A paleoclimatology tool is shown to give false positives

A popular tool to trace Earth’s oxygen history can give false positives

From the GEORGIA INSTITUTE OF TECHNOLOGY and the “settled science” department:

For researchers pursuing the primordial history of oxygen in Earth’s atmosphere, a new study might sour some “Eureka!” moments. A contemporary tool used to trace oxygen by examining ancient rock strata can produce false positives, according to the study, and the wayward results can mask as exhilarating discoveries.

It’s relatively simple to take a chromium isotope reading on a rock sample. First, crush the sample. Second, dissolve the pulverized rock in acid. Then place the solution in a mass spectrometer, which determines how much chromium 53 and how much chromium 52 is in the sample. CREDIT Georgia Tech / Christopher Moore

Common molecules called ligands can bias the results of a popular chemical tracer called the chromium (Cr) isotope system, which is used to test sedimentary rock layers for clues about atmospheric oxygen levels during the epoch when the rock formed. Researchers at the Georgia Institute of Technology have demonstrated in the lab that many ligands could have created a signal very similar to that of molecular oxygen.

“There are some geographical locations and ancient situations where measurable signals could have been generated that had nothing to do with how much oxygen was around,” said Chris Reinhard, one of the study’s lead authors. Though the new research may impact how some recent findings are assessed, that doesn’t mean the tool isn’t useful overall.

Rock record tool

“We’re not trying to revolutionize the way the tool is viewed,” said Yuanzhi Tang, who co-led the study. “This is about understanding its possible limitations to make discerning use of it in particular cases.”

Tang and Reinhard, both assistant professors of biogeochemistry in Georgia Tech’s School of Earth and Atmospheric Sciences, published their team’s results in a study on November 17, 2017, in the journal Nature Communications. Their work was funded by the NASA Astrobiology Institute, the NASA Exobiology program, and the Agouron Institute.

“On a global level, the chromium isotope system is still a great indicator of atmospheric oxygen levels through the ages,” Tang said. “The issue we exposed in the lab is more local with isolated samples, especially during eras when there wasn’t much atmospheric oxygen.”

Leaping ligands

Without a dominant oxygen presence, ligands likely made a great reactive substitute, as the researchers demonstrated in reactions with chromium. Like oxygen, ligands strongly attract electron pairs, which is what characterizes them as a chemical group.

And like reactions with oxygen, reactions with ligands enable metals like chromium to move around more easily in the world. In this case, the researchers were interested in organic ligands, ligands that contain carbon.

They were more apt to match oxygen’s mobility effect on chromium that made it end up as the signals in sedimentary rock that scientists, today, look for as a sign of ancient atmospheric oxygen.

Here’s roughly how the chromium isotope system works, followed by how organic ligands could make for false positives.

Chromium rollercoaster

The Earth is an enormous chemical laboratory performing reactions in conditions varying from arctic cold to volcanic heat, and from crushing ocean depths to no-pressure upper atmospheres. Winds and waves sweep around materials like turbulent conveyor belts, depositing some in sediments that later turn to stone.

Chromium’s ticket for the rollercoaster ride into sedimentary rock was usually an oxidizing agent that made it more soluble and better able to float, and atmospheric oxygen was an ideal oxidizer. The chemical reaction, which can be found in the study and involved manganese oxide handing off oxygens to chromium, would be a little like adding pontoons to chromium compounds.

For billions of years, Earth’s atmosphere was nearly devoid of O2, but after oxygen began increasing, especially in the last 800 million years, it became the domineering oxidizer. And characteristics of chromium deposits in ancient layers of rock became a great indicator of how much O2 was in the atmosphere.

Today, researchers test deep rock layer samples for the relation between two chromium isotopes, 52Cr, by far the most common Cr isotope, and 53Cr, to get a read on oxygen presence across geological eras.

“You powder the rock up; you dissolve it with acid, and then you measure the ratio of 53Cr to 52Cr in the material by using mass spectrometry,” Reinhard said. “It’s the ratio that matters, and it will be controlled by a range of complex processes, but generally speaking, elevated 53Cr in ocean sediment rock tends to indicate oxygen in the atmosphere.”

