EARLY HOLOCENE CONDITIONS AT THE WESTERN MARGIN OF CAPE COD BAY

A paleoenvironmental model is proposed for the western shoreline of Cape Cod Bay, Massachusetts, from about 9,000 through 5,500 years B.P. Central to this reconstruction is the integration and correlation of the glacial sediments, radiometric chronology and pollen stratigraphy of an exposed peat deposit located near Nameloc Heights, Plymouth, Massachusetts. Four Carbon-14 dates provide a chronology and suggest an average rate of peat accumulation of about 6 cm/100 years from 8,800 through 7,400 years B.P., decreasing to 3 cm/100 years from 7,400 to 5,500 years B.P. This sequence of radiometric dates provides a temporal framework within which the pollen taxa of the peat deposit can be correlated to the established floristic zones of the southern New England region.     

A diatom record of environmental change in Fryxell Basin,Taylor Valley,Antarctica, late Pleistocene to present

We present a diatom record from a sediment core taken in Lake Fryxell, Taylor Valley, Antarctica. Six zones were defined using diatom assemblage changes that indicate varying limnological conditions. The early lake stage, ca. 35,000 cal years BP, is characterized by Mayamea atomus f. permitis, a species rarely reported in modern Antarctic Dry Valley environments. An extended period from ca. 35,000 to 19,000 cal years BP is characterized by low diatom abundance, with dominant taxa Luticola spp., Muelleria spp., and Diadesmis contenta. The modern assemblage was established ca. 13,000 cal years BP, after two relatively brief transitional stages. One key species for this recent period, Navicula lineola var. perlepida, is absent in surface sediments and the modern environment, indicating an environmental change within the last several centuries. The diatom assemblage is compared to modern diatom communities in Dry Valley streams, which provide the most complete information on diatom distributions in this region. Although precise environmental interpretation of the core is hampered by limited knowledge of environmental constraints on many of the diatom taxa present in the lake core, the data provide important new insights into the history of Glacial Lake Washburn.     

The postglacial environmental development of Raffles Sø, East Greenland: inferences from a 10,000 year diatom record

A 341 cm long sediment sequence was recovered from the unofficially named Raffles Sø on Raffles Ø, outer Scoresby Sund region, East Greenland. The sediment sequence consists in the upper part (0–230 cm) of a stratified gyttja enriched in organic carbon and biogenic silica whereas the lower core part (235–341 cm) is composed of terrigenous, consolidated glacio-limnic sediments. ¹⁴C-AMS measurements indicate that the sediment sequence represents the entire Holocene lake history from 10,030 calibrated radiocarbon years.The geochemical parameters (opal, total organic carbon (TOC), total nitrogen (TN)) and the total diatom concentration show similar developments during the Holocene, and reflect changes in biological production and nutrient input into the lake. These records clearly reveal a broad Holocene TOC-opal-maximum interval between 5200 and 1800 cal. yrs BP.The diatom flora consisted of 66 taxa representing 20 genera but only seven taxa were abundant and, sometimes, these were monospecifically dominant during the Holocene. In the sediment core from Raffles Sø four successive stratigraphical zones can be distinguished. Accumulation of diatom valves began at 9900 cal. yrs BP with a Stephanodiscus minutulus (Kütz.) Cleve and Möller dominated assemblage (stratigraphic zone 1) followed by a diatom flora dominated by Cyclotella pseudostelligera Hustedt and, less frequently, by Diatoma tenuis Agardh (9400 until 5900 cal. yrs BP, zone 2). Cyclotella sp. A, a taxon which belongs to the Cyclotella rossii-comensis-tripartita-complex, was the dominant floral element between 5200 and 1800 cal. yrs BP (zone 3). From 1800 cal. yrs BP, the periphytic taxa Fragilaria capucina var. gracilis (Østr.) Hustedt and F. capucina var. rumpens (Kütz.) Lange-Bertalot attained highest relative abundances, also almost monospecifically (zone 4).The distribution and composition of the diatom assemblages in the sediment record from Raffles Sø probably reflect past variations in the extent of the lake-ice cover during the growing season. More or less ice-free conditions during summer may have prevailed during the early Holocene until ca. 1800 cal. yrs BP, which allowed growth of planktonic diatoms (Cyclotella taxa) in the pelagic lake region. From 1800 cal. yrs BP, colder conditions lead to a perennial lake-ice cover with a small ice-free moat in summer which favored the growth of periphytic, littoral species (Fragilaria capucina varieties).     

