QUATERNARY SEA LEVEL CHANGE in the CARIBBEAN: the IMPLICATIONS for ARCHAEOLOGY Sherri Baker Littman

QUATERNARY SEA LEVEL CHANGE IN THE CARIBBEAN: THE IMPLICATIONS FOR ARCHAEOLOGY Sherri Baker Littman

ABSTRACT Human habitation of previously subaerial land surfaces is well documented. Geophysical and geological meth ods have been applied to elucidate the presence of associated geomorphic features. Records from varved sediments of bottom, anoxic waters yield clues to ancient climate records stored at the bottom of the Cariaco Basin. A sea level curve based upon uranium series (230TH) for Acropora palmata corals off the island of Barbados provides high precision chronology of sea level change since the last glacial maximum. The prograding delta of the Orinoco River has tremendously impacted sites on the island of Trinidad. Tectonic settings, isostasy, coastal hydrodynamics, fluvial and sediment discharge and accumulation rates, along with biostratigraphy and lithostratigraphy are necessary to build a valid chronostratigraphy when examining once subaerial archaeological sites. These combined methods provide valuable clues to the "peopling" of the Caribbean and should be employed when searching for prehistoric sites and evaluating settlement models.

INTRODUCTION Evidence for Quaternary sea level change has been, at best, tenuous. Since scientific methods for determining sea level change are becoming more widely applied, this scenario is finally beginning to change. Studies show that although sea level has fluctuated eustatically, generalizations regarding specific regions, such as the Caribbean, cannot be made. Hence, this study will briefly examine the regional affects of sea level change on three areas within the Caribbean; the offshore area surrounding the island of Barbados, the Cariaco Basin (offshore Venezuela) and the Orinoco River delta and its affects on the island of Trinidad. Several models used in attempting to gauge sea level have been suc cessful, but must be applied on a regional basis. This paper is presented as a model or primer to be used for the purposes of determining the extent of sea level change and what the implications are for prehistoric archaeological sites. The most widely used methods that measure sea level change are varied, but combined, they can help build a valid picture for prehistoric environments. Examining coastal sediment accumula tion and erosion is crucial when measuring sea level change. Pollen sampling is another important tool. Marine fossil identification and corals can be used to help build a valid chronostratigraphy, much like dendrochronology. Corals and speleothems exhibit annual growth bands, thus providing calendar year chronologies. However, fossil corals can provide more specific data through C14 and U/TH230 dating. The combination of these methods allows for an accurate reading of sea level trans gressions and regressions. Additionally, geophysical methods, such as side scan sonar and sub bot tom profiling, can accurately detect prehistoric site locations. The above methods help to 'ground truth' or verify geophysical studies for site localities.

