NUMBER 4 1) . 35 



Agariim clathnitiitji (Adey and Hayek, 201 1), with mostly open 

 C. compactmn terrain. Laboratory experiments to confirm this 

 analysis are now underway; however, it seems unlikely that hght 

 variation is a significant direct growth rate factor in the mid- 

 depth range of C. compactmn and C. nereostratum. 



There is no anatomic or reproductive indication that growth 

 in C. compaction ceases during low temperatures. Growth ex- 

 periments currently underway have shown at least short term 

 growth in total darkness at 2°C. Also, growth curves suggest 

 that under the null light conditions below sea ice in northern 

 winters growth continues for a short period. Figure 27 shows a 

 10-year mean plot of molar Mg/Ca in four C. compactiim crusts 

 collected off Quirpon in northernmost Newfoundland. These 

 data were derived from equally spaced points in vertical sec- 

 tions. Here sea ice usually arrives along the coast in late January 

 and remains until sometime in early April, when temperatures 

 are near or below -1°C (Figure 23). The cold and warm peaks 

 are virtually symmetrical, both occurring at midlevels of solar 

 radiation, making this a temperature proxy with little direct 

 light component. Farther north in northern Labrador and Baffin 

 Island, these curves are quite asymmetric, suggesting that with 

 much longer sea ice intervals (December-June), growth stops be- 

 cause of a lack of sufficient stored photosynthate. 



Interfilament Calcieication 



The large and coarse mterfilament (outer wall) crystals ap- 

 pear to be growing downward (perithallium) and upward (epi- 

 thallium) from the fracture plane between meristem cells (Figure 

 28). It seems likely that these crystals are precipitated in the 

 small cavities that ring each filament m the meristem calcifica- 

 tion zone, perhaps because of removal of CO, by the overlying 

 photosynthetic epithallium. From decalcified paraffin sections 

 and SEM imaging of the meristem fracture zone, it seems likely 

 that no organic material is present in this interfilament area. This 

 process could involve a secondary mechanism of ion pumps de- 

 livering calcium ions to and removing hydrogen ions from the 

 interfilament spaces of the fracture zones. However, the meristem 

 fracture plane is directly below the primary photosynthetic tis- 

 sue of these plants (epithallium), and the overlying carbonate 

 (within the epithallium) appears to be made up of large, but thin, 

 vertical sheets that likely provide a quite porous connection to 

 the water at the surface of the plant. Also, in the Labrador Sea, 

 the perithallium interfilament crystals are only formed during 

 summer in any significant quantity. Thus, the interfilament wall 

 component could represent the CO, removal and carbonate for- 

 mation suggested by Ries (201 1). 



FIGURE 27. Ten-year mean molar Mg/Ca from four C. compactum specimens taken off Quirpon, northern 

 Newfoundland (sample QP4-3; see text). See curves for the Mg/Ca molar ratio (proxy for temperature) in 

 Figure 3. 



