What I Learned Today

On the final day of our field trip, we headed to New York Canyon, one of the best places to see the Triassic-Jurassic boundary. We hiked up a twisting path to the top of a ridge, and then one by one began to spot fossils. On the ground. Just lying around in bits and pieces. Everywhere! These were a combination of bivalves, gastropods, and ammonites — lots of ammonites. Ammonites are commonly used for biostratigraphy, the practice of dating a particular layer by the type of ammonites it holds. They’re particularly useful for this because they were evolving so quickly that a particular shape (type of chambers, amount of ridges on the outer shell, etc.) can pinpoint dates more precisely than most other fossils can.

Most of the fossils were trace fossils, or imprints of an ancient organism (as shown at left). However, we did find some whole pieces that were undoubtedly the silicified remnants of actual ammonites. I found one about 1 cm in diameter. It would not be an exaggeration to say that the whole ridge we were on was littered with these things — so much so that we were making slower and slower progress up to the top and then down the other side. Our guides were urging us on to the next ridge over, so that we could look back at the actual boundary layer from enough distance to really see it, but it was awfully hard to press onward when everyone was oohing and ahhing over their latest finds.

At right is a beautiful slice through a gastropod fossil (also silicified). I was struck by how almost unrealistically perfect it seemed — as if it had been drawn, not real! The sparkle effect induced by the silicification makes it even more of a Hollywood fossil.

We did make it up to the other ridge, and turned around to regard the boundary layer itself. There were several holes that had been dug in and around the boundary, presumably by other researchers looking not for fossils but for evidence of a carbon isotope excursion to help pinpoint the boundary itself. As important as this is, it was somewhat less exciting than finding fossils. We never did find the smooth-shelled ammonite that is supposed to be the lowest (oldest) ammonite, appearing just after the transition to the Jurassic. An educational day nonetheless!

The second day of our Geology 601 field trip led us to the (in)famous Alamo Breccia. A breccia is a rock that contains a bunch of other angular pieces of rock, all jumbled together, and generally forms after something breaks up the original rock and then cements the rubble together. The Alamo Breccia is notable because several researchers have hypothesized that it was created by a massive meteor impact. So out we went in the early, crisp (31 degrees F) Nevada morning to see if we could find the evidence that pointed to this interpretation.

We started by clambering up to the exposed rock face shown in the top image. And we did find breccia — lots of it (see right)! Some types of evidence for a possible impact were not things we could observe at the outcrop. One such “smoking gun” would be the presence of “shocked” quartz grains, which are grains with many parallel fractures, which could be created by an impact shock wave. This requires a microscope to confirm. Another way that we identify past impacts is by the presence of anomalously high levels of iridium. Iridium is rare in terrestrial rocks, but much more common in meteorites; if one impacted, its debris would leave an iridium signature in the surrounding area. This has been tested in this region, but so far only very low levels of iridium have been observed. However, the researchers in favor of an impact-related explanation for the breccia have also claimed to have found impact-created lapilli, which are little spherical beads of rock that was liquefied and then cooled by the impact. We think we found these (sorry, no picture).

We also found stromatopoloids (see left)! These have nothing to do with the impact hypothesis, except that they are found scattered all around and torn into pieces, so one could argue that something came through and caused the shallow sea environment to change suddenly; other explanations, such as tectonic or volcanic activity, could also apply. These little guys are a kind of sponge, and what you’re seeing is a silicified body fossil, the organism itself. I’m interested due to my current study of stromatolites, which are sedimentary layers that, in some cases, are created as a side effect of bacterial growth. Very similar appearances, but entirely different things. (Am I the only one who thinks this looks weirdly like an astronaut footprint?)

At our next stop, we found a hydrothermal vein where entirely abiotic stromatolites had formed. See the resemblance, in structure if not color? In this case, no life was involved. We know this because we have petrographic thin sections of this rock, and under the microscope you can see that they’re composed entirely of crystal growth (not bacterial layers). Still, this was another fun collecting frenzy (this is a very important specimen).

After lunch, we drove west across Nevada to Hawthorne, and sunset found us at Walker Lake, where abiotic tufa (see left) and carbonate stromatolites are found in abundance. We all stood around and watched while another grad student hacked off pieces of stromatolite for her dissertation research, placing each piece in a sample bag.