By the way, these Cr isotopes are stable and don’t undergo radioactive decay, thus the system does not work the way radiocarbon dating does, which relies on the decay of carbon 14.

Chemical imposter

In the lab, with a small assortment of organic ligands, Tang’s group showed that reactions of chromium with ligands led to 53Cr/52Cr signals that closely mimicked those stemming from oxygen-chromium reactions.

“Ligands have the capability to mobilize chromium as well,” Tang said. “In fact, ligands might be a significant factor in controlling chromium isotope signals in certain rock records.”

Organic ligands were probably around long before Earth’s atmosphere filled up with O2. And today, hundreds of millions of years after the reactions occurred, it’s basically impossible to find out if oxygen or ligands were at work.

Little discrepancies

If not accounted for, ligand reactions can distort small details in rock records about atmospheric oxygen, and they may have already.

Like paleontologists, who catalog ancient animal bones and other fossils, geologists keep massive, digitized archives of rock that they study to learn more about Earth’s ancient geological history. Scientists began testing physical samples of them with the Cr isotope system around 2009 and adding the results to the records.

“Since then, some discrepancies have turned up,” Reinhard said. “Ancient soil layers were showing evidence of oxygen when it probably shouldn’t have been there. Other samples from the same period weren’t showing that signal.”

But some researchers confronted with odd Cr signals have thought they had perhaps stumbled upon a radical find, and they developed explanations for how O2 may have been surprisingly bountiful on the lonesome spot where a particular rock layer formed, while molecular oxygen was scant on the rest of the globe. Others puzzled that atmospheric O2 levels may have risen much earlier than overwhelmingly broad evidence has indicated.

“A lot of that could be chalked up to other chemical processes and not to interactions with oxygen,” Reinhard said.

The study may serve as a cautionary tale about how to view Cr isotope data, especially when they leap off the page.

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43 thoughts on “A paleoclimatology tool is shown to give false positives

    • My thought exactly.

      It’s the ratio that matters, and it will be controlled by a range of complex processes, but generally speaking, elevated 53Cr in ocean sediment rock tends to indicate oxygen in the atmosphere.

      I take the above statement to mean that there are lots of ways error can creep in. Ligands could cause one such error.

      A lot of science is built on the complicated analysis of scanty evidence. For example, it seems that every time we send a probe to a planet something turns up that causes astronomers to exclaim: “This totally changes our understanding.” Nope. It shines a light on their lack of understanding and on their willingness to speculate nevertheless. link

      • “A lot of science is built on the complicated analysis of scanty evidence.”

        reminds of …

        “One gets such wholesale returns of conjecture out of such a trifling investment of fact.”
        Mark Twain

      • Another way to think about it is to consider the ‘new’ discovery (in this case ligands) as having been shown to increase the size of the error bar in the measurement attempted. eg, instead of an error of plus or minus 5% which might have been assumed, the new error is shown to be much greater – perhaps + or – 40%! The exact size of the error bar remains to be determined but there is no doubt it is bigger than previously thought.

    • Perhaps this entire process has its issues, of which this is just one.
      How about contamination of the sample specimens? Being ignorant of the correct method of crushing rock for this sort of measurement is a marble mortar/pestle the best method. Do the samples come in contact with metallic instruments (spatulas, spoons, platens, etc?) which are invariably stainless steel (which contains chromium).

    • Keep personal and political and ideological gain out of it, and “the science” is never settled. It remains a search.

  1. Makes me suspicious now about all science claims after ‘malicious’ claims by climate scientists. Climate claims are made by politically driven agenda’s but everyday science can be driven by faulty assumptions and throw in some human error/ego, and you have a recipe for poor science. If main stream climate science can tell us anything, it is to be skeptical about absolutely everything. I sure am now.

    • Your reaction is absolutely illogical and unwarranted – no need for suspicion or scepticism – this is fine science.

      • It looks like you didn’t even read the article. If faulty science can lead to faulty findings, and the duty of science is to be skeptical, then I think you may have your comment 100% backwards.

