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Site 1215

Site 1215 is located north of the Molokai Fracture Zone in 5406-m water depth. It is situated on typical abyssal hill topography in an area of thin but continuous sediment cover. Based on magnetic lineations, basement age at this site should be in the youngest part of C26r, or ~58 Ma. This site was the northernmost of the transect drilled on crust of this age during Leg 199. Double APC/XCB coring at Site 1215 recovered a sedimentary section from red clays at the seafloor to hydrothermal sediments immediately above basalt. The 70-m-thick sediment section consists of pelagic red clay (Unit I; ~0–26 mbsf) overlying 42 m of clayey calcareous ooze with chert (Unit-II; ~26–68 mbsf) and a thin basal unit of hydrothermal sediment (Unit III; ~68–70 mbsf) over basalt. The upper and lower units of the section are unzoned biostratigraphically, but calcareous fauna and flora and magnetostratigraphy indicate that Unit II is complete (to the zone/chron level) from the upper Paleocene through lower Eocene section (NP8 through NP12; C25n–C23n or ~56.2–52 Ma). Porosity values increase steadily with depth from ~73% near the seafloor to ~91% at the bottom of the red clay unit (25.8 mbsf) and then decrease steadily to values near 58% at the base of the Unit II carbonates (67.2 mbsf). The one sample analyzed from the lithologic Unit III hydrothermal sediments has a porosity of ~82%. Interstitial pore water profiles from Site 1215 primarily reflect the dissolution of biogenic silica, alteration of underlying basalt, and extremely low levels of labile organic matter available for oxidation.

Red Clay Transition

A light to dark color change occurs in the upper portion of lithologic Unit I (red clay) and has been observed in cores of pelagic clay throughout the north central Pacific Ocean. Light absorption spectroscopy, elemental solid phase chemistry, and both discrete and GRA bulk density data from Site 1215 indicate a downcore transition from illite to smectite between ~5 and 10 mbsf consistent with a change in the source of wind-blown dust from Asia (illite rich) to America (smectite rich) during late Miocene time.

Recovery of Chert with Host Sediments

Recovery of chert nodules together with their host sediments at Site 1215 represents a new achievement in ocean drilling. Despite intervals where chert fragments blocked the core barrel and occasioned "flow-in" and core disturbance, composite data from the MST and color reflectance show that cores from the two holes at Site 1215 provide a continuous overlap to ~50 meters composite depth (mcd).

Composite Lower Eocene Section of Clay–Calcareous Ooze Cycles

The interval between ~30 and ~50 mcd at Site 1215 shows a clear succession of cycles in color reflectance and physical properties data on a decimeter scale that appears modulated in amplitude as well as in thickness. This amplitude modulation, when combined with biostratigraphic time control, is consistent with a climatic forcing related to climatic precession (~19,000- to 23,000-yr period). In the calcium carbonate–bearing sediments, both nannofossil assemblages and planktonic foraminifers are strongly affected by dissolution, but the former provide well-constrained biostratigraphic control. The benthic foraminifers present are characterized by hyaline calcareous tests, are better preserved than the planktonic foraminifers, and are promising for shore-based paleoecological and paleooceanographic studies.

First Paleocene/Eocene Boundary Section from the Central Tropical Pacific Ocean

An interval of dark brown (10YR 2/2) nannofossil clay at ~54.7 mbsf in Hole 1215A (interval 199-1215A-8H-3; 128–148 cm) is interpreted to represent the first P/E boundary section to be recovered from the central tropical Pacific Ocean. Calcareous nannofossil biostratigraphy indicates that the boundary occurs within NP9, and the P/E benthic extinction event is observed to occur between ~54.5 and ~55.5 mbsf.

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