DEFINITION OF EFFECTS OF TRANSGRESSIONS AND REGRESSIONS

During sea-level transgressions, continental shelves are covered

by water, and available habitats are enlarged, increasing

the diversity of fauna. Transgressions are generally

associated with greater diversification of species. Regres-

sions cause extinctions through loss of environments, both

shallow marine and beach. There is a close association

between Phanerozoic extinctions and sea-level lowstands.

Salinity fluctuations also affect diversity—the formation of

evaporites (during supercontinent dispersal) causes reduction

in oceanic salinity. For instance, Permian-Triassic rifts

formed during the breakup of Pangea had lots of evaporites,

(up to 4.4 miles, or 7 km thick), which lowered the salinity

of oceans.

Supercontinents affect the supply of nutrients to the

oceans, and thus, the ability of life to proliferate. Large

supercontinents cause increased seasonality and thus lead to

an increase in the nutrient supply through overturning of the

ocean waters. During breakup and dispersal, smaller continents

have less seasonality, yielding decreased vertical mixing,

leaving fewer nutrients in shelf waters. Seafloor spreading

also increases the nutrient supply to the ocean; the more

active the seafloor spreading system, the more interaction

there is between ocean waters and crustal minerals that get

dissolved to form nutrients in the seawater.

See also GREENHOUSE EFFECT; ICE AGE; PLATE TECTONICS.

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