Revised depth scale, splice tie points and chron boundaries of ODP Hole 151-907A and Site 162-907

The magnetic polarity stratigraphy at Site 907 obtained from the shipboard pass-through magnetometer and from discrete samples is readily interpretable back to the onset of the Gilbert Chron (5.89 Ma). From this level to the base of the section at ~14 Ma, the interpretation is corroborated by silicoflagellate datums with predictable correlation to polarity chrons. The resulting magnetostratigraphic interpretation differs from those proposed in the Leg 151 (Hole 907A) and 162 (Holes 907B and 907C) Initial Reports volumes. An important hiatus in the 7-10 Ma interval at Site 907 caused sedimentation to slow or cease for ~2.7 m.y. We have revised the shipboard correlation among the three holes at Site 907, resulting in a new composite section splice and recalculation of composite depths. For Site 985, magnetostratigraphic interpretation is possible down to ~150 meters below seafloor (mbsf) (C3An/C3Ar) at ~6 Ma. There are no useful biostratigraphic datums from Site 985 to support this interpretation; however, the interpretation is supported by the correlation of Sites 985 and 907 using natural gamma data from the shipboard multisensor track. Below ~150 mbsf at Site 985, drilling-related deformation at the onset of extended core barrel drilling precluded magnetostratigraphic interpretation.

Geochemistry of ODP Site 162-907, North Atlantic

A geochemical analysis is conducted on hemipelagic sediments at ODP Leg 162, Site 907, North Atlantic. On the basis of major and minor element concentrations, the sequence is divided into five units. Geochemical data reveal that the sediments originated from two specific source areas, i.e., continental icerafted debris (IRD) and Icelandic basalt. In the upper part (lithological units I and II, 0 to 63.1 meters below sea floor (mbsf)), sediments were derived from continental IRD, whereas in the lower part, sediments (lithological units III, IV, and V, below 63.1 mbsf) comprise mixture of continental IRD and minor supply from the Icelandic basalt. The ratio of TiO2/Al2O3 to SiO2 content and the Th to Ti/Al molar ratio clearly provide a key to discriminate provenances. The change in source area is most likely related to the oceanographic and climatic evolution in the North Atlantic since the middle Miocene. Biogenic fossil-barren intervals (Units II and V) are considered as a consequence of dissolution caused by oceanic circulation. The timing of IRD initiation confers with that of geochemical analysis. Total organic carbon to total nitrogen (C/N) ratio shows a striking variation in the entire core. The C/N ratios exceed 10 below approximately 196 mbsf (lithological unit V) with a gradual downward increase. This suggests that terrigenous organic matters have been supplied from the neighboring continents. The total organic carbon to total sulfur (C/S) ratio also shows such possibility as well as diagenetic changes in Units IV and V. The carbonate-barren intervals presented in Units II and V, and intermittently in Units III and IV are interpreted as a consequence of dissolution effect related with climatic variation and deep-water circulation. Additional low surface productivity was considerable.