SITE 1092

Site 1092 is located on the northern Meteor Rise in the PFZ, ~3° north of the present-day position of the Polar Front. The water depth (1976 m) places the site above the regional carbonate lysocline and CCD and within a mixing zone between upper NADW and CPDW. The impetus for drilling Site 1092 came from the record obtained at ODP Site 704, located only 34 nmi southeast of Site 1092. The goal of Site 1092 was to improve upon Site 704 by recovering a continuous upper Miocene to Pleistocene section.

Three holes were drilled by the APC to depths of 188.5 mbsf (Hole 1092A), 168.9 mbsf (Hole 1092B), and 165.5 mbsf (Hole 1092C) with high recovery in all holes (97%, 86%, and 91%, respectively). A continuous spliced section was constructed to 188.5 mcd. The sediments consist of pale brown-green to pure white nannofossil ooze with varying mixtures of diatomaceous and foraminiferal ooze and mud. One lithologic unit was defined and divided into two subunits at ~54 mcd on the basis of a change from alternating calcareous and diatomaceous ooze above to dominantly nannofossil ooze below. Calcium carbonate is present throughout the sediment column, varying from 16.7 to 94.6 wt% with an average value of 80.2 wt%. Total organic carbon contents vary between 0 and 0.70 wt% with an average value of 0.16 wt%.

The section at Site 1092 ranges in age from the Pleistocene to the early Miocene. Sedimentation rates varied from 10 to 29 m/m.y. during the Pliocene-Pleistocene and between 4 and 38 m/m.y. during the Miocene. The pattern of sedimentation-rate change at Site 1092 is similar to Site 704, except that Site 1092 rates are considerably lower. For example, the Pleistocene section at Site 1092 appears to be compressed by half relative to Site 704. The pattern is also similar to that at Site 1088, including the hiatus in the middle Miocene, except that the lower Pleistocene section is more expanded at Site 1092. Winnowing in the upper and mid Pleistocene sediment is indicated by well-sorted foraminiferal sands and sedimentation rates averaging 10 m/m.y. Several hiatuses punctuate the sediment record at Site 1092: (1) a lower Pliocene hiatus at 65 mcd spans the interval from ~3.8 to 4.6 Ma; (2) one or more hiatuses occur across the Miocene/Pliocene boundary that was tentatively placed between 70 and 75 mcd; and (3) a hiatus at 178 mcd spans the earliest late Miocene to the middle middle Miocene from ~11 to 13 Ma.

The paleomagnetic inclination records are highly discontinuous in the upper 60 mbsf because of drilling disturbance in poorly consolidated nannofossil ooze. Below 70 mbsf, the polarity reversal stratigraphy is well resolved where a particularly good upper Miocene sequence is indicated. Correlation of the polarity sequence at Site 1092 awaits detailed shore-based biostratigraphy.

The redox characteristics of Site 1092 can be characterized as generally oxic or suboxic throughout the section. The major cations (Ca, Mg, and Sr) in interstitial waters vary similarly to those from Sites 1088, 1090, and 704 (Froelich et al., 1991b).

Measurements of physical properties show evidence of distinct cyclicity throughout the record. In the upper 35 mcd, large-amplitude variations in color reflectance mirror alternations between siliceous and carbonate sediments during the Pleistocene, whereas the signal is dampened below in sediments dominated by nannofossil ooze. Lithologic cyclicity in the upper Miocene section of Site 1092 may offer the opportunity to test the new late Miocene time scale derived by Shackleton and Crowhurst (1997) at Site 926 (Leg 154).

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