GeoLog

The absorption of atmospheric CO₂ by the oceans results in a decline in ocean pH, hence ‘ocean acidification’, and reduces carbonate ion availability. This presents a problem to calcifying organisms (those that deposit calcium as either calcite or aragonite as hard parts) because they cannot produce their shells, valves (in the case of bivalves), or tests (in the case of diatoms) as readily.

To explain this, we need a little chemistry. When CO₂ dissolves, it combines with water to form carbonic acid (H₂CO₃). This then breaks down to form bicarbonate (HCO₃^-) when one hydrogen ion is lost, and then carbonate (CO₃^2-) as the other hydrogen ion is lost. This carbonate is the important stuff, as it combines with calcium to form the calcium carbonate (CaCO₃) used by bivalves to produce shells. If something (such as the ocean) is more acidic, there must be more hydrogen ions available. These hydrogen ions interfere with the calcification process as they bond with carbonate, meaning there is less available for shell formation.

Calcification: carbonate chemistry in action!

This process is relatively well established for a number of calcifying organisms, although there are exceptions to (the coccolith, Emiliania huxleyi, for example) and the response to elevated CO₂ levels is not uniform across species.

Much of current research has focussed on the effect of constant CO₂ levels on calcification, but what about animals that live in environments where the CO₂ concentration is constantly changing? The availability of carbonate in estuaries is particularly variable as CO₂ concentrations vary seasonally (there’s a greater carbon load in the winter as storms wash nutrients into rivers), diurnally and with the tide. The impact of elevated CO₂ levels on an organism is also dependant on its life stage; something that is particularly true of bivalves.

Bivalve larvae. Photo credit: Minami Himemiya (source).

Bivalves spend the first part of their life in the plankton, first as a veliger (a relatively amorphous looking ciliated blob) and then as a pediveliger (that same blob, but this time with an identifiable foot) before metamorphosing into a miniature adult. During these larval stages, they are particularly vulnerable to ocean acidification and, until recently, both the reasons behind this, and the longer-term implications of this vulnerability, were unclear.

This is where doctors Christopher Gobler and Stephanie Talmage come in. They took to the lab to tackle why larvae are especially vulnerable to acidification and what this means for them in both the short and long term. It’s impossible to take a look at how all bivalves respond to acidification, though, so to tackle these questions, two bivalve species, the hard-shelled clam (Mercenaria mercenaria) and the Atlantic bay scallop (Argopecten irradians) joined the team.

The Atlantic bay scallop, Argopecten irradians. Photo credit: Rachael Norris and Marina Freudzon (source).

Using their RNA:DNA ratio as a proxy for growth and the uptake of a radioactive calcium isotope, ⁴⁵Ca, to estimate calcification, Gobler and Talmage found that growth in the presence of elevated CO₂ results in individuals of a smaller size. This is because there is less calcium available for uptake. Their findings, revealed that high CO₂ concentrations, not only affected size, but also negatively impacted bivalve physiology, as individuals reared in these conditions were found to have thinner shells. Shells are an important defence against predators and the reduction in shell thickness (and hence strength) may put them at greater risk from predation.

The higher the CO2, the slower the calcium uptake: calcium uptake rates of larval Atlantic bay scallop, Argopecten irradians, under different CO2 concentrations over a 12-hour period (Gobler and Talmage, 2013).

When transferred from a high CO₂ environment to an environment with an ambient CO₂ concentration, larvae grew faster than those in ambient conditions throughout the whole of their development. However, this higher growth rate doesn’t compensate for the low calcification rate during larval stages, as their final is still smaller than individuals reared in ambient conditions at all life stages. This “legacy effect” presents a significant problem for adult bivalves, due to the detrimental impact of reduced calcification on their defences.

By Sara Mynott, EGU Communications Officer

Reference:

Gobler, C. J. and Talmage, S. C.: Short- and long-term consequences of larval stage exposure to constantly and ephemerally elevated carbon dioxide for marine bivalve populations, Biogeosciences, 10, 2241-2253, doi:10.5194/bg-10-2241-2013, 2013.

