Kath Grey: Microfossil clues to Acraman Impact-induced climate change

Kath Grey talks about her meticulous PhD work on microfossils from drill core from central Australia and the recognition of how they relate to a giant meteorite impact almost 600 million years ago.

Transcript

00:01 Kath

The microfossils were spectacular, very complex. I think I named about 60 species that hadn’t been named before.

00:09 Julie

Welcome to The Rocks Beneath Our Feet. In this series, five geologists talk about their years devoted to working for the Geological Survey of Western Australia. From understanding early life, to the tectonic processes that shaped our planet, and making the maps that unearth our understanding of Western Australia’s geology, they reveal their shared passion for discovering the stories in the rocks beneath our feet.

I’m Julie Hollis.

In this episode, Kath Grey talks about her meticulous PhD work on microfossils from drill core from central Australia and the recognition of how they relate to a giant meteorite impact almost 600 million years ago.

00:48 Kath

When I went to do my PhD in Sydney, which was in 1991, I was lucky because I was co-working with Clive Calver. He was at the Tasmanian Geological Survey. He was interested in doing the carbon isotope record. So he did the stable isotope record and I actually did palynology, back to my old field of palynology, looking at the microfossils, with bit of stromatolite stuff chucked in. But mainly it was because we had a really good drill core that had been drilled in the Amadeus Basin and in South Australia. And we were able to go and work in the core library. We got plenty of support from the other geological surveys to do this. We were able to go and work in the core library in Alice Springs. And six weeks in the core library in Alice Springs is not the nicest thing, with the Ross Ranges fading away into the distance around you.

01:48 Julie

The spectacular scenery around Alice Springs is enough to make any field geologist wish they were out mapping.

01:54 Kath

You know drill core is absolutely vital material because you know exactly how things fit in relation to each other. And we spent another long period in the core library in Adelaide sampling. And it was all done very methodically. And we selected which samples we thought were going to produce the best results. We noted any stromatolites that were in the core. There weren’t that many. But mostly it's very fine siltstone to claystone. That preserves, very well, the plankton that’s floating around at the time. That's probably algae, single-celled algae that form the phytoplankton.

And like any palynology, they’re superb for doing biostratigraphy because you get hundreds of specimens out of a single sample. You only need about 10 centimeters of core at the most. And you dissolve it in acid, using a combination of very nasty acids, like hydrofluoric. You end up with just organic material, which you mount on a slide. And then you do the tedious work logging it down the microscope and identifying everything you can.

Clive and I cut the samples in half. And he took half to do his chemostratigraphy and I took half to do the palynology. So we knew exactly what the results he was getting and how they fitted to ours.

03:24 Julie

Could you describe what chemostratigraphy, lithostratigraphy, and biostratigraphy are?

03:31 Kath

Lithostratigraphy is the basis for studying sedimentary rocks. It's looking at what type of rock it is. Is it a sandstone, a limestone, dolomite? And of course, it's all based on the theory of superposition. So a layer of rock is succeeded by another layer of rock and is succeeded by another layer of rock. So they’re all in age order. And from that, you put together rocks that are related and you try to match the rocks up on the basis of the rock type, but also of the rock relationship across the basin.

Chemostratigraphy is looking usually at the amount of carbon isotopes. You also use strontium isotopes. And what you do is measure the ratio of two isotopes and you go up the core spotting them. So you end up with a curve You can very definitely see these transgressions in the isotope curves. And you can draw lines through them represent periods when the atmosphere in particular was all the same round the world.

04:43 Julie

Another way of correlating rocks from different locations that were formed at the same time.

04:48 Kath

And then biostratigraphy’s using living organisms or fossils of living organisms to put together a time scale. Individual species only live for a short period of time. Some live a bit longer. But most of them only have a very definite time period in which they were in existence and then they become extinct. So if I said in the simplest terms that I'd found the dinosaur, anybody who knew anything about geology would know that I was talking about something in the Jurassic to Cretaceous Period. And if I told you the name of the dinosaur you'd be able to pin it down tighter. And you don't just use one species, you use a combination of species. And you talk about zones, which are a zone where you've perhaps got six species that all occur together. And that gives you the time period in which they occur. And obviously if you can fit that into a lithostratigraphy and the chemostratigraphy, then it’s a very good evidence that, you know, you're on the right track, you've got everything tied together. 

