1 00:00:03,600 --> 00:00:08,640 This is Geology Bytes with Oliver Strimple Compared to our 2 00:00:08,640 --> 00:00:11,920 neighbours in the solar system, the Earth has experienced A 3 00:00:11,920 --> 00:00:15,200 remarkably constant temperature over the four and a half billion 4 00:00:15,200 --> 00:00:19,320 years since its creation. Venus has experienced runaway 5 00:00:19,320 --> 00:00:23,720 heating with surface temperatures of about 460°C, 6 00:00:24,320 --> 00:00:27,960 while on Mars the temperature fluctuates between 20 and 7 00:00:27,960 --> 00:00:32,400 -150°C. We owe this relative temperature 8 00:00:32,400 --> 00:00:36,040 stability to our atmosphere, which contains greenhouse gases 9 00:00:36,040 --> 00:00:40,000 such as carbon dioxide that are transparent to visible and near 10 00:00:40,000 --> 00:00:43,280 infrared light but opaque to thermal infrared. 11 00:00:44,320 --> 00:00:47,920 Too little of such gases and the Earth turns into a snowball. 12 00:00:48,480 --> 00:00:53,000 Too much and the planet cooks. Since volcanoes continuously 13 00:00:53,000 --> 00:00:56,440 inject greenhouse gases into the atmosphere, the relative 14 00:00:56,440 --> 00:00:59,880 temperature stability of the Earth implies that some feedback 15 00:00:59,880 --> 00:01:02,800 mechanism regulates the amount of greenhouse gas in the 16 00:01:02,800 --> 00:01:06,480 atmosphere. It is widely accepted that over 17 00:01:06,480 --> 00:01:10,400 geological time, this atmospheric thermostatic control 18 00:01:10,640 --> 00:01:14,200 is provided principally by the weathering of rocks containing 19 00:01:14,200 --> 00:01:18,240 silicate minerals, a process that draws carbon dioxide out of 20 00:01:18,240 --> 00:01:21,440 the atmosphere. But how exactly does this 21 00:01:21,440 --> 00:01:24,400 process function to regulate atmospheric temperature? 22 00:01:25,160 --> 00:01:27,720 And how does it depend on the changes that have occurred 23 00:01:27,720 --> 00:01:31,560 through geological time, such as the growth of continental crust, 24 00:01:31,880 --> 00:01:35,520 the formation and destruction of supercontinents, and the rise 25 00:01:35,520 --> 00:01:40,000 and fall of mountain chains? Susan Brantley has conducted 26 00:01:40,000 --> 00:01:43,600 extensive studies of the reactions between water and rock 27 00:01:43,720 --> 00:01:47,960 in the field and in the lab. Recently, she has scaled up our 28 00:01:47,960 --> 00:01:51,840 findings to a regional and global scale so as to estimate 29 00:01:51,840 --> 00:01:54,840 the overall temperature sensitivity of the Earth to 30 00:01:54,840 --> 00:01:58,600 silicate rock weathering. She is a professor in the 31 00:01:58,600 --> 00:02:02,440 Department of Geosciences at Pennsylvania State University. 32 00:02:03,360 --> 00:02:06,000 Susan Brantley, Welcome to Geology Bytes. 33 00:02:06,280 --> 00:02:08,240 Thanks, Oliver. Nice to talk to you this 34 00:02:08,240 --> 00:02:10,759 morning. Can you remind us as to what 35 00:02:10,759 --> 00:02:13,600 chemical processes are responsible for the weathering 36 00:02:13,600 --> 00:02:17,480 of a silicate rock and how that removes carbon dioxide from the 37 00:02:17,480 --> 00:02:21,040 atmosphere? Sure, When rock comes to the 38 00:02:21,040 --> 00:02:23,640 surface of the Earth, when it's exposed, it first has to 39 00:02:23,640 --> 00:02:27,720 fracture and break apart. That makes the surface exposed 40 00:02:27,720 --> 00:02:32,800 to air and water, and then minerals dissolve in the air and 41 00:02:32,800 --> 00:02:35,320 water. And that dissolution for 42 00:02:35,320 --> 00:02:38,480 silicate rocks pulls carbon dioxide out of the atmosphere 43 00:02:38,960 --> 00:02:41,760 and then holds it dissolved in the water. 44 00:02:42,400 --> 00:02:46,120 And then that water runs out of the soil and joins a stream and 45 00:02:46,120 --> 00:02:49,720 flows in a stream to the ocean. And eventually, over geologic 46 00:02:49,720 --> 00:02:54,080 time, that CO2 that was in the water recombines with calcium or 47 00:02:54,080 --> 00:02:57,760 magnesium in the ocean and then precipitates as carbonate rock. 48 00:02:58,280 --> 00:03:02,160 So really it's a sequestration process that produces rock out 49 00:03:02,160 --> 00:03:05,600 of the CO2 in the atmosphere. But this process operates over 50 00:03:05,600 --> 00:03:09,880 geological time, so how do you go about studying this in the 51 00:03:09,880 --> 00:03:12,680 lab? You take a flask and put some 52 00:03:13,000 --> 00:03:16,840 rock powder, you know, rock that you've ground up into the flask. 53 00:03:16,840 --> 00:03:19,120 And then you measure how the water chemistry changes with 54 00:03:19,120 --> 00:03:21,480 time. And then sometimes we might take 55 00:03:21,480 --> 00:03:25,760 a column and put the rock powder in the column and flow the water 56 00:03:25,760 --> 00:03:27,600 through. But either way, you're looking 57 00:03:27,600 --> 00:03:30,360 at how the chemistry of the water changes with time and you 58 00:03:30,360 --> 00:03:32,080 try to do that in a very controlled way in the 59 00:03:32,080 --> 00:03:35,120 laboratory. And then if you look at that and 60 00:03:35,120 --> 00:03:38,440 change different minerals or you change the conditions, the 61 00:03:38,720 --> 00:03:41,760 chemistry of the water or the temperature, you can really 62 00:03:41,760 --> 00:03:45,040 start to create a data set of rock weathering or mineral 63 00:03:45,040 --> 00:03:47,160 weathering based on laboratory data. 64 00:03:47,520 --> 00:03:51,680 Did you run these dissolution experiments by grinding up 65 00:03:51,800 --> 00:03:55,000 different rock types, For example granite and basalt? 