        “A lot of that could be chalked up to other chemical processes and not to interactions with oxygen,” Reinhard said.

        The study may serve as a cautionary tale about how to view Cr isotope data, especially when they leap off the page.

  2. “It’s the ratio that matters, and it will be controlled by a range of complex processes, but generally speaking, elevated 53Cr in ocean sediment rock tends to indicate oxygen in the atmosphere.”

    A range of complex processes controls the ratio which only tends to indicate oxygen in the atmosphere??? And they are talking about ocean sediment rocks “indicating” oxygen in the atmosphere. What that tells me is that maybe they can tell whether or not oxygen was present in the overlying water and not how much is present in the atmosphere in any precise measure.

    This may be just one example of “proxies” not really doing what their promoters claim they do.

  3. A tour de force! Wow, this is a terrific approach to educating the public on arcane scientific processes. In a few words one can immediately understand how Cr isotopes are used to measure Oxygen in ancient atmospheres. Thank you for this! My comments tend to be curmudgeonly on modern science. This is a gem. Also, corrupted climate scientists would never make this discovery, or if the did, they would bury it. They are only interested in supporting memes and don’t want to make conclusions even with er or bars. They entertain only what they hey can call “robust”.

  4. A proxy involves a chain of assumptions, any of which could falsify the output. For this reason, I regard the uncertainty of proxy measurements to be widely understated.

  5. When I try (or at least tried, no use beating a dead horse) to explain why I am a CAGW skeptic, I bring up stuff like this.

    People who don’t go beyond the headline (or worse, get their science from social media, or worse-than-that, late night “comedy” shows) think that scientists are simply taking today’s Planet Earth Temperature, and comparing it to a thermometer reading from one million years ago and seeing a Very Scary Increase.

    Then again, not many STEM students out screaming at the skies because Trump one. Too busy taking worthless classes like math and statistics, and not Very Important Topics like Gender Studies and How To Win The Oppression Olympics.

    • er…read “won” for “one.”

      I am a reformed grammar cop. Just getting this in before the unreformed show up…

  6. [Found and rescued your original comment. It was trashed by Word Press’s IP blacklist filter…which is itself a blackbox of sorcerous mystery! In the future, if this happens, just post a note to the mods and we can hold our noses and dive into the Trash folder to rescue the wayward comment. And yes, I do mean “hold our noses”. It’s pretty much the online equivalent of the Death Star’s trash compactor. But, we’re here to help. That’s why we get paid the big bucks! -mod]

    Many dating methods require knowledge of problems for which adjustments must be made.

    But ancient O2 levels can be estimated by a variety of proxies besides chromium, to include S.

    For most purposes, it’s not important to know whether mid-Proterozoic O2 levels were less than 0.1% of the modern atmosphere or more. Probably they were less.

    Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

    http://science.sciencemag.org/content/346/6209/635

    But they boomed in the late Proterozoic, perhaps reaching about one-fifth of today’s level (21 percent) by 600 million years ago in the Ediacaran Period.

    http://forces.si.edu/atmosphere/02_02_04.html

    This increase has been suggested as a cause of the Cambrian Explosion, but not convincingly. Like the Triassic Explosion, the Cambrian followed a mass extinction event.

    It was also thought until recently that animals evolved to take advantage of higher O2 concentrations in the Many dating methods require knowledge of problems for which adjustments must be made.

    But ancient O2 levels can be estimated by a variety of proxies besides chromium, to include S.

    For most purposes, it’s not important to know whether mid-Proterozoic O2 levels were less than 0.1% of the modern atmosphere or more. Probably they were less.

    Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

    http://science.sciencemag.org/content/346/6209/635

    But they boomed in the late Proterozoic Eon, perhaps reaching about one-fifth of today’s level (21 percent) by 600 million years ago in the Ediacaran Period.

    http://forces.si.edu/atmosphere/02_02_04.html

    This increase has been suggested as a cause of the Cambrian Explosion, but not convincingly. Like the Triassic Explosion, the Cambrian followed a mass extinction event.

