Antarctic Bottom Water with Brian Romans

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Travis

What do know about Antarctic Bottom Water When Virginia Tech's Brian Romans reached out to me about this topic, all I really could think was that it sounds low and cold.

And while I'm pretty sure those two things are true, I'm also very sure that there is quite a bit more to this topic, and I'm thankful to Brian for coming on the podcast to help confirm that suspicion. Brian is a professor of Sedimentary Geoscience in Virginia Tech's College of Science. His research interests include the study of ancient and modern sedimentary systems, using outcrop subsurface and earth surface data to help better understand the past and predict future geological problems related to tectonic and or environmental shifts. Brian and I talked about what Antarctic bottom water is and how do you go about studying water that's at the bottom of the ocean. We also talked about what he's trying to specifically learn there and how Antarctic bottom water can provide some unique clues about the Earth's history as well as maybe some things we can use to better predict the future. So this podcast is both geology and oceanography mixed in with a little bit of detective work. I'm Travis Williams and this is Virginia Tech’s curious conversations.

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Travis

So I know that you're working on Antarctic bottom water, which I don't know anything about. It sounds like it could be a great name for a band, but I kind of think that that's probably not what you're studying. maybe that's a great place to start this conversation. Simply, simply what is that?

Brian

Yeah, sure. Of course. So in oceanography, the ocean has all sorts of different currents and layers to it, and they are given different names. And so Antarctic bottom water is pretty much what it sounds like. It's, it's water that's at the bottom of the ocean and then is sourced from Antarctica. And so the way this works is in the ocean, these different layers of the ocean, these different currents are all governed by different densities. Like if you were to separate, no olive oil and water, right? Olive oil is denser. And so Antarctic bottom water is the densest, what's called water mass in the global ocean. So it's cold, which makes it denser and it's salty, which makes it even more dense. And so it's so dense, it occupies literally the bottom of the ocean. So it's along the seabed and it goes from Antarctica all the way north to into the North Atlantic ocean and into the middle of the Pacific ocean. So it's called Antarctic bottom water because that's where it comes from. But it's a globally important water mass as it's called. And it's important to learn about because it relates to what's called global ocean circulation, which can then modify our climate. And it also stores carbon, dissolved carbon in the deep ocean, sort of keeping it out of the atmosphere. So this is where oceanography intersects with climate science. And then I'm a geologist, which I can get into how kind of I got into this as well. But that's basically what Antarctic bottom water is and why it's important. It sounds like it is kind of the foundation to a lot of our oceans is what it sounds like. Yeah. People have studied it for decades, sort of physical oceanographers and trying to track it and trace it and its activity, its behavior. And it can be a bit of a, you know, maybe a canary in the coal mine in a sense for global climate change.

Travis

Well, how does one go about studying the bottom of the ocean and the water that's there? How did you go about studying it?

Brian

Yeah. So, you know, these kind of big kind of processes on our planet, you can go about studying them in different ways. When it comes to oceanography, you can go out in a boat and you can drop instruments down there and you could measure temperature and salinity and velocity of the current. You could build computer models, which people do build simulations of the physics. I'm a geologist from the department of geosciences. So what we like to do is look at the record of past behavior. Me and my students in my group study sediments and sedimentary rocks. And so they're like basically what's left over after processes in the past. You know, just like paleontologists look at fossils in the rock record, we're looking at these past physical processes that are left in these Sediments. So what we did is several years ago is on a ship that's outfitted with a drill rig and, and pouring equipment is went to these places, offshore Antarctica and took sediment cores into the seabed and brought those up. And we have lengths of core that go a couple hundred meters, know, several hundred feet. And that gets us back in time. So it's kind of like a time machine. And so we focused on reconstructing this. Antarctic bottom water history through that time. sounds awesome. You sound kind of like a, like a, like an ocean detective almost trying to figure out all these clues, right? Yeah. It's, you know, the subfield is called paleoceanography. know, like, you know, paleobiology past life. this is past, past oceanography basically, and, using the geologic record. it's a, it's a really interesting intersection of earth science disciplines. It's really you know, fun to learn about.

Travis

Aside from being, I guess, at the very bottom of the ocean, is there anything else that makes the Antarctic bottom water a really good barometer for changes that have occurred during the Earth's history?

Brian

Yeah. So, you know, how we think it forms and how we understand it to form today is that it doesn't start at the bottom. So it actually starts at processes at the surface and like right offshore of Antarctica today. And basically, you know, you've probably heard of glacial ice, right? You know, glaciers that come down on land and some of them come down into the ocean. But the ocean, especially in the poles, also has what's called sea ice. And so that's ice that forms out of the seawater. And usually in the winter more than in the summer, but it can be seasonal. And when sea ice forms, it preferentially freezes fresh water because salt water is harder to freeze, right? You need much cooler temperatures. When that happens, it leaves the water that is sort of underneath the sea ice even saltier and almost basically like a brine, you could call it. And that makes it denser. And so it flows, it flows down through this process. And it, and this is where you want to picture. this really salty, dense water is flowing kind of offshore along the continental shelf. And then it cascades off into the deep ocean.

