Site 609

Abstract

SITE 609Core recovery (%): 52 Oldest sediment cored:Sub-bottom depth (m): 190. 49°52.7'N, 24° 14.3 'W, at a water depth of 3884 m. Four holes were drilled at the site, three of which yielded sections of significant length; two cores were taken from the fourth during the process of feeling for bottom. Hole 609 was cored with the hydraulic piston corer (HPC) to a sub-bottom depth of 130.6 m (1.9 Ma) and then cored with the extended core barrel (XCB) to 399.4 m (7.0 Ma). Hole 609B was cored with the variable-length hydraulic piston corer (VLHPC) to 128.4 m (1.8 Ma) and then XCB-cored to 354.7 m (6.0 Ma). Hole 609C was XCBcored from 123.2 (1.75 Ma) to 190.4 m (2.8 Ma). Recovery averaged 75% in Hole 609, 87% in Hole 609B, and 52% in Hole 609C.Contorted layering occurs to a depth of 0.1 to 1.0 m in the tops of most cores, and some are entirely contorted. These disturbances appear to be related to pitching of the ship induced by remotely generated swell. Local weather at the site during operations was relatively calm, in general, and never bad. Paleomagnetic and lithologic tie-lines between holes indicate that we may have a 100% complete composite section to about 130 to 160 m sub-bottom (~2 Ma). Between-core verification of a 100% complete section is made very difficult by the low recovery, the contorted layering, and by complicated coring-related thickening and thinning of the layering from hole to hole (see Ruddiman et al., this volume). All calcareous microfossil zones are well represented, with no hiatuses evident. Preservation of calcareous material is generally very good. The paleomagnetic stratigraphy is exceptional throughout, with very high intensities. The upper 170 m consists of interbedded oozes, marls, and muds of the Pleistocene and upper Pliocene, extending to the top of the Gauss (2.47 Ma) and deposited at a rate of about 69 m/m.y. These are the glacial cycles of the North Atlantic. From 170 (2.47 Ma) to 240 m (3.2 Ma) are upper Pliocene muddy nannofossil oozes deposited at a somewhat faster rate (89 m/m.y.). The muddy component is tentatively attributed to an influx of redeposited fine basaltic-volcanic debris from Maury Channel, the main axis of which lies some 35 km to the east and some 225 m deeper than Site 609. This suggests a possible initiation of downslope deposition in the Maury Channel at about 3.4 Ma, and could be related to the first local glaciation of Iceland. The remainder of the section (240-399.4 m) is lower Pliocene and upper Miocene nannofossil ooze grading to nannofossil chalk; some deformation occurs in the upper Miocene chalks. Deposition rates in this unit are about 32 m/m.y. between 240 and 280 m (3.2-4.5 Ma) and about 56 m/m.y. between 280 and 348 m (4.5-5.7 Ma). No reliable data are available from lower levels. In general, deposition was pelagic, with significant enhancement by local redeposition of contemporaneous sediments as a result of gentle current activity. BACKGROUND AND OBJECTIVESSite 609 is on the upper-middle eastern flank of the Mid-Atlantic Ridge, roughly 450 ...