A French and Algerian study team seeks markers of underwater earthquakes off the Algerian coast. The team also matched the site’s paleoseismic history to land-based historical reports. Wayne Deeker reports.

The Mediterranean Sea represents the boundary between the African and Eurasian plates. Yet the fault segment off the Algerian coast is one of the most active in the western Mediterranean. It is associated with a series of moderate to large earthquakes: about 22 magnitude 6+ events since 856 CE. According to historical records, very large events in Algeria have been rare, though approximately one third of documented earthquakes would have been Mw 7+.

One of the most destructive was also the most recent. In May 2003, the Algerian coast experienced a Mw 6.8 earthquake, centred on the town of Boumerdès. It killed more than 2,300 people, injured some 10,000, and did all the usual structural damage earthquakes do in such countries. It also caused a tsunami which affected the western Mediterranean.

Building damage in the city of Boumerdes, Algeria (Credit: Ali Nour CGS; National Center of Applied Research in Earthquake Engineering)

The authors of a 2012 study published in EGU’s Open Access journal Natural Hazards and Earth System Sciences wanted to understand more about this event and earthquake recurrence intervals in the area. They found interesting clues deep underwater, where the quake likely caused landslides that resulted in almost 30 breaks of submarine telecommunications cables.

The researchers say it is especially worthwhile to investigate earthquake threats in areas that, like those surrounding the Algerian fault, have irregular, long-recurrence patterns because people there will be less prepared than where earthquakes are frequent. Another good reason motivates this research. While 2003 was a very well documented earthquake, most studies of its impact were conducted on land, with little attention paid to the effects and signatures offshore. This is unsurprising, as submarine earthquakes are difficult to monitor in real time, especially during catastrophic events. Therefore, the seafloor signatures of such events have remained elusive because they are occasional, erosive, and are a complex combination of many processes.

The damage to Algerian submarine telecommunications cables provides a clue to what happened underwater in 2003. While cable breaks often denote a definite time and place, pinpointing an event which can be interpreted in context with the landscape, the 2003 breaks were vague in all but one case. The location of the recovered cables also did not correspond to the point of damage, which suggests erosive action of turbidity currents (streams of rapidly moving sediment-rich water that deposit to form so-called turdibite sedimentary beds). Satellite images confirmed this, yet the flow pattern must have been quite complex, apparently following different paths along the underwater scarp system.

Resolving this complexity is challenging and, to do it, the researchers focused partly on seafloor morphology, in particular on the signs of seafloor rupture and instability, such as submarine landslides. Once the researchers started looking, they found the scarps had been affected by numerous landslides. The research team also found evidence of significant sediment transport, confined between salt domes and scarp walls. Sonar images showed signatures of high-energy events: ditches formed by erosion, indicating flow direction, plus perpendicular structures interpreted as pebble or gravel waves. These features probably constitute the sought-after signs of undersea earthquake activity, which can now be matched to other sites.

Direct sediment sampling compliments the scans. At the foot of the scarp slopes, the sediments are too mixed up to reveal any bedding that might be dated. Further out, this is not the case, and the presence of  turbidite beds alternating with another type of sedimentary deposit allows for some cautious dating of those layers. Preliminary results, from layers within the upper 1.5m of the core, show at least eleven turbidites accumulated during the Holocene, giving an average recurrence interval of about 800 years in this area. This matches the main seismic cycle on land, supporting the view that large earthquakes in Algeria are the main responsible for the large turbidity flows.

The authors concluded that the 2003 cable breaks were, for the most part, caused by the passage of a turbidity current triggered by the earthquake. The likely path of currents depends on the roughness and the irregularities of the sea floor: seafloor scarps in some cases deflect turbidity flow paths, while perched basins may trap them. The scarps seem prone to sediment failure, and are potential additional sources of cable breaks.

By Wayne Deeker, freelance science writer

“Irish coast” by Lena Noack, distributed by the European Geosciences Union under a Creative Commons licence.

