Lake Record Research Articles

Climate change and human occupation in the northernmost Chilean Altiplano over the last ca. 11 500 cal. a BP

AbstractThis interdisciplinary study represents an approximation towards understanding how regional human cultural systems may have been affected by climate change in the northernmost Chilean Altiplano (>3600 m) over the last ca. 11 500 cal a BP. We compare the archaeological record from Hakenasa cave with the lake record from Lago Chungará sediment cores, located 50 km to the south. By integrating both of these archives in conjunction with regional palaeoclimate and archaeological data, we provide new evidence for the role of changing environmental and climatic conditions in human settlement patterns. The first human occupation of the entire Altiplano occurs at Hakenasa and is dated to 9980 ± 40 14C a BP (11 265–11 619 cal. a BP), and took place under wetter regional climate conditions. An archaeologically sterile deposit occurs at Hakenasa between 7870 and 6890 cal. a BP. Constituted by sands and gravels, these sediments are interpreted as a flood event. This time period is synchronous with alternating short dry and wet events recorded in the Lake Chungará sedimentary sequence. Human activity resumes and increases in importance at Hakenasa by ca. 6000 cal. a BP. This corresponds to wetter conditions indicated by the Chungará record. Even though the lake record indicates intense volcanic activity over the last 6000 cal. a BP, this had little or no impact on the human population present at Hakenasa. This study shows that even in this extreme environment human settlement patterns do not always respond in a linear fashion to environmental change. Copyright © 2008 John Wiley & Sons, Ltd.

A Lateglacial and Holocene lake record from the Nianbaoyeze Mountains and inferences of lake, glacier and climate evolution on the eastern Tibetan Plateau

Our 12.81m long record from Lake Ximencuo provides insight into the Lateglacial and Holocene evolution of the lake and the Nianbaoyeze glacier in its catchment, and the regional climate history since ∼19kyr. Lake Ximencuo was a permanent deep lake throughout its history. In contrast to numerous lakes on the Tibetan Plateau which experienced shallow lake levels or even desiccation during the Lateglacial, Lake Ximencuo was fed by large quantities of meltwater in the Lateglacial. The existence of glaciated upper catchment areas is apparently a prerequisite for lakes on the Tibetan Plateau which maintained relatively high water levels during the generally cold and dry periods following the global LGM (∼21kyr). A minor re-advance of the Nianbaoyeze glacier was recorded coeval with the Greenland Stadial 2a (∼16.4 and 14.5kyr), followed by rapid warming apparently synchronous with the Greenland Interstadial 1. Warmest conditions were recorded at Lake Ximencuo during the early Holocene which was punctured by a remarkable pulse of climate deterioration around 8.3kyr. This spell represents probably the 8.2kyr event of the North Atlantic region, suggesting that it had a significant impact on the Tibetan Plateau. Colder conditions of longer duration occurred from 4.7 to 3.7kyr, apparently in phase with numerous records signalling colder and drier conditions on the Tibetan Plateau. Two minor spells of colder conditions and probably catchment erosion were recorded in the late Holocene between 2.0 and 1.4kyr and between 0.5 and 0.1kyr with the latter representing the Little Ice Age. It is unclear, whether human activities may have accelerated or even solely triggered the late Holocene erosion events.

Palaeoenvironmental changes during the last 1600 years inferred from the sediment record of a cirque lake in southern Patagonia (Laguna Las Vizcachas, Argentina)

