The Ocean Habitat
163
L amellibranchs
Gastropods
30·
, .... , .. " .. " Arctic and
Acadian Species
------ Virginian Sp.
- - Caribbean and
Carolinian Sp.
60 ·W
Fig. 6.6. Diversity of benthic shelf organisms as a function of latitude, shelf off the East Coast. An
Arctic fauna with few species in the north is being replaced by species-rich temperate and tropical
faunas to the south. (Each line equals ten species.) Note the sharp break at Cape Hatteras, where
warm Southern waters (Gulf Stream) leave the shelf. Arctic waters penetrate down to Cape Cod.
Gastropods show a greater change than bivalves (= lamellibranchs or pelecypods). The clams live
mostly l1'ilhin the sediment, in contrast to the snails. Hence they are more protected from inclement
conditions. lA. G. Fischer, 1960, Evolution 14: 64; modified]
Can we reconstruct the temperature distributions from a study of the remains of
organisms?
In shallow seas and on the shelf, benthic carbonate-secreting organisms (Fig. 6.7)
leave a record of changing temperatures in the oxygen isotope composition of the
calcareous shells and skeletons, as first demonstrated by the geochemist S. Epstein
and his associates (Chap. 7.3.2). In colder water, relatively more of the heavier
isotope 18 0 is incorporated into CaC03, changing the 18 0;160 ratio accordingly (Fig.
6.8).
163
L amellibranchs
Gastropods
30·
, .... , .. " .. " Arctic and
Acadian Species
------ Virginian Sp.
- - Caribbean and
Carolinian Sp.
60 ·W
Fig. 6.6. Diversity of benthic shelf organisms as a function of latitude, shelf off the East Coast. An
Arctic fauna with few species in the north is being replaced by species-rich temperate and tropical
faunas to the south. (Each line equals ten species.) Note the sharp break at Cape Hatteras, where
warm Southern waters (Gulf Stream) leave the shelf. Arctic waters penetrate down to Cape Cod.
Gastropods show a greater change than bivalves (= lamellibranchs or pelecypods). The clams live
mostly l1'ilhin the sediment, in contrast to the snails. Hence they are more protected from inclement
conditions. lA. G. Fischer, 1960, Evolution 14: 64; modified]
Can we reconstruct the temperature distributions from a study of the remains of
organisms?
In shallow seas and on the shelf, benthic carbonate-secreting organisms (Fig. 6.7)
leave a record of changing temperatures in the oxygen isotope composition of the
calcareous shells and skeletons, as first demonstrated by the geochemist S. Epstein
and his associates (Chap. 7.3.2). In colder water, relatively more of the heavier
isotope 18 0 is incorporated into CaC03, changing the 18 0;160 ratio accordingly (Fig.
6.8).