Tag Archives: papers

The 1991 Heard Island Feasibility Test

MV Cory Chouest the ship used for the experiments detailed below.  Image credit: US Navy (public domain, via Wikipedia).
MV Cory Chouest, the ship used for the experiments detailed below. Image credit: US Navy (public domain, via Wikipedia).

Batten the hatches and hang on to the hand rails, because this installment of science at/on/near Heard Island is going to be a wild ride! We’ll explore a paper entitled The Heard Island Feasibility Test,[1] and along the way we’ll make ports of call in climate science, oceanography, and physics. I encourage you to check out a copy of the paper, either at your local (research) library or online. It’s really well-written! There’s also a pre-experiment lecture given by the study’s lead author which is freely available online, and details the rationale behind the study and the expected results.

In 1991, scientists were concerned about global warming. They were very interested in measuring the ocean temperature—oceans can store much more heat than the atmosphere, so while the atmosphere may not warm quickly in a changing climate, the oceans are likely to capture most of the heat. Additionally, water has a high heat capacity (the amount of energy it takes to raise its temperature by a degree), which is why it takes so long to bring a pot of water water to a boil on the stove.

Measuring the ocean temperature seems fairly straightforward: put a thermometer in the ocean, and log the temperature. Scatter a bunch of stations around the world and it’s done, right? Wrong.

The problem with using a thermometer (or many thermometers) to measure the ocean temperature is that there are many small-scale features which can influence the measured temperature. The variability of these measurements is likely to be quite high, and they each measure only a small place— extrapolating to the whole ocean isn’t necessarily justified.

How, then, can a measurement be made which yields an average temperature over a huge volume of ocean?

Sound. Ocean temperatures can be measured with sound. This is an amazing world in which we live!

In water, the speed of sound will vary depending on temperature, pressure, and (to a limited extent) salinity, and be in the ballpark of 1.48 km/s. With variations in speed of 4–5 m/s/°C, a +5 m°C (0.005 °C) change in temperature results in a -0.1 s change in travel time over a 10 Mm (10,000 km) path.[1] Have an acoustic source emit a signal, measure the signal at a distant receiver, and the time delay will yield an apparent average speed of sound. Shifts in these speeds due to warming of about 5 m°C/yr would theoretically produce measurably earlier arrival times.

Speed of sound measured at various depths in the Pacific Ocean north of Hawaii.  Image credit:  Nicoguaro (CC-BY-SA); data from the 2005 World Ocean Atlas.
Speed of sound measured at various depths in the Pacific Ocean north of Hawaii. Image credit: Nicoguaro (CC-BY-SA); data from the 2005 World Ocean Atlas.

One potential problem with all this is the part about receiving a sound signal 10 Mm away from its source. However, the temperature and pressure profile of the ocean cause a minimum in sound velocity at a depth of 500–1,000 m (for mid/low-latitude oceans). This low-velocity region, termed a SOFAR channel acts as a waveguide or a duct, where sounds within it tend to stay within rather than dispersing.[2] Low-frequency sounds (50–100 Hz)are not attenuated or absorbed much by the water, so long-distance reception of these sounds might be possible.

The feasibility test was designed as a proof-of-concept for ocean-wide acoustic reception. Using powerful low-frequency transducers on loan from the U.S. Navy, the scientists would be able to send the signals and have receivers around the world listening for them.[3] Unfortunately for the scientists, the transducers could only operate to a depth of 300 m. That meant that a high-latitude site needed to be found, where the SOFAR channel—that special place which enables long-distance reception—is much closer to the surface.

Heard Island was chosen as a transmission site, because the direct sound paths (mostly, but not entirely, great circles) would reach across both the Pacific and Atlantic oceans.

No major field work is complete without a little drama, though. Late in the planning and preparation phase, the US National Marine Fisheries Service notified the researchers that permits were required to mitigate threats to marine mammals from the powerful sounds. The Australians (Heard Island is an Australian territory) required the permits too. A second vessel was chartered and biologists were assembled to monitor marine mammal activity and fulfill the responsibilities associated with the permits.