    Higher O2 concentrations in the Neoproterozoic Era were until recently also thought to account for the evolution of animals, but it’s now known that sponges require so little oxygen that they could have evolved much earlier. Since they commonly form symbiotic relationships with cyanobacteria, many sponges actually produce a surplus of O2.

    • Mod,

      Thanks a million for dumpster dive into the domain of demented depravity.

      Wish I knew what has caused all my prior Cmbrn Explsn and Edcrn comments to be tossed into Outer Darkness.

      Wish I could literally thank you with a million for all the nasty work you do. Or at least a thousand.

  7. Yet again, my comment is lost in cyberspace. I’m never allowed to post anything containing the phrases Cmbn Explsn or Edcrn Period here, so will see if leaving out internal vowels helps.

    Many dating methods require knowledge of problems for which adjustments must be made.

    But ancient O2 levels can be estimated by a variety of proxies besides chromium, to include S.

    For most purposes, it’s not important to know whether mid-Proterozoic O2 levels were less than 0.1% of the modern atmosphere or more. Probably they were less.

    Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

    http://science.sciencemag.org/content/346/6209/635

    But they boomed in the late Proterozoic, perhaps reaching about one-fifth of today’s level (21 percent) by 600 million years ago in the Edcrn Period.

    http://forces.si.edu/atmosphere/02_02_04.html

    This increase has been suggested as a cause of the Cmbrnn Explsn, but not convincingly. Like the Triassic Explsn, the Cmbrn followed a mass extinction event.

    It was also thought until recently that animals evolved to take advantage of higher O2 concentrations in the Many dating methods require knowledge of problems for which adjustments must be made.

    But ancient O2 levels can be estimated by a variety of proxies besides chromium, to include S.

    For most purposes, it’s not important to know whether mid-Proterozoic O2 levels were less than 0.1% of the modern atmosphere or more. Probably they were less.

    Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

    http://science.sciencemag.org/content/346/6209/635

    But they boomed in the late Proterozoic Eon, perhaps reaching about one-fifth of today’s level (21 percent) by 600 million years ago in the Edcrn Period.

    http://forces.si.edu/atmosphere/02_02_04.html

    This increase has been suggested as a cause of the Cmbrn Explsn, but not convincingly. Like the Triassic Explsn, the Cmbrn followed a mass extinction event.

    Higher O2 concentrations in the Neoproterozoic Era were until recently also thought to account for the evolution of animals, but it’s now known that sponges require so little oxygen that they could have evolved much earlier. Since they commonly form symbiotic relationships with cyanobacteria, many sponges actually produce a surplus of O2.

    • If oxygen didn’t kickstart animal evolution, then it might have been the Snowball Earth episodes of the Sturtian and Marinoan glaciations, which might have been a single prolonged Cryogenian Period event, c. 717 to 643 Ma, possibly with some respite in its middle.

      Discovery of Possible Earliest Animal Life Pushes Back Fossil Record

      https://www.nsf.gov/news/news_summ.jsp?cntn_id=117502

      Since 2010, there have been claims of proto-sponge fossils older than 650 Ma, and definite fossils from a bit younger than that, at ~635 Ma, ie the early Edcrn Period, which followed the Cryogenian.

      • And of course, before animals could evolve, their unicellular, colonial ancestors the bacteria- and detritus-eating choanoflagellates had to evolve.

      • Glad you appreciate them.

        No, but I’m interested in it, studied it in college and write articles in the popular press on it, but never a scientific paper.

        It’s maybe remarkable, or not, that the evolution of animals is better understood than that of some metazoan phyla. There was a controversy not too long ago challenging the primacy of sponges, but that has been cleared up.

        The origin of animals: Can molecular clocks and the fossil record be reconciled?

        http://onlinelibrary.wiley.com/doi/10.1002/bies.201600120/full

        The Phylum Porifera is actually paraphyletic, since the calcareous sponges gave rise to the eumetazoa (animals with radial and bilateral symmetry, like us). Properly speaking, pylogenetically, eumetazoa and the calcareous sponges form a natural group as the sister clade of the siliceous sponges. Calcium for shells and bones has proved a success. Silicon, not so much. Apparently animals with glass skeletons shouldn’t throw stones.