Among geoscientists, the beautiful island of Ireland is best known for its Giant’s Causeway, an area with some 40,000 polygonal columns of layered basalt that formed 60 million years ago as a result of a volcanic eruption. But another recognisable feature of the Emerald Isle, is its lush green vegetation, a product of the island’s mild climate and frequent rainfall.

It was on a rare sunny day of a two-week trip to Ireland in September 2010 that Lena Noack from Germany’s Institute of Planetary Research in Berlin capture this stunning landscape. Lena says: “I took the photograph on a holiday, a round-trip all over Ireland I did together with two friends. We wanted to see the beautiful nature the island is famous for and found it almost everywhere! The trip was an unforgettable experience, and I would love to go back one day.”

The photo beautifully showcases Ireland’s colours: the calm blue of the sea and the bright green of the pastures. It was captured along the south-western coast of the island in the Dingle peninsula.

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.

In this month’s Geosciences column, Mona Behl discusses a recent paper on the effects of anthropogenically-induced climate change on the planet’s oceans. 

A recent study led by scientists at the Scripps Institute of Oceanography, University of California San Diego, suggests that observed changes in ocean salinity are inconsistent with natural climate variations and can be attributed to human-induced climate change.

Average salinity measured near ocean surface (Source: NASA)

Co-authored by Peter J. Gleckler, Benjamin Santer, and Paul Durack of the Lawrence Livermore National Laboratory in Livermore, California, and Tim Barnett of The Scripps Institution of Oceanography, the paper, entitled “The fingerprint of human-induced changes in the ocean’s salinity and temperature fields,” was published in the AGU journal Geophysical Research Letters on 2 November 2012. This research builds on the studies conducted by Barnett et al. (2005) and Pierce at al. (2006), examining warming in the upper 70 m of the ocean and concluding that it could also be explained by human-induced climate change. By simultaneously examining temperature and salinity fields, this new work highlights that the observed changes in the ocean are consistent with human forcing of the climate.

The oceans constitute 71% of the global surface area, store 97% of the world’s water, receive 80% of all surface rainfall, and absorb 90% of the Earth’s energy. Salinity, along with temperature, determines the ocean’s density and therefore plays a vital role in guiding ocean currents from the equator to the poles. By redistributing heat around the world, ocean currents have a profound influence on global climate. Scientists monitor salinity in the world’s oceans to determine how evaporation and precipitation patterns are changing. To that end, it has been suggested that decreased salinity at high latitudes in the Atlantic is consistent with observed changes in precipitation in high latitudes. Ocean salinity is also a signature of the global hydrological cycle, which is one of the most important elements of the climate system. Human induced climate change leads to an increased polarisation of the global water cycle, causing arid regions to become drier and high rainfall regions to become wetter. A change in the water cycle poses a substantial risk to human societies and ecosystems, affecting food availability, stability, access, and use.

Salinity and temperature correlations related to human-induced climate change (Source: Pierce et al. 2012)

Pierce et al. (2012), using a technique called ‘detection and attribution,’ compared observed changes in salinity and temperature to 11,000 years of model simulations. Detection is a process of demonstrating that the observed changes in ocean salinity and temperature are substantially different, in a statistical sense, from the changes that may arise due to natural variability of the ocean-atmosphere system, brought on by volcanic eruptions, solar fluctuations, or regular climatic patterns. Attribution, on the other hand, establishes whether the detected changes in ocean salinity and temperature are caused by natural variability (internal or external) or external forcing, such as human-induced changes in atmospheric composition due to an increase in greenhouse gases or changes in land cover. The detection and attribution analysis is, therefore, a rigorous technique to verify computer model simulated changes in ocean salinity and temperature. The findings from this study show that the changes in ocean salinity and temperature over the top 125m are inconsistent with the natural causes of climate change. The observed changes can, however, be detected and attributed to anthropogenic forcing of climate change.

