Calcium isotope (δ44/40Ca) composition of Morozovella velascoensis during the Paleocene Eocene thermal maximum ocean acidification event

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Author(s): Kitch, G. D.; Jacobson, A. D.; Hurtgen, M.; Sageman, B. B.; Harper, D. T.; Zachos, J. C.
Author Affiliation(s): Primary:
Northwestern University, Evanston, IL, United States
University of California Santa Cruz, Santa Cruz, CA, United States
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: Ocean acidification (OA) events are transient disruptions to the carbonate chemistry of seawater that involve decreases in pH, [CO32-] and carbonate mineral saturation states (Ω). Numerical modeling studies predict that the Ca isotope (δ44/40Ca) composition of primary marine carbonate should be sensitive to OA1, and recent evidence from the rock record may support this hypothesis2. Boron isotope (δ11B) data for the planktonic foraminifera Morozovella velascoensis indicate that the Paleocene-Eocene Thermal Maximum (PETM; ≈55 Mya) was an interval of pronounced OA3, although the Ca isotope composition of the bulk carbonate record appears to show post-burial diagenetic effects4. To further evaluate the Ca isotope proxy, we used a high-precision (2σSD=±0.04 ppm), double-spike (43Ca-42Ca) TIMS method5 to measure δ44/40Ca values of well-preserved M. velascoensis tests spanning the PETM. M. velascoensis tests (250-355 µm) were picked from samples recovered during ODP Leg 198, Site 1209 on Shatsky Rise in the equatorial Pacific. Five M. velascoensis tests were combined per sample, dissolved, spiked, and analyzed using a Triton TIMS. Repeat dissolutions of ten samples gave δ44/40Ca values within ±0.04 ppm of the original measurements. Method and procedural blanks were negligible. δ44/40Ca values are elevated, even before the negative carbon isotope excursion (CIE) that marks the PETM. When δ11/10B values decrease during the CIE, δ44/40Ca values remain elevated, but then decrease by ≈0.10 ppm as δ11B values return to pre-CIE levels. The apparent inverse correlation between δ44/40Ca and δ11B values suggests that Ca isotope fractionation by M. velascoensis was sensitive to OA. A decrease in pH indicated by lower δ11B values is consistent with higher δ44/40Ca values (decreased fractionation) due to elevated [Ca2+]/[CO32-] ratios and reduced W. The Ca isotope composition of pristine foraminiferal calcite may have potential for reconstructing [CO32-]. The current, preliminary dataset may indicate changes in [CO32-] prior to the CIE. 1. Nielsen et al., 2012. 2. Du Vivier et al., 2015. 3. Penman et al., 2014. 4. Griffith et al., 2015. 5. Lehn et al., 2013.
Year of Publication: 2017
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Acidification; Alkaline earth metals; Ca-44/Ca-40; Calcium; Cenozoic; Isotope ratios; Isotopes; Leg 198; Metals; Morozovella; Morozovella velascoensis; North Pacific; Northwest Pacific; ODP Site 1209; Ocean Drilling Program; Pacific Ocean; Paleocene-Eocene Thermal Maximum; Paleogene; Shatsky Rise; Stable isotopes; Tertiary; West Pacific
Coordinates: N323900 N324000 E1583100 E1583000
Record ID: 2018074806
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