Geoarchaeological and archaeological research

Map of Babeldaob with the location of investigated earthworks
Map of Babeldaob with the location of investigated earthworks C. Hartl-Reiter

To shed light onto the above-mentioned research questions, an interdisciplinary project funded by the German Research Foundation was started in January 2019. It is a cooperation between the Institute for Ecosystem Research at Christian-Albrechts-University and the Commission for Non-European Archeology of the German Archaeological Institute. The aim is to use geoarchaeological and archaeological methods to investigate the Babeldaob earthworks and to close some of the existing research gaps. For this purpose, a total of 14 earthworks on Babeldaob were selected, which differ in their size, gestalt, and topographical location. The aim was to get the broadest possible picture of the Babeldaob earthworks. After the documentation of the earthworks and the investigation of a large number of test pits in select locations, the next step involves surface excavations in predetermined spots. The main objective is to investigate the function and use of probable special purpose areas.

Digital documentation
The first step in the investigation was the exact documentation of the respective earthworks. For this purpose, two documentation campaigns were carried out in January 2019 and September 2019. The earthworks were mapped photogrammetrically with a drone to document the exact location, size, shape and gestalt. For this, we used the structure-from-motion method, in which overlapping aerial images are digitally generated into 3D models. The result is georeferenced orthophotos and digital elevation models with high resolution and accuracy.

The orthophotos can then be used to read back three-dimensional coordinates at any point on the earthwork and to make digital transects and profiles. Orthophotos (PDF, 15 MB)

Drone flight
Drone flight A. Kühlem

The digital elevation models capture even small details such as shallow depressions or elevations in the relief. The color-coded models reveal that in many cases the earthworks are significantly larger and more complex than can be seen with the naked eye. Digital elevation models (PDF, 20 MB)
In addition to the scientific documentation, drone videos of the various earthworks were recorded. These serve to illustrate the monumentality of the features in the landscape Video (Right click, open link in new window). The earthworks are so extensive and complex that it is impossible to see them in their entirety from the ground. The video recordings show just how imposing and diverse the complexes are. This is especially important for the involvement of the local population, as the earthworks of Babeldaob are endangered by modern building projects and agriculture.

Test pits
After evaluating the digital elevation models, the locations for the test pits were determined on all selected earthworks. In addition, there were a few profiles where parts of the earthworks were cut during road construction.

Drone flight with our Palauan cooperation partners
Drone flight with our Palauan cooperation partners D. Schäffler

A total of 37 test pits measuring 1 m x 2 m, and seven profiles were excavated and documented on 10 of the selected earthworks. For this purpose, the areas with the highest probability of original surface preservation were selected. Our initial assumption was that after centuries of erosion, most of the original surfaces had been washed down the slopes. It quickly turned out that this was not the case: the detectable erosion and deposition of eroded material was within the margins of a few centimeters at most. The Palauan builders had created monuments of remarkable stability.

The test pits were usually about 1 m deep, the maximum depth was just over 2 m. In all but two cases we were still within the terrace fill at that level. In all test pits the matrix down to the bottom of the excavated layers was mixed with charcoal and ceramic shards, attesting to the anthropogenic nature of the fill. The large number of ceramic fragments within the terrace fill and on the surfaces is remarkable. It became clear that the form of the earthworks is a product of applied material. Humans once removed the soft saprolite and crushed it. Then they reapplied it in several layers, whereby the original surface relief was changed significantly. In the formerly hilly terrain, not only horizontal surfaces and terraces hillsides were created, but also crowns with steep slopes and so-called knols, hemispherical structures. The crowns and knols are in many cases surrounded by encircling trenches dug into the soft saprolite. In some cases, these trenches show multi-phase developments.

 

Test trench at the Ngermedangeb earthwork in Ngatpang
Test trench at the Ngermedangeb earthwork in Ngatpang A. Kühlem
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Flattened ceramic vessel in situ
Flattened ceramic vessel in situA. Kühlem

 
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Saprolite bedrock (detail)
Saprolite bedrock (detail) A. Kühlem

 

In the upper areas of the test pits, fertile garden soils and a large number of planting pits were found. The analysis of the sediment samples from these contexts will provide information about the type of plants that were once cultivated here. Evidence for the origin of the fertile soils and how the soil quality was optimized for cultivation is also subject to laboratory analyzes.

Transect through the encircling trench at the base of the crown. Ngerbuns el Bad earthwork, Aimeliik
Transect through the encircling trench at the base of the crown. Ngerbuns el Bad earthwork, Aimeliik H.-R. Bork

 

Sampling
The analysis of soil samples is paramount in being able to answer the above-mentioned research questions, especially questions about the function of the earthworks.

Grain size composition
The first set of surface sediment samples were taken as part of the initial survey. The aim was to determine the composition of the grain sizes.
All samples were free of lime, as was to be expected with the high rainfall rates in Palau. The clay contents of the samples vary between 10% and almost 30% and thus have properties similar to loess soils. The silt content lies between 50% and 85%. This means that the soils have good water storage capacities and are therefore suitable for the cultivation of crops. Whether the earthworks were really used for horticultural purposes and what types of plants were involved can be demonstrated by the occurrence of plant microfossils.

Microfossils
To clarify the possible function of the earthworks for horticultural purposes, the soil samples taken are examined for phytoliths and starch residues. Phytoliths are microscopic silica crystals that are found in many plants. They take on very diagnostic, plant-specific shapes that can be compared to reference collections. These crystals, just like the starch molecules of crops, are preserved in the sediment and can provide evidence of cultivated plants centuries after the earthworks were abandoned. The samples from the planting pits are of particular interest here.

Human deposited loam with ceramics and charcoal in the matrix
Human deposited loam with ceramics and charcoal in the matrix A. Kühlem
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Planting pit in the profile of one of the test trenches
Planting pit in the profile of one of the test trenches
A. Kühlem
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Microscopic image of a banana phytolith
Microscopic image of a banana phytolith
M. Tromp, A. Kühlem

Dating
A total of 82 charcoal samples for radiocarbon dating were taken in the various test pits. The aim is to relate the earthworks examined by us to the existing chronologies and to investigate whether certain earthwork types or certain uses can be assigned to a certain phase. Dating using optical luminescence (OSL) is also planned for future work.

Area excavations
Area excavations are planned for the next phase of the project. After evaluating the results from the documentation campaigns and the test pits, lager surface trenches will be excavated in particularly interesting areas. The focus here is to investigate the use and function of the earthworks. A few oral traditions report, for example, that the crowns were used for signal fires, that elites were buried in the knols, or that gods and demigods resided on the highest points of the island. The surface excavations will show possible evidence for ritual uses and diachronic changes.

 

Introduction
Challenges of an Island Ecosystem
The human factor
The monumental earthworks of Babeldaob
The first contact with the outside world
Brief history and state of research
Open questions
Geoarchaeological and archaeological research
Involvement of the local population
Project data