Early Earth Atmosphere and Environment This lecture is designed to help the students learn about Earth’s early atmosphere and environment. Most of the information is from what is currently known about “extreme” environments like hydrothermal vents, that we as humans would not consider suitable for life. These types of environments are currently our best representations of what early Earth may have looked like. A good starting point for this lecture is to ask the students about our current atmospheric gases and how they compare to what they know about gases that would be present when the planet is forming. Another starting topic to discuss with students is how different the activity of volcanoes was in early Earth and what that might mean for the atmosphere.

Earth’s Early Atmosphere From Degassing A depiction of Earth’s atmosphere where volcanoes are more active than today and releasing gases into the atmosphere. This diagram shows how gases other than H and He entered the atmosphere. Currently, volcanoes still release these gases into the atmosphere but they are less active today. We can see volcanoes on the surface of Earth, are there volcanoes any where else on Earth? M.A. Allison,A.T. DeGaetano and J.M. Pasachoff (2006) Earth Science, Holt

Underwater Volcanoes – Hydrothermal vents A map showing locations of hydrothermal vents which are similar to volcanoes but underwater. The circles indicate where vents have been found, there are likely to be more vents but it is costly (in terms of money and time) to explore for the vents. Vents are often found where new sea floor is being formed. What do they look like and are they releasing similar gases to the ocean? http://www.divediscover.whoi.edu/images/hottopics_biogeo_map.gif

http://upload. wikimedia http://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Blacksmoker_in_Atlantic_Ocean.jpg/220px-Blacksmoker_in_Atlantic_Ocean.jpg A picture of a hydrothermal vent. The plume is a mixture of hot water, gases and minerals.

CO2, N2 A schematic of a current hydrothermal vent. If we ignore temporarily the components containing oxygen, to reconstruct Early Earth’s ocean, we see similar gases and compounds coming out of the hydrothermal vent as volcanoes. Suggesting the ocean was also very different than what we see it as today. Without large quantities of oxygen present in the water certain elements like iron (unable to rust) could stay dissolved in the ocean. http://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Deep_sea_vent_chemistry_diagram.jpg/400px-Deep_sea_vent_chemistry_diagram.jpg

Time (billion yrs ago) Here is a diagram showing calculated values for iron and oxygen concentration in the ocean over time. Iron concentrations were higher in the ocean when there was little to no oxygen present. At this time Fe was found in the more soluble reduced form Fe(II). As the oxygen concentration increased the iron oxidized (from Fe(II) to Fe(III)) which is less soluble, and turned in to rust (the reaction of Fe and O). This made iron precipitate (form a solid and sink to the bottom) decreasing the concentration in the ocean. The concentration of Fe in the ocean is of interest since many biological processes like photosynthesis, nitrogen fixation, DNA biosynthesis and electron transfer proteins require Fe. When life on Earth was evolving there was plenty of iron in the ocean and it made sense to use it. Currently Fe has a very low concentration in the ocean and it means many organisms are in need of iron. Why hasn’t life evolved to use less Fe? The enzymes and proteins that require Fe change very slowly making biology “stuck” to need high concentrations of Fe.

This diagram shows the rise in oxygen concentration (%PAL- % of present atmosphere level) over time. The main processes for producing oxygen is photosynthesis. Photosynthesis uses carbon dioxide to produce carbohydrates for energy. We can see that as oxygen is increasing carbon dioxide is decreasing. How do we know these events occurred? In order to reconstruct Earth’s history, when humans were not around to make measurements, we can examine the geological record for answers. Modified from Introduction to Atmospheric Chemistry, p. Hobbs, 2000

This is a picture of the banded iron formations This is a picture of the banded iron formations. The red-orange bands show large accumulations of iron in the sediment. The Fe was oxidized which made it less soluble and it precipitated out of the water column. This is an example of how we know oxygen levels have changed over Earth’s lifetime. http://www.geol.lsu.edu/dutrow/mingy/BIF2.jpg

One of the organisms responsible for producing oxygen billions of years ago are cyanobacteria – ancient bacteria. This is a picture of fossilized cyanobacteria called a stromatolite http://www.sharkbay.wa.gov.au/tourism/what_to_see_and_do/images/stromatolites_lge.jpg