So can picture basically like an underwater waterfall in a sense of a dense waterfall in the ocean along the bottom of the ocean. And so that's what drives this Antarctic bottom water is this flowing across the shallow water and then cascading off into the deep water. And when it does this in some places, it gets funneled into submarine canyons and it becomes quite vigorous enough that it can pick up sediment and then deposit that sediment. that's, that's what we cored into to get the record of these. So kind of like, you know, a flood on land would leave a bunch of mud and debris around for there's the record of that flood. But this is just in the bottom of the ocean and offshore Antarctica. so if you see a flood like that, what are you able then to kind of think back that may have caused that? What, how do you, how do you go about kind of putting the pieces together, the puzzle? Yeah. Yeah. It's a great question. So, you know, when we, We took these cores and the ones that we focused on for this study, I should also say before I forget that the work, this work was led by a former PhD student in my group, Natalia Varela. She's now doing a postdoc up the road at UVA, but don't hold it against her. We will, we will not. But I just want to give her a shout out because she led, led this work and the paper that came out of it. And so what we did with the core data is that when we looked at the cores, you know, that were brought up under the ship and all this measurements were taken and high resolution photographs and all this information. We saw these thin, like few millimeter thin layers of slightly coarser grain sediment. And so these cores are pretty much all mud, almost featureless, you know, mud. And then they're interrupted by these little layers of slightly coarser, sometimes in very fine sand. And so these, we interpreted as vents that were related to what I was talking about, these cascading dense waters. And so we basically tallied up how many there were, where they were in the depths of the core. And then we had some idea of, so this record goes back just over 3 million years ago. And so we could put those events ⁓ into a context of time, and kind of when there was a lot of them, and actually was more curious of when they stopped is when they were absent, which I can talk about. We were kind of looking at, we called it a barcode graph of kind of basically when these events happened, when they were absent, when they were frequent, when they were infrequent and related that to the history of this process.

Travis

Do we know what the events might've been? Can we speculate at all?

Brian

I mean, yeah, it's a great question. In this case, we interpret, envision them to have been particularly large, sort of high magnitude of these dense cascading events. And so it's hard to know if they were related to, you know, like one season or if they were varying over sort of decades to centuries. We can't quite get that detailed of time. But when we look at sort of a two or three million years, we start to put together a picture of that history. So we can't say that maybe this was like an earthquake or a meteor hit or something that specific maybe. No. And I think in this case, we don't interpret those kinds of processes because these little events are pretty much all the same. So we looked at their thickness and we sampled them and measured their particle size and they're all basically the same. It's just when they occur and when they don't. So we interpret this process, which we can see today, right? people, oceanographers have documented this process today. So we feel good about making that link to the past. that makes a lot of sense. If it's more consistent, it's probably not something crazy that doesn't happen on a consistent basis. what is our,

Travis

You mentioned today, the stuff that is happening in the ocean. these similar cascading undersea waterfalls, they happening right now?

Brian

That's a great question. Yes. What's called dense water cascading does happen today and what physical oceanographers have been able to observe and measure. We don't have a lot of measurements because it's difficult in the deep ocean to do that kind of observation. But there is some data and some information that this is happening today. But if these kind of bigger events maybe only happen once every few decades or so, then we're very limited in what we have from our observations of the past 20, 30, or 40 years. That's where the long-term record becomes valuable. And what we found that was, think, kind of the most interesting is that, like I said, it's when these events are in the record, but then there's these curious times where they stop. And there's just none of these. And there was two of these intervals in particular that we looked at. And when we started comparing when that was to other past climate records from other workers and other kinds of data. These coincided with times of exceptional warm, of when it's thought and estimated to have been anomalously warm compared to what was before and after for that time. And so this kind of goes with this idea that oceanographers have looked at the last decade or so that with warming of the Southern Ocean, the ocean around Antarctica, that this Antarctic bottom water process could be slowing down because remember it comes from the sea ice formation. So if you're not getting as much sea ice and these processes, you have this kind of chain reaction effect. And so we think we have examples in the past of at least two of these exceptionally warm periods where this Antarctic bottom water process potentially shut down for some time.

Travis

Do we know what that might do to the rest of the ocean? Like how do we fit that into the context of everything?