The two ships sailed as originally scheduled on January 9, 1991, but neither the American nor Australian permits had been issued. With a scheduled transmission start of January 26th, there wasn’t much room for delay. Fortunately, the permits arrived just in time: January 18th and January 25th. I bet the scientists were very tense during the voyage from Perth/Fremantle (Australia) to Heard Island.

An unscheduled 5-minute equipment test the day before the first scheduled transmission was received in Bermuda, and shortly thereafter at Whidbey Island (near Seattle, and almost 18 Mm away). Basic feasibility was already shown!

Signals were sent in a 1-hour-on, 2-hours-off pattern. Some of the transmissions were a continuous-wave (CW) 57 Hz tone (to avoid 50 Hz and 60 Hz power noise), while others were a mixture of several different frequencies near 57 Hz. For details on these transmission modes I refer you to the paper.

Transmissions for the experiment were aborted on the 6th day—ahead of schedule—when a gale and 10-m swells caused one acoustic source to be lost from the string and fall to the ocean floor. The other sources were badly damaged. Conditions in the Southern Ocean can make field work there very difficult.

One thing I found surprising, but makes plenty of sense upon consideration, was that rather than staying in one fixed location, the ship towed the sources along at 3 kt (5.5 km/h, 3.5 mph). This makes sense once you think about the wind and waves in the Southern Ocean, and how, to maintain control of the ship, the vessel must be underway. Being broadside to the swell in a high sea is extremely dangerous.

In this experiment, the receivers were sensitive enough to detect the Doppler shift from the ship’s movement. In fact, the Doppler shift combined with the known path of the ship (from GPS) allowed the azimuth of the signals to be determined. For many of the signals, it was on the expected heading (not quite a great circle due to the non-spherical Earth and the inconsistent depth of the SOFAR channel). At Whidbey Island receiver array, though, the signals arrived from a bearing of 215°, not the 230° predicted. In that case, the signal appears to have taken a longer path southeast of New Zealand, rather than through the Tasman Sea and between Australia and New Zealand.

Fortunately for all involved, there was little impact noticed on the marine mammals.[4] Despite the low observed impacts, the authors make recommendations for the Acoustic Thermometry of Ocean Climate project to reduce adverse effects to marine life. Using shorter-range transmit/receive pairs, the total power needed can be reduced significantly. Additionally, with temperate waters having a deeper SOFAR channel, the transmitters can be bottom-mounted at depths of around 0.5–1 km, which will help physically separate them from the near-surface-dwelling marine mammals.

In short, the Heard Island Feasibility Test was a resounding (pardon the pun) success. Ocean acoustic temperature measurement is possible, and measurements were made in the North Pacific for a decade, from 1996–2006.

This paper was a really interesting one, and fairly accessible (scientifically) to someone not in the field of signal processing or oceanography. I enjoyed reading it, and suggest you take a look at it if you’re at all interested. My summary here has skipped over large parts which detail the nature of the propagation and the signal processing aspects.

***
[1] Munk, W. H., Spindel, R. C., Baggeroer, A., Birdsall, T. G. (1994) “The Heard Island Feasibility Test” J. Acoust. Soc. Am. 96 (4), p. 2330–2342. DOI 10.1121/1.410105

[2] This phenomenon is analogous to atmospheric ducting of radio waves, which can cause TV and FM radio stations to be heard far beyond their normal range, and for weather radar to pick up ground clutter far from the station.

[3] This sounds almost analogous to the upcoming VK0EK ham radio expedition to Heard Island, where radio operators (including myself) will have stations around the world listening for their signal.

[4] Bowles, A. E., Smultea, M., Würsig, B., DeMaster, D. P., Palka, D. (1994) “Relative abundance and behavior of marine mammals exposed to transmissions from the Heard Island Feasibility Test” J. Acoust. Soc. Am. 96 (4), p. 2469–2484. DOI 10.1121/1.410120

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Preferential Preservation of Phytoliths

Scanning electron microscope image of an elephant grass phytolith after dry-ashing.[1]  Image credit: Benjamin Gadet (CC-BY-SA).
Scanning electron microscope image of an elephant grass phytolith after dry-ashing.[1] Image credit: Benjamin Gadet (CC-BY-SA).

