Using Landsat Imagery to Analyse Land Cover Change in the Njoro Watershed, Kenya

In developing nations where resources are scarce and increased population pressures create stress on available resources, methods are needed to examine effects of human migration and resultant changes in land cover. Widespread availability and low cost of remotely sensed imagery and Geographic Information Systems (GIS) are making such methods a reality to develop quantitative resource mapping and land cover change detection in developing nations (Sheng et al., 1997). However, difficulties arise in tropical regions when trying to analyse traditional vegetation bands (Bands 3 and 4), or indices such as NDVI because saturated pixels limit spectral distinction

Earth: Atmospheric Evolution of a Habitable Planet

Our present-day atmosphere is often used as an analog for potentially habitable exoplanets, but Earth's atmosphere has changed dramatically throughout its 4.5 billion year history. For example, molecular oxygen is abundant in the atmosphere today but was absent on the early Earth. Meanwhile, the physical and chemical evolution of Earth's atmosphere has also resulted in major swings in surface temperature, at times resulting in extreme glaciation or warm greenhouse climates. Despite this dynamic and occasionally dramatic history, the Earth has been persistently habitable--and, in fact, inhabited--for roughly 4 billion years. Understanding Earth's momentous changes and its enduring habitability is essential as a guide to the diversity of habitable planetary environments that may exist beyond our solar system and for ultimately recognizing spectroscopic fingerprints of life elsewhere in the Universe. Here, we review long-term trends in the composition of Earth's atmosphere as it relates to both planetary habitability and inhabitation. We focus on gases that may serve as habitability markers (CO2, N2) or biosignatures (CH4, O2), especially as related to the redox evolution of the atmosphere and the coupled evolution of Earth's climate system. We emphasize that in the search for Earth-like planets we must be mindful that the example provided by the modern atmosphere merely represents a single snapshot of Earth's long-term evolution. In exploring the many former states of our own planet, we emphasize Earth's atmospheric evolution during the Archean, Proterozoic, and Phanerozoic eons, but we conclude with a brief discussion of potential atmospheric trajectories into the distant future, many millions to billions of years from now. All of these 'Alternative Earth' scenarios provide insight to the potential diversity of Earth-like, habitable, and inhabited worlds.Comment: 34 pages, 4 figures, 4 tables. Review chapter to appear in Handbook of Exoplanet

Groundwater table fluctuations recorded in zonation of microbial siderites from end-Triassic strata

In a terrestrial Triassic–Jurassic boundary succession of southern Sweden, perfectly zoned sphaerosiderites are restricted to a specific sandy interval deposited during the end-Triassic event. Underlying and overlying this sand interval there are several other types of siderite micromorphologies, i.e. poorly zoned sphaerosiderite, spheroidal (ellipsoid) siderite, spherical siderite and rhombohedral siderite. Siderite overgrowths occur mainly as rhombohedral crystals on perfectly zoned sphaerosiderite and as radiating fibrous crystals on spheroidal siderite. Concretionary sparry, microspar and/or micritic siderite cement postdate all of these micromorphologies. The carbon isotope composition of the siderite measured by conventional mass spectrometry shows the characteristic broad span of data, probably as a result of multiple stages of microbial activity. SIMS (secondary ion mass spectrometry) revealed generally higher δ13C values for the concretionary cement than the perfectly zoned sphaerosiderite, spheroidal siderite and their overgrowths, which marks a change in the carbon source during burial. All the various siderite morphologies have almost identical oxygen isotope values reflecting the palaeo-groundwater composition. A pedogenic/freshwater origin is supported by the trace element compositions of varying Fe:Mn ratios and low Mg contents. Fluctuating groundwater is the most likely explanation for uniform repeated siderite zones of varying Fe:Mn ratios reflecting alternating physiochemical conditions and hostility to microbial life/activity. Bacterially mediated siderite precipitation likely incorporated Mn and other metal ions during conditions that are not favourable for the bacteria and continued with Fe-rich siderite precipitation as the physico-chemical conditions changed into optimal conditions again, reflecting the response to groundwater fluctuations

