Mar. 12th, 2017


More gems from the "Origins" course

Little factoids, surprises, etc., keep popping up -- stuff that just enlarges what I have already learned, or give it a new light, or are completely new to me.

-- I didn't know just how effective carbon burial can be for conserving oxygen in the oceans. On average, for every mole of carbon atoms that reach the sea floor, one mole of oxygen atoms is conserved because it doesn't have to combine with the carbon.

-- There's disagreement about when cyanobacteria evolved. Some scientists claim it must have been only shortly before the Great Oxidation Event, others that they evolved as much as 500 million years before that.

-- You may read claims that before the GOE, Earth had a methane atmosphere. If, like me, you're reading The Cambrian Explosion, you'll read that in fact we have very little idea what Earth's primeval Archean atmosphere was like. Now, Tais Wittchen Dahl in the course, lesson 6.2, claims that there is sulfur isotope evidence that Earth's atmosphere was, indeed, high in methane gas! And also high in hydrogen, while nearly devoid of oxygen.

-- Dahl keeps showing a graph of oxygen levels (compared to amount of banded iron formations) which differs from what I've learned about the "boring billion". Apparently, even during the boring billion, there was much more oxygen in the atmosphere than there had been before the GOE. There just wasn't enough. (An article from ScienceNews claims that in fact, there was far less -- less than 0.1 percent of today's percentage! That's practically no oxygen, period, similar to what the Archaean levels were. Obviously, this is a cutting-edge topic, not settled yet.)

-- 99 percent of all the Earth's free oxygen is in the atmosphere, only one percent in the oceans.

-- If the atmospheric oxygen level dipped to below half of what it is today, the oceans would be largely anoxic. (I have my doubts about that last; we see evidence of oxygenated shallow waters before the Cambrian, and I'm not convinced that we had as much as half of present-day levels back then. But I agree that the oceans were anoxic throughout the Boring Billion.)

-- Something really interesting: cyanobacteria do release some oxygen while alive. So there would be a narrow zone of oxygenated water close to cyanobacteria mats (where presumably other one-celled organisms that needed free oxygen might live.)

-- Incidentally, that same ScienceNews article claims that all photosynthesizing bacteria do, in fact, emit oxygen. It's just that some of them emit very little, and when they die, the decay of their bodies uses up as much oxygen as they emitted, resulting in a net-zero change in oxygen levels. Obviously, if the bacterium's corpse gets buried in sediment, it doesn't use up any oxygen decaying, which allows a net gain in oxygen. This is a form of carbon burial, of course.

-- The main way that the deep ocean gets oxygenated is when cold water or saltier water from above sinks, due to water from the land feeding into the ocean OR the ocean circulation patterns.

-- So, you've got a photic zone in the oceans at the top, where photosynthesizing plankton can produce oxygen. But the ocean depths are net consumers of oxygen, because it's too dark for photosynthesis and everything drops down there to rot. If there is more plankton on the top, more stuff drops down to the bottom to rot, consuming oxygen there. How much plankton-bacteria there was at any time would depend on how much of their favorite nutrients there were -- which in turn would depend upon what and how much washed off the continents into the oceans. Soooo, the amount of nutrients washing into the ocean would dictate how much oxygen there was in the ocean depths.

-- Dahl is studying redox-sensitive elements to find proof that the oceans were anoxic even after the GOE: sulfur, chromium, molybdenum. These elements can dissolve in water if oxygen is present; if it's absent, they'll precipitate out in solid form. Now, rivers on the continents could have oxygen. They would dissolve these elements out of the rock and gravel and carry them to the sea. When they hit the anoxic oceans, out they'd pop, and drop to the sea floor. Because of this, Dahl has more evidence that the oceans were still anoxic. He noticed that after the GOE, there were more of these precipitated elements in the sediments. To him, that means that more of them were getting dissolved into the rivers, because there was at least a little oxygen in the atmosphere.

-- Bizarrely, Dahl claims that the deep oceans didn't get oxygenated until the Phanerozoic -- the Silurian and Devonian periods! This is the time period during which vascular plants evolved and started to cover the land, limiting the amounts of nutrients for bacteria as well as sulfur, chromium, and molybdenum could flow into the oceans. Possibly he simply means that the deep oceans didn't get anywhere near modern levels of oxygenation until then, because he then goes on to point out something interesting . . .

-- Interestingly, those weird Ediacaran life forms lived in fairly deep water -- not necessarily abyssal depths, but deeper than you'd expect for photosynthesis, or oxygen-breathing animals of that time. Dahl guesses that they lived in places of upwelling deep waters. At this time, the deep oceans did have some oxygen, higher than 1 percent of modern levels.

-- Then he claims that vascular plants, with their deep roots, actually increase weathering on land! You can see there's contradiction here.

-- Remember that ocean oxygen levels depend not only upon carbon burial, but on nutrient levels (which affect carbon burial -- again, more plankton means more little corpses falling to the bottom, taking their carbon with them, so there is less carbon to react with oxygen and lock it up).

-- Large predatory fish that could move quickly first evolved during the Devonian Period, when oxygen levels were extremely high.

-- Birds and mammals originated during another oxygen spike. Like fast-moving predatory fish, both clades need plenty of oxygen.

-- Of the five major extinction events of the Phanerozoic, four of them are associated with a drop in oxygen levels. (Cause or effect? He appears to think the marine anoxia was the cause)

-- Here's how zooplankton increase carbon burial and make it faster. Animals, unlike plants, produce shit. Zooplankton shits out little fecal pellets. These are much heavier than the corpses of phytoplankton, so they sink to the bottom quickly, delivering their load of carbon to be sequestered from the air. We don't know when they first evolved, unfortunately, given that they leave very little in the way of fossils. But when they appeared, carbon burial sped up.

-- During the Neoproterozoic, there is evidence for small chunks of carbon that stayed floating in the water column. This had stopped by the Cambrian Period.

-- Organic matter from algae in the photic zone was always destroyed and never preserved in sediments until 590 million years ago. After 530 million years, they become common in sediments. Dahl suggests that the reason is: zooplankton began gobbling them up and crapping out the remains at that point, so that they were more likely to be preserved without further decay. Again, decay=removal of oxygen from water. So, animals' bowel habits led to more oxygen in the water, which in turn led to more habitat for animals, which in turn . . . However, this probably did not lead to atmospheric oxygenation, since the total rate of carbon could not increase -- I assume that this means there was only so much phytoplankton available for animals to consume, but I could be wrong.

-- An oddity: sponges can survive on only one percent of current oxygen levels in the water. The Boring Billion might have had this much oxygen in the surface waters, at least. Yet, there's no evidence of sponges until after the Boring Billion had ended. Why? Reason unknown.

March 2008

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