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.

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Geoscientist’s Toolkit: Dilute Acid

Folded outcrop of marine sediments in Berkeley, CA.  Image credit: Laikolosse (CC-BY-NC).
Folded outcrop of marine sediments in Berkeley, CA. Image credit: Laikolosse (CC-BY-NC)

When looking at sedimentary rocks in the field, one of the questions which may come up is whether or not a rock is a carbonate, such as in the outcrop pictured above. Although it is easy to determine that with an electron microprobe in the lab, there is a faster field test method: using dilute hydrochloric acid.

Sedimentary geologists will often carry a bottle of 0.1 M HCl and a watchglass with them in the field. A chip of the rock in question can be broken up and placed on the watchglass. When the acid is added, a carbonate will fizz as the acid releases carbon dioxide. This is the same process which makes a baking soda volcano erupt.

In some of my field work in the Texas Panhandle, I encountered a white layer among the redbeds. This bed was not gypsum, as many of the other white beds were. Because I was looking for volcanic ash deposits, not carbonates, an acid test was performed in the field. Unfortunately for me, the ground up sample started fizzing, so I knew it wasn’t the volcanic ash I wanted to find.

Where on Google Earth #492

For WOGE #491, Ole showed us a Jurassic/Cretaceous endorheic basin in Argentina, where little water-erosion has taken place since the deposition of the tephra and lavas.

While the forests in WOGE #491 were petrified, the forests of WOGE #492 are still very much alive.
woge_492

Find this place on Google Earth, then post the latitude and longitude in the comments, along with a description of the geology, geography, or other interesting -ologies of the place. The first commenter to correctly identify the place will host the next WOGE (guest-hosting can be arranged). A list of previous WOGE selections is available here (kmz) and here (twitter).

Geoscientist’s Toolkit: Radiosonde

Radiosonde launch.  Image credit: Laikolosse (CC-BY-NC).
Radiosonde launch. Image credit: Laikolosse (CC-BY-NC).

Last weekend, I had the opportunity to tour the National Weather Service‘s Weather Forecast Office in Chanhassen, MN. One of the highlights of the tour was the upper air sounding facility, which launches a radiosonde (weather balloon) twice daily (at 0000 UTC and 1200 UTC, which is 7:00 local time here in Minnesota during the summer).

Radiosondes provide crucial information about the state of the atmosphere by measuring pressure, temperature, relative humidity, wind speed, and wind direction. Wind speed and direction are determined by the movement of the balloon as measured by GPS. Observations are transmitted back to the launching station via radio, where they are merged with all the other observations around the world. The merged observations from the most recent soundings are fed into the numeric weather prediction models used by your local weather service office (and your local TV meteorologist) to create forecasts and evaluate severe weather threats.

Radiosonde instrument packages are fairly small—about the size of a thick book, like Anna Karenina, but mostly styrofoam. A wire protrudes from the instrument package which acts as the radio antenna and holds the temperature sensor away from the body of the radiosonde. The instruments are suspended on a line beneath a parachute, which is in turn held up by a large helium-filled balloon, perhaps 1.5 m in diameter (~5′). Upon release, the balloon will rise around 5-8 m/s (11-18~mph), and can reach a peak altitude of more than 38 km (23.5 miles), well into the stratosphere. At these altitudes, where pressure is less than .5% of the surface pressure, the balloon can expand to a diameter of 7.5 m (~25′).

Temperature and dewpoint profiles are often plotted as functions of pressure (closely related to altitude). In the lower part of the atmosphere, the troposphere, temperature gradually decreases as altitude increases. This is why mountains have a snowline. Beyond the troposphere lies the stratosphere. In the stratosphere, the temperature tends to increase with altitude, due to the absorption of UV light by ozone. Pictured below is a temperature profile from Chanhassen, MN, on a clear, warm day.

