Tag Archives: peer-reviewed research

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.

Glaciers and City Buses

Central and southern portion of Heard Island, including Gotley glacier.  Image credit: NASA ISS.
Central and southern portion of Heard Island, including Gotley glacier. Image credit: NASA ISS

Heard Island is covered by 41 glaciers.[1] Some of these glaciers, such as Stephenson Glacier, are retreating rapidly, while others, like Gotley glacier in the southwest, have remained steady since 1947 when detailed surveys began.

The glaciers on Heard Island, like glaciers everywhere, act much like a conveyor belt, a giant river of ice. Snow (and rain) falling near the top solidifies into ice and flows downhill, until either the ice melts and flows away as a stream, or the ice meets the ocean and calves (breaks away).

Unlike the conveyor belt analogy, the snow does not only fall at the very head. Indeed, at times, the whole glacier can receive snow, or could be melting at the surface. On average, though, the snow accumulates in the eponymous accumulation region, and melts in the depletion region.

Perhaps a better analogy for a glacier is not a conveyor belt, but a city bus which runs into, and through, a city. Toward the beginning of the route, people board the bus, but very few leave. In the middle of the city, some people come and others go. Finally, as the bus leaves the city toward the surrounding area, more people leave than get on, and after reaching the end of the route, the bus is empty of passengers.

To really get a grasp of how glaciers work, you might want to try this Java-based simulation (helpful activity guide). One aspect of glaciers that becomes apparent in the simulation is that the surface of a glacier moves more rapidly than its base. There’s a tool to let you drill a virtual hole in the ice, then watch as the hole gets stretched out as the glacier flows downhill.

Now that you’ve spent a while playing around with the simulation—I certainly have—let’s take a look at the glaciers of Heard Island.

As I mentioned before, some glaciers are retreating, while others are relatively unchanged. Several factors influence the size of glaciers: precipitation amount, freezing level (of the atmosphere) [generally related to sea level surface temperature], relative humidity, albedo (reflectivity), and the thickness of rocky debris cover.

Precipitation and the freezing level are fairly straightforward factors: more snow and cooler temperatures yield larger glaciers. But there are complications! In a windy environment, such as that of Heard Island, snow doesn’t just pile up evenly as it falls; it drifts. Snow that falls in one place may end up being picked up by the wind and deposited somewhere downwind. This effectively adds precipitation to downwind areas, and removes it from upwind areas.

Relative humidity is important to glacier size, because even cold, dry air can cause snow and ice to sublimate—turning directly from solid to gas. In places like the McMurdo Dry Valleys in Antarctica, this effect keeps the valleys from filling with snow and ice. On Heard Island, a shift in the wind, or a more steady wind direction, can cause different areas to be affected by these dry winds, or to be affected more than in years past.

Here are some of Ruddell’s comments on the matter of why some glaciers are retreating and others do not:[1]

The accumulation, distribution and snowline elevation on many Heard Island glaciers appears to be influenced strongly by the re-distribution of snow by the wind. The prevailing wind direction is westerly and there is less likely to be a re-distribution of snow to low elevations on the westerly facing glaciers… Further, the re-distribution of snow by wind on west-facing glaciers is likely to be impeded not only by wind speed and direction but by severe crevassing (e.g., Gotley Glacier).

When the Heard Island expedition arrives this (austral) November, the glaciers can be observed from close range, rather than by satellite. An additional topic of interest is documenting the advance of vegetation toward recently de-glaciated areas.

During the 1947-2000 period, the glaciers on Heard Island showed an overall reduction in area of 12%.[1] The trend has been toward retreat, and with temperatures increasing about 0.8 °C since 1947, the glacier areal coverage in 2015 is almost certain to be lower still.

***

[1] Ruddell, A. “An inventory of present glaciers on Heard Island and their historical variation”, in Heard Island: Southern Ocean Sentinel (Eds K. Green and E. Woehler) Surrey Beatey & Sons, 2006, p. 28-51.

Paying the Bills with Open Access

Mean and median NSF grant sizes, adjusted for inflation.  Image credit: NSF.
Mean and median NSF grant sizes, adjusted for inflation. Image credit: NSF.

Last week, I shared some of my opinions on open access academic journals (or journal articles). I received the following response on Twitter:

.@i_rockhopper Good points here! OA is great, but who pays for publication in these journals? Publication costs are not favored in budgets.
Elizabeth Herndon @emh824

It is a very important part of the picture, and I have a few ideas which might be considered. Before discussion gets any further, though, I will say that there is no single magical way to make it work. Like with addressing climate change, there will need to be many small actions which collectively bring about some needed reforms—and maybe some bigger actions along the way.*[1]

My response here is based on my experience in the volcanology, geochemistry, and petrology areas of geoscience/geophysics. I know that open access varies greatly between disciplines. Some disciplines, like the NIH-funded biomedical research, have mandatory public access regulations, while others such as physics seem to have a culture of open access (arXiv.org).[3,4] Other disciplines, such as atmospheric chemistry & physics, are generally published in newer, fully open-access journals.

Open access publishers need to recoup their expenses, which are not trivial (significant IT costs, editorial staff, administrative staff). PLOS One and the EGU journals, among others, do this by charging article processing charges (APCs). These can be very significant: a 10-15 page paper with a couple figures and a table or two may cost $1-2k.

To put this number in context, let’s consider the typical (median) NSF award. The 2013 awards are summarized in this report (1.6 MB pdf) from the NSF, which is an interesting read.[5] For 2013, the median annualized award amount was $130k [see report, page 19]. If you have a 3-year grant which you use to publish two papers at $1300 each[6], that will use 1% of your budget.

