Unravelling the ocean’s secrets

Oct 30

In the past century, humans’ mastery of synthetic fertiliser production has allowed us to turn inert nitrogen into a super-fuel that helped double Earth’s food production capacity. But how much has this upended the natural nitrogen cycle? A tiny weather station on a peninsula just south of Cape Town is helping researchers understand change at a planetary scale.

Written by: Leonie Joubert
Photographs by: Sam Reinders

The ocean and atmosphere are one coupled system. Atmospheric chemist Dr Katye Altieri studies the boundary layer, an ephemeral space where these two mediums meet and mingle, to see what their chemical make-up can tell us about changes at a planetary scale.

An air sample, drawn from an instrument intake positioned above the Cape Point weather station, runs through a cryotrap which freezes and traps moisture, and cleans up the sample to allow for better analysis. Altieri is interested in the nitrogen aerosols present in the sample, not the inert nitrogen gas.

The historic Cape Point weather station is part of a collection of atmospheric monitoring stations that has allowed scientists to track global atmospheric carbon dioxide levels for decades. These GAW stations — part of the Global Atmospheric Watch network — have tracked the buildup of other climate-altering greenhouse gases and monitored changing ozone levels over time.

An ion chromatograph at the Cape Point weather station allows researchers at the University of Cape Town’s marine biogeochemistry laboratory to look at the amount of nitrogen in samples collected at this globally significant sampling point.

Casper Labuschagne, senior scientific researcher with the South African Weather Service, oversees the Cape Point met station, which is part of the Global Atmospheric Watch network.

It’s a wild winter’s morning, and the Cape of Storms’ mood is matching its reputation. An icy gale howls around the historic lighthouse and a small but globally important weather station on the end of a finger of peninsula just south of Cape Town.

Today’s weather shows why some at the University of Cape Town (UCT) marine biogeochemistry lab have an in-joke: why hop on a research ship and spend weeks trawling the Southern Ocean to collect Antarctic air samples, when you can sit here and wait for the Antarctic to come to you?

One unremarkable looking piece of instrumentation that juts into the atmosphere a few metres above the station — about 230m above the waves which crash against the sandstone rocks at the foot of the nearby cliffs beneath this historic nautical landmark — is sucking in some of that frigid air so that researchers can collect a sample of it and pick its contents apart to unravel some unexpected secrets.

Nitrogen bulks up Earth’s atmosphere to the tune of 78 percent, although this isn’t what atmospheric chemist Dr Katye Altieri is interested in. Atmospheric nitrogen – the gas that makes up most of each breath we take – is inert. Altieri’s sampling is to gather the nitrogen aerosols, the reactive nitrogen particles that are put into the air by, for instance, industry, pollution from car exhaust fumes, fertilised soils, and fires, all of which have changed the natural nitrogen cycle globally. The air moving through the instrumentation is filtered to specifically capture the nitrogen particles, while the nitrogen gas passes through and of no consequence to this study.

This sample must be collected from what atmospheric scientists call Earth’s ‘boundary layer’, an ephemeral realm in the lower part of the atmosphere where the ocean and air meet and mingle.

Air within the boundary layer is well-mixed around the entire planet, covering both continental land masses and oceanic areas, explains Altieri, and this is the sweet spot to sample in order to get a proper representation within this enormous body of air.

The boundary layer’s height can vary, depending on the meteorological forces at play. It is this part of the atmosphere that Altieri is interested in, and why she needs a high-volume air sampler that can capture the different sized aerosol particles.

Upending the natural nitrogen cycle

Synthetic fertiliser production is only about a century old, but this, together with burning of fossil fuels, has fundamentally changed the natural nitrogen cycle around the world dramatically, both at a local scale and a planetary one.

Industrial processes draw inert nitrogen from the air and convert it into ammonia which, once added to plants, dramatically boosts their growth. This is one of the central pillars of the so-called Green Revolution, which took off from the 1960s onwards. Together with mechanised farming techniques and new high-yield hybridised crop strains, this kind of synthetic fertiliser technology is why global food production can support eight billion people, when Earth’s food production capacity without nitrogen fertiliser is only about three to four billion.

