Geochemical trends and Milankovitch cycles within sediment from the North Atlatnic Ocean

Abstract

A geochemical study of sediment samples taken from North Atlantic Ocean Drilling Program (ODP) site 980 was undertaken to evaluate possible Milankovitch cycles and their inter-relationships with climatically determined geochemical variations. Wide variations were found in both total organic carbon (TOC) and carbonate carbon throughout the column; TOC varied from about 0.1 to 0.6 %, and the carbonate content from about 8 to 83 %. Possible causes of such extreme fluctuations include time-dependent changes, either in Milankovitch cycles of biogenic oceanic productivity or in the dissolution patterns of carbonate. To evaluate the underlying reasons for these large fluctuations we analyzed the spectral patterns of both organic carbon and carbonate, but with all TOC and CaCO3 data interpolated at every 2.5 kyr and detrended prior to analysis of the spectra. The resulting spectra were evaluated in the upper (2.5-472.5 ka) and lower (475-945 ka) sections of the column, and in the sequence as a whole (2.5-945 ka). The results show a strong 100-ky Milankovitch cycle that is particularly apparent within the upper section (2.5-472 ka). However, the tilt and precession cycles are not always clear; a potential source of error is the lack of age control in the high-frequency range, where there was insufficient sampling. On the other hand, it is possible that variations in % CaCO3 fail to reflect tilt and precession cycles because of some local environmental condition, and further study is necessary to ascertain the precise cause of the 100-ka cycle and its application in paleoceanographic interpretation.