As I was looking through the recently published papers in PLoS ONE (all open-access!), I came across an interesting article on the preservation of phytoliths.[2] It is an interesting and well-written paper, and is quite accessible—both in terms of copyright and of science content.

Plants often have little bits of rock in them, called phytoliths (phyto- plant, -lith rock). Phytoliths are formed within the plant by precipitating SiO2 in a non-crystalline form (opal). These microscopic stones can help maintain the structure of the plant, perhaps among other functions. They also preserve well, because SiO2 (glass, essentially) generally doesn’t react chemically with much in the environment.

Just like with fossilized bones or impressions of leaves, the size and shape of phytoliths can be used to identify the plant (or family of plants) which is producing them. If phytoliths are found in the geologic or archaeologic record, they can be used to determine what kinds of plants were in the area, or were being eaten. They also contain small traces of carbon, which can be used for radiocarbon dating (back to ~40 ka) or 13C isotope analysis.[3]

This paper is looking at what happens to various phytoliths in the archaeologic or geologic record, and whether there are preservation biases (some phytoliths being destroyed more easily than others).

The authors took samples of four different types of modern, living plants. These samples were then burned away in a 500°C furnace, leaving just ash and the microscopic rocky bits. With some further, relatively gentle treatment, they were able to isolate the phytoliths. Some of these phytoliths were mounted on microscope slides and counted to determine the relative abundance of different sizes and shapes.

Isolated phytoliths were partially dissolved for six weeks, and the Si content of the liquid was measured. The partially dissolved phytoliths were dried, mounted on microscope slides, and they too were counted to determine relative abundance of the different sizes and shapes after treatment.

Phytoliths which were small, and had a large surface-area-to-volume ratio, tended to be preferentially dissolved—this is not an unexpected result, but is important. The authors argue that based on the Si solubility, the degree of preservation can be assessed (high Si solubility means better preservation); in situations where the Si solubility is low, some of the more delicate phytoliths are likely to be missing, and a count of phytoliths under those circumstances would yield biased results.

But don’t take my word for it! Read the paper. It’s better written than my short explanation, and a fine example of scientific scholarship.

[1] Parr, J.F.; Lentfer, C.J. & Boyd, W.E. 2001, ‘A comparative analysis of wet and dry ashing techniques for the extraction of phytoliths from plant material’, Journal of Archaeological Science, vol. 28, no. 8, pp. 875-886. DOI: 10.1006/jasc.2000.0623

[2] Cabanes D. & Shahack-Gross R. (2015) Understanding Fossil Phytolith Preservation: The Role of Partial Dissolution in Paleoecology and Archaeology. PLoS ONE 10(5): e0125532. DOI:10.1371/journal.pone.0125532

[3] Looy, C.V.; Kirchholtes, R.P.J.; Mack, G.H.; Van Hoof, T.B. & Tabor, N.J. 2011, ‘“Ochoan” Quartermaster Formation of North Texas, U.S.A., Part III: First Sign of Plant Life‘ Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 383.

A Window into the Mantle (Part 2)

Heard Island, February 23, 2015.  Scale: 250 m/pixel.  Image credit: excerpted from NASA GSFC (Aqua/MODIS).
Heard Island, February 23, 2015. Scale: 250 m/pixel. Image credit: excerpted from NASA GSFC (Aqua/MODIS) (warning: ~5 MB!).

Previously, I wrote about some of the challenges of studying the mantle. I also wrote about mass spectrometers—this was not accidental, as they were used heavily in the research discussed here. If you have not read those items already, you should do so before continuing. Also, if you are not familiar with isotopes, you may wish to get more familiar with those as well.

Although Big Ben is the dominant feature on Heard Island (seen above with a bow wave and some poorly-defined Von Karman vortices), there is a smaller volcanic edifice, Mt. Dixon, on the Laurens Peninsula (to the NW, right in the bow wave from Big Ben). Mt. Dixon is home to many lava flows, which can be seen on Google Earth, and are believed to be as young as 200 years or less.[1]

The major-element composition (Si, K, Na) of the lavas from Big Ben and Mt. Dixon can be quite different.[2] Big Ben generally has basalt and trachybasalt composition (low SiO2, moderate K2O + Na2O), while the Mt. Dixon and the other cones on the Laurens Peninsula show a much wider range, from basanite to trachyte (wide range of SiO2, generally higher K2O + Na2O).

