Pollen: Our Little Time Machines
We’ve already covered two areas of research that we used to better understand and visualize Wyoming in the Eocene through Miocene (56 – 5.3 million years ago), from paleobotany to leaf margin analysis. This time, we’re looking at something much smaller, but just as important: pollen.
Palynology is the scientific field that deals with pollen, but as it means “the study of particles that are strewn” or “the study of dust”, it also covers fungal spores, charcoal, algae, and any other really small but identifiable thing. For this post we’ll focus on pollen, but I just wanted you to know that there’s a whole wide world of tiny specs out there.
Actually, before I get too carried away explaining palynology research, let’s go over some pollen facts. Each pollen grain is a male haploid mini-plant that contains the male gametes, or sperm. All plant species produce pollen in the hope that the grains will find eggs (called “ovules”), transfer the gametes during pollination, and fertilize the eggs so they become seeds. The seeds then get dispersed and (hopefully) germinate into a new generation of plants. Since plants aren’t mobile and can’t very well walk into a bar to find a mate, they rely on external forces like wind, water, or animals to disperse their pollen. Plant parents also can’t really nurture their offspring into adulthood, so the odds of success for any given seed are low; however, since pollen isn’t very energetically costly to produce, plants tend to make far more of it than could realistically turn into mature plants, just to ensure that at least one seed will eventually make it. This means there’s a ton of unused pollen floating around, just waiting to be collected in sediments and examined by scientists millions of years later. As UWyo’s palynologist (pollen specialist) Tom Minckley put it, “I study failed reproduction”.
The plant’s failure makes for the scientist’s success: pollen is a great proxy for the past. Since pollen spores are so light and easily transported on the air, studying them expands our spatial scale from the local to the landscape. Macro-fossils tell us about the individual plants that lived in an exact location; pollen fossils, on the other hand, provide a picture of the larger landscape. A leaf can only fall within a certain radius of its tree, but pollen can float for miles. It’s the difference between standing at the lake’s edge to look at vegetation, and climbing to the top of a nearby hill for a 360º view of the surrounding area. Macrofossils tell us about the wetter environments where plants preserve more easily, but pollen records reveal both the wetland species AND the upland, drier species. This way, scientists have an idea of vegetation composition away from the water’s edge. Besides, pollen is the most common terrestrial fossil, so you’d be crazy not to see what stories it could tell! In fact, scientists have been using pollen as a proxy for understanding the past for at least 100 years.
If pollen only told us about plant distribution in space that would be cool enough, but since it also accumulates in layers, you get a great view of species composition through time. Uniformitarianism and the Law of Superposition tell us that in an un-deformed stratigraphic sequence, you’ll find old things at the bottom and new things at the top. This is how layers of sediment deposits work, whether you’re looking at rocks or mud cores. Palynologists can take samples from exposed rock faces or from lake beds, and by comparing the pollen from the bottom layers to the top layers, they can tell when and which species were around in a given area. Sometimes, if they know how often sediments were laid down, they can look at species turnover at a really precise time scale. If it’s a really productive lake with lots of plants and algae and no current, pollen can stack up at a rate of 1 centimeter every 10 years; on the other end of the spectrum, it could take 200 years to get that same centimeter of pollen.
As I mentioned, palynologists can collect “dry” samples from exposed sedimentary rock faces or “wet” samples from the mud at the bottom of lakes. Tom Minckley does both, which means he gets to go to some pretty spectacular places for his research. If you’re looking for information since the last glacial period in Wyoming (about 10,000 years ago), you go to a lake and collect a core. For really old time periods like the cretaceous (146-65 million years ago), you go to exposures. To take a dry sample, Dr. Minckley carefully scrapes out sections from various layers, making sure not to contaminate between layers. To take a wet sample, he and his students and collaborators will go out on a boat in a lake, stick a long tube down into the muck on the bottom, and bring it back up. It’s kind of like sticking a straw into a scoop of ice cream: when you remove the straw, you also remove a column of ice cream. The mud core then gets wrapped in plastic and taken back to the lab.
In the lab, the researchers take a little transect from the mud core or a little sample from the rock scrapings, and treat it with acids and bases to dissolve the rock and inorganic materials. What’s left fits in a little fluid vial, and to the naked eye just looks like floating specs of dirt. Under a microscope, however, you’ll see pollen grains, fungal spores, and bug bits. Using reference books and experience, scientists try to identify the pollen down to the lowest taxonomic level, which is often family or genus. They don’t have to count and ID every single grain in a sample, just enough to be reasonably sure that if they kept counting, no new taxa would show up.
Once the ID work is done, it’s time for some analysis. In a cubic centimeter of lake mud, you can expect to find tens of thousands of pollen grains. Every meter of mud, then, is worth 40-50 hours of basic analysis time in the lab, or 100 hours for a full analysis. You need to know quite a bit about the reproductive strategies and lifestyles of modern-day plants in order to make inferences about past ecosystems and environmental change based on pollen samples. For example, say your pollen sample has quite a lot of pine pollen, and a little spruce pollen. You might think it came from a pine-dominated forest, unless you were aware that pines produce a ton of pollen that disperses much more easily than spruce, so pine tends to be over-represented in the pollen record. Knowing this, you can now infer that there had to have been a lot of spruce trees in order for you to get even that little bit of pollen. Similarly, you can look at a sample and say, “gee, there was a lot of sagebrush here, so it must have been pretty dry”, since sagebrush is found in xeric (a.k.a. dry) environments.
So pollen: pretty amazing, right? I think so. That said, there are some limits to what stories pollen can tell. For one thing, pollen needs the same conditions for preservation as leaves and branches do: it is best is they settle in sedimentary environments like lake beds, or oxygen-poor environments like bogs, so that they don’t rot before having a chance to become fossils. (Pollen can be well preserved in caves as well, if it is dry and no animals stir up the dusty floors). Essentially, if pollen falls and there’s no place preserve it, it doesn’t show up in the fossil record. So there are a lot of places we don’t know what the vegetation composition was in the past.
Another limitation is that pollen is either dispersed by wind (anemophily), water (hydrophily), or animals (zoophily), and these strategies don’t preserve equally well in the fossil record. For example, when bats go from flower to flower drinking nectar and spreading pollen, most of the pollen grains end up in their fur and don’t survive the millennia. So animal-pollinated species are under-represented in the fossil record, and anemophilous species are over-represented. Even within anemophilous taxa, some are under-represented: for instance, short grasses are don’t get their pollen blown very far, so a grassy patch may not show up in the fossil record if it’s not near enough to a lake or bog. You can even be in a meadow and get only 10% grass pollen.
Despite these limitations, pollen has been instrumental in revealing Wyoming’s riverside (i.e., riparian) habitats from the Eocene, Oligocene and Miocene. It’s amazing that something so small can paint a picture of a sweeping landscape.
For more on pollen fossils:
Florida Museum of Natural History website: “What is Palynology?” http://www.flmnh.ufl.edu/pollen/whatispalynology.htm
Wing, S. (1998). Tertiary vegetation of North America as a context for mammalian evolution. In C. Janis, K. Scott, and L. Jacobs (Eds.), Evolution of tertiary mammals of North America, volume 1: terrestrial carnivores, ungulates, and ungulatelike mammals (pp 37-60). Cambridge: Cambridge University Press.