66 00:03:55,440 --> 00:04:00,240 We wanted to make it even more simple by isolating not just the 67 00:04:00,240 --> 00:04:03,080 rock type, but whatever mineral was in the rock. 68 00:04:03,080 --> 00:04:07,880 So granite, for example, is mostly feldspar and quartz, 69 00:04:07,880 --> 00:04:10,840 let's say. So we might just grind up the 70 00:04:10,840 --> 00:04:12,920 feldspar and look at how the feldspar dissolves. 71 00:04:12,920 --> 00:04:17,000 And basalt can have feldspar, it can have olivine, it can have 72 00:04:17,000 --> 00:04:19,320 pyroxene. But if we separate the minerals 73 00:04:19,320 --> 00:04:21,720 and we thought we could have a very precise measure of the 74 00:04:21,720 --> 00:04:25,520 dissolution rates of individual minerals and then we should be 75 00:04:25,520 --> 00:04:28,360 able to put those back together in some kind of model to make 76 00:04:28,360 --> 00:04:31,760 predictions that predict how fast the weathering is happening 77 00:04:31,760 --> 00:04:34,360 out there in that soil that you're digging in your backyard. 78 00:04:34,800 --> 00:04:37,760 One of the things you were looking for was to see how 79 00:04:37,760 --> 00:04:40,760 temperature dependent these various processes are. 80 00:04:41,280 --> 00:04:45,760 What did you find on that score? In almost every case, as you 81 00:04:45,800 --> 00:04:48,360 increase the temperature, certainly for the silicates, 82 00:04:48,720 --> 00:04:52,720 minerals dissolve faster and so there's a standard process by 83 00:04:52,720 --> 00:04:56,240 which you measure some chemical process as a function of 84 00:04:56,240 --> 00:05:00,000 temperature and then calculate the temperature dependence. 85 00:05:00,000 --> 00:05:03,520 So the temperature sensitivity, and with that temperature 86 00:05:03,520 --> 00:05:07,400 sensitivity you can suggest, but any temperature you want to 87 00:05:07,400 --> 00:05:10,360 extrapolate to down in temperature, up in temperature, 88 00:05:10,640 --> 00:05:12,800 how fast the rate should proceed. 89 00:05:13,040 --> 00:05:16,360 So we've done that with a number of minerals, and my lab only 90 00:05:16,360 --> 00:05:19,160 produced a relatively small number of rates. 91 00:05:19,160 --> 00:05:23,400 But if you compile those with all the data collected by people 92 00:05:23,400 --> 00:05:26,280 around the world, there's a pretty hefty data set now for 93 00:05:26,280 --> 00:05:29,480 mineral dissolution rates. And can you give me a ballpark 94 00:05:29,520 --> 00:05:33,240 of how that rate varies with temperature? 95 00:05:33,840 --> 00:05:38,240 The rate double S with 10s of degrees of temperature rise. 96 00:05:38,600 --> 00:05:42,480 It is a significant effect what the temperature does to the 97 00:05:42,480 --> 00:05:45,680 process, and so that's very important when you start to 98 00:05:45,680 --> 00:05:49,600 think about how the Earth Thermo regulates how the Earth's global 99 00:05:49,600 --> 00:05:51,360 temperature has been regulated over time. 100 00:05:51,640 --> 00:05:55,360 I said in the introduction that you've just been scaling up the 101 00:05:55,480 --> 00:05:59,120 findings in the lab up to a regional and even global scale. 102 00:05:59,520 --> 00:06:04,200 So how can you actually apply a lab result to the incredibly 103 00:06:04,200 --> 00:06:08,480 messy and complicated and diverse situation that prevails 104 00:06:08,480 --> 00:06:11,760 on a global scale? There's lots of papers in the 105 00:06:11,760 --> 00:06:14,840 literature where people measured the temperature dependence of 106 00:06:14,840 --> 00:06:18,200 dissolution in the laboratory, and then there's some estimates 107 00:06:18,200 --> 00:06:21,040 for the temperature dependence of dissolution. 108 00:06:21,080 --> 00:06:24,720 You know, weathering in soils. And then there's temperature 109 00:06:24,720 --> 00:06:28,040 dependences that have been measured for watersheds, and in 110 00:06:28,040 --> 00:06:29,720 general they contradict one another. 111 00:06:29,800 --> 00:06:33,200 I would like to be able to make those numbers make sense one to 112 00:06:33,200 --> 00:06:35,800 the other. And so I moved from just 113 00:06:35,800 --> 00:06:39,040 measuring in the laboratory to actually looking at how can we 114 00:06:39,040 --> 00:06:42,280 measure weathering in the soils? And then most recently, how can 115 00:06:42,280 --> 00:06:44,360 we measure weathering in watersheds? 116 00:06:44,520 --> 00:06:48,280 And so I've tried to answer the question that you just asked me 117 00:06:48,280 --> 00:06:50,360 in both directions. I've done it from the direction 118 00:06:50,360 --> 00:06:52,880 of can I predict it from the laboratory? 