06:00 Julie

So back to Clive and Kath’s chemostratigraphy, biostratigraphy, and lithostratigraphy on the drill core.

06:07 Kath

It was all, you, know meticulously documented, the whole lot. I got a really superb succession. The microfossils were spectacular, very complex with lots of spines and different types of processes sticking off them. I think I named about 60 species that hadn’t been named before.

06:30 Julie

Wow.

06:31 Kath

That matched beautifully. It matched the material in the Adelaide Rift complex, matched the material in the South Australian Officer Basin, and it matched the Amadeus Basin. And I could tie it all together. In addition to that, for some of the material, there were well logs and detailed lithostratigraphy. So it was a case of putting the whole picture together in this big sort of mosaic and seeing what happened. And we got absolutely consistent results. So the lithostratigraphy, which formed the basis for the original subdivisions, any well log data, any geophysics or anything like that, Clive's chemostratigraphy and my stromatolite and palynology biostratigraphy matched up beautifully for the whole basin. So we now have a complete record that we can use. Particularly having drill core, that was the real key to the whole thing. Because you know exactly where you are in, within a drill hole.

07:4233 Julie

Yep.

07:34 Kath

You know, the depths, you know all the details. I felt very proud of the fact that there's very solid evidence for the way that came out.

07:41 Julie

Yep.

07:42 Kath

That also threw up a very interesting thing in that there's a major change in the lower part of the drill holes. The fossils are  mainly just simple rounded things. And there's hundreds of them. It's not just based on one or two specimens. And then there's a sudden change and you start getting all these all these spiny things and there's a very distinct development of the spiny forms, a rapid change and a huge increase in the number of species.

I was quite lucky because the survey had just got a big plotter and it was the first time I managed to plot my drill hole data sort of in one picture. And I'd piled it all into the plotter and it started coming out. And I can remember phoning Malcolm

08:31 Julie

That’s Malcolm Walter, Kath’s PhD superviser.

08:35 Kath

saying, “You're not going to believe this but the change from the simple leiospheres to the spiny ones is coinciding with the Acraman Impact Event.” And he said, “Oh, come on, you know, I don't believe it.” 

The Acraman Impact Event refers to a very large asteroid impact which occurred in the Gawler Craton in South Australia. It was about 580 million years ago, but we don't have any very tight dating on it. We can only say that because we know the age of younger and older rocks. The bolide that hit, we're not sure what it was. But it's probably an asteroid. It was about four point eight kilometers in diameter. So it’d be as big as Perth’s CBD. And the initial crater that it formed was about 45 kilometers in diameter. And then the whole crater collapses and you end up with what's called the collapse crater. That was about 90 kilometers. So if you sort of hit Perth, the whole of greater Perth would have been within the crater.

And it also threw a huge blanket of material out that traveled several hundred kilometers. Again if we're thinking of WA as the analogy, it would have reached Geraldton and Albany with this blanket of material. And that's known as the Acraman impact ejecta horizon.

10:03 Julie

Yep.

10:04 Kath

The, the crater is about half the size of Chicxulub, which is the one in Mexico that's associated with the extinction of the dinosaurs. So it was a pretty big one. The rocks that it hit were the Gawler Volcanics, and in particular a unit called the Yardida Dacite, which is quite distinctive, lithologically. It's got a sort of pinkish tinge to it. The ejecta layer landed in seas on either side of the Gawler Craton, which was probably land at the time. And the seas were depositing a greenish coloured siltstone. So the ejecta layer’s quite obvious within this siltstone.

10:45 Julie

As a pink layer.

10:47 Kath

And the history of how it was recognized is quite interesting. Vic Gostin, of Adelaide University and his PhD student Peter Haines,

10:57 Julie

Who is now a senior geologist at GSWA, where he has been working for almost 20 years.

11:02 Kath

who was mapping in the Flinders Ranges, found this layer of volcanic rocks, some of them quite big, up to about 50 cm across in the middle of this siltstone succession. And initially they thought it might be a volcanic horizon and it would be worthwhile getting a date on it so they could date the whole succession more precisely. So they sent it off for dating and were very surprised when the dates came back because, instead of being around 800 million years, which was what we were expecting, it was 1550 million years old. And the only rocks that they knew about anywhere around that were that age were in the Gawler Craton. And that was about 200 kilometers away. So it was nearly twice the age that it should have been.

