Climate drivers of diatom distribution in shallow subarctic lakes

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A late glacial to present diatom record from Lake Euramoo, wet tropics of Queensland, Australia

A new diatom record from Lake Euramoo on the Atherton Tableland, north Queensland, Australia is used to assess regional climate change and variability and their links to forcing at a local to global scale. The major factor driving diatom composition in the approximately fifteen thousand-year record appears to be regional moisture availability. Patterns of diatom preservation and other indicators, particularly sediment organic content, suggest that permanent deep water formed at the site from ca. 15,000 cal. yr BP. However, between 13,800 and 11,500 cal. yr BP, there was a notable phase of lower lake levels and effective precipitation. The timing and duration of this phase does not correspond to large-scale climate phenomena such as the Antarctic Cold Reversal or the Younger Dryas and supports emerging evidence for a variable climate regime in the south-west Pacific during the late glacial transition.The Early to Mid Holocene record is one of remarkable stability with 5000 years of sustained dominance by the planktonic diatom Aulacoseira ambigua. Conversely, the Mid to Late Holocene record is marked by distinct diatom variability superimposed on a series of sustained shifts in composition. Accentuated Late Holocene climate variability may aid in explaining intensified land use in indigenous populations and also suggests that Europeans may have arrived in the landscape at the time it was most vulnerable to perturbation.     

Lake Sediment Records on Climate Change and Human Activities in the Xingyun Lake Catchment,SW China

Sediments from Xinyun Lake in central Yunnan, southwest China, provide a record of environmental history since the Holocene. With the application of multi-proxy indicators (total organic carbon (TOC), total nitrogen (TN), δ¹³C and δ¹⁵N isotopes, C/N ratio, grain size, magnetic susceptibility (MS) and CaCO₃ content), as well as accelerator mass spectrometry (AMS) ¹⁴C datings, four major climatic stages during the Holocene have been identified in Xingyun′s catchment. A marked increase in lacustrine palaeoproductivity occurred from 11.06 to 9.98 cal. ka BP, which likely resulted from an enhanced Asian southwest monsoon and warm-humid climate. Between 9.98 and 5.93 cal. ka BP, a gradually increased lake level might have reached the optimum water depth, causing a marked decline in coverage by aquatic plants and lake productivity of the lake. This was caused by strong Asian southwest monsoon, and coincided with the global Holocene Optimum. During the period of 5.60–1.35 cal. ka BP, it resulted in a warm and dry climate at this stage, which is comparable to the aridification of India during the mid- and late Holocene. The intensifying human activity and land-use in the lake catchment since the early Tang Dynasty (∼1.35 cal. ka BP) were associated with the ancient Dian culture within Xingyun’s catchment. The extensive deforestation and development of agriculture in the lake catchment caused heavy soil loss. Our study clearly shows that long-term human activities and land-use change have strongly impacted the evolution of the lake environment and therefore modulated the sediment records of the regional climate in central Yunnan for more than one thousand years.     

The Dry Holocene Megathermal in Inner Mongolia

The paleoclimate since 14 kyr BP (¹⁴C age) was reconstructed based on a 16.22-m-long sediment core collected from Lake Yanhaizi, a saline lake located near the northern limit of the East Asian summer monsoon in Inner Mongolia. Coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated. These sediments have low organic carbon contents but high maturity indices, indicating that they were deposited in an arid environment. By contrast, based on high organic contents and low maturity indices, fine sediments were deposited during periods of high lake stand in a humid environment. It was in general dry between 8.0 and 4.3 kyr BP. The above dry and wet phases are consistent with those recovered from the arid-semiarid transition zone elsewhere, but are unlike the widely perceived humid Holocene Megathermal reported in east China and the newly reconstructed record in the alpine Retreat Lake in Taiwan. The discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon. On the other hand, much enhanced evaporation over higher monsoon precipitation at Lake Yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon. This also accounts for the fact the high-temperature Holocene Megathermal, as revealed in the Okinawa Trough and the northern South China Sea, is correlated to the dry phases at Lake Yanhaizi. Conversely, the 4-2-kyr BP coldest period in the Holocene corresponds to a wet phase at Lake Yanhaizi.     

