 So, I'd like to introduce our speaker today, which is a member of the Geography Department, as well as the U.S. Geological Survey, wearing two hats, which is neat, and oversees the Palinology Lab, which is even more wonderful, David Wall. And he is also a member of our, I don't know what your title is, but you're part of our open group. And so today, he's going to be presenting some of his current research that he is doing in Mesoamerica in the Maya Lomans, as the title says, and he's going to be telling us about climate and land use change in the Maya Lomans, I'm assuming in the past. That's right. Good. Okay, well, welcome, David. Thank you so much for starting our computer off. All right. Thank you, Christine. Yeah, thank you guys all for coming out. As Christine said, I've been doing some work in the Maya Lomans going on about 20 years now. And so I'm actually really excited to take this opportunity to step back and kind of synthesize the work that I've been doing, looking at environmental change, climate change, pretty much throughout the Holocene, but really kind of focusing in on questions that revolve around cultural trajectory in the Maya region in the Maya Lomans. And so what I'm going to do is basically start off with the background of the Maya Lomans, kind of the physical components of it, touch on some of the cultural history and the archeology of the areas that I've been working on. And then I'll go a little bit through the field work techniques and lab methods that we use and then get into the data. And I'm actually going to wrap up with a very new, pretty exciting find that we have that looks like we might be able to link the sediment record to a warfare event and the epigraphic record, which is kind of first of its kind, certainly in the New World, linking written record to changing environments. So before I get started, I just want to kind of point out this is definitely a multi- institutional, multi-collaborative effort. Roger Burn, I'd like to just give a nod to. He passed away, unfortunately, in March. It was really critical in my development of interest in prehistoric agriculture and environmental change in Mesoamerica. Other than that, working with a lot of different folks, archeologists, Richard Hansen is the director of the Mirador Project, Francisco Stradibeli, director of the Holmo Project, and a whole list of folks that have contributed data, contributed time and effort to what I'm going to be showing you guys today. Okay, so this map here, just to really quickly situate ourselves geographically, the Maya Lomans essentially is all of the area of this map of the Yucatan Peninsula that's off of this volcanic axis to the south here. And the Yucatan Peninsula is this very large tertiary limestone platform that uplifted. And it's really dominated, aside from the granitic intrusion here of the Maya Mountains, it's dominated by karst topography, so this limestone topography. And that really means two things for the point of this talk. One is water bodies are really in surface water is pretty scarce, right? You have this limestone, it's very porous rock, a lot of the water percolates through. So you don't have a lot of lakes and you don't have a lot of rivers. There's a lot of subterranean rivers and there's sinkholes and cenotes, and lakes are not so common up in the heart of the karst platform here. And so basically what that draws to mind are two things. One is the possibility of water shortages and drought in the past, affecting the Maya, which I'll touch on. And the other is in terms of my work, I'm looking for permanent standing bodies of water. So in a sense, it kind of catches catch can. It's not like you can just go down and find a spot right next to a site that you're interested in exploring the history of and just start coring. Precipitation is highly seasonal today. So there's essentially a six month drought every year down here. So we get a big wet season from late May, early June, through to sometime in November. And then it really kind of shuts off. And there's some winter rain that can kind of come through from the northern latitudes, but you really get a very stark dry season down here. If you've ever been out there in the dry season, water procurement can be an issue when you're not in a town. And it's also, there's a very steep precipitation gradient. So down in the southern part of the peninsula, we get about two and a half meters of rain during that wet period. It's very wet, very moist, and it drops off as one moves up towards the northwest corner. Here's the really the driest part. It can drop as low as, you know, 500 millimeters a year, which is barely enough to support rain fed agriculture. Along that precipitation gradient, we see vegetation gradient. So in the south, we've got big, high, close canopy, tropical evergreen forest. And as one moves north, we move out of that. Height of the canopy comes down, starts opening up a bit, get more of an understory. You start seeing a lot of deciduousness on the big forest trees. They drop their leaves in the dry season, likely a pollination strategy. They flower also when the leaves are dropped. And then that just grades down. The canopy comes down as one moves north, and then finally up in the northern part of the peninsula, it's this leguminous thorn scrub. It's really just kind of an open, very low canopy, maybe 10, 15 feet. Lots of undergrowth, lots of scrubby vegetation. Okay, so zooming in on the center part of that, here we're looking at the northern part of Guatemala, the Petén District. And this is where I've been doing all my work. And I'm just going to kind of tee off by talking about this area that's up here in the very north central part of Petén. It's known as the Mirador Basin. It's not really a basin. I'm not going to get into that right now, but it's a very important area in terms of early Maya development. And it's a really incredible area. It's about 2,200 square kilometers of roadless wilderness. So Richard Hansen has been doing his best to try to keep this kind of unimpacted. It's part of the Maya Biosphere Reserve. But importantly, out within this area, there are a number of very large, very early Mayan sites. So these sites listed here are all pre-classic sites. They date to the initial settlement can show up in the middle pre-classic, but really kind of had their boom in the late pre-classic. And the namesake site for the Mirador Basin is El Mirador. And it's really an incredible place. It's very large. This is an artist's reconstruction. Terry Rutledge drew this for National Geographic. It's kind of giving an idea of the scale. I should point out that Nakbe is about 12 kilometers away. Recently, there's been a big effort to fly LiDAR down in the Maya Lowlands. And it's just been an