甘肃平凉地区约80万年以来的植被与气候变迁*

将该地约80万年以来的植被发展和气候变化历史划分为14个大的阶段.其中距今约80-78万年、20-14万年、10-1万年三个阶段的植被主要为荒漠草原,气候干冷;距今约78-64、55-46、14-10万年三个阶段植被主要为落叶阔林,气候温暖湿润;其它阶段的植被和气候处于上述二者之间.全新世早、中期,本区气候较现代温湿,晚期趋于温凉半干旱.     

Mid and late holocene diatoms from ‘Water Supply Lake’, Baffin Island,N.W.T., Canada

Diatom analyses from a 40 cm sediment core from Water Supply Lake, northeastern Baffin Island, reveal 67 taxa of primarily benthic forms, strongly dominated by small Fragilaria spp. No major stratigraphic changes are noted in the diatom floral record, which encompasses the last 7000 years. In the uppermost 2 cm, there are increases in: a) F. virescens var. subsalina, b) the ratio of symmetric: asymmetric valves of F. construens, and c) species richness. These are interpreted as reflecting anthropogenic modifications of the lake (chlorination and deepening) associated with its use as a source of water for the community of Pond Inlet since 1979.     

Environmental variability in Lake Naivasha,Kenya, over the last two centuries

Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by ²¹⁰Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm⁻² year⁻¹ correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.     

Holocene diatom flora and stratigraphy from sediment cores of two Antarctic lakes (King George Island)

The diatom stratigraphy of Holocene sediment cores from two Antarctic lakes (Mondsee, Tiefersee; 62° 10 S/58° 50 W) of King George Island was investigated. The diatom assemblages were dominated by cosmopolitan species. The flora was composed of three main components: 1) taxa of submersed bryophytic habitats, which also occur in the bryophilic diatom flora of southern South America; 2) species of various terrestrial habitats, including some specific subantarctic taxa; 3) species distributed in coastal inland waters influenced by sea-spray.Changes in sediment pattern and diatom species composition seemed to reflect climatic changes. At least three different core sections were distinguished in both lakes: Two sections rich in mosses occur between approx. 7000–4700 B.P. and from 3200 B.P. to present, in general resembling present day conditions. In between is a section of high allochthonous content with generally low diatom concentrations and rising percentages of aerophilic, halophilic and alkaliphilic diatoms. An increase of melt water discharge, possible sea-spray influence, and lake- level fluctuations are discussed.Three new taxa are described: Achnanthes metakryophila nov. spec., Achnanthes renei nov. spec., Navicula australomediocris nov. spec. The taxonomy of selected taxa is discussed.     

Holocene paleoclimate changes determined using diatom assemblages from Lake Long,King George Island,Antarctica

Formation of Lake Long, King George Island, Antarctica started about 4,000 years B.P., after which the diatom community changed in response to environmental shifts driven by climatic oscillations (warm/wet and cool/dry). Successive sequences of diatoms in a 7.5-m drill core were divided into 11 assemblage zones by cluster analysis. The most obvious change was an alternation of major dominants, Achnanthes minutissima, Fragilaria alpestris and Fragilaria pinnata v. antarctica according to the climatic oscillations in the late Holocene. Variations in diatom assemblages clearly reflect two warm periods, a single cool period, and three transition periods. The recent warm period (zones 2 and 1) has persisted for approximately 450 years, perhaps sufficiently long to suggest the imminent onset of a new transition period. A recent high TOC (total organic carbon) value in the core reflects a warm period in Antarctica during the late Holocene.     

Assessing aquatic macrophyte community change through the integration of palaeolimnological and historical data at Loch Leven,Scotland

Submerged macrophytes have a critical role in lake ecosystems affecting nutrient cycling, sediment stability, and community composition across multiple trophic levels. Consequently temporal changes in the composition of submerged plant populations can have profound ecological implications and key significance from the perspective of lake conservation. By focusing on macro-remains of aquatic macrophytes and extensive historical plant records spanning the last approximately 180 years, this study seeks to evaluate a combined historical-palaeolimnological approach for establishing pre-disturbance macrophytes communities in Loch Leven, Kinross, Scotland and to provide new information regarding temporal trends in its macrophyte vegetation as well as potential drivers of change. Some 81% of the species historically recorded for the core site (east side of St. Serf’s Island) were found as macro-remains. Potamogeton taxa were underrepresented, whereas remains of Elatine hexandra, a small species never recorded historically were found. The core sequence showed good agreement with known floristic changes including an early (pre- ca. 1850) loss of Isoetes lacustris and Lobelia dortmanna and a more recent (post-1910) shift to dominance by Potamogeton and Chara taxa associated with eutrophic conditions. A clear pattern in the relationship between macrofossil principal component analysis (PCA) and loss on ignition suggested a key control of sediment conditions on macrophyte community structure. In particular the major macrophyte community change of the mid-nineteenth century was concurrent with a substantial increase in organic matter, likely linked to a historic lake lowering (early 1830s) which would have beached the former gravel-sand shoreline leading to a much siltier lake littoral. Although recent monitoring data show signs of ecological recovery our study illustrates that Loch Leven remains a long way from its reference state as a lake with characteristic soft-water macrophytes. To achieve a full recovery, sediment properties would need to change in addition to nutrient reduction. Consequently restoration strategies will need to compromise between the desirability of achieving the pre-disturbance state and what is feasible and practicable. Our study shows the clear potential role of a combined palaeolimnological-historical approach for informing lake management decisions.     