Laguna Las Vizcachas is a cirque lake located at the margin of an extra-Andean volcanic plateau in southern Patagonia, Argentina, within the area of steppe and semi-desert east of the Andes. The number of paleoenvironmental records is still limited in this region. Sediments of this lake were studied in order to obtain multi-proxy information about the paleoenvironmental history of this site for the ‘Medieval Climate Anomaly’ and the ‘Little Ice Age’ chronozones. In combination with results from other sites across southern Patagonia, our data enhance the understanding of spatial patterns of past hydrological changes and contribute to distinguishing between the signals of temperature and precipitation. As Laguna Las Vizcachas is situated at 1100 m a.s.l. in a cool ‘mountain climate’, the lake system is more sensitive to changes of temperature and winter ice cover than other sites from lower elevations in this region. Our interpretation of the multi-proxy dataset is based on signals of clastic sediment input, lake productivity, organic matter sources and preservation, dilution effects and early diagenetic overprint. The record reveals a period of enhanced fluvial runoff resulting from higher precipitation from the 12th until the end of the 14th century as inferred from high concentrations of Ti, Ca, and from magnetic susceptibility. This may coincide with higher wind intensities as suggested by higher proportions of epiphytic diatoms which point to an enhanced lateral transport from their littoral habitat towards the coring position at the center of the lake. In comparison with other records from southern Patagonia, the results from Laguna Las Vizcachas suggest opposite precipitation regimes between the western and eastern parts of Patagonia during that time which corresponds partly to the ‘Medieval Climate Anomaly’ chronozone. However, this proposal is compromised by the chronological uncertainties of the different records under consideration. The diatom record of Laguna Las Vizcachas indicates temperature changes: highest proportions of benthic diatoms point to coldest conditions from the mid-15th until the mid-17th century, followed by relatively warm conditions until the mid-18th century as suggested by a decrease of benthic taxa and a conspicuous rise of the planktonic/non-planktonic diatom ratio that can be used as an indicator for the length or presence/absence of winter ice cover.

Bulk organic geochemistry of sediments from Puyehue Lake and its watershed (Chile, 40°S): Implications for paleoenvironmental reconstructions

Since the last deglaciation, the mid-latitudes of the southern Hemisphere have undergone considerable environmental changes. In order to better understand the response of continental ecosystems to paleoclimate changes in southern South America, we investigated the sedimentary record of Puyehue Lake, located in the western piedmont of the Andes in South–Central Chile (40°S). We analyzed the elemental (C, N) and stable isotopic (δ 13C, δ 15N) composition of the sedimentary organic matter preserved in the lake and its watershed to estimate the relative changes in the sources of sedimentary organic carbon through space and time. The geochemical signature of the aquatic and terrestrial end-members was determined on samples of lake particulate organic matter (N/C: 0.130) and Holocene paleosols (N/C: 0.069), respectively. A simple mixing equation based on the N/C ratio of these end-members was then used to estimate the fraction of terrestrial carbon (ƒ T) preserved in the lake sediments. Our approach was validated using surface sediment samples, which show a strong relation between ƒ T and distance to the main rivers and to the shore. We further applied this equation to an 11.22 m long sediment core to reconstruct paleoenvironmental changes in Puyehue Lake and its watershed during the last 17.9 kyr. Our data provide evidence for a first warming pulse at 17.3 cal kyr BP, which triggered a rapid increase in lake diatom productivity, lagging the start of a similar increase in sea surface temperature (SST) off Chile by 1500 years. This delay is best explained by the presence of a large glacier in the lake watershed, which delayed the response time of the terrestrial proxies and limited the concomitant expansion of the vegetation in the lake watershed (low ƒ T). A second warming pulse at 12.8 cal kyr BP is inferred from an increase in lake productivity and a major expansion of the vegetation in the lake watershed, demonstrating that the Puyehue glacier had considerably retreated from the watershed. This second warming pulse is synchronous with a 2 °C increase in SST off the coast of Chile, and its timing corresponds to the beginning of the Younger Dryas Chronozone. These results contribute to the mounting evidence that the climate in the mid-latitudes of the southern Hemisphere was warming during the Younger Dryas Chronozone, in agreement with the bipolar see-saw hypothesis.

