A TALE OF PREHISTORIC CLIMATE CHANGE

Florida, with an average elevation of six feet above sea level, tops the list of states at risk of flooding due to climate change. Over three-quarters of the Sunshine State’s twenty million residents live on or near its 1,350mile coastline, and the National Oceanic and Atmospheric Administration reports that sea levels are already rising more than a third of an inch a year. Even modest projections show Florida’s sea levels up to seventeen inches higher by 2030. While politicians and urban planners debate how to deal with this, archaeologists are looking to the past to see how its earliest inhabitants adapted to changing sea levels long before high-rises filled the streets of Miami.

The Lower Suwannee Archaeological Survey (LSAS) was launched in 2009 to investigate prehistoric sites along Florida’s northern Gulf Coast, fifty miles north of Tampa. Its goal, said project leader Ken Sassaman, an archaeologist with the University of Florida, is more than just recording and interpreting data from sites that may well be underwater in the near future. It’s also to use this information to help contemporary policy makers respond to climate change and its consequences. “In our lifetime, we could see half of lower Florida under water,” Sassaman said. “We’re trying to understand how people who lived through this sort of change anticipated different futures.”

The LSAS study area spans twenty-six miles of coastline, in the center of which is the mouth of the Suwannee River, the only major gulf-draining river in the state that hasn’t been altered by canals or dams. The study area encompasses 54,000 acres of federal land in two national wildlife refuges, Lower Suwannee and Cedar Keys. Together they protect dunes, salt marshes, and tidal creeks, with low islands and oyster reefs offshore.

The low-relief landscape created abundant and productive estuarine and intertidal habitats, which helped draw the first humans here in the late Holocene. It has also been sensitive to the slightest changes in sea level. Geological studies

University of Florida graduate students and members of the Friends of the Lower Suwannee National Wildlife Refuge conduct test excavations on the steep slope of the south ridge of Shell Mound in 2012.

In consultation with the Seminole Tribe of Florida, archaeologists from the University of Florida, the National Park Service, and the Florida Bureau of Archaeological Research teamed up in March of 2013 to salvage a 4,500-year-old cemetery of thirty-two individuals at McClamory Key.

have shown that, since the end of the Ice Age, ocean levels have risen by roughly 300 feet, shrinking the Florida peninsula by about half. Within the past 4,500 years, the rate of increase has slowed, and sea levels have stayed within five or six feet of the modern range. But as long as people have lived on the north Gulf Coast of Florida, they have had to deal with a shoreline that has changed over time, and for the most part they’ve done it by moving inland.

Many prehistoric settlements were in former coastal areas that are now underwater. Of the 112 documented archaeological sites, the LSAS team has done test excavations at twenty-five of them. AMS and radiocarbon assays from charcoal, nut shells, animal bones, and marine shells recovered from these sites have yielded dates between 2600 B.C. and A.D. 1300, with most concentrated in the first millennium A.D. Overall, Sassaman said, the sites have a history of centuries of relative stability punctuated by periods of abandonment and relocation, sometimes apparently as a result of climate events. The earliest inhabitants lived a subsistence existence in communities of a few dozen people that, when necessary, could move easily. Geological evidence shows a significant climate event around A.D. 1 – 200 caused a rapid sea level rise of one and one-half to three feet, enough to move the local shoreline up to two miles inland. Immediately thereafter, residents moved from the coast and coastal islands farther inland.

The Middle Woodland period (ca. A.D. 100 – 500) was a time of dramatic cultural change throughout the Southeast. Small groups came together and moved into larger and more permanent settlements with plazas and elaborate ceremonial and burial mounds. In the northern Gulf Coast of Florida, intensive “terraforming”—the construction of platform and burial mounds from sand and shell, some over six feet high, as well as straight and ring-shaped shell deposits up to 230 feet in diameter—took place, perhaps partly in response to the rising seas.

Half a dozen of these civic-ceremonial centers are located in or near the LSAS study area. The two largest, called Crystal River and Garden Patch, were both built around A.D. 100 – 500. Crystal River, twenty-five miles south of the river delta, was almost five miles inland. It probably had the largest population, Sassaman said, up to 150 people. Garden Patch, twelve miles north of the delta and about 1.2 miles inland, was the next largest, with perhaps as many as 100 residents. Every civic-ceremonial center was preceded by a cemetery

In 2014 and 2015, University of Florida field school students excavated test units on the top of Shell Mound to document the emplacement of oyster shells around A.D. 550 to modify the mound into the U-shaped configuration seen today.

that was already in use for hundreds of years. In all, almost thirty percent of the LSAS sites contained human remains, some of which are about 4,500 years old. Many had secondary burials, meaning human remains were dug up and moved.

