Use of Pb/Pb ratios to investigate the surface integrity of peat cores used to study the recent depositional history and geochemical behaviour of inorganic elements in peat bogs

The well characterised temporal trend in the ²⁰⁶Pb/²⁰⁷Pb atom ratio of atmospheric lead deposition in Scotland during the 20th century was used to investigate the surface integrity of several cores collected by different methods from Flanders Moss ombrotrophic peat bog, central Scotland, during 1996–2001. Based on ²⁰⁶Pb/²⁰⁷Pb profile comparisons, in conjunction with identified ²¹⁰Pb radionuclide inventory deficits for two of the cores, it was deduced that 25 ± 7 yrs worth of material was missing from the more seriously affected core. After allowing for an appropriate vertical offset based on ²⁰⁶Pb/²⁰⁷Pb profile matching, the subsequent matching of profiles of titanium, lead, sulfur, arsenic, iron, phosphorus and manganese in three cores for which total concentration data were available was excellent. Without such offset correction, erroneous conclusions could have been drawn concerning the recent historical record of anthropogenic lead and arsenic deposition, the position of the redox boundary, which controls geochemical cycling and enrichment of iron, and the nutrient recycling status of manganese and phosphorus in the near-surface vegetation. Topographic, vegetative and coring (both device and operator) influences may have been responsible, thus endorsing the use of reliable, multiple core sampling and the use of lead isotope ratio profiles, supplemented by appropriate radionuclide data, in both assessing and ensuring the surface integrity of peat cores.     

Recent organic matter accumulation in relation to some climatic factors in ombrotrophic peat bogs near heavy metal emission sources in Finland

Accumulation of organic matter (OM) was studied in four ombrotrophic peat bogs in Finland: Harjavalta (vicinity of a Cu–Ni smelter), Outokumpu (near a closed Cu–Ni mine), Alkkia (Ni-treated site) and Hietajärvi (a pristine site). At each sampling site, two peat cores (15 × 15 × 100 cm) were taken. Age-dating of peat was determined using ²¹⁰Pb method (CRS model). The local annual temperature sum and precipitation for the past 125 years were modeled. The objective was to compare recent net accumulation rates of heavy metal polluted ombrotrophic peat bogs with those of a pristine bog, and to study the relationship between weather and net accumulation rates. Based on ²¹⁰Pb age-dating, the upper 16-cm peat layer at Harjavalta, 35 cm at Outokumpu and 25 cm at Hietajärvi represents 125 years of peat formation, yielding the following average peat accumulation rates: Harjavalta 1.3 mm year^− 1, Outokumpu 2.8 mm year^− 1 and Hietajärvi 2.0 mm year^− 1. At the Alkkia site, the Ni treatment in 1962 had completely stopped the peat accumulation. Net accumulation rates were related to precipitation at Outokumpu, Harjavalta and Hietajärvi sites. In addition, emissions released from the nearby located Cu–Ni smelter could have affected negatively net OM accumulation rate at Harjavalta site.     

Isotopic-geochemical study of nitrogen and carbon in peat from the Tunguska Cosmic Body explosion site

Isotopic-geochemical investigations were carried out on peat samples from the 1908 Tunguska Cosmic Body (TCB) explosion area. We analyzed two peat columns from the Northern peat bog, sampled in 1998, and from the Raketka peat bog, sampled during the 1999 Italian expedition, both located near the epicenter of the TCB explosion area. At the depth of the “catastrophic” layer, formed in 1908, and deeper, one can observe shifts in the isotopic composition of nitrogen (up to Δ¹⁵N = +7.2‰) and carbon (up to Δ¹³C = +2‰) and also an increase in the nitrogen concentration compared to those in the normal, upper layers, unaffected by the Tunguska event. One possible explanation for these effects could be the presence of nitrogen and carbon from TCB material and from acid rains, following the TCB explosion, in the “catastrophic” and “precatastrophic” layers of peat. We found that the highest quantity of isotopically heavy nitrogen fell near the explosion epicenter and along the TCB trajectory. It is calculated that 200,000 tons of nitrogen fell over the area of devastated forest, i.e., only about 30% of the value calculated by Rasmussen et al. (1984). This discrepancy is probably caused by part of the nitrogen having dispersed in the Earth’s atmosphere. The isotopic effects observed in the peat agree with the results of previous investigations [Kolesnikov et al 1998a], [Kolesnikov et al 1998b], [Kolesnikov et al 1999] and [Rasmussen et al 1999] and also with the increased content of iridium and other platinoids found in the corresponding peat layers of other columns [Hou et al 1998] and [Hou et al 2000]. These data favor the hypothesis of a cosmochemical origin of the isotopic effects.     