Significant seasonal variation in the hydrogen isotopic composition of leaf-wax lipids for two deciduous tree ecosystems (Fagus sylvativa and Acerpseudoplatanus)

Compound specific hydrogen isotope ratios (δD) of long chain sedimentary n-alkanes, which mostly originate from the leaf waxes of higher terrestrial plants, are increasingly employed as paleoclimate proxies. While soil water is the ultimate hydrogen source for these lipids and the isotopic fractionation during biosynthesis of lipids is thought to remain constant, environmental parameters and plant physiological processes can alter the apparent hydrogen isotopic fractionation between leaf-wax lipids and a plant’s source water. However, the magnitude and timing of these effects and their influence on the isotopic composition of lipids from higher terrestrial plants are still not well understood. Therefore we investigated the seasonal variability of leaf-wax n-alkane δD values for two different temperate deciduous forest ecosystems that are dominated by two different tree species, Beech (Fagus sylvatica) and Maple (Acerpseudoplatanus).We found significant seasonal variations for both tree species in n-alkane δD values of up to 40‰ on timescales as short as one week. Also, the isotopic difference between different n-alkanes from the same plant species did vary significantly and reached up to 50‰ at the same time when overall n-alkane concentrations were lowest.Since δD values of soil water at 5 and 10cm depth, which we assume represent the δD value of the major water source for the investigated beech trees, were enriched in autumn compared to the spring by 30‰, whereas n-alkane δD values increased only by 10‰, we observed variations in the apparent fractionation between beech leaf derived n-alkanes and soil water of up to 20‰ on a seasonal scale. This observed change in the apparent fractionation was likely caused by differences in leaf water isotopic enrichment. Based on mechanistic leaf water models we conclude that changes in the isotopic difference between water vapor and soil water were the most likely reason for the observed changes in the apparent fractionation between n-alkanes and soil water.The large variability of n-alkane concentrations and δD values over time implies a continuous de novo synthesis of these compounds over the growing season with turnover times possibly as short as weeks. The signal to reach the soil therefore represents an integrated record of the last weeks before leaf senescence. This holds true also for the sedimentary record of small catchment lakes in humid, temperate climates, where wind transport of leaf-wax lipids is negligible compared to transfer through soil and the massive input of leaves directly into the lake in autumn.

A 1000 year mid-interglacial cold phase at about 400 ka BP in the Piànico lake record

This article was submitted without an abstract, please refer to the full-text PDF file.

A 2000 year varve-based climate record from the central Brooks Range, Alaska

Varved minerogenic sediments from glacial-fed Blue Lake, northern Alaska, are used to investigate late Holocene climate variability. Varve-thickness measurements track summer temperature recorded at Atigun Pass, located 41 km east at a similar elevation (r2 = 0.31, P = 0.08). Results indicate that climate in the Brooks Range from 10 to 730 AD (varve year) was warm with precipitation inferred to be higher than during the twentieth century. The varve-temperature relationship for this period was likely compromised and not used in our temperature reconstruction because the glacier was greatly reduced, or absent, exposing sub-glacial sediments to erosion from enhanced precipitation. Varve-inferred summer temperatures and precipitation decreased after 730 AD, averaging 0.4°C above the last millennial average (LMA = 4.2°C) from 730 to 850 AD, and 0.1°C above the LMA from 850 to 980 AD. Cooling culminated between 980 and 1030 AD with temperatures 0.7°C below the LMA. Varve-inferred summer temperatures increased between 1030 and 1620 AD to the LMA, though the period between 1260 and 1350 AD was 0.2°C below the LMA. Although there is no equivalent to the European Medieval Warm Period in the Blue Lake record, two warm intervals occurred from 1350 to 1450 AD and 1500 to 1620 AD (0.4 and 0.3°C above the LMA, respectively). During the Little Ice Age (LIA; 1620 to 1880 AD), inferred summer temperature averaged 0.2°C below the LMA. After 1880 AD, inferred summer temperature increased to 0.8°C above the LMA, glaciers retreated, but aridity persisted based on a number of regional paleoclimate records. Despite warming and glacial retreat, varve thicknesses have not achieved pre-730 AD levels. This reflects limited sediment availability and transport due to a less extensive retreat compared to the first millennium, and continued relative aridity. Overall, the Blue Lake record is similar to varve records from the eastern Canadian Arctic that document a cool LIA and twentieth century warming. However, the occurrence and timing of events, such as the LIA and Medieval Warm Period, varies considerably among records, suggesting heterogeneous climatic patterns across the North American Arctic.

