Oxygen isotope analysis of diatom silica and authigenic calcite from Lake Pinarbasi, Turkey

There is increasing interest in the ¹⁸O/¹⁶O ratio of diatom silica, particularly for lakes where carbonates are absent. Here we compare the ¹⁸O/¹⁶O ratios preserved in diatom silica and authigenic calcite from an open, spring-fed, freshwater lake core from Turkey which spans marine oxygen isotope stage 3. The two sets of isotope data show contrasting trends in spite of their mutual dependence on the water ¹⁸O/¹⁶O ratio and lake-water temperature. The most likely explanation for this divergence is difference in seasonality of biological productivity mediated by the strongly continental climate of the Anatolian plateau. Diatom silica and authigenic calcite are precipitated from solutes in the lake-water at different times of the year. Diatom productivity follows a well-defined seasonal cycle, peaking first and most importantly in the spring and then in the autumn. The precipitation of calcite follows productivity by all forms of photosynthetic organisms that deplete CO₂ but in most lakes this occurs during the summer months. The ¹⁸O_calcite curve shows mean summer temperature maxima at ca. 30–35 and ca. 58 ka BP while the intervening data represent a period of relatively cool summers. The ¹⁸O_diatom curve shows bipolar results (15–20 and 29–33), which suggests that at least two discrete sources or processes contributed to the oxygen composition of the diatoms but probably involved a dilution mechanism to shift the isotopic values. The most likely source of depleted water is from snow entering the lake during the spring thaw. We infer that many authigenic calcite curves from regions with markedly seasonal climates may be temporally limited to a few summer months and that diatom silica provides complementary data on seasonally-specific water isotopic composition rather than a substitute for analyses based on carbonate.     

The indication of morphological degradation of streams and rivers using Simuliidae

Blackfly communities from five German stream types out of two ecoregions (small and mid-sized siliceous gravel-bed mountain streams of the Central mountains, ecoregion 9, and organic type brooks, small and mid-sized sand-bottomed Central lowland streams of ecoregion 14) are compared. Ecoregional, as well as stream type-specific biocoenotic differences are discussed. The presence of Prosimulium sp. was restricted to mountain streams, whereas Simulium lineatum seemed to prefer mid-sized sand-bottomed Central lowland streams, and S. vernum showed a clear preference for Central lowland streams. An Index of Morphological Stress (IMS), derived from 22 geo-hydromorphological parameters recorded during sampling in the field, is presented. According to the IMS values, sites are divided into morphologically ‘unstressed’ sites (high or good morphological quality) and ‘stressed’ sites (moderate, poor or bad morphological quality), and biocoenotic differences of the two categories are discussed. Two stream types and the entire data set showed significantly higher numbers of taxa at ‘unstressed’ sites. Linear Multiple Regression (LMR) was used to identify geo-hydromorphological parameters that significantly explain the variance of the three most constant taxa, Prosimulium sp., P. hirtipes and Simulium sp. in the LMR model.     

Palaeoclimatic implications of isotopic data from modern and early Holocene shells of the freshwater snail Melanoides tuberculata, from lakes in the Ethiopian Rift Valley

Carbon and oxygen isotope ratios in the shells of the freshwater snail Melanoides tuberculata yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the snails' life span. Melanoides is particularly important because it is widespread in Quaternary deposits throughout Africa and Asia and is ubiquitous in both fresh and highly evaporated lakes. Whole-shell and incremental growth data were collected from modern and fossil shells from two lakes in the Ethiopian Rift Valley. 18O values in the modern shells from Lake Awassa are in equilibrium with modern waters, while 18O values in subfossil shells from the margins of Lake Tilo indicate high rainfall during the early Holocene. Sequential analysis along the growth spiral of the shell provides information on seasonal or shorter-term variability of lake water during the lifetime of the organism.     

Climatic and non-climatic effects on the δO and δC compositions of Lake Awassa, Ethiopia, during the last 6.5 ka

A comparison of a 6450 ¹⁴C yr δ¹⁸O and δ¹³C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing δ¹⁸O and δ¹³C values from 4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after 2800 BP when δ¹⁸O increases again, reflecting more arid conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both δ¹⁸O_calcite and δ¹³C_calcite immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. δ¹⁸O, δ¹³C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual–decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past.     

Stereotypical Constructions of the Maori ‘Race’ in the Media1

Since the colonisation of Aotearoa / New Zealand by Pakeha, Maori have been stereotypically imagined as the Black Other. One of the principal mechanisms for the continued perpetuation of this racialised discourse is the representation of ‘race’ through stereotypes in the media. ‘New’ cultural geography provides a theoretical foundation to interrogate stereotypical (re)formations of Maori identity. I conclude that the use of the stereotype in the (re)formation of Maori identity further demarcates and constrains Maori within the perennial position of the Black Other. Only through a retheorisation of difference (which moves beyond the construction of the Maori ‘race’ through stereotypes), can the impact of racialisation be effectively resisted.     

Seasonal changes in vocalization during behavior of the Atlantic bottlenose dolphin

Four vocalizations (whistles, buzzes, quacks, and pops) were quantified during three behavioral categories (socializing, traveling, and feeding) of the bottlenose dolphin in the Newport River Estuary, North Carolina. This study tested the hypothesis that specific vocalizations of dolphins in this area are associated with specific behavioral categories during the summer and fall. Approximately twice the total number of vocalizations per unit time occurred during the fall as compared to the summer. Each type of vocalization had a distinctive and consistent acoustical structure. The relative frequency of the different vocalization types was significantly dependent on the behavior of the dolphins. Pops were consistently associated with feeding, quacks with socializing, and buzzes with traveling. Whistles, the most common vocalization, were associated with traveling and feeding during the summer and socializing in the fall. Dolphins spent more time involved in undirected activity (socialization) during the summer but spent more time traveling in search of food and feeding during the fall in preparation for migration out of the estuary and movement south.     

Quantifying climatic change through the last glacial–interglacial transition based on lake isotope palaeohydrology from central Turkey

Questions remain as to the nature of climatic change through the last glacial–interglacial transition in the eastern Mediterranean region, particularly the relative contribution of evaporation and precipitation to regional water balance. Here changes in oxygen isotope values through this time period from Eski Acıgöl, a crater lake in central Turkey, are investigated using hydrological and isotope mass balance models. These allow changes in evaporation and precipitation to be quantified and their relative importance evaluated. We show that it is the volumetric flux rate of water passing through the lake system and not the precipitation-to-evaporation ratio per se which controlled the stable isotope record in Eski Acıgöl. Early Holocene precipitation is shown to be much greater than that during both the latter part of the last glaciation and the present day. We test these calculated values against other records in the same region, firstly with other lake records in Anatolia, the Konya basin and Lake Van, and secondly with isotope-inferred palaeo-precipitation data from Soreq cave in Israel. This reveals a contrast between pre- and post-LGM precipitation values in Turkey (wetter and drier, respectively) and also suggests that during the last glacial–interglacial transition there was a more marked precipitation gradient than at present between northern/interior and southern/coastal parts of the eastern Mediterranean region.