Sunday, 23 March 2014

How to find sea ice once it's gone

Climate reconstructing; it sounds elusive. Climate, that’s weather averaged out over a period of 30 years. But weather is so evanescent! The wind, sunshine, rainfall we experience one day; how much is left of that the next day? Let alone 30 years later.  Climate reconstruction therefore has to rely on indirect measures. You want to know how windy it was at a given place, over a period of time? Find a place where wind-blown dust would accumulate, and measure its grain size through time. It evidently takes stronger winds to transport larger grains. And you have to be careful that you’re not looking at a record of dust availability, but if you’ve ruled that out, you may have managed to reconstruct  the elusive wind. Trying to reconstruct sunlight? You might manage to find pollen of various species of plants; some that need more sunlight than others. Rainfall? Try to find a dripstone cave; the stalagmites in it are likely to have recorded that in their isotopic signature. 

One of the big and attention-grabbing factors of modern climate change is sea ice. It is vanishing rapidly from the Northern hemisphere (see, for instance, here), and, due to its reflective properties, that is not something that can or should go unnoticed (see, for instance, here). But if you want to understand it, and thus try to reconstruct its behaviour in the past; how would you grasp that? If it melts it’s gone. But it’s not more gone that the sunshine or the rain. Of course we have ways…

 Thick sea ice

Big icebergs that have calved from a glacier are easy to trace. During their time as a part of the glacier, they pick up material of all kinds – from clay to boulders. And while they melt, they deposit that on the sea floor as so-called ice rafted debris. An iceberg will float into the open sea, where rivers can’t reach, rather soon, so if you find coarse sediment in a region like that, you know who the culprit must have been. But these icebergs are not the big player in the climate system; that’s the sea ice that froze straight from the sea water. That covers the big expanses of the polar oceans, and reflects all of that light back into space. How to get a handle on that?

Quite a lot of sea ice is so thick it doesn’t let any sunlight through. This means that not much can live in the sea water underneath. Any nutrients below sea ice will just float around, uneaten. But when they reach the edge of the ice, all the photosynthesizers will pounce on that opulence. The ice edge is often the most biologically vibrant part of a polar sea. And quite often, the ice edge will be where two water masses meet: an incoming warm current will stop ice advance in its way, and the mixing waters will makes the situation even more dynamic. Early on, researchers realised that, and figured that if you just trace the movings around of the high densities of marine microorganisms who like fresh food you are likely tracing the ice edge. But these organisms are not always very well preserved. And even if they are; it is a somewhat indirect measure. There may well be other reasons there is high productivity somewhere. Before you know it, you are tracing a plume of nutritious wind-blown dust…

Nonionellina labradorica; a foraminifer often found near the ice edge

Later on, people figured out that something quite relevant lives attached to the (thinner) sea ice: sea ice algae. And they contain biological compound, which they called IP25, and which is not found anywhere else, and which is very resistant to degradation. Find this in your sediment, and know for sure there has been sea ice overhead. This kicked sea ice reconstruction into a higher gear. Never before had something measurable in sediment been so unequivocally linked to the presence of sea ice! Drill a transect of cores, date them, measure this organic compound through the cores and you know how far the ice has reached over the time period covered by the sediments in your core (provided you chose your transect wisely). So have we finally found a method of environmental reconstruction without caveats? Well no, of course not. First of all, if you don’t find this compound it doesn’t necessarily mean there was no sea ice; it might just have been so thick it didn’t let any light through, so the algae couldn’t live under it. And another issue is: when do you consider an area of sea covered by ice? If it’s entirely covered? When it’s mostly covered? What are you really reconstructing when you find this IP25

 Sea ice algae. Source: NOAA

Fortunately, there are people who will just tirelessly run marine sediments through mass spectrometers, always looking for more chemical compounds that might be indicative of something interesting. And some found a set of additional biomarkers associated only with open water. That allowed us first to distinguish between the absence of IP25 due to absence of ice, and due to overbearing presence of ice. And, helped by calibration with satellite imagery, other workers later even managed to use both components to quantify how much of the water had been covered by ice during the growth of the algae that had produced the IP25. This means we now have ways of getting a handle on where there has been how much sea ice over long periods of time, which would hardly have been dreamed of during the turn of the century. So it takes 21th century organic geochemistry to know when there has been how much sea ice at a given location. But in the end, you’re just asking some algae…

 Partial sea-ice cover

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