Monday, 21 May 2012

How to make a sea level record

“So how do you know how high sea level has been in the past?” I get that question a lot. It’s a good question; if you’re not a sea-level scientist (because that’s cheating) you might wonder how something as elusive and evanescent as sea level could be traced back in time. One may think of marine features above the reach of modern tides such as raised beaches, or coral reefs, or signs of wave erosion, such as wave notches. And as signs of sea levels having been lower one can think of drowned ruins. But these are all isolated indicators. What if you want to know what happened over a long period of time?
A fossil coral reef in a forest, which is clearly a terrestrial environment; a clear indicator of relative sea level having been higher in the past. Picture taken in Windley Key Fossil Reef Geological State Park

When geologists and geographers want to have a continuous record of something they often stick a corer into the ground.  If you pick the place where you do that right, you get a core comprised of sediment that has been deposited over the time period you are interested in, and contains the information you need. So just suppose you want a sea level record that goes back 500 years, how would you go about that? You need to find a place near current sea level where sediment has been deposited in a calm environment for quite a while, and where there are things in the sediment that record the position of the sea. Think about it: not many places will do. Many coasts are erosive. Many are energetic. Many just contain sand, with dead shells, that have washed ashore. So what did the scientific community come up with? The salt marsh.

 A salt marsh
A salt marsh is an environment that gets flooded periodically by tides, and where salt-tolerant plants grow, trapping sediment. If sea level rises slowly, they can build up; if it drops slowly, they can build out, provided there is space for that. If sea level changes too fast the marsh will become either a fresh-water environment, or drown.
On the surface of salt marshes not only plants grow; they tend to host a plethora of micro-organisms too. Among these are foraminifera and diatoms. Foraminifera are small animals that build a shell, either of calcite or of particles they find. Diatoms are algae that build a kind of shell from silica. And there are countless many species in either group. And all of these species have their own preference for how often they are in the water and how often they are dry, and how salt the water is they live in, and many more of these factors. In other words: they are picky about where they live with respect to sea level. And the good thing is: they take that pickiness with them into their graves. The organisms themselves decay, but under favourable circumstances their shells are preserved. Ready for a passing geographer to interrogate…
A sediment core from a salt marsh

What we do is roughly as follows: we take samples from the surface of the marsh over a transect from high to low. We also take a core, trying to get the location right so that the entire period of interest is preserved in its sediments. Using surveying equipment we find out at what elevation these are all taken from.
Later, in the lab, we determine which species of the microorganisms of our choice we find in the surface samples, and their absolute and relative abundance. That tells us at what elevation the various species live. Then we determine the species assemblages in samples taken from the sediment core. Given that we have established at what elevation interval these species live we then know at what elevation these samples must have been deposited. And if we, for instance, take a core and find an assemblage typical for an elevation of 30 cm above sea level at half a meter down in the core, we know that when these sediments were deposited sea level was 80 cm below the current land surface. And if you do that for every centimetre of the core, you get a rather detailed record. If you then manage date the core with good precision you have your sea level reconstruction!

This is what you would then see through your microscope. A salt marsh foraminifer is encircled.

We tend to look for salt marshes with a tide gauge nearby; these record sea level in real time. The oldest records go back hundreds of years; if our reconstructions match these records we have faith in them, and assume they also describe the period from before the start of the tide gauge record as reliable.
So after all that work, what are the questions we want to answer with this kind of research? There are many. One is, for instance, if we find accelerations in sea level rise of which the timing is consistent with the industrial revolution as a possible cause. Another one is how fast such accelerations in the past have been; it might be good to know how fast such processes can take place. Another one is if we can attribute specific sea level changes to specific ice caps; if so, we get some idea of what to look out for. If the West Antarctic Ice Sheet has collapsed in the past, and we can figure out how fast that went, we know what to get ready for. It may happen again! And how we can distinguish between the various ice caps (there are many other sources of sea level change, of course, but in the long run the ice caps are the biggest players) is material for another entry…

The author on a body of ice. Not an ice cap, admittedly.

No comments:

Post a Comment