Counting Microfossils and Grains of Ice-rafted Debris

This figure from a paper in the journalScience⁽¹⁾ represents a comparison of the measurements of lithic concentrations and percentages of a microfossil from a deep-sea core, with the oxygen isotope record and age model for an ice core. The deep-sea sediment core, VEMA 23-81⁽¹⁾ was recovered from the North Atlantic and is in the collection archived at Lamont-Doherty; the ice core was taken at Summit, Greenland by the Greenland Ice-core Project (GRIP)⁽²⁾. Core sites are shown in the map below. The graph on the right of the figure is an illustration of the percentage of the foraminifera (a microfossil) Neogloboquadrina pachyderma (left coiling)⁽³⁾ (known affectionately as "N. pachy left") found in the sediment layers of the core greater than 150 ultramicrons in size relative to all foraminifera greater than 150 ultramicrons in size. The graph in the middle of the figure illustrates the number of grains per gram of terrestrial material carried into the ocean by drifting glacial ice (ice-rafted debris)⁽¹⁾. The graph on the left is the isotope record (shown as O¹⁸) of the ice core, the result of work by GRIP on the ice core. "Radiocarbon" ages for the cores are indicated on the far right⁽¹⁾, and on the far left⁽²⁾. The dating indicated on the inner-left is by counting the layers in another ice core taken at the same site at Summit, Greenland by the Greenland Ice Sheet Project Two GISP2)⁽⁴⁾.

Below is an image of a slide of sediment used in the above study. It is full of grains of ice rafted debris from glacial ice that were individually counted to give the percent grains per gram of sediment. There is also an image of the foraminifera "N. pachy left" which was picked and counted from the samples of sediment to produce the percentage of "N. pachy left" greater than 150 ultramicrons in size relative to all foraminifera greater than 150 ultramicrons in size.

The results of the work on the sediment core demonstates that calving of icebergs increased at recurring intervals of 2,000 to 3,000 years in the North Atlantic Ocean. The pacing correlates with warm-cold oscillations in the ice cores. Each cycle of 2,000 to 3,000 years records synchronous discharges of ice from different sources not associated with sea-surface temperatures. The findings suggest that possibly a climate-driven mechanism affecting the atmosphere caused the rapid oscillations in air temperatures above Greenland and in the calving of ice from more than one ice sheet.

An example of ice-rafted debris. There are many different grains of minerals to be counted.

Foraminifera Neogloboquadrina pachyderma (left coiling), the microfossil used for counting and dating in this paper.

Papers Referenced:

(1) Bond, Gerard C. and Lotti, Rusty (1995) Iceberg discharges into the North Atlantic on millennial time scales during the last glaciation. Science 267, pp. 1005-1010.

(2) Dansgaard, W., Johnsen, S., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjšrnsdottir, A.E., Jouzel, J., Bond, G. (1993) Evidence for general instability of past climate from a 250-kyr ice core record: Nature, v. 364, p. 218-220.

(3) Bond, G.C., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J., and Bonani, G. (1993) Correlations between climate records from North Atlantic sediments and Greenland ice: Nature, v. 365, p. 143-147.

(4) Mayewski, P.A., Meeker, L.D., Whitlow, S., Twickler, M.C., Morrison, P., Bloomfield, P., Bond, G., Alley, R., Gow, A., Grootes, P., Meese, D.A., Ram, M., Taylor, K., and Wumkes, W. (1994) Changes in atmospheric circulation and ocean ice cover over the North Atlantic during the last 41 k years: Science, v. 263, p. 1747-1751.

See More Results of Work on Cores

Home Page