Continental Tropics Research Articles

Using luminescence dating to constrain lake sediment records: A new age model for the 1.38 Ma lake Malawi drill core, Eastern Africa

The 2005 Lake Malawi deep scientific drill core is the longest and most continuous high-resolution record from the continental tropics, extending to ∼1.38 Ma. While extensive sets of paleoclimate proxy data have been generated from this core, a gap in directly dated sediment, between ∼74 ka and ∼590 ka (28–167 m below lake floor), has limited the understanding of climatic drivers in this system. Previous age models fill the gap in direct dates by tuning to the global δ18O stack and interpreting paleomagnetic excursions, but these methods and the resulting models remain disputed. We fill this gap in chronology using luminescence dating, with 31 samples collected at ∼4 m resolution in this section of the core. Luminescence dating can sometimes be limited by relatively large uncertainties (typically ∼7%) and difficulty estimating the water content history of samples. We overcome these limitations by employing a high sampling density and using the sediment record to understand changes in water content during the burial period. This yields a vastly improved and robust age model that indicates changes in sedimentation rates not discernible in prior age models.

Occupancy winners in tropical protected forests: a pantropical analysis.

The structure of forest mammal communities appears surprisingly consistent across the continental tropics, presumably due to convergent evolution in similar environments. Whether such consistency extends to mammal occupancy, despite variation in species characteristics and context, remains unclear. Here we ask whether we can predict occupancy patterns and, if so, whether these relationships are consistent across biogeographic regions. Specifically, we assessed how mammal feeding guild, body mass and ecological specialization relate to occupancy in protected forests across the tropics. We used standardized camera-trap data (1002 camera-trap locations and 2–10 years of data) and a hierarchical Bayesian occupancy model. We found that occupancy varied by regions, and certain species characteristics explained much of this variation. Herbivores consistently had the highest occupancy. However, only in the Neotropics did we detect a significant effect of body mass on occupancy: large mammals had lowest occupancy. Importantly, habitat specialists generally had higher occupancy than generalists, though this was reversed in the Indo-Malayan sites. We conclude that habitat specialization is key for understanding variation in mammal occupancy across regions, and that habitat specialists often benefit more from protected areas, than do generalists. The contrasting examples seen in the Indo-Malayan region probably reflect distinct anthropogenic pressures.

Aboveground carbon accumulation by second‐growth forests after deforestation in Hawai'i

Successional processes ultimately determine and define carbon accumulations in forested ecosystems. Although primary succession on wholly new substrate occurs across the globe, secondary succession, often following storm events or anthropogenic disturbance, is more common and is capable of globally significant accumulations of carbon (C) at a time when offsets to anthropogenic carbon dioxide (CO2 ) emissions are critically needed. In Hawai'i, prior studies have investigated ecosystem development during primary succession on lava flows, including estimates of C mass accumulation. Yet relatively little is known regarding secondary succession of Hawaii's native forests, particularly regarding C mass accumulation. Here we documented aboveground C mass accumulation by native- and nonnative-dominated second-growth forests following deforestation of mature native lowland rainforests in the Puna District of Hawai'i Island. We characterized species composition and stand structure of three distinct successional forest stand types: those dominated by the native tree, Metrosideros polymorpha ('Ōhi'a), and those dominated by invasive nonnative trees, Falcataria moluccana (albizia) and Psidium cattleianum (strawberry guava). We compared M. polymorpha-dominated and F. moluccana-dominated second-growth forests to adjacent mature M. polymorpha-dominated forests as well as young M. polymorpha-dominated forests undergoing initial stages of primary succession on 36-years-old lava fields. Aboveground carbon density (ACD) values of mature primary forest stands (171 Mg/ha) were comparable to those of mature continental tropical forests. M. polymorpha-dominated second-growth stands attained nearly 50% of ACD values of mature primary forests after less than 30 years of post-disturbance succession and exhibited aboveground carbon accumulation rates of ~3 Mg C·ha-1 ·year-1 . Such rates were comparable to those of second-growth forests in continental tropics. Rates of ACD accumulation by second-growth forests dominated by nonnative F. moluccana stands were similar, or slightly greater than, those of M. polymorpha-dominated stands. However, M. polymorpha individuals were virtually absent from stands dominated by either P. cattleianum or F. moluccana. Taken together, results demonstrated that re-establishment and rapid accumulation of C mass by M. polymorpha stands during secondary succession is certainly possible, but only where populations of nonnative species have not already colonized areas during early stages of secondary succession.

