Cobalt‐based age models of pelagic clay in the South Pacific Gyre

Abstract

Dating pelagic clay can be a challenge due to its slow sedimentation rate, post-depositional alteration, and lack of biogenic deposition. Co-based dating techniques have the potential to create age models in pelagic clay under the assumption that the flux of non-detrital Co to the seafloor is spatially and temporally constant, resulting in the non-detrital Co concentrations being inversely proportional to sedimentation rate. We apply a Co-based method to the pelagic clay sequences from Sites U1365, U1366, U1369, and U1370 drilled during Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre. We distinguished non-detrital Co from detrital Co using multivariate statistical partitioning techniques. We found that the non-detrital flux of Co at Site U1370 is approximately twice as high as that at the other sites, implying that the non-detrital Co flux is not regionally constant. This regional variation reflects the heterogeneous distribution of Co in the water column, as is observed in the present day. We present an improved approach to Co-based age modeling throughout the South Pacific Gyre and determine that the Co-based method can effectively date oxygenated pelagic clay deposited in the distal open ocean, but is less reliable for deposition closer to continents. When extending the method to geologically old sediment, it is important to consider the paleolocation of a given site to ensure these conditions are met.

Environmental rock‐magnetism of Cenozoic red clay in the South Pacific Gyre

“…This indicates that the proportion of biogenic magnetite to terrigenous magnetic minerals increases there. The high k ARM /SIRM ratios of Site U1365 are consistent with the predominance of biogenic magnetite inferred from the prominent central ridge on the FORC diagrams, in particular below about 8.0 mbsf Dunlea et al [2015b] gives younger ages for the changes (supporting information Figure S5). The high k ARM /SIRM ratios of Site U1365 are consistent with the predominance of biogenic magnetite inferred from the prominent central ridge on the FORC diagrams, in particular below about 8.0 mbsf Dunlea et al [2015b] gives younger ages for the changes (supporting information Figure S5).…”

“…Variations of magnetic susceptibility and RREY content at Sites U1365 and 596 agree well on the common age axis (supporting information Figure S1). Co concentration profiles at Sites 596 and U1365 based on their own age models, Zhou and Kyte [1992] and Dunlea et al [2015b], respectively, do not agree with each other (supporting information Figure S2a), although they are expected to match if sedimentary environments of the two sites are similar. We calculated sedimentation rates at Site U1365 below 6 mbsf according to the ages transferred from Site 596.…”

A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific

“…In the absence of large hydrothermal inputs (see Sect. 4.2.5; Swanner et al, 2014), burial of Co-depleted sediments on the margin is needed to balance extremely high Co / Al ratios in Pacific pelagic sediments, which collect Co scavenged from the water column (e.g., Dunlea et al, 2015;Goldberg and Arrhenius, 1958). In contrast to depleted Co along the South American shelf, the Co / Al ratio in shelf sediments from the western margin of the Pacific appears crustal (Table 2).…”

Dust, volcanic ash, and the evolution of the South Pacific Gyre through the Cenozoic

“…Furthermore, slight differences in our own age model determinations can impart variations in the PAAS MAR (e.g., see Figure S1 for the PAAS MAR record using variations in the age model described in Dunlea et al . []).…”