Arctic Ice and Wood: What’s the Connection?

Abstract swirls of blue, green, tan, and some pink

Photo: Warmer freshwater mixes with cold seawater in the Mackenzie River in Canada. NASA/USGS

This is the second in a series of posts by Associate Curator of Invertebrate Zoology Janet R. Voight as she heads out on an expedition to Norway. There, she and colleagues will look for a wood-boring clam, Xylophaga dorsalis, to study its, well, poo. Read the first post and stay tuned for upcoming posts to find out what they discover. 

Does a melting ice cap help us find a rare clam species?

The high Arctic is witnessing massive changes, and the most notable is the melting of the ice cap. As a biologist who studies deep-sea, wood-boring bivalves (Xylophaga dorsalis), I see open water at the top of the world as a way for these animals to dramatically extend their range. Rivers in Siberia and northern North America carry a lot of wood during summer floods after their ice breaks up in spring and gouges their forested shores. That wood floats into the Arctic Ocean, and then much of it is beached. Arctic beaches have always had lots of wood; anthropologists think that the native peoples built their boats and poles for summer homes from driftwood. When winter sets in, any wood floating in the high Arctic gets locked in ice. If the ice doesn’t melt, that wood stays locked up. When the ice melts, it is freed and has a chance to sink to the bottom.  

bering_strait_sea_ice.jpg

Sea ice exits the Arctic Ocean through the Bering Strait. NASA

What’s on the bottom of the Arctic Ocean now?

Well, like most of the seafloor, we really don’t know for sure. However, marine archaeologists have found wooden shipwrecks from the nineteenth century in the Arctic Circle that look like they just sank. Elsewhere in the world’s oceans, wood-boring clams would have found those deep-sea shipwrecks and eaten them in a comparative flash. Think about the photos you’ve seen of the wreck of the RMS Titanic: there are “rusticles,” rust formations that look like icicles, but not a trace of the wood that once graced the ship. Why are these wrecks intact? Apparently, wood-boring bivalves either can’t get to them or can’t survive when they do.  

Why not? We know shallow-water wood borers (shipworms) don’t like cold. Despite being able to tolerate pretty low salinity (watered down seawater), they do not range far north. At high latitudes, deep-sea wood-borers that don’t mind the cold live at shallower depths. There’s no reason that they can’t live at greater depths, too.

Why don’t the clams eat those wooden shipwrecks?

We're heading to Norway to find out. We have some ideas. First, I suggested in a scientific paper a year or two ago that if they don’t have food, they can’t live. Not exactly rocket science, I admit, but these borers eat wood and only wood, so the secret is knowing where there is wood. Their swimming larval stage has to find wood before their time is up. If they can’t find wood, there is no plan B—they die. If the Arctic ice cap is hiding the wood, wood-borers can’t find it. Of course, that was the way it used to be. Now that the ice cap is melting, more wood will be released, get waterlogged, and sink to the bottom. And more borers will be there waiting…

The other thing is that the clams can’t survive in extremely low salinity. If the seawater becomes too diluted by ice and snowmelt, it might kill them. Because the wood-borers live deeper, no one has ever tried to figure out what they can tolerate. Our target species, Xylophaga dorsalis (the wood paddock) is recorded as deep as 350 meters but also as shallow as two meters. We don’t know if the larvae are swimming in the shallow or the deep water.

To test these opposing ideas, we will try to keep live bivalves in their wood in laboratory aquaria and change the saltiness of the water to see if it stops them. If they seem to tolerate the changes we impose, we will conclude that they are limited by wood availability. This may predict that reduction of the ice cap will allow them to move into the Arctic, where they will bore into and destroy sunken wood in the Arctic Ocean.  

Why’s that a big deal?

In addition to the wrecks I mention above, culturally important shipwrecks (remember the Viking ship at the Museum?) exist under the seas around Scandinavia. These ships are a big part of Scandinavian heritage; it’s part of who the people are. As the Arctic ice cap melts due to climate change, those deep-sea wood-boring clams will be able to colonize those wrecks that have lain under the seas for centuries, destroying that heritage.

Read on in Part 3: How We Think About Clam Poo: Ideas Matter

Funding for this project was provided by the Robert A. Pritzker Center for Meteoritics and Polar Studies established by a grant from the Tawani Foundation.