It is very likely that the results of this new study will contribute to the next report of the Intergovernmental Panel on Climate Change, scheduled to be released in phases beginning in 2013.

The research was funded by the United States Department of Energy and National Ocean and Atmospheric Administration (NOAA).

For more background, check out this video by Climate Central, explaining the relationship between ocean salinity and climate.

By Mona Behl, Visiting Fellow with the American Meteorological Society Policy Program

The text of this week’s Imaggeo on Mondays comes from the photographer herself, Madlen Gebler, who tells us the tragic story behind this stunning picture.

“Glimpse of heaven” by Madlen Gebler, distributed by the European Geosciences Union under a Creative Commons license.

This picture was taken on the 2nd of March 2008 on board the research vessel Polarstern during the expedition ANT XXIV-3. After a four-week cruise we arrived in Atka Bay, Antarctica, in front of the German Antarctic research station Neumayer. I’ve never seen a sunrise like the one I saw that day and captured in this photograph; it was simply amazing. That morning the sunlight was reflected by the ice in a way which made it sparkle like diamonds with colours from yellow to red.

Shortly after taking this picture, I was standing together with a colleague, Willem Polman, at the bridge’s portside window. We were talking about how privileged we are because our work gives us the chance to visit Antarctica, the most fascinating continent on Earth, in my view. This picture is named “Glimpse of heaven” because it was the last glimpse of heaven before we went through hell. Just three hours after this picture was taken, Willem Polman died together with Stefan Winter in a helicopter accident.

Scientists are privileged people in that their work takes them to amazing places; but we should never forget that sometimes we pay a very high, too high, price for our work. I dedicate this to Willem and Stefan – we missed you sadly during the rest of the cruise.

By Madlen Gebler, Alfred Wegener Institute (Germany)

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.

“Keanae coast” by Martin Mergili, distributed by the European Geosciences Union under a Creative Commons licence

Geologically speaking, Hawaii is a very dynamic archipelago. Each of its islands is an exposed peak of a large undersea mountain range formed by volcanic activity starting about 28 million years ago as the Pacific plate moved slowly in northwest direction over a geological hotspot in the Earth’s mantle. Big Island and Maui, the southeastern most islands, are therefore the youngest and geologically most active of the archipelago.

Martin Mergili (BOKU University in Vienna, Austria), the author of today’s stunning photo, says of these two islands: “Whilst Big Island, the main island of the Hawaiian archipelago, is growing through lava flows into the sea, in Maui the waves are slowly eroding the coast. There is still volcanic activity on East Maui which, therefore, maintains its appearance of a shield volcano. However, in a few million of years it will have been reshaped by erosion and rather resemble today’s island of Kauai.”

This photo is a prime example of Hawaiian natural beauty. The lush green vegetation, the dark volcanic rocks and the white and blue of the rough ocean combine to create a colourful yet somber print. “The picture shows the shoreline of the Keanae Peninsula which is part of the rugged, windward northeast coast of Maui. Here, ancient lava flows from the Haleakala volcano (which is visible in the background) meet the roaring Pacific Ocean,” Martin says.

He took this dramatic photograph in Maui in August 2010 during a holiday journey – “as far as a geoscientist can spend real holidays on Hawaiian islands”, he jokes. More of Martin’s pictures of beautiful Hawaiian landscapes can be found at www.mergili.at/worldimages.

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.

“Patagonian blues” by Agathe Lisé-Pronovost, distributed by the European Geosciences Union under a Creative Commons license.

If you are feeling the Monday blues, this peaceful photograph of a Patagonian lake might be just what you need to light up your day. Patagonia is known for its rich volcanic history and dramatic landscapes, and this scene is no exception. It shows a lake in the Pali-Aike volcanic field on the Argentina-Chile border, located north of the Strait of Magellan: the beautiful Laguna Potrok Aike.

Agathe Lisé-Pronovost was lucky enough to visit the site in the Austral spring of 2008. “As part of my PhD, I was participating in the scientific drilling operations of PASADO, the Potrok Aike maar lake sediment archive drilling project in the framework of ICDP, the International Continental Scientific Drilling Program,” she says.