As was pointed out earlier, funding agencies don’t like pay for publication, because it’s money you’re not spending on doing science. On the other hand, they should insist on open access because they want to maximize the number of people benefiting from the research (“broader impacts”). It is likely government funds are used to buy access to commercial journals (many times over, once for each public university and national lab), so why not reduce those costs and pay to publish things freely in the first place? Compared to the cost of bombing various countries with the latest drones, it wouldn’t take much to open up academic publishing.

There are a couple other options I can think of which may or may not work:

  • Department-level funding. This would be great, but I wouldn’t hold my breath waiting for it to happen.
  • University-level funding, such as from the library. Change could happen here, because a move to widespread open access would greatly reduce subscription costs. The transition is difficult, which commercial publishers will take advantage of.
  • Professional societies funding their own journals. The Geological Society of America and the American Geophysical Union each have their own journals. Using a portion of memberships and other society income to support open access journals could bring about the open access change that is needed. I applaud the work the European Geophysical Union is doing with their journals (expensive though they are).[7]
  • The PeerJ model. While it hasn’t been thoroughly tested yet, a flat rate of $99 for a lifetime membership and 1 paper/year seems like a good deal. It’s too bad they don’t publish geoscience.

It all seems to boil down to a problem of externalities. To the funding agency, publication costs are an externality. For libraries, open access isn’t cost-effective until they can ditch the expensive subscriptions. For established researchers, the costs of for-profit journal subscriptions are an externality. Early-career researchers are pressured by the hiring/tenure system to publish in the established, for-profit journals. And finally, the commercial publishers have a huge interest in making sure their lucrative business remains intact, and will act to make the barrier to open access [seem] as high and painful as possible.

Without some form of external impetus, widespread adoption of open access will probably remain elusive.

Those are my thoughts on funding open-access publication as of today. With further thought and discussion, they will evolve. In particular, I am looking forward to reading Cory Doctorow’s book, Information Doesn’t Want to Be Free, which treats on the issues of copyright and getting paid for creative works in the internet age.[Review]

I would be interested in hearing what your thoughts are on paying for open-access publishing, either in the comments below (open for 14 days), or on Twitter (@i_rockhopper).

* Due to the large number of footnotes here, they will be numbered, not asterisked.
[1] Bonus fact: global warming, or climate change, is really happening.[2]
[2] Bonus opinion: Global warming should be addressed sooner rather than later, with a major eye toward reducing our energy consumption and fossil fuel burning.
[3] I may have misrepresented the time frame under which NIH-funded work becomes publicly available in my previous post. The submission to the PubMed database must be immediate, but public access to that work may be delayed by up to one year to keep commercial publishers profitable (and libraries stuck in the position where they must maintain expensive subscriptions to stay current).
[4] Yes, arXiv is a pre-print, non-peer-reviewed repository.
[5] One unexpected tidbit I found in it was that the number of senior research personnel being supported on NSF grants is up 48% since 2005.
[6] Universities often charge overhead on grants of ~50%. This can be a lucrative “revenue stream” for underfunded “public” institutions (which may not have as much state funding as the public thinks).
[7] The EGU journals are also giving a significant financial incentive to use LaTeX (€5/page).

The Importance of Openness

Open prairie.  Image credit: Laikolosse (CC-BY-NC).
Open prairie. Not the same kind of openness as discussed below, but it looks nicer. Image credit: Laikolosse (CC-BY-NC).

On March 18, the National Science Foundation took a small step toward advancing the state of science in the world by announcing a new public access plan (more details here). It is a good start, but leaves plenty of room for improvement.

Academic publishing is dominated by for-profit publishers (Elsevier, Wiley, Springer, and others), who rake in the big bucks.

Here’s how their racket works. Academic researchers need to be able to read about the findings in their field and related fields, so rather than paying $30-40 per article, the institutional library will negotiate a year-long contract.* Because the publisher has a monopoly on research in their journals, the libraries don’t have much leverage during negotiations. Publishers will sell “bundled” journal packages, which include the journals people actually read and use, as well as a whole bunch that are extremely infrequently read. This manipulates the cost-per-article and cost-per-journal statistics.

For the researchers, library costs are generally an externality. Combine that with the need to publish in established journals if you want to land a tenure-track job, get tenured, or get promoted, and the researchers have all kinds of motivation to publish in the for-profit journal.

The system perpetuates. Researchers only publish in for-profit journals to keep their jobs, and the for-profit journals keep milking the library for everything it’s worth, safe to point out that people should publish with them because that’s what everyone reads (because that’s where people publish). It’s a vicious cycle, just like in computer software, where people write software for Windows because that’s what people use, and people use Windows because that’s what people write software for. None of that is to say that Windows is a good operating system, just a fairly well entrenched monopoly.

In the past few years, there has been an increased awareness of the need to break this cycle. Particularly, the National Institues of Health has moved to requiring that papers be made available free-of-charge on PubMed immediately upon acceptance for publication in any peer-reviewed journal. The program has been well-received by the academic community, and means that more people have access to the results of federally-funded research.

However, the NSF has been lagging behind the NIH in this front. Their recent move is a good step in the right direction. However, it still gives a 1-year embargo to preserve the profits of publishers. The rule doesn’t go into effect for quite a while, either. Only grant applications which were due or submitted after January 1, 2016 will be subject to the rule.

A few issues remain. Access to articles published before 2017 (6 months to review grant applications, plus 6 months to paper, a very generous estimate) will still be paywalled. Some of the literature I’m highlighting here on the blog is from the mid-1980s, yet access still is $30-40/article.

The real solution is open-access journals, such as PLOS One, PeerJ, or the family of journals published by the European Geophysical Union. Because of the NIH requirement, the biological and medical sciences have seen a great deal of inroads for open-access journals. Sadly, the geological sciences have generally lagged behind.

* The terms of which, both financial and otherwise, are protected under non-disclosure agreements.

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).

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).