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Radiocarbon and Stable Isotope Evidence for Changes in Sediment Mixing in the North Pacific over the Past 30 kyr

Published online by Cambridge University Press:  16 November 2017

Kassandra M Costa Open the ORCID record for Kassandra M Costa [Opens in a new window] ,
Jerry F McManus  and
Robert F Anderson
Kassandra M Costa*
Affiliation:
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA
Jerry F McManus
Affiliation:
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA
Robert F Anderson
Affiliation:
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA
*
*Corresponding author. Email: [email protected].
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Abstract

Deep-sea sediment mixing by bioturbation is ubiquitous on the seafloor, and it can be an important influence on the fidelity of paleoceanographic records. Bioturbation can be difficult to quantify, especially in the past, but diffusive models based on radioactive tracer profiles have provided a relatively successful approach. However, a singular, constant mixing regime is unlikely to prevail in a region where dynamic oceanographic changes in the bottom water environment are a consequence of paleoclimatic variability. In this study, foraminiferal stable isotopes, radiocarbon (14C) dating, and 230Th fluxes are utilized to understand the sediment mixing history in the easternmost region of the North Pacific. In the uppermost sediment, a 12,000-yr offset between planktonic foraminifera species N. incompta and G. bulloides is observed that coincides with age plateaus at 2000–2500 yr for N. incompta and 15,000–16,000 yr for G. bulloides despite coincident glacial-interglacial shifts in δ18O of benthic species. These age plateaus, particularly for G. bulloides, are a result of changing foraminiferal abundance related to assemblage shifts and carbonate preservation changes since the last glacial period, providing a window into the extent of mixing in the past. The 14C and stable isotope results can be simulated using an iterative model that couples these changes in foraminiferal abundance with variability in mixing depth over time. The best-fit model output suggests that the deepest, or most intense, mixing of the past 30,000 yr (30 kyr) may have occurred during the Holocene. Even though changes in mixing affect the 14C and δ18O of planktonic species that have dramatically varying abundance, substantial age control is nevertheless provided by δ18O measurements on the more consistently abundant benthic foraminifera Uvigerina, thus allowing the construction of a reliable chronology for these cores.

Keywords

bioturbationNorth Pacificradiocarbonstable isotopes
Type
Research Article
Information
Radiocarbon , Volume 60 , Issue 1 , February 2018 , pp. 113 - 135
Copyright
© 2017 by the Arizona Board of Regents on behalf of the University of Arizona 

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