Earth and Atmospheric Sciences, Department of


Date of this Version



Published in Marine Geology 209 (2004) 31–43. Copyright 2004, Elsevier. ISSN: 0025-3227. DOI: 10.1016/j.margeo.2004.06.005. Used by permission.


Upper Paleocene (zone CP8b) cyclic sediment from Ocean Drilling Program (ODP) Site 1051 on Blake Nose, western North Atlantic, alternates from white carbonate-rich to green carbonate-poor in response to precessional forcing. Compositional differences between the two bed types are minor: mineral and nannofossil composition of the beds vary subtly, but grain size of the terrigenous component and biogenic silica content remain constant. Iron content determined by sediment magnetic susceptibility and iron intensity determined by a core-scanning XRF correlates negatively with carbonate content and are higher in green beds. Kaolinite content of green beds is slightly higher as well. Green beds exhibit lower evenness values than white beds for nannofossil assemblages and are more dominated by the species Coccolithus. Dominance by Coccolithus indicates more mesotrophic conditions over Blake Nose during deposition of the green beds, which correlates with slightly higher iron and kaolinite contents.

The absence of change in terrigenous grain size and the absence of any indication of planktonic blooms indicate that there was a minor, most likely eolian, input of iron and kaolinite during deposition of the green beds. The source area was most likely northwest Africa, which supplied iron and kaolinite when source areas deflated under drier conditions. With this scenario, kaolinite is an indicator of drier climate and source area erosion, rather than the warmer and/or wetter conditions under which it forms in soils.

During precessional minima or perihelion summer (the opposite of our current configuration), there was an increased range of seasonal temperatures and an increase in the intensity of summer and winter monsoon circulation. As a consequence, there was more rainfall in northern Africa and drier conditions in the continental interiors at mid-latitudes. Green bed deposition occurred during precessional maxima when the North Atlantic was cooler, northwest Africa drier, and the eolian flux to the western North Atlantic slightly increased.