From Plant Press, Vol. 14, No. 4 from October 2011.
For over 40 years, the National Museum of Natural History has been the largest repository in the world of a calcified, lichen-like group of red seaweeds called corallines, mostly collected by Walter Adey. Early in his career, Adey found that like most woody plants (unlike all other red algae), corallines have an intercalary meristem (cambium) that produces an overlying (in this case photosynthetic) epithallium (= bark) and builds up a thick underlying perithallium (= wood). However, there the analogy ends. In the cellulosic walls of coralline cells, dense layers of calcite crystals are formed to produce a “limestone” rather than wood. In the tropics, corallines can produce reefs, often called algal ridges. In the 1970s, through core-drilling, Adey was able to show continuous coralline carbonate build up of up to 3,500 years.
Since Adey’s early work, it has been known that species of the Arctic/Subarctic coralline genus Clathromorhum, growing on rocky bottoms, accumulates many thick yearly layers (200-400 um thick) of cellular calcite. Equally interesting, just as tree rings, the thickness of the coralline “rings” depends upon water climate (mostly temperature). However, of even greater interest, the calcite, structurally a chemical lattice-work, holds trace elements that are indicators of water and atmospheric physical and chemical state. The most important indicator is magnesium, which replaces the calcium in the crystalline lattice as a function of temperature (the higher the temperature, the more the lattice vibrates, and the greater the amount of magnesium replacing the chemically similar calcium). While this has been published since the 1960s, the instrumental techniques capable of utilizing this information as an indicator of past sea water temperature was lacking.
During the last decade, a group of geochemists at the University of Toronto, led by Adey’s colleague Jochen Halfar, applied a new instrumental tool, laser/mass spectrography to corallines. A laser beam, passed from younger to older layers on the vertical surface of a sectioned coralline, vaporizes a groove. As it moves, the vaporized carbonate is transferred to a mass spectrograph, where the amount of magnesium relative to calcium can be measured with considerable precision. This provides a proxy measurement of the ambient water temperature at the time the cells formed their calcite crystals. Recently it has been demonstrated that by measuring the replacement of calcium by barium, the ambient water salinity can also be determined.
The Labrador Sea is a primary source for the thermohaline current (also known as the global ocean conveyor belt). This density current, starting as cold Atlantic Deep Water and returning as the Gulf Stream, is a major heat transfer system, and is a key to much of global climate. Since it is driven by water density, created primarily by temperature and salinity, knowledge of these parameters, and the flow of Atlantic Deep Water in the past, can provide crucial climate information on what to expect in the future. Particularly concerning, as the Greenland Ice sheet melts with global warming, will density instability produce rapid flip-flops of climate?
Clearly, the measurement of changes in coralline growth bands and their chemical composition (called schlerochronology) can give us a climate archive and an understanding of how the Labrador Sea and the thermohaline current have changed in the past. However, the oldest specimens in the Museum coralline collection provided information only back to 1912. The large collection of North Atlantic specimens in the Museum, taken through the 1960s and early 1970s, were collected to provide a measure of relative species abundance, in turn to document ecological and biogeographic patterns; determining thickness of carbonate had not been part of the collection protocol.
In the summer of 2010, using the research vessel Alca i, Adey returned to the Labrador Sea coasts of Labrador and Newfoundland with a team of divers to determine ecological and geographic patterns of variation in thickness in Clathromorphum compactum. Specimens up to 300 years old were found. It was shown that within the now more finely understood ecological framework of this target species, that iceberg scouring of the bottom provided an important limitation. The coralline crusts would continue to grow for thousands of years, but if large waves and icebergs were to scour the bottom, even once in 100 years, then corallines of significant age would be lacking.
During the summer of 2011, Adey and his crew, returned to central Labrador with the Alca i. Here, an archipelago of islands provided a wide variety of environmental conditions that would provide protection from storm waves and icebergs, and yet be far enough from the mainland to avoid sediment burial. Here, the geomorphological factors could also be examined. Steep eroding rocky cliffs that would provide an unstable bottom and talus or a thick overlying glacial till would provide an endless supply of coralline eroding cobbles and pebbles. Armed with new ecological and geomorphological understanding, the divers were able to recover specimens nearly twice as thick as those of 2010. Since these sites were further north, in colder water, growth rates were slower and annual layers thinner; the age of specimens was expanded nearly three times, to over 800 years. Since divers could spend only a few hours per day working at these cold temperatures, and only a small fraction of potentially optimum bottoms could be examined, it seems likely that much older specimens remain to be found in future expeditions.
During the winter, between the summer expeditions, while studying Clathromorphum anatomy with SEM, Adey has been able to show considerably greater complexity of both tissue anatomy and cell wall construction than previously known. The laser beam width of 65um is too coarse, even with a rapid scan and sampling rate, to differentiate the finer parts of the anatomy; this technique, although a great advance, is still missing a large part of the considerably more precise climate archive written by the corallines into their intricate crusts as calcite crystals. A new tool, an EDS microprobe, could make elemental analyses in a 2-5µm spot. Given the ability to carry out even more precise analyses of Labrador Sea corallines, an accurate archive of past climate, in one of the most sensitive climate-determining regions of our globe, can be obtained. This can provide input for the sophisticated climate models that will help us manage our uncertain future.
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