Temperature (right) and dewpoint (left) profile from Chanhassen, MN, on a clear, warm evening.  The x-axis is showing temperatures (°C), while the y-axis is showing pressure (in hPa; 1000 hPa is roughly surface pressure near sea level).  Image credit: University of Wyoming Department of Atmospheric Science.
Temperature (right-most dark trace) and dewpoint (left-most dark trace) profile from Chanhassen, MN, on a clear, warm evening. The x-axis is showing temperatures (°C), while the y-axis is showing pressure (in hPa; 1000 hPa is roughly surface pressure near sea level). For those curious about the purple and green lines, see here and here. Image credit: University of Wyoming Department of Atmospheric Science.

With a little training (e.g. this web course), a meteorologist can look at a temperature/dewpoint profile and identify whether it is sunny, cloudy, or if severe storms are likely.

Radiosondes are a great way to get detailed data about the state of the atmosphere. Within the US, there are roughly 90 balloon sounding stations, and there are many more stations around the globe. If I had more time and the funding to do it, I would love to send radiosondes up from Heard Island—it certainly is an under-sampled area of the globe.

Where on Google Earth #490

In WOGE 489, Ole took us to Ubundu, in the Democratic Republic of the Congo, where the muddy Lualaba River is joined by two clearer tributaries and flows over the Boyoma Falls (previously Stanley Falls).

Here I present WOGE 490:
woge_490

Find this place on Google Earth, then post the latitude and longitude in the comments, along with a description of the geology, geography, or other interesting -ologies of the place. The first commenter to correctly identify the place will host the next WOGE (guest-hosting can be arranged). A list of previous WOGE selections is available here (kmz) and here (twitter).

Counting Birds on Heard Island

Rockhopper penguins (Eudyptes chrysocome) on Heard Island.  Image credit: K. Lawton.
Rockhopper penguins (Eudyptes chrysocome) on Heard Island. Image credit: K. Lawton.

Heard Island is a wonderful place for birds. Indeed, Heard Island and the McDonald Islands are the only sub-Antarctic islands without introduced macrofauna, and which have had very little human influence (part of why they are listed as a UNESCO World Heritage Site).[1] Freedom from human influence makes these islands scientifically very interesting, because it is extremely rare to have a site which is that pristine and that isolated.

The species diversity for birds on Heard Island is not very high, even though the number of birds is very large—in the millions.[1,2] Only 19 species breed on the islands, with another 28 species recorded as visitors or seen at sea within the region around the islands.[2] Among the species breeding on the island are four types of penguins (king, gentoo, macaroni, and [my personal favorite,] rockhopper), and three species of albatross (wandering[?], black-browed, and light-mantled sooty). Two species, the Heard Island sheathbill and the Heard Island cormorant, are endemic to Heard Island and the McDonald Islands.

To study the populations of these birds over time, it is necessary to take a census of the populations periodically. On the face, this seems relatively straightforward, and will be familiar to anyone who has participated in the Audubon Society‘s Christmas Bird Count or the Cornell Lab of Ornithology‘s eBird program [ed.: both are excellent citizen science projects you can do in your area, and I highly encourage you to get involved with them.].

On Heard Island, surveying the bird populations is very difficult. Not only do some of the species look similar (e.g. rockhopper and macaroni penguins), but there can be vast colonies of them. Heard Island is home to an estimated 1 million breeding pairs of macaroni penguins alone![2] Adding to the challenge, some species nest in burrows underground, so a photograph of the area may not help estimate the numbers like it might when the birds and nests are visible. With the exception of the penguins, birds on Heard Island can fly, and will often do so. If you have ever watched birds in your backyard, you may know it can be difficult to count the number of chickadees (or hummingbirds, or other species) visiting, because they move around a lot and look alike.

King penguin (Aptenodytes patagonicus) colony on Heard Island.  Image credit: Eric Woehler.
King penguin (Aptenodytes patagonicus) colony on Heard Island. Image credit: Eric Woehler.

