1,721,195 research outputs found
Stratigraphy, age and correlation of two widespread Late Holocene tephras preserved within Lake Kutubu, Southern Highlands Province, Papua New Guinea
No full textSediment cores were retrieved from Lake Kutubu, the largest upland lake in Papua New Guinea, to assess palaeoenvironmental baselines. Two prominent tephra layers were encountered within the cores. Using a combination of core stratigraphy, sediment properties and geochemical characterization (electron microprobe and laser ablation inductively coupled-plasma mass spectrometry techniques) we were able to correlate these tephras with Tibito and Olgaboli tephras, both previously described at adjacent distal localities. Glass shard compositional data of Tibito tephra from Lake Kutubu, Kuk swamp and from proximal volcaniclastic successions from Long Island are indistinguishable and this similarity strongly supports Long Island as the eruptive source. For Olgaboli tephra, the location of its eruptive source is less certain but the glass shard compositional data indicate a Karkar Island source. The occurrence of Tibito and Olgaboli tephras in Lake Kubutu sediments extends their known distributions further south-west into the southern Highlands region and affirms their importance as inter-regional stratigraphic markers, of late Holocene age, that have considerable potential for use in future palaeoenvironmental, archaeological and volcanic hazard and/or impact studies. Copyright © 2017 John Wiley & Sons, Ltd. Copyright © 2017 John Wiley & Sons, Ltd
Stratigraphy, age and correlation of Lepue Tephra: a widespread c. 11 000 cal a BP marker horizon sourced from the Chaitén sector of southern Chile
Full text linkWe describe the stratigraphy, age and correlation of a prominent tephra marker, named Lepue Tephra, extensively distributed in north-western Patagonia. Lepue Tephra is well dated at c. 11 000 cal a BP from numerous lake and soil cover-bed sequences and its recognition is useful for assessing the rate and timing of deglaciation as well as associated environmental changes in this region during the last glacial termination and early Holocene. Lepue Tephra has attributes typical of a complex and compositionally zoned phreatomagmatic eruptive. While the initial rhyolitic phase can be readily distinguished from multiple eruptive products sourced from the adjacent Volcan Chaiten, the main erupted end member is of basaltic-andesitic bulk composition-similar to younger tephras sourced from Holocene monogenetic cones adjacent to the Volcan Michimahuida massif (tMim). Lepue Tephra can be correlated to an equivalent-aged pyroclastic flow deposit (Amarillo Ignimbrite) prominently distributed in the south-eastern sector of tMim. The source vent for these co-eruptive events is obscured by an extensive ice field and is currently unknown. The widespread radially symmetrical distribution of Lepue Tephra centred on tMim cannot be attributed solely to volcanological considerations. Reduced Southern Hemisphere westerly wind influence interpreted from climate proxies at the time of eruption are also implicated.Iniciativa Cientifica Milenio
P02-51
NC120066
Fondecyt
1151469
1160488
Victoria University of Wellington Science Faculty Research Grant
Aberystwyth University Research Fund
PIP CONICET
2011 031
Palaeolimnology of Adelaide Tarn, a ~14,000-year-old low-alpine glacial lake, northwestern South Island, New Zealand
No full textA palaeolimnology study has been carried out on a ~14,000 calendar [cal]-year-old low-alpine glacial lake, Adelaide Tarn. The lake is located in a cirque basin at ~1260 m elevation in the Tasman Mountains, just below present-day treeline, within Kahurangi National Park, northern South Island, New Zealand. A 5.6-m long sediment core was retrieved from the lake by staff of GNS Science. The chronology of the core was constructed from 15 ¹⁴C dates obtained via AMS on 14 samples of in-situ plant macrofossils and one sample of bulk organic sediment. The core was divided into lithozones 1, 2, and 3 from the base through to the top of the core. Lithozone 1 (5.6–4.8 m) comprises inorganic (carbon content 0.3 to 4%) grey silts with gravel (Munsell colour 5Y 6/1) and these sediments, primarily composed of angular particles identifed by scanning electron microscopy (SEM), date to a little earlier than ~13,932 cal yr BP. Lithozone 2 consists of organic (carbon content up to 15%) brownish black (10YR 2/3) silt and clay and extends from ~13,932 to ~7709 cal yr BP. In lithozone 2, diatoms appear, as identified by SEM. Lithozone 3 is made up of dark brown (10YR 3/4) organic silt (carbon content 5 to 10%) and extends from ~7709 cal yr BP through to the top of the core which has an age estimated to be a little older than ~700 cal yr BP on the basis of a ¹⁴C date (~926 cal yr BP) at 13 cm depth and a pollen record that shows neither a Polynesian deforestation signal (i.e. the sediments likely pre-date ~700 cal yr BP) nor European adventives (i.e. the sediments pre-date ~1840 AD). Lithozone 3 is diatom-rich with subordinate angular particles. Intermittent yellow/brown (10YR 6/8) laminae occur throughout lithozones 2 and 3; one lamination occurs near the top of lithozone 1. These yellow/brown laminae comprise mainly angular clastic particles (evident in SEM micrographs) and show a slight increase in sand compared with non-laminated sediments, and the laminae are inferred to reflect terrigenous input to the lake as a consequence of storms or during intense rainfall events. These possible storm events may correspond with ENSO events as described/identified in Lake Tutira in Hawke’s Bay.
