Smelling sulfur

Shelly and I completed our last round of sampling at Tent Island yesterday. Normally we get a bit of wind protection from the island but not this time, it was possibly the coldest we’ve been in the field on this trip! Two weeks ago when we sampled ice cores from that spot we saw the first signs of the spring ice algal bloom; a slight brown coloration to the bottoms of the ice cores. By now the algal bloom is in full swing, with a thick mat of algae covering the bottom of each core. The growth is so thick that water flooding out of our drill holes is a soupy brown color, very much like the surface of a stagnant marsh pond (except this water isn’t stagnant of course; fresh, nutrient rich water is circulating in from the Ross Sea).

Everything takes longer on a cold, windy day and we had a couple of extra samples to collect. By the time the helicopter came to pick us up we were exhausted. After a good night’s sleep we’re ready for our final sampling effort – our fourth and hopefully final attempt on Taylor Glacier! Right now the weather looks promising, if it holds for a few more hours we’ll have it…

Whenever we bring samples back from the field we start melting them immediately so that we can filter them as quickly as possible. When I opened our sampling bins last night to start melting the samples I was greeted with a very strong odor that I should have anticipated. The samples, particularly the more algae rich samples from the ice core bottoms, reeked of sulfur. The smell was very much like what you might find at a tidal mud flat, but much stronger. The source of the smell is the same in both cases; a compound with the fancy name dimethylsulfanopropianate, or DMSP for short. DMSP is a small hydrocarbon molecule that also contains the element sulfur. When algae or phytoplankton are stressed (for example when trapped in newly forming ice, or on a rapidly drying mudflat) they release this compound. In sea ice DMSP is though to help the ice algae cope with the rapidly changing salinity caused by ice formation.

DMSP has received a lot of attention in recent years because it has some implications for regional and global climate. Many marine bacteria can use DMSP as a source of food, a process which converts DMSP to the volatile compound dimethylsulfide (DMS). DMS is probably the largest pool of sulfur in the atmosphere with a biological source (there are large industrial inputs, as well as natural inputs that don’t directly involve biology). In the atmosphere DMS converts to sulfuric acid, a very efficient nucleator of the droplets that form clouds (check out this YouTube video for a further explanation of how cloud nucleation works).

Clouds are basically water vapor, and water vapor is a very potent greenhouse gas. Scientists are still unsure of how exactly clouds impact climate, and how clouds might change as climate changes. One idea is that high altitude clouds have a different impact on climate than low altitude clouds. Both reflect and absorb solar energy and energy radiated from the Earth’s surface. Clouds must radiate the energy they absorb, how that energy is radiated is determined by the cloud’s temperature. One implication might be that cold high altitude clouds warm the atmosphere less than warm low altitude clouds. Consider a bloom of ice algae in relatively warm water underlying a cold atmosphere and producing copious quantities of a powerful nucleator. The resulting clouds will be very low in the atmosphere.

We don’t really know how communities of ice algae and ice microbes are changing with a changing climate, nor do we know how communities of phytoplankton and marine bacteria are changing. Without that knowledge we can’t predict how biology will influence cloud formation in the future. Taking it a step further we can see the full circle: climate changes biology, which changes clouds, which impacts climate, which further changes biology. It’s an interesting problem, there are three different interactions here that we know very little about!