). The motivation was to improve the chronology of this recent section in connection with significant fluctuations observed in sediment accumulation rates, particle size distribution and primordial radioisotope (i.e. 40K and 232Th) composition. Previously only an extrapolation of a broad-range OxCal age-depth model, which was based on 8 AMS radiocarbon dates from the deeper part of a parallel sediment sequence and tentatively validated for the upper part using the double peaks of the 137Cs activity concentration distribution, was available for the studied section (1–24 cm). Parallel to the previous 137Cs measurement, 210Pb and 226Ra (for a more detailed, 210Pbex-based chronology), 241Am (for an additional time-marker), as well as 40K and 232Th concentrations have also been determined by gamma-spectrometry. In case of the 210Pbex-based chronology, due to a large deviation from a pure exponential distribution, the Constant Flux (CF) model has been used for the calculation of sediment ages and accumulation rates. Although the broad-range OxCal and the CF model were broadly similar down to 22 cm, the 210Pbex-based ages are clearly superior in terms of uncertainty in the uppermost 12 cm, while the broad-range model has smaller uncertainty below 20 cm (>150 years). The CF model gave an average mass accumulation rate of (0.08 ± 0.03) g cm−2 yr−1 for sections 0–11 cm, and (0.03 ± 0.01) g cm−2 yr−1 for sections 12–22 cm, respectively. Significant changes have been observed in the depth distribution of both the particle size distribution and the elemental/isotopic composition of the sediment record, most likely related to the variation observable in the intensity and volume of precipitation in the catchment. The obtained high-resolution records of Lake Bolătău, including multiple radioisotopes, can serve as a regional benchmark for similar studies.