Tertiary Oceans: the Onset of the (Northern) Ice Age 261
Ocean, that is, to the "cold box" of the ocean modelers. Thus, the average ocean
water has to be cold (which it is, at present). Since the cold water sphere becomes the
global environment, while the tropical waters are isolated, deep-sea and cold-water
faunas become global, while tropical ones become more provincial.
As concerns the heat budget, one leitmotif of Tertiary ocean history is the increased displacement of the intertropical convergence zone (ITCZ, the heat equator)
to the north of the Equator. There are several causes for this. One is the whitening of
the Antarctic continent. This has the effect of pushing climatic zones northward.
Another is the northward movement of large continental masses which sets up monsoonal regimes favorable for northward heat transfer. The uplift of Tibet and the
Himalayas as a consequence of the collision of the Indian with the Eurasian Plate had
additional climatic consequences, for example, the strengthening of monsoons, and
an increase in weathering. A fourth factor is the peculiar geographic configuration in
both major ocean basins, Atlantic and Pacific, which provides for deflection of westward-flowing equatorial currents, sending them northward to strengthen the Gulf
Stream and the Kuroshio. The end result is that the southern hemisphere is robbed of
heat by the northern hemisphere: glaciers in southern New Zealand are in walking
distance from the seashore, at a latitude which corresponds to that of the vineyards of
Bordeaux! The significance of this asymmetry for paleoclimatology was emphasized
long ago by James Croll in his book Climate and Time (1875).
One important aspect of this planetary heat asymmetry is the fact that the North
Atlantic tends to deliver deep water to the southern exchange around Antarctica. On
the whole, it receives shallower water in return, not directly from the Antarctic, but
via various routes along which this water was warmed. Thus, the North Atlantic uses
deep water formation as a heat pump: warm water in, cold water out.
When was this "Nordic heat pump" first turned on? We do not know for sure, but
there are certain clues from chemial asymmetries between Pacific and Atlantic
(basin-basin fractionation, Sect. 7.6.5). Whenever the North Atlantic makes cold
deep water, and sends it off to the southern mixing ring, it also sends nutrients and
dissolved silica out of the basin. Also, in order to achieve the necessary density for
making deep water, salinity needs to be increased by sending water vapor to the
North Pacific. In tum, this sets up an esturine deep circulation pattern there, with high
nutrient and silica values at depth (Sect. 7.6.1). The result is that siliceous sediments
become rare in the North Atlantic and abundant in the North Pacific, whenever the
Nordic heat pump is in action. From the record of silica deposition, we see that the
Pacific-Atlantic asymmetry greatly increased between 15 and 10 million years ago
("silica switch"). At that time the Nordic heat pump started working hard (Sect.
9.5.5).
9.5.4 The Onset of the (Northern) Ice Age. Once the Nordic heat pump was turned
on, the north-to-south asymmetry was greatly strengthened - the ice age could proceed in Antarctica, while being kept out of northern latitudes. Eventually, however,
the overall cooling caught up with the north as well, between 3.5 and 2.5 million
years ago (Fig. 9.13). Northern glaciation set in, and the planetary climate moved into
the glacial cycles which we discussed earlier. Again, the Plio-Pleistocene cooling was
Ocean, that is, to the "cold box" of the ocean modelers. Thus, the average ocean
water has to be cold (which it is, at present). Since the cold water sphere becomes the
global environment, while the tropical waters are isolated, deep-sea and cold-water
faunas become global, while tropical ones become more provincial.
As concerns the heat budget, one leitmotif of Tertiary ocean history is the increased displacement of the intertropical convergence zone (ITCZ, the heat equator)
to the north of the Equator. There are several causes for this. One is the whitening of
the Antarctic continent. This has the effect of pushing climatic zones northward.
Another is the northward movement of large continental masses which sets up monsoonal regimes favorable for northward heat transfer. The uplift of Tibet and the
Himalayas as a consequence of the collision of the Indian with the Eurasian Plate had
additional climatic consequences, for example, the strengthening of monsoons, and
an increase in weathering. A fourth factor is the peculiar geographic configuration in
both major ocean basins, Atlantic and Pacific, which provides for deflection of westward-flowing equatorial currents, sending them northward to strengthen the Gulf
Stream and the Kuroshio. The end result is that the southern hemisphere is robbed of
heat by the northern hemisphere: glaciers in southern New Zealand are in walking
distance from the seashore, at a latitude which corresponds to that of the vineyards of
Bordeaux! The significance of this asymmetry for paleoclimatology was emphasized
long ago by James Croll in his book Climate and Time (1875).
One important aspect of this planetary heat asymmetry is the fact that the North
Atlantic tends to deliver deep water to the southern exchange around Antarctica. On
the whole, it receives shallower water in return, not directly from the Antarctic, but
via various routes along which this water was warmed. Thus, the North Atlantic uses
deep water formation as a heat pump: warm water in, cold water out.
When was this "Nordic heat pump" first turned on? We do not know for sure, but
there are certain clues from chemial asymmetries between Pacific and Atlantic
(basin-basin fractionation, Sect. 7.6.5). Whenever the North Atlantic makes cold
deep water, and sends it off to the southern mixing ring, it also sends nutrients and
dissolved silica out of the basin. Also, in order to achieve the necessary density for
making deep water, salinity needs to be increased by sending water vapor to the
North Pacific. In tum, this sets up an esturine deep circulation pattern there, with high
nutrient and silica values at depth (Sect. 7.6.1). The result is that siliceous sediments
become rare in the North Atlantic and abundant in the North Pacific, whenever the
Nordic heat pump is in action. From the record of silica deposition, we see that the
Pacific-Atlantic asymmetry greatly increased between 15 and 10 million years ago
("silica switch"). At that time the Nordic heat pump started working hard (Sect.
9.5.5).
9.5.4 The Onset of the (Northern) Ice Age. Once the Nordic heat pump was turned
on, the north-to-south asymmetry was greatly strengthened - the ice age could proceed in Antarctica, while being kept out of northern latitudes. Eventually, however,
the overall cooling caught up with the north as well, between 3.5 and 2.5 million
years ago (Fig. 9.13). Northern glaciation set in, and the planetary climate moved into
the glacial cycles which we discussed earlier. Again, the Plio-Pleistocene cooling was