I’ve been promising a blog post about phosphorus in Taylor Valley, so here goes. At this point in the season, we’re a little frantic processing samples, packaging up boxes of soil and rock to send home, returning all of our gear, and cleaning the lab. But I’ll find the time to write this post, since it’s something I’m invested in.
So why are we interested in phosphorus in the first place? Phosphorus is a limiting nutrient in many ecosystems, but the Taylor Valley lake basins are some of the most phosphorus deficient systems in the world. Understanding how phosphorus enters the Taylor Valley system and becomes available to organisms is thus critical to understanding the controls on life in this extreme environment.
In addition to general phosphorus deficiency throughout most of the valley, there is also an interesting and well-established phosphorus gradient up the valley. Down in the lowest-elevation Fryxell basin, there is a relative abundance of phosphorus compared to the higher elevation Hoare and Bonney basins. Why does this gradient exist? Multiple ideas have been proposed: perhaps it is due differences in climate, age of the soils, or till type from which the soil has formed. This last explanation – that the composition of the till determines the amount of phosphorus available – is one that seems to fit best with the data. And it is this claim that I hope to address with the samples that I have collected this season.
If the till type results in such a strong phosphorus gradient, then the till in the Fryxell basin must be significantly different from the till covering the rest of the valley. Indeed, the Ross Sea till that covers the Fryxell basin contains kenyte, an unusual phosphorus-rich volcanic rock originating from Mt. Erebus, the volcano that dominates Ross Island. The Taylor till found throughout the rest of the valley, on the other hand, contains no kenyte. Is Mt. Erebus responsible for providing the Fryxell basin with its higher levels of phosphorus? Using the rock and soil samples I’ve collected, we hope to use isotopic signatures to differentiate apatite (the most common phosphorus-bearing mineral) that stems directly from Mt. Erebus from apatite stemming from other rock types.
We are not only interested in how apatite gets into the Taylor Valley system; we are also curious about its fate. How do microorganisms interact with the apatite in the soil? Are microorganisms responsible for releasing phosphorus from the apatite? As a start, we’ve set up a little experiment (involving soil sausages) that will stay out in the field until next season. The sausages are tubes full of soil, apatite grains, and glass beads (used as controls), cleverly held together with red clips. The semi-permeable tubing we used will allow the environment to interact with what’s inside the sausages while keeping everything nicely contained for retrieval next year. When we collect the apatite grains next year, we hope to be able to see evidence of how the grains have been altered by microorganisms. We’ll look for organisms on the grains themselves, as well as signs that the microbes have been eating away at the mineral.
On Friday, Eric and I went out to F6 camp to bury the sausages. We picked a location near Von Guerard stream – a wet soil but a stable location. We buried the soil incubations five centimeters below the surface, marking them with tags and recording the exact location. Next year we’ll return to dig them up. I hope both that they are intact, and that they give us more insight into the ways in which phosphorus moves throughout the Taylor Valley system.
Next step? Packaging up my samples to be shipped back to Dartmouth, and figuring out how to process them once we’re all back in Hanover.