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A possible constraint is 613C in planktonic foraminifera from north of the polar front. 
This signal covaries with the deep ocean signal, which may suggest that preformed nutrients in 
NADW are not constant (Mix and Fairbanks, in press). Choosing between the second and third 
hypothese, however, depends on the mechanism for the global deep-ocean signal. If the 13c¢ 
budget reflects the transfer of forest biomass to the ocean in glacial time (Shackleton, 1977), 
polar planktonics reflects this global signal, and the second scenario is correct. If 13¢ js 
linked to PO4q in shelf organics (Broecker, 1981), polar planktonics reflect preformed nutrients 
in NADW, and the third scenario is correct. The truth probably lies somewhere in the middle, 
and both effects operate. 