A Holocene paleosalinity diatom record from southwestern Saskatchewan, Canada: Harris Lake revisited

Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-¹ throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.     

Distribution and floristics of desert plant communities in the lower reaches of Tarim River, southern Xinjiang, People's Republic of China

The distribution and abundance of desert plant communities were examined in the lower reaches of the Tarim River, southern Xinjiang. Eighteen plant species were collected at 18 sites along a sequence of increasing ground-water depths in six transects. Except for Tamarix ramosissima and Populus euphratica, which were distributed across nearly all sites, most of the investigated species had low frequency of occurrence. Correspondence analysis (CA) of the 18 species revealed a separation of growth forms into distinct groups corresponding to different ground-water levels. Three clearly defined groups of growth forms were strongly associated with three distinct ground-water zones, ranging from <3, 3–5 and >5 m, respectively. Four taxa groups were found to correspond to the three ground-water zones and to several other environmental factors. Canonical correspondence analysis (CCA) of the 18 species and seven environmental variables would suggest a major botanical gradient exists relating to ground-water depth and secondary gradients exists that include soil moisture, pH and to a lesser extent alkalinity and mineralization.     

Environmental Changes at the Desert Margin: An Assessment of Recent Paleolimnological Records in Lake Qarun, Middle Egypt

Lake Qarun has been profoundly affected by a combination of human activities and climatic changes during the past 5000 years. Instrumental records available for the 20th century show that during most of this period both lake water level and salinity increased and that by the late 1980s lake water salinity was approximately that of seawater. Sediment cores (c. 1 m long) were collected from this shallow (Z_max 8.4 m) saline lake in 1998 and the master core (QARU1) was used to examine the potential of paleolimnology for reconstructing the recent environmental history of the site. According to ¹³⁷Cs and ²¹⁰Pb radio-assay, the recent sediment accumulation rate in QARU1 was around 5 mm year⁻¹ during the latter half of the 20th century but radionuclide levels were low. Spheroidal carbonaceous particles (SCPs) were present in the upper c. 30 cm of QARU1 and indicates contamination by low level particulate pollution, probably beginning around 1950. The record of exotic pollen (Casuarina) indicated that sediment at 51–52 cm depth dated to around 1930. Otherwise the pollen spectra indicated a strongly disturbed landscape with high ruderals and increased tree planting particularly since c. 1950. Diatom records were strongly affected by taphonomic processes including reworking and differential preservation but typical marine diatoms increased after the 1920s. Instrumental records show that the lake became more saline at this time. Freshwater taxa were present at approximately similar abundances throughout the core. This distribution probably reflected a combination of processes. Reworking of ancient freshwater diatomites is one likely source for freshwater diatoms in QARU1 but some taxa must also be contributed via the freshwater inflows. Overall, the diatom stratigraphy indicated increasingly salinity since the 1920s but provided no firm evidence of lake eutrophication. Diatom inferred salinity reconstructions were in only partial agreement with instrumental records but inferred for the lower section of the core (pre 20th century to the 1960s) accord with measured water salinity values. Surficial sediments of Lake Qarun contain environmental change records for the 20th century period but high sediment accumulation rate and pollen reflect the high degree of human disturbance in the region. Because of poor preservation and evidence of reworking, the relationships between diatom records and past water quality changes require careful interpretation, especially in the upper section of the core. Nevertheless, early to mid 20th century measurements of increasing lake water salinity are well supported by sediment records, a change that is probably linked to ingress of saline ground water     