Climate Versus In-Lake Processes as Controls on the Development of Community Structure in a Low-Arctic Lake (South-West Greenland)

The dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A ¹⁴C and ²¹⁰Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic–inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (δ¹³C, δ¹⁸O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18–32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12–17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.     

Lake Ecosystem Responses to Holocene Climate Change at the Subarctic Tree-Line in Northern Sweden

A Holocene sediment sequence from Lake Seukokjaure, a subarctic lake at tree-line in northern Sweden, was analyzed to assess major changes in the structure and functioning of the aquatic ecosystem in response to climate change and tree-line dynamics. The compiled multi-proxy data, including sedimentary pigments, diatoms, chironomids, pollen, biogenic silica (BSi), carbon (C), nitrogen (N) elemental and stable-isotope records, and total lake-water organic carbon (TOC) concentration inferred from near-infrared spectroscopy (NIRS), suggest that the Holocene development of Lake Seukokjaure was closely coupled to changes in terrestrial vegetation with associated soil development of the catchment, input of allochthonous organic carbon, and changes in the light regime of the lake. A relatively productive state just after deglaciation around 9700 to 7800 cal years BP was followed by a slight long-term decrease in primary production. The onset of the local tree-line retreat around 3200 cal years BP was accompanied by more diverse and altered chironomid and diatom assemblages and indications of destabilized soils in the catchment by an increase in variability and absolute values of δ¹³C. An abrupt drop in the C/N ratio around 1750 cal years BP was coupled to changes in the internal lake structure, in combination with changes in light and nutrient conditions, resulting in a shift in the phototrophic community from diatom dominance to increased influence of chlorophytes, likely dominated by an aquatic moss community. Thus, this study emphasizes the importance of indirect effects of climate change on tree-line lake ecosystems and complex interactions of in-lake processes during the Holocene.     

Paleoenvironmental changes in northwest Mongolia during the last 27?kyr inferred from organic components in the Lake Hovsgol sediment core record

We studied organic components in the X106 sediment core (length 130.3 cm, water depth 236 m, 50°53′01″N, 100°21′22″E) from Lake Hovsgol to elucidate the biological production, source of organic components, and paleoenvironmental and paleolimnological changes during the last 27 kyr in northwest Mongolia. Total organic carbon (TOC) contents (0.20–0.70%) in the core of the last glacial period increased dramatically and attained 3.16–5.85% in the postglacial period (Holocene), together with the increase of the contribution of terrestrial organic matter. Biological production (both terrestrial and aquatic production) based on the TOC contents in the Holocene was 14 times higher than that in the last glacial period. The B?lling-Aller?d warm period and Younger Dryas cool period were both observed at depths of 55–50 cm (ca. 15–13 cal kyr BP) and 50–45 cm (ca. 13–11 cal kyr BP), respectively. We propose here a terrestrial/aquatic index (TAI) for organic matter in lake sediments. The TAI values suggest that terrestrial organic matter in the bottom of the core was less than 10%, increased to 48% in the B?lling-Aller?d warm period, decreased abruptly to 20% in the Younger Dryas cool period, and again increased to 30–40% in the Holocene. Normal-C₃₁ alkane (a biomarker of herbaceous land plants) and n-C₁₈ alkanoic acid (marker of plankton) decreased from the last glacial period to the Holocene, whereas n-C₂₃ alkane and n-C₂₂ alkanoic acid (a marker of higher vascular plants) increased from the last glacial period to the Holocene. Scarce herbaceous plant vegetation, such as Artemisia spp. of the lake basin in the last glacial period, changed into an abundance of higher woody plant vegetation (e.g., Pinus spp., Betula spp. and/or Larix spp.) in the Holocene. Stanol/sterol ratios suggest that relatively high oxygen tension of the lake bottom in ca. 27–22 cal kyr BP decreased from this age to the present, though benthic organisms are still abundant.     