Are Terrestrial Biological Invasions Different in the Tropics?

Most biological invasion literature—including syntheses and meta-analyses and the resulting theory—is reported from temperate regions, drawing only minimally from the tropics except for some island systems. The lack of attention to invasions in the tropics results from and reinforces the assumption that tropical ecosystems, and especially the continental tropics, are more resistant to invasions. We have critically assessed biological invasions in the tropics and compared them with temperate regions, finding relatively weak evidence that tropical and temperate regions differ in their invasibility and in the traits that determine invader success and impacts. Propagule pressure and the traits that promote adaptation to disturbances (e.g., high fecundity or fast growth rates) are generally favorable to invasions in both tropical and temperate regions. We emphasize the urgent need for greater investment and regional cooperation in the study, prevention, and management of biological invasions in the tropics.

Honey Bees in the Tropics Show Winter Bee-Like Longevity in Response to Seasonal Dearth and Brood Reduction

Upon their first introduction to Americas in 1956, African honey bees (Apis mellifera scutellata) hybridized with the previously introduced and already established European honey bees (EHBs). The resulting Africanized honey bees (AHBs) have spread through the continental tropics of the Western Hemisphere. The expansion of AHB has been constrained in temperate climates generally thought to be because of a lack of key adaptations required for successful overwintering. A drastic increase in longevity during broodless periods is crucial to colony survival. In the temperate context, broodlessness happens in winter. While tropical regions do not experience winters as temperate zones do, seasonal changes in the abundance of floral resources cause variation in brood levels throughout the year. Here we use an island population of AHB in Puerto Rico (gAHB) to test the capacity of tropical-adapted honey bees to alter their longevity in different seasons, as well as under brood manipulation. We found that worker longevity in the gAHB colonies increases in the wet season (maximum longevity ca. 88 days vs 56 days) in response to dearth of floral resources. A more pronounced increase in longevity was observed in response to manipulative reduction of the amount of open brood (maximum longevity 154 days). In addition, long lived gAHB demonstrated the signature winter bee-like hypopharyngeal gland size (average acini diameter 100.8 ± 6.2 µm at 65 and 70 days of age, N=26), intermediate between forager (88.7 ± 5.9 µm, N=24) and nurse (129.5 ± 8.1 µm, N=24) gland size. We showed that gAHBs do not lack the adaptation to alter their longevity seasonally, though the magnitude of changes is less intense than those observed in EHBs during temperate winters. This suggests that increased longevity in response to limited capacity to rear brood is a shared character of Africanized and European honey bees.

Land–atmosphere interactions in the tropics – a review

Abstract. The continental tropics play a leading role in the terrestrial energy, water, and carbon cycles. Land–atmosphere interactions are integral in the regulation of these fluxes across multiple spatial and temporal scales over tropical continents. We review here some of the important characteristics of tropical continental climates and how land–atmosphere interactions regulate them. Along with a wide range of climates, the tropics manifest a diverse array of land–atmosphere interactions. Broadly speaking, in tropical rainforest climates, light and energy are typically more limiting than precipitation and water supply for photosynthesis and evapotranspiration (ET), whereas in savanna and semi-arid climates, water is the critical regulator of surface fluxes and land–atmosphere interactions. We discuss the impact of the land surface, how it affects shallow and deep clouds, and how these clouds in turn can feed back to the surface by modulating surface radiation and precipitation. Some results from recent research suggest that shallow clouds may be especially critical to land–atmosphere interactions. On the other hand, the impact of land-surface conditions on deep convection appears to occur over larger, nonlocal scales and may be a more relevant land–atmosphere feedback mechanism in transitional dry-to-wet regions and climate regimes.