119 00:06:53,120 --> 00:06:55,800 And then I've done it from the perspective of what can you 120 00:06:55,800 --> 00:06:58,800 measure in the field and then pull out of that field 121 00:06:58,800 --> 00:07:01,800 measurement, what is a number that is comparable to something 122 00:07:01,960 --> 00:07:05,160 in the laboratory. And with those two approaches, 123 00:07:05,160 --> 00:07:08,320 you can start to make some sense out of this puzzle. 124 00:07:08,720 --> 00:07:10,440 Let's look at different environments. 125 00:07:10,440 --> 00:07:15,760 You have, say, a relatively flat river, plain or floodplain, and 126 00:07:15,760 --> 00:07:17,360 then you would have a mountainous region. 127 00:07:17,360 --> 00:07:21,400 I mean, how did you subdivide the surface of the earth in 128 00:07:21,400 --> 00:07:23,760 order to be able to do this scaling up? 129 00:07:24,160 --> 00:07:27,880 Under some conditions be the overall rate of weathering is 130 00:07:27,880 --> 00:07:29,680 limited by one process or another. 131 00:07:30,120 --> 00:07:33,880 So you just said what about in floodplains sort of flatlands 132 00:07:33,880 --> 00:07:38,280 versus mountainous areas. In the flatlands, sometimes what 133 00:07:38,280 --> 00:07:42,480 we see is very, very thick soils that are depleted of the 134 00:07:42,480 --> 00:07:44,280 minerals of interest. So they're depleted of 135 00:07:44,280 --> 00:07:49,680 Feldspar's for example. And that's because the soil is 136 00:07:49,680 --> 00:07:55,600 shielding the reactive minerals that are at depth from corrosive 137 00:07:55,920 --> 00:07:59,040 rainwater, basically. So the rain comes in and it may 138 00:07:59,040 --> 00:08:02,280 or may not get all the way down to the bottom of the soil where 139 00:08:02,280 --> 00:08:04,160 the reactive mineral is still there. 140 00:08:04,160 --> 00:08:07,600 So it may not ever actually see any feldspar if all the feldspar 141 00:08:07,600 --> 00:08:11,040 has been removed from the soil. So in the flatlands, there 142 00:08:11,040 --> 00:08:14,440 actually may be very little reactive mineral surface area 143 00:08:14,600 --> 00:08:16,400 that's interacting with corrosive rain. 144 00:08:16,720 --> 00:08:20,000 Whereas if you go to a mountaintop where there's very 145 00:08:20,000 --> 00:08:23,680 little soil buildup, then the feldspar, let's say, or whatever 146 00:08:23,680 --> 00:08:26,400 reactive mineral you're interested in is exposed to the 147 00:08:26,400 --> 00:08:29,080 rain. And so in the former case, the 148 00:08:29,080 --> 00:08:33,320 flatland, that's erosive transport limited because if the 149 00:08:33,320 --> 00:08:36,320 erosion rate were just faster and the soil was removed, then a 150 00:08:36,320 --> 00:08:38,200 lot of that reactive mineral would be exposed. 151 00:08:38,760 --> 00:08:42,320 And then in the mountain, we say that it's kinetic limited 152 00:08:42,520 --> 00:08:47,200 because the erosion is so fast in essence that the reactive 153 00:08:47,200 --> 00:08:50,680 mineral surface area is right at the surface interacting with the 154 00:08:50,680 --> 00:08:53,680 rain. So that's one way to simplify 155 00:08:53,680 --> 00:08:57,600 these complex natural systems is to think of these sort of end 156 00:08:57,600 --> 00:09:01,400 member situations where you're either limited by very, very 157 00:09:01,400 --> 00:09:05,080 slow erosion or actually limited by kinetic weathering. 158 00:09:05,920 --> 00:09:08,960 And then did you estimate the proportion of the Earth's 159 00:09:08,960 --> 00:09:14,040 surface in which each of the erosive limited and kinetic 160 00:09:14,040 --> 00:09:18,800 limited regimes prevail? And I presume you also have to 161 00:09:18,800 --> 00:09:22,400 do something about gazettes or frozen areas. 162 00:09:22,400 --> 00:09:25,480 How do you scale that up? When we started applying this 163 00:09:25,480 --> 00:09:29,640 paradigm of this kinetic limit and erosive transport limit, it 164 00:09:29,640 --> 00:09:32,400 allowed us to start to see why some of the data in the 165 00:09:32,400 --> 00:09:35,760 literature made sense or didn't make sense or was comparable 166 00:09:35,760 --> 00:09:39,320 across these different scales. And then we tried to make the 167 00:09:39,320 --> 00:09:42,080 watersheds bigger and bigger and started to think about the globe 168 00:09:42,720 --> 00:09:46,040 and at the global scale. Then we had to come up with how 169 00:09:46,040 --> 00:09:49,160 much is erosive transport limited, how much is kinetic 170 00:09:49,160 --> 00:09:51,920 limited. But as you also mentioned, 171 00:09:52,040 --> 00:09:56,800 something like 1/2, the globe also has such a small amount of 172 00:09:56,800 --> 00:09:59,440 runoff going through the system. So it's such a small amount of 173 00:09:59,560 --> 00:10:02,920 basically rainfall that the fluxes are very, very small. 174 00:10:02,920 --> 00:10:05,800 So we ended up having to estimate how much of the globe 175 00:10:05,800 --> 00:10:07,640 was runoff limited. It's just not getting enough. 176 00:10:07,640 --> 00:10:11,240 Runoff versus kinetic limited versus erosive transport 177 00:10:11,240 --> 00:10:13,840 limited. And then we had to think about 178 00:10:13,840 --> 00:10:17,440 how much of the Earth's surface is characterized by different 179 00:10:17,440 --> 00:10:19,680 rock types. So how much is granite, how much 180 00:10:19,680 --> 00:10:23,160 is basalt, how much is sediment, and how much is carbonates? 