Sedimentary pellets as an ice-cover proxy in a High Arctic ice-covered lake

Sediment aggregates (“sedimentary pellets”) within the sedimentary record of Lake A (83°00′ N, 75°30′ W), Ellesmere Island, Canada, are used to construct a 1000 year proxy record of ice-cover extent and dynamics on this perennially ice-covered, High Arctic lake. These pellets are interpreted to form during fall or early winter when littoral sediment adheres to ice forming around the lake’s periphery or during summer through the development of anchor ice. The sediment likely collects in ice interstices and is concentrated in the upper ice layers through summer surface ice melt and winter basal ice growth. The pellets remain frozen in the ice until a summer or series of summers with reduced ice cover allows for their deposition across the lake basin. Sedimentary pellet frequency within multiple sediment cores is used to develop a chronology of ice-cover fluctuations. This proxy ice-cover record is largely corroborated by a record of unusual sedimentation in Lake A involving iron-rich, dark-orange to red laminae overlying more diffuse laminae with a lighter hue. This sediment sequence is hypothesized to represent years with reduced ice cover through increased chemocline ventilation and iron deposition. During the past millennium, the most notable period of inferred reduced ice cover is ca. 1891 AD to present. Another period of ice cover mobility is suggested ca. 1582–1774 AD, while persistent ice cover is inferred during the 1800s and prior to 1582 AD. The proxy ice-cover record corresponds well with most regional melt-season proxy temperature and paleoecological records, especially during the 1800s and 1900s.

Arid and humid phases in southern Spain during the last 4000 years: the Zoñar Lake record, Córdoba

A multiproxy study of sediment cores from Zoñar Lake (37°29′00″N, 4°41′22″W, 300 m a.s.l.) supported by 11 14C AMS dates provides the first high-resolution centennial-scale reconstruction of past humidity changes in southern Spain during the last 4000 years. Arid periods occurred prior to 2.9 cal. kyr BP and during 1.3—0.6 cal. kyr BP (`Mediaeval Climate Anomaly'). The most humid period occurred during 2.6—1.6 cal. kyr BP encompassing the late Iron Age—Iberian and Roman epochs. Two humid periods of lower intensity occurred between 0.8 and 0.6 cal. kyr BP (AD 1200—1400) and about 400 cal. yr BP (around AD 1600) coinciding with the onset of the `Little Ice Age'. Humid conditions are synchronous with a decline in solar output and seem to correspond to atmospheric patterns similar to negative NAO phases. Arid conditions show better correlation with northern Africa climate evolution, suggesting a possible link to subtropical dynamics. The geographic location of Zoñar Lake and the robust chronology provides an opportunity to improve our understanding of the climate evolution in mid latitudes during the late Holocene and to evaluate subtropical and high latitude factors in Mediterranean climate evolution.

Carbonates in the sedimentary record of saline Tsagan-Tyrm Lake, west Baikal region: New type of high-resolution paleoclimatic signals