11:54 Julie

Yep.

11:55 Kath

And then they discovered that you could recognize the same horizon as a very thin layer but in a couple of drill holes in the Adelaide Rift Complex. And later it turned up in one in the Officer Basin, which was 500 kilometers to the west of the Gawler Craton. And whilst Vic and Pete were scratching their heads trying to decide what all this meant, another geologist from Adelaide University, George Williams, was also sitting there scratching his head because he’d just recently received some geophysical maps. And geophysics was fairly new at that time. And there was a huge anomaly centered around Lake Acraman. And he couldn't work out what on Earth this strange circular anomaly was. And eventually the three of them put the story together and realized that George had found the crater that had supplied the rocks for the Acraman Impact Event.

And initially, from my point of view, this line was a very useful thing because it provided a synchronous timeline through the drill core that I was looking at. And I was using that to actually hang my palynology results off the line. 

13:12 Julie

By this, Kath means a single line representing the Acraman Impact Event, that she could draw through all of the drill core.

13:20 Kath

If you fit this into this mosaic I was talking about, you can narrow it down more, even though you haven't got absolute dates on your drill core. You go in the core library, the amount of core’s overwhelming. But we were able to predict which core trays it should be in.  And then I realized that the changes I was seeing in the microfossils was associated with this particular line. 

13:48 Julie

Yep.

13:49 Kath

I put the story together about how the impact might have affected the organisms that were living there. The original organisms,

14:00 Julie

The smooth-walled leiospheres 

14:02 Kath

I don't think we're able to reproduce very successfully 

14:05 Julie

When compared with the more-complex spiny ones that had proliferated after the impact.

14:10 Kath 

But I've since found one species occurring below the impact layer. And that is what you would expect.

14:20 Julie

Kath explains giving the dinosaur extinction and rise of the mammals as an example.

14:24 Kath

The mammals were around, but in small numbers and not easily spotted, during the age of the dinosaurs. And it was only after the dinosaurs had been wiped out that the mammals took over. I think we're looking at something similar.

The idea was then to test it with other drill holes. And a PhD student in Sweden, Sebastian Willman, ran tests through several other drill holes and got very similar results to mine, with the same change and a rapid expansion of the different species at that period of time.

14:58 Julie

Right.

14:59 Kath

And it looks as if the spiny acritarchs are resting cysts, 

15:03 Julie

Could you say what an acritarch is?

15:06 Kath

Acritarch is a catch-all phrase. It actually means we don't know what this is. We're not sure of the affinities of these creatures. They appear to be plants that photosynthesize, most of them. There’s probably a few other things mixed in there that could be protist cysts and various other types. They’re algae, so they’re one step above the cyanobacteria, which are the main organisms that occurred earlier in the Precambrian.

Cyanobacteria and other bacteria don't have a membrane around their nucleus, whereas algae have a membrane around their nucleus. That's the main difference. And in particular, the algae can use other methods of reproduction. So a resting cyst is just one stage in their life cycle. Normally, they’d just split in half and produce two halves, which is the same way the cyanobacteria do it. But then they got these additional phase which allowed them to protect themselves.

In some cases, they appear to produce a lot of smaller specimens inside the parent cyst. The offspring then, there's probably 20 or 30, are released when the cyst springs open. So they've got slight differences in their reproduction.

And of course that, they then gave rise to things like seaweeds and eventually to plants. And a lot of the plants we know today have their origins with the algae. Whereas the bacteria just continues on its own way and proliferates as bacteria.

These cellular organisms produce a very hard surrounding, if conditions are right. And those tend to be low temperature, lack of light. And when those conditions get bad lack of nutrients, they encyst and they can then settle to the bottom and stay down until conditions improve. And then they can hatch out. So in a way they hibernated through the worst conditions of the Acraman Event.

The amount of dust and cloud would probably have blotted out the sunlight for quite some time. I once flew through a sandstorm on the way to Adelaide,

17:30 Julie

Yep.

17:31 Kath

to give a lecture on the Acraman and I suddenly realized that the opaque view out of the window was probably what had happened. There was no sun to speak of and it was probably enough to give the spiny acritarchs the advantage over the cyanobacteria, which had been the main organisms up to now, up to then.

17:53 Julie

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