58 It is necessary that local environmental events be considered when predicting the location of archae ological sites, which is why regional variations on sea level change are crucial. Global geostrophic events affect gravity, force and pressure on the geoid, thereby resulting in regional variations such as hurricanes, eolian deposition, tectonics, isostasy, subsidence, fluvial sediment accumulation rates and varying ocean currents, such as ENSO (El Nino Southern Oscillation). Since coastal geomorphology throughout the Caribbean varies greatly, a detailed discussion of each regions geologic history is beyond the scope of this paper. Based upon what we do know, both geologically and archaeologically what types of environ mental changes occurred that affected island coastlines, resulting in paleoshorelines? What does the location of littoral archaeological sites tell us about sea level, and did sea level fluctuate enough dur ing the Quaternary to inundate prehistoric coastal habitation areas? As we can see from present-day settlement patterns, people have always migrated to coastlines. Certainly, during prehistoric periods, an abundance of marine fauna and proximity to waterways were key factors for settlement location. Recent geological and geophysical studies show that that indeed, prehistoric humans settled along now submerged paleo river channels and coastlines. Technically speaking, the late Pleistocene marked the movement of humans into the New World. Recent articles on New World migration suggest that the correlation of radiocarbon dates, cal ibrated into calendar years, pushes back earliest settlement date to 13,000 (cal BP). Scientists have retested atmospheric levels of radiocarbon since new evidence has been introduced into the study of paleoclimatology. This new evidence comes from pollen imbedded in Greenland ice cores, in addition to Caribbean corals and sediments from lakebeds that have aided in dating sea level changes. Late Quaternary climate was highly unstable and prone to large, rapid oscillations that occurred with in a few decades. This climatic variability was particularly pronounced during the last Ice Age when ice sheets, that covered northern latitudes, led to exposure of the continental shelves. Global climate variations, indicated by Milankovitch and Daansgard/Oescher (D/O) cycles, are primary causes of changes in the geosphere. Additionally, the Great Ocean Conveyor and thermohaline circulation of the North Atlantic cause variations in climate cycles. Milankovitch cycles, confirmed by Greenland ice cores, are gravitational pulls from planets, which have caused subtle changes in the orbit of the Earth. These alterations have resulted in differing intensities of sunlight that lead to dramatic variations in climate. Milankovitch cycles show oscillations in climate roughly every 20, 40 and lOOka and are con comitant with glacial and interstadial periods in geologic history. The more obvious intervals are those that occur approximately every lOOka, with the shorter oscillations occurring at 20 and 40ka intervals. Much like Milankovitch cycles that show variations in climate, D/O cycles show warm phas es (interstadials). These D/O cycles are punctuated by the cold conditions of the Last Glacial Maximum (LGM) and are demonstrated by their bistable behavior. The Great Ocean Conveyor and thermohaline circulation of the North Atlantic provide valuable data for D/O cycles through the analysis of marine organisms such as benthic formanifiera. These foraminifera show dextral and sinis tral coiling of benthic marine organisms (such as N. pachyderma), in response to warming and cool ing ocean temperature variations. D/O cycles show that warm, interstadial climates alternated

59 between cool stadial climatic episodes, and resulted in a severely cold Heinrich event approximately 7,200 years ago. These climatic changes do appear to be synchronous between Greenland (shown by ice cores) and the Cariaco Basin off the coast of Venezuela, where annual laminations or varves, can be preserved in both marine and lucustrine sediments. Marine varved chronologies are limited in that they usually only represent fragments of temperature change within the last 80 ka (for example, 9-15 ka ago in the Cariaco Basin). However, lacustrine environments are ideal for preservation for the length of duration of a lakebed. All of these cycles can be used as paleoenvironmental proxies that are vital when attempting to reconstruct and predict the locations of prehistoric archaeological site.

METHODS Geophysical methods that help to identify landscapes that were ideal for human settlement involve the use of high resolution acoustic reflection and side scan sonar. Seismic data from these are processed using standard geophysical methods to delineate hard ground surfaces and drainage sys tems that were subaerially exposed during the Pleistocene. Side scan sonar is an acoustic technique that records topographic images of the sea floor, thus creating sonogram mosaics. Sonar is a good device for mapping the topography of an area because boulders and rocks are good reflectors. A typical sonar frequency is 100 kHz. However, sonar leaves an acoustic shadow, hence, the need to use an additional method such as sub bottom profiling. This is an acoustic profil ing technique (seismic reflection) that is utilized to penetrate the sub-sea floor and then image geomorphic and geologic features. The technique relies on the contrast of the acoustic impedance of sub- bottom features. For example, sediment layering, outcrops and incised valleys are routinely detected with this method. Most sub-bottom profiling uses a very low frequency, such as 3.5 kHz. The lower the frequency, the better the penetration of the sea floor fades. A higher frequency could give overall accuracy, but would cause rapid attenuation, thus decreasing penetration due to considerable lag time in layers of clay. Basically, the shortest pulse length that does not sacrifice resolution is best. Subsurface investigations of areas presumed to contain cultural deposits can help to determine whether they are allogenic in nature. This is why 'ground truthing' is crucial to any seismic explo ration. Coring is the preferred method of 'ground truthing', whether coring is conducted on a terres trial landscape or a submerged one. Coring permits the recovery of significant volumes of sediment so that their character can be ascertained without alteration. In addition, coring can isolate erosional and ravinement surfaces that can help develop a relative sequence of sea level change. The affects of inundation of an archaeological site are influenced by factors such as depth of burial of a site, salinity and length of inundation. Sediment analyses of cores can be informative in dis cerning these factors. The use of elemental, mineralógica!, chemical, mechanical, pollen and inverte brate analyses of soils and sediments can help to discern archaeological deposits as well as geological origins. Elemental analysis can depict organic compounds, such as phosphates, calcium and sodium. Phosphates, shown to have a higher concentration in Anthrosols (human habitation sites), may still be present in inundated sites but can be significantly reduced by continuous inundation. Organic com pounds, calcium and sodium, may still be present but can also be reduced by a leaching out process. As sites become covered by water and then silt, decomposition of organic matter will deplete oxygen,