The reasons for catastrophic climate warming have been a matter of heated debates for the past few decades. The issue of the decisive role of anthropogenic factors or natural fluctuations in modern warming can be solved correctly only from paleoclimatic reconstructions, which are mainly based on study of bottom sediments of oceans, seas, and continental lakes. Despite significant intensification of paleoclimatic studies in recent years, one can sense a deficit in reliable data on past climates, especially for intracontinental regions of the large Asian continent. Reference data on this region are represented by the results of deep drilling within the framework of the International Baikal‐Hovsgol Drilling Project in the two largest lakes of the Baikal rift zone (Baikal and Hovsgol). Sedimentary sections of these lakes represent a unique continuous record of climatic and environmental changes in Central Asia for a few million years. In terms of detail, the sections are comparable with records obtained from oceanic muds and Greenland and Atlantic ices. However, intracontinental regions can be characterized by significant local climatic variations related to topography and atmospheric circulation. Therefore, we should examine a significantly greater number of objects. The most promising objects for paleoclimatic studies are the present-day systems of small saline lakes, which can exist only in arid and semiarid conditions [1‐3]. These lakes can provide higher resolution records, because they differ from large basins by lesser conservatism relative to the external impacts. Owing to small dimensions, they rapidly respond to any climatic variations. This paper reports the results of study of the evaporate sediments of one small saline lake in the Ol’khon region (western coast of Lake Baikal) with carbonatetype sedimentation. We attempted to reveal natural assemblages of low-temperature carbonates, crystallochemical and structural features of individual carbonate phases, and their formation sequence in lacustrine sediments depending on the past climatic and environmental variations.

Abrupt changes in the water balance of tropical West Africa during the late Quaternary

Understanding the causes of tropical climate variability requires an understanding of not only the temporal patterns of these changes, but their spatial extent and magnitude. In this study, we employ a water balance modeling approach to simulate the magnitude of hydrologic changes associated with the geologic record of past lake stands at Lake Bosumtwi, West Africa. Our results indicate that lake level lowering during the late glacial (16 ka) was likely the result of a ∼20% decrease in precipitation, consistent with general circulation modeling (GCM) results and existing paleohydrological estimates in Africa. In contrast, our observation‐based estimate of hydrologic change during the early Holocene (∼5–10 ka) is at odds with the magnitude and direction of hydrologic changes predicted by GCM results, suggesting that the models may not adequately reproduce the processes associated with large‐scale changes in the West African monsoon. Our results for drought events at 12 and 8.7 ka are also inconsistent with GCM predictions of a precipitation dipole over West Africa, but depend strongly on assumptions about other hydrologic parameters such as relative humidity and temperature. Our water balance model also indicates that significant hydrologic changes occurred in response to changes in orbital forcing at ∼70–80 ka, possibly associated with an eccentricity‐driven shift in precession. Drought conditions also occurred during the same time as the European “Little Ice Age,” indicating a connection between relatively minor Northern Hemisphere cooling and drought conditions in Africa. These results suggest that the West African monsoon has the potential to respond in a highly nonlinear way to forcing involving changes in the solar radiation over Africa.

Reduced early Holocene moisture availability in the Bayan Har Mountains, northeastern Tibetan Plateau, inferred from a multi-proxy lake record

A late glacial and Holocene lake sediment record from the northeastern Tibetan Plateau was investigated using ostracod and geochemical analysis. After Lake Koucha was established about 16 cal ka BP, oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka BP clearly suggesting that the water body was a closed-basin lake. From about 10 cal ka BP, the salinity of the lake rose continuously until hyperhaline levels were reached about 7.5 cal ka BP. This increase in salinity was accompanied by a falling lake level. Extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8.5 cal ka BP causing a unique, thin gravel-containing layer within the continuous fine-grained lake sediments. The age control of our record does not allow us to correlate this event exactly to the widely recorded 8.2 cal ka BP event, however, the recovered sediments from Lake Koucha provide strong evidence that climate changes around 8.5 cal ka BP had significant environmental implications in northeastern Tibet. After about 7.3 cal ka BP, the available moisture significantly increased causing a continuous salinity decline until the present-day freshwater conditions of a through-flow lake were established about 6.5 cal ka BP in Lake Koucha. This major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte-dominated to a phytoplankton-dominated lake. A slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4.3 and 2.0 cal ka BP, in phase with numerous records from monsoonal Asia. The reconstructed overall change in moisture availability from dry conditions before about 7.3 cal ka BP to wetter conditions afterwards contradicts most palaeoclimate records from the Tibetan Plateau and its surroundings which show that wetter conditions in the early Holocene were replaced by drier and probably cooler conditions in the mid and late Holocene. However, in the semi-arid region of Lake Koucha with a mean annual precipitation of about 500 mm, the insolation-driven strengthened monsoon in the early Holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability. Our record provides further evidence for a regionally non-uniform response of the available moisture evolution to the overall monsoonal signal in the Holocene climate history of Central Asia.

