Neotropical Eocene coastal floras and 18 O/ 16 O‐estimated warmer vs. cooler equatorial waters

The history of the earth's sea‐surface temperature (SST) in equatorial regions during the Tertiary is unsettled because of uncertainty as to the presence and extent of glaciers during the Paleogene. The 16 O trapped in glaciers and subsequently released back to the ocean basins as meltwater during interglacials aifects the 18 O/ 16 O ratio of sea water, one of the variables that must be known for oxygen isotope paleotemperature analyses of calcareous fossils. Estimates of SST range from ~ 18 to 20 C, assuming an ice‐free earth, to ~ 28 C assuming glaciers were present in the Paleogene. Low latitude SST presently averages 28 C, so the former estimate gives a value 8 to 10 C cooler than present, while the latter gives a value as warm or slightly warmer than present. The figures are important for interpreting terrestrial vegetational history because the temperature differential between low and high latitudes is a major factor in determining global climates through the control of poleward transfer of heat. The middle(?) to late Eocene Gatuncillo Formation palynoflora of Panama was deposited at the ocean‐continental interface at ~9° N latitude. The individual components and paleocommunities are distinctly tropical and similar to the present vegetation along the Atlantic coast of southern Central America. This is consistent with data emerging from other recently studied tropical coastal biotas and represents a contribution from paleobiology toward eventually resolving the problem of Eocene equatorial marine environments. Collectively, the evidence is beginning to favor a model of Eocene SST near present values.