A salt marsh in the Outer Hebrides provides an “almost unbroken record” of sea level changes over the last 2,500 years, scientists have said.
Researchers studying the marsh at Gress on the Isle of Lewis had originally been searching for deposits left by the Storegga tsunami, a massive wave triggered by an underwater landslide off Norway around 8,200 years ago.
Instead they were surprised to uncover 2,500 years of geological history preserved less than two metres beneath the surface, recording how sea levels have changed since around 500 BC.
They said the findings provide one of the most detailed records yet of how Scotland’s coastlines have responded to shifting climates and changes in the elevation of land since the end of the last Ice Age.
Dr Uisdean Nicholson, a sedimentary geologist from Heriot-Watt University’s School of Energy, Geoscience, Infrastructure and Society, said: “Salt marshes are incredibly sensitive to changes in sea level, so they preserve a detailed history of how coastlines evolve over time.
“And this one at Gress turned out to hold an almost unbroken record of the past 2,500 years.
“It’s amazing to get a snapshot of what the environment was like so far back in time, when the Persian empire was at its greatest extent and the Roman Republic was just getting started.”
The team took sediment cores from the marsh, which is located on the east coast of Lewis, and examined the microscopic algae and ancient plant remains layer by layer.
They said the analysis indicates the relative sea level at Gress has risen by around 1.2 metres over the past 2,500 years, with the rate of rise beginning to accelerate slightly around AD 950.
Professor Natasha Barlow, a coastal consultant at engineering consulting firm Haskoning, said, “Our results show that there is little or no ongoing land uplift following the melting of the British Irish ice sheet around 20,000 years ago on Lewis.
“Rather, sea level rise over the last 2,000 years is due to ongoing ice melt and warming of the ocean.”
Professor Sue Dawson, an expert on geohazards from Geography at the University of Dundee, said: “This painstaking work gives us an understanding of the way the rising and falling sea level has influenced the vegetation and the algae that grow and live in our coastal environments, which helps to unlock the sea level story”.
The team also said the research fills a gap in north-west Scotland’s sea level data, and said it will help refine future predictions for the region’s coastline.
Dr Nicholson said: “There are many other salt marshes across the Hebrides and around the Scottish coast that have barely been studied.
“They’re extraordinary natural archives, and there’s still a chance one of them is old enough to have preserved evidence of the Storegga tsunami.”
The team included researchers from the University of Leeds and Nanyang Technological University in Singapore.
The findings are published in journal The Holocene.