  8. This is hardly world-shaking. There are several proxies for atmospheric oxygen levels (Uranium and Iron oxidative states, Sulfur isotopes etc) and Cr52/Cr53 has not been used much.

  9. weirdly I find nothing about paleoCLIMATOLOGY in this paper as the title suggests.

    Could be? but I am sceptical of the title until I see evidence that there are significant

    PALEOCLIMATOLOGY papers that rely on oxygen levels and that rely on this proxy.

    Imagine being skeptical of your skepticism…

    na..

      • CO2 is 0.04% of the atmosphere. O2 is 21%. Without CO2, no cyanobacteria or plants. Without O2, no aerobic microbes, fungi or animals. Nor BIFs or red beds. Looks as if both gases affect climate.

      • How about NASA’s take on the subject?

        Figures, captions, references and artwork removed.

        https://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/

        Oxygen is one of the most significant keys to deciphering past climates. Oxygen comes in heavy and light varieties, or isotopes, which are useful for paleoclimate research. Like all elements, oxygen is made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons. All oxygen atoms have 8 protons, but the nucleus might contain 8, 9, or 10 neutrons. “Light” oxygen-16, with 8 protons and 8 neutrons, is the most common isotope found in nature, followed by much lesser amounts of “heavy” oxygen-18, with 8 protons and 10 neutrons.

        The ratio (relative amount) of these two types of oxygen in water changes with the climate. By determining how the ratio of heavy and light oxygen in marine sediments, ice cores, or fossils is different from a universally accepted standard, scientists can learn something about climate changes that have occurred in the past. The standard scientists use for comparison is based on the ratio of oxygen isotopes in ocean water at a depth of 200-500 meters.

        What climate factors influence the ratio of oxygen isotopes in ocean water?

        Evaporation and condensation are the two processes that most influence the ratio of heavy oxygen to light oxygen in the oceans. Water molecules are made up of two hydrogen atoms and one oxygen atom. Water molecules containing light oxygen evaporate slightly more readily than water molecules containing a heavy oxygen atom. At the same time, water vapor molecules containing the heavy variety of oxygen condense more readily.

        As air cools by rising into the atmosphere or moving toward the poles, moisture begins to condense and fall as precipitation. At first, the rain contains a higher ratio of water made of heavy oxygen, since those molecules condense more easily than water vapor containing light oxygen. The remaining moisture in the air becomes depleted of heavy oxygen as the air continues to move poleward into colder regions. As the moisture reaches the upper latitudes, the falling rain or snow is made up of more and more water molecules containing light oxygen.

        Ocean waters rich in heavy oxygen: During ice ages, cooler temperatures extend toward the equator, so the water vapor containing heavy oxygen rains out of the atmosphere at even lower latitudes than it does under milder conditions. The water vapor containing light oxygen moves toward the poles, eventually condenses, and falls onto the ice sheets where it stays. The water remaining in the ocean develops increasingly higher concentration of heavy oxygen compared to the universal standard, and the ice develops a higher concentration of light oxygen. Thus, high concentrations of heavy oxygen in the ocean tell scientists that light oxygen was trapped in the ice sheets. The exact oxygen ratios can show how much ice covered the Earth.

        Ocean waters rich in light oxygen: Conversely, as temperatures rise, ice sheets melt, and freshwater runs into the ocean. Melting returns light oxygen to the water, and reduces the salinity of the oceans worldwide. Higher-than-standard global concentrations of light oxygen in ocean water indicate that global temperatures have warmed, resulting in less global ice cover and less saline waters. Because water vapor containing heavy oxygen condenses and falls as rain before water vapor containing light oxygen, higher-than-standard local concentrations of light oxygen indicate that the watersheds draining into the sea in that region experienced heavy rains, producing more diluted waters. Thus, scientists associate lower levels of heavy oxygen (again, compared to the standard) with fresher water, which on a global scale indicates warmer temperatures and melting, and on a local scale indicates heavier rainfall.