Late Holocene changes in the humic state of a boreal lake and their associations with organic matter transport and climate dynamics

An important factor in the ontogeny of boreal lakes is the development of their humic state through terrigenous input of organic matter (carbon) that affects strongly the functioning and structure of these ecosystems. The long-term dynamics and role of humic substances for these systems in relation to climate are not clear. In this study, a boreal lake from southern Finland was investigated using paleolimnological methods, including diatom, chironomid and geochemical analyses, for Late Holocene changes in the humic state. The aim was to examine the relationship between sediment biogeochemistry and climate variation. Consistent trends were found in diatom-inferred total lake-water organic carbon (TOC) and in the ratio of humic/oligohumic chironomids. Sediment geochemistry provided further evidence for the limnological development of the lake and related long-term climate trends in the region. The results indicated three distinct phases with differing humic state; the beginning of the record at ca. 4,500 cal year BP was characterized by extremely humic conditions coinciding with warm and dry climatic conditions, a meso-oligohumic period between ca. 3,000–500 cal year BP with increasing allochthonous organic matter transport and cooler and wetter climate, and recent period with polyhumic (TOC >10 mg L^?1) lake status and warming climate. This study shows that instead of straightforward linear development, boreal lakes evolve through dynamic humic stages related to climate and lake-catchment coupling processes. As the changes in the humic state are ultimately climate-driven, the ongoing climate change probably has a major influence on boreal lakes through direct and indirect effects on organic carbon transport, utilization and accumulation.     

Holocene vegetational and coastal environmental changes from the Lago Crispim record in northeastern Pará State, eastern Amazonia

Vegetational and coastal environmental changes have been interpreted from a 600cm long and 764014C yr B.P. old sediment core from Lago Crispim located in the northeastern Pará State in northern Brazil. The radiocarbon dated sediment core was studied by multi-element geochemistry, pollen and charcoal analysis.Holocene Atlantic sea-level rise caused an elevation of local water table, which allowed the formation of organic deposits in a probably former inter-dune valley. Dense, diverse and tall Amazon rain forest, and some restinga (coastal vegetation) covered the study area at the beginning of the record at 764014C yr B.P. Mangrove vegetation developed along rivers close to the core site at that time. Subsequent decrease in less mangrove vegetation near the study site indicates a sea-level regression, beginning since around 700014C yr B.P. Lower sea-levels probably favoured the formation of a local Mauritia/Mauritiella palm swamp at 662014C yr B.P. Oscillations of higher and lower sea-level stands probably changed the size of the local palm swamp area several times between 6620 and 363014C yr B.P. Sea-level transgression at around 363014C yr B.P., caused marked coastal environmental changes: the development of mangroves near the site, the replacement of the local palm swamp by a Cyperaceae swamp, the substitution of the surrounding former Amazon rain forest and some restinga vegetation mainly by salt marshes. High amount carbonised particles suggest a strong human impact by burning on the coastal ecosystems during this late Holocene period.Highest concentrations of NaCl and also Ca, Mg and K in the upper sediment core indicate that the Atlantic was close during the late Holocene period. The core site, which is today 500m from the coastline and only 1-2m above modern sea-level, was apparently never reached by marine excursions during the Holocene.Less representation of mangrove since ca. 184014C yr B.P., may be related due to a slightly lower sea-level or to human impact in the study region. The modern shallow lake seems to be formed recently by road construction, forming an artificial dam.     

