Climatic fates in the ooze

While governments scramble to provide the laziest climate-change commitments ahead of the UN conference in Paris later this year, the world is being honed to confront how life about land will change as the atmosphere and surface and heat up. But for another world – a world that has often shown up its terran counterpart in sheer complexity – scientists are far from understanding how things will change over the next 85 years.

Climatologists and oceanographers were only recently able to provide a rounded explanation for why the rate of global warming slowed in the late 1990s – and into the 2010s: because the Pacific Ocean was absorbing heat from the lower atmosphere, and then palming it off to the Indian Ocean. But soon after the announcement of that discovery, another team from the US armed with NASA data said that the rate of warming hadn’t slowed at all and that it seemed that way thanks to some statistical anomalies.

Irrespective of which side is right, the bottomline is that our understanding of the oceans’ impact on climate change is poorly understood. And although it hasn’t been for want of trying, a new study in the journal Geology presents the world’s first map of what rests on the oceans’ floors – a map that’s been updated comprehensively for the first time since the 1970s.

The ocean floor is in effect a graveyard of all the undersea creatures that have ever lived, but the study’s significance for tracking climate-change lies with the smallest of those creatures – the tiny plankton, inhabitants of the bottommost rungs of the oceanic food chain. Their population on the surface and pelagic zones of the oceans increases with the abundance of silica and carbon, and when they die or the animals that eat them die, the float into the abyss – taking along a bit of carbon with them. This is the deceptively simple mechanism called the biological pump that allows the world’s larger waterbodies to absorb carbon dioxide from Earth’s atmosphere.

Digital map of major lithologies of seafloor sediments in world’s ocean basins. Source: doi: 10.1130/G36883.1
Digital map of major lithologies of seafloor sediments in world’s ocean basins. Source: doi: 10.1130/G36883.1

The new map, made by scientists from the University of Sydney and the Australian Technology Park, shows that contrary to popular beliefs, the oceanic basins are not settled by broad bands of sediments as much as there are pockets of them, varying in size and abundance due to a variety of surface characteristics and with the availability of certain minerals.

A photomontage of plankton. Credit: Kils/Wikimedia Commons, CC BY-SA
A photomontage of plankton. Credit: Kils/Wikimedia Commons, CC BY-SA

For example, diatom ooze – not watery eidolons of muck sticking to the underside of your shoe but crystalline formations composed of minerals and the remains of calcium- and silica-based plankton called diatoms – is visible in widespread patches (of light-green in the map) throughout the Southern Ocean, between 60º and 70º S.

The ooze typically forms in the 0.8-8º C range at depths of 3.3-4.8 km, and is abundant in the new map where the temperatures range from 0.9º to 5.7º C. Before this map came along, oceanographers – as well as climatologists – had assumed these deposits to be lying in continuous belts, like large undersea continents. But together with the uncertainty in data about the pace and quanta of warming, scientists had been grappling with a shifting image of climate change’s effects on the oceans.

The locations of diatom ooze also contribute to a longstanding debate about if the ooze settles directly below the largest diatom populations. According to the Australian study’s authors, “Diatom ooze is most common below waters with very low diatom chlorophyll concentration, forming prominent zones between 50° S and 60° S in the Australian-Antarctic and the Bellinghausen basins”. The debate’s origins lie in the common use of diatoms to adjudicate water quality: some species proliferate only in clean water, some in polluted water, and there many species of them differentiated by other preferred environments – saline, acidic, warm, etc.

The relative abundance of one species of plankton over the other could, for example, become a reliable indicator of another property of the water that scientists have had trouble measuring: acidity. The dropping pH levels in the oceans are – or could be – a result of dissolving carbon dioxide. While some may view the oceans as great benefactors for offsetting the pace of warming by just a little bit, the net effect for Earth has continued to be negative: acidic waters dissolve the shells of molluscs faster and could drive populations of fishes away from where humans have set up fisheries.

Ocean acidification’s overall effect on the global economy could be a loss of $1 trillion per year by 2100, a UN report has estimated – even as a report in the ICES Journal of Marine Science found that 465 studies published between 1993 and 2014 sported a variety of methodological failures that compromised their findings – all of precise levels of acidity. The bottomline, as with scientists’ estimates of the rate of pelagic warming, is that we know that the oceans are acidifying but are unsure of by how much.

The new map thus proves useful to assess how different kinds of ooze got where they are and their implications for how the world around them is changing. For example, as the paper states, “diatom oozes are absent below high diatom chlorophyll areas near continents”, where sediments derived from the erosion of rocks provides a lot of nutrients to the oceans’ surfaces – in effect describing how a warming Earth posits a continuum of implications for contiguous biospheres.

The Wire
August 13, 2015