Some questions about Anderson and Prell (1993):

1. What is SPECMAP?

This is a stacked d¹⁸O record that is a proxy for changes in global ice volume. It is made by superimposing records from several sites on top of one another and averaging. This eliminates site-specific anomalies.

The records serves as a chronology because it can be 'tuned' to the orbital components that force changes in ice volume.

2. Why is the benthic isotope record (d¹⁸O) less noisy (smaller amplitude variations) than the planktonic one?

The benthic habitat is nearly free of temperature change, so the only factor affecting d¹⁸O values is the global change in ice volume. Local and seasonal temperature changes can introduce noise to the planktonic record.

3. Why might the cycles of shell concentration be more regular than those of percent Globigerina bulloides abundance?

Shell concentration represent the response of the entire planktonic foraminifer assemblage. As such it represents a combination of the response of several different ecologies to environmental forcing. In contrast, the abundance of G. bulloides is the response based on only one species ecology. The smoothing of the response is conceptually somewhat like the 'stacking' of SPECMAP.

4. How can it be that shell accumulation rates are higher during interglacials when sedimentation rates are much lower?

Terrigenous material inflates glacial sediment accumulation. Aeolian dust from the relatively drier continents (remember: the monsoon is weak) and erosion from the exposed shelves (remember: sea level is lower) dilute shell concentrations during glacials. Also upwelling is reduced and fewer shells are being produced.

5. Why does the nearshore site examined in this study record 100 kyr and 23 kyr cycles while sites further offshore record ~50 kyr cycles?

The movement of the offshort 'jet' causes the offshore sites to experience upwelling and downwelling at somewhat irregular intervals. There is positive Ekman pumping (upwelling) NW of the jet and negative pumping (downwelling) SE of it. This has the effect of "smearing" and obscuring the orbital signal.