> Yep. If fossiliferous rocks are subducted, the fossils will be buried in the mantle until eventually, those fossils are somehow transformed beyond being recognizable as fossils (i.e. they're mixed enough, melted or not).

Your use of ‘eventually’ is kinda misleading here. Any fossiliferous rocks would be at the top of a subducting slab and so if they didn’t get scraped off onto the overriding plate they would already be sheared upon entering the mantle; not to mention right at the slab-mantle interface where it won’t be long at all before the heat finishes them off.

> So if we are still in an ice age, then the gradual increase in global temperatures is all part of that

For the last 10,000ish years the Earth has been in an interglacial, ie. the least cold stage of an ice-age. Glacial-interglacial cycles have been occurring for the last 2 million years or so, which is the Quaternary Ice Age.

The increasing global mean temperature in very recent times is on top of being in the warmest part of an ice age, it cannot be attributed to part of the natural cycle. The natural cycle would be due to start cooling sometime in the next few thousand years and transition back to a glacial episode. Anthropogenic warming has eliminated that possibility and the fear is that the Earth could exit Ice Age mode entirely and switch into hot-house mode with virtually no ice at the poles at all, correspondingly higher global sea levels and a lot more energy in the climate system for extreme weather events to become a regularity.

> So where do scientists draw the line between the global warming caused by humans, and the global warming that’s part of the natural cycle of things?

Somewhere around the year 1900. Take your pick, determining an exact date is a bit of a moot point by now.

> Goldschmidt classification is nowhere near absolute, which is why we can have gold in the crust.

I thought that’s more to do with the following factors:

(1) chemical bonding subtleties mean that partition coefficients are never perfectly into exclusively one phase or the other

(2) core formation is far from a perfectly efficient process with regards to taking certain elements from other layers — even if partition coefficients were perfectly weighted to siderophile, reactions don’t run to completion without being diluted or interrupted, not least because the Earth was not completely molten for very long (if at all).

(3) the crust today (particularly the continental crust) has been modified extensively since whatever was left behind immediately after core formation. Transport and concentration of certain elements from the mantle to the crust and into more localised bits of the crust to form ore deposits has had ~4 billion years of geological processing to occur.

(4) most of the gold that exists in the crust today is thought to have been delivered to the Earth from space after core formation — the late veneer hypothesis eg. Dauphas & Marty, 2002

Is the Goldschmidt classification really so lacking? I know it was developed a long time ago but i thought it was only ever meant to be a broad classification scheme? Seems to fo a good job of that and it does allow for elements to have mixed classifications.

> 1a. A quick look at that last glacial maximum shows time frames on 10’s of thousands of years. So the ice would be accumulating for around ~10-15 thousand years then declining to where we see it today possibly.

A fair bit too quick for the ice accumulation phases. 10,000-15,000 years sounds more like the length of an interglacial, ie. what we’re in right now. It has always taken several times the length of an interglacial to transition to the next glacial maximum (though transitions back into interglacial are typically more abrupt). Maybe you are reading the graphs the wrong way around? 15,000 years is about the length of the phases in the last million years when going from glacial to interglacial.

Even before the Mid-Pleistocene Transition — before the glacial-interglacial cycle started turning from a 40,000 year cycle into the current 100,000 year cycle — it was taking at least 20,000 years to get from minimum to maximum ice volume.

For the last million years or so we are within the territory of the 100,000 year cycle, with much more drawn out changes (at least going from interglacials to glacials). For instance the previous interglacial ended about 115,000 years ago and the last glacial maximum began about 30,000 years ago.

Atlas of the Underworld would be interest to you — a research project which integrates seismic tomography datasets to produce an atlas of the mantle all the way down to the edge of the core, and thus all the known subducted slabs.

> The D” (D double prime) layer, the lowermost zone of the mantle, was describe to me in grad school as the “subducting slab graveyard”.

It’s still fairly unexplained what exactly the D” prime layer is — whether it’s made from a build up of old semi-molten slabs or if it’s even compositionally different from the rest of the lower mantle at all is not yet settled. Even the idea of what the whole lower mantle in general is composed has evolved a lot since the discovery of the D” layer; in part due to new types of high pressure minerals being proposed as important parts of the mineralogy but also due to the ever increasing heterogeneity of the mantle as it gets probed at slightly higher resolutions with time.

>This layer was also hypothesized to insulate the core enough to cause heat anomalies large enough to create break thru hotspots in some places that give rise to features like the Hawaiian or Yellowstone hotspots.

I’m not sure that a slab-graveyard interpretation of the D” layer would provide thermal insulation at all — subducted slabs are colder than surrounding mantle material, even by the time they reach that depth; this would have the effect of increasing the thermal gradient (and thus heat loss) at the core mantle boundary rather than insulating; though in a roundabout way this can cause Rayleigh-Taylor instabilities (ie. thermally buoyant regions) elsewhere at the core-mantle-boundary. Seismic tomography makes a convincing case that the Hawaiian hot-spot has origins at the core-mantle-boundary, possibly from such a mechanism (or maybe because the physics of the fluid outer core just happen to create hotter and ‘colder’ regions of the CMB). The origins of the Yellowstone Hotspot are even more enigmatic, seismic methods employed by Yuan & Dueker, 2005 traces what is likely the Yellowstone plume down to only 500 km depth (over 2000 km higher than the CMB). Either a lower mantle counterpart to this plume existed in the past but doesn’t today, or the origin was/is at some point in the upper mantle.

It looks increasingly like the two huge continent scale structures known as LLSVPs which rise up from either side of the CMB and extend hundreds of kilometres through the mantle could be providing the sort of insulating process that you describe — whereby rising plumes get temporarily stuck underneath them and build up heat and/or material before leaking around the LLSVP edges to continue towards the surface. The whole thrust of the research from Torsvik et al, 2006 was establishing how surface expressions (in particular large igneous provinces) of plumes can be traced back to the margins of LLSVPs. The Yellowstone plume does not fit in with this model, but then that would make sense with it not having a deep mantle origin as origins at or near the CMB would be the ones to get ‘stuck’ underneath LLSVPs.