6 



SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



Calcification 



FIGURE 2D. Vertical fracrure of C. coinpactum from southern 

 Labrador with late mature asexual (multipored) conceptacles. The 

 conceptacie to the right shows a roof and sporangial pore, exposed 

 by the breakout of the conceptacie cap (epithallial plug overlying 

 developing conceptacles). The roof of the postmature conceptacie to 

 the left is growing in with new meristem. 



can provide an archive of water climate that is often undisturbed 

 for centuries by grazers or other organisms. 



Most corallines form reproductive structures in peripher- 

 ally uncalcified packets, called conceptacles, which can be scat- 

 tered or densely arrayed across the crust surface; conceptacles 

 are integral with perithallial tissue but can either be buried below 

 the surface or protuberant above the crust surface. Patterns of 

 origin and development, relative to the perithallial tissue, vary 

 widely between genera. In some genera, conceptacles originate 

 deep within the perithallium, sloughing off caps of overlying 

 perithallial carbonate at maturity (e.g., Phymatolithon and 

 Leptophytiim; Adey, 1964, 1966a). In Clathromorphum con- 

 ceptacles originate in the meristem, develop upward, as allowed 

 by the growth of the surrounding vegetative tissue, and remain 

 buried in that carbonate-encasing tissue at least until maturity 

 (Adey, 1965). Some species of Clathromorphum, including 

 C. compactum and C. nereostratitm, bury their conceptacles 

 with continued growth; successive years leave layers of con- 

 ceptacie "holes" in the crust that document seasonality (Fig- 

 ures 2B, 2D, 2E). The principal Clathromorphum species with 

 archival value initiate conceptacie development in the early 

 autumn, apparently regardless of latitude. In the later stages 

 of conceptacie maturation, to make room for the enlarging 

 sporangia, the calcite in the immediately surrounding tissue 

 is first dissolved, and then the remaining organic tissues are 

 crushed by the enlarging sporangia; roughly 30%-50% of a 

 conceptacie cavity's height and 50% of its width result from 

 this crushing. 



The skeletal carbonate in corallines forms as prismatic high- 

 magnesium calcite crystals, perpendicular to the filament axis, 

 within the organic wall of each cell (Adey, 1998). In most coral- 

 line genera, a thin layer of tabular crystals forms between the 

 individual filaments and serves as a "glide plane," allowing dif- 

 ferential cellular extension (growth; Adey et al., 2005). Clath- 

 romorphum species lack differential cellular growth and have 

 a very different mode of calcification, which we will describe in 

 detail below. In the Subarctic and its fringes, some Clathromor- 

 phum species can be very long-lived (at least many centuries). 

 Given favorable environmental and geomorphological circum- 

 stances, C. compactum and C. nereostratwn can build crusts 

 with a substantial thickness of biogenic, high-magnesium car- 

 bonate (Figures 2B, 2F). With the convergence of an enhanced 

 understanding of species ecology and preservational geomor- 

 phology, multiple complementary analytical techniques, and a 

 full understanding of cellular anatomy, limitations to the climate 

 archive potential of these species are finally being overcome. 



The Need for an Arctic-Subarctic Archive 



Much of the climate variability in the North Atlantic is dic- 

 tated by the North Atlantic Oscillation (NAO), which is a hemi- 

 spheric meridional oscillation in atmospheric mass with centers 

 near Iceland (low) and over the subtropical Atlantic (high; Hur- 

 rell, 1995). However, the Labrador Sea and its coastal currents 

 (the West Greenland Current and the Labrador Current) func- 

 tion somewhat independently of the NAO. Variability in North 

 Atlantic Deep Water formation in the Labrador Sea impacts both 

 the global Thermohaline Circulation and the cold and relatively 

 fresh Labrador Current (Drinkwater and Mountain, 1997; Col- 

 bourne, 2004). Future climate predictions depend on a clear un- 

 derstanding, reaching back many centuries, of the relationships 

 among the dominant climate parameters: the NAO, water tem- 

 perature and salinity, the North Atlantic Deep Water formation, 

 and Labrador Current mass transport. 



The main branch of the Labrador Current flows along 

 the edge of the Labrador and Newfoundland shelf (Lazier and 

 Wright, 1993), whereas an inshore branch follows the various 

 cross-shelf saddles and inshore troughs (Colbourne, 2004). 

 Rapid climate changes beginning in the late 1980s produced an 

 enhanced outflow of low-salinity waters from the Arctic and a 

 general freshening of the Labrador Current (Greene and Persh- 

 ing, 2007). Recent and substantial evidence for rapid changes 

 in North Atlantic climate include increased Arctic air tempera- 

 tures (Thompson and Wallace, 1998), decreased sea ice extent 

 (Comiso, 2006), record minimum sea ice (Gersonde and Vernal, 

 2013), increased melting of the Greenland ice sheet, and freshen- 

 ing of the North Atlantic (Curry and Mauritzen, 2005). Under- 

 standing the mechanisms responsible for the recently observed 

 changes in the strength of the Labrador Current and climate 

 variability in the subarctic Atlantic requires analysis of long cli- 

 mate time series (Sutton and Flodson, 2003). 