absolute game changer. It's incredible. It's basically, the last couple of decades, the total station mapping is, the further they'd push out into the forest, it was just settlement all the way. If people were not down in a marsh or in a wetland doing the mapping, there were structures everywhere. And so now the LiDAR is showing that it looks like El Mirador and Nakbe were likely connected. I mean, there are causeways that connect them, but now we're just seeing continuous settlement. And it's massive. And El Mirador has very large structures. This pyramid right here is called El Tigre. This one in the background is Ladonta. It's only about 70 meters high, but it's arguably the largest pyramid in the world in terms of cubic meters of fill. So 2,800,000 cubic meters of fill. You basically go up the first platform and walk another 10 minutes and then go up the second platform another five minutes. It's just, it's incredibly large. So just to kind of hit on that point some more. So Tigre, which I showed is the second largest pyramid at El Mirador. If you were to pick up El Tigre and drop it in downtown Tikal, it would basically cover the Great Plaza and the entire North Acropolis. So this is one pyramid at El Mirador for comparison with kind of the downtown region of this classic period site of Tikal. So again, here's Mirador and its heyday around 80, 100, late pre-classic. Everything's going fine and this is what El Mirador looks like today, right? So we've got Ladonta off in the background. Here's El Tigre, there's Los Monos, another pyramid there, which that kind of stark contrast kind of raises this question, which I think pretty much anyone going far back in time who's ever wandered into one of these sites asked themselves, what happened? What is going on here? What happened to the people who lived here? And so there are a number of different collapse theories. I have collapse in quotes because it implies, I don't know, a rapidity that I don't think we have constrained in terms of the rate at which places were abandoned. It's something that I've actually tried to do some work on and get at. But we do know there was widespread abandonment. There was widespread abandonment at the end of the classic period and it looks like there were abandonments prior to that at the end of the late classic, possibly before. And so I've kind of broken these collapse theories out into two main categories here. You can think of non-environmental ones. I do want to say this before I push forward. It's pretty easy to see that these are intertwined, right? Like we're not going to pull one of these out and say this is the singular cause or even the primary cause. We can look at warfare and that's going to be linked to collapse of trade routes. You can have warfare increased because of climate variability. There's an interwoven connection between all of these theories. But here we have them. These are kind of the major ideas around what may have caused this abandonment in the southern Maya Lolan. Warfare is a very prominent theory. There's a lot of evidence. Arthur Demarest has done a lot of work down in the Pitesh Batun region showing increased warfare, increased frequency, intensity of warfare during the terminal classic period. It's not entirely clear how much is inner polity versus revolt. There's some kind of scapegoat king models where the peasants may be rising up because they're unhappy with, you know, the king has pitched himself as having a connection to the gods and then, oh, there's a drought and people might overthrow the elites. But there is plenty of evidence for warfare. We have drawings of it. We have epigraphy. We have war statements. There's several war statements, burning, chopping, entering the cities. But other than that, there's evidence of fortifications. There's not a lot of firm archeological evidence for what warfare meant on the ground. And I'm going to loop back to this as I kind of close out the talk. Other theories here, collapse of trade routes, which again could be linked to warfare. A lot of the trade routes were likely on waterways. So again, if we have climatic variability, things are getting drier. That could feed into that overpopulation. I'm not even going to get into environmental change. So climate variability, drought. I'll be talking about this. There's kind of a prominent theory that multi-decadal droughts during the terminal classic period may have played a role in the trajectories down here. Environmental degradation, another one. And finally, disease is a compelling idea. There's very little evidence, no evidence for it. It's a mechanism that would explain widespread abandonment across the kind of geographic scales we're talking about. Again, some of these are easier to test, these hypotheses than others. And like I said, I'm working on lake sediment cores. They can get a lot of these environmental questions here in terms of climate variability, environmental change. And so if I kind of frame the research questions that I'm working under, one of them would be what's the occupational slash agricultural history of this region? So with the lake sediment cores, we have a lot of preservation of materials, of fossil materials, different chemical signatures that can do much to augment the archaeological record. You know, we're in the human tropics here. There's not a lot of cultural material preserved. We're able to, if we can link a sediment sequence to a site or to an area, it can actually add a lot, actually deeper in time. So I put this slash agricultural here because we're able to actually look at agricultural activity pretty clearly in the sediment record. And so we can look at settlement and abandonment through this lens of evidence of nearby agricultural history or activity. What were the environmental impacts of pre-Hispanic settlement? So again, looking at this notion of environmental degradation, what can we say about when we know people are here, what do we see happening on the landscape? What can we say about that? And then finally, looking at these drought hypotheses, is there any evidence for climate variability associated with periods where we see demographic decline or population transitions near the sites that we're working at? Okay, so here's the Yucatan Peninsula again. And so in order to answer those questions, what I've done is basically set out to create a network of sites across the southern Maya Lolan. So these are all published records. We have a handful of other cores in the coolers down in Menlo Park. And we also have other sites targeted for future work. But the idea here is, you know, the Maya Lolan's are a pretty heterogeneous environment. And by creating a network of sites, we're able to kind of parse out what might be a local signal from a more regional