W4937 Cenozoic Paleoceanography
Lab 1
Due Mar. 14, 2013
Introduction
This lab will introduce some of the basic methods which are used to establish age control,
stratigraphic correlations and paleoclimate signals in ocean sediment cores. This lab
provides an overview of some of the methods which were applied to study sediment cores
in the 1960s when there was intense debate about the number and the magnitude of past
ice ages. Because there were successive ice ages it was reasoned that the land record of
glacial moraines was probably incomplete, with each subsequent glaciation erasing
evidence of prior glaciations. Ocean sediments accumulated slowly but continuously and
thus were seen as a way to reconstruct the full history of earth paleoclimate changes.
At issue though was how to extract paleoclimate information from ocean cores. In the
1947 Harold Urey at the Univ. of Chicago discovered the temperature-dependent oxygen
isotopic fractionation in calcite and reasoned that one could measure past ocean
temperatures using foraminiferal 18O. In 1955, Cesare Emiliani, then Urey’s student,
measured 18O variations in a Caribbean core and he found multiple glacial-interglacial
18O cycles. He attributed the ~1.5-2.0 per mil oscillations entirely to SST changes,
which were found to be large…too large!
Thinking such large SST changes were unreasonably large, Dave Ericson and Gusta
Wollin at Lamont proposed in the early 1960’s that foraminiferal species assemblages
could be used to reconstruct past changes in Caribbean SSTs. The approach noted that the
~27 different foram species in the world ocean are zoned by temperature and water mass
characteristics. One species, Globorotalia menardii, was known to be most abundant in
warm tropical oceans and thus its relative abundance could be used to qualitatively
indicate temperature changes. (The transfer function method of using the full set of foram
species to calculate SSTs was developed later (also at Lamont) in the late 1960’s by John
Imbrie and Nilva Kipp.)
Ericson and Wollin observed that the relative abundance of G. menardii changed
dramatically downcore for cores collected from the Caribbean basin and in a way which
was consistent, only very roughly as you’ll see, with the isotope results from Emiliani. G.
menardi abundances were higher in the coretops and then decreased to zero at a level
which was radiocarbon dated near the time of the Last Glacial Maximum. Further
downcore, abundances of G. menardii varied periodically into the more distant past,
suggesting earlier glacial and interglacial cycles. They examined many other cores and
noted that these cores showed identical variations, providing a way to correlate between
adjacent cores.
-1-
G. menardii stratigraphy
This lab exercise will have you relive the initial discoveries by Ericson and Wollin.
You’ll apply what you’ve learned in the class so far using a well-studied sediment core
from the central equatorial Atlantic Ocean.
Imagine you have a sediment core in front of you and you have to establish some way to
a) reconstruct past ocean conditions, and b) correlate between sediment cores. Ericson
and Wollin did this by counting the relative abundance of the tropical species G.
menardii downcore.
G. menardii is readily distinguished from other foram species by the following
morphological characteristics:
 medium to quite large in size (400-1000µm)
 5-6 wedge shaped chambers in the final whorl
 circular to sub-circular peripheral outline
 convex on both sides
 prominent peripheral keel
 aperture is a low arch with a large plate-like umbilical tooth
 wall calcareous, densely perforated with irregularly sized and shaped pores, nonspinose
Globorotalia menardii
Similar looking, but not menardii:
 (G. flexuosa) last few chambers flex inward
 (G. tumida) shell shape is more elliptical and is much thicker
 (G. ungulata) rare species, the high umbilical face and thin, shiny test wall
differentiates this species from menardii, tumida and flexuosa
Ericson and Wollin noted that there were distinct intervals where G. menardii were
present-abundant or nearly absent (defined as <1%) in the Caribbean. Based on
radiocarbon dates (only for the last ~40 ka) and the positions of paleomagnetic reversals
-2-
they established a rough chronology. They defined a zonation scheme as follows
(updated with more current age estimates):
Zone Age (ka BP)
description
Z
Y
X
W
abundant G. menardii, warm
G. menardii absent, cool
abundant G. menardii, warm
G. menardii absent, cool
0-9
9-80
80-135
135-160
Lab exercise (work in pairs):
1) Measure relative abundances of G. menardii in core Vema (VM) 30-40. Count
between 100-150 specimens per slide, keeping track of the number of G. menardii
and all “other” forams.
2) Calculate and plot the percentage of G. menardii versus depth, labeling the E&W
zones. Make a table and plot the age-depth curve for this core.
Once you have completed this, send an email with these results to Peter deMenocal
(peter@ldeo.columbia.edu) and he’ll then send you more data to complete the lab
exercise.
3) Use your age model to calculate a timeseries from the planktonic 18O. Assume
that you’re Cesare Emiliani now and calculate how large the SST changes would
be if the entire 18O signal is due to temperature changes. The modern SST for
this site is 25°C and the kinetic 18O fractionation effect for calcite due to
temperature is -0.23 per mil per degree centigrade (that is, calcite 18O increases
with decreasing temperature). Are these changes in tropical ocean SSTs are
reasonable?
4) Now you’ve morphed into Dave Ericson and you’re confronting your former self
Cesare Emiliani. You’ve measured variations in the foram species abundances
and you can only support about 3°C cooling for each of the last glacial cycles; the
interglacial SSTs were about the same as today. How do you reconcile the
planktonic 18O signal data, the isotope-derived SST changes, and the foram
assemblage-derived SST changes? Comment on what you now think about the
relative contributions of SST and global ice volume to the observed planktonic
18O signal.
5) Now plot the 18O timeseries adjacent to the orbital insolation curve calculated
for the summer season at 65°N. Recall that orbital forcing of global climate was
still a very new idea back in the 1960’s. What are your first order observations
about the timing and amplitudes of the 18O changes relative to the orbital
insolation forcing curve. How many ice age cycles were there over the last 150
ka? How would you reconcile the G. menardii % and 18O data?
6) Lastly, you’ve also measured CaCO3% in this core VM30-40. Apply your age
model to this dataset too and plot it next to the 18O and G. menardii percent
records. The core was recovered in a deep basin (3706m) in the central tropical
Atlantic. What do you think the CaCO3% variations are trying to tell you? What
-3-
additional measurements might you make to test your hypotheses and what would
they tell you?
-4-