Where things really get interesting is in looking at the isotopes. Specifically, Barling et al. looked at the isotopes of Sr, Nd, and Pb isotopes.[2,3] Some of those isotopes (86Sr, 144Nd, and 204Pb) are stable and non-radiogenic. That is, they do not decay away, nor are they formed from radioactive decay. The other isotopes studied (87Sr, 143Nd, 206Pb, and 207Pb) all are stable, but are the products of radioactive decay (87Rb, 147Sm, 238U, and 235U, respectively).

The ratio of radiogenic/non-radiogenic isotopes can be used to identify different sources, sort of like fingerprinting. To get high concentrations of radiogenic isotopes means that the rock’s history includes lots of the radioactive parent. Low concentrations of radiogenic isotopes means that the source rock has relatively little of the radioactive parent.

This is important, because although isotopes of an element are chemically similar, different elements behave differently from a chemical standpoint. Some are more often found in the crust than the mantle, while others are the opposite, depending on the compatibility of the element in mantle minerals.* Uranium is generally incompatible, and preferentially moves into the continental crust. Crustal rocks, would be likely to have a high ratio of radiogenic to non-radiogenic lead (product of uranium decay). Mantle rocks would have a lower ratio of 206Pb/204Pb, and similarly for 207Pb/204Pb.

Zindler and Hart (1986) proposed that oceanic basalts can be treated as mixtures of four components, each having a distinct chemical (and isotopic) composition.[4, via 2] Barling and Goldstein found that the Heard Island lavas exhibit a range of compositions consistent with mixing between two sources.[2] Neither of those sources matches the compositions suggested by Zindler and Hart. For the first Heard Island source, three explanations are given why that may be the case:

  1. The Heard Island source is a mixture of two Zindler and Hart sources
  2. That same Heard Island source is a fifth distinct mantle source
  3. It’s more complicated; the two Zindler and Hart sources in question define a spectrum, and the Heard Island source lies along that spectrum

Barling and Goldstein (1990) favored case 3, which they argue is reasonable given that recycling continental crust is likely to give a wide range of isotopic compositions.

Barling et al. (1994) built off of the results presented by Barling and Goldstein (1990), and focused on two main questions:

First, what is the origin of continental crustal signatures in oceanic basalts; are they inherited from the mantle source region, or are they caused by shallow contamination? If they originate in the mantle, how much continental material is present, how is it distributed and in what form, and how and when did it become incorporated into the mantle? Second, what are the origin and timing of enrichment of the sub-Indian Ocean mantle?

Perhaps some clarification is needed about what is at issue. Since it is clear there is some continental influence expressed by the Heard Island lavas, where in the history of that magma did mixing with continental crust occur? Was there a chunk of intact continental material relatively near the surface which partially melted as the basalt came upward through it? Or was there continental material which has been mixed in to the mantle beneath the Indian Ocean? If that occurred, when, and under what conditions?

Their data, and particularly the lead isotopic data (207Pb, 206Pb, and 204Pb), lead them (pardon the pun) to conclude that the component with a high-87Sr/86Sr is derived from marine (ocean) sediments subducted into the mantle at least 600 Ma before present, and probably 1–2 Ga. Modeling of the Sr isotope ratios and total concentrations, along with thermodynamic considerations, suggest that partial melting followed by partial crystallization from the magma is unlikely. That is, recycled crustal material is needed to make things work.

Barling et al. (1994) found that the overall isotopic compositions of the lavas suggest, if crustal material is indeed being recycled into the mantle, the subduction occurred around 1–2 Ga. That timing makes it far too early to be related to subduction beneath the paleo-supercontinent Gondwana.

Finally, the paper closes with the suggestion that, although Heard Island and Kerguelen Island are separated by 440 km, the two may be manifestations of the same plume head and hotspot. They note that the distance between the islands is quite small for separate hotspots, yet is obviously large for being just one hotspot. Perhaps the 2015 Heard Island expedition can collect samples which will give insight into resolving this question.

