Tropical rainforests on the deep sea floor?

Professor Kai Hinrichs is coming from Germany to talk to us about archaea (single celled, microrganisms with no cell nucleus or organelles) on the bottom of the ocean. He describes himself as

'a biogeochemist studying the interactions between microbial life and the carbon cycle on a range of spatial, temporal and molecular scales. I am interested in which and how microbes shape element cycles and what the related environmental consequences are. In order identify and ideally quantify microbial processes, my research group studies the information encoded in distributions and isotopic compositions of organic biomarker molecules in geological and environmental samples, ranging in size from 1 to ~100 C-atoms. My group consists of closely collaborating biologists, chemists, geochemists, and marine geoscientists. In our research projects, we combine analyses of environmental samples with experimental, laboratory-based approaches. Our current research foci encompass the deep subsurface biosphere, methane biogeochemistry, life in extreme environments, development and application of new geochemical-analytical techniques, prokaryotic membrane lipid taxonomy, and the study of paleoenvironments associated with major perturbations of the C-cycle.'

The reading this week is an early nature paper Hinrich wrote discussing the decomposition of a methane hydrate (ice forming in sediment that contains large amounts of methane ), where the methane is being consumed by archaea. As no organism can consume methane without oxygen being present, scientists think that the consumption of methane must be done with a consortium of bacteria with some being sulfate (SO4) reducing (providing the oxygen) and some being a reverse type of methanogen (consuming methane rather than producing it). Hinrichs shows using very negative carbon isotopes that his archaea are feeding on methane and he suggests methane consuming archaea have evolved alongside methane producing archaea. Their presence is important if you want to estimate future or past methane concentrations in the atmosphere (faint sun paradox or at Permian-Triassic Boundary)- because when balancing carbon reservoirs consumption is just as important as production.

We're not in Kansas anymore....

This weeks lecturer will be discussing early Earth - going back in time to the point at which the first whiff of oxygen appeared.

The Hadean Era was the first in Earth history, extending from the first formation of continental crust, which began some time around 4.4 billion years becoming persistent soon after, to approximately 3.5 billion years. The earliest terrestrial environments were harsh: Levels of atmospheric oxygen around 1% were too low to sustain an ozone layer, without which there would have been little protection from solar radiation. High levels of atmospheric carbon dioxide and methane would have created a strong greenhouse effect, with global temperatures estimated to have been between 30 and 50°C. Oceans formed early, between 4.4 and 3.9 billion years, from condensation of atmospheric water vapor. Estimates suggest that the earliest oceans were hot (between 80 and 100°C) and acidic.

University of Colorado researcher Stephen Mojzsis explains what Earth might have looked like back then:

"Before 4 billion years ago, the Earth would not be recognizable for the pale blue world that we are familiar with today. Indeed, although we now understand that there were significant landmasses already present by that time, the denser carbon dioxide-rich atmosphere would have given the sky a reddish-tinge. The oceans, with a much higher concentration of iron than our contemporary oceans, would look a dark greenish-blue and these oceans would have bathed hundreds of small continents akin to New Zealand or the Japan arc,"

This weeks reading is a paper published by Mojzsis about evidence for life on earth at 3.4 billion years ago. Mojzsis measured carbon isotopes in inclusions within apatite grains. The isotopes (δ13C) were light – meaning they were enriched in 12C relative to 13C. Organisms turning CO2 into organic matter favor the 12C atoms over the 13C atoms because metabolic processes involving the lighter isotope occur faster. So inorganic carbon (C not fixed by organisms) has a δ13C of ~-10‰, carbonaceous fossils have a δ13C of ~-20 to -35‰ and photosynthesizers, ~-50 to -60‰.

