Getting to the core of geoarchaeology

GPS positioning palaeoenvironmental cores on Northumberland Bog © ARS Ltd 2024
Describing sediments in a geoarchaeological core © ARS Ltd 2024
Subcontractors providing windowless core samples from tracked rig © ARS Ltd 2024

What is sediment coring?

One of the most powerful ways of exploring the past on an archaeological site is through geoarchaeology. This uses an understanding of the physical environment to better understand the archaeology contained within it. And a key element of this approach is our ability to reconstruct past environments through a programme of what’s known as sediment coring.

There are different approaches to coring, from manual ‘push-pull’ systems through to tracked mechanical rigs. However, by extracting material from the ground in ‘cores’, they can all recover continuous records of sedimentation that contain archives of past environmental conditions through the physical and chemical properties of the sediments, as well as the microscopic remains of plants and animals.

The deeper the core, the further back in time the record goes, and by applying specialist analytical techniques we can uncover details of changing vegetation, land clearance, climate, water levels, and more, through time.

Coring allows us to reconstruct how the local landscape has changed, which helps put the archaeological record into wider environmental context.

How can geoarchaeological coring investigations help?

The aim of this technique is to recover samples of sediment. These are usually extracted as long, continuous cylinders of sediment just a few centimetres wide.

As we record what we found, we begin with a simple description of sediment colour, particle size (clay, silt, sand, pebbles, etc) and this gives us an initial insight into the physical processes that led to the deposition of the sediment – for example, a river system (alluvial), downslope movement (colluvium) or perhaps older glacial material. This helps to provide a picture of the landscape of the site as it evolved. We can even spot previous soils horizons that might warrant further examination for archaeological remains.

How coring works

Some of the best places for geoarchaeological coring occur along the valley floors, wetlands and bogs as these have continual supplies of sediments and biological material (fossils) that allows the record to accumulate in the first place.

The most common biological approach to palaeoenvironmental reconstruction, due to the common occurrence of their remains, is through plant fossils such as leaves, wood, twigs, nuts and berries which can survive in waterlogged conditions, as can the microscopic remains of spores and pollen. This is useful because plants are particularly sensitive to natural changes in climate as well as human-induced management of the landscape. Other biological remains include insects, molluscs, crustaceans and a range of single-celled organisms each of which requires the attention of specialists. Recent advances in DNA research even allow us to identify the presence of particular groups of plants and animals through the discovery of distinct molecules within the sediment (eDNA), even though there may be no physical remains.

To compliment the evidence from these biological remains, we are also able to assess changes in the chemical composition of the sediments (through hand-held X-Ray Fluorescence), which may relate to natural physical processes of erosion and deposition, but we can also recognise human activities such as metal working.

One of the most important aspects of any coring programme is the need to establish a chronological framework for the palaeoenvironmental record – that is, how old are these sediments? We can use carefully selected plant remains for carbon-14 dating.

Portable OSL dating – saving time and money in the field

At ARS we are pioneering the use of portable Optically Stimulated Luminescence (pOSL) equipment in the field to establish relative changes in sediment accumulation rates and help identify horizons for full OSL dating at specialist laboratories.

For full OSL dating to work, correct sampling and a detailed understanding of stratigraphy is critical. This means that archaeologists can often struggle to take samples in the field that produce useful dates. We’ve found that using pOSL is a fast and efficient way to establish whether sediments can be dated by OSL in the first place. It helps to discriminate between ‘natural’ sediments and those that have been humanly derived or displaced, while also identifying sedimentary layers that may not be visible to the human eye. So, using it prior to taking full OSL samples allows for more reliable and better targeted dates, while also saving money on samples that might otherwise fail.

Learn more about how this approach fits into our Landscape Prospection Service (LPS) here >

Learn more about paleoenvironmental analysis here >

Archaeological Research Services Ltd