ORBITAL GEOMETRY & OCEANIC CHANGE
World Data Center A-Paleoclimatology at the National Geophysical Data Center maintains an archive of data relevant to Pleistocene climate. These data include the Earth's insolation, the CLIMAP and SPECMAP data sets, and many long records from the Ocean Drilling Program (ODP).
Long-term climate change (tens to hundreds of thousands of years) appears to be paced and in some cases, forced, by variations in the latitudinal distribution of incoming solar radiation due to changes in the earth's orbital geometry. This relationship is particularly striking for the pattern of alternating glacial-interglacial conditions during the Pleistocene. The marine record has provided a critical test of one of these models, formulated by Milankovitch in the early 1930s.
Components of orbital geometry - Eccentricity
The path of the earth around the sun is slightly elliptical, rather than circular. The eccentricity of the ellipse changes periodically with time, with components having periods of ~100 ky and ~413 ky. The eccentricity is defined as the ratio of the focal length of the ellipse (the distance between the foci) to the length of its major axis; the eccentricity of the earth's orbit about the sun has ranged from less than 0.01 to more than 0.05 over the past 600 ky (Figure OG-1, 2):
Figure OG-1. Exaggerated range of eccentricity of the earth's orbit.
Figure OG-2. Variations in the eccentricity of the earth's orbit during the past 600,000 years.
As is suggested by the exaggerated ellipses of Figure OG-1, variations in eccentricity change the flux of radiation received by the entire planet at different times of the year, but there is no latitudinal effect. In actuality, because the earth's orbit is nearly circular, the radiation effect of eccentricity by itself is very small.
Obliquity
The rotational axis of the earth is not normal to the ecliptic (the plane of the earth's orbit - Figure OG-3). The present tilt is 23.5º (the latitudes of the Tropics of Cancer and Capricorn). This tilt or obliquity has ranged from 22.1 to 24.5º over the past 700 ky (Figure OG-4):
Figure OG-3. Exaggerated range of tilt of the earth's orbit.
Figure OG-4. Variations in the obliquity of the earth's orbit during the past 600,000 years.
The exaggerated tilts of Figure OG-3 illustrate how this parameter affects the seasonality of the earth (the length of day as a function of latitude and season); when the tilt is large, there is more contrast between summer and winter, and the tropics and polar zones both increase in extent at the expense of the mid latitudes. It is easy to see how this parameter interacts with eccentricity - summers will be hot in the hemisphere tilted towards the sun when the earth is at its closest approach to the sun.
Precession
Over time, the earth's rotational axis rotates or precesses around the normal to the ecliptic. If the earth's orbit were circular, this would have no climatic consequences. Because precession changes the locations on the orbit (i.e the times of year) where the hemispheres experience summer and winter, however (Figure OG-5), the coupling of precession and eccentricity produce variations in the latitudinal distribution of radiation with periods of 23 ky and 19 ky (Figure 17-6).
Figure OG-5. Exaggerated view of the impact of precession on the energy flux to the earth.
Figure OG-6. Variations in the precession of the earth's orbit during the past 600,000 years.
Why does the earth's orbital geometry change over time? Two factors are involved:
1. Changes in the Earth's spin due to tidal friction (resulting in a lunar recession rate of 10-9 m/sec).
2. Weakly chaotic changes in the orbits of the inner planets.
Estimates of 1. from direct laser ranging to the moon, from Babylonian, Hellenic, Chinese, and Islamic historical data on the precession of the equinoxes, and from the number of days in a solar and lunar month recorded in corals spanning the past 450 million years, are reasonably well known and the effects of the remaining uncertainty have been assessed.
Chaotic changes are inherently unpredictable, but their effect back to about 100 to 200 my is small (Figure OG-7). Prior to about 100 my, the exact motion of the solar system cannot be calculated, so only the ratios of the orbital parameters, not the absolute age of a particular configuration, can be calculated.
Figure OG-7. Variations in the dominant frequency of precession (left) and tilt (right) of the earth during the past 200 million years (after Berger et al., 1992)
For the past five or so million years, very detailed insolation curves for 65º N have been prepared (e.g. Figure OG-8 shows the history for the past 600,000 years) and correlated with the geologic record to allow individual events to be dated to within a few thousand years - an accuracy of better than 0.1 percent over 5 million years. The relative ages of two closely spaced events can be determined even more precisely. Residual uncertainties relate both to the orbital parameters, and the offset in time, or lag, between a change in incoming radiation and the response of the geologic and oceanographic proxies that are measured in sediment cores.
The sedimentary record preserves only indirect evidence of past oceanographic conditions. Much of the paleoceanographic work of the past three decades has focussed on identifying and validating proxies for these conditions. Some of the proxies that we will refer to include:
|
|
|
|
Microfossil census data (factor analysis, transfer functions) |
SST, SSS, mixed layer depth, bottom water O2, bottom water corrosiveness (CO3=) |
|
Fragmentation of foram tests |
Bottom water corrosiveness, CO3=* |
|
Oxygen isotopic composition of foram tests |
Vol. of continental ice sheets, SST, bottom water T, SSS |
|
Carbon isotopic composition of foram tests |
Productivity, water mass mixing |
|
Cd in foram tests |
PO4-3 content of SW, water mass formation |
|
Mg in foram tests |
SST |
|
Ba in foram tests |
Productivity |
|
Opal flux |
Productivity |
|
Excess Al in carbonate ooze |
Productivity |
|
Quartz (dust) abundance and flux |
Wind patterns and intensities |
|
Alkenone indices |
SST |
These will be discussed in more detail during the future lectures.
|
|
Oceanography 540 Pages |