signal. So when we see kind of coherence in the transitions of these records across a bigger geographic space, we can say more about regional processes than local ones. And so these are all kind of acronyms for these lakes that we're working on. This is Lago Puerto Arturo. This is Lago Pashban. And Guadazacatol. Laguna Eknaub. And Laguna Ya Loach. And I've also highlighted some other very well-dated, good paleoclimate records, the locations of these sites on this map as well. So this is a speleothem from Sabna. Lake Chichan Kenab, which was kind of the original lake sediment core that pushed the drought hypothesis forward. Punta Laguna, another lake sediment core. All of these notably coming from the northern Maya Lolan's. And then we have a speleothem from down here in southern Belize, Yaqbalam, that has again a very well-dated sequence that we're going to look at as we move forward here. Okay, so in terms of doing the fieldwork, it's always exciting. A lot of you may have experiences like this, but we basically bring all our gear down to a small town, pile it all into the back of a pickup truck until it's overflowing, and then pile eight grown men into the back for a two to four hour ride on a dirt road, which is always quite exhilarating. Usually they're in the wet season, so we push our way through the mud. A lot more fun when you get to drive. This always cracks me up. So you break down out there. So we snapped our leaf springs. And these guys, it took us five minutes to be back on the road. So we went out in the jungle, carved off a stick, lifted the truck up, propped it in there, and we move on. You know, there's no stopping the train when you're that far out there. So the end of the road's usually a very small village. Get out here, in this case, it's Karmalita. And that's where phase two starts. We get all the equipment loaded onto mules and push off for another 40 mile walk through the forest. Again, it's the wet season, often basically wading through water in the low lying areas. But we're aiming for the field camp, which again, a lot of you guys probably have experienced with these types of field camps. They're pretty significant production. So in the case of Hanson's group out at Mirador, he's often hosting two to 300 people out there for two to three months. It's a big operation. It's very remote. And this is basically civilization when we're out there. We've got the lab set up, we've got the kitchen, our little camp zone for the summer. But as I mentioned, the lakes are kind of few and far between. So this is base camp for me. We basically put the gear back on the horses and then get the machetes out and push another day or three further out to get to the lakes. Carrying our coring platform, which I've named the Fitzcarral, though. Everything breaks down and fits on the mules. Get that constructed, get it out and anchor it into a lake. It's pretty common to find old mahogany dugout canoes tucked in the reeds, which we take advantage of if we can find them. And so then our coring itself is done with a Livingston piston core. So it's all hand-operated equipment. It's not mechanized. We always use plastic liner tubes to kind of maintain the integrity of the cores to get them out of the jungle and back to the lab. But we basically load the barrel up and just push it down into the mud. The clay's pretty stiff in patins, so sometimes it takes, you know, a fair amount of pushing to get a good 40-centimeter section. So, but at the end of the day, if all goes well, you've pushed through the entire sediment sequence. In this case, we have seven meters of mud. Here's the sediment-water interface showing nice and clear there. Seven meters down and then a replicate core that basically catches the junctions of each of these sections. So we have complete recovery going all the way down. Then we get them back to the lab and we do essentially as many different analyses as we can on the core. So a big one that we use, Christine mentioned that we have the pollen lab over in geography, is pollen analysis, reconstruct the vegetation, mostly in the watershed nearby the lake. We can enumerate charcoal fragments and reconstruct fire histories and see how that interplays with the vegetation history. We look at physical properties. We look at what the sediment is composed of. Like here we have these carbonate bands that are interbeted with organic rich material. We do these analyses that kind of parse out how much clay is in the core, how much organic material, magnetic susceptibilities of proxy for erosion. So it's a torriginous material coming into the lake has its own magnetic signature. We do stable isotope analysis on organic matter to look at what types of plants we're growing in and around the lake. We also do analyses on carbonate to look at hydroclimate variability in the past. And age control is provided by radiometric dating. So we do lead 210 on the surface sediments and a lot of radiocarbon dating. This is something that we've really been emphasizing and pushing forward a lot on the need for very high resolution and accurate age control and age depth modeling. So there's some nice statistical packages that have come out recently, Clam and Bacon and more recently one called Plum where you can put in your radiocarbon ages and get the distribution, the probabilities of an age really kind of constrained because you're able to add in some more variables in terms of where those samples came from. And then we always do resolution, digital photography and X-rays to just get a nice archive of the records of the sediment material. Okay, so I just want to say one thing about ZiaPollin before I push forward into the data. ZiaPollin is an incredibly important tool for us because of some characteristics that it has. One is once you move out of the highlands in Mexico and western Guatemala where Teosinte currently grows, we don't have any Zia growing in the wild down in the lowlands here in Guatemala. It's a completely domesticated plant, corn is anyway, and it has to be planted to grow. So it's not growing in the wild and importantly it's identifiable down to the species level. So most pollen taxa, unfortunately, especially in the tropics you can only get down to the family level which causes some lack of resolution in terms of our ability to interpret what those pollen types can tell us. And yet others can go down to the generic level but here we have this ability because Zia has been selected for so long the corn pollen itself is really dramatically different than other grass pollen and so there's a number of characteristics that allow us to identify it. It doesn't travel very far from the parent plant so all of those combined means that when you see ZiaPollin in the sediment we know we have a very clear