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certain_people t1_ja5zi0n wrote

The dating doesn't give you the gas levels or temperatures.

Instead separate samples are taken through an ice core, or a sediment core, or a rock sequence. Separate analyses are done on the samples to measure the greenhouse gas levels, temperatures, and ages.

Sediment samples might have carbon isotopes measured as a proxy for CO2, ice cores might have gas bubbles analysed, there's a whole suite of different measurements possible. The dating is totally separate and just puts some age pins on those measurements.

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clamshellconundrum t1_ja6apou wrote

And those samples are compared to samples of similar age throughout the world to get a clear picture of global atmosphere levels during any one time.

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SupernovaTheGrey t1_ja8hdwv wrote

How do they account for dissolved C02 and CO2 offgassing into the bubbles over time?

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certain_people t1_ja8jhly wrote

Basically there are tests they can do to determine if that has happened, and where it has that data is dumped.

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Mdork_universe t1_ja683sh wrote

Greenhouse gas dating is done with ice cores—mostly from Antarctica glaciers. Those date back to at least 2 million years ago. Bubbles of atmosphere get trapped in the ice, which allows us to study ancient atmospheric composition during the growth and decline of various ice ages in the past. Radioactive dating is what we can use on fossils or certain types of rock. The other, easier form of dating is by the use of index fossils. Also, stratigraphy rears its ugly head to help date strata, and drive geology students mad.

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TurqoiseDays t1_ja7e4r8 wrote

There are other ways of estimating* atmospheric co2 for dates prior to we can access using ice cores. Off the top of my head some include alkenones, boron isotope ratios in microfossils, stomatal counts in fossil leaves. They do have significantly greater uncertainty than direct ice core measurements of course.

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KronoMakina t1_ja6gz2g wrote

Wouldn't 2 million years worth of ice be a lot thicker? How do we know the ice didn't melt and renew in 2 million years through warm or cold phases?

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ShadowDV t1_ja6jrpx wrote

The ice sheet is up to 3 miles deep. Antarctica is also a desert getting less than 2 inches of precipitation a year. Also, the glacial ice flows out. Imagine pouring something very viscous, like honey on top of a bowling ball. That kind of like how continental glaciers flow.

So as the ice flows out, the annual sheet accumulation thins out, to where 1 year’s worth of accumulation can be like 1mm thick. And those layers can be read like tree rings for the age in an ice core.

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llanthas t1_ja6rs84 wrote

Would it be safe to assume that precipitation levels observed in the past 50-ish years have been consistent for millions?

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ShadowDV t1_ja6sae8 wrote

In the case of Antarctica, at the South Pole, yes. Landmasses at poles of a planet are generally going to be dryer as the weather patterns are not as affected by the Coriolis effect nearly as they are at higher latitudes.

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Undercover_in_SF t1_ja8g2z9 wrote

To add to the other response you received, precipitation doesn’t have to be the same. The annual warming/cooling of the seasons leaves a mark like a tree ring (I’m simplifying), so they can differentiate precipitation years regardless of how much from each year.

None of it is exact, so they triangulate lots of different measurements to increase confidence.

For example, if you know a big volcano erupted 1,000 years ago, you’d look for ash at the depth the layers tell you is 1,000 years and see how accurate you were.

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Retepss t1_jaa0hct wrote

As long as there was some precipitation during the year. IIRC, the one assumption we have to make is that there was SOME precipitation for most years on Antarctica. Which doesn't seem unreasonable.

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Goldenslicer t1_ja7hyz9 wrote

How do we know any particular depth corresponds to a particular age?

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BlueFlannelJacket t1_ja7k8l3 wrote

That's where the radioactive dating comes in. It uses the half life of known elements to measure how long ago that ice formed, and those 2 measurements together can be used to figure out the environment at a certain time.

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Retepss t1_jaa6xnx wrote

There are lots of clues. One of the simpler ones is counting seasons. I've only seen ice cores from the Arctic, which don't go as far back, but just looking at them you can tell summer ice from winter ice. Counting the layers give you years. There are also more accurate ways to measure the difference (you can look into what delta 18Oxygen means).

You will lose count if there was a period where summers got warm enough to melt more ice than was formed during the winter, but you can then use the other methods to try and correct for that.

Even so, being of by a 100 years isn't too bad when you are counting 100000.

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Pandarmy t1_ja6lpux wrote

The other reply has some great info but I can add a bit. Another way we know it hasn't melted is radioactive dating. I'll use carbon dating as the example since I'm most familiar with it. Carbon-14 is radioactive (half life 5700 years) and naturally present at a rate of 1 ppt. If a substance has gas exchange with the atmosphere, it will keep that 1ppt amount of carbon-14. If not, that number will fall as the carbon decays. Since the ice sheet has a much lower percentage of C-14 (or other radioactive element they are testing) it means the ice must have been there for a long time.

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Undercover_in_SF t1_ja8ggxq wrote

And accuracy decreases the farther back you go. Once you get around 50,000 years you’re at 10 half lifes, so any residual C14 is so small it’s not accurate.

