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

Pore water composition of volcanogenic sediments from across the Central American Subduction Zone

Copyright © 2009 Elsevier B.V. All rights reserved. ScienceDirect® is a registered trademark of Elsevier B.V. Abstract of GEOFLUIDS VI., GEOFLUIDS VI. Sixth International Conference on Fluid Evolution, Migration and Interaction in Sedimentary Basins and Orogenic Belts Conference on Fluid Evolution, Migration and Interaction in Sedimentary Basins and Orogenic BeltsSubduction Zones are among the most dynamic tectonic environments on Earth. Thus, the Central American Suduction Zone became the focus of research cruise Meteor 66 since here the sediment cover is strongly influenced by the highly explosive volcanism along the Central American Volcanic Arc. The main focus of this study is to analyse the influence of alteration reactions on the chemical composition of pore waters extracted from volcanogenic surface sediments. During research cruise RV Meteor 66, a program of sampling and analysis of pore waters from retrieved sediment cores was carried out. Alkalinity, sulphate, calcium, magnesium, potassiumand silica concentration-depth profiles of deep-sea sediment coresweremeasured to define the ash alteration reactions that occur in surface sediments. The concentrationprofiles ofmajor cations showed no clear effects of ash alteration, which is in contrast to investigations performed on deep sediment cores (N100 m depth) during the Deep Sea (DSDP) and Ocean Drilling (ODP) Programs. However, no significant difference in dissolved Ca and Mg could be observed between the ash-bearing cores and the ash-free reference core, confirming that the distribution of alkaline earth elements in the surface sediments investigated in our study is not significantly affected by ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawaterderived pore waters. Apart from the generally low reactivity of the investigated of the investigated sediments, two interesting observations could be made: (1) Potassium pore water concentrations are enriched in background sediments and depleted in ash-bearing sediment layers; (2) Ash alteration seems to affect dissolved silica contents in pore waters, causing concentrations about two times higher than in the ash-free reference core.Ulrike Schacht, Steffen Kutterolf, Oliver Bartdorff, and Emelina Corrales Corder

Microorganisms and climate change:terrestrial feedbacks and mitigation options

Microbial processes have a central role in the global fluxes of the key biogenic greenhouse gases (carbon dioxide, methane and nitrous oxide) and are likely to respond rapidly to climate change. Whether changes in microbial processes lead to a net positive or negative feedback for greenhouse gas emissions is unclear. To improve the prediction of climate models, it is important to understand the mechanisms by which microorganisms regulate terrestrial greenhouse gas flux. This involves consideration of the complex interactions that occur between microorganisms and other biotic and abiotic factors. The potential to mitigate climate change by reducing greenhouse gas emissions through managing terrestrial microbial processes is a tantalizing prospect for the future