“During the Late Archaic period, cemeteries were being removed to higher and drier land as sea level rose,” Sassaman said. Only then did the living relocate nearby. He sees this as clear evidence they were planning for the future, motivated by a variable climate. The new construction doesn’t seem random or contingent on available space. “It looked like they were being smart,” he said, “because building right on the coast would be really vulnerable. Instead of waiting for the crisis, the big storm surge, they’re getting out in front of it.”

Tom Pluckhahn of the University of South Florida, who directs work at Crystal River, said there’s no better explanation for why they moved inland. “There’s not much evidence for conflict, and there wasn’t much farming. Occam’s Razor says people are moving either because of sea level rise or increased frequency of storms, similar to what we’re seeing now.”

The civic-ceremonial centers were connected to places hundreds of miles away, as shown by the amount and diversity of artifacts found in cemeteries and mounds. Palmetto Mound has a nearly-ten-pound piece of galena, the biggest piece found in Florida, that came from Missouri, and large cutting tools made of greenstone that likely came from eastern Alabama. Pottery stamped with the same identifiable maker’s marks—carved wooden paddles printed into the clay—has turned up at Garden Patch, the Block-Sterns site in Tallahassee, and Kolomoki in Georgia. Over a dozen soapstone vessels from a cemetery at Bird Island, a site at the north end of the study area, were sourced over 300 miles north.

All but one of the centers in the study area were depopulated by A.D. 700, although bodies and grave goods were still being deposited in cemeteries as late as A.D. 1300. “We have a pretty good sense that settlement following the abandonment of centers was dispersed, small-scale, and mobile—what appears to be a return to earlier times,” said Sassaman. There is no archaeological consensus for why the centers were abandoned. The only exception was a site

called Roberts Island, which was terraformed into a civicceremonial centeraround A.D. 700 - 800.

Sassaman suspects that investing in large-scale construction and infrastructure was ultimately a liability in the long term. Even though it likely spurred innovations like mariculture, building the civic-ceremonial centers also anchored residents in place. At the same time, the ritual infrastructure and cemeteries undoubtedly made it more likely that residents would try to hold out in the face of rising sea levels. “It’s hard to walk away from so much symbolic capital,” he said. He also emphasized that the effects of rising sea levels extend beyond whether or not a site is underwater. “The estuarine ecosystem, navigability of waterways, and more are affected by changing water levels so a place may be abandoned not because it’s too wet or vulnerable to flooding, but because the last storm infilled a channel or impacted local oyster beds.”

The flexibility and resilience of a nomadic existence may have been critical to survival in an environment as variable as the northern Gulf Coast. “We are increasingly learning the lesson that once you’ve made significant infrastructure investment, the harder it is to adapt,” Pluckhahn said.

The presence of Hopewell artifacts also suggests the possibility that immigrants from the Midwest brought new ideas and beliefs that weren’t compatible with coastal existence over the long term. “What happens when you take a nature religion that’s developed in one environmental context and transfer it to one that’s radically different?”

Kenneth Sassaman (blue cap) and several researchers take core samples in Horseshoe Cove to document changing sea levels over the past 4,500 years.

A Civic-Ceremonial Anomaly

Not all of the new civic-ceremonial centers were located inland. On the shoreline about eight miles south of the river delta, the Shell Mound site centers on a C-shaped mound roughly 400 feet in diameter and about twenty-three feet above sea level. Part of the mound was built on top of one end of a large parabolic dune open to the southwest. Archaeologists estimate the mound contains the shells of up to 1.2 billion oysters, making it the largest shellwork in the study area. Evidence suggests that the site’s residents practiced oyster mariculture.

Shell Mound rose to prominence about A.D. 400. Sassaman thinks it was built on the shore to be close to a large cemetery called Palmetto Mound, which occupies an island a third of a mile west across a narrow channel. Palmetto Mound, which contains hundreds of burials, is the largest cemetery found from its time period in the lower Southeast, according to Sassaman. Some burials predate Shell Mound by at least 800 years.