Traces of cometary material in the area of the Tunguska impact (1908)

A short overview of the studies of the authors and their colleagues performed over many years, which resulted in the discovery of traces of cometary matter in the peat at the epicenter of the Tunguska catastrophe in 1908, is given here. In the epicenter of the Tunguska cosmic body (TCB) explosion, the shifts in the isotopic composition of hydrogen and carbon relative to their values for the upper and lower layers of the same column were found in the catastrophic layers of peat grown up in 1908. These shifts cannot be attributed to any known terrestrial processes: the conservation of mineral and organic dust in peat, peat humification, the emission of hydrocarbon gases from the Earth, climate changes, and other physical and chemical processes. In the catastrophic layers of the control peat columns, the isotopic shifts are absent. The isotopic data agree well with the increased concentration of iridium and other platinum-group elements in the same peat layers, which is a reliable indicator of the presence of cosmic material in terrestrial objects. The cosmogenic character of the isotopic effects is confirmed by the presence of “dead” carbon (not containing radioactive ¹⁴C) in the catastrophic layers. To provide the shifts observed in the isotopic composition of carbon, cosmic carbon preserved in peat should be isotopically superheavy—from +50‰ to +60‰ according to calculations. Such isotopically heavy carbon is absent both on the Earth and in ordinary meteorites. It occurs only in individual mineral phases of CI carbonaceous chondrites, close to cometary dust in chemical composition, ratios of the content of iridium and other platinoids and rear-earth elements also points to the cometary nature of the TCB. In the near-catastrophic peat layers, the anomalous increase of the concentration of many volatiles was detected, which also suggests that the TCB was a cometary core. The studies of the content and the isotopic composition of nitrogen in the peat revealed traces of heavy acid rains induced by the flyby and explosion of the TCB.     

Finding of probable Tunguska Cosmic Body material:: isotopic anomalies of carbon and hydrogen in peat

Method of a search for traces of Tunguska Cosmic Body (TCB) material using layer-by-layer analysis of the isotopic composition of light elements in peat has been offered. Four peat columns sampled at the explosion epicentre indicated significant carbon and hydrogen isotopic effects in its near catastrophic layers. The shifts, opposite in direction, for carbon (Δ¹³C reaches +4.3‰) and hydrogen (ΔD reaches −22‰) cannot be attributed to any known terrestrial reasons (fall-out of terrestrial dust and fire soot; emission from the Earth of oil–gas streams; climate changes, humification of peat, and so on). Moreover, the isotopic effects are clearly associated with the area and with the time of the 1908 event. They are absent in the uppermost and the lowest peat layers and also in the control peat columns sampled at the remote places. Since calculated δ¹³C value for an admixture of carbon (+51–64‰) is very high, these effects may not be explained by contamination of peat with material similar to ordinary chondrites or achondrites, too. Such heavy carbon occurs in the most primitive CI and CM types of carbonaceous chondrites. However, C/Ir ratio in a cosmic admixture is 10,000 times as many as in CI chondrites that points to cometary nature of the TCB. The isotopic effects are in agreement with the increase of the Ir content observed in peat, but, at the same time, small content of Ir points to the low content of dust in the Tunguska comet that sharply differs it from Halleys comet.     

Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: Patterns and climatic signals

Although several proxies have been proposed to trace the course of environmental and climatological fluctuations, precise paleoclimate records from the tropics, notably from Africa are still sorely lacking today. Stable carbon isotopes (δ¹³C) in tree rings are an attractive record of climate. In this study, the patterns and climatic signals of δ¹³C ratios were determined on tree rings of deciduous (Acacia senegal, Acacia tortilis, Acacia seyal) and an evergreen (Balanites aegyptiaca) species, from a semi-arid Acacia Woodland in Ethiopia. δ¹³C inter-annual patterns are synchronous among the co-occurring species. A declining trend with time was observed in δ¹³C, notably for B. aegyptiaca, which could be due to anthropogenic increases in atmospheric CO₂ concentration and decrease in atmospheric δ¹³C. Tree ring δ¹³C values of all the species revealed significant negative correlation with precipitation amount but not with temperature and relative humidity. The δ¹³C series of the deciduous species shows a higher correlation (r = − 0.70 to − 0.78) with precipitation than the evergreen species (r = − 0.55). A master δ¹³C series, composed of the average of the three Acacia species, displayed stronger significant correlation (r = − 0.82) than any of the individual species δ¹³C series. The weak relationship between temperature and δ¹³C in this study indicates that photosynthetic rate is not a significant factor. Moisture stress, however, may have a direct impact on the stomatal conductance and explain the strong negative relationship between δ¹³C and precipitation. The results demonstrate the potential of δ¹³C in tree rings to reflect physiological responses to environmental changes as a vehicle for paleoclimatic reconstruction, which is important to understand tree response to past and future climate change.     

Platinum group element abundances in a peat layer associated with the Tunguska event, further evidence for a cosmic origin

We have measured excesses of Pd, Rh, Ru, REE, Co, Sr, and Y in a peat column from the Northern peat bog of the 1908 Tunguska explosion site. Earlier, in this peat column the presence of an Ir anomaly at the event layers (30- depth) has been found (Planet Space Sci. 48 (1998) 179). In these layers, Pd, Rh, Ru, Co, Sr, and Y show pronounced anomalies of a factor 4-7 higher than the background value. In the event layers there are also good correlations between the siderophile platinum group elements (Pd, Rh, Ru) and Co, indicators of cosmic material, which imply they might have the same source, i.e. the Tunguska explosive body. The patterns of CI-chondrite-normalized REE in the event layers are much flatter than those in normal peat layers and different from those in the nearby traps. Furthermore, in these layers the patterns of CI-chondrite-normalized PGEs and the element ratios (e.g. C/Pd, C/Rh, and between some siderophile elements) give evidence that the Tunguska explosive body was more likely a comet, although we cannot exclude the possibility that the impactor could be a carbonaceous asteroid. We have estimated the total mass of a solid component of the explosive body up to 10³-10⁶ tons.     

Mired in the past — looking to the future: Geochemistry of peat and the analysis of past environmental changes

Peat cores from ombrotrophic bogs have been used as a valuable archive to study environmental change for over a century. Much of this focus on the peat record has been on biological proxies of environmental change, such as pollen and peat-forming macrofossils, but there is growing interest in the geochemical record to study environmental changes. Several studies of long-term peat records in Europe have reconstructed past changes in atmospheric lead pollution, for example, and the general cohesiveness of the results and their agreement with known historical trends in metal production exemplify the best potential of peat geochemistry as an environmental archive. Based on the success with lead, a current emphasis in peat reconstructions is to assess the record of past mercury deposition and results thus far show generally consistent trends, e.g., a pre-anthropogenic mercury accumulation rate of about 0.5–1.5 μg Hg m^− 2 year^− 1. Despite this general consistency there is increasing concern that there may be diagenetic effects on the quantitative record of some metals, which can be inferred based on a strong relationship between mercury and other organically bound elements and proxies for peat decomposition (C/N ratio). However, it is possible that changes in decomposition and the alteration of some metal records could provide climatic information. A few recent studies show that closer examination of the geochemical matrix, in some cases along with biological proxies, can provide valuable information on landscape changes and climate; for example, partitioning metals into different weight fractions and source regions can be applied to climate studies. The best interpretations of the peat geochemical record in the context of environmental and climate change will likely come when geochemical and biological records are considered simultaneously.     