181 00:10:23,200 --> 00:10:26,120 Because in general, we're interested in the silicate 182 00:10:26,120 --> 00:10:28,040 weathering as opposed to the carbonate weathering. 183 00:10:28,840 --> 00:10:32,000 Wow, it sounds like extremely ambitious undertaking to try and 184 00:10:32,000 --> 00:10:34,640 quantify all those different things, but perhaps the data is 185 00:10:34,640 --> 00:10:38,720 already out there. But broadly speaking, then, what 186 00:10:38,720 --> 00:10:43,400 does introducing these different considerations due to the 187 00:10:44,080 --> 00:10:46,320 temperature sensitivities you measure in the lab? 188 00:10:46,320 --> 00:10:49,320 Does it make it more temperature sensitive, less temperature 189 00:10:49,320 --> 00:10:51,840 sensitive, or some of one and some of the other? 190 00:10:52,160 --> 00:10:56,720 When we went from laboratory scale up to watershed scale, and 191 00:10:56,720 --> 00:11:00,160 I'm talking relatively small watersheds now, I'm not talking 192 00:11:00,160 --> 00:11:03,720 sort of global watersheds, What we saw was that the temperature 193 00:11:03,720 --> 00:11:07,320 sensitivity actually increased from the laboratory up to 194 00:11:07,320 --> 00:11:10,800 watersheds. It's not a huge increase, but it 195 00:11:11,000 --> 00:11:14,520 actually makes some sense because just as I've expressed 196 00:11:14,520 --> 00:11:18,200 in the lab, we can constrain it so that the only thing that's 197 00:11:18,200 --> 00:11:22,280 happening is dissolution. But when we go up in scale, 198 00:11:22,400 --> 00:11:26,360 other processes come in that just aren't even present in the 199 00:11:26,360 --> 00:11:28,840 lab. And so as those other processes 200 00:11:28,840 --> 00:11:31,400 come in, they can be also temperature sensitive. 201 00:11:31,480 --> 00:11:34,080 It actually makes it look like the activation energy, which is 202 00:11:34,080 --> 00:11:37,680 what we call this temperature sensitivity, is higher in a 203 00:11:37,680 --> 00:11:40,200 watershed than it is in the laboratory. 204 00:11:40,680 --> 00:11:45,160 But then when we tried to go up to global scale, because so much 205 00:11:45,160 --> 00:11:49,640 of the earth is dry, when you try to get temperature 206 00:11:49,640 --> 00:11:52,840 sensitivity for the whole globe, temperature sensitivity 207 00:11:52,840 --> 00:11:57,080 decreases again, mostly because of this dry land. 208 00:11:57,240 --> 00:12:00,360 The fluxes are just so small and dry lands that the temperature 209 00:12:00,360 --> 00:12:04,440 sensitivity is pretty small. In your recent paper, you write 210 00:12:04,440 --> 00:12:07,880 that the other processes that come into play as you scale up 211 00:12:07,880 --> 00:12:12,440 from the lab to the watershed include solute transport, clay 212 00:12:12,440 --> 00:12:17,560 precipitation, biotic activity, disaggregation, fracturing, and 213 00:12:17,560 --> 00:12:20,960 erosion. And that these processes in turn 214 00:12:20,960 --> 00:12:25,160 can be influenced by climate factors such as temperature run 215 00:12:25,160 --> 00:12:29,440 off and precipitation, and by weatherability factors such as 216 00:12:29,440 --> 00:12:32,400 lithology, porosity, permeability, type of 217 00:12:32,400 --> 00:12:35,520 vegetation, and position within a landscape. 218 00:12:36,320 --> 00:12:38,440 That's a lot of processes to consider. 219 00:12:39,560 --> 00:12:43,400 You characterize these processes using a standard formula for the 220 00:12:43,400 --> 00:12:46,280 temperature dependence of reaction rates called the 221 00:12:46,280 --> 00:12:50,480 Arrhenius equation, in which the temperature sensitivity is 222 00:12:50,480 --> 00:12:54,360 represented by an activation energy for each process or set 223 00:12:54,360 --> 00:12:58,040 of processes. But in any particular setting, 224 00:12:58,440 --> 00:13:01,920 isn't just one of the processes likely to be the overall rate 225 00:13:01,920 --> 00:13:05,040 determining step? Is that what determines the 226 00:13:05,040 --> 00:13:08,840 activation energy? The activation energy does 227 00:13:08,880 --> 00:13:11,280 characterize something about the rate limiting step. 228 00:13:11,800 --> 00:13:14,600 But if you have a complex process that doesn't really even 229 00:13:14,600 --> 00:13:17,240 have a rate limiting step, it can have so many processes that 230 00:13:17,240 --> 00:13:21,320 are all confounded to it, it can be very hard to interpret what 231 00:13:21,320 --> 00:13:23,320 that activation energy actually means. 232 00:13:23,920 --> 00:13:28,360 Broadly speaking, then, you try and parameterize all those many, 233 00:13:28,360 --> 00:13:32,760 many different effects in terms of a single activation energy 234 00:13:32,760 --> 00:13:35,360 that you then plug into the equation that is meant to 235 00:13:35,360 --> 00:13:38,280 indicate the blended temperature sensitivity. 236 00:13:38,800 --> 00:13:41,520 That's right. And in a way it's like treating 237 00:13:41,520 --> 00:13:44,640 each of these larger spatial scale systems the way a chemist 238 00:13:44,640 --> 00:13:47,680 would treat it, without being able to isolate every single 239 00:13:47,680 --> 00:13:49,560 process and measuring them all separately. 