Engineering quantum dots for electrical control of the fine structure splitting

We have studied the variation in fine-structure splitting (FSS) under application of vertical electric field in a range of quantum dots grown by different methods. In each sample we confirm that this energy splitting changes linearly over the field range we can access. We conclude that this linear tuning is a general feature of self-assembled quantum dots, observed under different growth conditions, emission wavelengths and in different material systems. Statistical measurements of characteristic parameters such as emission energy, Stark shift and FSS tuning are presented which may provide a guide for future attempts to increase the yield of quantum dots that can be tuned to a minimal value of FSS with vertical electric field

An interlaboratory study of TEX86 and BIT analysis of sediments, extracts and standard mixtures.

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility ±3-4°C when translated to temperature) but a large spread in BIT measurements (reproducibility ±0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0°C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the "true" (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values

A mid-Holocene thermal maximum at the end of the African Humid Period

The termination of the African Humid Period (AHP) about 5 thousand years ago (ka) was the most dramatic climate shift in northern and equatorial Africa since the end of the Pleistocene. Based on TEX86 paleotemperature data from lake Turkana, Kenya, we show that a temperature shift of 2-4 degrees C occurred over the two millennia spanning the end of the AHP, with the warmest conditions occurring at similar to 5 ka. We note a similar shift, though of a smaller magnitude, in other East African temperature records from Lakes Malawi and Tanganyika, as well as Mt. Kilimanjaro. Additionally, we document the temperature history for the last 220 years from Lake Turkana that indicates the thermal anomaly at 5 ka was warmer than the present day Lake Turkana temperatures and on par with modern temperatures of Lakes Tanganyika and Malawi. We suggest that the thermal response at the end of the AHP may be linked to local insolation during September-November, when local air temperature rises to an annual maximum over Lakes Malawi and Tanganyika and a secondary maximum over Lake Turkana and Mt Kilimanjaro. September-November insolation peaked at similar to 5 ka and likely caused air and water temperatures in the region to rise to maxima at that time

Vertical and temporal variability in concentration and distribution of thaumarchaeotal tetraether lipids in Lake Superior and the implications for the application of the TEX₈₆ temperature proxy

This study investigated the vertical and temporal distribution of Thaumarchaeota derived core isoprenoid glycerol dialkyl glycerol tetraether (GDGT) lipids through sampling and analysis of both suspended particulate matter from the water column at different times in the annual cycle and a 3 year long record of settling particles in two sediment traps at different depths at an open lake location in Lake Superior. Results from these analyses suggest that Thaumarchaeota were present throughout the water column during times of overturning, but mainly resided below the depth of the thermocline (20-40 m) during the period of thermal stratification. Fluxes of thaumarchaeotal produced GDGTs were highly periodic and mainly occurred during two periods of the annual cycle (winter and late spring/early summer). A covariance of both branched and isoprenoid GDGT fluxes with the mass accumulation flux combined with the observation that those periods of maximum fluxes were associated with increased BIT index values, however, suggest that these two periods of elevated fluxes may be related to an influx of resuspended particles transported from shallower near shore regions of Lake Superior. During all sampling periods TEX86 inferred temperatures from SPM were in good agreement with in situ water temperatures of the depths at which the SPM was sampled. The observed range of TEX86 inferred temperatures in 3 years of settling particles is relatively small and does not show significantly higher inferred temperatures during the thermally stratified period, indicating that the sedimentary TEX86 signal during the summer thermally stratified period mainly originated from depths below the relatively shallow thermocline. Additionally, TEX86 values during the winter period of increased fluxes did not capture the decrease in water temperatures observed throughout the water column during this period, and thus may be a further indication that the thaumarchaeotal lipid flux was the result of sediment focusing. Flux-weighted TEX86 inferred temperatures from both sediment traps were in good agreement with TEX86 temperatures from surface sediments from the same location in Lake Superior. Both flux weighted TEX86 temperatures from the sediment traps and average TEX86 temperatures from surface sediments were similar to averaged measured water temperatures at below similar to 40 m depth within the error of the lacustrine TEX86 calibration. Based on the observed depths of Thaumarchaeota in the water column, TEX86 values in sediments of Lake Superior likely reflect a combination of mixed-season and sub-thermocline temperatures. This is effectively the same as the annual averaged water temperature observed at depths below 40 m in Lake Superior. Thus, trends in TEX86 inferred temperatures in sediment records of Lake Superior, and similar lakes, are likely to reflect subsurface temperature variability rather than that of surface temperatures