***

[1] Quilty, P. G.; Wheller, G. (2000) Heard Island and The McDonald Islands: a Window into the Kerguelen Plateau. Papers and Proceedings of the Royal Society of Tasmania. 133 (2), 1–12.

[2] Barling, J.; Goldstein, S. L. (1990) Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature 348:59-62, doi 10.1038/348059a0.

[3] Barling, J.; Goldstein, S. L.; Nicholls, I. A. (1994) Geochemistry of Heard Island (Southern Indian Ocean): Characterization of an Enriched Mantle Component and Implications for Enrichment of the Sub-Indian Ocean Mantle. Journal of Petrology 35:1017-1053, doi 10.1093/petrology/35.4.1017.

[4] Zindler, A.; Hart, S. (1986) Chemical Geodynamics. Annual Review of Earth and Planetary Sciences 14:493-571, doi 10.1146/annurev.ea.14.050186.002425.

* This turns out to be crucial for things like uranium-lead dating, where the mineral zircon generally crystallizes with 10-1000 ppm U, but does not incorporate Pb. All the Pb found in a zircon can be assumed to come from uranium decay or laboratory contamination (which has a known isotopic composition).

Geoscientist’s Toolkit: LaTeX

A LaTeX file.  Image/text credit: Bill Mitchell.
A LaTeX file. Image/text credit: Bill Mitchell.

LaTeX is a typesetting program used for preparing documents—generally articles and books, but sometimes posters and presentation slides. It is available for Linux, Mac, and Windows, and is free, open-source software. The primary output file format these days is PDF, but other options are available.

When you are putting together a document with figures, citations, and sections which get moved around, it is tough to use a common word processing program and maintain sanity. However, because LaTeX is a markup language (like HTML, the HyperText Markup Language), it is explicit which text is grouped where. For instance, suppose you are trying to have both superscripts and subscripts following a letter, such as in CO32-. If you need to edit the superscripts or subscripts in Word, it can get confused easily. In LaTeX, it is explicit which parts are superscript and which are subscript. A little more work up front saves a lot of frustration later.

Above is a small excerpt from my dissertation, written in LaTeX. I am not sure I would have survived grad school had I attempted to write my dissertation in a word processor.

Yes, there is a learning curve to using LaTeX, and you don’t see changes in the finished document immediately when you make them in your text editor, but there are tons of advantages.

First, the format is all plain text, so it will be readable for a long time and across platforms (although the OpenDocument formats are attempting to make word processor documents future-compatible). Plain text is also very convenient when combined with things like version control software. Track changes isn’t just for word processors!

LaTeX separates the content from the formatting. Most of the formatting is done automatically. Yes, you manually specify that something is a emphasized, or is part of a quote, or a heading, but LaTeX will make sure that the formatting is consistent throughout (unless you intervene), and the defaults are generally good.

One place where LaTeX really shines is in mathematical equations. Greek letters and many mathematical symbols are input as commands such as \beta or \sqrt{n}, so your hands need never leave the keyboard. Once typeset, the equations are neat and properly sized.

Many journals accept submissions in LaTeX, if they do not outright encourage its use, because it is easy to keep the formatting consistent from article to article. The fonts will match, the font sizes will match, and in general things are awesome and look professional.

I have given several presentations made with LaTeX (pdf output). The outline slides are automatically maintained, and slide headers/footers can show where in the presentation you are. Those indicators link to the sections if you click on the section name, and it’s all done automatically. LaTeX is totally worth the effort of learning, and do it soon while you can take your time and experiment. Writing your dissertation while learning LaTeX is a recipe for unhappiness.

So, now that you’re ready to get started, here are some tutorials and reference materials:

It really bothers me when the justification for doing something slow, inefficient, and expensive is “that’s what most people use, and I can’t be bothered to learn something new.” There comes a time to do things differently, and a good ecosystem is one where there are several options based around open standards. Case in point: USB ports are great! The proprietary charger connection on my (old-school) phone? Awful. Lock-in is expensive. Choose open source.

