Tag: saltmarsh

Comparing loss-on-ignition, acid decarbonation and combustion elemental analysis for measuring organic carbon concentrations – recommendations for best practice

After testing various lab protocols we have a strong preference for coupling accurate and precise instrumental measurements with combustion pre-treatments to isolate and measure organic carbon. This work was published in Organic Geochemistry.

Quick and cost-effective measurements of organic carbon concentration are fundamental for a variety of environmental projects, but not all methods produce reliable results. Poor quality baseline data could compromise an entire study.

Measuring the total amount of carbon in a sediment sample is usually carried out by an elemental analyser, which makes quick, accurate measurements of carbon and nitrogen. However, sediments can contain carbon in a range of forms – plant matter, mineral-bound organics, microbial biomass, shells, carbonate minerals, and more. Sometimes distinguishing all these sources might be criticial, but often the split between organic and inorganic carbon is sufficient, and important since the organic and inorganic carbon cycles are very different.

For this study, we investigated three ways of measuring the amount of organic and inorganic carbon in a sediment, with a focus on young materials such as saltmarsh carbon.

  1. Loss-on-Ignition: Burning the samples a fixed temperatures to combust the organic matter (at about 500 °C) and decompose inorganic carbon (at 800 – 900 °C). Use the change in mass to estimate the amont of carbon present.
  2. Acid digestion followed by elemental analysis: Removing the inorganic carbon by reacing with acid, and then measuring the remaining organic carbon.
  3. Combustion followed by elemental analysis: Remove organic carbon in the furnace, measuring the remaining inorganic C, calculating the OC lost.

Perhaps the most important finding should not have been a surprise – Loss-on-Ignition relies on combusting organic matter, but wants to measure organic carbon, therefore a conversion between the two is needed. There are lots of conversions in the literature, and they all produce different results. Our testing showed that some common conversions are out by more than 50%, and we cannot recommend using Loss-on-Ignition for current or future work.

We also showed that using acid to remove inorganic carbon also removed up to 40% of the organic carbon, so we would avoid using this technique unless it’s imperative to measure the organic matter directly, for example in stable carbon isotope analysis.

As a final observation, this work was enhanced by contributions from a cohort of undergraduate student co-authors who generated real-world saltmarsh data. For this, I am very grateful.

Rapid carbon accumulation at a saltmarshrestored by managed realignment exceeded carbon emitted in direct site construction

In the last few years, I have expanded my research into the burial of carbon in saltmarsh environments, especially around the UK. This is the first paper I have published on the topic. The paper is available Open Access via the journal website.

A composite map/aerial image of the Steart Marshes saltmarsh site

Saltmarshes, which is an coastal wetland which is flooded and drained by saltwater brought in on the high tide, are natural features acrosst the UK and around the world. In the UK, many saltmarshes were drained to form farmland, with a sea defence built between the drained marsh and the river or estuary. Rising sea levels threaten the reclaimed marshes, and the nearby fields, towns and villages, with flooding. Often it is decided that retreating from the drained land is the best way to protect other, more valuable, assets nearby. Through a process called “managed realignment”, the sea defences are breached and the tide returns to the saltmarsh.

Realigned saltmarshes are often lower than the local high tide level, and are rapidly filled with sediment and saltmarsh plants when the water returns. This creates a habitat that can attract wetland birds and, since the sediment has organic carbon associated with it, also generates and opportunity to bury carbon in the marsh.

This paper investigates two things: how much organic carbon was buried on a realigned saltmarsh in the first years after it was created, and how does this carbon burial compare to the emissions generated by the construction of the site.

Samples were collected from Steart Marshes, a site in Somerset, UK, that was flooded in 2014. The samples were analysed for their total carbon and organic carbon content using analytical facilities here at Manchester Met. The carbon concentrations were scaled up to the entire site using sedimentation data calculated from laser scans of the marsh collected at different time points.

There has been a very rapid build-up of sediment at Steart Marshes since the sea defences were breached

We found that the organic carbon burial rate (19 tonnes per hectare per year) was very high compared to other saltmarsh sites, mostly because the sediment built up very rapidly (75 mm per year) after the sea defences were breached. The organic carbon buried on site is much greater than the carbon emissions generated by the diggers and bulldozers used to make the new marsh, and so it seems that there has been a net climate benefit by creating the marsh.

However, the next piece of the puzzle is to fully understand the types of organic carbon being buried on the site. Not all carbon has the same climate benefit associated with it, and so further work is required to properly calculate the climate change mitigation potential of restoring saltmarshes.