Crenarchaeotal membrane lipids in lake sediments : a new paleotemperature proxy for continental paleoclimate reconstruction?

Paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy. A novel sea-surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota. This proxy will provide an independent continental paleotemperature tool that will allow paleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change

Large temperature variability in the southern African tropics since the Last Glacial Maximum

The role of the tropics in global climate change is actively debated, particularly in regard to the timing and magnitude of thermal and hydrological response. Continuous, high-resolution temperature records through the Last Glacial Maximum (LGM) from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments. Here we used the TEX₈₆ paleotemperature proxy to reconstruct mean annual lake surface temperatures through the Last Glacial Maximum (LGM) in Lake Malawi, East Africa (9°–14°S). We find a ∼3.5°C overall warming since the LGM, with temperature reversals of ∼2°C during the Younger Dryas (12.5 ka BP) and at 8.2 ka BP. Maximum Holocene temperatures of ∼29°C were found at 5 ka BP, a period preceding severe drought in Africa. These results suggest a substantial thermal response of southeastern tropical Africa to deglaciation and to varying conditions during the Holocene

Organic geochemical records of environmental variability in Lake Malawi during the last 700 years, Part I: The TEX₈₆ temperature record

We have applied the TEX86 paleothermometer to produce a surface water temperature record for Lake Malawi spanning the past 700 years. Over much of the record temperature fluctuates from similar to 24-27 degrees C with a mean of similar to 25 degrees C however, there has been a substantial increase in temperature of similar to 2.0 degrees C during the past similar to 100 years. The TEX86 temperature record reveals a strong similarity to the instrumental record: both records demonstrate warming (similar to 0.7-1.4 degrees C) over the past similar to 50 years as well as a cooling anomaly around 1959. Comparison of the TEX86 temperature record with the proxy records of primary productivity suggests that wind induced upwelling and/or precipitation have a strong influence on the surface temperature of Lake Malawi

Branched glycerol dialkyl glycerol tetraethers in lake sediments: Can they be used as temperature and pH proxies?

A series of surface sediments from 82 lakes of variable water depth and size was analyzed for glycerol dialkyl glycerol tetraethers (GDGTs) in order to investigate the potential of the MBT/CBT (methylation ratio/cyclization ratio of branched tetraethers) as a continental palaeothermometer in lacustrine environments. Branched GDGTs dominated in most sediments, as indicated by the high branched vs. isoprenoid tetraether (BIT) values. We observed that CBT and MBT varied substantially. Mean annual air temperature (MAAT reconstructed) and pH values were calculated using the CBT and MBT values and the calibration from the global soil data set. The MBT/CBT inferred temperatures were considerably lower than measured values. Nevertheless, there was a significant correlation between MAAT reconstructed and MAAT observed on site, although there was still considerable scatter (r(2) 0.47). Lacustrine sediments integrate organic remains of organisms in a lake and its drainage basin, thereby offering a unique opportunity for calibrating MBT and CBT, as small scale variability is averaged out. Since the source of the branched GDGTs in the lake sediments is ambiguous, it is not clear whether only temperature and pH in the catchment area are the driving factors. Therefore, even in lake sediments with high concentrations of branched GDGTs the straightforward application of MBT/CBT as palaeoproxies may be difficult given the uncertainties regarding their source and origin

A progressively wetter climate in Southern East Africa over the past 1.3 million years

African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial–interglacial change intensified worldwide1, 2, 3. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases4. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°–14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals