To the upper left is a photo of the unsplit core when we first opened it onboard the ship. The siliceous, glass-like diatoms were so numerous our shipboard core splitter could not cut through the core. When the hair-like, 1 to 4 millimeter long diatoms become tangled in such large masses, they form into layered mats. This is the first core with a mat of diatoms found in the North Atlantic (thin layers of diatoms are more common). The figure below on the left shows the fibrous diatoms; the middle figure from a paper in the journal Geology(1) is a scanning-electron microscope photomicrograph showing individual mats of the diatoms appearing as thin sheets of paper. To the right is a photo of a section of the split core(2) showing the layers (or laminae) which vary in color and texture. These variances reflect changes in microfossil composition of the layers other than diatoms, or introduction of other material such as ash from volcanic eruptions or debris from melting ice bergs(3,4).
Oxygen isotopes were obtained from the benthic foraminifer Cibicidoides weullistorfi . Abundances of Neogloboquadrina pachyderma (left coiling), were derived by counts of planktonic foraminifera. These records combined with other climate proxies indicate that the 3 meter-thick section of diatoms was deposited during the previous glacial period, known as substage 5e, covering roughly from 115,000 to 125,000 years ago. The climate was similar to our present climate. The large deposit would further indicate substantial geographic stability of ocean fronts in the North Atlantic during this period of time.(1) Papers Referenced: (1) BodŽn, P and Backman, J. (1996) A laminated sediment sequence from the northern North Atlantic Ocean and its climatic record. Geology 24, pp. 507-510. (2) Archive photo of core EW9303-17 (1993) Lamont-Doherty Earth Observatory Deep-Sea Sample Repository Core Log. (3) Bond, G.C., Broecker, W., Lotti, R., and McManus, J., Abrupt color changes in isotope stage 5 in the North Atlantic deep sea cores: Implications for rapid change of climate-driven events, in Kukla, G.J., and Went, E., eds., Start of a glacial: Berlin, Springer, p. 182-205, 1992. (4) Cortijo, E., Yiou,P., Labeyrie, L., and Cremer, M., Sedimentary record of rapid climatic variability in the North Atlantic Ocean during the last glacial cycle. Paleoceanography, v. 10, p. 911-926, 1995.
For more information, contact Rusty Lotti Bond (curator@ldeo.columbia.edu). Comments are welcomed. Last update of this page was January 20, 2001. |
When the deep-sea core was first opened aboard the Research Vessel MAURICE EWING, most of us thought we had cored an old trash dumping site. But how did it get into 3,233 meters (1.75 miles) of water off the coast of Greenland? And more curiously, if it was a modern trash site, how did it become buried by 300 cm (nearly 10 feet) of deep-sea sediment? It didn't take long to figure out that what we had was not a three-meter thick layer of wet newspaper (or worse), but billions of layers of the fibrous, hair-like diatom Thalassiothrix longissima (see below left). The thin laminae may tell a story of interannual environmental and climatic variation.(1)
Diatoms are the largest composition of most of this section of core, but on closer examination of at least one laminae, there is only 45% diatoms (biogenic opal) by weight. Per BodŽn and his coworker Jan Backman from Stockholm University also
found that of this particular layer, there was also 5% calcium carbonate and 0.5% organic carbon. In the carbonate component there were many foraminiferas and calcareous nannofossils. The other 50% of weight probably include microscopic-sized minerals. Another laminae contained an ash layer; dominance of certain nannofossils changed at the interface of two laminae; polar foraminifera appear in another; the dark color of some intervals indicate a glacial origin(3,4). 