Billion years, Plate tectonics, Geology, Earth science RS session – Deep carbon cycle by Dietmar Müller
So this is a large group effort between myself and the whole bunch of co. Authors listed down here, let’s see going to projection mode now, you’re all familiar with the great ocean conveyor belt, which is a well known concept, describing the global network of ocean currents, distributing heat and moisture around the globe, but there’s another lesser known, but much older and Slower belt, the earth’s carbon conveyor belt and that’s depicted here, it shifts massive amounts of carbon between the deep earth and the surface and plays a pivotal role in earth’s climate, the evolution of life and things like diamond formation. This belt is controlled by mid ocean ridges and deep sea trenches. It moves just a few centimeters per year and it can take tens of millions of years to complete a loop shedding and collecting carbon at various locations along the way. Carbon is released into the oceans and the atmosphere at mid ocean ridges during seafloor spreading uh, then carbon gets stored in the plate and the mantle where carbon is assimilated from the collecting mantle into the lithospheric mantle carbon is assimilated into the crust by hydrothermal circulation and Sequestered into deep sea sediments and that collects pelagic biogenic, sediments there’s, also serpentinization, going on when slow, very slow spreading occurs, also where the plate bends before entering the subduction zone and the serpentinization process accumulates additional carbon in the plate. So we’re going to have a quick look um, how this conveyor belt works and how it completes your loop and what happens to the carbon that ends up being subducted.
Of course, some of it comes back out along volcanic arcs, but some keeps on going into the departmental. So what controls this carbon um conveyor belt is mid ocean ridge length and subduction zone length shown up here. Then we have mean spreading rates and convergence rates and the product of mid ocean ridge, length and spreading rate gives us crust production rates and then from the production rates we ultimately obtain. Consumption rates along subduction zones. What’S also important is the mean age of the subducted ocean crust and the age is important, because most of the processes that accumulate carbon in oceanic plates are age dependent in different ways and so we’ve got to know the age area distribution of the plate through time. We need to know two other tectonic parameters. We we need to know something about the prevalence of sediment accretion versus erosion along subduction trenches, because this determines how much of the sediment that arrives at a trench actually keeps on going down versus sediment, that’s being accreted to the overriding plate contributing to cluster growth. And so we have a shortcut to approximate this ratio through time. Then the last element here is the shallow slab dip. The slab dip at subduction zones is important because the uh, how much it this the plate, curves its abduction zones, controls how much it falls. Um and uh, and this in turn drives the serpentinization along the outer rise of the plate. So this is a little video and that ben mather has put together um that is focused on the driving forces of the carbon conveyor belt.
So what’s shown here is the spreading rates along mid ocean ridge segments in red and convergence rates in blue, and we see um pangea, going through a cycle of breaking up mid ocean ridge lengths doubled in this process and vastly accelerated the workings of the carbon conveyor Belt now what this next video is showing is the carbon area density through time, and you see time playing on the upper left here of oceanic plates and you see not a hell of a lot of carbon was sequestered into plates early on and we see brighter And brighter colors popping up here as time goes by meaning that more and more carbon uh gets sequestered into oceanic plates. The reason for this is uh found in the evolution of biogenic pelagic carbonate producers. These old critters didn’t exist before the cretaceous period, and so before. The cretaceous period sequestration of carbon into the plate was entirely driven by processes in the crust and the mantle and the sediments might have had a bit of organic carbon, but they didn’t have any carbonates. So this ends up being important as we’ll see in a minute. So if we look at the carbon area density in the oceanic lithosphere through time from olds 250 million years ago, down here to young present, so the the these bright pink colors that show up here, um exemplify this enormous growth of the oceanic um biogenic carbon reservoir. That didn’t exist at all um in the triassic and jurassic.
So now what we can do we can. We can compute some summary statistics. We can have a look at the carbon flux into the plate um through time. So we have time from 250 million years ago to the present on the x axis and we have a carbon flux into the plate on the y axis at the top, the gray is the total and the green and reddish components are the components in the crust And the mantle and the blue is the carbonate sediments and you see how this component shoots up in the late cretaceous and the cenozoic when these carbonate produces and evolved and then developed the capacity to ultimately dominate a carbon sequestration of the oceanic carbon conveyor belt. So then we can look at the subduction flux as well. Of course, as we accumulate carbon on the plate is then takes some time for these parcels of ocean crust to arrive at the subduction zone. So, with some delay uh, we we see a very significant increase in the subduction of carbonate, sediments that’s, the blue graph um here the blue curve on the lower graph and while um crustal and mental carbon. This is the green and the red curve down. Here are much more important before and the only reason why there is a peak of subducting, crustal and metal per carbon in the mid cretaceous is because the speed of the global plate tectonic system went into overdrive into a slab superflux mode.