Discovery of probable Tunguska cosmic body material: anomalies of platinum group elements and rare-earth elements in peat near the Explosion Site (1908)

Ten Sphagnum fuscum peat samples collected from different depths of a core including the layer affected by the 1908 Tunguska explosion in the Tunguska area of Central Siberia, Russia, were analyzed by ICP-MS to determine the concentrations of Pd, Rh, Ru, Co, REE, Y, Sr, and Sc. The analytical results indicate that the Pd and Rh concentrations in the event- and lower layers were 14.0–19.9, and 1.23–1.56 ppb, respectively, about 3–9 times and 3 times higher than the background values in the normal layers. In addition, the patterns of CI-chondrite-normalized REE in the event layers were much flatter than in the normal layers, and differed from those in the nearby traps. Hence, it can be inferred from the characteristics of the elemental geochemistry that the explosion was probably associated with extraterrestrial material, and which, most probably, was a small comet core the dust fraction of which was chemically similar to carbonaceous chondrites (CI). In terms of the Pd and REE excess fluxes in the explosion area, it can be estimated that the celestial body that exploded over Tunguska in 1908 weighed more than 10⁶ t, corresponding to a radius of >60 m. If the celestial body was a comet, then its total mass was more than 2×10⁷ t, and it had >160 m radius, and released an energy of >10⁷ t TNT.     

A peat core based estimate of Late-glacial and Holocene methane emissions from northern peatlands

We estimate the intensity of Late-glacial and Holocene methane emissions from peatlands based on their paleo net primary production (PNPP). The PNPP is derived from the carbon accumulation rates of the studied bog profile (Etang de la Gruère, Switzerland), which are corrected for the degree of peat degradation. The obtained PNPP curve is taken as a proxy for methane emissions. It shows relatively high values (90 g C m^− 2 yr^− 1) early in the Bolling/Allerod and drops to low values (40 g C m^− 2 yr^− 1) during the Younger Dryas cold period. With the onset of the Holocene the PNPP increases strongly up to 150 g C m^− 2 yr^− 1 around ca. 10,000 Cal. yr bp. This is followed by a decline to minimum values (30 to 40 g C m^− 2 yr^− 1) between 6500 and 4000 Cal. yr bp. Thereafter, the PNPP starts to increase again to reach its highest value (175 g C m^− 2 yr^− 1) around 1000 Cal. yr bp.The PNPP curve correlates well with the evolution of the atmospheric methane concentrations as derived from Greenland ice-cores. For example, minima in atmospheric methane reported during the Younger Dryas and around 5200 Cal. yr bp are coinciding with the lowest values of PNPP and the negative atmospheric methane peak at 8200 Cal. yr bp corresponds to a marked decrease in PNPP.Our PNPP curve suggests that the methane emissions from northern peatlands evolved similar to those of low latitude wetlands and together they largely determined the evolution of atmospheric methane throughout the Late-glacial and the Holocene. The abruptness of the rise of atmospheric methane at the end of the Younger Dryas probably points to an additional source (e.g. marine gas hydrates), but very early in the Holocene the peatlands have likely become the dominant source of atmospheric methane.     

Sensitive response of desert vegetation to moisture change based on a near-annual resolution pollen record from Gahai Lake in the Qaidam Basin, northwest China

We present a 50-year pollen record at near-annual resolution from Gahai Lake in the Qaidam Basin on the northeastern Tibetan Plateau. Chronology of a 22-cm short core was established by ²¹⁰Pb and ¹³⁷Cs analysis. The pollen results at 0.5 cm intervals show large changes in Artemisia/Chenopodiaceae (A/C) ratios from < 0.2 to 0.95 in the last 50 years. High (low) A/C ratios represent increase (decrease) in steppe pollen production, which correspond to high (low) relative humidity observed at nearby Delingha weather station. On the basis of good correspondence with instrumental records and carbonate content from Gahai Lake, we conclude that A/C ratio is sensitive to moisture change and can be a very useful index in reconstructing paleoclimate of arid regions. Comparison with pollen and snow accumulation data from Dunde ice core suggests that effective moisture at low and high elevations shows the opposite relationship when mountain precipitation was extremely high, possibly due to topography-induced uplifting and subsiding air dynamics.     