60 rendering a site anaerobic. Metals that are present, shown by XRF (x-ray fluorescence spectroscopy), serve as stable markers of human habitation areas. Under burial conditions, metals convert from oxide forms to insoluble sulfide forms, thus resisting further leaching, serving as good markers of human habitation areas. While samples show concentrations and results of these chemicals, few comparative studies of offshore sites have been conducted. Therefore, the paucity of data makes comparative analysis difficult. However, the combined results of these methods, can help to ascertain previously inhabited areas. Mechanical (textural analysis) illustrates percentages and size of sand, silt and clay and can help differentiate storm beds from beach deposits. Thus, erosional environments can be ascertained from depositional environments. Pollen analysis from sediments can decipher between warm, decid uous forests and cold, arboreal coniferous woods. Palynological studies have proven to be useful in paleoenvironmental reconstructions since they can serve as markers to climate changes. Additionally, marine invertebrates vary in habitation environments. Some thrive on an abundant amount of salin ity while others thrive in deep-sea environments. Analyses of invertebrate species is important, par ticularly when examining sea level changes since they are easily identified, thus, lending clear guide lines for estuarine/marsh/deep sea environments.

CASE STUDIES OF SEA LEVEL VARIATIONS Evidence for the last glacial maximum (LGM), during the late Pleistocene (20ka) shows that mean sea levels were -120 m below sea level (bsl) stands of the present. Eustatic curves that have been recent ly developed for Barbados, come closest to representing late Wisconsinin sea level changes. These published curves, based on TH230 and C14 dating of Acropora palmata from Barbados coral reefs, has provided high precision chronology of this sea level change. Based upon these dating techniques, con clusive evidence shows that glacial melting at the end of the Pleistocene did not occur all at once. The sea level curves are useful in assessing regional sea levels up to roughly 7,000 BP. The location of Barbados in the western tropical Atlantic minimizes the effects on sea level from changes in the con figuration of the geoid. As Wisconsinin ice sheets disintegrated, the changing gravitational attraction of the ice sheets would have resulted in changes in configuration of the geoid, the result of atmospher ic changes. The island of Trinidad has been experiencing an apparent surge of rising sea level on the southern side of the island. At the same time, it has been noted that sea level appears to be lowering on the northern side of the island. Not only have residents become aware of these changes and dif ferences. It has been the subject of research by climatology faculty at the University of Toronto and by archaeologists working on the island. At the Quemada Point site, on the southern-most point of the island, artifacts from archaeological sites are being exposed by rising water levels. It was at this loca tion that artifacts, most likely originating in South America, were found by this author. Numerous pottery sherds, along with a serpentine celt, were excavated here. The fact that they are eroding out of the high tide line, combined with the statement from landowners that sea level has been encroach ing on private property, provides evidence that sea level has dramatically been rising. In contrast to this scenario, researchers have noted that sea level is having the opposite affect on the northern part of the island.