indicator of nearby agriculture. People are there, they're planting corn and it's nearby. And so that's a very powerful tool that we've used in terms of trying to reconstruct as I mentioned earlier these agricultural and settlement histories. Okay so I'm just going to show this this lake here is I'm going to be showing a lot of data from Lago Porto Arturo. It's kind of a mid-sized lake, it's about one and a half square kilometers, this crescent shape of very deep water out in the middle of nowhere. I was a little alarmed to see the clandestine airstrip near it after I got out from the jungle but nonetheless it's got a very shallow the whole centered area likely a delta that formed through its history that's just dominated by emergent sedges and that's going to come into the story in a little bit. It also has an island on it that was just rife with classic period artifacts and tipped over stelae but this is a site that has a record that goes back about 8,700 years so we've got kind of a pre-settlement pre-agricultural run-up to the transitions that we see. Alright so turning to the data here what we've got here are the pollen curves I've got these two profiles here grouped together represent several of those families I mentioned that together represent high forest the forest taxa we have grasses weedy taxa the kinapods and asters mostly agricultural weeds anytime there's a white inlay that's ambrosia, ragweed which is a very common agricultural weed and over here we have the corn pollen now I'm starting this I truncated this so this record goes back as I mentioned to 8,700 years and what we see over and over again in this area is coming out of the glacial period probably around 11,000 years ago the forests moved in and dominated so from this point down we really have a domination of high forest in all of the records not that there's that many that actually go back through the entire Holocene but what we see is starting around 5,000 years ago we start seeing a decrease in forest taxa and there's a big debate as to whether that was driven by climate change things getting drier or the initial arrival of people and agriculturalists and that's a whole other seminar to dive into the data on that but what we can look at here is starting around 4,600 or so years ago we have our first corn pollen showing up in the record and along with that we start seeing at least with this group of families of forest species a decrease a long slow decrease heading up into the middle part of the early pre-classic and so this is kind of the initial evidence of folks moving into the area predating settlement at Knock Bay which is the oldest settlement we have up in the Mirador area and then starting shortly after 3,500 years ago we get really kind of more consistent presence of corn and we see this very stark and clear decrease in forest taxa through this period and it's coupled with this increase in grasses and increase in weedy taxa and this inverse relationship between agricultural weeds and evidence of an open environment, disturbed environment with closed canopy forest taxa is something we can think of as an ecological signature for settlement and conversely when it all reverses here we can see it as an ecological signature of abandonment and so what we've got here is basically a 2,500 year record of settlement around Lago Puerto Arturo one unknown right now with tropical palinology and palinology in general we don't really know how regional this record is, we don't know how far out from the lake we can actually extrapolate these data and say this is representative of Shulnaal which is one of the nearby sites or El Akiyote but what we do see here is that fitting quite well with the sequence of myocrinology we've got this period of a lot of disturbance through here within that we look at magnetic susceptibility as I mentioned is a proxy for erosion and we see kind of coherence with big peaks of magnetic susceptibility coming in where we see peaks of disturbance indicators in terms of the palinology and then also that's where we have the presence of the corn pollen and this is pretty early on this is around 3,000 years ago and so you know likely caused by well one major factor I think why we see so much more erosion early on so you notice here this is 2,000 BP this is a transition to the classic period and there's far less erosion happening and that's likely tied to the fact that when people first came into the area there was a lot of soil developed it hadn't really been disturbed by humans at least in a way that would really open up the forest and allow for it to erode down into the lake basins what was causing that opening we don't know a ton about agricultural strategies in the area in terms of swidden or slash and burn versus more intensive methods like terracing or raised or ditched fields especially that early on but there was likely some swidden agricultural happening so we've got opening of the forest possibly on the slopes around the lake leading to that I will say I mentioned that LiDAR earlier is again, blowing this wide open people have been looking for evidence of raised and ditched fields in the low-lying Bajos of the central part of the Yucatan Peninsula for decades and now it's showing up quite clearly canals across the Bajos terraces all over the place it's really really exciting time to be interested in prehistoric my agricultural strategies so this is one possible cause of that erosion we see a lot of disturbance forest opening the other thing I want to point out this is again Terry Rutledge's reconstruction of El Mirador but you'll notice the city is basically paved in lime plaster and that is not artistic license out at Mirador we have plaster floors that are decimeters thick in sequences coming up from the late pre-classic up through the late classic and that production of lime plaster had an impact on the environment in a way that is often overlooked Tom Schreiner who is actually an affiliate of RF at one point did his dissertation ethnographic research looking at quantifying the amount of fuel would needed to create calcium hydroxide to get the quick line so everything was weighed all on the input side and everything was weighed on the output side and the punchline from his dissertation was it something on the order 2.6 square kilometers would have had to have been clear cut to pave El Tigre one time and so it's unlikely they did that but if you think about the fuel would needed to pave El Tigre one time and then consider that it has about five coats of plaster on it and that's just one structure of hundreds and all of the water catchment plastering that was done throughout the cities the deforestation we see these kind of low forest values through here kind of come into more perspective regardless of