Potrok Aike is a maar lake, a water-filled volcanic crater that originated from an eruption in which groundwater came into contact with hot lava or magma. “It has a maximum diameter of 3.5 km and, at 200 metres depth, is the deepest crater of the Pali Aike volcanic field,” Agathe explains.

“It is a key site for paleoenvironmental studies because the sediments accumulate rapidly and it is located at 52 degrees of latitude south, on one of the only landmasses in the path of the strong Southern Hemisphere westerly winds.” Being highly susceptible to paleoclimatic changes, the lake is important in understanding the natural phenomena that cause climatic variability.

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.

This week’s guest post introduces a book recently published by Cambridge University Press.

Written by William I. Newman, a Professor at the University of California, Los Angeles, Continuum Mechanics in the Earth Sciences provides an introduction to continuum mechanics and essential mathematical and physical approaches in the Earth sciences. It also contains problem sets and worked examples, altogether providing a valuable first step towards understanding continuum mechanics, related tensor notation, and mathematical-background concepts. Clearly structured and merging basic with advanced topics, this textbook will capture the attention of both expert researchers and beginners in the area.

Hardback; ISBN: 9780521562898; Publication date: March 2012; 194 pages; Price: £40 (~€50)

The text is divided into nine main chapters, with the first three covering geometrical definitions of the material body and the response of materials under different forces. These definitions start with a review of essential mathematics for continuum problems, with the purpose of their geometrical descriptions in addition to covering rotation, transformation, and kinematics. The text continues, mainly in Chapter 2, by covering the most important physical quantities in continuum mechanics, like temperature, force, and stress.

The fourth chapter is devoted to fundamental laws and equations. This part of the text starts by introducing new terminology and is followed by the derivation of conservation laws. The following subsections cover some well-known constitutive equations, describing the internal mechanical, thermal, and other properties, of the constitutive quantities of the continuum materials. These parts are followed by thermodynamic considerations, which play a key role in the geosciences, particularly in the context of flows in Earth materials where the temperature can undergo dramatic change.

Chapter 5 is dedicated to linear elastic solids. It defines a body of material as elastic if at each body point the strain is a one-to-one function of stress at that point regardless of its history of loading. This chapter continues with discussions on statics and dynamical equations of isotropic bodies, homogeneous deformation equations, and the role of temperature on body deformation. It ends with a quick review on microscopic structure of crystals and their behaviour.

The next two chapters, on classical fluids and geophysical fluid dynamics, discuss the motion of fluids and their behaviour under stationary and dynamic inertial environments. Examples include the interior of Earth as well as the motion of the oceans and atmospheres.

The book ends with two chapters covering computations in continuum mechanics and nonlinearity in the Earth. The first one deals with partial differential equations used in numerical methods with the purpose of solving complicated real-word problems of continuum bodies rather than using simplified and linearized solutions.

Finally, the last chapter starts with some comments on the role of nonlinearity and its manifestation in the Earth sciences and continues by reviewing both friction, as the oldest mechanical aspect, and fracture, as one of the most challenging aspects of continuum mechanics. The last subsections cover percolation models, used to demonstrate self-organization under specific conditions, as well as fractals, used for the generation of noticeably realistic details of the objects.

Although on the whole an informative volume, this book unfortunately does not provide sufficient rigorous examples to work with, as is common in other continuum mechanics books. In addition, more examples about applications of continuum mechanics into real life Earth science problems could have made the book a little more interesting for student readers even in other fields within engineering.

By Arash Maghsoudloo, Research Assistant at Middle East Technical University

“Smiling with the smiling fishies” by Lee Miller, distributed by the European Geosciences Union under a Creative Commons license.

Jacques Cousteau once said, “I have seen other places like Sipadan, 45 years ago, but now no more. Now we have found an untouched piece of art.”

Indeed, the ‘wall of life’ shown in this picture suggests an untouched world, where schools of fish abound in a pure turquoise ocean.