61 Since Trinidad is an extension of the South American mainland where the Orinoco River Delta is locat ed, it is entirely probable that sea level transgression and regression is a factor of subsidence. The Quemada Point site is located 11 mi from the coast of South America. The water that is surging onto Quemada Point is purely sediment filled. It is clear that the prograding delta from the Orinoco is pro ducing this sediment, as it spills billions of tons into the surrounding sea. With this amount of sedi ment discharge, it is likely that subsidence is occurring, thus causing local sea levels to rise. When subsidence of this magnitude occurs, the result is the creation of the opposite affect in the northern part of the island, tilting it upward to appear that sea level is lowering. The weight of the sediment discharge is such that it is creating a seesaw affect of the island. The Cariaco Basin, off the northeastern coast of Venezuela, has recently been the attention of scientists trying to explain paleoclimate. The Cariaco Basin is 1,400 m deep, 160 km long, 70 km wide, and lies on the Venezuelan Continental Shelf. This is an anoxic basin that has recorded climate changes over several dozen millennia, within its sediment layers. A joint U.S.-Venezuelan research effort has provided evidence that links sediment records with surfacial ocean processes. Additionally, this basin shows seasonal variations in hydrographie properties. This basin is bounded by a sill that connects Margarita Island to Cobo Cadera. Because the sill restricts water motion and the lateral flux of material below 140 m, the basin forms a natural sediment trap. Varved sediments that accumulate within the bottom, anoxic waters, have provided detailed records of seasonal change over several dozen millennia. Hydrographie parameters, exhibited by surface salinity changes show a strong ENSO connection. Wind speed has a direct correlation with seasonal cycles of ENSO and can be meas ured by salinity isopleths. For the Caribbean Sea, it is possible that vertical displacement of the Subtropical Underwater mass that enters the southern Caribbean is controlled by geostrophic flow through the basin. These variations may be related to small amplitude fluctuations in the meridional component of the wind. Since the Cariaco Basin serves as a trap for sediment, it provides abundant data on phyto- plankton that inhabit the region. In addition, chlorophyll concentrations that provide clues to season ality can be measured. Thus, surface transport of these organics, along with sediments, has provided researchers with data that tells us much about settling velocities in this basin. Hence, clues to the bur ial of organic-rich archaeological sites can be deciphered if this approach is used.

SUMMARY In short, late Quaternary sedimentation mostly occurred in environments similar to those observed today. Paleoclimate records and geologic data show chronology in the form of sea level curves for the Quaternary. Barrier islands, sandy beaches, fluvial dynamics and marine facies that result in paleoshorelines, all provide the evidence necessary for detecting not only submerged archaeological sites, but also for helping us to reconstruct settlement patterns. These sea level curves show us that a surge of glacial melt waters occurred about 12ka (BP). At this time period, sea level was -60m msl (mean sea level), before reaching its current stand. Since déglaciation began ca 14,000 BP, causing rising sea levels from their full glacial maximum stands, vast areas of the continental shelves that were exposed, were drowned by sea transgressions. This was fol-

62 lowed by a pause in melting and then another meltwater surge approximately 9ka BR These contin ual vs episodic rates in sea level change are the results of a catastrophic ice sheet decoupling that resulted in a eustatic sea level change. This is termed 'rapid overstep' and has proven to provide a clear, geologic record that is important for understanding the expansion of peoples across the earth. Sea levels and shorelines stabilized by about 4,500-5,000 yrs BP (during the Ceramic/Archaic period), allowing coastal habitation to occur adjacent to coastlines and rivers. Thus, human occupation areas that were along coastlines at that time are now submerged. We can see that research in reconstructing paleoenvironments, using non-invasive modern technology, gives us answers that are not only important in historic terms, but also gives us valuable information on regional trends in the sediment process of submerged archaeological sites. It is the hope that geomorphic and geophysical studies will continue to add information that will assist in painting a picture of not only when human occupation of the Caribbean began, but also what the envi ronment was like. Within the sediments lies buried the evidence that documents the great event of the expansion of the human species into the colonization of the Americas.

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