what type of agricultural strategies that we were being utilized at the time okay so looking back at this sequence of these pollen profiles here we can also see what looks like pretty clear evidence of a decline in disturbance near the lake notably here at the end of the classic very abrupt this is the abandonment of the area permanent we see no more corn through the rest of the record no more weedy taxa grasses drop off precipitously I think there's about 160 years between that data point and that one so the forest bounce back amazingly quick we don't have the taxonomic resolution to know what sort of species difference there was from the pre-disturbance forest down here and what came back but I think I was pretty surprised to see this and this is replicated across most of these sites but anyways interestingly we have some compelling evidence for abandonment at the lake pre-classic transition which there's also archaeological evidence for at El Meridor a lot of those pre-classic sites were abandoned at the end of the late pre-classic you guys probably know more about this than I do but I understand there's a lot of cultural transitions at the end of the middle pre-classic into the late pre-classic in terms of pottery styles you know the onset of the notion of divine kingship and these types of things and so seeing what looks like another demographic decline and transition right around that switch from middle to late pre-classic as well is an interesting find so looking specifically at this classic to post-classic transition one of the things I've tried to do is to get a very good age control on that and try to constrain that ecological signature of settlement and abandonment across these sites and that's what we're looking at here so this red shaded area shows from these three sites basically the last corn pollen present at Iguata Zacotal up through the last one that I found at Laguna Yaloche and the idea is to try to get at how rapid this collapse was you know with the archaeological record we're looking at the last inscription that has happened to survive right and so that's fraught with obvious problems in terms of just preservation in terms of piecing together the sequence of abandonment through the myelolins I'm not sure I've done much better here we have about 140 year period radiocarbon has its own ranges and you can't really constrain it tightly but again we see that very clear decrease in grasses and increase in forest taxa coming right at the last appearance of zeo pollen in the record so again this is that kind of signature of abandonment or classic period abandonment in the area okay so turning to climate which has been a hot topic in the myelolins so in 1995 Dave Hodel who was at the University of Florida at the time published a paper in Nature got the cover of Nature from a sediment corridor that he worked up in the northern myelolins from Lake Chichang Kanab and it's an isotope based record also looking at other precipitates and densities of precipitates in the record of gypsum but we're going to just quickly kind of talk through how this isotope work the model behind looking at isotopes and carbonates for hydroclimate variability so the basic ideas here we got kind of three main isotopes of oxygen 16, 17 and 18 oh 16 is by far the most abundant 99.9 something percent so when we have a lot of precipitation and very little evaporation it's the water I should back up this is a very simple simple system there are a lot of variables that can make this model very noisy some of them are minimized by the fact that we're down in the tropics and we can assume relatively stable temperature and relatively stable moisture source but so when we have a closed basin system where evaporation is the prime mechanism for lowering lake level that's the idea behind this model that we're working on here so again we've got a lot of precipitation very low evaporation kind of wet climate lake levels high the ratio of oh 18, oh 16 is going to be very low so you have it's enriched in oh 16 so oh 16 is lighter so when we have evaporation that is evaporated out and so as lake levels drop through evaporation we see an increase or an enrichment of oh 18 and you can see the system is pushed even further here the beauty is we can look at any carbonate material has oxygen in it and that's a snapshot of what the lake water ratios of oxygen isotopes were at the time so we can look at fossil shell material and reconstruct hydro climate variability to a certain degree using these techniques so again Puerto Arturo we've got this 8700 year record and it shows this nice sequence of we know that because of the Earth-Sun relationships this is called the thermal maximum in the northern hemisphere this mid-holstein period and we would expect to see more moisture down here and that's exactly what we see so we see again we've got wetter conditions with the lighter values here and drier with the heavier values so we've got this whole period of more moisture than what follows and then starting around 6200 years ago we start seeing this trajectory towards drier conditions so we've got this decreasing moisture interestingly this is right when we see corn agricultural coming into the area through this whole period so the late Holocene is characterized by these relatively dry conditions so one of the nice things about using a multiproxy approach is from the same lake you can produce data sets to test your findings and that's exactly what we did here so I mentioned that that shallow delta in Lago Puerto Arturo that's currently dominated by sedges well what we've done here is ratioed the sedge pollen Cyperaceae with water lily pollen which water lily prefers deeper water 2-3 meters deep gets its root down through that and also relatively clear water and if we think of that as a lake level proxy as well as our delta 018 we can see that these actually track pretty well so we can essentially lean on this as a pretty good lake level reconstruction at least from Lago Puerto Arturo I'm going to put a really noisy diagram up here but I think it's worth talking through so this is that same curve here in red is the Lago Puerto Arturo isotope curve and what I've done is I've plotted it against a sea surface temperature reconstruction from the Gulf of Mexico where we have warm water penetrating in the loop current up into the Gulf of Mexico where again if we've got a big warm pool in the Gulf of Mexico we expect more precip more moisture in the air similarly in the eastern Atlantic sea surface temperature reconstruction warmer as wetter you get more basically just more loading of the atmosphere with water and so I'll talk about these two first so what we see here is really quite an amazing correlation neither of these age curves are tuned to the