Why Are Scientists Exploring Ancient Drowned Shorelines in the Gulf of Mexico?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until recently has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
The origin of the first people to inhabit North and South America has been a subject of controversy for decades. In 1927, archaeologists working near Folsom, New Mexico discovered a stone spearhead point embedded in the rib cage of an extinct bison. This discovery provided direct proof that humans and large extinct mammals co-existed for a time, and that humans had arrived in North America by the end of the Pleistocene epoch (about 11,000 years ago). Several years later, distinctive long spear points were discovered at an archeological site near Clovis, New Mexico, along with bones of prey dated to as much as 11,200 years ago.
During the years following discovery of sites at Folsom and Clovis, a theory developed that became widely accepted as fact, even though there was very little supporting evidence. This theory proposed that the first humans to enter North America were a small group of hunters that migrated from Asia by walking across a land bridge between Asia and North America about 13,500 years ago, passing through an inland ice-free corridor in western Canada. These pioneers, called “Clovis people,” carried thrusting spears tipped with specialized stone points that made them very successful hunters of the large mammals that inhabited North America. Their success allowed the first group to rapidly expand throughout North and South America, and, according to the theory, after approximately 1,000 years the Clovis people exterminated 33 genera in North America and more than 50 genera in South America. The key element of this theory is that the “Clovis people” were the first human inhabitants of North and South America; hence this theory has come to be known as the Clovis-First model.
There are several problems with the Clovis-First model. One problem is that the distinctive stone spear points that are a key part of the model have not been found in Siberia which is supposed to have been the point of departure for the pioneers. A related problem is that the stone points found in the United States appear to be older than points found in the far north. In addition, other stone projectile points, shaped differently than the Clovis points but just as old as the Clovis specimens, have been found on sites in the United States. Possibly the most serious problem for this theory is the discovery of sites in North and South America that are much older than the Clovis sites.
A further challenge to the theory is the proposed timing of the “Clovis migration.” When humans first arrived in Siberia about 32,000 years ago, there were ice-free corridors along the Pacific coast and inland east of the Canadian Rockies which provided a conduit to the Americas. By about 24,000 years ago these corridors were closed by ice. The coastal corridors were probably open again by 15,000 years ago, but the interior corridor did not re-open until 14,000 - 13,500 years ago. A paleoamerican settlement in southern Chile named Monte Verde was in existence 14,600 years ago, so it is unlikely that the humans who settled there had migrated via the inland corridor; but, they could have arrived via the coastal route.
Recently, molecular genetics has provided new insights into the origins of the first Americans, and archaeologists have studied new sites and re-visited others with new methods. Genetic evidence now suggests that members of a single population of modern humans (Homo sapiens) entered North America from Siberia sometime between 30,000 and 13,000 years ago. Most studies suggest that the migration occurred less than 22,000 years ago, and probably involved boats. Once they reached the Pacific Northwest they could have continued dispersing south along the coast to eventually reach Chile, as well as dispersing eastward along the margins of the continental ice sheets. Some of the oldest archaeological sites discovered in North America are in the eastern United States, and there are numerous quarry-campsites in the southeastern states. Quarries are locations where raw rock material was mined and processed to make tools. If a reliable source of water is nearby, residential sites are often found near the quarry. These observations may indicate that these sites were occupied longer than sites in other regions.
Coastal areas were almost certainly inhabited by early Americans, but are difficult to explore because the coastlines of 15,000 years ago are now under more than 300 feet of water! As the last ice age drew to a close, melting ice sheets caused a rapid rise in sea level just as the first Americans were entering the New World. Drowned settlements may contain well-preserved artifacts that can provide important new information about how the first Americans lived and when they arrived at various locations in North and South America. The eastern Gulf of Mexico in the vicinity of the Ocala Uplift Zone is particularly promising as a potential location for drowned coastal settlements because:
- There are numerous onshore archaeological sites along the same zone that are more than 13,000 years old;
- Clusters of similar sites have been identified along drowned shorelines in the northern Gulf of Mexico;
- Rivers that discharge into this part of the Gulf do not carry large quantities of sediment, so sites should not be deeply buried;
- The coastal shelf is gently sloped which would reduce the impact of waves and storms; and
- Rapid sea level rise would have reduced the impact of waves as coastal settlements were inundated.