240 00:13:50,080 --> 00:13:54,120 In the early days, people measured very complex processes 241 00:13:54,440 --> 00:13:57,640 and then over time we simplified them more and more and try to be 242 00:13:57,640 --> 00:14:00,440 very reductionist. And now what we're trying to do 243 00:14:00,440 --> 00:14:03,160 is put it all back together again and try to understand 244 00:14:03,160 --> 00:14:05,600 these really big, complex systems again as I. 245 00:14:05,600 --> 00:14:09,320 Mentioned in the introduction, there have been enormous changes 246 00:14:09,320 --> 00:14:12,720 in the Earth's surface over geological time, such as the 247 00:14:12,720 --> 00:14:16,000 growth of continental crusts and the formation and destruction of 248 00:14:16,000 --> 00:14:19,520 supercontinents and the rise and fall of mountain chains. 249 00:14:20,080 --> 00:14:23,120 Would these changes be expected to affect the temperature 250 00:14:23,120 --> 00:14:26,600 sensitivity of the thermostat, and if so, in what way? 251 00:14:27,160 --> 00:14:29,560 In the points of Earth's history, when there were really 252 00:14:29,560 --> 00:14:34,400 large supercontinents, very large fractions of the continent 253 00:14:34,720 --> 00:14:37,360 may have been very dry. Because as the rain comes in 254 00:14:37,360 --> 00:14:40,720 from the oceans and gets rained out, you end up having big 255 00:14:40,720 --> 00:14:42,400 deserts in the middle of big continents. 256 00:14:42,400 --> 00:14:44,520 And you can see that even on today's Earth. 257 00:14:45,200 --> 00:14:48,360 To do an analysis like the one I did, one would have to think 258 00:14:48,400 --> 00:14:52,480 about what would we expect the dry land fraction would be when 259 00:14:52,480 --> 00:14:55,840 there are big supercontinents. And then you mentioned big 260 00:14:55,840 --> 00:14:58,440 mountain building events. There have been times like 261 00:14:58,640 --> 00:15:02,160 uplift of the Hamayas where you know mountains were going up 262 00:15:02,160 --> 00:15:06,040 very, very fast, exposing a lot of mineral to weathering. 263 00:15:06,480 --> 00:15:10,200 And there's been an ongoing argument for quite a while about 264 00:15:10,600 --> 00:15:13,080 how the mountain building effects weathering. 265 00:15:13,320 --> 00:15:17,040 From one point of view, one might argue that more reactive 266 00:15:17,040 --> 00:15:21,560 minerals should cause higher rates of weathering, and so it 267 00:15:21,560 --> 00:15:24,600 could affect the temperature sensitivity of the planet if 268 00:15:24,600 --> 00:15:26,920 more of the planet was kinetic limited. 269 00:15:27,800 --> 00:15:33,680 So would it be fair to say that during the prevalence of 270 00:15:33,680 --> 00:15:36,840 supercontinent, say every 500 million years in the Wilson 271 00:15:36,840 --> 00:15:41,960 cycle, that you get a period when the drawdown is less 272 00:15:41,960 --> 00:15:44,840 effective because you have all these dry interiors and so that 273 00:15:45,600 --> 00:15:49,600 the carbon dioxide might build up in the atmosphere and you get 274 00:15:49,600 --> 00:15:52,880 a little bit warmer. And conversely when you have a 275 00:15:52,880 --> 00:15:57,200 lot of mountain chains forming that the weathering is more 276 00:15:57,200 --> 00:16:00,280 efficient and so you get a draw down of the carbon dioxide and 277 00:16:00,280 --> 00:16:02,240 it cools off. Is that something people have 278 00:16:02,240 --> 00:16:05,360 looked to see? I think what you said is our 279 00:16:05,360 --> 00:16:08,680 baseline set of hypothesis. And of course when they dig into 280 00:16:08,680 --> 00:16:11,160 that, there's a lot of other variables. 281 00:16:11,160 --> 00:16:14,640 So where is this supercontinent? Is it high latitude, low 282 00:16:14,640 --> 00:16:17,200 latitude, Northern hemisphere? It depends. 283 00:16:17,640 --> 00:16:21,640 And in terms of mountain building events and higher rates 284 00:16:21,640 --> 00:16:26,760 of drawdown, higher temperature sensitivities, again, it depends 285 00:16:26,880 --> 00:16:29,640 what's the rock type. It depends where the mountain 286 00:16:29,640 --> 00:16:32,000 building's occurring. And then there's this other 287 00:16:32,000 --> 00:16:35,800 wrinkle that we haven't really talked about and that is that 288 00:16:35,920 --> 00:16:39,840 there's not that much CO2 in the atmosphere and yet the CO2 in 289 00:16:39,840 --> 00:16:43,800 the atmosphere has been maintained at relative constancy 290 00:16:43,800 --> 00:16:46,800 over periods of time. So all these processes have to 291 00:16:46,800 --> 00:16:49,960 be balanced. So even if a mountain chain goes 292 00:16:49,960 --> 00:16:53,200 up and let's say there's a increased temperature 293 00:16:53,200 --> 00:16:56,800 sensitivity because there's more kinetic limited landscape, if 294 00:16:56,800 --> 00:17:00,840 that were to pull too much CO2 out of the atmosphere, the globe 295 00:17:00,840 --> 00:17:02,680 could quickly become ice covered. 