Paleo-roothole facilitated transport of aromatic hydrocarbons through a Holocene clay bed

A field study involving high-resolution core sampling of a 0.5−2 m thick clay bed was undertaken at a contaminated former industrial facility in the UK to establish the nature and significance of preferential contaminant flowpaths. In contrast to most previous research, the focus was upon a buried aquitard, in this case a Holocene lagoonal clay located 6 m below ground surface and overlain by a sand aquifer impacted by historic nonaqueous phase liquid hydrocarbon spills. The study, involving 11 cores over a 630 by 150 m area, demonstrated that the presence of paleo- (i.e., preupper sand) rootholes controlled the degree of dissolved-phase benzene penetration into the aquitard. Where homogeneous, largely paleoroot-free clay is present (hydraulic conductivity 3 × 10−5 m/d.), contaminant concentrations in the clay decline rapidly with depth: modeling showed the dominant transport process to be diffusion. In other cores, elevated benzene concentrations deep in the clay require advection to have occurred, presumably along preferential pathways. The latter were shown by thin sectioning, core slice mapping and 3-D X-ray tomography to be organic matter lined rootholes of <2 mm aperture. The significance of such preferential pathways was confirmed quantitatively by measuring hydraulic conductivity (0.04 m/d) and calculating flux, the latter being over 10 times greater than expected from steady state diffusion. Our study hence demonstrates paleoenvironmental control of modern-day contaminant transport through a clay aquitard. It is suggested that many subaerial unconformities in mudrocks, especially those associated with even rudimentary paleosol development, would lead to permeability enhancement and therefore afford substantially reduced protection against migrating contaminants. In contaminated site investigations, it is hence necessary to consider the aquitard paleoenvironment and not just the main rock type present

Soil Carbon Isotope Values and Paleoprecipitation Reconstruction

Anthropogenic climate change has significant impacts at the ecosystem scale including widespread drought, flooding, and other natural disasters related to precipitation extremes. To contextualize modern climate change, scientists often look to ancient climate changes, such as shifts in ancient precipitation ranges. Previous studies have used fossil leaf organic geochemistry and paleosol inorganic chemistry as paleoprecipitation proxies, but have largely ignored the organic soil layer, which acts as a bridge between aboveground biomass and belowground inorganic carbon accumulation, as a potential recorder of precipitation. We investigate the relationship between stable carbon isotope values in soil organic matter (δ13CSOM) and a variety of seasonal and annual climate parameters in modern ecosystems and find a statistically significant relationship between δ13CSOM values and mean annual precipitation (MAP). After testing the relationship between actual and reconstructed precipitation values in modern systems, we test this potential paleoprecipitation proxy in the geologic record by comparing precipitation values reconstructed using δ13CSOM to other reconstructed paleoprecipitation estimates from the same paleosols. This study provides a promising new proxy that can be applied to ecosystems post‐Devonian (∼420 Ma) to the Miocene (∼23 Ma), and in mixed C3/C4 ecosystems in the geologic record with additional paleobotanical and palynological information. It also extends paleoprecipitation reconstruction to more weakly developed paleosol types, such as those lacking B‐ horizons, than previous inorganic proxies and is calibrated for wetter environments.Plain Language SummaryRainfall is very important to plant health and function. When plant material is deposited onto the ground, it becomes soil. This soil retains records of plant chemistry. We tested whether this plant chemistry recorded amount of rainfall over a wide range of environments, and found that soil chemistry does record rainfall. When tested in fossil soils, the soil chemistry as it remained of prior plant deposition could be used to calculate ancient rainfall, millions of years ago.Key PointsSoil carbon isotope values can be used to reconstruct precipitationReconstructed paleoprecipitation using carbon isotope values of organic matter are comparable to reconstructions with other proxiesThese soil isotope geochemistry findings validate prior work linking aboveground plant biomass isotope ecology and precipitationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167532/1/palo21004.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167532/2/palo21004_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167532/3/2020PA004158-sup-0001-Supporting_Information_SI-S01.pd