A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
13
now recognized that the small variations in received solar radiation caused
by these Milankovitch cycles are important drivers of climatic variability
[53].
The strongest evidence that Milankovitch cycles induce changes in ice
volume during glacial periods comes from the analysis of oxygen isotope
fractionation in marine sediment cores. These show variability with strong
signals at frequencies corresponding to cycles of 23 kyr and 41 kyr; recent
studies have also demonstrated lower frequency cycles at -100 kyr and
-400 kyr [54].
Although ice volume is only one factor influencing the marine sediment
oxygen isotope signal, there is general agreement that orbital variations
have been responsible for Milankovitch cycles in the volume of polar ice in
the Pleistocene glaciation. Although it would seem reasonable to assume
that similar Milankovitch cyclicity in ice volume will have occurred
throughout the Cenozoic glaciation of Antarctica, evidence is hard to come
by. There is, however, some evidence from varYing in marine sediment
cores for Milankovitch cyclicity in the Mesozoic [55].
We can therefore conclude that superimposed on large-scale changes in
ice volume, such as that suggested for the Pliocene, will be shorter-term
fluctuations on time-scales in the range 23 to -400 kyr. If these fluctuations
in ice volume were substantial it is possible that they may have driven
significant changes in available shallow-water habitat. This in turn could
be a mechanism for diversification through allopatric speciation, through a
mechanism termed by Valentine [56] the climatic diversity pump [57].
Milankovitch-driven cycles in habitat area may have been a mechanism
contributing to the high diversity of some taxa in the Southern Ocean
[5,6,58]. At present, however, we do not have the evidence from the
marine glacial record to determine the extent of high-frequency variations
in the ice volume of Antarctica.
What Has Been the Role of Sudden Climatic Events in the
Evolution of the Southern Ocean Fauna?
High resolution analyses of ice-cores from Greenland have shown that at
certain periods, the regional terrestrial climate has changed very rapidly
[59,60], confirming the rapid changes first demonstrated by climatic
reconstructions from subfossil Coleoptera (beetles) [61,62].
What is not at all clear is the extent to which such rapid changes might
occur in the sea. Whilst the thermal mass of water means that changes in
the temperature of bulk seawater must inevitably be slow, it has become
clear that climatic change in the sea may be exhibited through sudden
changes in patterns of current flow. Evidence for such changes is
13
now recognized that the small variations in received solar radiation caused
by these Milankovitch cycles are important drivers of climatic variability
[53].
The strongest evidence that Milankovitch cycles induce changes in ice
volume during glacial periods comes from the analysis of oxygen isotope
fractionation in marine sediment cores. These show variability with strong
signals at frequencies corresponding to cycles of 23 kyr and 41 kyr; recent
studies have also demonstrated lower frequency cycles at -100 kyr and
-400 kyr [54].
Although ice volume is only one factor influencing the marine sediment
oxygen isotope signal, there is general agreement that orbital variations
have been responsible for Milankovitch cycles in the volume of polar ice in
the Pleistocene glaciation. Although it would seem reasonable to assume
that similar Milankovitch cyclicity in ice volume will have occurred
throughout the Cenozoic glaciation of Antarctica, evidence is hard to come
by. There is, however, some evidence from varYing in marine sediment
cores for Milankovitch cyclicity in the Mesozoic [55].
We can therefore conclude that superimposed on large-scale changes in
ice volume, such as that suggested for the Pliocene, will be shorter-term
fluctuations on time-scales in the range 23 to -400 kyr. If these fluctuations
in ice volume were substantial it is possible that they may have driven
significant changes in available shallow-water habitat. This in turn could
be a mechanism for diversification through allopatric speciation, through a
mechanism termed by Valentine [56] the climatic diversity pump [57].
Milankovitch-driven cycles in habitat area may have been a mechanism
contributing to the high diversity of some taxa in the Southern Ocean
[5,6,58]. At present, however, we do not have the evidence from the
marine glacial record to determine the extent of high-frequency variations
in the ice volume of Antarctica.
What Has Been the Role of Sudden Climatic Events in the
Evolution of the Southern Ocean Fauna?
High resolution analyses of ice-cores from Greenland have shown that at
certain periods, the regional terrestrial climate has changed very rapidly
[59,60], confirming the rapid changes first demonstrated by climatic
reconstructions from subfossil Coleoptera (beetles) [61,62].
What is not at all clear is the extent to which such rapid changes might
occur in the sea. Whilst the thermal mass of water means that changes in
the temperature of bulk seawater must inevitably be slow, it has become
clear that climatic change in the sea may be exhibited through sudden
changes in patterns of current flow. Evidence for such changes is