other this is based on the original age model output from the original data sets very tight correlation with Gulf of Mexico sea surface temperatures and eastern Atlantic SSTs and that all kind of plays into our understanding is synoptic climate drivers which is a big question with the Maya area what's driving climate variability so early to mid-holocene the Atlantic there's a very tight coupling between lake level Puerto Otero and sea surface temperatures across the tropical north Atlantic and what we start seeing is around 4,000 to 3,500 or so years ago as the ITCZ starts to move south I mentioned those Earth-Sun relationships with that thermal the ITCC is moving south and what looks like happens at this point is we start to see more inso-activity so and this is played out across records that I'm working on in the western U.S. and others in terms of reconstructing the history of El Niño it looks like it really began expressing itself strongly as both El Niños and La Niñas in the late Holocene so you know basically over the last 3,000 years and so what it looks like in terms of bringing it back to the mile world with the late pre-classic and classic period we have much more of a Pacific influence and therefore increased variability this inso variability is much stronger and higher frequency than climates from before so this is again kind of showing that around 3,500 years ago we've got this increased eastern tropical Pacific influence in terms of climate that's happening on the Yucatan Peninsula so if we want to look at the records of climate reconstructions we can compare across the entire Maya Lolan so this is the whole Yucatan Peninsula here so again this is Lago Pertoroturo this is only going back 3,000 years Yaloche another site that I worked on had a very distinct band of carbonate precipitating out Puta Laguna and Chichankinabra those lakes from the northern Yucatan Peninsula this is Dave Hodel and Jason Curtis's work that really kind of put these terminal classic droughts on the map Saab Nas is Martín Elizalde Peleo Speleothem reconstruction and then Yaak Balum and we can see that what started with this one record right here in 1995 there is indeed evidence of drier conditions through this terminal classic period from you know around 800 to 1000 AD the Speleothem's the age control in them are much better than the radiocarbon dated lake sediments and so these are really interesting it's interesting to see that you know this peak of dry conditions comes a little later than what we see elsewhere again we're paleoclimatologists we're really trying to rectify these to suss them out to see what these signals mean are they time synchronous are they reproducible you know notably at Pertoroturo there isn't a this isn't the biggest dry indicator here at the late classic period and to me that's telling of the variability just on the landscape with precip I also highlighted a couple other you know I mentioned that what looked like in the pollen record demographic shift right here at the late pre classic classic transition there's some evidence for drier conditions there there's archeological evidence as well up near Mirador and then also just this larger peak down here in the middle pre classic so we again we do have replicability in this term is terminal classic droughts how much they played a role in the abandonment is it's difficult to say one of the things that I've looked at is the persistence of corn pollen at the sites that I've worked at at least and the corn pollen is persistent through the peak of dry conditions that we see here you know up closer to you know 1,000 to you know 900 BP so people are there beyond these droughts I don't know if we can say that perhaps the droughts were causing issues you know there may have been social breakdown or you know socioeconomic changes and the corn pollen I'm seeing is just the last remnants of the last folks that hung around or perhaps you know things didn't really start breaking down until after the drought it's difficult to say it is of interest to note that following the drought in nearly all the signals is a very dramatic not so much here but shift to extremely wet conditions and so this is kind of where my mind is with this stuff right now is that this is when the corn pollen falls out is when you see this post multi-decadal droughts shifting to extremely wet conditions and it might just be too much variability right these increased variability that we see in the in the late Holocene that I was talking about so that's kind of where we are with climate so I'm going to dive into the world of Maya warfare here which is not something I know a ton about this is all relatively new but as we've been developing our most recent sediment core we have this finding that kind of led me down this path and what I'm understanding and what I'm being told by the archaeologists that I'm working with is that very little is known about the reality of pre-Hispanic Maya warfare right so we have drawings that depict it we know there are captives taken you know big school of thought that suggests that these were mostly you know small scale raids or they were mostly focused on the elites so most of the archaeological evidence that we have is tied to the royal compounds or very rapidly built fortifications around the royal compounds you know Arthur Demarest again has a lot of evidence from Cancun and around the Petesh Batun region showing massacres essentially of royals of elites all of them dating to the terminal classic you know post-800 AD and that's led him to develop this kind of this idea that increased warfare you know may have played a role in the abandonment we see that I think his model is mostly focused on as royal families grew and more and more princes were being anointed and they all each went and took their own little fiefdom that you know basically trade routes got tight resources got tight and warfare basically increased but other than that you know we've got epigraphic data we have war statements which I'm going to touch on but there's a handful from the classic period from the late classic period Poulouille is one which means burning another one is chopping another one is entering so there's a lot of epigraphic data that says on this date we entered that city and so we know that a battle took place but we really don't know much about what that means and so what we're going to look at here is a tale of two cities of Weitz-Nah and Naranjo and so I have been working at a lake near Weitz-Nah so a classic period site it sits up on top you can kind of see this one of these big escarpments you see these trending escarpments in the impotent so it sits on the top of one of those escarpments in fact I think Weitz-Nah means like mountain house or something it's an amazing