Magnetostratigraphic evidence for post-depositional distortion of osmium isotopic records in pelagic clay and its implications for mineral flux estimates

Abstract

Chemical stratigraphy is useful for dating deep-sea sediments, which sometimes lack radiometric or biostratigraphic constraints. Oxic pelagic clay contains Fe–Mn oxyhydroxides that can retain seawater 187Os/188Os values, and its age can be estimated by fitting the isotopic ratios to the seawater 187Os/188Os curve. On the other hand, the stability of Fe–Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original 187Os/188Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of 187Os/188Os ages of pelagic clay has never been tested. Here we report inconsistency between magnetostratigraphic and 187Os/188Os ages in pelagic clay around Minamitorishima Island. In a ~ 5-m-thick interval, previous studies correlated 187Os/188Os data to a brief (< 1 million years) isotopic excursion in the late Eocene. Paleomagnetic measurements revealed at least 12 polarity zones in the interval, indicating a > 2.9–6.9 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the 187Os/188Os chronology leads to unrealistically high values. These results suggest that the low 187Os/188Os signal has diffused from an original thin layer to the current ~ 5-m interval, causing an underestimate of the deposition duration. The preservation of the polarity patterns indicates that a mechanical mixing such as bioturbation cannot be the main process for the diffusion, so diagenetic redistribution of Fe–Mn oxyhydroxides and associated Os may be responsible. The paleomagnetic chronology presented here also demands reconsiderations of the timing, accumulation rate, and origins of the high content of rare-earth elements and yttrium in pelagic clay around Minamitorishima Island.

Secular Variations in Provenance of Sedimentary Components in the Western North Pacific Ocean Constrained by Sr Isotopic Features of Deep‐Sea Sediments

“…Petit‐spot volcanos are also among the closest sources of volcanic material in the area around Minamitorishima Island (e.g., Hirano et al., 2019), although the reported ages of petit‐spot volcanos around Minamitorishima Island are markedly younger (<3 Ma; Hirano et al., 2019) than the onset of Unit II deposition (Nozaki et al., 2019; Ohta et al., 2020; Usui & Yamazaki, 2021). Model 4 considers the mixing of the terrigenous component, BCP, hydrogenous Mn oxides, and petit‐spot basalt (Table 2).…”

“…Here, the additional input of volcanic material to the study area is considered in terms of spatiotemporal changes in the geographical setting. The 1st REY peak and Unit II were deposited between the latest Eocene and the Eocene‐Oligocene boundary, and between ∼34 and ∼15 Ma, respectively (Nozaki et al., 2019; Ohta et al., 2020; Usui & Yamazaki, 2021). During these intervals, the input of volcanic materials from the Izu‐Bonin‐Mariana arc increased.…”

“…Although there are uncertainties owing to possible erosion(s) (Tanaka, Nakamura, Yasukawa, Mimura, Fujinaga, Iijima et al., 2020; Tanaka, Nakamura, Yasukawa, Mimura, Fujinaga, Ohta et al., 2020), it is suggested that Unit I was deposited after 12 Ma, Unit II was deposited between at least ∼15 and ∼34 Ma, and the 1st REY peak was deposited just before the Eocene–Oligocene boundary. On the other hand, Usui and Yamazaki (2021) reported paleomagnetic constraints for the initiation of the 1st REY peak deposition, from 38.615 Ma to 34.999 Ma during the latest Eocene, indicating that further investigations would be required to determine the precise age and sedimentation rate of the REY peak.…”

“…Strontium isotope ratios in pelagic seawater were homogenous at each age, regardless of the location, because the residence time of Sr (>10 6 years) is sufficiently longer than the timescale of ocean circulations (several hundred to 1,500 years; Broecker & Peng, 1982;Frank, 2002;Ravizza & Zachos, 2014). However, considering the depositional ages of Units I and II, and the 1st REY peak (Middle Miocene to Pliocene, Oligocene to Early Miocene, and the latest Eocene to the Eocene-Oligocene boundary, respectively; Nozaki et al, 2019;Ohta et al, 2020;Usui & Yamazaki, 2021), the 87 Sr/ 86 Sr values of the fish teeth could be higher (0.709123 in Unit I, 0.708015-0.708837 in Unit II, and 0.708022-0.708189 in the 1st REY peak) than those in seawater at each plausible time interval of deposition (0.708250-0.709050 during the Middle Miocene to Pliocene, 0.707850 to 0.708250 during the Oligocene to Early Miocene, and 0.707800 to 0.707900 during the latest Eocene to the Eocene-Oligocene boundary; Veizer et al, 1999;McArthur et al, 2012McArthur et al, , 2020. Therefore, it is likely that the fish teeth did not perfectly preserve the marine Sr isotopic values at the time of deposition.…”

Non‐Chained, Non‐Interacting, Stable Single‐Domain Magnetite Octahedra in Deep‐Sea Red Clay: A New Type of Magnetofossil?

“…This may be related to global cooling and increased eolian flux at that time. Such a trend is not fully consistent with a long‐term record of eolian flux from the central Pacific (Janecek & Rea, 1983), which suggests an increase from ∼25 Ma, but this inconsistency may be due to the age uncertainties (Usui & Yamazaki, 2021).…”