When the first Americans arrived in Florida, sea level was much lower and there was more than twice as much dry land as exists today. The climate was considerably drier, and water was scarce. Not surprisingly, early American settlements that have been discovered in the state are almost always associated with reliable water supplies such as rivers and springs. These areas would also have been attractive to animals, increasing the likelihood that human hunters would be able to find food. Florida is sometimes nicknamed the “sinkhole state” because the limestone that underlies much of the state is gradually dissolved by acidic water (normal rainwater is slightly acidic) that creates underground caves. These caves sometimes collapse to form sinkholes. Sinkholes provide natural reservoirs for fresh water, and some of the most artifact-rich paleoamerican sites are located near these formations. This association means that archaeologists looking for early American settlements along drowned shorelines of Florida can look for sinkholes as indicators of promising sites.
The Northeastern Gulf of Mexico 2008 expedition focused on ancient river channels in the vicinity of the Florida Middle Grounds, which are now several hundred feet below the Gulf’s surface. Major accomplishments included:
- Locating and high resolution mapping of a large, essentially intact, and infilled ancient river channel several kilometers long east of the Florida Middle Grounds;
- Identifying additional stream and river channels, some with clearly visible deposits along the banks;
- Locating and identifying more than 100 sinkhole features filled with material in stratified layers that may contain human artifacts as well as plant and animal material from early American times; and
- Mapping nearly 10 square kilometers of intact, shallow-water, nearshore sand ripples/ridges adjacent to an area that would have been Florida’s shoreline when sea level was at it lowest point during the last glacial period.
The latter achievement is particularly important because it shows that the oldest shoreline is extraordinarily well preserved, as well as portions of later shorelines that the expedition was able to observe. Most interesting, though, are the infilled sinkholes with multiple layers of distinct strata. These sinkholes potentially are a treasure trove of new information about the late Pleistocene landscape and environment, as well as about the humans who lived there.
Key questions for the Exploring the Submerged New World 2009 expedition include:
- Is there evidence of human activity at sites identified in 2008 that is visible to diver and/or video examination?
- Does the previously identified ancient river channel form a “T” intersection with another river channel (three modern terrestrial sites where rivers form a “T” junction have extensive paleoamerican settlements with hundreds of thousands of artifacts)?
- Is the previously identified ancient river channel the former course of the modern Suwannee River?
- Are several “channel-like” features identified in 2008 tributaries of the ancient river channel (areas where major and minor tributaries converge also have a high potential as locations for paleoamerican settlement sites)?
Sonar: Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a transducer. The transducer may be mounted on the hull of a ship, or may be towed in a container called a towfish. If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. The system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and an experienced operator can use the strength of the echo to make inferences about some of the object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. This is a general description of “active sonar”. “Passive sonar” systems do not transmit sound pulses. Instead, they “listen” to sounds emitted from marine animals, ships, and other sources (click here for more information about sonar systems).
Side-scan Sonar: Side-scan sonar systems use transducers housed in a towfish, usually towed near the sea floor, to transmit sound pulses directed sideways, rather than straight down. Return echoes are continuously recorded and analyzed by a processing computer. These data are used to construct images of the sea floor made up of dark and light areas. These images can be used to locate seafloor features and possible obstructions to navigators, including shipwrecks (click here for more information). The Exploring the Submerged New World 2009 expedition used a Klein Associates model 3000 side-scan sonar system rated for a depth of 1,500 meters.