view from up there you can just look out across this whole low-lying Bajo so there's about a hundred meter escarpment this red line here shows the watershed for Laguna Econab and Laguna Econab sits at the bottom of this very steep escarpment and so all these black dots are structures at Francisco Estrada Valley has mapped in so this is kind of the scenario we're looking at and one of the reasons we targeted this lake was because it sits right at the base of the cliff below Weitz-Nah and likely all of the sediment washing off that cliff is going to tell us the story of what happened up here in the city and so we got a seven meter core that was the one I showed you guys earlier in those plastic tubes and it turns out it has not surprisingly an incredibly high accumulation rate so this is how many centimeters per year accumulating you know kind of through the lake classic kind of peeking here in the lake classic a greater than a centimeter and a half per year and for comparison the range was 20 to 40 years per centimeter of sediment so that was the resolution we could look at here we're diving in into a much higher resolution accumulation rate and looking at the sediment itself what we're looking at here is influx of clay and influx of organic matter and carbon nitrogen isotope so both of these are what's washing into the lake in basically mass per unit area per unit time so it's grams per centimeter square per year and carbon nitrogen is a tool that we can use to look at what type of organic material is in the sediment so heavier values indicate more terriginous terrestrial plants and values of 10 and less indicate algal blooms in the lake right and so most lakes somewhere along that spectrum but what we can see is that during this classic period particularly during this peak of erosion that's coming in between 650 and 700 AD that we have tons of erosion coming in lots of activity there's tons of corn pollen kind of pouring through there and filling the lake up rapidly so I mentioned we do charcoal reconstructions and so this is our charcoal influx curve for the entire 1700 year record from ECNOB and what stands out is this one peak right here at 780 and it stands out not only because it's twice as large as any of the other peaks but the sediment itself was just loaded with massive pieces of charcoal unlike anything I've seen in any lake I've worked on 5 millimeters long you know this is from that level this is what another level looks like a typical level of charcoal or counting you know the entire cubic centimeter of sediment and you might have you know 50 or 80 or 100 charcoal fragments this is just loaded with charcoal and it's a 3 centimeter thick horizon of it so it's just this massive deposition of charcoal that we happen to find a seed in so we're able to get a direct radiocarbon date from 316 centimeters down and so if we just calibrate that radiocarbon date the median probability is basically 710 and when we plug it into our age model using CLAM again these kind of statistical age modeling tools I mentioned that really kind of they take the fact that you've got an age below and an age above and helps constrain where the line is going to go through the probability distribution CLAM kind of put this age at 693 so I'm looking at all these data here we have the abandonment 700 AD it's a little bit early for these terminal classic droughts what's happening here well Francisco and Alexandre Toko Binine who I mentioned is an epigrapher we're out in 2016 excavating wheat's gnaw and so they found two stelae and on stelae 2 there was a nice clear emblem glyph for wheat's gnaw so we now know what the ancient Maya called wheat's gnaw which was Balam Joel which I think means Jaguar Ear but what's exciting about that along with the stelae they have excavated most of wheat's gnaw at least the big monuments at wheat's gnaw and all of them show signs of intense burning so the royal palace Francisco notes is basically the plaster is burned off the walls it's the hottest fire he's seen inside a structure they were just out there this summer excavating as far as they could to the east they watched our up here all of it basically burned and not rebuilt so I understand there's some questions around ritual burning of structures when maybe another building phase is going to happen and I think that's something we're going to need to sort out but I think what we're looking at is likely the invasion of wheat's gnaw by Noranjo so I mentioned these war statements so it's not uncommon for there to be emblem glyphs and place names on stelae that nobody knows where they are because it's so rare to find an emblem glyph in the city or in the site that it's at so as soon as we had Balamjoal in our pocket Alex has done a ton of work here and he knew immediately that the king of Noranjo basically bragged about going in and burning wheat's gnaw on May 21st, 697 AD so now we have for the first time ever what we think is a picture of what Poulouie looks like so there's literature on this where people are saying well the burning, it was probably maybe they burned the royal palace maybe it was this kind of ritualistic they went in, it was a raid they got some captives instead what we think we're looking at is basically this is the charcoal curve up against that erosion influx, the clay influx and then there's an erosion point for the decline there so we start seeing things dropping off corn pollen is present pretty consistently for another hundred years but clearly there was a demographic shift things began to decline at 700 AD at this burn horizon people persisted until I think corn pollen falls out of the record completely around 1000 AD and so this is interesting evidence that perhaps these battles that we see these war statements for were more widespread, they involved more of the population and had broader impacts than elites on elites in these battles amongst the royal palaces I think the Maya have this long coming out of a long period it started with the peaceful forest dwellers and I think it's been a long time kind of shaking the notion that they were out there kind of doing something, you know so again this is a really exciting find we're kind of still putting the final touches on what we think it means what we've got what we think is the first very clear environmental evidence for the impacts of a discrete event and linking between the sediment record, the paleo record and the epigraphy so that's it, thank you oh thank you yep so I've had diatomists look for diatoms in these lakes but because of the extremely basic nature of the water they don't preserve well which is a real shame every time I get a new core I send samples off hoping that they will be present really they're really thin and not very useful, diagnostic