Peer-Reviewed Research: Terrestrial Vegetation and Environments on Heard Island

Kerguelen cabbage (Pringlea antiscorbutica).  Image credit: B.navez, CC-BY-SA, via Wikimedia commons.
Kerguelen cabbage (Pringlea antiscorbutica). Image credit: B.navez, CC-BY-SA, via Wikimedia commons.

Previously I’ve mentioned rocks, glaciers, a volcano, penguins, and elephant seals, but what about plants on Heard Island? That has been well-covered (pardon the pun) by Bergstrom and Selkirk (2000), who published their findings in an open-access journal.[1]*

Vegetation on Heard Island is generally categorized into six groups (“communities”), which reflect the general microhabitats and species makeups of the area. Here are a couple of brief descriptions:

Poa cookii maritime grassland” is characteristic of nutrient-enriched, animal influenced environments (Hughes 1987, Scott 1988). This community is dominated by the small tussock grass Poa cookii (Hughes 1987), with the nitrophiles Callitriche antarctica and Montia fontana also common.

“Feldmark communities” are characterised by less than 50% vegetation cover. Hughes (1987) described feldmark as having high relative vasular plant diversity but low species abundance, with predominant plants being Azorella selago, Poa kerguelensis, Colobanthus kerguelensis, Pringlea antiscorbutica, bryophytes and lichens. Scott (1988) recorded feldmark on well-drained areas of high altitude/high wind exposure, areas of recent glacial retreat, flat valleys likely to be subject to cold air drainage, and geologically recent lava flows.

After establishing some terminology about what types of communities are there, the authors move into the environmental factors that influence the plants on Heard Island.

Animal-derived nutrients are one factor. “A nutrient gradient is apparent, diminishing away from coastal seal and bird-breeding, resting, and hauling-out areas. Areas affected by direct manuring by seals, penguins, cormorants and giant petrels are generally devoid of vegetation, reflecting toxic nutrient levels and physical damage to plants.”

The types of rocks present, and the geochemistry of those rocks, could control what plants will thrive there. However, that has not been studied in detail on Heard Island (yet! [as of 2000]).

Salt spray from the ocean can make life difficult for plants, as can debris blowing in the wind. The depth to which roots can be sunk varies greatly, from almost nothing on the lava flows to more than 50 cm on moraines and other areas of loose sediment. Water availability also is important; some areas with poor drainage form pools, while other areas of loose rocky material drain very quickly. Snowmelt provides water throughout the summer, although precipitation is frequent throughout the year.

Movement of the rock/soil surface, such as through landslides, frost-heaving, and sediment accumulation can disrupt plant activity. Animals can trample (and eat!) plants, in addition to “adding nutrients”. Of course, the general climate influences, such as temperature (warmer toward sea level) and sunlight (more clouds on west side, more sun on east side) also play a role.

Bergstrom surveyed the plant diversity and abundance quantitatively during the 1986/1987 Australian National Antarctic Research Expedition to Heard Island. Almost 500 quadrats (1×1 m squares) were surveyed, and included three main vegetated areas of the island: Laurens Peninsula (northwest), Spit Bay (southeast), and Long Beach (south-southwest). By placing these quadrats on random (or as random as practical) ice-free locations, representative population statistics can be tabulated. Some species occur primarily clustered with certain other species, and others (e.g. Azorella selago) are widespread.

This survey of terrestrial flora provides an excellent baseline from which to study the changes in populations as the climate warms and glaciers recede on Heard Island. Through this kind of work, scientists can find how the plants are responding to changing conditions and new areas to colonize.

Beyond this research are some big questions: how did plants first arrive on Heard Island? Where did they come from? Which species were first to arrive? When did they arrive, and how long had Heard Island been above the ocean when they came?

[1] Bergstrom, DM and Selkirk, PM (2000) Terrestrial vegetation and environments on Heard Island. Papers and Proceedings of the Royal Society of Tasmania, 133 (2). pp. 33-46. ISSN 0080-4703

* Open access journals are a great way to ensure that research is widely accessible. I am considering outlining my views on academic publishing in a later post (this footnote is only so large).