How the Sahara Became Dry

A continuous lake record elucidates how Saharan climate changed gradually from humid to today's desert conditions.

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.

Characterizing changes in the sedimentary environment of a varved lake sediment record in southern central Finland around 8000 cal. yr BP

AbstractThe varved lake sediment record of Lake Nautajärvi in southern central Finland indicates fluctuations in the annual sedimentation pattern and lake environment around 8000 cal. yr BP. Annual accumulation of catchment‐derived mineral matter peaks a number of times and there are changes in the deposition rate of organic matter. Based on evidence from the Lake Nautajärvi sediment record, the climate was characterised by a millennial‐scale colder period that terminated at one or several cooling events around 8000 cal. yr BP. During one of these events, diatom composition undergoes a change from planktonic species dominance to more littoral species dominance at 8160 cal. yr BP, and a recovery around 8010 cal. yr BP. The timing of this 150‐year‐long diatom anomaly is convergent with increased catchment erosion and decreased organic matter accumulation. We interpret that during such a period, the environment was characterised by colder winter temperatures, prolonged duration of ice cover, higher than normal spring discharge and rapid development of thermal stratification in the spring, coupled with reduced water circulation and shorter growing seasons. Copyright © 2008 John Wiley & Sons, Ltd.

Study of the varve record from Erlongwan maar lake, NE China, over the last 13 ka BP

In a sediment sequence from Erlongwan maar lake that spans the last 13 ka BP, two main varve types can be recognized: biogenic varves (from the present to ∼11.2 ka BP, 0–632 cm) and clastic varves (from ∼11.2 to ∼12.7 ka BP, 632–700 cm). Based on the dominant types of algae contained in the sediment, the biogenic varves can be classified into dinocyst-biogenic varves (0–63 cm) and mixed (dinocyst and diatom)-biogenic varves (214–632 cm). In this paper, the formation process and components of the varve are described, the possible reason for the types of varve changing throughout the record is discussed and a high-resolution varve chronology is established spanning the last 13 ka BP. Although further varve counting and error assessment are needed, the results presented here represent a solid foundation for studying the palaeoclimate record of Erlongwan maar lake.

Pleistocene pluvial lakes of the American West: a short history of research

Abstract Scientific investigations of Pleistocene pluvial lakes in the American West occurred in five phases. The pioneer phase prior to 1870 saw former lakes identified by missionary priests, fur trappers, military expeditions and railroad surveyors. The classic phase, between 1870 and 1920, linked initially with independent surveys and, after 1879, with the United States Geological Survey and with irrigation and mining ventures, saw most lakes identified and described by such worthies as Gilbert, Russell, Gale, Waring and Thompson. A consolidation phase from 1920 to 1955 provided synthesis and new data but, in the absence of age controls, saw much speculation about temporal links between pluvial lakes, glacial stages, and climate forcing. The initial dating phase between 1955 and 1980 saw radiocarbon dating applied to late Pleistocene lakes and their Holocene relics and successors. The integrative phase since 1980, supported by enhanced field, remote sensing, laboratory and dating techniques, has seen an array of issues involving pluvial lakes linked to changes in regional ecology and global climate. In the above sequence, progress from one phase to the next reflected changes in the intellectual climate and advances in scientific methods. Today, we reflect on the episodic but cumulative increase in knowledge about late Pleistocene pluvial lakes, especially for Lake Bonneville, Lake Lahontan and the eastern California lake cascade. The record of earlier Pleistocene lakes, in some cases successors to Miocene and Pliocene lakes, is less certain because of deformation and erosion or burial. Continuing challenges involve evaluation of the Pleistocene lake record as a whole in the context of late Cenozoic tectonic and climate change, and of contemporary environmental and water-resource issues.

