NUMBER 40 . 2 7 



capable of dissolving already emplaced carbonate. In the concep- 

 tacle fertile cylinder, during the formation of sporangia from the 

 meristem, carbonate is not laid down in the cell walls of intra- 

 sporangial filaments; 100 pm away in the vegetative tissue, nor- 

 mal calcification continues to occur. As sporangia enlarge, they 

 are able to dissolve a large volume of cell wall carbonate in the 

 surrounding and underlying vegetative tissue. These decalcified 

 tissues are crushed by the final enlargement of sporangia prior to 

 spore formation. Later, after spores are released, the thin, empty 

 sporangial walls become calcified (Figures 12B, 13A). Likewise, 

 the inner walls of meristem cells and the interfilament space are 

 (in summer) emplaced in the meristem fracture zone with car- 

 bonate; when cut off by cell division, this carbonate joins the 

 underlying perithallium. Then, two to five cells into the perithal- 

 lium, passages (fusions) are excavated out of calcified cell walls. 



laterally, to join the lumina of adjacent cells. Especially in the 

 autumn, when sporangia begin development, these fusions may 

 extend laterally for several cells (Figure 1 1 B). Although the num- 

 ber of fusions varies considerably at the autumn "break," when 

 sporangial formation is initiated, they are often so abundant 

 that they develop a plane of weakness that fractures horizontally 

 when breaking stresses are applied when creating fracture sec- 

 tions (Figure 1 1 A). 



Where bivalve boring (and carbonate etching) and deeply 

 grooved sea urchin grazing occur, breaking the "seal" of living 

 tissue over dead carbonate, alteration of carbonate chemistry is 

 likely. These diagenetic zones have ecological signatures, often 

 with the formation of epi- and endophytes on the dead crust. Also, 

 in thick clathrostromes, occasional fractures develop because of 

 wave or moving substrate stresses. Alteration of carbonate along 



500- 



GOM ^NE NFS. Lab. N. Lab. Arctic Bay 

 ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I 



42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 



Latitude (°N) and Subregion 



FIGURE 21. Clathromorphum compactiim. Regional yearly crustal accretion of haphazardly selected specimens and years as a function of 

 latitude in the northwestern North Atlantic, Labrador Sea, and Arctic Bay, northern Baffin Island. The red squares are individual specimens. 

 In Newfoundland (NF) and Labrador, each data set included one sample from Hiatella-excheA burrows; otherwise, data are from analyses of 

 conceptacle layers on SEM mosaics. 