Shell Mound’s residents dug hundreds of pits, some up to six feet deep and six feet across, that were backfilled with sand, pottery sherds, and vertebrate remains such as sea turtles. The pits were probably used for cooking during feasts, and Sassaman found that one large pit had an unusually high proportion of mullet bones, as well as the bones of at least eighteen water birds such as ducks, grebes,

These exotic items were found in a pit feature in Shell Mound: (clockwise from top) drilled shark teeth, a quartz flake, chert flakes, mica, a quartz crystal, and the premolar of a panther with ground roots (middle).

herons, egrets, and spoonbills. Half the samples were white ibis, and four of those were juveniles, likely from rookeries on islands to the south. They would have reached that age in June, around the summer solstice, when sea turtles would have been coming ashore to lay eggs. Taken together, Sassaman thinks this could be evidence of large social gatherings, perhaps ritual feasts at a spiritually significant time of year. —Julian Smith

Sassaman said. “Does it serve people well, make them more or less vulnerable? It could be a religious sensibility that does not take into account that change is inevitable, like earlier coastal people had.”

It’s a challenge to draw a direct parallel between thirdcentury coastal dwellers and the modern residents of Miami, according to Neill Wallis of the Florida Museum of Natural History, who directed work at the Garden Patch site. But it’s not impossible. “The main lesson to learn is planning for the future, anticipating the social requirements you’re going to need,” he said. “It’s pretty obvious that the inhabitants of the Lower Suwannee saw sea level change was part of life, not an unexpected event, and planned for it.”

The Intergovernmental Panel on Climate Change predicts that sea levels will rise eleven to twenty-four inches by 2100 even with aggressive emissions reductions. But the organization admits that this doesn’t factor in the collapse of the Greenland or West Antarctic ice sheets, since there’s simply no precedent for events of that magnitude. One key lesson for modern policy and planning experts, Sassaman said, is that the past 4,500 years of relative climate stability are not a good benchmark for the future. A better analog is 12,000-11,000 years ago, when sea level change was so dramatic it would have been obvious within as little as five to ten years. “It is not enough to look back to see forward,” he said. “We have to look back at conditions that parallel those projected for the future.”

Planners can design coastal communities and related infrastructure to take into account, and even take advantage of, changing sea levels. “We could plan for the inundation of coastal settlements with landscape modifications and set-asides that accommodate rising water,” he said. “The Dutch do this well, creating capacity in flooded areas for mariculture, transportation, and even recreation.” Sassaman acknowledged that the simplest solution—not building on the coast—isn’t going to happen. “We can’t give up on the coasts,” he said. “We can still have access, but maybe we don’t need to be right on the most vulnerable places, like those people in A.D. 200. I don’t want to sound like Chicken Little, but the water’s coming up, there’s no question. We’re going to keep seeing impacts like we saw these past months.”

Coastal archaeological sites are particularly vulnerable, as the team has witnessed firsthand. In 2017, Hurricane Irma

A winter storm overturned these oak trees on the shore of McClamory Key in December 2012. Four years later Hurricane Hermine stripped away up to a foot of the island’s seventh-century A.D. midden

spared the Lower Suwannee area, but in 2016, Hurricane Hermine brought a six-foot storm surge and eighty-miles-per-hour winds that scoured eight to twelve inches from McClamory Key, where the team had excavated a cemetery a few years earlier. Derrick Key, a nearby island, has vanished completely.

Sassaman still has long stretches of coastline to survey, and there are more sites to examine on offshore islands and at the mouth of the Suwannee River. The island town of Cedar Key, population 702, sits on the remains of another civic-ceremonial center. In the future he plans to deploy underwater archaeologists to look for evidence of near-shore settlements. He is also interested in how large a role astronomical cycles played in residents’ decision-making over time.

A graduate student is investigating what “traditional” subsistence looked like before the centers emerged. “My sense is that we will see little, if any, difference in before-andafter conditions, supporting the idea that the abandonment of civic-ceremonial centers cannot be reduced simply to a downturn in the ecological production capacity,” Sassaman said. “It may have been more of a sociopolitical problem— which, of course, is not mutually exclusive of environmental challenges.”

If there is a silver lining to the story of early life on the north Gulf Coast, Sassaman said, it may be that the same social networks that brought objects and beliefs from outside also gave people somewhere to go when things got difficult. “We think that when they left the coast, they already had well-established networks to draw on to relocate.”

Maybe we should be thinking about places like South Florida in the same way, he said, to help prepare for the inevitable. “We could start by analyzing their social networks. Let’s ask every person: who do you know that doesn’t live in Miami? What’s your relationship, and would they mind if you stayed with them?”

JULIAN SMITH is the author of Smokejumper: A Memoir by One of America’s Most Select Airborne Firefighters. He is a frequent contributor to American Archaeology.