A collapsed mine at Chain Valley Bay has featured in an international research project to investigate the capacity of saltmarshes to store carbon dioxide.
Researchers from seven institutions analysed carbon stored in more than 300 saltmarshes across six continents, collecting data going back 6000 years, and found that saltmarshes subject to sea level rise stored two to four times as much carbon dioxide in the top layers of sediment, and five to nine times as much in lower levels.
The University of Wollongong team analysed sediment from a saltmarsh at Chain Valley Bay where the collapse of a mine shaft in the 1980s resulted in the shoreline subsiding by a metre in a matter of months, effectively simulating rapid sea-level rise.
“Our paper shows that carbon storage by coastal wetlands is explicitly linked to sea-level rise,” Associate Professor Kerrylee Rogers, from the University of Wollongong's School of Earth, Atmospheric and Life Sciences, said
“Saltmarshes on coastlines subject to sea-level rise had, on average, two-to-four times more carbon in the top 20cm of sediment, and five-to-nine times more carbon in the lower 50-100cm of sediment, compared to saltmarshes on coastlines where sea level was more stable over the same period.”
Sea-level rise leads to increased carbon sequestration due to what the researchers call “accommodation space” – the area available for a wetland to store mineral and organic sediments.
In coastal wetlands, the boundaries of the accommodation space align with the upper half of the intertidal zone, roughly between the mean sea level and the high-tide level.
As the sea level rises, so does the mean sea level and the high-tide mark, increasing the accommodation space in which carbon can be stored.
“Capture and storage of carbon by saltmarshes has a dual benefit. It removes CO2 from the atmosphere, mitigating greenhouse gas emissions, and the organic carbon that is accumulated builds the elevation of the wetland as sea levels rise,” Professor Rogers said.
“Just like coral reefs, coastal wetlands can keep pace with low to moderate rates of sea-level rise, though are vulnerable to drowning under the high rates of sea-level rise projected by the Intergovernmental Panel on Climate Change.”
The researchers compared carbon storage in saltmarshes in Europe and North America, which have experienced a consistent rise in relative sea levels over the past 6000 years, to those in the southern hemisphere, where relative sea levels have been comparatively stable.
Those in Europe and North America have deep and highly organic sediments, while those on southern hemisphere coastlines have less organic carbon and a higher proportion of minerals in their sediments.
Co-author Dr Patrick Megonigal, a Senior Scientist at the Smithsonian Environmental Research Center, said, “Scientists know a fair amount about the carbon stored in our local tidal wetlands, but we didn’t have enough data to see global patterns."
"By synthesizing the local data, we discovered that soil carbon sequestration is linked to patterns of sea level rise at continental and global scales.”
The site of the collapsed mine at Chain Valley Bay at the southern end of Lake Macquarie provided an ideal opportunity to test the hypothesis by analysing sediment from a saltmarsh in an area where rapid sea level rise had occurred.
They found the relative sea-level rise led to a fourfold increase in organic material in the sediment, much of it carbon.
Relative sea level refers to the position of the sea relative to land, as opposed to eustatic sea level, which refers to the volume of water within oceans. If land next to the sea rises or falls then the relative sea level will change even if the eustatic sea level remains the same.
The differences in relative sea level rises between the hemispheres over the past 6000 years is due to the melting of vast ice sheets that covered North America and Eurasia during the last glacial period and the resulting flexure of the Earth’s mantle as the weight of the ice lifted. Continents in the southern hemisphere were less affected by ice sheet loads.
“Saltmarshes on the tectonically stable coastlines of Australia, China and South America may be the sleeping giants of global carbon sequestration,” Professor Rogers said.
The study was published in the March 7 edition of the prestigious science journal Nature.