A late Quaternary lake record from the Qilian Mountains (NW China): evolution of the primary production and the water depth reconstructed from macrofossil, pollen, biomarker, and isotope data

The history (45–0 ka BP) of the aquatic vegetation composition of the shallow alpine Lake Luanhaizi from the NE Tibetan Plateau is inferred from aquatic plant macrofossil frequencies and aquatic pollen and algae concentrations in the sediments. C/N (range: 0.3–100), δ¹³C (range: −28 to −15‰), and n-alkane measurements yielded further information on the quantitative composition of sedimentary organic matter. The inferred primary production of the former lake ecosystem has been examined in respect of the alternative stable state theory of shallow lakes [Scheffer, M., 1989. Alternative stable states in eutrophic, shallow freshwater systems: a minimal model. Hydrobiological Bulletin 23, 73–83]. Switches between clear and turbid water conditions are explained by a colder climate and forest decline in the catchment area of Lake Luanhaizi. The macrofossil-based reconstruction of past water depth and salinity ranges, as well as other organic matter (OM) proxies allowed climatic inferences of the summer monsoon intensity during the late Quaternary. Around 45 ka BP, conditions similar to or even moister than present-day climate occurred. The Lake Luanhaizi record is further evidence against an extensive glaciation of the Tibetan Plateau and its bordering mountain ranges during the Last Glacial Maximum. Highest lake levels and consequently a strong summer monsoon are recorded for the early Holocene period, while gradually decreasing lake levels are reconstructed for the middle and late Holocene.     

Authors' Reply

Abstract— Jull et al. propose an alternative interpretation of our depth vs. ¹⁴C data measured on a peat core from the central Tunguska impact site (Rasmussen et al., 1999). We find that the proposed alternative is untenable.     

Mid–late Holocene monsoon climate retrieved from seasonal Sr/Ca and δO records of Porites lutea corals at Leizhou Peninsula, northern coast of South China Sea

South China Sea (SCS) is a major moisture source region, providing summer monsoon rainfall throughout Mainland China, which accounts for more than 80% total precipitation in the region. We report seasonal to monthly resolution Sr/Ca and δ¹⁸O data for five Holocene and one modern Porites corals, each covering a growth history of 9–13 years. The results reveal a general decreasing trend in sea surface temperature (SST) in the SCS from 6800 to 1500 years ago, despite shorter climatic cycles. Compared with the mean Sr/Ca–SST in the 1990s (24.8 °C), 10-year mean Sr/Ca–SSTs were 0.9–0.5 °C higher between 6.8 and 5.0 thousand years before present (ky BP), dropped to the present level by 2.5 ky BP, and reached a low of 22.6 °C (2.2 °C lower) by 1.5 ky BP. The summer Sr/Ca–SST maxima, which are more reliable due to faster summer-time growth rates and higher sampling resolution, follow the same trend, i.e. being 1–2 °C higher between 6.8 and 5.0 ky BP, dropping to the present level by 2.5 ky BP, and reaching a low of 28.7 °C (0.7 °C lower) by 1.5 ky BP. Such a decline in SST is accompanied by a similar decrease in the amount of monsoon moisture transported out of South China Sea, resulting in a general decrease in the seawater δ¹⁸O values, reflected by offsets of mean δ¹⁸O relative to that in the 1990s. This observation is consistent with general weakening of the East Asian summer monsoon since early Holocene, in response to a continuous decline in solar radiation, which was also found in pollen, lake-level and loess/paleosol records throughout Mainland China. The climatic conditions 2.5 and 1.5 ky ago were also recorded in Chinese history. In contrast with the general cooling trend of the monsoon climate in East Asia, SST increased dramatically in recent time, with that in the 1990s being 2.2 °C warmer than that 1.5 ky ago. This clearly indicates that the increase in the concentration of anthropogenic greenhouse gases played a dominant role in recent global warming, which reversed the natural climatic trend in East Asian monsoon regime.     

