W4937 Marine Sediments and Stratigraphy Lab

W4937 Marine Sediments and Stratigraphy Lab

Feb. 25, 2005

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 d18O. In 1955, Cesare Emiliani, then Urey’s student, measured d18O variations in a Caribbean core and he found multiple glacial-interglacial d18O 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.

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, non-spinose

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 they established a rough chronology. They defined a zonation scheme as follows (updated with more current age estimates):

Zone Age (ka BP) description

Z 0-9 abundant G. menardii, warm

Y 9-80 G. menardii absent, cool

X 80-135 abundant G. menardii, warm

W 135-160 G. menardii absent, cool

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 () 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 d18O. Assume that you’re Cesare Emiliani now and calculate how large the SST changes would be if the entire d18O signal is due to temperature changes. The modern SST for this site is 25°C and the kinetic d18O fractionation effect for calcite due to temperature is -0.23 per mil per degree centigrade (that is, calcite d18O 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 d18O 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 d18O signal.

5)  Now plot the d18O 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 d18O 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 d18O 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 d18O 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 additional measurements might you make to test your hypotheses and what would they tell you?

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