Thursday, September 23, 2010

Message in a bottle

Station 12 (i.e., "W")
04:15 EST
39° 3.409' N, 69° 22.632' W
winds: 4 knots
seas: calm

Ship time moves quickly. We finished our transect along the Nova Scotia margin, steamed through Igor’s wake (see picture at left), and have completed half of our stations on our present transect. We are currently on Station “W”, positioned at the intersection of the 3000 m isobath and a straight line (“Line W”) connecting Woods Hole to Bermuda. This will be our highest priority station because it has been continually observed since 2004 and provides a nice record of temporal variability in the NW Atlantic margin. And that is why we are here. To paraphrase the Cruise Planning Synopsis, our goal is to better understand the movement of organic particles (i.e., particles that are mostly carbon, like finely ground dirt or the soft tissues of our bodies) in the Northwest Atlantic margin. The implications of this work are surprisingly far reaching. It should improve our understanding of sedimentation processes, the formation of energy-rich deposits, the interpretation of sediment cores, the origins of organic material found just off of our coast, and the global carbon cycle in general just to name a few. How can we trace so many pathways? Consider the following.

Imagine you walk to the shore for the first time and look out to the sea. It is only natural to start asking big questions with small words. “Where did this water come from?” Not so very long ago people tried to figure this out by throwing corked bottles into the ocean. A letter inside would inform lucky beachcombers on distant shores of the bottles’ origins, with instructions for returning them. In this way, the bottles’ paths could be traced and currents could be mapped. Similar work is still carried out today. Check out Curtis Ebbesmeyer’s book, “Flotsametrics and the Floating World,” to learn how yellow rubber ducks helped map the Pacific. More sophisticated means of tracing the ocean can be found in NOAA’s incredible ARGO array of drifters (Coincidentally, one of our cruise participants, Larry George of WHOI, builds ARGO floats and will be repairing a spray glider recovered during our last transect).

But how, exactly, can we infer the history of organic particles sinking through the ocean? Fortunately, nature has been sending-off the particles with tiny corked bottles of their own: the atoms, isotopes, and molecules of which they are composed. By quantifying these constituents, we are able to constrain their histories. For example, my project will use radiocarbon (14C) measurements to trace the history of organic molecules dissolved in seawater (collectively called dissolved organic matter, or DOM). Since radiocarbon is an isotope of carbon that falls apart at a predictable rate (5,730 year half-life), we can infer that radiocarbon-rich molecules are fairly new while radiocarbon-depleted molecules are likely older. In this way, it has been shown that organic molecules dissolved in the deep ocean are 4,000 to 6,000 years old on average! In addition to tracing time, radiocarbon can be used to trace source material. If, for example, a particle sinks noticeably faster than its radiocarbon content would suggest, we might infer that older organic material was incorporated along the way. With a little more evidence, we might even be able to figure out where that material came from, how it got there, and more importantly, what it means in a broader context.

Check out the following resources for more information:
links from the Radiocarbon journal homepage
Willard Libby’s classic book, “Radiocabon Dating”
Edgar Allen Poe’s short story, “MS. in a Bottle”
ask me a question…

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