The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 126

1.029

1.028

1.027

1.026

1.025

1.024

1.023

-2 3 8 13 18 23 28

Temperature ( ° C)

Sea-waterdensityandtemperature.Asseawatercoolsitgetsmoredensebutasthetemperatureapproaches
freezingpointitgetsdenserataslowerrate.Therefore,adegree-coolerwaterincoolseashaslessnegative
buoyancythanadegree-coolerwaterinwarmseas,whichwillhaveagreatertendencytosink.

Fig. 4.1

the aforementioned sub-Arctic North Pacific scenario. Meanwhile, in today's world
there is less deep water returning to the surface bringing with it carbon dioxide. More
carbon dioxide remains trapped within ocean deep water. This tendency for lower
atmospheric carbon dioxide after 2.7 mya added an additional cooling forcing factor
to the already cooling global climate.
As we shall see later in this chapter when discussing the Broecker thermohaline
circulation (section 4.5.3), today there is some climatically important mixing of
surface and deep waters but this takes place only in special places where the sea
is both more salty (and hence more dense) and cool. It is this combination of salt
(halinity) and temperature (thermality) affecting density that drives the Broecker
thermohaline circulation, and not temperature and density by themselves.
The idea that changes in sea-water temperature/density relationships changed
2.7 mya is substantiated by biological evidence other than that from the sub-Arctic
North Pacific (Sigman et al., 2004). This is exemplified by two examples that relate
to both the northern and southern hemispheres. First, opal is a mineral composed of
amorphous silica and forms a constituent of the cell wall of a group of phytoplankton.
It is a useful palaeo-indicator because it is comparatively resistant to decomposition.
Second, living organisms prefer to take up the nitrogen isotope 14 Nto 15 N from the
fixed biosphere pool of nitrogen. So, when there are more living organisms consum-
ing nitrogen the ratio between the two isotopes changes. Looking at sediments around
Antarctica and the Arctic, Sigman et al. found that both the rate of opal production
and the nitrogen isotope proportions changed around 2.7 mya. This signifies a change
in nutrient mixing, hence surface- and deep-water mixing necessary to support a
large phytoplankton population. Finally, this scenario ties in with ocean sediments
that suggest that ice rafting also commenced about this time. Therefore, it would
appear that ice rafting began around a time of changing surface- and deep-water