On the Level
I am often struck by the shock with which many in the media seem to greet the news that global sea level may be rising. No, I am not a global warming denier. We are using up the Earth’s precious carbon resources in a hasty, thoughtless manner, and this is bound to have negative outcomes. Climate change is just one of these, and sea level rise is obviously a likely outcome of climatic warming.
What I find somewhat surprising is the static view of the world that seems to be so deeply ingrained in so many people. The media in general remind me of a bunch of oldtimers sitting on the porch of the general store, talking about “young people these days” and “whatever will they think of next.” Note to the media: constant change is one of the basic truths that underlie the geological record. If the sea isn’t rising, it is falling. And if the world isn’t getting warmer, then it is getting colder.
One of the most interesting aspects of sea level change is that it is a local phenomenon, as well as a global one. On a local scale, the sea can be rising in one place while it becomes lower elsewhere. This is because sea level is relative: it is linked to both the rise and fall of the ocean’s surface, and the rise or fall of the shore on which we happen to be standing (the world’s landmasses are in both horizontal and vertical motion, though at rates that we cannot usually perceive directly).
I thought of this last week, as my family was visiting old haunts near the mouth of the Bay of Fundy, New Brunswick (for those of you who have read my other posts: yes, I did look for jellyfish the whole time, but unfortunately I didn’t find any). What I did see was a fabulous geological story that I had not previously been aware of.
One day we took the ferry from Grand Manan Island to Whitehead Island, then wandered to the beach near the Whitehead lighthouse. As we walked along the beach near the water line, I began to notice wood sticking out of the gravel. At first, I thought that this was random driftwood. But then I began to notice that most of the pieces of wood were oriented near-vertical. Each of them rose out of the gravel into the air in exactly the way that floated-in pieces of driftwood do not. When I saw tree stumps sticking out of the beach in places, I developed an inkling of the true story: the wood represents the remnants of an old forest that is gradually disappearing beneath the sea, suggesting that the relative sea level is rising on this shore of Whitehead Island.
Later, I saw information describing a much better-preserved “drowned forest” and peat layer along the shore in front of Castalia Marsh, on Grand Manan itself. The wood, which has been carbon dated to more than three thousand years old, consists of stumps, roots, and other wood from conifers such as hemlock or spruce. These are embedded within the thick peat, which is itself composed of an immense quantity of compressed plant fibre. My friend Dick Grant, a retired geologist, kindly offered to show us the deposit at Castalia. So my family met up with Dick and his daughter Hannah, with dogs in tow (or, more realistically, the dogs were towing us).
The peat layer is only exposed when the sea is several feet below the high tide mark. Tree stumps and roots are visible within the peat, and when one sees the entire assemblage it is remarkably easy to imagine the ancient community. The wood is wonderfully preserved and unmineralized. When you hold a piece of it, it is very difficult to appreciate that it comes from trees that grew long before the time of the Roman Empire. The dogs were not burdened by these thoughts, and the two labs occupied themselves with pulling wood out of the peat and tearing it to bits with their teeth. It clearly had a superb vintage bouquet.
This deposit appears to be part of a fascinating, continuing geological story. The peat in front of the modern beach ridge may have been deposited in a low-lying wood, and perhaps also under saltmarsh conditions. If sea level is rising in this area, then the beach is moving landward over the peat, and the peat to seaward is continuously eroding away.
Behind the beach is a semi-enclosed embayment, and behind that is saltmarsh and low coniferous trees. The seaward edge of the saltmarsh also exhibits a thick, eroded peat deposit. Meanwhile, trees along the front of the low-lying wood are now dead, possibly killed by increasingly salty conditions. All of these features appear to be consistent with a gradual sea level rise; the wood-salt marsh-embayment-beach could be considered as a conveyor belt that is being gradually fed into the sea.
Anyway, it seems to make a nice story, at least to my eye. But much detailed scientific work would need to be done to determine if such an interpretation is, in fact, correct. As Dick and I discussed all of this, the sea was beginning to rise over the peat. The dogs were engaged with their own concerns, as important to the grand scheme of things as any of our scientific considerations:
I hadn’t really considered dogs as agents of erosion, but seeing how they worked on the wood in this deposit, I have had to modify this opinion (I will try to add a video clip after I get home).
© Graham Young, 2009
(For general information on Castalia Marsh and the geology of Grand Manan, I referred to J. Gregory McHone’s Grand Manan Geology: Excursions in Natural History, which was kindly provided by the author.)