Another challenge on Heard Island is that many places are quite inaccessible to humans. Sheer cliffs, unstable slopes, and glacial crevasses keep people away from nesting areas.

Even the act of counting birds can put some of them at risk: if a nesting pair is disturbed by a human—even 100 m away—they may fly off the nest long enough for a scavenger to fly in and eat the egg or the chick.[2]

Detailed surveys of the breeding areas can provide important information about the migrations of the birds. Antarctic terns, which had been banded at Bird Island, South Africa, were later observed nesting on Heard Island.

The strategy for counting birds is a little complicated. For small numbers of birds (in my backyard at home, or rare birds on Heard Island), they can be counted individually; this works up to around 50 birds. Beyond that, the numbers are likely to become estimates, though high-resolution photographs of nesting colonies provide a record which can be carefully scrutinized later for more exact numbers. When estimating birds in the field, you would count the number in a small area (one binocular field-of-view, or a 10×10 m region of ground. That gives an estimate of the birds-per-area. Multiplying by the larger area (as multiples of the smaller one) would give an estimated total number of birds.

King penguins (Aptenodytes patagonicus) and elephant seals (Mirounga leonina) in front of Lambeth Bluff, Heard Island.  Image credit: Eric Woehler.
King penguins (Aptenodytes patagonicus) and elephant seals (Mirounga leonina) in front of Lambeth Bluff, Heard Island. Image credit: Eric Woehler.

At sea, another method is used. Because birds are not standing or sitting on a nest, they are counted (species and number) for ten-minute periods, with the location, date, and time noted.

When I go to Heard Island, I will do some birding. My life list has no penguins on it, and I intend to change that. I will not have the time to do a full survey of the birds, but will probably get at least a few estimates of a colony or two. I also intend to do some at-sea counts if the weather and sea-state permit. These observations will be added to the eBird database when I return to civilization after the expedition. One additional challenge I will have is that all of the birds of Heard Island will be life birds for me, and I am not especially good with my field identification of new birds. If you know of a good field guide, please let me know; I’d love to have a good reference to bring with me.

***

[1] http://whc.unesco.org/en/list/577 UNESCO organization. Retrieved May 17, 2015.
[2] Woehler, E. J. “Status and conservation of the seabirds of Heard Island”, in Heard Island: Southern Ocean Sentinel (Eds K. Green and E. Woehler) Surrey Beatey & Sons, 2006, p. 128-165.

Geoscientist’s Toolkit: Frantz Magnetic Separator

Frantz magnetic separator.  Image credit: Bill Mitchell (CC-BY).
Frantz magnetic separator. Image credit: Bill Mitchell (CC-BY).

When a sample for geochemical analysis gets in to the lab, often one of the first priorities is to separate the mineral(s) of interest in the rock (e.g. zircon, potassium feldspar, or quartz) from the other minerals. Once a rock has been crushed and milled to single-grain size, the sample is ready for separation.

One of the first methods employed is magnetic separation, which will separate the more magnetic (paramagnetic) minerals from the less magnetic (diamagnetic).

For magnetic separation, a Frantz magnetic separator is used (see figures). It has a chute which is tilted both down its long axis (right to left in the picture) and its short axis (far to near in the picture). With the chute alone, the samples would end up in the non-magnetic (near, pink) bucket from gravity. However, a strong electromagnet is used (big black things above and below chute) which holds the paramagnetic materials up against the force of gravity, directing them into the far chute.

Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon).  Image credit: Bill Mitchell (CC-BY).
Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon). Image credit: Bill Mitchell (CC-BY).

Both the feed chute and the main chute have vibrating motors attached, so that the grains get slowly bounced around and move gradually down. The electromagnet provides enough force to keep the paramagnetic minerals in the upper (far) part of the main chute. By adjusting the current running through the electromagnet, the threshold for magnetic/non-magnetic can be controlled.