Arkell.1956.Jurassic..

CLIMATE 617
general temperature of the sea was lower at the time of the Lias than during
the Upper Jurassic. This is suggested by the general scarcity of limestones
and of coral reefs before the Middle Jurassic and by an increase in the
average size of fossil insects during the period. It should not be overlooked
however, that the Greenland flora about which so much has been said
lived in early Lias times.
The existence of seasonal changes in what are now temperate and
Arctic regions seems indicated by growth-rings in fossil wood, dissepiment
rings in certain fossil corals and by ledging or ringing of the growth-lines
of certain pelecypods. (For references see Arkell, 1935, p. 102.)
When we speak of the waters of the whole earth being warmer in the
Jurassic than at present, there are two areas about which there must be
reservations, since at present there is no evidence, namely, the middle of
the Pacific and the middle of the South Atlantic. If some theory of
geophysics were to demand that the poles were situated in these areas
in the Jurassic, and that in consequence of the poles lying in great oceans
(the Arctic Ocean is a diminutive land-locked sea in comparison) there
were no permanent (or greatly reduced) ice caps, there would not seem to
be any positive evidence against this. But to be sufficiently far from the
rich Jurassic faunas of the Alaska peninsula and from the corals in the
Torinosu Limestone of Japan, the Pacific pole would have to be not
more than 20° or 30° north of the present equator if on such a meridian
as would bring the other pole reasonably central to the South Atlantic.
In that case the Jurassic equator would have lain on about latitude 6o°-70°
on the European side, namely north of the Orkneys and through Iceland
and Scandinavia, which is too far north to suit the facts. The ideal position
for the Jurassic equator, assuming displaced poles, would be through
southern Europe, which would bring the north pole up to latitude 40°-50°
in the Pacific, too near Alaska and Japan. Any less drastic displacement
of the north pole, which would locate it over any of the lands surrounding
the Arctic Ocean, must be ruled out unless the climate was in general
much warmer, owing to the ring of Jurassic faunas which encircles that
ocean and to the absence of any signs of glaciation.
All things considered, therefore, the most probable explanation of the
warm temperature of the Jurassic is that which depends on receipt of
more solar radiation, as already postulated by Dubois (1895).* His theory
has the special advantage that the effects of increased solar radiation would
have a disproportionately great effect in high latitudes, owing to the
efficiency of the distributary mechanism of currents and winds.
Therefore, in order to melt the ice caps and render the Arctic Ocean
inhabitable by a large and varied fauna, it is not necessary for the
sun to heat the equatorial zone to inconveniently high temperatures.
Moreover, in middle and high latitudes currents are said to have a greater
warming effect than winds, so that the sea would tend to be warmer than
* 'Continental submergence' (Berry, 1929) does not seem a suitable or adequate
explanation for the Jurassic, at least.
http://jurassic.ru/