A middle Miocene benthic foraminiferal stable isotope record from extensively recrystallised carbonate sediments of IODP Site U1336 in the Equatorial Pacific

Author(s): Voigt, J.; Hathorne, E. C.; Holbourn, A. E.; Frank, M.
Author Affiliation(s): Primary:
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Other:
Christian-Albrecht-University, Institute of Geosciences, Kiel, Germany
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English. 2 refs.
Summary: The elemental and isotopic composition of foraminifera is widely used for reconstructing oceanic and climatic conditions in the past. However, ancient foraminiferal tests are altered after deposition through replacement of the original biogenic calcite by secondary (inorganic) calcite. Therefore, it is important to quantify changes in the elemental and isotopic composition of recrystallised tests to assess the reliability of proxy data. Here, we present benthic foraminiferal stable isotope data from IODP Site U1336 where the geochemistry of bulk carbonates and associated pore waters suggests extensive recrystallisation resulting from an enhanced thermal gradient. In sediments older than 20.3 Ma the 87Sr/86Sr ratios of bulk carbonates and associated pore waters exhibit lower values than contemporaneous seawater indicating the incorporation of Sr originating from older carbonates recrystallised deeper in the section. Furthermore, the generally lower Sr/Ca ratios of bulk carbonates from Site U1336 also suggest extensive recrystallisation. Despite the extensive recrystallisation at Site U1336, the stable isotope composition (δ13C and δ18O) of benthic foraminifera (C. wuellerstorfi and C. mundulus) from the middle Miocene (13-16 Ma) is in good agreement with existing records (e.g. Holbourn et al. 2007, Tian et al. 2013). The carbon-isotope events of the Monterey Excursion (including CM 3b, CM 4a, CM 5 and CM 6) can clearly be identified. The CM 3b event displays the highest δ13C values with a maximum of 1.78 ppm at 15.61 Ma which is in accordance with values measured from Sites 1237 (Nazca Ridge off Peru) and U1337 (706 km southeast from U1336) of 1.72 and 1.74 ppm, respectively at 15.60 Ma. The Middle Miocene cooling at 13.91-13.84 Ma marks the onset of ice-sheet expansion over Antarctica and the drastic increase in δ18O (0.86 ppm) at Site U1336 during that cooling event (CM 6) is comparable to that at Site 1237 (0.79 ppm) (Holbourn et al. 2007) and U1337 (1.00 ppm) (Tian et al. 2013). During this transition the δ13C of Site U1336 increased by 0.87 ppm which is 0.21 ppm higher than the changes at Sites 1237 and U1337. The temporal resolution for most of our record is 16.7 kyrs so far and is too coarse to discern orbital cyclicity. However, in the part of the record with a higher resolution of 4.4 kyrs eccentricity cycles are visible. The results obtained from these extensively recrystallised sediments indicate that much of the original geochemical signal has been retained and benthic foraminifera can be reliably used for isotope analysis. Holbourn A., Kuhnt W., Schulz M., Flores J.-A., and Andersen N. (2007) Orbitally-paced climate evolution during the middle Miocene "Monterey" carbon-isotope excursion. Earth and Planetary Science Letters 261, 534-550. Tian J., Yang M., Lyle M. W., Wilkens R., and Shackford J. K. (2013) Obliquity and long eccentricity pacing of the Middle Miocene climate transition. Geochem. Geophys. Geosyst. 14, 1740-1755, doi:10.1002/ggge.20108.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Cenozoic; Cores; East Pacific; Expedition 320; Expeditions 320/321; Foraminifera; IODP Site U1336; Integrated Ocean Drilling Program; Invertebrata; Marine sediments; Microfossils; Middle Miocene; Miocene; Neogene; North Pacific; Northeast Pacific; Pacific Ocean; Protista; Sediments; Tertiary
Coordinates: N074204 N074204 W1281515 W1281515
Record ID: 2015083282
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