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Geosciences Column: Did Mediterranean salt change the global climate?

The latest Geosciences Column is brought to you by Annabel Slater, who describes a time of dramatic change in the Mediterranean. Slater shares the results of a recently published Climate of the Past study and sheds light on how – in the context of global climate – a little salt can go a long way…

Many of us worry about the effects of too much salt on our health, not its effects on global climate. Often the amount we get goes up and down as we sprinkle miserly, health-conscious pinches into cooking, or give in and gladly seize the salt shaker when there’s a cone of chips in hand. Over 5 million years ago, the Mediterranean Sea also experienced salty extremes, swinging from periods of hypersalinity to salinities so low, it approached freshwater. Now, for the first time, scientists from the UK and Germany have shown that this variable Mediterranean salt diet could have had weighty consequences for global climate.

Around 5.96 – 5.33 million years ago during the latest Miocene period, the Messinian, tectonic changes closed off the Mediterranean Sea from the Atlantic Ocean. The salinity of the Mediterranean Sea fluctuated dramatically as water evaporated, increasing salt concentrations up to ten times what they are today.  At other times, regions of near-fresh water accumulated as water flowed in from surrounding rivers and lakes. This event is known as the Messinian Salinity Crisis, and its story is written out in sequences of evaporites and rich fossil records of brackish marine species in the region.

An artist’s impression of the Messinian Salinity Crisis. (Credit: Wikimedia Commons user Paubahi)

An artist’s impression of the Messinian Salinity Crisis. (Credit: Wikimedia Commons user Paubahi)

It had long been thought that, during the event, the Mediterranean Sea remained isolated from the Atlantic and even dried up completely. But recent research suggests that periodically, the connection was remade and outflows of either hypersaline or brackish Mediterranean water flowed into the Atlantic Ocean, like it does today through the Strait of Gibraltar.

The Atlantic Ocean hosts a major ocean current, the Atlantic Meridionial Overturning Circulation (AMOC), which circulates warm surface water from the mid-Atlantic to the higher Arctic latitudes. This looping current is powered by changes in water density, based on temperature and salt content. The more salt there is in water, the denser it is, and the deeper it sinks. The movement of the AMOC influences worldwide ocean circulation and climate. So what would have been the consequences of receiving this variable Mediterranean salt diet?

Led by Ruza Ivanovic, a team of scientists from the University of Leeds and the University of Bristol, UK, and GEOMAR Helmoltz Centre for Ocean Research, Germany, ran several simulations to investigate the effects of fluctuating salt input. They used a climate prediction model known as HadCM3 from the UK Met Office to generate several scenarios of Mediterranean water outflow.

Their simulations showed that any Mediterranean outflows during the Messinian Salinity Crisis would cause significant cooling. Outflows of the saltiest, densest water would cause a shift in the AMOC pattern, and cooling of a few degrees as far north as the Labrador and the Greenland-Iceland-Norwegian seas. But outflows of the freshest water would completely shut down the AMOC and generate a bipolar climate effect. Strong cooling reaching –8 degrees Celsius would occur in the northern seas, with weak warming occurring in the south. This simulation also showed a lasting southern shift in precipitation patterns.

The Mediterranean Sea today. (Credit: NASA/Eric Gaba)

The Mediterranean Sea today. (Credit: NASA/Eric Gaba)

These are the first major findings to unveil a connection between the Messinian Salinity Crisis and worldwide changes in climate. The simulations also show that flows or lack of flow from the Mediterranean Sea had more impact on the AMOC then, in comparison to conditions today. Palaeoclimate researchers investigating this relationship will now know where to look and also what to look for to find more evidence. As the North Atlantic sea surface temperature showed the most variability throughout all the simulations, the team think this region could be key to confirming their modelled results.

And as more evidence is uncovered about the connections between the Mediterranean and the Atlantic, the team will be able to refine their simulations to find out more about the duration and reach of the Messinian Salinity Crisis’ effects.

With much about the cause and nature of the Messinian Salinity Crisis still unknown, this research is a new step towards unpicking the mystery. One thing seems clear – either a little or a lot of salt can go a long way.