Diurnal patterns in lightning activity over South America

AbstractSatellite observations of lightning flash distribution data are used to examine the diurnal cycle of lightning activity over the tropical and subtropical regions of South America. A harmonic analysis is used to study the spatial variations in the peak and strength of diurnal lightning activity across this area. Results show that in the northern and central regions of South America, the times of maxima in lightning activity was concentrated from late afternoon to evening hours (between 14:00 and 18:00 local time), which may be associated with the peaking of the local convective activity connected with heating of the surface caused by daytime insolation. In subtropical South America, particularly in the area limited by 25°S, 35°S of latitude and 70°W, 50°W of longitude, the time of maximum lightning activity was shifted to nocturnal hours, extending from close to midnight to early morning hours. This behavior can be associated to the peak in mesoscale convective systems in the region which occurs in the morning hours. The annual flash densities in the tropical and subtropical parts of the continent were found to have comparable magnitudes. However, the contribution of the continental tropics to the global electric circuit dominates over the continental subtropics contribution throughout all seasons, since the surface covered by the tropical region is more than twice the area covered by the subtropical region.

Influence of future climate and cropland expansion on isoprene emissions and tropospheric ozone

Abstract. Over the 21st century, changes in CO2 levels, climate and land use are expected to alter the global distribution of vegetation, leading to changes in trace gas emissions from plants, including, importantly, the emissions of isoprene. This, combined with changes in anthropogenic emissions, has the potential to impact tropospheric ozone levels, which above a certain level are harmful to animals and vegetation. In this study we use a biogenic emissions model following the empirical parameterisation of the MEGAN model, with vegetation distributions calculated by the Sheffield Dynamic Global Vegetation Model (SDGVM) to explore a range of potential future (2095) changes in isoprene emissions caused by changes in climate (including natural land use changes), land use, and the inhibition of isoprene emissions by CO2. From the present-day (2000) value of 467 Tg C yr−1, we find that the combined impact of these factors could cause a net decrease in isoprene emissions of 259 Tg C yr−1 (55%) with individual contributions of +78 Tg C yr−1 (climate change), −190 Tg C yr−1 (land use) and −147 Tg C yr−1 (CO2 inhibition). Using these isoprene emissions and changes in anthropogenic emissions, a series of integrations is conducted with the UM-UKCA chemistry-climate model with the aim of examining changes in ozone over the 21st century. Globally, all combined future changes cause a decrease in the tropospheric ozone burden of 27 Tg (7%) from 379 Tg in the present-day. At the surface, decreases in ozone of 6–10 ppb are calculated over the oceans and developed northern hemispheric regions, due to reduced NOx transport by PAN and reductions in NOx emissions in these areas respectively. Increases of 4–6 ppb are calculated in the continental tropics due to cropland expansion in these regions, increased CO2 inhibition of isoprene emissions, and higher temperatures due to climate change. These effects outweigh the decreases in tropical ozone caused by increased tropical isoprene emissions with climate change. Our land use change scenario consists of cropland expansion, which is most pronounced in the tropics. The tropics are also where land use change causes the greatest increases in ozone. As such there is potential for increased crop exposure to harmful levels of ozone. However, we find that these ozone increases are still not large enough to raise ozone to such damaging levels.