When we had very fast c flow spreading and very long ridge, lengths and equivalently very high subduction speeds, so we got these two peaks and so now we’re going to have a look at the carbon burial grounds. We’Ve heard about the slab barrier grounds earlier today, and so what’s colored here are, is the carbon area density um, essentially in the upper mantle, and that reflects the cumulative uh subduction of carbon into the mantle, so the brighter the colors, the more carbon has been inserted Into the mantle and the little inset graph shows the mean evolution of carbon sequestration into the mantle through time, so there’s not much happening while pangaea was assembled uh in the triassic and early jurassic, then, as pangaea started breaking up, we form all these new mid ocean. Ridges and the plate tectonic system starts going faster and ultimately, we store more carbon in the oceanic plates and we also subduct more carbon to the mantle and so much of what we see here is really driven by carbon sequestration into the mantle in the last sort Of 130 million years, i would say, because there wasn’t really a hell of a lot going on before this time. Now what we can also do. We can then look at the carbon that’s going down, so we know that’s going down into the mantle, but what happens to it afterwards right now, some of this carbon um. It will be degassed um at some sub arc depths that’s the upper 125 kilometers right and we figure out how much carbon each one of these reservoirs that we have discussed releases um by using thermodynamic modeling.
We use the the perplex software to do this we’re working with chris gonzalez and veronica gorczak at uwa for this purpose, because they are experts in thermodynamic, modeling and, and these models tell us um that the crust and the mantle um get nearly a hundred percent devolatilized Into the shallow mantle right as on their way down, they lose all the carbon relatively quickly, whereas the sediments are um. Surprisingly resistant to devolatization, once they’ve lost much of their water, carbon tends to stick to these sediments right and so what we are showing um. So so the gray graph here is the total carbon slab out flux into the subargumental, and so this stuff will then either ultimately reach. The atmosphere or metals will percolate into the overriding plate and get stuck as magmas or cause volcanism or cause metamorphic reactions right, and so one of the intriguing things um that and that the um that the carbon that’s going down into the mantle does is. It helps form diamonds, but diamonds mostly form at depth, between 200 and 300 kilometers. So we need to look at the carbon that isn’t devolatilized at very shallow depths, but that keeps on traveling into the deeper mantle right and then i gradually. It also makes its way into the surrounding mantle and it and it forms um a belt um in in the uppermost mantle where carbon is enriched as accompanied by other wallet tiles and what we are showing on the right hand.
Side here is the transport of carbon to these deeper mental depths and and the blue is again the sediments and the green and the and the reddish colors are the other components. We can see right away how the um and the the blue curve dominates what’s happening here. So the much of the carbonate sediments that go down in larger and larger quantities in the late cretaceous and the cenozoic and they don’t get devolatilized at shallow depths. They keep on traveling to deeper depths, but then, ultimately, they get dispersed um in the upper part of the upper mantle, or some of it also makes it down into the transition zone, and – and so what we observe is so we have these two peaks. So this is when a sedimentary carbon first becomes really prominent and being subducted um in the mid cretaceous. And then it goes even higher here, um in the um in the latest, cretaceous early, uh tertiary right and and turns out that these two peaks are actually related to two uh kimberlite and peaks in north america, because north america and south america are two of the Primary places where there’s a lot of sedimentary carbonate going down the subduction zone, sorry um and it turns out in north america, we find a lot of young diamonds formed in the late cretaceous and paleogene, and they are shown on this map from a paper by beam. Nadal in lithus 2003, so there are these bells of diamonds that parallel the subduction zone, western north america and and we think that these diamonds exist and because co2 rich fluids were released from the subducting slab and made their mostly from carbonate sediments.
They helped generate carbonated melts by interacting with the deeper parts of the mantle wedge peridotites, creating a shallow metal carbon reservoir and then, ultimately, this becomes the source of kimber like carbonate magnetism, especially during periods of shallow slab subduction, which is what we had in the late Cretaceous and before 50 million years ago, exactly at the time when these things were in place and those of you familiar with the work by tron, torresvick and others, um who had a paper in nature in 2010, focused on kimberlites and especially diamond differs kim kimballites are Being mostly related to amantha plumes and the edges of lsvps, you will notice that there’s um a cloud of points uh on his map here that he couldn’t explain. These are all the north american diamonds right that are clearly not related to metal, plumes or lsvps right, and we think that we have a story for these diamonds and we now understand why they are there.