By Annabel Slater, Freelance Science Writer

Reference:

Ivanovic, R. F., Valdes, P. J., Flecker, R., and Gutjahr, M.: Modelling global-scale climate impacts of the late Miocene Messinian Salinity Crisis, Clim. Past, 10, 607-622, 2014.

Geosciences Column: Meshing models with the small-scale ocean

The latest Geosciences Column is brought to you by Nikita Marwaha, who explains how a new generation of marine models is letting scientists open up the oceans. The new technique, described in Ocean Science, reveals what’s happening to ocean chemistry and biology at scales that are often hard to model…

Diving into the depths of the ocean without getting your feet wet is possible through biogeochemical modelling – a method used by scientists in order to study the ocean’s living systems. These simulated oceans are a means of understanding the role of underwater habitats and how they evolve over time. Covering nutrients, chlorophyll concentrations, marine plants, acidification, sea-ice coverage and flows, such modelling is an important tool used to explore the diverse field of marine biogeochemistry.

Barents Sea plankton bloom: sub-mesoscale flows may be responsible for the twisted, turquoise contours of this bloom (Credit: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC)

Barents Sea plankton bloom: sub-mesoscale flows may be responsible for the twisted, turquoise contours of this bloom (Credit: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC)

There is one outstanding problem with this technique though, as the very-small scale or sub-mesoscale marine processes are not well represented in global ocean models. Sub-mesoscale interactions take place on a scale so small, that computational models are unable to resolve them. Short for sub-medium (or ‘sub- meso’) length flows – the smaller flows in question are on the scale of 1-10 km. They are difficult to measure and observe, but their effects are seen in satellite imagery as they twist and turn beautiful blooms of marine algae.

Sub-mesoscale phenomena play a significant role in vertical nutrient supply – the vertical transfer of nutrients from nutrient-rich deep waters to light-rich surface waters where plankton photosynthesise. This is a major area of interest since the growth of marine plants is limited by this ‘two-layered ocean’ dilemma. But the ocean is partially able to overcome this, which is where sub-mesoscale flows come in. Sub-mesoscale flows are important in regions with large temperature differences over short distances – when colder, heavier water flows beneath warmer, lighter water. This movement brings nutrient-rich water up to the light-rich surface. Therefore, accurately modelling these important small-scale processes is vital to studying their effect on ocean life.

Global chlorophyll concentration: red and green areas indicate a high level or growth, whereas blue areas have much less phytoplankton. (Credit: University of Washington)

Global chlorophyll concentration: red and green areas indicate a high level or growth, whereas blue areas have much less phytoplankton. (Credit: SeaWiFS Project)

A group of scientists, led by Imperial College’s Jon Hill, probes the technique of biogeochemical ocean modelling and the issue of studying sub-mesoscale processes in a paper recently published in the EGU journal Ocean Science.  Rather than simply increasing the resolution of the models, the team suggests a novel method – utilising recent advances in adaptive mesh computational techniques. This simulates ocean biogeochemical behavior on a vertically adaptive computational mesh – a method of numerically analysing complex processes using a computer simulation.

What makes it adaptive? The mesh changes in response to the biogeochemical and physical state of the system throughout the simulation.

Their model is able to reproduce the general physical and biological behavior seen at three ocean stations (India, Papa and Bermuda), but two case studies really showcase this method’s potential: observing the dynamics of chlorophyll at Bermuda and assessing the sinking detritus at Papa. The team changed the adaptivity metric used to determine the varying mesh sizes and in both instances. The technique suitably determined the mesh sizes required to calculate these sub-mesoscale processes. This suggests that the use of adaptive mesh technology may offer future utility as a technique for simulating seasonal or transient biogeochemical behavior at high vertical resolution – whilst minimising the number of elements in the mesh. Further work will enable this to become a fully 3D simulation.