296 00:17:02,720 --> 00:17:05,680 And we don't always see that. So, well, what happens when a 297 00:17:05,680 --> 00:17:08,680 mountain chain goes up and it causes this higher temperature 298 00:17:08,680 --> 00:17:12,359 sensitivity? Is it perhaps modulated by other 299 00:17:12,359 --> 00:17:14,160 processes? And one of the big arguments out 300 00:17:14,160 --> 00:17:16,960 there right now is, you know, you can't just think of the 301 00:17:16,960 --> 00:17:20,920 world as being silicate rock. There's also a lot of pyrite out 302 00:17:20,920 --> 00:17:23,560 there. Pyrite is iron sulfide, and when 303 00:17:23,560 --> 00:17:26,560 pyrite weathers, it produces sulfuric acid. 304 00:17:26,920 --> 00:17:31,200 And weathering driven by pyrite also happens as mountains go up. 305 00:17:31,520 --> 00:17:34,920 And so there's other complexities that may actually 306 00:17:34,920 --> 00:17:39,080 cause not only CO2 drawdown, but actually caused some CO2 release 307 00:17:39,080 --> 00:17:42,080 during mountain building events. What's about the effect on 308 00:17:42,080 --> 00:17:45,680 weathering of life forms, both microscopic and macroscopic? 309 00:17:46,480 --> 00:17:49,960 We've done some experiments where we've put in bacteria and 310 00:17:50,400 --> 00:17:52,720 into our flask experiments with minerals. 311 00:17:52,720 --> 00:17:55,240 We've done some column experiments with plants. 312 00:17:55,640 --> 00:17:59,960 And in general, what you see at the lab scale sort of at that 313 00:17:59,960 --> 00:18:03,560 small scale is there's usually an acceleration of weathering. 314 00:18:03,920 --> 00:18:07,440 But when you go up in scale, if you think about your backyard, 315 00:18:07,440 --> 00:18:10,880 if you have any kind of a slope in your backyard, you're often 316 00:18:10,880 --> 00:18:13,760 advised to put plants on that slope because it'll hold the 317 00:18:13,760 --> 00:18:17,680 soil in place and can move your system from something that could 318 00:18:17,680 --> 00:18:22,760 be kinetic limited to a system that is covered with soil that 319 00:18:22,760 --> 00:18:24,440 has very little reactive mineral in it. 320 00:18:24,440 --> 00:18:26,200 So it could be more erosive transport limited. 321 00:18:26,680 --> 00:18:30,760 So really, with the lab scale, biology tends to accelerate 322 00:18:30,760 --> 00:18:33,680 weathering, but as you go up in scale, it can have both effects. 323 00:18:33,680 --> 00:18:35,880 It can accelerate weathering, but it can also slow down 324 00:18:35,880 --> 00:18:40,560 weathering, and the jury's out as to how that has played out at 325 00:18:40,560 --> 00:18:43,280 higher and higher spatial scales of analysis. 326 00:18:43,760 --> 00:18:49,000 Why is it so important to know the overall temperature 327 00:18:49,000 --> 00:18:52,680 sensitivity of the Earth's geological thermostat? 328 00:18:53,320 --> 00:18:57,560 Broadly speaking, I suppose the higher the sensitivity, the 329 00:18:57,560 --> 00:19:02,040 quicker any temperature excursion can be damped over 330 00:19:02,040 --> 00:19:05,000 geological time and brought back to its steady state value. 331 00:19:05,520 --> 00:19:09,720 Is that something that we need to know both to understand 332 00:19:09,920 --> 00:19:13,400 geological history, but also to understand what's happening 333 00:19:13,400 --> 00:19:14,840 today? Sure. 334 00:19:14,840 --> 00:19:20,320 Well, what we're doing today is totally changing this cycle that 335 00:19:20,320 --> 00:19:23,360 we're talking about. South geologists think about the 336 00:19:23,360 --> 00:19:27,960 CO2 in the atmosphere and the global temperature as being 337 00:19:27,960 --> 00:19:32,800 modulated by how fast volcanoes release CO2 and how fast 338 00:19:32,800 --> 00:19:35,320 weathering pulls the CO2 back out of the atmosphere. 339 00:19:35,760 --> 00:19:37,560 But that's over a million year time scales. 340 00:19:37,560 --> 00:19:39,360 That's a very long, slow process. 341 00:19:39,600 --> 00:19:44,280 If the Earth is just left on its own to respond, CO2 will be 342 00:19:44,280 --> 00:19:47,000 drawn down out of the atmosphere by weathering. 343 00:19:47,360 --> 00:19:49,560 But it could take hundreds of thousands of years. 344 00:19:49,960 --> 00:19:54,840 And right now we're in this big dialogue around the world about 345 00:19:54,840 --> 00:19:58,240 how are we going to stop using fossil fuels. 346 00:19:58,440 --> 00:20:01,840 But what's important is that there's still so much CO2 out 347 00:20:01,840 --> 00:20:04,560 there in the atmosphere, and that's going to be still acting 348 00:20:04,560 --> 00:20:06,720 like a thermostat. The greenhouse effect is going 349 00:20:06,720 --> 00:20:10,280 to maintain the warming that we're observing, and it's going 350 00:20:10,280 --> 00:20:12,200 to be a very long time to pull that out. 351 00:20:12,200 --> 00:20:15,440 And so this basic understanding of the Earth system has spurred 352 00:20:15,440 --> 00:20:19,760 a conversation about. Well, what can we do to pull the 353 00:20:19,760 --> 00:20:21,920 CO2 out of the atmosphere? Because the Earth is not going 354 00:20:21,920 --> 00:20:26,360 to do it fast enough for us. There are silicate rocks on the 355 00:20:26,360 --> 00:20:29,800 surface of Venus. Why have they not been able to 356 00:20:29,800 --> 00:20:32,960 provide thermostatic temperature control there? 357 00:20:33,320 --> 00:20:38,440 Venus is closer to the Sun. It's rotation is slower, so part 358 00:20:38,440 --> 00:20:42,720 of it is facing the sun for longer periods of time, and the 359 00:20:42,720 --> 00:20:46,040 temperature at the surface of the planet has been such that it 360 00:20:46,040 --> 00:20:49,200 has lost most of its water, if not all of its water. 