Biological Oceanography

(^) The resulting overall range of summer isolation at 65°N is from approximately 410
to 500 watts m−2, a huge variation. Ellipse eccentricity acts to “modulate” the effect of
precession of the equinoxes, which is possibly the reason that eccentricity variation
acts as the switch between glacial and interglacial times (see Ruddiman 2007a).
Pulses of ice formation and loss are also evident in ice-volume proxies at the obliquity
period, 41 kyr.
(^) The time course of the amount of glacial ice piled on the land is best represented
among available “fossil variables” (ice-volume “proxies”) by changes in the level of
oxygen-18 (^18 O) in the ocean as recorded in glacial ice (Fig. 16.10) and fossil
carbonate (foraminifera, coral). Lighter H 216 O molecules are more likely to evaporate
than H 218 O molecules, and more likely to be included in glacial ice. Thus, seawater
becomes enriched in ^18 O as glacial accretion progresses. As ^18 O level rises in the
ocean during ice accumulation, it also rises in water evaporating, so that stacks of
glacial ice, old near the bottom and young at the top, record global ice volume as
changes in ^18 O content, determined from the oxygen in the water itself. Measurement
is done by mass spectrometry, and the result expressed as a fractional difference from
the oxygen in a standard. This fraction is called δ^18 O (“del-O-18”) with units of parts
per thousand (‰). Deuterium in water rises by the same mechanism and is measured
as δD, which can be more accurately calibrated in antarctic ice as a temperature index
than can δ^18 O. Profiles of both variables have been developed for ice cores from
central Antarctica and central Greenland – sites where lateral ice movement is
minimal and stacking with age is close to vertical – as well as from montane glaciers
worldwide. Age is determined from ^14 C levels and other dating techniques, with
special precision when annual layers of dust deposit can be counted.
(^) Equilibration of ^18 O : ^16 O between water and dissolved CO 2 will occur in a matter
of hours via carbonic acid and bicarbonate formation. Thus, oxygen in carbonate
included in new coral or foraminiferan shells will partially record the water values of
(^18) O : (^16) O. Equilibration among different global oxygen pools (water, atmospheric gas,
dissolved O 2 , carbonate) by exchange on time scales of a few thousand years means
that all will record relative removal of ^16 O into ice. For ocean carbonate, the record
will show some lag, and lag will vary with depth of shell deposition (say, benthic
forams vs. surface ones) and with latitude, since there are several ^18 O-enriching
cycles of evaporation and precipitation along the route from the equator to the ice
sheets.
(^) A composite foraminiferan δ^18 O “stack” has been generated by Lisiecki and Raymo
(2005), extending back 5.3 million years. (Fig. 16.11 presents 3.6 Mya). It shows that
throughout that long interval, there were many cycles of glaciation, most of them
rather restricted in the volume of ice accumulated and initially with dominance of the