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Entries in geology (5)

Wednesday
Jan262011

Poseidon smiles

The swells died away this morning and the science crew was thrilled to make it back in the water.  As I type, lead scientist Rodrigo Moura is in the sphere and geologist Alex Bastos is in the back and they’re 800ft down on the south edge of the Abrolhos Platform.  They’re exploring a steep sandy slope (like, 45 degrees steep) expecting sponges and fishes and collecting sediment samples for the geological context and to measure the quantity and origins of the organic matter, which is important to understand how much the ecosystem there relies on plankton above or on its own productivity at the bottom.

 

In the photo above, Jim “Sully” Sullivan sits at the Com-Track for the submersible, while Brazilian scientist Paulo Sumida (U. Sao Paulo) takes notes.  Com-Track is a station on the bridge of the ship, where sub crew staff and scientists can track the position of the sub and communicate with the scientists onboard.  Its not that straightforward, though, because there’s no cable to the JSL and radio and other EMF frequencies don’t travel through water.  Instead, communication takes place embedded in acoustic signals, with limited voice coms and a system of “pings” to serve as acknowledgement/agreement.  The whole shebang is tied into the GPS system for the ship, which is what made it possible for Bill Baxley to embed the sub track in his lovely 3D model.  If all goes according to plan, I’ll send an update on what they find later today

 

Sunday
Jan232011

Rocking the Abrolhos

This is the first post from onboard the R/V Seward Johnson, currently at 18 deg 6 min S, 38 deg 25 min W, somewhere off the coast of Bahia State, Brazil.

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Alex Bastos likes rocks.  Biologists may scoff, but the truth is that rocks and geology provide the context for biology in the oceans, much the same way as a playing field provides the context for a game of football.  In an expedition concerned with coral reef ecology, then, it’s important to have a thorough understanding of geological processes, because they shape the history of the reef and can help predict its future.  Dr. Bastos, who hails from the Federal University of Espirito Santo, is a marine geologist with a special interest in the Abrolhos.  He is trying to understand what has happened in the prehistoric past and what is happening on the reef today by reading the shape of the bottom features (what geologists like to call morphology) and reconstructing a story and past and present events on the Abrolhos shelf.  Scientists call this sort of activity “inferring process from pattern”, and in terms of marine geology it brings with it some special challenges.  The biggest of these is that, for the most part, Alex can’t see the morphology he wants to study, certainly not as easily as a terrestrial geologist might fly above a mountain range and read the faults and synclines of the landscape to understand what geological forces shaped the land.  No, he has to construct his story a piece or two at a time, like putting together a jigsaw puzzle, often from indirect evidence.  He gets his jigsaw pieces from some pretty cool technology though, including seismic equipment, side scan sonar and by drilling cores out of the sediment.

Side scan sonar is an especially great tool that has become really popular in research circles in recent years.  It uses sound to reconstruct bottom morphology by sending out high frequency noises and then gathering back the echoes off the bottom, much the same way bats find food.  What makes side scan different from regular sonar is that instead of aiming the sound straight down under the boat, it is directed away to each side.  Since the sound hits objects and bounces back at an angle, any structure of any height will cast a sort of sound “shadow” on the side facing away from the boat.  The resulting image is a plan view, but with shadows that reveal shapes and structures, rather like taking a photo from an aeroplane late in the day, when shadows are long on the ground.

Side-scan reveals and ancient river channel. Img: Alex BastosBy combining studies of morphology with reconstructions of sea level at different times in geological history, Alex tells a story about the way the Abrolhos platform evolved.  Around 40 million years ago the main playing field was established by a flood of volcanic basalt that spread out to create the basis of the platform.  In the ensuing geological periods, carbonate rocks (i.e. limestone) were deposited, implying that reefs existed there at times in the distant past (my assertion in a previous post that there were no reefs on Abrolhos before 8,000 years ago was wrong).  During the last ice age, the sea level was over 100m lower than its current height, so much of the platform, including the locations of all the current shallow coral reefs, was above the surface.  At that time, a central depression in the platform was probably still filled with water, forming a shallow coastal lagoon opening to the south.  The Caravelas River probably drained into this lagoon from the north, cutting a number of different channels through the limestone as it meandered over time.  The river also provided a source of sediment that fanned out off the edge of the platform from the mouth of the lagoon.  All three of these features – the shallow depression, the channels in the limestone and the sediment fan – are still there on the bottom of the sea and help to tell the story.  At the end of the last ice age came the last great transgression, when sea level rose to approximately its current levels.  The entire platform was flooded, and corals once again colonized the platform and began to grow up towards the surface to where we see them today.