Sub-bottom Profiler: Sub-bottom profiling systems are used to study layers of sediment or rock under the seafloor. The basic principle is similar to sonar: a transducer sends a sound pulse vertically downward toward the seafloor, and a receiver records the return of the pulse after it has been reflected off the seafloor. Parts of the sound pulse will penetrate the seafloor and be reflected by different sub-bottom layers. Sub-bottom profiling systems use a stronger sound signal than echolocation and lower sound frequencies. The signal from some “chirp” profilers are able to penetrate more than 100 meters into the sea floor. The sound pulse is often sent from an airgun towed behind the ship. Some sub-bottom profilers use several hydrophones towed behind the ship (called a towed array or streamer) which detect the reflected sound signal when it reaches the surface. The time it takes the sound to return to the ship can be used to calculate the thickness of the layers in the seafloor and their position (sloped, level, etc). Sub-bottom profilers also give some information about the composition of the layers. The Exploring the Submerged New World 2009 expedition used a Klein Associates model 3310 Sub Bottom Profiler attached to the side-scan sonar towfish.
SCUBA – As anyone knows who has ever tried, it is impossible to breathe underwater through a snorkel, pipe, or hose that is more than a few feet long. This is because the surrounding water exerts pressure on a diver’s body that is equal to one atmosphere (about 14 pounds per square inch) for every 33 feet of depth. To breathe through a long snorkel underwater, the muscles we use to fill our lungs have to overcome this water pressure – and they are not designed to do that.
Conventional SCUBA technique overcomes this problem by using a device called a demand regulator (invented in 1943 by Jacques Cousteau and Emile Gagnan). A demand regulator supplies air at a pressure that matches the pressure in the water (ambient pressure), so there is no additional pressure for breathing muscles to overcome. A diver using this type of regulator breathes gas at higher pressures than at the surface. Instead of a normal pressure of one atmosphere, a diver at 33 feet is breathing air pressurized to two atmospheres; at 66 feet, the pressure is three atmospheres, and so on. But there are several problems with this system.
One problem is that if a diver takes a lungful of air at a depth of 66 feet and then ascends to 33 feet, the external pressure drops from 3 atmospheres to 2 atmospheres. Since the volume of a gas is inversely proportional to the pressure of the gas, as the pressure drops the volume will increase. So the air in the diver’s lungs would expand, and if the diver was holding his breath the expanding air could rupture his lungs. When this happens, air bubbles can enter the bloodstream causing a condition known as an air embolism. If the bubbles block important blood vessels, the result can be paralysis or death.
Another problem is that as the pressure of a gas increases, the solubility of that gas in a liquid increases as well (Henry’s law). So if a diver breathes air from a demand regulator at 66 feet for a while, her blood will contain twice the amount of dissolved gases from the air than it did at the surface. Again, the problem comes when the pressure is reduced. Anyone who has ever opened a can of soda knows what happens when you suddenly release the pressure on a liquid containing dissolved gas: bubbles form in the liquid. If the diver rapidly ascends from 66 feet, the dissolved gases in her blood may form bubbles, creating a problem that is somewhat similar to an air embolism in that critical blood vessel may become blocked. This condition is called decompression sickness or “the bends,” and was first seen in miners working in pressurized coal mines (it was also a problem for workers constructing the Brooklyn Bridge, who spent hours working underwater in pressurized iron boxes called caissons, so yet another name for the condition is “caissons disease”). Since air is about 70% nitrogen, more nitrogen is dissolved in the blood than other gases and the bubbles of decompression sickness are bubbles of nitrogen gas. Oxygen isn’t believed to be involved, since much of the oxygen dissolved in a diver’s blood is quickly bound by hemoglobin, and normal metabolism reduces blood oxygen concentration.
Divers avoid decompression sickness by closely monitoring their dive time and depth, since they both affect the amount of gas that dissolves in the blood. Decompression tables and dive computers show how long a diver may stay at a particular depth without having a high risk of decompression sickness. If they stay longer than this time, then they have to return to the surface in stages, stopping for a specific amount of time at shallower depths (“decompression stops”) to allow the nitrogen to diffuse out of their blood without forming bubbles. For more information on technical diving, visit the Cayman Islands Twilight Zone Expedition web page.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.