Espécies de aves não nativas no Brasil

Many non-native species of birds have been introduced beyond the limits of their natural geographic ranges, where they may subsequently establish viable populations and perhaps become invasive. Much attention in the literature has been devoted to the study of bird introductions, but relatively little of that attention has been focused on introductions to the continental tropics. Here, we compile published data on records of non-native bird species introduced to the wild in Brazil, with the aim of organizing and standardizing available information to facilitate future studies in this context. A total of 59 bird species were classified as introduced in Brazil, with one further species believed to be a recent natural colonist. Published information implies that 14 species have established or probably established non-native populations in Brazil, while a further 10 species have possibly established non-native populations. In this context, we map the non-native distributions of non-native species of birds in Brazil. Additional research is needed to confirm the status of these species, as well as to establish practical measures to combat and control introductions.Key words: alien birds, biological invasions, exotic species.

Characteristics of Precipitating Convective Systems Accounting for the Summer Rainfall of Tropical and Subtropical South America

AbstractTen years of Tropical Rainfall Measuring Mission precipitation radar data are used to study the physical properties of the precipitating cloud systems that account for the summer rainfall of tropical and subtropical South America. Radar echoes in the continental subtropics tend to be of an intensely convective nature, especially at the eastern foothills of the Andes where diurnally forced deep convective cells of small horizontal scale form when moist low-level flow is driven toward the foothills in connection with a midlatitude disturbance. As the disturbance moves east over the La Plata basin, nocturnal convective systems of larger horizontal scale with wide stratiform regions occur in a zone of general convergence. Precipitation in the continental tropics is generally produced by convective systems with greater stratiform composition. At the northeastern foothills of the central Andes, radar echoes of nocturnal convective systems of medium to large horizontal scale occur where moist low-level flow is lifted over the foothills. Growth of systems to large size is inhibited by daytime divergence at the foothills. Over the Amazon basin, daytime systems are also smaller than nocturnal systems. Radar echoes of precipitation over the Brazilian Highlands are generally smaller in horizontal scale, more convective, and mostly occur during the afternoon over elevated terrain. In the oceanic South Atlantic convergence zone, radar echoes grow to extremely large sizes. They are highly stratiform in nature and occur during all times of the day except late evening when convergence is weakened as a response to continental heating.

Introduced Alien Plant Species in the Neotropics: the Panama Case

Non-insular tropical biomes appear relatively resistant to invasive alien species (IAS). While some argue fewer IAS in continental tropical communities is the result of the complexity of species-rich communities (e.g., Elton's biological resistance hypothesis), others suggest lack of IAS might reflect fewer invasion opportunities, which could change with time. In effect, deforestation may lead to the simplification of tropical habitats, thus reducing biological resistance. Little is known about the current status of alien plants species, factors contributing to their spread and IAS in non-insular tropical systems. Here I report on the status of alien plant species in relation to area, number of native and endemics species, population, forest cover and cultivated area across provinces of Panama. Alien plant species comprise nearly 4 percent of the flora and was positively correlated with the number of native plant species (r = 0.84, P < 0.001) and while this pattern runs counter to Elton's premise, it is consistent with other landscape-scale studies. In Panama, the number of alien plant species is explained by population density (r= 0.91, P < 0.01) and potentially linked to disturbance, albeit coarsely, as the proportion of aliens negatively correlates with forest cover (r = 0.69, P < 0.05). Thus, despite high diversity, these results portray disturbed tropical forests becoming dotted with introduced alien species. While few species seem to become invasive in the continental tropics, rapid land use change could promote the success of IAS representing serious consequences for tropical countries' economies and biodiversity.

