Description: We present a high-resolution X-ray fluorescence (XRF) core scanner record for the expanded middle Eocene section from Integrated Ocean Drilling Program (IODP) Expedition 302 (ACEX) drilled on the Lomonosov Ridge, central Arctic Ocean. The division of the middle Eocene into two units (subunit 1/6 and unit 2) is seen in the cyclical behavior of the elements as well as the changing interelemental correlations and their relationship to physical property measurements of bulk sediment. Al, Ti, and K strongly correlate throughout the record, while the behavior of Fe, Mn, and Si is more complex. Utilizing sediment geochemistry calibration to ground truth the XRF data, we suggest the middle Eocene Arctic Ocean was predominately euxinic, although periodic oxygenation of bottom waters must have occurred during unit 2 (49.7–45.4 Ma). Initially, the sediments are rich in biogenic silica (unit 2), but there is a pronounced shift to terrigenous dominated sediment accumulation in subunit 1/6. We report changes in the elemental concentrations of these elements and investigate the relationship between Fe content and pyrite. Additionally, we explore the potential change in paleoenvironmental conditions across the unit boundary.

Description: Drilling a complete deep crustal section has been a primary yet elusive goal since the inception of scientific ocean drilling. In situ ocean crustal sections would contribute enormously to our understanding of the formation and subsequent evolution of the ocean crust, in particular the interplay between magmatic, hydrothermal, and tectonic processes. Ocean Drilling Program (ODP) Leg 206 was the first of a multileg project to drill an in situ crustal section that penetrated the gabbroic rocks of the Cocos plate (6°44.2'N, 91°56.1'W), which formed ~15 m.y. ago on the East Pacific Rise during a period of superfast spreading (〉200 mm/yr) (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). During Leg 206, the upper 500 m of basement was cored in Holes 1256C and 1256D with moderate to high recovery rates. The igneous rocks recovered are predominantly thin (10 cm to 3 m) basalt flows separated by chilled margins. There are also several massive flows (〉3 m thick), although their abundance decreases with depth in Hole 1256D, as well as minor pillow basalts, hyaloclastites, and rare dikes. The lavas have been slightly (〈10%) altered by low-temperature hydrothermal fluids, which resulted in pervasive dark gray background alteration and precipitation of saponite, pyrite, silica, celadonite, and calcium carbonate veins. Here we present a geochemical analysis of the CaCO3 recovered from cores. The compositions of ridge flank fluids within superfast spreading crust will be determined from these data, following the approach of Hart et al. (1994, doi:10.1029/93JB02035), Yatabe et al. (2000, doi:10.2973/odp.proc.sr.168.003.2000), and Coggon et al. (2004, doi:10.1016/S0012-821X(03)00697-6).