Comparison of different meshes produced by adaptive simulations: (a) Bermuda, taking the amount of chlorophyll into account (b) the original adaptive simulation at Bermuda, without taking chlorophyll into account (c) adaptive simulation at Papa, taking the amount of detritus into account (d) the original Papa simulation, without taking detritus into account. (Credit: Hill et al, 2014)

Comparison of different meshes produced by adaptive simulations: (a) Bermuda, taking the amount of chlorophyll into account (b) the original adaptive simulation at Bermuda, without taking chlorophyll into account (c) adaptive simulation at Papa, taking the amount of detritus into account (d) the original Papa simulation, without taking detritus into account. (Credit: Hill et al., 2014)

The fruits of this adaptive way of studying the small-scale ocean are already emerging as the secrets of the mysterious, sub-mesoscale ocean processes are probed. The ocean holds answers to questions about our planet, its future and the role of this complex, underwater world in the bigger, ecological picture – adapting to life and how we model it may just be the key we’ve been looking for.

By Nikita Marwaha

Reference:

Hill, J., Popova, E. E., Ham, D. A., Piggott, M. D. and Srokosz, M.: Adapting to life: ocean biogeochemical modelling and adaptive remeshing. Ocean Sci., 10, 323- 343, 2014

Real life Minesweeper

Reading GeoLog when you should be working? We are all guilty of a little procrastination, but, sometimes, the parallels between science and the games we play to postpone the next write-up are closer than you’d think. Victor Archambault, a scientist from US Radar, reveals how playing Minesweeper mimics the way geoscientists analyse data in the field…

We have all played the infamous Minesweeper that comes with our computer, but few realise the principles of the game are used in a variety of fields and by scientific communities worldwide. In the game, the player is given numbers to make educated guesses as to where the mines will be in order to both avoid the dangers and uncover all the other tiles. This principle is no different from real life, where trained industry workers and scientists use electromagnetic waves to get clues about what might be under the surface. This could mean finding pipelines running through the foundations of a building, excavating an archaeological site, or even trying to identify and disarm a minefield.

The game. (Credit: Victor Archambault)

In the game. (Credit: Victor Archambault)

The Minesweeper game places nice neat little numbers everywhere that are both clear and easy to read but this isn’t the case with Ground Penetrating Radar (GPR), the technique used by scientists and other professionals to find out what’s underground. Depending on what you’re looking for, and what materials you’re penetrating, the images can be anything from a strange-looking pattern of waves, similar to a heart rate monitor at the hospital, to a rough 3D rendering straight from a cartoon.  This picture shows the variation in what you may see as you look into the internal structure of a cement supporting wall. As you can see, there are multiple ways to view it, which helps us make our most accurate guess.  This can be very useful in construction or city planning, allowing people to know what is currently there to use and what should be avoided.

In real life. (Credit: Victor Archambault/USRadar)

…And in real life. (Credit: Victor Archambault/US Radar)

The diagram below shows a GPR device as it is pushed along a surface. The waves spread downward in a fan-like shape and you can see an object before you are directly above it and after you have walked over it. Careful attention is needed to be sure not to miss any small artifacts you may be searching for.  The more constant and consistent the material is, the more complete and efficient the data will be for the user to read. Just as Superman cannot see through lead, the “radar mower” will struggle to see through certain types of materials – such as moist and clay-filled soils that have higher soil electricity conductivity.

'Mowing' the lawn to get a look at what lies under the surface. (Credit: Victor Archambault/USRadar)

“Mowing” the lawn to get a look at what lies under the surface. (Credit: Victor Archambault/US Radar)

Another way professionals use this technology is in oceanic plane crashes where large bodies of water are needed to be scanned for wreckage to help locate survivors. This involves a large, highly equipped plane to fly over the water – just like the ground scanning counterparts – and scan the ocean surface and below for clues as to the whereabouts of suspicious dots or shadows. It is more complex than most ground GPR designs because all elements of the radar need to be locked in place and it requires precise measurements for position correction.

Using electromagnetic waves in our daily lives continues to be more and more productive.  From catching a car that’s speeding to seeing a prenatal baby in the womb, we can see its implications to help us and better humanity. Minesweeper is a popular way to procrastinate, I hope next time you kick back and relax with a game, you look at it with more of a scientific eye! 