Brian

Yeah, that's right. So that's where oceanographers can then kind of use maybe models or simulations to ask that question, right? What happens or what happened, whether it's in the past or projecting into the future, right? And there's a few things that could happen, but most importantly, this Antarctic bottom water is a really important component of the global circulation. If a water mass flows from one area to the other, that means you have return flow. Right? because it's all kind of connected. Sometimes people call it the Earth's circulatory system. Just like there's atmospheric circulation with jet streams and all these patterns, the ocean has similar. And so if that is slowing or even kind of shuts down, that basically could cascade through the global ocean and impact other currents. And so we don't know exactly. There's different hypotheses and different sort of studies of what that could do could be disrupted well beyond Antarctica. And as I mentioned beginning, another thing Antarctic bottom water does, a service, so to speak, is that it sort of stores carbon that would otherwise be in other parts of the earth system, like the atmosphere. And so if it's not active and sort of, and doing that, transporting and moving dissolved carbon and keeping it in the deep ocean, that could have a feedback on the atmosphere. So the ocean's an atmosphere connected in that sense. Although we might not know exactly what could happen, it would be disruptive and sort of unpredictable to the global climate potentially. It sounds like it's in our best interest to keep those things moving is my basic understanding. Yeah. And that's, you know, that's why there's a lot of interest when it comes to earth science and climate sciences, to understand the behavior of them, both at present in the past, like our group does, and then forecasting into the future, like say climate modelers might do. And that all comes together to try to get the best understanding we can.

Travis

You mentioned that your focus kind of shifted from geology to this study that's, is geology, but it's ocean-based. How did that happen? How'd you get interested in the ocean?

Brian

Yeah, that's a great question. Yeah, my training, if I think back to grad school, which is now 20 years ago in the thousands, I was looking at rocks out in the field, hiking around. And I still do that work as well. But as I learned about these ancient sediments, kind of how they, where they came from and how they formed and learning that these were, these were sediments that were deposited in the deep ocean. And now, now they're uplifted into mountains, you know, because of geology. And I got more interested in, well, it would be really interesting to actually go out to the ocean and, you know, look at these sediments more directly, right? They're still in the deep ocean, right? So they're kind of younger geologically. And so when I became a professor here at Virginia Tech, which was 2011, I started applying to go out on these expeditions in what's called scientific ocean drilling. So out on a vessel that, you know, looks like a drill rig ship that industry might use for resource extraction, but it's outfitted for science. And so there's all these labs on board. And so from 2012 through 2023, I went on three different expeditions to the North Atlantic to Antarctica. And then the most recent was offshore Greenland to also study ice sheet ocean processes. you go to the cold parts of the ocean. That's where there's a lot of, there's a lot of things to learn firstly, that we don't know as much about potentially. And also our poles, the Arctic and the Antarctic are sort of feeling and responding to climate change more, you know, noticeably or more measurably than other parts of the planet. whereas around the equator or even kind of where we live, temperate latitudes, there may be some effects that it could be really variable depending on where you are. The poles are where things are changing really rapidly. So there's a lot of interest in understanding how the polar systems respond to change as can we learn as much as we can about those to help us think about the future better.

Travis

That is super cool that you got to go to both of those areas that when I was a kid, I don't know, I didn't even imagine going to a place like that. didn't even know people could go to places like that. So the fact that you've been several times is really cool.

Brian

Yeah. The way I did it, of course, was I never actually stepped foot on Antarctica or Greenland. I was in a boat that came from, you know, we sailed from New Zealand all the way to Antarctica. So that was about nine or 10 days. For Greenland, we sailed from Iceland and that was, you know, four or five days. Although I didn't step foot on these places, I kind of, I count it as going. I would count it for you. I think that's awesome. What a, a, what a crazy, fun adventure to go on in search of these clues.

Travis

I am curious what in this space, as you were looking back in time and history and looking through clues and trying to figure it out. What, what gives you hope?

Brian

Yeah, really good question. I think the thing that I've really come out of my participation and involvement in this program. And the science for this goes on for many years, I should say. Like I'm even still working on samples and ideas from expedition over a decade ago. But the thing that kind of comes out of that is how international it is, you know, just, you know, many, many countries involved and the scientists from different nations coming together. So with all the geopolitical strife that could be going on in any given era, right? That's really, ⁓ you know, potentially chaotic and confusing and dangerous even. think science and scientists, you know, have a vernacular getting together to focus on the things that they think are interesting and important to learn about. So I've really enjoyed that part of this kind of work. And thanks to Brian for helping us better understand Antarctic bottom water.

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Travis

If you someone you know would make for a great curious conversation, email me at traviskw@vt.edu. I'm Travis Williams and this has been Virginia Tech's Curious Conversations.

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