NUMBER 40 . 23 



FIGURE 17. Clathromorphum nereostratum from the Aleutian island of Amchitka. (A) SEM of vertical fracture with overlying epithal- 

 lium, underlying perithallium, and meristem with a break in calcification. Inner cell layer of radial crystals (red arrow); vertically oriented 

 interfilament crystals (blue arrow). (B) SEM looking downward into meristem with epithallium removed. A smooth organic material 

 coats the inner wall (red arrow); large deltoid interfilament crystals (blue arrow), apparently with no integral or overlying organic mate- 

 rial, lie between the inner wall organic material. (C) Unfractured meristem cells showing the inner-wall inner membrane (IM) and outer 

 membrane (OM), the fractured calcification zone (CZ), and the precipitation cavity (PC). (D) Clathromorphum nereostratum from Rat 

 Islands, Aleutian Islands. Summer perithallial cells showing cell fusions formed after burial in the perithallium (horizontal arrows) and 

 pit connections (vertical arrows) between cells of a filament. 



The regional growth rates presented in Figure 21 are plotted 

 as a function of yearly mean regional temperature in Figure 22. 

 The Arctic Bay temperature is an estimate based on a 9-month 

 sea ice interval of -1.8°C and a 3-month ice-free interval of 

 3.5°C. As we will show in a future paper (Halfar and Adey, un- 

 published), when sea ice is consistently present longer than about 

 2 months, growth ceases, probably because of a lack of suffi- 

 cient stored photosynthate. This drop in expected growth rela- 

 tive to temperature is producing a highly significant proxy for 

 sea ice cover. Using the straight-line (GOM to southern Labra- 

 dor) part of the yearly growth curve with the greatest amount of 



information (Figure 22), converted to monthly rates and plotted 

 against the regional temperature curves (Figure 23A), monthly 

 regional growth curves can he derived (Figure 23B). Total yearly 

 growth and the rate of conceptacle maturation (based on the 

 regional height requirement for maturation) can then be summed 

 from the monthly rates. The latter match the data presented in 

 Table 1 . Although laboratory-produced growth rates for Clath- 

 romorphum compactum are not yet available, those for C cir- 

 cumscriptum (Adey, 1970, 1973) suggest that at temperatures 

 below 5°C-6°C, temperature (and not light) is the primary en- 

 vironmental factor controlling vertical growth. Monthly growth 