Influences of herbaceous vines on community characteristics in pioneer succession stages

In the continental tropics, herbaceous vines and lianas are roughly equal in abundance, and the former are even more abundant in the temperate region. However, only little attention has been paid to the study of biological and ecological characteristics of herbaceous vines. In particular, research about effects of herbaceous vines acting as a biological control on plant communities has not been carried out in depth. Herbaceous vines are widely distributed and abundant also in humid subtropical areas, especially in the early stages of forest community succession, but their ecology is little known. The aim of our study is to understand the effect of local herbaceous vines on community characteristics in pioneer succession stages. The hypothesis was tested that herbaceous vines would have predominantly negative effects on co-occurring species, thereby reducing their diversity. Based on a quadrate method, a detailed survey of shrub and herb communities covered by herbaceous vines was conducted in the Jinyun Mountain Nature Reserve of Chongqing, SW China. The sample plots were selected based on the numbers and coverage of vines, distinguishing among high vine coverage plots, middle vine coverage plots and low vine coverage plots. All species in the plots with different herbaceous vine coverage were identified and measured. The measurements for each species included number, average height and coverage. Because of abundant tree seedlings in the habitat of forest edge plots, we only recorded the number of tree seedlings in those plots to evaluate the overall effects of vines on tree seedling regeneration. After the field investigation, herbaceous vines and other species in the plots were harvested respectively, then oven dried and weighed. The results showed that herbaceous vines had high productivity and produced a lot of branches, which caused above-ground competition and mechanical stress to other species. Herbaceous vines seriously affected species composition and species importance values of self-supporting species. In all three habitats, the number of species and families in low coverage samples was larger than that in high coverage samples, and furthermore the identities of species were different between them. Species richness significantly decreased with increasing herbaceous vine coverage, illustrating that some species disappeared. Herbaceous vines reduced species diversity of communities, and as a result, community complexity was decreased, which might also decrease community stability. Biomass of communities of self-supporting species significantly decreased with increasing herbaceous vine coverage, which suggested that herbaceous vines significantly decreased community productivity. The number of seedlings also significantly decreased with increasing herbaceous vine coverage, and seedlings were mainly distributed in lower coverage samples. Herbaceous vines reduced the light exposure in the understory, which may be the mechanistic explanation for the negative influence of vines on the performance of tree seedlings. It was concluded that herbaceous vines affected seedling quantitative dynamics and distribution, and inhibited the natural succession from shrub and herb communities to tree communities. Thus herbaceous vines not only had significant influences on community characteristics in pioneer succession stages, but also on subsequent succession stages.

Late Pleistocene temperature history of Southeast Africa: A TEX 86 temperature record from Lake Malawi

We present a TEX 86-derived surface water temperature record for Lake Malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past ∼ 74 kyr with millennial-scale resolution. Average temperature during Marine Isotope Stage (MIS) 5A was 26.5 °C, with a range from 25.7 to 27.3 °C, comparable to Holocene temperatures. MIS 4 was a relatively cold period with temperatures generally decreasing from 25.5 °C at 68 ka to a minimum of 20 °C at ∼ 60 ka, 1.5–2 °C colder than the Last Glacial Maximum (LGM). Termination of MIS 4 is characterized by a rapid increase of 3–4 °C in only ∼ 0.5 kyr. Temperatures were relatively stable throughout MIS 3 at the resolution of this study, with an average of 23.8 °C and a range from 25.1 to 22.9 °C. The lack of millennial-scale temperature variability during MIS 3 suggests that Lake Malawi's documented response to the bipolar seesaw (Brown et al., 2007) is not reflected in its thermal history. Our temperature estimates for the LGM and Holocene are consistent with a previously published TEX 86 record from Lake Malawi with a temperature of ∼ 22.6 °C for the LGM, ∼ 25–26 °C in the mid Holocene and ∼ 25–28 for the late Holocene. In general the present extended TEX 86 record indicates that temperature variability in tropical East Africa during late MIS 5 and MIS 4 was as great as that associated with the deglaciation and Holocene. A decrease in Southern Hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during MIS 4, but after 60 ka other factors, such as the extent of the polar ice sheets, or atmospheric CO 2 may have forced temperature in tropical Africa to a greater extent than local summer insolation.

