Lab 6: Paleoceanography

Reading:

• Textbook Chapters 8 (p. 130-133), 9 (p. 149-152)
• Zachos, J. C., Pagani, M., Sloan, L., Thomas, E., and Billups, K., 2001. Trends, Rhythms and Aberrations in Global Climate 65 Ma to Present. Science, vol. 292, p. 686-693
• Notes on the use of oxygen and carbon isotope use in paleoceanography (handout).

Goals of this lab: learn basic principles of use of stable isotopes of carbon and oxygen in paleoceanography, as well as basic climate development during the Cenozoic.

1. Microscope study of material in the >63mm size fraction.
2. Interpretation of isotope records.

1. The 4-well slide contains 4 samples: A, B, C, and D. Two of these samples are Recent, two have a numerical age of about 56 Ma; one of the Recent and one of the old samples is from high latitudes; the other is from low latitudes. Use figure 2 from the reading (Science paper) to see how warm surface waters were at that time at high latitudes (temperature of high latitude surface waters is about the same as temperature of deep waters globally). Two of the samples are from a high latitude (~65^oS), two are from within 4^o of the equator. The most common objects in each of the samples are planktonic foraminifera. Planktonic foraminifera live in various latitudinal zones (see figure below for Recent species, p. 151-152 text book) with higher diversity at lower latitudes. Use the plate to determine which of the samples are Recent, of which of these two is from high, which from low latitudes. Define what objects other than planktonic foraminifera are present.

A and B are recent samples; A is tropical, B from high latitudes. Specimens labeled 1, 2 as well as 3-5 are present in sample A. There are also radiolarians and diatoms. In sample B there are only specimens as in 16-17 (different sides of same species), as well as terrigenous material (large quartz grains), which fell from melting icebergs; there are also radiolarians in this sample.

Planktonic foraminifera from the recent oceans, arranged by latitudinal species assemblage. Plate from J. P. Kennett, 1982, Marine Geology (Prentice-Hall), Figure 16-1.

2. Now determine which one of the remaining samples is low and which one is from high latitude, assuming that the diversity gradient (more species at low latitudes) was valid in the past, but keeping in mind that the temperature gradients between high and low latitudes were much smaller 56 Ma ago.

Sample C is the tropical sample, with many specimens having specimens which are not rounded, but have sharp edges and spines. Sample D is the sample from high latitudes, dominated by rounded forms. Note the absence of ice-rafted material in the polar sample, and the presence of more variability in the planktonic foraminifera than in the modern polar sample.

Use the notes on on isotopes to answer the following questions:

• Use the temperature equation to derive the deep-water temperature of the average ocean at about 55 Ma during the short peak in warmth, when d¹⁸O reached values of about &endash;0.5 ^o/_oo. Assume that there was no ice, and use the appropriate oceanic values of d¹⁸O _water (see notes on on isotopes).
  • t=16.9 - 4.38 (-0.5+1.26) + 0.10(-0.5 + 1.26)² = 13.6^oC (this is very warm for poles!)
• Now use that same equation (assuming no polar ice caps) to calculate the temperature at about 33.5 Ma (when d¹⁸O_calcite reached values of about +3.2 ^o/_oo. First, assume that there still were no ice caps to get the ocean water values of d¹⁸O. Is the temperature that you get realistic in view of the fact that you just assumed that there were no ice caps? (remember that deep-water ocean temperatures are equal to surface water temperatures at the poles).
  • t=16.9 -4.38 (3.2 + 1.26) + 0.10 (3.2 + 1.26)² = -0.6^oC. This is a negative temperature (freezing; although sea water is salt and still liquid at this temperature). This result is thus inconsistent with the assumption of 'no polar ice caps'.
• Then assume an oxygen isotope composition as in the present oceans, and recalculate the temperature of the deep waters.
  • t=16.9 - 4.38 (3.2 + 0.28) + 0.10 (3.2 + 0.28)² = 2.9^oC.
• Present average deep-ocean temperatures are close to 1-2^oC. Does the temperature that you found suggest that ice sheets at 33.5 Ma were similar in size to the present ones or not?
  • Found temperature is slightly on the high side, but not very far off, in view of the various uncertainties. Probably the polar ice sheet was slightly smaller at that time.
• The carbon isotope composition of benthic calcite is much lower than that of planktonic calcite (see handout). Just after the extinction at the end of the Cretaceous, however, this difference disappeared and the two groups of organisms have almost the same value. What caused this?
  • At the end of the Cretaceous oceanic productivity collapsed, probably as a result of darkness after a meteorite impact. Oceanic primary producers are mainly microscopic algae with a very short life span, which die off during a few months darkness. This change in carbon isotopes thus most likely reflects the collapse of the biological pump (thus major impact of all ocean life).
• What is the effect of fossil fuel burning on the carbon isotopic composition of benthic and planktonic foraminiferal shells?
  • It wil add isotopically light carbon (enriched in ¹²C) to the atmosphere, which then will exchange this carbon with the dissolved inorganic carbon reservoir in the oceans. This dissolved inorganic carbon then will be used by plantkonic as well as benthic foraminifera: isotopically light carbon has been added to the complete oceanic reservoir.
• During the very warm period about 55 Ma ago, there was a very rapid shift (over less than about 1000 years) to lower carbon isotope values in planktonic and benthic foraminifera worldwide. Could such a rapid change have been caused by a change in productivity in the oceans?
  • No; changes in productivity will affect the gradient between planktonic and benthic forms, not both. Such an overall change must have been caused by addition of isotopically light carbon to the ocean-atmosphere system. In this specific case, the isotope effect is so large that even complete burning of all the world's life would not be enough! Much speculation on this event (with implications for future global warming) is ongoing (click here for more information).