A negative excursion at ≈14-16 Ma in seawater osmium isotope record; implications for paleoceanographic studies using Fe-Mn crusts

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Author(s): Goto, K. T.; Tejada, M. L. G.; Suzuki, K.
Author Affiliation(s): Primary:
Geological Survey of Japan, Tsukuba, Japan
Japan Agency for Marine-Earth Science and Technololgy, Yokosuka, Japan
Volume Title: AGU 2017 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2017; American Geophysical Union 2017 fall meeting, New Orleans, LA, Dec. 11-15, 2017. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English. 5 refs.
Summary: Osmium isotope stratigraphy is a recently proposed method to determine the depositional age of Fe-Mn crusts [1, 2]. Seawater Os isotope (187Os/188Os) is roughly determined by the balance of riverine Os inputs with radiogenic value (187Os/188Os = ≈1.4), and mantle-derived and extra-terrestrial Os inputs with non-radiogenic value (187Os/188Os = ≈0.12) [3]. Secular variation of global seawater Os isotope (seawater Os isotope curve) has been reconstructed by the analysis of pelagic sediments and exhibits large variations ranging from 0.2 to 1.0 with several negative excursions [3]. Hence, the depositional age of Fe-Mn crusts can be approximately estimated by fitting their Os isotope depth profiles to the seawater Os isotope curve (Osmium isotope stratigraphy). However, this method allows multiple interpretations which are partly due to the lack of high-resolution seawater Os isotope curve [1, 2]. For example, the available seawater Os isotope curve does not exhibit negative anomaly during the Miocene, which contrasts with Os isotope records of Fe-Mn crusts [4]. In the present study, we obtained a high-resolution Os isotope record of Miocene seawater using hemipelagic sediments from IODP Expedition 351 Site U1438. We found a small negative Os isotope anomaly as low as 0.7 from sediments deposited at ≈14-16 Ma. The magnitude of this anomaly is similar to those reported from Fe-Mn crusts. Although the extrapolation of Be-10 ages for Fe-Mn crust indicate a younger age for the anomaly (≈11 Ma) [4], we could not find any discernable isotope anomaly at ≈11 Ma. Our finding is consistent with the timing of major eruption of the Columbia River flood basalts (CFRB) which could provide non-radiogenic Os to seawater at ≈14-16 Ma [5]. Hence, we suggest that the observed isotope anomaly reflect eruption and subsequent weathering of the CFRB. As the similar Os isotope anomaly is commonly found from Fe-Mn crusts, the Os isotope anomaly at ≈14-16 Ma could be used as a key event to constrain depositional ages of Fe-Mn crusts. In the presentation, we will also discuss the significance of our findings to paleoceanographic studies using Fe-Mn crusts. References: [1] Klemm et al., 2005 EPSL; [2] Nielsen et al., 2009 EPSL; [3] Peucker-Ehrenbrink & Ravizza, 2012 The Geologic Time Scale 2012; [4] Klemm et al., 2008 EPSL; [5] Hopper et al., 2002 GSA Bulletin
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Expedition 351; Ferromanganese crusts; IODP Site U1438; International Ocean Discovery Program; Isotope ratios; Isotopes; Izu-Bonin Arc; Metals; North Pacific; Northwest Pacific; Os-188/Os-187; Osmium; Pacific Ocean; Platinum group; Stable isotopes; West Pacific
Coordinates: N272300 N272301 E1341907 E1341905
Record ID: 2018074732
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