Exotic plant invasion in the highly degraded upland landscape of Hong Kong, China

Plant communities in the continental tropics have suffered less from exotic plant invasions than their oceanic island counterparts. Most studies have focused on near-pristine communities. By contrast, we examine the resistance of semi-natural continental plant communities in Hong Kong, which have been suffering from chronic and massive human impacts. We compiled a list of all naturalized non-native species recorded in Hong Kong and then sampled the plant communities for exotic species along roadsides, a stream through semi-natural vegetation, and in semi-natural vegetation away from both roads and streams on Tai Mo Shan, Hong Kong’s highest peak (957 m). Similar surveys were repeated in other areas of Hong Kong. More than 162 naturalized exotic plant species have been recorded in Hong Kong. On Tai Mo Shan, 29 exotic species were recorded in roadside vegetation, with the diversity but not percentage cover declining significantly with altitude. Fifteen exotic species were found along the stream, including two not found along the roadside. Only six exotic species were found away from roads and streams, all in unshaded areas disturbed by feral cattle. In all surveys, no exotics were found in closed woody vegetation or in open areas without feral cattle, except for one species. The shade-tolerant tropical Asian tree Syzygium jambos was found invading along some streamsides without anthropogenic disturbance. Despite centuries of massive human impacts, exotic plant invasions in Hong Kong are still largely confined to habitats that suffer from chronic human disturbance. Feral cattle promote invasion where people are absent, but this problem still seems potentially reversible. Only Syzygium jambos is of possible current conservation concern.

Pacific Island Forests: Successionally Impoverished and Now Threatened to Be Overgrown by Aliens?1

ABSTRACT Indigenous forests in remote islands are generally impoverished of secondary successional tree species. After canopy disturbances, the same indigenous tree species seem to resume dominance by a process known as “autosuccession” or “direct succession.” Primary forest tree species are mostly colonizer species. Mature island forests are difficult to categorize as either pioneer, successional, or climax forests by their canopy species composition. Climax forests, which characterize mature forests in less-isolated areas, are typically of distinctly different canopy species composition than the pioneer forests. In central Canada, for example, pioneer pine forests are replaced in succession by mixed hardwood/softwood forests under exclusion of fire. This process is known as “normal replacement succession” or “obligatory succession.” Another well-known ecological concept distinguishes between “primary” and “secondary” forests in the continental tropics. Secondary forests are formed by fast-growing relati...

A mesoscale structure of meteorological fields in the tropopause layer and in the lower stratosphere over the southern tropics (Brazil)

Some results of studying thermodynamics of the tropical tropopause and lower stratosphere performed with the help of the stratospheric M-55 Geofizika research aircraft during the international TROCCINOX experiment in 2005 in Brazil are considered. New data on a mesoscale structure of the wind, temperature, and humidity fields over the continental tropics are derived.

African Drilling Project may offer climate

Africa's Lake Malawi, the site of an upcoming scientific drilling project, could provide data on climate variability and the evolution of the region's aquatic organisms, according to researchers from four U.S. universities who have received a $2‐million grant through the National Science Foundation's Earth System History program.“We believe that Lake Malawi will provide a unique, continuous, and high‐resolution (annual‐to‐decadal) record of past climates in the continental tropics over the last 800,000 years,” said Christopher Scholz, an Earth scientist at Syracuse University in New York, and principal investigator on the project. Researchers from the University of Arizona, the University of Minnesota at Duluth, and the University of Rhode Island also have received NSF support for the drilling project, which is scheduled to begin in January 2003.