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Pandarmy t1_ja9bocd wrote

Right, which is why they probably use potassium or krypton instead of carbon. I'm just not as familiar with their isotopes or halflives which is why I used carbon in my example.

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Thundahcaxzd t1_ja6yajf wrote

The lighter oxygen isotope O16 evaporates more readily than the heavier O18. Thus, when temperatures are higher and there is more evaporation, O18 gets concentrated in liquid water and O16 gets concentrated in the atmosphere. the water in the atmosphere falls as precipitation and some of it falls on ice caps and stays there. In the ocean, organisms such as foraminifera use water to help build their shells. These shells settle into the sediment after they die. Scientists drill cores into the ice caps and ocean sediments. Using a mass spectrometer, they measure the ratio of oxygen isotopes in the ice and foraminifera shells which gives us a proxy of how much evaporation was taking place, and therefore the temperature.

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Skarr87 t1_ja7wzw7 wrote

This is the correct answer. I would also like to add that O18 will precipitate out of the atmosphere faster as the temperature cools so as the temperature gradient decreases the ratio favors O16 more and more. So taking samples from all over we can get gradients for the temperature around the world at a particular timeframe.

Corals and animals with shells in the ocean make it out of calcium carbonate or silicon dioxide. The concentration of O18 to O16 in shells is dependent on the temperature of the water do to biological and chemical processes. So this is another check to corroborate ice core values.

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Undercover_in_SF t1_ja8gno5 wrote

Since O16/O18 ratio is stable, does this let you go back farther than C14 carbon dating?

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Skarr87 t1_ja8kwp4 wrote

Oxygen isn’t used for dating the samples. The oxygen ratio is for temperature. The stability of this isotopes means that we can be confident they haven’t changed since being deposited. To date the cores we use layers, chemistry, and radiometric dating.

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Busterwasmycat t1_ja80g88 wrote

The dating is not used to define the temperature or other chemistry. It is used to put the sample into a time context. Most dating using radioactive decay as a "stopwatch" has a window of time where it works, and only works if the radioactive element and its daughters get captured at the time the mineral, rock, or organic residual got made, and any changes after formation to concentration of those atom types in that sample is only because of decay).

The date we would get from measurement and calculation refers to the date that the measured component got created. Dating the age of a mineral derived from erosion of an igneous rock will yield the age of the igneous rock, not the age of the sediment, so sediments tend to be more difficult to date by radioactive means. Some minerals do form in the sediments at (about) the time of sediment deposition, and dating those minerals, when possible, will give a decent age for the sedimentation. Generally speaking, radioactive dating is used to put date limits on sequences (using some cross-cutting relationships with datable units like dikes or sills; younger than that 5 million year old rock but older than this 3.2 million year old rock). Ash layers from volcano eruptions are actually really good for that, because the ash layer is a distinct layer and a fixed time and covers a very large area, and the age of the ash is the age of the sediment it is in, so it can be a very good marker unit for a lot of different places in the area.

Some fossils are actually pretty good for dating, but short time windows tend to rely on microscopic fossils like foramnifera that are a lot of work to find and ID). Carbon dating (which only works on fairly young materials, like 60,000 years old or less, about) isn't actually dating the sediment either, but the age of the piece of wood or plant leaf or whatever is going to be the same (to the precision we can measure) as the sediments it got buried by and within.

Mostly, though, we either use other non-radioactive methods to date the sediment or material. Ice ores are dated sort of the same way as trees using tree rings. The rings (layers) get counted. It isn't always counting right from surface, sometimes the section or the sample is compared to other sequences already counted and matched to a window of time without counting all the way back until today in that one sampling program.

The point here is that dating is not the same analysis as the measurements used to define chemical conditions of atmosphere and oceans. There are lots of ways to measure temperature and chemistry, some of which are very direct (when you can get an actual, real sample of preserved atmosphere in some ice and analyze it) and some are indirect (using stable isotope equilibrium among different species to define the temperature of equilibration (=formation) assume the minerals formed from sea water did so in equilibrium with that sea water and thus with each other). Similar things respecting ocean chemistry is revealed by major and trace element contents of minerals formed from that sea water. So we can get a pretty decent idea, within certain windows or ranges (small error limits), about what ocean chemistry and temperature was at the time of the deposition of sediments in that ocean.

It isn't just one thing and "POOF" the conditions are known for a given time. It is a lot of work involving measurement from a lot of samples from a lot of different places and times, and fitting all the data into how things have changed (or stayed the same) through time.

We keep doing sampling and analysis of different sediments, or ice, or whatever we can get our hands on, from different places and different times, and adding that data to what we already know, and this allows us to know more precisely, better, what actually happened. The more information we have, the more certain we can be in our understanding of things. It takes time and lots of work by lots of different people though.

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[deleted] t1_ja75n01 wrote

[deleted]

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_PM_ME_PANGOLINS_ t1_ja79622 wrote

This is not true. Radiocarbon dating works on anything that exchanges carbon with the atmosphere, which includes air bubbles trapped in ice cores. You can approximate the year when the exchange stopped.

Other kinds of radioactive dating work on different materials, often rock.

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