It was another fabulous, full day, and I could barely keep my eyes open through dinner. One more day to go!

Our Geology 601 field trip this spring took us into the wilds of Nevada. The class, called “Crises in Earth History”, examines past mass extinctions and the evidence for such in the rock record. Nevada contains some great field sites associated with these events. Our first day focused on the Permian-Triassic boundary. While the P-T boundary itself is not visible in the southwestern U.S. (due to a lack of deposition at the time or a later erosional event), we were able to visit the late Permian in Arrow Canyon and the early Triassic in the Muddy Mountains.

This was a fantastic experience! I’ve never gone fossil hunting before, so I didn’t know what to expect. I also haven’t taken any paleobiology classes, so I didn’t have the expertise needed to identify the fossils myself. But luckily, I was out there with several other grad students and professors with exactly that knowledge. We saw the broken-up arms of crinoids (at right), radial corals (below left), and even fossilized tree roots (below right).

We hiked further into the canyon, which flourished with present-day life as well (notably, lots of gorgeous barrel cacti). I found an interesting specimen that turned out to be (in the best guess of the assembled professoriate) bladed gypsum crystals. As usual, the line between biotic and abiotic isn’t always obvious. However, even this abiotic find excited interest in the group because of the implications for the local environment (gypsum tends to form in shallow marine waters).

We had lunch in a fabulous slot canyon that felt like it came right out of a Tolkien book, with towering rock walls only 20 feet apart, a fringe of bright green grass along the base of both sides, and the canyon itself leading off into mysterious twists that practically demanded further investigation. However, time was limited and we headed off to our next stop.

At the Muddy Mountains, we got to observe the early Triassic. A recent flash flood had wiped out part of the road and cut into the surrounding hills to provide us with a remarkable glimpse into newly exposed beds. We found an incredible shell bed, where the fossils (bivalves) were literally just falling out of the rock (see image at right, lower layer).

Across the road, early Triassic “red beds” (shale) were in evidence, alternating with carbonates (pale layers). This has been taken as evidence of fluctuating (local) sea levels, as red-beds are generally believed to have formed above water and carbonates below it. However, if you look closer, you can also see thin green shale layers between the carbonate and the red beds — on both sides of the carbonate, in fact. The green color, instead of red, could indicate that it was formed in anoxic (low or no oxygen) conditions, but this is unlikely to happen above water, and even in the water usually would only happen in very deep water, while the carbonates form in shallow water. We were all standing around scratching our heads, and someone suggested that the green shale could be an alteration of the red shale, caused by contact with the carbonate. This looks plausible visually: I noted that the green layers were thicker where the carbonate layers were thicker, as if the thicker carbonate could affect more of its surroundings. On the other hand, no one could come up with a chemical reaction that the carbonate would induce on the shale to reverse its oxidation.

We moved further up the road, and turned the corner to find the real jackpot. Trace fossils are fossil structures that record not the body of a organism but marks it made while burrowing or crawling or otherwise disturbing soft sediment. We were told that this particular outcrop had, somewhere in it, the trace fossil asteriacites, a star-shaped imprint where a starfish-like creature rested. Immediately, we all tackled the outcrop with enthusiasm, sorting through bits of rock to find these centimeter-scale fossils. The sedate meandering observational pace transformed into an Easter-egg hunt or garage-scale atmosphere, and just minutes later the first find was declared. At left is one I went home with. Whee!

Descending back to the road, I almost stepped on a bit of present-day life. I hesitated mid-step when the ground below me seemed to shimmer, then realized that it was occupied by a baby rattlesnake (about one foot long). It was the first rattlesnake I’d ever seen, and I was eagerly zooming in for pictures, when another student suggested that I back away because, after all, they are venomous, and apparently (she said) the baby ones are actually more dangerous than adults because they have no control over how much poison they inject, so they just shoot you full of everything they’ve got. I backed away regretfully (I was fascinated by the evidence of its last meal) but did get a few photos!

Overall, the extinction associated with this boundary is believed to have been the biggest in Earth’s history, with up to 96% of marine species going extinct. And indeed, we observed that Arrow Canyon’s fossils showed much more diversity than those in the Muddy Mountains (although I was surprised to see so many Triassic fossils — this section must have been a good bit after the actual boundary). An excellent start to the field trip!