Multiple components of the sediment archive were analysed to reconstruct the history of the lake and its catchment. Properties included were X-radiography, grey-scale, magnetic susceptibility, grain size, plant macrofossil assemblages, organic carbon content, and isotopes δ¹⁵N and δ¹³C.
The results show four phases of climate variability (Adelaide Tarn climate events, AT-CEs) from the onset of lake formation ~14,000 cal yr BP. AT-CE1: early sediments ~14,000 cal yr BP show enhanced terrigenous input (fine-grained grey silts, with gravel layers, of lithozone 1), consistent with fluvio-glacial in-wash during retreat from the Adelaide Tarn basin of a local cirque glacier. AT-CE2: from ~13,932 to ~10,000 cal yr BP, erosion of the catchment is much diminished, indicated by decreases in modal grain size and magnetic susceptibility, and lake productivity concomitantly commences as shown by the δ¹³C and δ¹⁵N isotope records. Poaceae species dominate the macrofossil plant assemblages with no tree-species found. AT-CE2 (~13,932 to ~10,000 cal yr BP) is inferred to be one of warming but the catchment was not yet stable and little or no soil formation occurred that would allow habitation by tree species. AT-CE3: the third palaeoenvironmental phase, from ~10,000 to ~2400 cal yr BP, is marked by an expansion of Nothofagus (Nothofagus menziesii and Nothofagus fusca) forest with minor constituents of Libocedrus biwillii and Phyllocladus alpinus. This phase is inferred to be one of sustained warm conditions and a stable catchment. Adelaide Tarn is situated 250 m higher than the present-day altitudinal limit of N. fusca and the presence of macrofossils of this species in the sediment record suggests that between ~6400 and ~2400 cal yr BP the mean annual temperature was possibly ~1.2˚C warmer than present based on an observed environmental lapse rate of 0.47˚C/100 m. There was slight increase in denitrification from ~8000 through to ~3000 cal yr BP, suggesting that primary productivity increased in the lake. AT-CE4: from ~2400 cal yr BP, the fourth phase, the plant assemblage shifted to Poaceae-dominated; cold-sensitive taxa were forced to descend, marked by disappearance of forest species from the plant macrofossil assemblage. The climate is inferred to have deteriorated and the tree-line descended to near the present-day position.