361 00:20:49,520 --> 00:20:52,160 And so the atmosphere is basically a CO2 atmosphere. 362 00:20:52,160 --> 00:20:55,520 So there may be rock and there may be CO2, but there isn't the 363 00:20:55,520 --> 00:20:58,280 water there to allow the weathering to proceed and pull 364 00:20:58,280 --> 00:21:01,640 that CO2 out. And I guess on Mars the 365 00:21:01,640 --> 00:21:04,720 situation is there's just not much of an atmosphere at all. 366 00:21:04,800 --> 00:21:06,800 Right. There has been weathering on 367 00:21:06,800 --> 00:21:09,880 Mars, our Rovers that we've put up on the planet and looked at 368 00:21:09,880 --> 00:21:12,840 the geology. There's been beautiful evidence 369 00:21:12,840 --> 00:21:17,040 that there have been times on Mars earlier in Mars's history 370 00:21:17,040 --> 00:21:20,080 when there actually was very deep weathering that occurred, 371 00:21:20,080 --> 00:21:25,080 and now it's not occurring. Coming back to the question of 372 00:21:25,120 --> 00:21:29,600 global warming that we're facing here on Earth, and your point 373 00:21:29,600 --> 00:21:32,920 that the time scale for this geological drawdown is extremely 374 00:21:32,920 --> 00:21:40,000 long, does your work help inform the attempts to sequester carbon 375 00:21:40,000 --> 00:21:45,840 dioxide by the use of enhanced weathering, which is one of the 376 00:21:46,200 --> 00:21:48,400 many techniques that are being proposed now? 377 00:21:48,760 --> 00:21:52,800 One way to do this is to dig up a bunch of rock, grind it up, 378 00:21:53,000 --> 00:21:55,320 and then put it out on fields and let it weather. 379 00:21:55,680 --> 00:21:58,560 And that's such a simple thing to imagine. 380 00:21:58,560 --> 00:22:01,960 We know how to dig up rock, We know how to transport it, we 381 00:22:01,960 --> 00:22:05,080 know how to grind it. We know how to disseminated on 382 00:22:05,080 --> 00:22:06,720 fields. We do that all the time with 383 00:22:06,720 --> 00:22:09,840 lime. So many farmers fields are lime, 384 00:22:09,880 --> 00:22:13,080 so they take limestone and grind it up and put it just really 385 00:22:13,080 --> 00:22:16,040 common. And so people have said why 386 00:22:16,040 --> 00:22:19,920 don't we dig up, especially basalt, which is a rock type 387 00:22:19,920 --> 00:22:21,960 that has a lot of calcium and magnesium. 388 00:22:22,240 --> 00:22:26,320 It's actually a very good set of minerals to pull CO2 out of the 389 00:22:26,320 --> 00:22:28,480 atmosphere. Why don't we dig up a lot of 390 00:22:28,480 --> 00:22:32,720 basalt, grind it up, transport it to farmers fields, put it out 391 00:22:33,120 --> 00:22:35,520 and let it pull the CO2 out of the atmosphere? 392 00:22:35,880 --> 00:22:39,040 And people are doing this. People are trying this in the 393 00:22:39,040 --> 00:22:42,880 Midwest, people are trying it in small plots around the world. 394 00:22:43,240 --> 00:22:48,160 And people are trying to measure then whether CO2 that's pulled 395 00:22:48,160 --> 00:22:50,520 out is what they might have predicted. 396 00:22:50,840 --> 00:22:54,640 And then they have to compare the CO2 that's pulled out to the 397 00:22:54,640 --> 00:22:59,680 total calculated CO2 of digging up the rock, grinding the rock, 398 00:22:59,680 --> 00:23:02,120 transporting the rock, and then spreading the rocks. 399 00:23:02,120 --> 00:23:04,880 So there's like all those processes in there that you have 400 00:23:04,880 --> 00:23:09,000 to figure out how much CO2 was released by the hydrocarbons 401 00:23:09,000 --> 00:23:11,440 that you used to do all those kinds of processes as well. 402 00:23:12,040 --> 00:23:15,800 This could be successful. You could run this system in the 403 00:23:15,800 --> 00:23:19,040 way that you'd pull more CO2 out than you'd put in, in terms of 404 00:23:19,040 --> 00:23:22,160 the digging, grinding, transporting and spreading. 405 00:23:22,520 --> 00:23:25,760 I discussed enhanced weathering in an earlier episode with Phil 406 00:23:25,760 --> 00:23:29,800 Renforth and learn that to remove gigatons of carbon 407 00:23:29,800 --> 00:23:33,240 dioxide from the atmosphere per year, which is what we need to 408 00:23:33,240 --> 00:23:36,040 do in order to have a significant impact, we would 409 00:23:36,040 --> 00:23:38,840 have to do this on a truly gigantic scale. 410 00:23:39,160 --> 00:23:41,640 Right. Really large percentages of the 411 00:23:41,640 --> 00:23:46,400 farmland in the top agricultural countries in the world would 412 00:23:46,400 --> 00:23:51,200 have to be used to make a dent in the amount of CO2 that we put 413 00:23:51,200 --> 00:23:54,960 in the atmosphere. We may say, well this is too 414 00:23:54,960 --> 00:23:58,320 much rock to be dug up and transported and powdered and 415 00:23:58,320 --> 00:24:00,560 spread to solve the whole problem. 