I came across an article today describing the latest extraterrestrial ocean hypothesis — that one exists beneath the crust on Titan. That’s right, a subsurface ocean, in this case probably composed of water and ammonia. This is in contrast to the so-last-year news about methane lakes on Titan, which are widely accepted. Usually Enceladus or Europa get all of the press with regards to potential oceans, so this is pretty unexpected. From the article:

“Using data from the radar’s early observations, the scientists and radar engineers established the locations of 50 unique landmarks on Titan’s surface. They then searched for these same lakes, canyons and mountains in the reams of data returned by Cassini in its later flybys of Titan. They found prominent surface features had shifted from their expected positions by up to 19 miles. A systematic displacement of surface features would be difficult to explain unless the moon’s icy crust was decoupled from its core by an internal ocean, making it easier for the crust to move.”

I’m curious about these observations. I’m sure that the scientists involved have already applied an appropriately sized dose of skepticism to this subsurface ocean theory, but my gut reaction would be that an error in measurement is far more likely than a decoupled crust and core separated by a liquid ocean (where are you, Mr. Occam?). (In fact, if the displacements really are “systematic” then a measurement bias/error is an even more likely candidate explanation.) I’ll have to look for the upcoming article in Science!

My Master’s degree research seeks to analyze rock samples to determine whether the structures therein might have been created by life. I’ve looked at various information theoretic approaches to analyzing digital images of samples, and currently I’m testing some texture characterization algorithms to see if they might be useful. It turns out that while there is a lot of existing work debating whether a given rock sample (e.g., stromatolite) was the result of bacterial life or abiotic chemical processes, few people have actually tested biogenic (“created by life”) and abiotic samples side-by-side. One challenge is that stromatolites take a really long time to grow, so it’s not exactly a weekend project.

However, I’ve gotten involved with the USC microbiology lab, and due to the generous assistance of a couple of graduate students, we’re going to do exactly that, but with simple organisms and reactions rather than trying to grow an entire stromatolite. The abiotic samples have already been grown and imaged, and now we’re working on the biogenic ones. To learn how to do this, I reported to the microbiology lab this afternoon and spent two hours trying to absorb a whirlwind of new terminology, procedures, and concepts. Here’s what we did:

1. Create some “growth medium”. This involves mixing a powder with distilled water to create a “broth” in which bacteria will like to grow. We used “marine broth 2216” and water from the Nanopure Filtration System. We then stirred the mixture, which these days does not mean sticking a rod in and swirling it around manually. No, you put the glass jar on a “stirrer”, which when turned on moves its platform in gentle horizontal circles. That wasn’t enough to get the powder to completely dissolve, so we threw in a magnet (!) which is affected by the stirrer and spins around independently, thoroughly mixing the solution. Very cool.
2. Sterilize the medium. Here we screwed the lid on the glass jar, stuck “autoclave tape” on it, and then put it in the autoclave. The autoclave is effectively a pressurized oven that (in this case) warms its contents up to a toasty 121 C. (It’s pressurized so that the fluid can be pushed above its boiling point — clever!) It stays at 121 C for 15 minutes and then cools down. The autoclave tape has bands that go black after it’s been cooked, so that you know whether the contents are sterile or not.
3. Inoculate the medium with bacteria. We’re using a Mn-oxidizing bacteria (an erythrobacter) that is known to generate interesting structures. We pulled some out of cold storage (-80 C!) and pipetted it into the growth medium on a “clean bench”. The clean bench has a constant flow of air coming out to help minimize the chance of you contaminating your own samples. Again, instead of old-time manual pipettes, we had the aid of a “pipet aid”, which is a hand-held device that creates a tiny vacuum and with a push of a button can suck up or release precise quantities of fluid.

We then put everything away (the inoculated samples in a dark closet), and I’ll go back next week to work on the next steps. It turns out that I will also have to take “lab safety” training (certainly not a bad idea) that is spread out over three days, at 2.5 hours each day (!). This is inconvenient, to put it mildly, when one has a day job. :) But regardless, I definitely learned something new today. And there’s more to come.