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

• Published in: Radiocarbon
• Main Authors: Costa, Kassandra M, McManus, Jerry F, Anderson, Robert F
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2017
• Subjects: General Earth and Planetary Sciences; Archeology; Pacific; Planktonic foraminifera
• Online Access: http://dx.doi.org/10.1017/rdc.2017.91; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033822217000911

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 ( 14 C) dating, and 230 Th 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 δ 18 O 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 14 C 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 14 C and δ 18 O of planktonic species that have dramatically varying abundance, substantial age control is nevertheless provided by δ 18 O measurements on the more consistently abundant benthic foraminifera Uvigerina , thus allowing the construction of a reliable chronology for these cores.