The waters around Pulau Sipadan (Sipadan Island) are globally recognised as some of the most diverse on the planet, playing host to over 3,000 classified fish and coral species. This rich endemic biodiversity comes from millions of years of geographical isolation; Sipadan, like the Azores and Iceland, is an oceanic island with volcanic origins, formed by the growth of living corals on top of an extinct prehistoric volcano. Today it rises 600 m from the sea floor.

Lee Miller, postdoctoral researcher at the Max-Planck Institute of Biogeochemistry in Jena, Germany, took this picture in 2008, just before starting his doctorate. He describes his experience at Sipadan, “I went to Borneo (Malaysia) in search of the ‘wild’ — it did not disappoint. Along with pygmy elephants, crocodiles, orangutan, sharks, and sea turtles, I caught this glimpse of a school of barracuda off the eastern coast near Sipadan Island. This particular region is one of the only areas like this on Earth, as it combines a volcanic history with a steady nutrient supply from the deep water. This results in this amazing underwater diversity… along with those hundreds of teeth and eyes that are exchanging my stare in this picture.”

The island gets its name from the Siparan people, who used to go there to collect turtle eggs in the 19th century. Today, green and hawksbill turtles are commonly seen in the waters around Sipadan. One of the most unique sites in the area is the Turtle Tomb, an underwater cave system littered by the skulls and bones of turtles that got lost and drowned in its labyrinth of passageways.

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.

“Sunset on the Black Sea coast” by Gerrit de Rooij, distributed by the European Geosciences Union under a Creative Commons license.

In the context of human history, few bodies of water are as storied as the Black Sea, located at the juncture of Europe, Asia, and the Middle East. Countless cargo ships and frigates have sailed its waters, over 1,100 km in length from east to west, daunting enough that the Ancient Greeks believed its eastern shores (now Georgia) marked the edge of the known world.

However, perhaps the Black Sea’s most unique quality is that it is strictly meromictic; that is, its water column is divided into two separate layers that do not intermix, as they would in most other (holomictic) lakes. While the sea’s upper layer receives oxygen from the atmosphere, over 90% of its total volume is permanently anoxic, or heavily depleted of all oxygen.

The Black Sea’s stratification is enforced by its physical features, including by the topography of its basin, a vast, low-lying valley caused by the uplift of the Caucasus, Pontides, and Balkan mountain ranges after the collision of the Eurasian and African tectonic plates 5-12 million years ago. Many rivers also feed into the sea’s upper layers, keeping them generally cooler, less dense, and less salty than the deeper layer, which is fed by the warmer, saltier waters of the Mediterranean. Agricultural runoff in these rivers feeds phytoplankton, whose growth and decay keeps available oxygen at a premium.

EGU Hydrological Sciences Division President Gerrit de Rooij took this picture in August 2008 at the Romanian village of 2 Mai. He describes the moment of its capture, “It was during my holiday (the kids in the picture are my girls). There is not much geosciences in there, but with the sun, the atmosphere (visible through the scattered sunlight and the wind ripples on the water surface), the water, vegetation, and earth/soil all present, the link to hydrology and geosciences is straightforward. But I just made the photograph to capture the light of that morning and the light in the girls’ hair. With the elegant curve of the hill, the positioning of the horizon, and the sun just above it aligned with the hill crest to the left and one of the girls below, connected by the reflection band on the sea surface, I liked the geometric composition of the image.”

The anoxic zone of the Black Sea is of great interest to marine archaeologists because it is rife with well preserved shipwrecks, lacking the common wood-devouring microorganisms that inhabit most other oceans and seas around the world.

Imaggeo is the online open access geosciences image repository of the European Geosciences Union. Every geoscientist who is an amateur photographer (but also other people) can submit their images to this repository. Being open access, it can be used by scientists for their presentations or publications as well as by the press. If you submit your images to imaggeo, you retain full rights of use, since they are licenced and distributed by EGU under a Creative Commons licence.