Long-term investigations on the changes of the MPN values of bacterial communities participating in the sulphur cycle in Lake Velencei,Hungary

Lake Velencei is a shallow lake with a fairly variable water quality due to the effect of the earlier basin reconstruction and external factors, such as the weather and the organic matter loading from the catchment areas. For studying the changes in sediment of Lake Velencei, MPN technique, based bacteriological investigations, as well as measurements of water chemical parameters were performed. Microbial communities of aerobic thiosulphate-oxidising, anaerobic phototrophic, and sulphate-reducing bacteria taking part in the sulphur cycle were monitored between 1993 and 2001. In the western part of the lake the bacteriological results and the chemical parameters verified the presence of sulphuretum. In the first low water period of the studies the quantity of the bacteria contributing to the sulphur cycle was low in the sediment. After the increase of the water level the distribution of these microbes became somewhat homogeneous in the sediment. However, the different sediment regions might be characterised with dissimilar MPN values of the studied bacterial communities.

     

History of anthropogenically mediated eutrophication of Lake Peipsi as revealed by the stratigraphy of fossil pigments and molecular size fractions of pore-water dissolved organic matter

We investigated stratigraphic changes in fossil pigments and the molecular structure of the UV-absorbing fraction of pore-water dissolved organic matter in a sedimentary record from Lake Peipsi (Estonia/Russia) temporally covering the 20th century. The aims of the study were to define the onset of eutrophication in the lake and to track its course. An attempt was also made to reconstruct lake conditions before the intensive nutrient loading began. Fossil pigment analysis indicated that the eutrophication of the lake started in the 1960s and accelerated in the 1970s. Sedimentary pigments also indicate a continuing tendency of the lake ecosystem towards eutrophy in the 1980s and 1990s. However, changes in the molecular size structure of pore-water dissolved organic matter indicated that the contribution of autochthonous matter to the organic pool of the lake ecosystem had already started to increase around the end of the 1930s. We conclude that this rise was generated by a coincidence of several anthropogenic and natural factors. The pore-water data also show that a slight relative reduction in the autochthonous organic matter took place in the 1990s. A discordance in the paleodata obtained for the beginning of the 20th century complicates clear conclusions about earlier conditions in the lake. On the one hand, the qualitative characteristics of pore-water dissolved organic matter and the low concentration of chlorophyll a indicate that the phytoplankton biomass was low in Lake Peipsi during that period. On the other hand, the concentrations of marker pigments of specific phytoplankton groups are high, comparable with the values in the recent sediments. Possible reasons for the high levels of these pigments in the early 1900s sediments, such as a shift in the preservation conditions of organic substances and their transport from the lake’s catchment, are discussed.     

An 11?000-year lithostratigraphic and paleohydrologic record from Equatorial Africa: Lake Edward, Uganda-Congo

Sedimentologic and geochemical analyses of four cores from Lake Edward, Uganda-Congo, document a complex record of moisture balance in the Edward basin from 11?000 cal yr BP to present. Highly organic, diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early Holocene wet phase. Lake level variations within this period may have shifted due to tectonic lowering of the lake’s outlet level. The onset of mid-Holocene aridity, as documented by the appearance of authigenic calcite at 5200 cal yr BP, initiated a period of falling lake levels that culminated in a late Holocene lowstand between 4000 and 2000 cal yr BP. This lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site. Although different cores yield different ages for this regression, it appears that declining lake levels culminated in a maximum lake lowstand of −14 m. Lake levels then rose rapidly, attaining modern positions by 1700 cal yr BP. This lake level history suggests that although many paleoclimatic changes in Africa are apparently synchronous throughout northern hemispheric Africa, other events may be spatially heterogeneous. These patterns highlight the need for a well-dated network of paleoclimate sites within the African continent.     

A comparison of the palaeolimnology of Peipsi and Võrtsjärv: connected shallow lakes in north-eastern Europe for the twentieth century, especially in relation to eutrophication progression and water-level fluctuations