The Adelaide Tarn record is one of only a few spanning the last ~14,000 years in central New Zealand, and is especially useful in adding to the plant macrofossil records, which are rare. The Adelaide Tarn record was compared with the newly-published New Zealand climate event stratigraphy (NZ-CES), with the proposed tripartite subdivision of the Holocene, and with a number of records across New Zealand that are based on various proxies including speleothems and pollen assemblages. None of the NZ-CES events were clearly evident in the Adelaide Tarn record (apart from NZce-1), and the proposed boundaries of the subdivided Holocene (at ~8.2 ka and 4.2 ka) were not evident. A palynological record spanning the last ~12,000 years from Cropp Valley, western South Island, showed close consistency with the Adelaide Tarn record
Exploring the Spatio-temporal Relationship Between Two Key Aeroallergens and Meteorological Variables in the United Kingdom
No full textConstructing accurate predictive models for grass and birch pollen in the air, the two most important aeroallergens, for areas with variable climate conditions such as the United Kingdom, require better understanding of the relationships between pollen count in the air and meteorological variables. Variations in daily birch and grass pollen counts and their relationship with daily meteorological variables were investigated for nine pollen monitoring sites for the period 2000–2010 in the United Kingdom. An active pollen count sampling method was employed at each of the monitoring stations to sample pollen from the atmosphere. The mechanism of this method is based on the volumetric spore traps of Hirst design (Hirst in Ann Appl Biol 39(2):257–265, 1952). The pollen season (start date, finish date) for grass and birch were determined using a first derivative method. Meteorological variables such as daily rainfall; maximum, minimum and average temperatures; cumulative sum of Sunshine duration; wind speed; and relative humidity were related to the grass and birch pollen counts for the pre-peak, post peak and the entire pollen season. The meteorological variables were correlated with the pollen count data for the following temporal supports: same-day, 1-day prior, 1-day mean prior, 3-day mean prior, 7-day mean prior. The direction of influence (positive/negative) of meteorological variables on pollen count varied for birch and grass, and also varied when the pollen season was treated as a whole season, or was segmented into the pre-peak and post-peak seasons. Maximum temperature, sunshine duration and rainfall were the most important variables influencing the count of grass pollen in the atmosphere. Both maximum temperature (pre-peak) and sunshine produced a strong positive correlation, and rain produced a strong negative correlation with grass pollen count in the air. Similarly, average temperature, wind speed and rainfall were the most important variables influencing the count of birch pollen in the air. Both wind speed and rain produced a negative correlation with birch pollen count in the air and average temperature produced a positive correlation
Late Quaternary palynological investigations into the history of vegetation and climate in northern New Zealand
No full textThis thesis describes the vegetation and climatic changes over the past 20,000 years from pollen records at eight northern New Zealand lowland peat and lake sites, ranging from Taranaki to the Far North. The sites investigated are Umutekai Swamp (Taranaki), Lakes Rotomanuka, Rotokauri, and Okoroire (Waikato), Kopouatai Bog (Hauraki Plains), Lake Waiatarua (Auckland), Otakairangi Swamp (mid-Northland), and Trig Road Swamp (Far North). At sites from Auckland southwards, dating and correlation of the pollen records were enhanced by the occurrence of multiple tephra layers within the pollen-bearing sediments. The clearest picture of regional vegetation history and tightest chronologic control were obtained from the tephra-rich organic lake sediments of the Waikato lowlands.
Holocene vegetation changes were broadly consistent throughout this northern New Zealand region and indicate climates, which were initially moist, mild and equable, but became increasingly variable and probably drier overall during the late Holocene. Podocarpangiosperm forest was always prominent and Agathis australis forest expanded throughout the region north of latitude 38° S during the last 6,000 years. Kauri was especially prominent in the Waikato region during the 1000 years or so following the Taupo eruption of c.1800 years ago. At pollen sites from Waikato, Hauraki Plains, and Auckland, palynological evidence suggests that people began clearing forests as early as 800 years ago, but probably not much earlier.
Pollen records for the last glacial show less regional uniformity. South of Auckland, scattered tracts of Nothofagus or Libocedrus forest within a shrubland/grassland mosaic were succeeded, between c.14.5 and 10 ka by the regional expansion of podocarp-angiosperm forest, with Prumnopitys taxifolia initially prominent. North of Auckland, the pre-Holocene vegetation history is complicated by uncertain chronologies. Conifer-angiosperm forest with prominent A. austalis grew in the Far North during the last glacial, while in mid-Northland, a substantial period of Nothofagus forest, shrub and grassland communities may correspond to either the entire last glacial or to the late glacial.
Local variations in vegetation cover were maintained to some extent independently of regional climate, influenced by site specific factors including edaphic controls, hydroseral succession, and local hydrological changes caused by, e.g., lahar or lava flow, fluvial activity and sea level change. The influence of these local factors is most evident for the late glacial, during which period podocarp-angiosperm forest spread throughout northern New Zealand generally, but with considerable variation in timing even between nearby sites.