How Abrolhos might have looked during the last ice age; note the central lagoon opening to the south

Current bottom topography of Abrolhos; note central depression due south of the islands

Holocene sedimentary processes.

Unfortunately, morphology only gets you so far.  Knowing the history of the platform, many of Dr. Bastos’ questions now relate to what has happened since the last transgression, during the most recent geological period (the Holocene), and most of these need other methods to answer.  These questions include “Is all the sediment on the platform a relict of geological history, or are sediments actively being deposited to this day?” and “If they are still being deposited, do they come from the land via the river, or are they produced in the ocean by corals, coralline algae and other organisms?”  To answer these questions, Alex will use a combination of seismic methods and sediment cores.  Seismic surveys provide indirect evidence of the nature of sediments below the surface of the sea bed, and these can be calibrated or “ground-truthed” by taking sediment cores that will reveal a timeline of sediment history on the platform.  These geological questions played a large part in the choice of sites for the main transects on this expedition.  One is to the south, in the mouth of that ice-aged lagoon, on the edge of the shallow-sloping fan of sediment.  The other is in the north, on a much steeper shelf break where the platform gives way to the deep sea beyond.  These two locations not only have profoundly different geology, but it’s likely that organic processes differ between them too, as we shall see in a future post.

Applying the lessons

Alex was telling me that one of the more satisfying aspects of his work is translating geology data into benthic (bottom) habitat maps.  In other words, taking all that information from the sonar morphology, the seismic data and the sediment cores, and mapping out what different parts of the platform are like on the bottom.  This is important, because once scientists can say with confidence that Area X is a rhodolith bed, or Area Y is a steep-sided valley, then they can predict with good accuracy what sorts of organisms will live there.  That sort of information is of critical value when deciding where to take biological samples and even more so when the Ministry of the Environment needs to make decisions about the boundaries of protected areas, which they are currently doing.  In this way geology plays a very important role in both the biological sciences and in conservation decision making.

Monday
Jan172011

Which came first, the Abrolhos or the Abrolhos?

In just a couple of days I’ll join a group of scientists from Brazil, Australia and the US for an expedition to study the reefs of the Abrolhos platform, off Bahia state in Brazil.  When this trip was first mentioned I have to admit being confused.  That’s because, as a native Aussie, to me “the Abrolhos reefs” means a group of reefs and emergent islands off the remote Northwest coast of Western Australia.  So why are there two Abrolhos Reefs, and which came first?

My Brazilian friend and colleague Julia Todorov tells me that Abrolhos is a contraction of two Portuguese words, abro and olhos, meaning “open eyes” as in “Keep your eyes peeled, Marcos, lest you plough the ship into the reef!”.  That etymology is listed in several online sources.  The above Wikipedia link for the Aussie Abrolhos, however, says its not a true etymology, but I don’t see why not, since it applies just as well to reefs as it would to caltrops: basically, watch where you’re going!

Frederick de Houtman (Wikimedia commons)None of that explains why the Aussie reefs got the name, since the Portuguese did not explore Australia that we know of.  Nor does it explain which place name came first.  That’s a bit easier.  The Australian reefs are properly called the “Houtman Abrolhos” or “Frederick de Houtman’s Abrolhos” and were named by de Houtman, the captain of the Dutch East India Company ship Dordrecht in 1619.  He almost certainly named them after the Brazilian reefs, which he had previously sailed through in 1598.  The Brazilian reefs were already known and named at that time, so by name, the Brazilian Abrolhos came first.

