Comment by DemocracyFTW2 | Hacker News Reader

Comment by DemocracyFTW2 | Hacker News Reader

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This would be a good place to discuss (1) what 'old' water means; isn't Earth's H2O mainly circulating and, hence, 'old' in the sense that all molecules are? But of course water can react with other elements and likewise result from chemical reactions, so is it that what the mean? (2) How can you just fill a vial with some water and then go and analyze the sample in a mass spectrometer to see what other stuff is in there and then go and claim attested age for that sample without loosing sleep over the question that we can not possibly know all of which might have been going on with that water over the centuries, millennia, millions and, in this case, billions of years? Could I, if you gave me a vial of ultrapure water, fake that result in a well-equipped laboratory or at least write a peer-reviewed paper that details alternative mechanisms plausibly possibly present in geological conditions that would cause the water to attain said properties in a fraction of a billion years?

Sadly none of these questions are addressed in the article.

> isn't Earth's H2O mainly circulating and, hence, 'old' in the sense that all molecules are?

A lot of it may be, but some of it isn't.

You can buy quartz, for example that contain water or petroleum sloshing around inside [1] although some think the water could seep in and out over time.

But quartz with fluid inclusions (small bubbles containing water) are completely sealed and have the same composition as they were up to 3 billion years ago. "the fluid in the inclusion is an actual sample of fluid in which the mineral was growing" [2]

[1] https://en.m.wikipedia.org/wiki/Enhydro_agate

[2] https://www.gamineral.org/writings/enhydros-gray.html

To expand on this

Groundwater "age" is typically considered in terms of residence time, that is the period of time between entering the subsurface and exiting at some discharge location. These analyses are often based upon the hydrogeochemistry and the decay rate of known isotopes, hence the mass spectrometer.

Shallow, unconfined aquifers often contain "young" groundwater, on the order of decades or centuries. For example snow melts on a mountain, downhill into a flatter coarse grained area where it enters the groundwater system. The groundwater then moves downdip along the bedrock contour through and into the deeper alluvial basin, eventually naturally discharging into a river or is pumped out at a well.

It would be interesting to read their paper to understand how they (i.e. what isotope) can date back to 1.6 billion years.

https://books.gw-project.org/groundwater-in-our-water-cycle/...

Yeah, the article is really frustrating because it only addresses that question in a roundabout way. It says "The Precambrian Canadian Shield that stores vast mineral deposits is 2.7 billion years old, and once formed an ocean floor." - so I guess this water got trapped while the ocean floor was subducted or overlaid with sediments or whatever happened that brought it to its current depth?! So it's not the H2O molecules, but the whole liquid which has stayed there for that amount of time.

Referring back to your original question, it would be fascinating to find out what percentage of the water molecules in circulation on Earth today date back to the time when water first condensed out of the atmosphere (IIRC ~ 4 billion years ago) - but I'm not sure if that's possible.

One might also consider the question "If there is old water, does that mean there is also new water?" - additionally, how does one actually date a molecule? :-/

They are addressed reasonably well in Lollar's published work on the Kidd Creek water: https://www.tandfonline.com/doi/abs/10.1080/01490451.2019.16...

ETA: (Unfortunately, it's paywalled at a high price, which I didn't realized when I originally posted this. Apologies).

From the article:

> We might know water as H2O, forgetting everything that’s dissolved in it. For example, a small vial of the Kidd mine water transferred in 2020 to Ingenium—a Crown corporation that runs the Canada Science and Technology Museum in Ottawa—features sediment at the bottom. That’s iron oxide, which precipitated from the water after exposure to oxygen from the air.

I did read that sentence and its among the few instances where the article goes a little into the details of the evidence rather than the circumstances of their acquisition, like that journey down the mine shafts. The introductory sentence is fine and a good reminder given the likely audience. It does a good job for setting the stage. The next two sentences then thrash that hope by telling lots of irrelevant detail in the hope to tell an interesting story, and we're left with, ok, there's a certain amount of iron oxide in the water, which fell out because of contact with oxygen. This is literally what can happen to you when you bore a meter-deep well in your garden; in regions with iron in the ground, the water that comes up will have lots of iron oxide in it that will color everything in a reddish hue when drying. How does that address any one of my questions?

> 'old' in the sense that all molecules are

Trying to remember school chemistry lessons, when they explained pH, ... I think H2O is in equilibrium with H+ and OH-, perhaps with 1 in 10^7 of the molecules being disassociated at any time (hence pH 7), so a particular H2O molecule doesn't survive for very long. Minutes? Hours? Not years, I think.

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