Fire appears to have been an important factor in vegetation change throughout the period investigated, not just during the human deforestation era; peat swamp communities show a long history of association with fire, while in dryland vegetation, Agathis australis appears to have been especially affected by burning.
No unequivocal evidence was found for postglacial latitudinal migrations, but several plants show significant altitudinal range expansions during the last glacial compared with their present distributions in northern New Zealand, viz., Nothofagus menziesii, Libocedrus bidwillii, Phyllocladus aspleniifolius, and Halocarpus spp. Thus although vegetation communities at each locality have changed substantially over time, the flora of northern New Zealand remained essentially the same during the c.20,000 years before the human era.
Interpretation of the pollen records was assisted by principal components analysis (PCA) and by referring to modern pollen data and pollen-vegetation comparisons obtained from Waipoua Forest, Northland. PCA provides an efficient means of summarising and portraying large pollen datasets, and helps to clarify underlying environmental factors and temporal trends. PCA also generally supports pollen diagram zonations determined by eye. The Waipoua study indicated that the relationship between pollen and tree abundances is highly variable within forests, dependent largely on local site characteristics, especially the masking effect of strong local pollen sources. Nevertheless, quantified pollen-vegetation relationships averaged for the study area mostly accord with results from previous New Zealand modern pollen rain studies, while adding new information for the pollen representation of several prominent northern species. The Waipoua study indicates that pollen spectra rich in Agathis may be found where trees grow nearby, but Agathis pollen appears to be less widely dispersed than pollen of other New Zealand anemophilous taxa.
At several swamp sites, correlation of tephra layers and pollen-stratigraphic events reveals problems with radiocarbon chronologies which can not be satisfactorily resolved except by assuming contamination by modern carbon. Sites with a history of hydroseral succession, where swamp communities have developed in former lake basins, are especially prone to this contamination, presumably because root penetration of older sediments provides channels for downward movement of younger carbon. In such situations it may be unwise to date and correlate pre-Holocene sediments on the basis of radiocarbon alone. Periods of hiatus are not uncommon in lake and swamp profiles from northern New Zealand and it is possible that the record of the last glacial is missing or strongly compressed at many Northland sites.
Sedimentation rates also varied markedly between and within sequences, precluding the accurate estimation of pollen accumulation rates except at the Waikato lake sites where tephra sequences provide detailed chronological resolution. Even here, however, pollen concentration and accumulation rates appear to have been highly susceptible to short-term fluctuations in the sedimentation regime
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Holocene vegetation and volcanic activity, Auckland Isthmus, New Zealand
No full textA 12 000 to 4000 yr BP pollen and tephra-bearing profile from Auckland, New Zealand, provides insights into the vegetation history and evidence for early Holocene volcanic activity in this area centred on the Mount Wellington basaltic volcano. Possibly 500 yr separated initial scoriaceous ash deposition (ca. 9500 yr ago) and subsequent major lava flows (ca. 9000 yr ago) from Mount Wellington. The local vegelation, topography, and drainage patterns were substantially modified during this time, and damming by the lava flows resulted in the formation of Lake Waiatarua in a shallow valley head ca. 9000 yr ago. Diatom evidence indicates that this lake was initially deep (> 5 m) but was shallowing around 4000 yr ago. In contrast to the Mount Wellington eruptions, tephra deposition resulting from distant rhyolitic volcanic activity of the central North Island and Mayor Island has had little effect on the Auckland vegetation during this time interval (12 000–4000 yr ago).
Between ca. 12 000 and 10 000 yr ago, conifer-angiosperm forest was the predominant vegetation cover on Auckland Isthmus, but during the early Holocene, forest dominated by Metrosideros expanded, probably on to fresh volcanic surfaces resulting from the Mount Wellington eruptions. At this time, swamp forest communities developed in Waiatarua valley basin, and included species indicative of moist, mild, relatively frost-free climates. Some taxa show histories consistent with other records from the northern New Zealand region, including the rise of Ascarina lucida ca. 11 000 to 9000 yr ago, and its subsequent decline, and the expansion of Agathis australis (kauri) forest communities from ca. 6000 yr ago. Taken together the history of local and regional vegetation points to a mild, moist and weakly seasonal early Holocene climate, which subsequently became drier with greater seasonal temperature extremes
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