We applied a multi-proxy palaeolimnological approach to provide insights into the natural variability and human-mediated trends of two interconnected temperate large shallow lakes, Peipsi and Võrtsjärv, during the twentieth century. The history of the lakes was assessed on the basis of age-related changes in the sediment main constituents (water, organic matter and carbonate), sub-fossil pigments, diatom assemblages and organic matter dissolved in pore water. The temporal changes in the palaeodata indicate an increase of the in-lake biological production in both lakes from about the 1960s, suggesting enhanced nutrient inputs. In subsequent decades, the gradual increase of autochthonous organic matter becomes more obvious, indicating progressive eutrophication of the lakes. Palaeolimnological indicators from the sediment record of Lake Peipsi indicate a slight recession of the lake’s eutrophication in the 1990s but not for Lake Võrtsjärv. The results of the study also suggest that after the lakes became eutrophied, the climatically induced water-level fluctuations ceased to be the main driver determining the abundance of phytoplankton. Responses of the lakes to human-induced impacts are better recorded in the sediments of Lake Peipsi than in those of Lake Võrtsjärv, which is shallower of the two and where the wave-induced resuspension of deposits markedly smooths or erases the signals of environmental changes. The results of the investigation expand the knowledge on how large shallow lakes respond to human-mediated and natural perturbations, including those in the lake catchment areas and the capability of the lakes to store the chronology and sequence of these changes.     

Holocene history of environment and human impact on two islands in the Ostholstein lakeland area, Northern Germany

Holocene sediment cores from contemporary terrestrialised kettle holes on two islands in the Ostholstein lakeland area, Northern Germany, were investigated by palaeoenvironmental methods. Records from such isolated locations, such as these island mires, are an important source of information on small-scale vegetation changes. Pollen, non-pollen palynomorphs and macrofossils were used to reveal anthropogenic impact as well as lake level fluctuations. Changes in the peat decomposition and the ratio of organic to minerogenic content in the sediment facilitated correlation with past ground water table levels and accordingly with former lake levels. The palynological surveys indicate that human impact on the islands expanded in prehistoric times during phases of low lake levels or periods of stable hydrological conditions. The first distinct low level is recorded between ca. 8210 and 7740?cal.?yr b.p. Another major low level period was detected for the time span from approximately 5290 to 4600?cal.?yr b.p. After a subsequent period with a wetter climate, indicated by rising and fluctuating lake levels culminating at ca. 2420 to 2180?cal.?yr b.p., a third lowering occurred until about 660?cal.?yr b.p. This period was interrupted by minor fluctuations in lake levels from ca. 1770 to 1630?cal.?yr b.p. and from ca. 1370 to 1010?cal.?yr b.p.     

The influence of climate and sea-level change on the Holocene evolution of a Mediterranean coastal lagoon: Evidence from ostracod palaeoecology and geochemistry

Coastal lagoons provide an excellent basis for the study of processes controlling the evolution of a coastal zone. We examine the relative importance of these processes during the middle to late Holocene through a study of an 8.5 meter-long sediment record from the Albufera de Valencia (Spain). We combine sedimentological analyses with investigations into the palaeoecology, taphonomy and geochemistry (Mg/Ca, Sr/Ca, δ¹⁸O and δ¹³C) of ostracod valves in order to assess the effects of sea-level changes, storm events and effective moisture on the evolution of a Western Mediterranean coastal wetland. The late Pleistocene sediments represent a subaerial environment, which was followed by a hiatus in deposition. The first Holocene unit (～8700–7500 calendar yr. BP) is composed of typical lagoon-barrier and backshore sediments, deposited when seawater intruded into the lake and the climate was arid. The upper part of the sequence (between 7500 and 3400 yr.) is characterized by two sedimentary units, which correspond to Holocene progradation phases and humid climate associated with an increased freshwater influx to the lake accompanied by several high-energy events (palaeostorms). Overall, the record shows that an arid climate prevailed in the western Mediterranean area between 8400 and 7600 yr. The main marine transgression and accompanying progradational phases occurred between 7000 and 3400 yr., which is confirmed by other studies of coastal evolution along the Mediterranean coast. The multiproxy reconstructions demonstrate that controls on sedimentation and palaeoecology in this Mediterranean coastal lagoon were complex.     