Molecular cores of the high-latitude cloud MBM 40

Towards the high-latitude cloud MBM 40, we identify 3 dense molecular cores of M0.2–0.5 M, and sizes of 0.2 pc in diameter embedded in the H I cloud of 8 M which is observed to be extended along the northeast–southwest direction. The molecular cloud is located almost perpendicularly to the H I emission. We confirm the previous result of Magnani et al. that MBM 40 is not a site for new star formations. We found a very poor correlation between the H I and the IRAS 100 μm emissions, but the CO (1–0) and 100 μm emissions show a better correlation of W_CO/I₁₀₀=1±0.2 K km s⁻¹ (MJy sr⁻¹)⁻¹. This ratio is larger by a factor of ≥5 than in dense dark clouds, which may indicate that the CO is less depleted in MBM 40 than in dense dark clouds.     

The significance of coral reefs as global carbon sinks— response to Greenhouse

Coral reefs are net sinks for C, principally as CaCO₃ accretion. It is possible to predict quite accurately the rate of production, given adequate information about any particular reef environment. The best data set for an extensive region is that for the Great Barrier Reef (GBR). Careful analysis of this region and the incorporation of previously documented present day system calcification rates suggest net production (G) from G = 1 (kg CaCO₃ m⁻² yr⁻¹) for fringing reefs, to G = 1.9 for planar (infiled platform) reefs, G = 3 for ribbon reefs and lagoonal reefs. The 20,055 km² of reefs in the GBR are thus estimated to average G = 2.4, resulting in a total production of 50 million tonnes yr⁻¹. In a 50–100 year Greenhouse scenario of rising sealevel, we predict that recolonisation of present day reef flats will be extensive and prolific. Production will increase substantially, and this could be by as much as 40% (ranging from 0% for deep shoals to 180% for fringing reefs) to give 70 million tonnes yr⁻¹ if the rate of sealevel rise reaches or exceeds 6–8 mm yr⁻¹We estimate 115,000 km² of oceanic atolls worldwide. Drawing on points equivalence from the detailed analysis of the GBR, we estimate the atolls presently produce 160 million tonnes yr⁻¹. We predict that a similar 40% increase could be possible in the next 100 years or so resulting in a production of 220 million tonnes.Accepting an existing estimate of 617,000 km² for reefs worldwide, drawing from our projections from the GBR and the atolls, and making some assumptions about the remaining reef types (we suggest fringing reefs to dominate) we estimate global reef production at the present time to be 900 million tonnes yr⁻¹. Within the next 100 years or so, we suggest this rate could almost double to 1800 million tonnes. In the long term (several centuries) we predict that the continuing trend of recolonisation, particularly of fringing and planar reefs could result in the production of > 3000 million tonnes yr⁻¹ if rates of sealevel rise approaching or exceeding 6–8 mm yr⁻¹ are achieved. Eventually (> 500 yr), reefs could actually “drown” due to inability to match the rate of sealevel increase if that rate significantly exceeds 6–8 mm yr⁻¹.Thus, coral reefs at present act as a sink for 111 million tonnes C yr⁻¹, the equivalent of 2% of present output of anthropogenic CO₂. In the short term Greenhouse scenario (100 yr) we predict this could increase to the equivalent of 4% of the present CO₂ output. In the much longer term (several centuries), if all trends continue, this could increase to the equivalent of as much as 9% of the present CO₂ output.Unfortunately, we also predict that this considerable sink for C will be most likely of negative value in alleviating Greenhouse because of the immediate effect of CaCO₃ precipitation is to raise the P_CO2 of the surface oceans — ie, ot encourage CO₂ efflux to the atmosphere. We do not attempt to quantify this effect.Other Greenhouse changes such as seawater temperature increase, changes in cloud cover, increased rainfall and runoff, increased storm activity, and changes in dissolved CO₂ concentration and surface ocean circulation may complicate the reef response. However, we suggest that sealevel rise will be the dominant influence, at least during the next 100 years or so.     