By Victor Archambault, US Radar

Geosciences Column: Shifting the O in H2O

Wherever you are in the world’s oceans, you can identify particular bodies of water (provided you have the right equipment) by how salty they are. You can get a feel for how productive that part of the ocean is by measuring a few chemical components in the water column. And, year on year, you will see a recurring pattern in how things like temperature, salinity and oxygen content vary with depth. This last property – the oxygen content – is vital for life in the oceans, but recent decades have seen shifts in the amount available.

There is always more oxygen at the surface than there is at depth. When waves break they mix an abundance of tiny air bubbles into the water, providing oceans with their oxygen supply, which is mixed into the deep through large-scale ocean circulation and storms over winter. At the surface, algae make the most of the abundant light to photosynthesise, beginning the base of the marine food web and adding a little more oxygen to the water in the process. These microscopic plants are eaten by animal plankton (zooplankton), which are, in turn, eaten by other plankton, crustaceans, fish, and a plethora of other predators – none of which contribute to the ocean’s oxygen. Instead, they, and a multitude of microbes, slowly use up more and more of the supply as they respire and there comes a point in the water column where there is no longer enough oxygen for these aerobic animals to survive – the oxygen minimum zone (OMZ).

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

What marks the boundary of this zone is dependant not on the properties of the water, but the life that lives there – it is the point when marine organisms experience hypoxic stress, usually an oxygen concentration in the range of 60–120 μmol kg−1. Below this, life in the marine environment is very different indeed. Anaerobic microbes thrive below the OMZ, making the most of life in an environment where there is very little oxygen in each litre of seawater.

The boundary between oxygen-rich water and the OMZ is known as the oxygen limiting zone (OLZ), and during the day many small swimming species take refuge here to avoid their predators. In the Eastern Pacific, you reach the OLZ when there’s 60 μmol kg−1 oxygen in the water, and the OMZ when there’s a mere 20 μmol kg−1.

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

The depth of the OMZ depends on temperature. Because warmer water is capable of containing less dissolved gas than cold, the OMZ is found at shallower depths in the tropics, and occurs at shallower depths in the summer than it does over winter. Winter weather allows more oxygen to be mixed into the deep ocean as storms break down the sea’s stratification, bringing nutrients to the surface and replenishing supplies closer to the sea floor. However, when there’s a lot of production at the surface (which draws down the oxygen) and the replenishment at depth is slow, large oxygen minimum zones persist from year to year.

Recently though, the upper boundaries of these zones have been shifting to shallower depths, resulting larger hypoxic regions in the ocean. Since the 1960s, the OLZ in the Gulf of Alaska, for example, has shifted some 100 metres shallower. Why?

The oceans are absorbing more heat in response to climate change. Because high temperatures reduce oxygen solubility, they reduce the amount of dissolved oxygen at the surface. The increase in surface heat also creates stronger stratification in the ocean, making it harder for oxygen to be mixed deep into the water column, and reducing dissolved oxygen at depth. Ocean circulation systems are also in a state of change, with systems like the Atlantic meridional overturning circulation in decline. Such changes in ocean circulation will also affect the amount of oxygen that’s mixed into the deep sea.

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Working out whether this is part of a long-term trend is a difficult task, as records of deep ocean oxygen only stretch back to 70 years ago. Only a longer record of observations will help determine the trend, but for now we can be sure that shoaling oxygen minimum zones will change the amount of habitat available to species either side of the line between oxygen-rich and oxygen-poor.

By Sara Mynott, EGU Communications Officer

References:

Gilly, W. F., Beman, J. M., Litvin, S. Y., & Robison, B. H.: Oceanographic and biological effects of shoaling of the oxygen minimum zone. Annual Review of Marine Science, 5, 393-420, 2013

Smeed, D. A., McCarthy, G. D., Cunningham, S. A., Frajka-Williams, E., Rayner, D., Johns, W. E., Meinen, C. S., Baringer, M. O., Moat, B. I., Duchez, A., and Bryden, H. L.: Observed decline of the Atlantic meridional overturning circulation 2004–2012, Ocean Sci., 10, 29-38, 2014