Multidecadal- to century-scale arid episodes on the northern Altiplano during the middle Holocene

We present reconstructions of multidecadal- to centennial-scale shifts in the hydrologic balance of the northern Altiplano during the middle Holocene using stable isotopic, geochemical, and sedimentologic evidence from Lake Titicaca sediments. Large downcore changes in the δ¹³C_org of bulk organic matter, previously shown to be a site-specific lake-level proxy, indicate that extended periods of lake-level regression occurred in response to sustained reductions in mean annual precipitation and/or increases in air temperature. Supporting evidence from a floating interval of finely-laminated sediments indicates that salinity and stratification were enhanced at Lake Titicaca during the middle Holocene. The age model used in this study is based on 7 AMS ¹⁴C age dates estimated from bulk decalcified sediment from a 60 cm sequence spanning 7200–5600 ¹⁴C cal yr BP. Simulations from a hydrological-energy balance model of the Titicaca Basin indicate that the ～ 38 m drop in lake level that preceded the maximum lowstand at ～ 6200 yr BP would require a significant decrease in mean annual precipitation of up to 50% and/or a significant increase in mean annual temperature of up to 8 °C. Today, the Altiplano is often affected by drought during periods of enhanced summertime westerly wind flow, which reduces the delivery of moisture from the east. This investigation supports the idea of sustained periods of enhanced westerly air flow during the mid-Holocene and provides additional submillennial-scale information about the timing and magnitude of regional drought. Results of our frequency-domain analysis of the floating lamination interval indicate significant power at a decadal periodicity (10–12 yr) associated with the Schwabe cycle of solar activity. Frequency domain analysis also reveals power in a 75 year time series of modern lake level. We suspect that solar influence on large-scale atmospheric circulation features results in changes to the moisture balance in the Titicaca basin over decadal and possibly century-scale time periods.     

Last glacial–Holocene water structure in the southwestern Okhotsk Sea inferred from radiolarian assemblages

In order to reconstruct paleoceanographic conditions in the southwestern (SW) Okhotsk Sea, radiolarian assemblages were analyzed from 10 surface sediments and 5 sediment cores obtained from various water depths, ranging between 461 and 1348 m. We also measured geochemical components such as biogenic opal, calcium carbonate, total organic carbon, total sulfur and total nitrogen for one sediment core. These data imply that the complicated water structure in the SW Okhotsk Sea changed with time from the last glacial to Holocene. The glacial surface water was characterized by low primary production in the summer and expanded sea-ice coverage in the winter. During this time, the ventilation might have reached deeper than during the Holocene. During the major deglacial period including the melt-water pulses 1A and 1B, the deep-sea oxygen content had declined with less ventilation and/or more oxygen consumption with organic matter supply into the deeper depths. The intermediate layer has been well ventilated and has supplied large amount of organic matter during the last 20 kyrs, especially in the early-mid Holocene, however such condition ceased during 3 to 2 ka. The warm Pacific deep water has been present since 9 ka. The influence of warmer surface water such as the Soya Warm Current (SWC) increased in the coastal area during 7 to 4 ka and since 2 ka till present. Neritic-derived productivity might be extended at 2.2–1.8 ka and since 1.2 ka till present.     

A late pleistocene and holocene vegetation and environmental record from Lake Wangoom, Western Plains of Victoria, Australia

Pollen, microfaunal and sedimentological evidence from the top 20 m of sediment in a closed volcanic crater lake is used to construct a detailed record of vegetation and environmental conditions through the Holocene and a substantial part of the Late Pleistocene. Radiocarbon dating suggests that the sequence covers tha last 51,000 yr or so. High lake-levels and the presence of forest or woodland vegetation indicate that the Holocene and the basal few thousand years of the record experienced wet and warm conditions. The earliest period was succeeded by a long phase of lower but variable moisture levels before more arid conditions resulted in the replacement of forest and woodland by herbaceous vegetation and frequent lake drying. Periods of slight amelioration occurred between about 27,000 and 19,000 and before 15,000 radiocarbon yr ago. Maximum aridity occurred between about 19,000 and 10,000 yr B.P., a period incorporating the height of the last glacial. During the Pleistocene, the lake became gradually more saline but has been fresh during the Holocene, even under low lake-levels.

