Sub-bottom Sampling? A case for using systematic sampling for site prospection of submerged landscapes

By Chelsea Colwell-Pasch, Individual Associate Member

Concurrent with the rise in development of offshore renewable energy projects around the globe we are also seeing a rise in job opportunities for marine archaeologists as underwater cultural heritage (UCH) remains a major consideration (and hurtle) for these project types globally. A new focus on UCH and jobs for specialised archaeologists is very good, indeed, however the current ways of prospecting for UCH, while technologically advanced for decades, has a heavy focus/bias on super-bottom or proud anomalies (UCH above or partially above seabed). What of the UCH of submerged landscapes of recent debate and trending in publications globally? While submerged landscapes and their associated sites are not, by any means, new research nor is the threat to these sites. Underwater cables, oil rig infrastructure, scallop dredging and a litany of other types of underwater disturbances and developments have always been a threat. The difference is now more resources (archaeologists) are being provided in order to consider these sites prior to impact. Why does this matter, besides celebrating a win for UCH and new roles for our peers and colleagues? Because, while the UCH of submerged landscapes are being considered, yes, the methodologies used for site prospection are not the most suitable or appropriate for locating and delineating drowned terrestrial sites.
Enter systematic sampling.

Yes, the same systematic and standardized sampling we utilize for terrestrial prospection for sites in development projects and impact assessments. Based on predictive modelling typically, or regulatory requirements, probabilistic sampling is ideal for locations where very little information regarding the potential of an area is known (like underwater). Purposive testing is generally based on observed features or land forms and testing is based on ground-truthing said targets. Probabilistic or purposive sampling are both suitable reasons for sampling (better known as ‘testing’) a submerged landscape to identify UCH prior to impacts – no matter the sampling methodology, it’s the systematic and standardized SAMPLING for presence/absence of archaeological resources that I am insinuating is missing from the methodologies in use today. Sub-bottom sampling can be the same standard for UCH prospection, especially drowned sites, as it has been for terrestrial site prospection for over fifty years. The reason terrestrial archaeologists still use test pitting on a grid system to detect the presence/absence of material culture in a proposed development area (PDA) is due to two fundamental reasons: 1) it is considered best practice for most archaeological institutions, regulatory bodies, and cultural resource management (CRM) firms for site prospection and; 2) it WORKS!

Now I understand the problems with sampling, especially the tendency to over sample unnecessarily with undo costs and resources wasted by proponents and governments (our collective tax dollars!). I believe this happens when the data used to justify the sampling is incomplete, outdated, misinterpreted or just plain wrong. With the technology available today to map the seabed to a very high degree of accuracy, raise or drop the sea levels artificially on GIS, and map out and render (digitally) entire landscapes now resting offshore in over 30-meters (+) of ocean – the data, as they say, is good! At least it is, I argue, good enough to predict the potential for an area on the seabed to be a submerged landscape (there are tonnes of research and publications on submerged landscapes around the world – like Australia, Isreal, North America and Asia – and we have generally great maps of these areas already).

So…we have the areas of high-to-moderate potential identified. We have the development plans and necessary approvals for preliminary investigations, including prospection.

 What’s next? Diver survey? Coring? Side Scan? Geophysical investigation? All these methods have high data value and a place in UCH site prospection but how do we reach that same confidence interval systematic sampling provides for drowned sites? I know what you are thing…monitoring! Monitoring during construction is never a great way to discover an archaeological site. I doubt there is an archaeologist who will argue against that. I suggest that the application of systematic sub-bottom sampling is an appropriate and FEASIBLE means to ensure that sites are located and delineated prior to the negative impacts of any offshore developments on submerged landscapes with possible drowned archaeological sites. The same way we dig test pits on a grid on land to find a site, so can we underwater.

But how? Isn’t coring suitable enough? While coring is suitable to confirm a submerged landscape and guide areas of potential (looking at the drowned sediments and stratigraphy), it does not screen the volume of sediments sampled for material culture. What I am suggesting here is that we “dig test pits” on a grid placed on submerged landscapes underwater and screen the “test pit” sediments for the presence/absence of archaeological resources.

Ok, but still…..HOW?!

We have placed people on the moon surely, we can figure out how to systematically excise sediments on the seabed and screen/wash them for material culture. In New Brunswick (NB), Canada I have been using mechanical auger testing and screening (MATS) for years to sample areas of high-to-moderate potential that are too deep, too wet, too large, too contaminated, or have too tight a timeline to be sampled by shovel. I argue that a modified version of my MATS system can be used for offshore sub-bottom sampling. Alternatively, a large acrylic tube (sized based on regulations sized test pit for the project jurisdiction) that is implanted via Vibracore (or the like) then dredged out with a modified suction dredge (like those used in gold prospection) or hull-mounted archaeological dredge. The idea is that the material suctioned out would go into an appropriately sized screen and the finer sediments would get washed out and then the test pit ‘negative’ would be held in the acrylic tube to allow for, hopefully, recording of the stratigraphic profile of the test pit – just like on land. I have this exact set up in my offices garage currently (minus the Vibracore system) and I am just waiting for the perfect project and to be granted the necessary permissions to attempt sub-bottom sampling. The more expensive, higher volume mechanical, topside barge-fitted version (in a ‘rainy day’ file on my desktop) is a bit more of an investment of time, resources and, most importantly, regulatory buy in to make “the juice worth the squeeze”.

At the end of the day, our role is to ensure that UCH is recorded and not negatively impacted during development. There is no better way to be confident an area is clear of archaeological resources (in my humble CRM opinion) than systematic sampling especially for drowned sites not observed by divers or most remote sensing or geophysical surveys. So why are we not using proven, standardised methods for archaeological site prospection on landscapes that happen to be underwater as we do for their drier terrestrial counterparts?

 I absolutely think this juice is worth the squeeze. Should I get the ability to test my sub-bottom sampling apparatus (SSA) I will be sure to update everyone on another blog post. But for now, ask yourself if what we are currently doing and how we are doing it are the best ways to achieve our goals. In NB monitoring was so standard no one asked if it was the best alternative to finding sites if traditional shovel testing was not applicable or possible until MATS was piloted and is now a regulatory standard and alternative to “untestable, recommend monitoring”. Remember to not get stuck in status quo and tradition. We study the past; we need not be stuck in it as well. There are better ways, even if we must make them ourselves!

Categorised in: Deep Thoughts

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