The youngest lobe-dominated radio sources

We present an analysis of multi-epoch global VLBI observations of the Compact Symmetric Objects: 2352+495 and 0710+439 at 5 GHz. Analysis of data spread over almost two decades shows strong evidence for an increase in separation of the outer components of both sources at a rate of 0.2h⁻¹c (for q=0.5 and H=100h km s⁻¹Mpc⁻¹). Dividing the overall sizes of the sources by their separation rates implies that these Compact Symmetric Objects have a kinematic age 10⁴ years. These results (and those for other CSOs) strongly argue that CSOs are indeed very young sources that probably evolve into much larger classical doubles.     

Ska and Evla Computing Costs for Wide Field Imaging

I investigate the problem of high dynamic range continuum synthesis imaging in the presence of confusing sources, using scaling arguments and simulations. I derive a quantified cost equation for the computer hardware needed to support such observations for the EVLA and the SKA. This cost has two main components – from the data volume, scaling as D⁻⁶ (where D is the antenna diameter), and from the non-coplanar baselines effect, scaling as D⁻², for a total scaling of D⁻⁸. A factor of two in antenna diameter thus corresponds to 12 years of Moore’s law (18 month doubling time) cost reduction in computing hardware. For a SKA built with 12.5 m antennas observing with 1 arcsecond at 1.4 GHz, I find the computing load to be about 150 Petaflops (costing about $500 million in 2015). For 25 m antennas, the load is about 256 times lower, costing $2 million in 2015. This new cost equation differs from that of Perley and Clark (2003), which has scaling as D⁻⁶. This is because I find that the excellent Fourier plane coverage of the small antenna design does not significantly change the convergence rate of the Clean algorithm, which is already satisfactory in this regime.The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.     

The Color-Magnitude Relations of Late-type Spiral Galaxies and Their Disks

The 395 late-type spiral galaxies brighter than 15^m in r-band are selected from the Data Release 2 of Sloan Digital Sky Survey and the colormagnitude relations of these galaxies and their disks are investigated. It is found that the colors g − r, r − z and g − z of these galaxies and their disks are strongly correlated with the r-band absolute magnitudes, i.e., the more luminous galaxies (or disks) have the redder colors than the less luminous galaxies (or disks). And the correlation of galaxies is stronger than that of their disks.     

Chemical sources of haze formation in Titan's atmosphere

A prominent feature of Titan's atmosphere is a thick haze region that acts as the end product of hydrocarbon and nitrile chemistry. Using a one-dimensional photochemical model, an investigation into the chemical mechanisms responsible for the formation of this haze region is conducted. The model derives profiles for Titan's atmospheric constituents that are consistent with observations. Included is an updated benzene profile that matches more closely with—recent ISO observations (Icarus 161 (2003) 383), replacing the profile given in the benzene study of Wilson et al. (J. Geophys. Res. 108 (2003) 5014). Using these profiles, pathways from polyynes, aromatics, and nitriles are considered, as well as possible copolymerization among the pathways. The model demonstrates that the growth of polycyclic aromatic hydrocarbons throughout the lower stratosphere plays an important role in furnishing the main haze layer, with nitriles playing a secondary role. The peak chemical production of haze layer ranges from 140 to 300 km peaking at an altitude of 220 km, with a production rate of 3.2×10⁻¹⁴ gcm⁻² s⁻¹. Possible mechanisms for polymerization and copolymerization and suggestions for further kinetic study are discussed, along with the implications for the distribution of haze in Titan's atmosphere.