An increase in charcoal values is recorded, probably around 20,000 years ago, and this could have resulted from increased burning as a result of the activities of Aboriginal people. This may have also caused a change in understorey vegetation from one composed largely of Asteraceae, to grassland.

The record is compared with others from southeastern Australia and there is good correspondence through the last 20,000 or 30,000 yr but, before this, problems of dating and the condensed nature of most sequences prevent detailed correlation.     

An integrated approach to relate Holocene climatic, hydrological, morphological and vegetation changes in the southeastern Amazon region

The geomorphological analysis of the river Tocantins alluvial area was carried out by remote sensing techniques, jointly with studies of sedimentary facies, mineralogy and geochemistry, pollen and spores, C and N isotopes data and radiocarbon dating of a sediment core from a floodplain lake, near the river Tocantins in the southeastern Amazon region. The aim was to identify and discuss the relationship between the main geomorphological processes and climatic factors, as well as their influence on vegetation patterns. The data indicate three phases of sediment and organic matter accumulation associated with vegetation changes during the Holocene: in Phase 1 (>7760 ± 80 cal yrs b.p.), the channel was gradually being disconnected from its main course, accumulating organic matter from terrestrial woody plants; in Phase 2 (>7760 ± 80 to 5145 ± 175 cal yrs b.p.), the very low energy flows allowed the deposition of autochthonous organic mud from suspension. Herbaceous vegetation and a Mauritia palm-swamp dominated the study area, followed by an increase in the contribution of freshwater dissolved organic carbon (DOC) to the lake; Phase 3 (5145 ± 175 cal yrs b.p. until present) records the expansion of ferns and arboreal plants, and greater freshwater organic matter input into the lake with anoxic water conditions. The development of the studied lake occurred by geomorphological changes and the filling process of an abandoned channel according to hydrodynamics and vegetation changes which were probably influenced by climatic changes during the Holocene.     

A mid-Holocene environmental change in Amazonian savannas

Aim The main goal of this study was to investigate how climate and human activities may have influenced ecotonal areas of disjoint savannas within Brazilian Amazonia. Location Eastern Brazilian Amazonia, Amapá State. Methods The fossil pollen and charcoal records of two lakes in Amapá (Marcio and Tapera) were used to provide a Holocene palaeoecological history of eastern Amazonian savannas. Detrended correspondence analysis was used to enhance the patterns of sample distribution along the sediment core. Results Even though sedimentary hiatuses were recognized in the sediment cores from both lakes, a marked change in vegetation from closed forests with swamp elements to open flooded savanna at c. 5000 yr bp was evident from the pollen record. Charcoal analysis revealed a pattern of increased accumulation of charred particles coincident with the establishment of savanna vegetation, suggesting higher fire frequency near the lakes. Because the timing of the sedimentary hiatus overlapped with the highest Holocene sea level, which would have increased the local water table preventing the lakes from drying out, we infer that both lakes used to depend heavily on flood waters, and the sedimentary gap was caused by reduced discharge from the Amazon River, due to a dry period in the Andes, when precipitation levels markedly decreased between 8000 and 5000 yr bp . The lack of Andean pollen (probably river transported) in the sediment record after this event and the existence of similar records near the study site make this interpretation more appealing. The resumption of sedimentation in Lake Marcio, contemporaneous with falling sea level and increasingly wet conditions in the Andes after 5000 yr bp , indicates that Holocene sea‐level variation did not play an important role in maintaining lake levels. Main conclusions The study site recorded long‐term occupation by pre‐Columbian peoples. However, it is still unclear whether these disjoint savannas have an anthropogenic origin. Even though locally dry environmental conditions were inferred from both records, there is no evidence of a mid‐Holocene dry climate in eastern Amazonia. Instead, the Amapá record indicates a connection between Andean climate and eastern Amazonia, demonstrating the need for a better understanding of the impacts and magnitude of climate changes.