        Paleoclimatologists use oxygen ratios from water trapped in glaciers as well as the oxygen absorbed in the shells of marine plants and animals to measure past temperatures and rainfall. In polar ice cores, the measurement is relatively simple: less heavy oxygen in the frozen water means that temperatures were cooler. Oxygen isotopes in ice cores taken from mountain tops closer to the equator are more difficult to measure since heavy oxygen tends to fall near the equator regardless of temperature. In shells, the measurement is far more complicated because the biological and chemical processes that form the shells skew the oxygen ratio in different ways depending on temperature.

        Fossilized oxygen isotopes

        The shells of tiny plants and animals and corals are typically made of calcium carbonate (CaCO3), which is the same as limestone, or chalk, or silicon dioxide (SiO2), similar to the compound common in quartz sand. As the shells form, they tend to incorporate more heavy oxygen than light oxygen, regardless of the oxygen ratio in the water. The biological and chemical processes that cause the shells to incorporate greater proportions of heavy oxygen become even more pronounced as the temperature drops, so that shells formed in cold waters have an even larger proportion of heavy oxygen than shells formed in warmer waters, where the difference is less notable. This temperature-based skew effect means that the oxygen isotope make-up of shells would not precisely match the make-up of the ocean water in which they grew. Scientists must correct for this skew if they are to learn about the ratio of oxygen isotopes in the ocean waters where the shells formed.

        Correcting for the Skew: Scientists can correct for the temperature factor by looking at other chemicals in the shells. In coral, for example, the balance between strontium and calcium is determined by temperature. By measuring the amounts of these chemicals in the coral, scientists can determine the ocean’s temperature, then calculate how much more heavy oxygen the shells were likely to incorporate at that temperature. Discrepancies in the oxygen isotope ratio after the temperature correction reveal changes in the ocean’s local salinity, which is related to evaporation, rainfall and runoff, and global salinity—a measure of the total amount of ice in the world.

        The oxygen isotope ratio has the potential to tell scientists about past climate anywhere that the ratio is preserved in water chemistry or elsewhere. Scientists are moving forward to apply this powerful tool to more and more branches of paleoclimatology.

      • I suspect that Mosher is pointing to the test not being used in paleoclimatology studies. He did not suggest that they weren’t useful. He only suggested that this test wasn’t used in such studies.

        The proper response is finding studies that did use this method and posting them.

        Mosher: The range of skepticism is always challenging to explain. No matter what I have to take a step out on a ledge and point. If I attempt to fully explain the entire chain, I will never be listened to. If I don’t explain the entire chain, there will be gaps to lob grenades into.

  10. Use gloves and demineralized water in an agate mortar consisting of about 30 minerals and silica phases to grind up your specimens. Use of agate mortars is use of a filthy substance that is anything but benign and amorphous.

  11. “Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere.”

    https://en.wikipedia.org/wiki/Great_Oxygenation_Event

    This could explain local Cr signals indicating oxygenation, well before the early sinks of O2 (iron, organics) were saturated and free oxygen started accumulating in the atmosphere 2.3 billion years ago just before the Huronian glaciation.

    • Which is one reason why animals could have evolved much earlier, but apparently didn’t. Eukaryotes might already have evolved from endosymbiosis of bacteria in archaea as soon as 2.3 Ga (or even earlier), and possibly in modern form by 1.7 Ga.

      The key genetic innovations permitting colonial choanoflagellates to evolve into protosponges arose only once. The most important such development was signalling proteins which direct cells to differentiate. Modern sponges have several kinds of cells, in two layers with a jelly-like substance, mesohyl, between them. Unspecialized cells can transform into other types and often migrate between the main cell layers and the mesohyl in the process.

      Choanocytes (“collar cells”), flagellated cells which function as the sponge’s digestive system, are remarkably similar to the protistan choanoflagellates, the closest unicellular relative of animals. Choanoflagellates are capable of making the connective protein collagen, found in sponge mesohyl.

      When proto-sponges evolved remains controversial, but definitely before 635 Ma, probably before 750 Ma and possibly as early as 1.2 Ga, but the latter date is considered dubious, as based only on molecular clocks and not fossils.

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