The 'Canfield Ocean' Guy

You've made it when you have an ocean named after you or you have a wikipedia page (albeit a short one). The "Canfield Ocean", a sulfidic, partially oxic ocean existing for more than 40% of Earth history, between the Archean and Ediacaran periods, takes its name from his seminal paper (Nature, 1998). This finding has profound implications for biological evolution, and the history of atmospheric oxygen, carbonate sedimentology and climate. Canfield and his co-workers found convincing evidence that a prolonged, stable, oxygen-rich environment following the demise of the Canfield Ocean permitted the emergence of animals capable of movement some 550 million years ago. In Dons own words
"Fossil evidence suggests that animals probably evolved sometime around 600 million years ago, and large animals appear around 575 million years ago. Animals have an absolute requirement for oxygen for their respiration, so it has often been speculated that large macroscopic animals (those with the largest oxygen requirement) evolved when oxygen rose to permissible levels, which would be about 10% of those levels we have today. Our study supports this scenario by showing that the first occurrence of large respiring organisms, some of which are likely stem-group animals, emerged on the Avalon Peninsula in Newfoundland in concordance with oxygenation of this local environment."

On Friday Don will be talking about Oxygen Minimum Zones. The Oxygen minimum zone (OMZ), is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,000 metres, depending on location. Surface ocean waters generally have O2 concentrations in equilibrium with the atmosphere. Colder waters can hold more oxygen than warmer waters. As this water sinks from the surface into deeper waters it is exposed to a rain of organic matter from above. As aerobic bacteria feed on this organic matter they use oxygen as part of the bacterial metabolic process lowering its concentration within the surrounding water. Therefore, the concentration of oxygen in deep water is dependent on the amount of oxygen it had when it was at the surface minus depletion by the degradation of organic matter. The OMZ relates to new ways scientists are looking at the 'Great Oxygenation Event'.

The reading this week is another scientific paper which puts forward the idea the delivery of nutrients to the ocean following the last of the Snowball Earth Ice Ages (the Gaskiers glaciation) feed an algal (cyanobacterial) bloom that provided enough oxygen to fuel multicellular life - in other words animals. Don and his co-authors come up with this idea because the geochemistry of sediments before the Gaskiers glaciation is different from the geochemistry of the sediments afterwards.

Video primer about oxygen and photosynthesis

The Extinction happened on Tuesday at 9:15am

Sam Bowring is a professor at MIT who specializes in Uranium-lead zircon geochronology and has gotten really good at it. The precision that he is getting on his dates are amazing. Basically zircon is a mineral (ZrSiO4) that accepts element such as U which has a large ionic radius into its structure, but rejects Pb (lead). Zircon has an almost ubiquitous presence in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). U-Pb dating provides an age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range. The method relies on two separate decay chains, the uranium series from ²³⁸U to ²⁰⁶Pb, with a half-life of 4.47 billion years and the actinium series from ²³⁵U to ²⁰⁷Pb, with a half-life of 704 million years. Sam will be talking about two major extinctions that occurred on Earth.

Permo-Triassic Extinctions

A major extinction event occurred approximately 250 million years ago and marks the boundary between the Permian Period and the Triassic Period. At the end of the Permian more than 85% of all species in the oceans, approximately 70% of land vertebrates, and significant numbers of plants and insects vanished. The Permian extinction caused the most fundamental reorganization of ecosystems and animal diversity in the past 500 million years. The marine communities of today are largely a result of the recovery following this extinction. In addition, dinosaurs and mammals arose in the aftermath of the extinction.

A better understanding of how long the end-Permian extinction and its recovery took will allow for new insights and better understanding into the role of mass extinctions in evolution. Mass extinctions are marked in the fossil record by the abrupt disappearance of taxa, sometimes associated with a discrete "boundary bed"; in the case of the end Cretaceous extinction this bed is a layer rich in impact ejecta with distinctive chemical signatures. A critical question is how abrupt such extinctions really were. A satisfactory answer must involve statistical analysis of the stratigraphic and fossil record. Differences in sediment accumulation rate and the preservation potential of organisms can lead to an artificially abrupt and/or drawn out extinction signal, especially if the extinction is of short duration, say, less than 1 million years. Because sedimentation rates vary, stratigraphic thickness does not convert to time directly. Therefore, understanding an extinction requires constraining its tempo by combining high-precision geochronology with paleontological studies. Sam has found that the carbon isotope event associated with the major shift in carbon reservoirs as organisms disappeared from the planet was 165 thousand years long which indicates that there was a catastrophic addition of 12-carbon into the earths system. Furthermore he has found that most of the extinctions occurred in less than 1 milion years.

P-T Video featuring Sam Bowring