The chemical alchemy involved here means that there is much more bio-available nitrogen moving through the Earth’s system than before – roughly double what was in the pre-industrial nitrogen cycle, says Altieri.

‘We’ve now added as much human-produced nitrogen to the system as was there naturally,’ she explains.

At a local scale, this can result in farm run-off that over-nitrifies the immediate surrounds, particularly rivers, a form of pollution that causes algal blooms and oxygen dead-zones in fresh water and marine systems.

In its active gaseous form — nitrous dioxide — contributes considerably to climate disruption. This greenhouse gas originates from farming practices, including synthetic fertiliser use and rotting crop residue, and has 300 times more powerful a global warming effect than carbon dioxide.

Changes in the nitrogen cycle on land and in terrestrial water courses are fairly well understood, because these systems are much easier to get to and study. Not so with the ocean.

Altieri’s work is part of a larger global collaboration that aims to better understand ocean-atmosphere changes in the nitrogen cycle.

She explains some of the interacting forces that science needs to understand better.

Just like adding synthetic fertiliser to a garden to get more growth, adding it to the ocean means that there are more nutrients available to speed up phytoplankton growth.

‘As the phytoplankton grows, it absorbs carbon dioxide from the atmosphere, which, from a climate perspective, is a good thing,’ says Altieri. ‘That’s weird, right? Pollution is bad, but the ocean drawing down carbon dioxide is good. Human pollution is helping the ocean become a better carbon sink.’

But once this push-pull has worked itself out, the third force comes into play: the global heating caused by nitrous oxide, which is altering the climate system.

While a pollution-boosted ocean carbon sink might counter global warming, atmospheric nitrous oxide pollution will pull the climate system in another harmful direction by causing more warming.

The more science knows, the better the projections get

Cape Point was long thought to be the meeting point of the Atlantic Ocean and Indian Ocean, and local lore still tells this romantic tale. But as marine science advanced in recent decades, oceanographers can now show that these two giant water bodies mix further east, around the Cape Agulhas region, which is the southern-most tip of the African continent.

Isotope studies like those done at the marine biogeochemistry lab contribute towards a growing sophistication in the understanding of planetary systems at such a large scale.

‘The ocean was once thought to be a passive recipient of nitrogen deposits,’ says Altieri. ‘People assumed that nitrogen just washed into the ocean from rivers or was dropped in with the rainfall. But the ocean also emits nitrogen gas.’

There are circular forces at play here, with nitrogen being deposited into marine waters in different ways, the ocean releasing some, and then these being recycled back into the ocean again.

‘There’s a natural component to this cycle, and then there’s the human-caused dumping on top of that. How much is natural? How much is human caused? And what does that matter?’

Isotope studies are one of the tools that scientists can use to answer some of these questions and are a large part of Altieri’s work which has already sharpened the focus of our understanding. In this case, ratios of nitrogen-15 to nitrogen-14, and oxygen-18 to oxygen-16 can tell us something about the possible source of the nitrogen-bearing compounds.

Parallel studies using nitrogen isotopes, taken from atmospheric samples in the Bermuda area and analysed by Altieri and colleagues from Princeton University, the Bermuda Institute of Ocean Sciences and others, have shown that the amount of natural nitrogen deposits into the ocean is much greater than previously thought.

Until now, it was believed that 80 percent of the total nitrogen making its way into the ocean was from human-linked sources. Now, this research shows that it’s only 27 percent.

‘We need to know what these kinds of numbers are, and how big the forces are,’ says Altieri.

Nitrogen isotope knowledge is so advanced that a trained eye can tell if the nitrogen aerosols in an air sample were shaped by lightning, from burning crop residue after harvest, a savannah wildfire, a coal-fired power station, or a vehicle.

It’s even possible to tell if, for instance, nitrogen resulting from burned biomass is from the African continent, or from India.

‘Knowing how much comes from each source lets us create strategic policy to reduce emissions from the biggest contributors to the problem,’ says Altieri.