Table 3. --Comparison of erosion agents and rates for Black Rock 

 (unpublished data of C. M. Hoskin, D. H. Mook , and J. K. Reed) 



Process Agent Average Rate of Substrate Erosion 



(mg cm" 2 yr" 1 ) 



E. 1 ucunter 



XFucket enclosures) 667 



Bioerosion Rock infauna 



(bucket enclosures) 183 



Intertidal microborers 

 (tethered limestone blocks) 26 



Physical Waves and currents 

 abrasion (tethered limestone blocks) 5 I _5 



Chemical Intertidal seawater 



solution (filter-protected, suspended 

 limestone blocks) 0-_2 



processes at Black Rock (table 3) indicates that biological activities are more 

 significant than physical and chemical processes in the degradation of reef 

 substrates. 



In terms of paleoecologi cal significance, we suggest that the most 

 preservable part of the bioerosive activity of E. lucunter at Black Rock will 

 be the band of bore holes. However, with SEM we searched for and did not find 

 appropriately sized scratch marks on the bottoms of bore holes which might be 

 expected from urchin teeth. The sediment produced by E. lucunter is initially 

 pelleted, but the pellets quickly disintegrate. Sand "From the pellets is most 

 abundant at sizes between 0.125 - 0.177 mm, but other processes may produce 

 sand of the same size. Approximately half (by weight) of the disintegrated 

 pellets is mud with no distinctive properties. Although the urchin population 

 density was as high as 100 adults m _ 2, the total amount of echinoderm skeletal 

 fragments in the sand fraction of nearby shallow water sediment was < 1%; Hine, 

 et a! . (1981, table 1, p. 288) did not include echinoderm skeletons in their 

 TTsting of carbonate constituents. 



Hine, et al . (1981, fig. 17-B, p. 278) have shown by seismic reflection 

 profiling that the southwest margin of the Little Bahama Bank in the vicinity 

 of Black Rock is a rocky surface, barren of sediment. Our seismic reflection 

 profiling confirms the seismic findings of Hine, et al . (1981), but from the 

 JOHNSON-SEA-LINK submersibles we have seen sediments a few centimeters thick 

 overlying hardgrounds on the western bank margin. Using the geometry of sand 

 bodies 3-12 km northeast of the bank margin as an indicator, Hine, et al . (1981, 

 fig. 14, p. 275, and table 1, p. 268) suggested that sand is transported onto 

 the Little Bahama Bank. Sediment produced by E. lucunter and the associated 



159 