Pronounced central uplift identified in the Bosumtwi impact structure, Ghana, using multichannel seismic reflection data

The Lake Bosumtwi impact structure is the youngest and best-preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes. Because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform, no significant backwash processes have modified the crater morphology. Not only may Bosumtwi contain the best-preserved central uplift structure on Earth, but it is the most accessible relatively large, young crater in the solar system generated in a large gravity field. There is a well-established link between the Lake Bosumtwi impact structure and the Ivory Coast tektite field, and the lacustrine sediments within the crater contain a unique 1 m.y. record of paleoclimate in the continental tropics south of the Sahel. Eight profiles of marine-type multichannel seismic reflection (MCS) data were acquired from the 8-km-diameter, similar to75-m-deep lake that fills much of the crater. These were augmented by wide-angle seismic data acquired with ocean-bottom hydrophones. MCS data reveal a well-defined central uplift near the northwest-central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat inside the crater. An intermediate velocity layer (3200 m/s) beneath the lacustrine sediment is interpreted as fallback breccia or a breccia-melt horizon. The measured apparent depth of the crater (d(a)) is 500 m, implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships. The Bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws, perhaps because of the effects, of a large gravity field.

A Non-linear Inverse Technique for Calculating Palaeotemperature and Other Variables Using Noble Gas Concentrations in Groundwater: The Last Glacial Maximum in the Continental Tropics Re-examined

We present here an error weighted non-linear inverse technique adapted from Menke (1984). This method fits palaeotemperature and up to two other variables, gives an estimate of the precision of the fitted variables and enables us to assess the confidence of the conceptual models with the input data. We illustrate the utility of this method with comprehensive data sets from the Pannonian basin, San Juan basin and tropical Brazil (Stute and Deak, 1989; Stute et al., 1995a,b). Solving for temperature and 'excess air' we demonstrate that the inverse method reproduces literature iterative techniques, but with a bias of up to 1.0~ This is due to error weighting placing more emphasis on the heavy noble gas concentrations in the temperature determination. Solving for either recharge salinity or altitude as a third variable, synthetic data sets reproduce input values. All the literature data produces a significant average negative offset from known recharge salinity and altitude. This offset in natural systems points to a significant and ubiquitous noble gas fractionation effect in all of the aquifer systems investigated. We also solve for fractionation by diffusive gas loss at recharge (Stute et al., 1995b). The tropical Brazil data set reproduces independently derived literature fractionation constants. Although this improves the data fit, a simple statistical test shows that diffusive fractionation alone cannot account for the sample noble gas abundance pattern observed in the tropical Brazil samples. Our derived errors of between +1.6 to _+6.3~ ( lc0 are significantly higher than the quoted literature error of +0.8~ This is due to both the inclusion of the third variable and the poor fit of the sample data to the conceptual model. When solving for fractionation in the Pannonian basin and San Juan data, we demonstrate that optimal recharge salinity for the combined data is now within error of meteoric water (Fig. 1). Palaeotemperature errors range between _+0.4 to _+2.5~ and _+0.8 to +2.4~ respectively. In both cases the data show a significant degree of improvement in the fit of the data to the model, the conceptual model is statistically consistent with both data sets, young samples are within error of the current day recharge temperatures, and samples previously considered outliers now agree with samples in the same age bracket. Despite the incomplete fit to the tropical Brazil data, it would appear that diffusive gas loss, at the very least, provides a necessary and reasonable proxy to this seemingly ubiquitous aquifer fractionation process.

A simulation model for the analysis of Space Station gas-phase trace contaminants

A simulation model for the analysis of gas-phase trace contaminants in the cabin air of the NASA Space Station Reference Configuration was developed at the NASA Langley Research Center. The model predicts changes in trace contaminant concentrations from both physical and chemical sources and sinks as a function of time. Simulations were performed in which values for relative humidity, temperature, radiation intensity, pressure, and initial species concentrations were constrained to values for these parameters measured and modeled in the continental tropics at the Earth's surface. Species concentrations simulated using the model compared favorably with concentrations in the continental tropics which demonstrated that the chemical mechanism in the trace contaminant model approximates changes in atmospheric species concentrations. The sensitivity of initial species concentrations to producing changes in additional species concentrations was also assessed. Results from the model indicated that chemical reactions will be important in determining the composition of cabin air in the Space Station. It is anticipated that the trace contaminant model will be useful in assessing the impact of experiments and commercial operations on the composition of the cabin air in the Space Station.