Putting the trivialities of human history aside for a moment, we might ask a bigger question: which Abrolhos ultimately came first? Y’know, biologically.  Which reef grew up from the seafloor first?  In short, it was a tie.  Both reefs showed a major growth spurt around 8,000 years ago in the midst of the “last transgression”, when sea level started rising as the ice caps melted away from the last ice age.  This is a pretty common pattern everywhere.  In fact, there are pretty much no extant coral reefs anywhere older than about 12,000 years, since they were all high and dry back then (the reef organisms having receded into what are now much deeper areas).

OK then, if the current reef communities of the Abrolhoses (?) are both about the same age, then which reef came first geologically?  Which one has the longest geological history?  Chalk that one up as a win in the Houtman column.  The current  Houtman Abrolhos islands and reefs sit atop limestone bedrock that is the remnant of a coral reef that grew in the same location in the Quaternary period, before about 125,000 years ago.  The Brazilian Abrolhos, on the other hand, sit atop a layer of flood basalts (i.e. volcanic rocks, solidified lava) that spread out across the edge of the continental shelf during the Eocene (>30 million years ago).  When scientists core into the reef, the oldest reef they find before they hit the volcanic layer is a bit over 7,000 years; suggesting that the Brazilian reefs are relatively much younger (see Dillenburg & Hesp, 2009

The Houtman Abrolhos in Australia. (Wikimedia commons)Aside from the name and the similar recent growth spurt, the Abrolhos reefs have little in common; Houtman Abrolhos is a faily typical Indo-Pacific reef with high coral, invertebrate and fish diversity growing on a relict of an even older reef, whereas Brazilian Abrolhos is species poor and dominated by just a few coral and fish species growing on a volcanic base.  Could the short geological history of the Brazilian Abrolhos account for the biological differences?  Maybe, but biogeography probably has a lot to do with it too.  Houtman Abrolhos are not too far from the Indo-Pacific center of diversity, the highest tropical diversity there is and source of much species richness throughout the Indo-Pacific, whereas Brazilian Abrolhos are remote and cut-off from other major centers of reef diversity.  There will be a lot more to talk about regarding the diversity in Brazilian Abrolhos in future posts.

So the Aussie Abrolhos has probably been around quite a bit longer, but the Brazilian Abrolhos has been known to people (European at least) longer by about 100 years.  Despite this, the Brazilian reefs are still poorly known, having come to research and conservation attention only for the last two decades or so.  Its fantastic to think that on this expedition we will still have so much to learn about such a unique ecosystem.  I look forward to reporting  from onboard the R/V Seward Johnson some new biology in the Brazilian Abrolhos, starting later this week.  I hope you’ll stick around and join in the conversation.

Monday
Mar222010

What do expectant parents and the Chilean earthquake have in common?

The recent Chilean earthquake was a disaster on a mind-boggling scale; one that had its genesis beneath the sea.  The temblor, and all those in Chile before it, including the biggest ever recorded anywhere, resulted from the Nazca plate sliding down under the South American plate, under the sea to the South West of Santiago.  Well, it doesn't exactly slide, I always imagined it would sound like a creaking door if you could speed up the process a few zillion times.  The upward pressure this collision puts on the South American plate is immense and produces the longest mountain range in the world, the Andes.

Anyway, this most recent slip, which shifted about 10 meters and registered 8.8 on the Richter scale, caused a small tsunami.  Now some researchers from Scripps and UCSD want to know whether it was because of the sea floor movement itself, or because the quake triggered undersea landslides ("slumping") that produced the wave.  They are going to do some nifty multi-beam sonar work to map the seafloor changes in unprecedented details.  Sonar technology has become a really cool tool these days; the same sorts of benefits that new parents reap when they ultrasound their new bundle of joy also give scientists a fantastic new view on the sea floor.  Just check out this example of a shipwreck revealed by NOAA's nautical survey side-scan sonar.

Friday
Dec182009

Try fitting *this* into a bedside lamp

Work reported in Nature today from a presentation at the annual AGU meeting shows easily the deepest underwater volcano ever filmed.  The eruption was filmed from a remote submarine at 1200m depth - far more than the previous 500m depth record - and shows lava bursting out onto the sea floor.  The discovery helps scientists understand how pillow basalts form and how sea floor materials are added to the oceanic crust.

Its hard to imagine how extreme that process is.  We're talking hot enough to melt lead, at pressures that would turn a styrofoam cup into a thimble!