Post-glacial vegetation history of the Mission Mountains,Montana

A pollen record from Rock Lake in the Mission Mountains, northwestern Montana reveals a four-zone sequence reflecting Holocene vegetation change. Chronologic control is provided by two well-known tephras, Glacier Peak (11 200 yr B.P.) and Mazama (6800 yr B.P.). The presence of Glacier Peak tephra above the basal inorganic sediments indicates deglaciation prior to 11 200 yr B.P. Colonizing vegetation (Zone I) after the fall of Glacier Peak tephra was dominated byArtemisia (sage) andAlnus (alder). The presence ofAbies needles,Picea needles, and oneTaxus needle in the core demonstrates that these taxa were at Rock Lake at the time Zone II sediments were deposited. The increase inPinus,Picea, andAbies pollen in Zone II (10 850-4750 yr B.P.) suggests warmer and drier conditions prevailed, and may record the Hypsithermal. The pervasiveness ofPicea andAbies pollen in Pollen Zone III (4750-3350 yr B.P.) suggests the emergence of the modern subalpine forest. Pollen Zone IV (3350 yr B.P.-present) is characterized by relatively little change in the pollen assemblages. One noted change, however, is the increase of Cyperaceae (sedge), which may indicate an expansion of shore-line around the lake, possibly reflecting increased precipitation.This is the 5th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Diatom succession in an urban reservoir system

A 43 cm by 5 cm diameter sediment core sample was obtained from Ford Lake reservoir in Washtenaw County, Michigan, and sectioned at 1 cm intervals. The purpose of this study was to determine whether diatom communities in this reservoir have undergone quantifiable changes in abundance and composition since its creation. Thirty-one cm of this core appeared to represent material deposited since the creation of the reservoir based on changes in diatom abundance, the physical composition of the sediment and the change in biogenic SiO₂ concentration. Fortyseven species of diatoms were identified total concentrations of diatom remains varied from 1×10⁴ g^-1 to 1×10⁷ g^-1. Prior to the establishment of the reservoir, the diatom flora was dominated by benthic taxa. Benthic diatoms were numerous throughout the entire core, but eutrophic taxa (e.g., Aulacoseira italica, Aulacoseira granulata, Stephanodiscus niagarae, Fragilaria crotonensis) dominated much of the core after the reservoir's creation. Total diatom density increased about tenfold in the about the first 10–15 years after the reservoir's creation before declining markedly.     

A high resolution multi-proxy record of pronounced recent environmental change at Baker Lake,Nunavut

Arctic aquatic systems are considered to be especially sensitive to anthropogenic disturbance, which can have cascading effects on biological communities as aquatic food-web structure is altered. Bio-indicators that respond to major limnological changes can be used to detect and infer major environmental change, such as climate warming, with the use of paleolimnological techniques. A multi-proxy approach was used to quantify recent environmental changes at Baker Lake, Nunavut, Arctic Canada. Analyses of fossilized remains of chironomids and diatoms were conducted on a sediment core of 20 cm in length sampled at 0.5-cm intervals. A new surface sediment training set of subfossil chironomid assemblages from 65 lakes across the eastern Canadian Arctic generated a robust (r _jack² = 0.79) surface water paleotemperature transfer function. The transfer function was applied to stratigraphic intervals from the Baker Lake sediment core to generate a paleotemperature reconstruction of sub-decadal resolution. The surface water temperature reconstruction inferred a 2°C increase in mid-summer surface water temperature for Baker Lake over the last 60 years, which was corroborated by the local instrumental record spanning the period of 1950–2007 AD. The chironomid record shows a recent decline of several cold-water taxa and appearance of warm-water indicators. This shift in community structure began circa 1906 AD, and intensified after 1940 AD. The corresponding fossil diatom record showed an increase in small planktonic Cyclotella taxa over the past 60 years, intensifying in the last 5 years, which also suggests a warmer climate and longer ice-free periods. The shifts in the diatom assemblages began later than the shifts in the chironomid assemblages, and were of lower magnitude, reflecting differences in the mechanisms in which these two indicators respond to environmental change.     

湖北利川二万多年来的古植被古气候演变

利用孢粉资料,讨论了利川地区古植被和古气候。     

A high resolution Holocene pollen record from Lago do Pires,SE Brazil: vegetation,climate and fire history

