OK, let’s make sure that we’re clear on the question first. By ‘heaviest’, I mean the element with the largest atomic number. By ‘naturally occurring’, I mean pretend that humans have never existed on this planet (probably would have turned out much nicer, eh?). Right, you can’t argue about the question anymore, and if you still want to, go and do it somewhere else – all I want now are arguments about the answer.

A lot of people (well, science-y types I guess) when posed with this question will say ‘uranium’.

But hang on, naturally occurring uranium can apparently undergo neutron capture reactions followed by beta decay to produce transuranium elements… but I’m struggling to find (primary) literature sources confirming just how far this process goes. Also note that plutonium is thought to be a primordial element (there might be some on Earth that has been around longer than the Earth itself) – the Nature paper is here and a potential rebuttal is here (thanks to Brett Thornton for pointing this out on Twitter).

After digging around Wikipedia for a while (and lots of the sources it links to), I’m fairly convinced that it is safe to say that neptunium and plutonium are found in nature – in naturally occurring uranium deposits and natural nuclear reactors such as the one found at Oklo in Gabon.

What about the next few elements though? Well, here’s what Wikipedia has to say:

Am: A few atoms of americium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits. [link]

Cm: A few atoms of curium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits. [link]

Bk: A few atoms of berkelium can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits, thus making it the rarest naturally occurring element. [link]

Cf: Very minute amounts of californium have been found to exist on Earth due to neutron capture reactions and beta decay in very highly-concentrated uranium-bearing deposits. [link]

The Wikipedia page on californium also states that: It is the heaviest element to occur naturally on Earth; heavier elements can only be produced by synthesis.

Notice that the wording for all of these elements is very similar. And they all cite the same (and only) source – John Emsley’s book Nature’s Building Blocks: An A-Z Guide to the Elements. I don’t have a copy of Emsley’s book to hand and so I’m still in the dark about primary literature sources to back up these claims (I’m not doubting the book, I’d just like to find the original sources).

When it comes to the next two elements, einsteinium and fermium, Wikipedia notes that: Einsteinium and fermium did occur naturally in the natural nuclear fission reactor at Oklo, but no longer do so.

The source of that claim? Emsley’s book. I really need to get my hands on a copy.

In fact, the Wikipedia articles kinda contradict themselves (I think). They say that einsteinium and fermium did occur in the Oklo reactor at some point, but each article also states that: Synthesis of einsteinium/fermium from naturally occurring actinides uranium and thorium in the Earth crust (sic) requires multiple neutron capture, which is an extremely unlikely event.

So, how far does neutron capture get you in nature? As far as fermium or not past plutonium?

I’ve kinda had this debate before (see here and here), but it was focused more on the number of naturally occurring elements, not the heaviest. Looking back at those posts does remind me to point out this link that states: All six of these elements (93-98) have been found in very small amounts in samples of uranium-rich pitchblende. Alas, there is no primary source to back up that claim either.

So, what is the heaviest element that has shown up on this notional human-free Earth that I’ve dreamed up? I think I’m sticking with plutonium for now, until someone points me in the direction of a literature source that says otherwise.

UPDATE: this pdf prepared by Argonne National Lab a few years back seems to suggest that anything above uranium does not occur naturally… but includes confusing phrases such as:

Although neptunium is essentially not naturally present in the environment, very minute amounts may be associated with uranium ores (huh, so does it occur naturally or not?!)

and

Essentially all the plutonium on earth has been created within the past six decades by human activities involving fissionable materials (but it does mention the Oklo reactor…)