416 00:24:00,800 --> 00:24:05,440 But in certain areas, possibly near big basalt deposits, it 417 00:24:05,440 --> 00:24:09,200 might be a really good process, especially because the ground up 418 00:24:09,200 --> 00:24:13,840 powder has nutrients in it. As you mentioned, you clearly 419 00:24:13,840 --> 00:24:17,560 had to make a lot of simplifying assumptions in order to build a 420 00:24:17,560 --> 00:24:20,680 model for how weathering depends on temperature on a global 421 00:24:20,680 --> 00:24:23,120 scale. Are there any aspects in 422 00:24:23,120 --> 00:24:26,400 particular that you'd like to add to your model to make it 423 00:24:26,400 --> 00:24:29,120 more realistic? One of the puzzles is the effect 424 00:24:29,120 --> 00:24:33,000 of biota. So plants and organisms and how 425 00:24:33,000 --> 00:24:35,840 that affects weathering. I mentioned that the laboratory 426 00:24:35,840 --> 00:24:39,320 scale, it seems to accelerate, but then as you get up to field 427 00:24:39,320 --> 00:24:42,720 scales, you often see that biota can decelerate weathering. 428 00:24:42,720 --> 00:24:47,080 So I did work on that to some extent in order to do this first 429 00:24:47,240 --> 00:24:50,400 estimate that we did as a global scale temperature sensitivity. 430 00:24:50,400 --> 00:24:53,000 But I'd like to work on that more and think more about again, 431 00:24:53,000 --> 00:24:56,000 scaling up biota. There's a lot of papers and all 432 00:24:56,000 --> 00:24:58,720 these different spatial scales. I'd like to see if I could do 433 00:24:58,720 --> 00:25:02,040 something similar and make sense out of why the different 434 00:25:02,320 --> 00:25:04,920 observations at different spatial scales don't always seem 435 00:25:04,920 --> 00:25:07,480 to make sense. What are you working on at the 436 00:25:07,480 --> 00:25:09,520 moment? One thing that I'm very 437 00:25:09,520 --> 00:25:13,840 intrigued by is this idea that soil can become depleted in 438 00:25:13,840 --> 00:25:16,520 minerals because of weathering. If you think of weathering as 439 00:25:16,520 --> 00:25:20,400 being top down, the rain comes from the atmosphere and moves 440 00:25:20,400 --> 00:25:22,800 down. Like A1 dimensional worldview 441 00:25:23,200 --> 00:25:26,400 that upper soil becomes depleted in their reactive minerals. 442 00:25:26,720 --> 00:25:30,440 Eventually, as you dig down, you come to a zone where there are 443 00:25:30,440 --> 00:25:34,800 reactive minerals and they are reacting sometimes at very, very 444 00:25:34,800 --> 00:25:38,040 slow rates if they're very deep. But that zone where they're 445 00:25:38,040 --> 00:25:41,040 reacting, I call that a reaction front or the weathering front 446 00:25:41,680 --> 00:25:45,480 and weathering front should be all around us in the landscape 447 00:25:45,480 --> 00:25:47,880 beneath our feet. So we can see the landscape that 448 00:25:47,880 --> 00:25:51,240 we walk on and we can see the morphology of that landscape. 449 00:25:51,240 --> 00:25:54,080 That's a geomorphologist study, is why the shape of the 450 00:25:54,080 --> 00:25:57,640 landscape looks the way it does. What we don't know is what does 451 00:25:57,640 --> 00:26:00,800 the landscape of that reaction front look like in the 452 00:26:00,800 --> 00:26:03,960 subsurface? So for each mineral in a rock, 453 00:26:04,160 --> 00:26:06,920 there will be a landscape, there will be a depth at which that 454 00:26:06,920 --> 00:26:12,560 mineral is dissolving, and how does that landscape for each 455 00:26:12,560 --> 00:26:17,280 mineral vary as you move across the landscape that we can walk 456 00:26:17,280 --> 00:26:19,360 on. So for example, at a Ridge top, 457 00:26:19,440 --> 00:26:23,520 you may only have to go very shallow before you start to see 458 00:26:23,520 --> 00:26:27,880 a reaction front versus if you go down to a stream, you might 459 00:26:27,880 --> 00:26:30,320 have a different depth down to the reaction front. 460 00:26:30,880 --> 00:26:34,160 And realistically, geochemists have never been able to study 461 00:26:34,160 --> 00:26:37,640 those landscapes because that three dimensionality of it is 462 00:26:37,640 --> 00:26:40,320 very difficult. And so recently I've worked with 463 00:26:40,320 --> 00:26:44,000 geophysicists who have all these tools that can look at 464 00:26:44,000 --> 00:26:47,880 landscapes in the subsurface and start to measure geophysical 465 00:26:47,880 --> 00:26:50,960 properties in the subsurface. So if we could understand 466 00:26:50,960 --> 00:26:53,840 distributions of these geochemical landscapes of 467 00:26:53,840 --> 00:26:57,160 reaction in the subsurface and relate that to the geophysical 468 00:26:57,160 --> 00:27:00,720 signals, we could map what the subsurface looks like and where 469 00:27:00,720 --> 00:27:03,680 weathering is happening. And I think that in itself would 470 00:27:03,680 --> 00:27:07,320 help us understand this puzzle of global weathering as well, 471 00:27:07,600 --> 00:27:10,920 namely how it's distributed across our landscapes. 472 00:27:12,080 --> 00:27:13,960 Susan Bradley, thank you very much. 473 00:27:14,280 --> 00:27:16,080 Thank you. It was a pleasure to talk to 474 00:27:16,080 --> 00:27:18,160 you. To see pictures and 475 00:27:18,160 --> 00:27:23,600 illustrations that support this podcast, go to geologybytes.com, 476 00:27:24,120 --> 00:27:27,440 where you'll also find transcripts and a subject matter 477 00:27:27,440 --> 00:27:31,400 index of all the episodes there. You can also give me feedback, 478 00:27:31,400 --> 00:27:35,160 which I welcome, as well as sign up to get my emails about new 479 00:27:35,160 --> 00:27:35,920 episodes.