Forest Changes in the Great Lakes Region at 5-7 ka BP

Pollen stratigraphy from 90 sites in and bordering the Great Lakes record the 5-7 ka history of forest development of the Great Lakes region. By 7 ka beech (Fagus grandifolia) had invaded the oak-hickory (Quercus-Carya) forest of lower Michigan and hemlock (Tsuga canadensis) and beech the white pine (Pinus strobus)-dominated forest of southern Ontario. At the same time, white pine replaced jack pine (P banksiana) as it expanded northward to the Clay Belt beyond its present-day range. Forest changes at 6 and 5 ka were dominated by range extensions of beech and hemlock in a northwesterly direction, by northward expansion of eastern white cedar (Cupressineae), and southward migration of white pine into the Michigan basin. The beech and hemlock migrations (160 m yr-1and 280 m yr-1, respectively) may have been influenced by the cool-moist climate generated by the Nipissing Great Lakes in combination with enhanced regional warming. White pine and eastern white cedar responded to regional warming and reduced precipitation, whereas birch (Betula) and alder (Alnus) may have been influenced more by fire activity caused by the warm-dry climate. The boreal-mixed forest ecotone was displaced 140 km northward at 5-7 ka compared to 60-70 km for the mixed-deciduous forest ecotone.

Size‐fractionated phytoplankton biomass and its implications for the dynamics of an oligotrophic tropical lake

Summary1. Size‐fractionated phytoplankton biomass was examined in relation to the hydrodynamics of tropical Lake Alchichica from 1999 to 2002.2. Alchichica is a warm monomictic lake, in which mixing takes place from late December to early March. The lake is oligotrophic (mean total chlorophyll‐a concentration 4.2 ± 4.2 μg L−1) and its phytoplankton biomass is dominated (72.3 ± 16.4%) by large individuals (>2 μm). The degree of dominance of the large size class (nano‐ and microplankton) over the small size class (picoplankton) throughout the year is mainly determined by the availability of silicate and the Si/N ratio in the hypolimnion prior to the mixing period.3. This is the first record of an oligotrophic tropical lake dominated by large size fractions of phytoplankton. Because of this dominance, the fate of most primary productivity is rapid sedimentation to the bottom followed by decomposition that promotes an anoxic hypolimnion.4. Our findings in tropical Lake Alchichica challenge the idea that oligotrophic waters are dominated by small phytoplankton, as has been well established for the oligotrophic ocean and temperate lakes.

On the contribution of phytoplankton and benthic biofilms to the sediment record of marker pigments in high mountain lakes

This study compares the marker pigment composition in three different lake compartments: the water column; the surface sediment biofilm (0–0.5 cm), where production and main degradation processes take place; and the deep sediment (15–17 cm), where the signal is finally stored and marker pigments are used as a proxy in paleolimnological studies. The aim was to evaluate which marker pigments better record in the sediment the relative contributions of planktonic and benthic production in high mountain lakes, taking into account the differential preservation existing among pigments. A survey of 82 high mountain lakes distributed along the major environmental gradients was carried out in the Pyrenees. Comparison of pigments between the three compartments revealed that both water column and sediment biofilm signals could be distinguished in old sediment layers, despite the observation that some of the most characteristic carotenoids that appeared frequently and in high concentrations in the water column were already less common or even absent from the sediment biofilm. In the shallowest lakes, pigment composition in the sediment biofilm was typical of photosynthetically active communities and their deep sediment samples were characterised by a substitution of fucoxanthin by diatoxanthin as the dominant diatom marker pigment. However, in the deepest lakes the pigment composition of the sediment biofilm and deep sediment was similar, characterised by marker pigments of mainly planktonic algal groups and pheophytins, which are typical pigments of decaying communities. Results are discussed in terms of how pigment source (planktonic or benthic) and pigment type (lability of molecule) interact to shape pigment composition in the sediment record of high mountain lakes.