22 . SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



FIGURE 16. Cbthruniurphiini cuinpactiun calcified wall structure. (A) Transverse fracture showing summer cells with large interfila- 

 ment calcite crystals between rings of small, radial, inner-wall crystals (crystals 1 pm long). (B) Longitudinal fracture of summer cells 

 showing large, angular interfilament calcite crystals and thin inner walls of fine, radially oriented crystals. (C) Winter cells of fine, radial, 

 inner-wall calcite crystals. Interfilament crystals are only at cell "corners." 



fractures (Figures I6B, 17C,D), interfilament crystals are much 

 larger (to 5-6 pm long) than inner-wall crystals and tend to be 

 arrowhead shaped (Figure 17B). 



A closer look at the etched valleys shows that the smaller, 

 radial, inner-wall crystals are more resistant to solution than the 

 larger, vertically oriented, interlamellar crystals. This is a differ- 

 ence in crystal morphology that results in winter cells that tend 

 to break out of the carbonate matrix as silt-sized, hollow grains 

 (Figures 19D, 20A). The inner-wall crystals are embedded in an 

 organic matrix that likely controls their development (Giraud 

 and Cabioch, 1979). Thus, they are somewhat protected from 

 dissolution (Ries, 2011). As discussed below, the interfilament 

 (interlamellar) crystals may be precipitated because of CO, re- 

 moval in the inorganic interfilament space and therefore lack 

 protection from dissolution from an organic matrix. 



Growth Rate 



On the basis of combined analyses of conceptacle bands 

 and cellular anatomy, the thickness of yearly bands in Clathro- 

 morphiim compactum and C. nereostratum can be defined using 

 mosaics of SEM images. Further combined with measurements 

 of the bivalve-etched ridges, as described earlier, regional growth 

 rates for Clathromorphitm compactum are shown in Figure 21. 

 There is a highly significant and consistent drop in overall crustal 

 growth rate from south (Gulf of Maine) to north (northern Lab- 

 rador and Arctic Bay, Baffin Island). The plants for which these 

 intervals were measured were selected haphazardly from avail- 

 able SEM sections having clearly defined conceptacle bands. No 

 effort has been made to compare the same years in each region, as 

 this would require a more defined ecological depth range for each. 