clam is just the name for I don't know what it stands for if it's even an acronym but it's an age modeling software so you can basically input your raw radiocarbon dates with the depths that they're found in and clam will basically run several Monte Carlo iterations through the entire string of radiocarbon dates and the probability associated with them and you can have it go through 30,000 times and it will pick the most common path through the distributions of the radiocarbon dates bacon is another one that's very useful where you can tell it I have a stratigraphic break here I have no reason to believe the sedimentation rate is the same above and below this break so it allows some more freedom for how the line gets fit but it's really bringing us forward with pinning down as much as we can the error that's associated with radiocarbon dates when we create these age depth models because it's the single most important thing when you're looking at all of these changes if you're kind of floating in time it's frustrating so it's moving forward but to answer your question I'm not sure exactly what clam stands for yeah you know today is really great climate change is there any evidence information you see that man's activity has any causal effect on climate change or is it so minimal that it would be the climate change that's actually a great question and yes there is so what happens in this area when you have solar radiation close canopy tropical forest is very dark and it's just going to absorb a lot of heat and that heat is going to be you know convecting and raising the air above the forest and causing precipitation clear cut the forest or cut the forest I should say and have a lot of grasses come in you've changed that albedo you've changed the lightness of the surface and a lot of that radiation is bouncing away decreasing the heat the convection potential and yes people Ben Cook has modeled that he's at NASA out on the east coast attempted to quantify it so looking at if we have large areas of very open forest rather than what was there before how does that affect the precipitation and so he's worked out some models looking at a very clear decrease in precip associated with that changing albedo with the vegetation and it's a fascinating threat right so we see corn coming in as things are getting drier and so there's this kind of chicken or the egg thing you know Roger Byrne was really big on looking at you know there's a lot of talk about the droughts I pointed this out things get very wet after those droughts too much precip in this area is also a big problem for pre historic or pre-Hispanic agriculture you know you get a lot of mold you get pests you get more weeds if the dry season is shorter you might not get a chance to burn your slash you know and so this idea that you see corn coming in and really spreading out pretty rapidly throughout the Yucatan Peninsula around 5,000 years ago that's when we were coming out of that very moist thermal maximum but again it's this chicken or the egg thing was it people came in and they started opening the forest and then you know at least with our discrete records we're seeing that drying it's likely a combination of two of my feeling is the big you know the ITCZ moving south and those big changes in the synoptic patterns are really what drove it and I think the dryer conditions of the late Halcyon really kind of opened up and eased the ability to kind of penetrate into the forest yeah so do you used to show how long or is that very common update that it was yeah so corn it has to be planted and tended to grow it's if you've ever tried to grow it it's like it needs the red carpet treatment fertilizer weeding it watering it so if it's it's unlikely that corn just kind of continue to grow in the fields after the fields were abandoned to your point though there's a possibility that and this is another kind of unknown with palinology that corn pollen may be reworked so corn pollen could maybe be in the soils we this is where I'm at with pollen analysis in general how long does pollen live on the soil surface or not live how long can it persist on the soil surface and so some of that might be material washing into the lake after the abandonment may be bringing in some pollen but it's pretty unlikely you can actually see pollen's pretty fragile when it's not deposited and encased in the sediment of a lake and so it gets pretty beat up it gets abraded so if it does if it is reworked and old it has holes in it you know the fungus and stuff starts eating it but so to your question it's unlikely that it was kind of persisting on its own but there's a chance that some of the pollen signal might be earlier stuff coming in later so to speak yeah kind of a similar question how do you account for accretion of these sediments on the lake floor I guess how do you account for the travel of the sediments through the water column to their final location of deposition yeah another great question that's something we think about when we choose our coring location so if you think there's different types of morphologies of lakes but typically there's a deep point in the lake and that's called the depot center and more material is going to make it into that depot center if it gets deposited on the slopes hopefully relatively quickly after it was initially deposited and so you actually have a higher sedimentation rate in the deepest part of the lake because this is called sediment focusing happens that's what we go for because the higher the sedimentation rate the less years there are in a centimeter sample the higher temporal resolution we have but there are stream inputs that can be bringing charcoal charcoal can sit on the soils for thousands of years so charcoal has some pitfalls right so you could have a massive rainfall event if you're coring right by a stream input it could be bringing charcoal in from the you know it's been sitting around the soils for decades or centuries but yeah ideally and people do this when they're working on lakes that you can drive up to and hang out in camp for a couple weeks you can actually quantify that and study it and put sediment traps out hauling traps out and push it through a wet season and a dry season and see where the diatom blooms are and where do the little carbonate critters grow those are all variables that are best constrained kind of hard when I've got two weeks at the lake before my food runs out but yeah short answer is you know we basically you want to pick a site that's further away from direct stream input I shoot for closed basin lakes because that simplifies that the hydrologic model greatly and then looking for the deeper part of the lake so you can kind of get the thickest sediment package let's say thank you again alright