Lago do Pires (17° 57 S, 42° 13 W) is situated at 390 m a.s.l. in the foothills of the Serra do Espinhaço, 250 km from the Brazilian Atlantic coast. The original vegetation of the study area has been almost destroyed by pastoral activity. Relicts of a dense 20–30 m tall tropical semidciduous forest are present only on a few hill tops. The dry season of the Lago do Pires region lasts for 4 months and the annual precipitation is 1250 mm. A high resolution pollen record from a 16 m long sediment core, composed of 77 samples, subdivided in 7 zones and 4 subzones, allows a reconstruction of Holocene paleoenvironments. For the early Holocene (9720-8810 B.P.), the results indicate that the region surrounding the lake was dominated by a herb savanna (campo cerrado) withCuratella americana (cerrado tree) and high fire frequency. Species ofCecropia, Urticales and a few others, form small gallery forests along the water courses. This vegetation pattern is consistent with a long dry season (perhaps 6 months) and a low annual precipitation. Between 8810 and 7500 years B.P. gallery forests expanded in the valleys and suggest a period of higher rainfall with shortened dry season (perhaps 5 months). Fire was less frequent. Reduction of gallery forests followed (7500-5530 B.P.), probably related to a return of drier climatic conditions (5–6 months dry season, lower precipitation). Fires were more frequent. Between 5530 and 2780 years B.P. in the vallyes were forests and on the hills still an open cerrado. The dry season probably was about 5 months and the rainfall was higher than in the previous period. Later (2780-970 B.P.) the more open cerrado on the hills changed to more closed cerrado. A dense and closed semideciduous forest existed in the region only in the latest Holocene period (since 970 B.P.) under the current climatic conditions. The vegetation was no longer influenced by fire. A very strong human impact by deforestation and use of fire occurred in the last decades. Today cerrado vegetation is generally restricted to central Brazil and exists in several small isolated Islands (Hueck, 1956) in the area of semideciduous forest in SE Brazil which were more widespread during the drier periods of the Holocene. The wettest period of Holocene occurs in the present millenium.This is the 4th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Responses of Fragilarioid-dominated diatom assemblages in a small Arctic lake to Holocene climatic changes, Russell Island, Nunavut, Canada

We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.     

Modern Diatom Assemblages in Surface Sediments from Estuarine Systemsin the Southeastern Buenos Aires Province, Argentina

Paleoecological reconstructions of Holocene sea-level changes in Argentinean coastal regions were based mainly on ecological data gathered from other regions, as there was a lack of information on modern estuarine diatom distributions. The aim of the present work was to assess the spatial variation of diatom assemblages in two representative estuaries of Argentina in order to gather ecological information for paleoecological reconstructions in the region. The two selected estuaries have different geomorphologic features and salinity regimes: Mar Chiquita Lagoon is shallow, which prevents the development of a stable salinity gradient as it occurs in the Quequén Grande River. Surface sediment samples were taken from selected stations representative of the environmental gradient from the inlet to the inner reaches of both estuaries. Cluster analysis defined three diatom zones at Mar Chiquita: marine/brackish assemblages dominate the inlet (zone I), where salinity, tidal range and current speed are higher. The brackish/freshwater tychoplankton Staurosira construens var. venter and Staurosirella pinnata dominate the inner lagoon (zone II), where environmental conditions are very variable and concentrations of suspended sediments are higher. Brackish/freshwater euryhaline diatoms dominate the headwaters (zone III). On the other hand, the Quequén Grande River was divided into three diatom zones: coastal taxa are distributed at the inlet (zone I), while the middle estuary (zone II) is dominated by brackish/freshwater euryhaline taxa. At the upper estuary region (zone III), freshwater diatoms dominate, and the halophobous Nitzschia denticula increased in abundance values. Diatom distributions were most closely related to the salinity gradient at Quequén Grande River than at Mar Chiquita Lagoon. Fossil data of a sequence from Mar Chiquita Lagoon (Las Gallinas Creek) were compared to the modern data set in order to search for analogies between fossil and modern diatom assemblages. DCA results showed that fossil diatom assemblages have modern counterparts. Most diatom assemblages of Las Gallinas Creek fall within Mar Chiquita zone III, representing a shallow brackish/freshwater environment, with low salinity fluctuations (~1–9‰) and no tidal influence. Therefore, our modern diatom data provide useful analogs to interpret paleoenvironments in the region.     

Middle to late Holocene chironomid-inferred July temperatures for the central Northwest Territories,Canada

We analyzed subfossil chironomids, sediment organic matter and sediment particle size data from a 1.11-m-long freeze core collected from Carleton Lake (unofficial name), located approximately 120 km north of the modern treeline. This well-dated core spans the last ca. 6,500 years. Two chironomid transfer functions were applied to infer mean July air temperatures. Our results indicated that the chironomid-inferred temperatures from this lake sediment record did not pass a significance test, suggesting that other factors in addition to temperature may have been important in structuring the chironomid community through time. Although not statistically significant, the chironomid-inferred temperatures from this site do follow a familiar pattern, with highest inferred temperatures occurring during the Holocene Thermal Maximum (~6–4 cal kyr BP), followed by a long-term cooling trend, which is reversed during the last 600 years. The largest change in the chironomid assemblage, which occurred between ca. 4,600 and 3,900 cal yr BP is possibly related to the well-documented northward advance and subsequent retreat of treeline in this region.