Abstract:
A detailed geochemical and sedimentological study was conducted to determine the origin and nature of cyclic color variations in lower Eocene pelagic sediments at ODP Site 690 in the Weddell Sea. From calcium carbonate measurements and X-ray diffraction analysis of closely spaced samples in a 1,5 m interval, it was determined that the sedimentary color cycles are a depositional feature resulting from the gradational transition between nannofossil ooze (light beds) and clay-rich naanofossil ooze (dark beds). Light units were found to contain higher amounts of carbonate related to increased nannofossil production, higher delta (super 18) 0 and lower delta (super 13) C values of the shallow surface dwelling Acarinina soldadoensis, and, of the total clay portion, a higher relative percent of smectite than dark beds. Dark beds were found to contain more clay and of the total clay portion, a higher relative percent of chlorite than lighter beds. The climatic interpretation of the carbonate isotopic values suggests that the carbonate-rich intervals were deposited during cold intervals with low productivity and the clay-rich intervals were deposited during warm intervals with high productivity. The cyclic sediments throughout the core appear to reflect alternating climates. On the basis of spectral analysis of digitized sediment color and magnetostratigraphic dating, the durations of the color cycles in the sediments are approximately 40 k.y., comparable to the obliquity period of the Earth's rotation. This suggests a possible orbital origin for the cycles during which high insolation phases would be responsible for the clay-rich intervals and low insolation phases would be responsible for the carbonate-rich intervals. Although further work is needed to refine climatic interpretations, the strong correlation between the cyclic sediments and the obliquity frequency suggests that orbitally-related changes in insolation are responsible for the significant environmental changes indicated by the climatic proxies in the southern high latitudes during a non-glacial episode.