Fire | Carbon Cycle | Rivers

Fires prime terrestrial organic carbon for riverine export to the global oceans

Matthew W. Jones, Alysha I. Coppola, Cristina Santín, Thorsten Dittmar, Rudolf Jaffé,
Stefan H. Doerr and Timothy A. Quine

Read our article in Nature Communications (2020)


Fires emit two billion tonnes of carbon each year, but they also leave behind around 250 million tonnes of carbon as burned residues, like charcoal and ash. Around half (usual range: 20-80%) of the carbon in these residues is in a particularly long-lived form — ‘black carbon’ — that is stored throughout the Earth System, including in oceans. In this study we estimated that about one-third of all black carbon has an oceanic fate.


Burned Carbon from Land to Sea

A surprisingly large fraction of the carbon stored in the ocean bears the chemical signature of burned vegetation.

Land-derived (terrigenous) carbon, including black carbon (BC), reaches the ocean, via rivers, after a chain of processes.

Land plants sequester CO2 from the atmosphere to organic carbon (OC) through photosynthesis.

Dead biomass carbon is added to soils as plants drop their leaves (or other components), or as plants die.

Soil carbon can be eroded and passed by rivers to the global oceans. This is how terrigenous particulate organic carbon (POC) reaches ocean sediments.

Soil carbon can also be solubilised to dissolved organic carbon (DOC). Some of this DOC is recycled by plants, some is degraded to carbon dioxide, and some is exported with water as it travels through river catchments

Only about 1% of all carbon sequestered by plants flows through rivers to the oceans.

The terrigenous OC can be stored in oceanic pools, and it can also pass between particulate and dissolved phases.

Fire transforms OC into BC, which has special chemical properties that slow its degradation to CO2. Consequently, BC passes through this chain of processes in a more conservative way than most terrigenous OC, and a large fraction of the terrigenous OC in the oceans bears the signature of burned vegetation.

With thanks to Nigel Hawtin for producing the figure.

Motivation

Fires leave behind carbon-rich materials, like charcoal and ash, which break down very slowly in soils (centuries to millennia). We care about this burned carbon because it is essentially ‘locked out’ of the atmosphere for the distant future – it breaks down to greenhouse gases extremely slowly in comparison to most unburned carbon.

We know that this burned carbon takes about 10 times longer to break down in the oceans than on land (millennia to tens of millennia).

Rivers are the conveyor belts that shift carbon from the land to the oceans, so they determine how long it takes for burned carbon to break down. We set out to estimate how much burned carbon reaches the oceans via rivers.


New Evidence for Regional Variability in the BC Content of River DOC

In their earlier paper, Rudolf Jaffé, Thorsten Dittmar and colleagues found only small deviations in the BC content of DOC in rivers across the world.

In our study, we analysed new data from global rivers to reveal that the BC content of DOC varies systematically across rivers at different latitudes and in different ecological regions.

Our data set incorporates 409 coupled measurements of DOC and DBC concentration in total, including 195 from 34 major rivers and 214 from 44 minor channels.

300 of the coupled observations are new and derive from 12 major rivers and 90 minor channels.

204 of the coupled observations are from new samples taken from (sub)tropical rivers (<30° N/S), which is an important advance because ~90% of global BC production occurs in the (sub)tropics and because (sub)tropical rivers contribute ~60% of the total global DOC export flux.

Black C content of dissolved organic carbon for rivers in different latitudes and ecoregions. Small points mark individual observations, large points mark averages, thick black lines show standard deviations, letters denote groups with statistically similar distributions, numbers denote sample size.
Red line of best fit and uncertainty range (standard error) from a linear model fitted following Jaffé, Dittmar and colleagues. Faded points show individual observations. The more prominent points show means and standard deviations for rivers in different latitudes and ecoregions.
Dotted line of best fit from a linear model fitted following Jaffé, Dittmar and colleagues. The full line (and shaded uncertainty range) shows the mean and standard deviation ratio of DBC:DOC for rivers in different latitudes and ecoregions.

Seasonality in the BC content of River DOC

The spatial variability is broadly seen even when seasonality is accounted for.


Global Export of Burned Carbon

We upscaled our findings to estimate that 18 million tonnes of dissolved burned carbon are transported annually by rivers.

When combined with an estimate for the amount of BC that is exported with with POC, the total rises to 43 million tonnes of BC per year.

Our findings show that a surprising amount – around 12% per cent – of all carbon flowing through rivers comes from burned vegetation.


Implications

Our results show that about one-third of all black C produced by fires reaches the oceans. By comparison, only about one per cent of carbon taken up by land plants ends up in the ocean. Hence we conclude that fires transform vegetation carbon to a form that is ‘primed’ for export to the oceans by rivers.

With wildfires anticipated to increase in the future because of climate change, we can expect more black C to be produced, flushed out by rivers, and locked up in the oceans.

This is a natural quirk of the Earth system – a moderating ‘negative feedback’ of the warming climate that could trap some extra carbon in a more fire-prone world. The feedback offsets some of the carbon that could be lost from vegetation stores in a more fire-prone world.

The production and transport of burned carbon is not the kind of process that can offset a large fraction of the carbon emitted by fossil fuel burning, but it is the sort of process that land and ocean models should include.

Black carbon production and export influence the carbon cycle on land and in the oceans, yet models tend to overlook these processes. We recommend modifications to models to account for the unique dynamics of black carbon and to predict responses to future climate change and fire regimes.


Processes in Action

Images from around the world help to visualise the transport of burned carbon from land to ocean.

A fire near the Salcha River floodplain south of Fairbanks, Alaska. Photo credit: John Lyons/Alaska Fire Service
A forest fire blazes alongside a river in Washington state. Photo credit: Washington DNR
Some burned carbon remains on the ground following a flood in the chaco of Bolivia. Trees mark the depth of the flood waters. Photo credit: Tim Quine
Rainfall on a steep valley slope bearing charcoal and ash following the extensive 2019/20 Tambo-35 wildfire in Australia. Photo credit: Jason Alexandra
Ash deposits line the banks of a river following the 2019/20 Green Wattle Creek wildfire near Sydney. Photo credit: Stefan Doerr 
A ‘tea’ of organic-rich water washes off a burned UK heathland and mixes with milky pond waters. Photo credit: Matt Jones

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