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

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.

Anomalous > 2000‐Year‐Old Surface Ocean Radiocarbon Age as Evidence for Deglacial Geologic Carbon Release

“…The glacial-interglacial trend in Gulf of California benthic-planktic 14 C ages are therefore consistent with an increased flux of geologic carbon during the last glacial termination, as predicted by several studies (Lund et al, 2016;Lund & Asimow, 2011;Ronge et al, 2016;Tolstoy, 2015). The timing of the benthic-planktic 14 C age reversals suggests an increased flux of 14 C-free geologic carbon into the Gulf of California as early as 25-kyr BP (Figure 4)-predating the rise in atmospheric pCO 2 but within the ≈25to 10-kyr BP time window for increased seafloor hydrothermal (and presumably volcanic) activity on Pacific mid-ocean ridges (Costa et al, 2018;Lund et al, , 2019. The occurrence of the foraminifera 14 C age reversals is also consistent with work, suggesting that "high temperature thermogenic reactions […] released large amounts of carbon" in the northern Gulf of California seafloor between 28-and 7-kyr BP (Geilert et al, 2018) (discussed in more details below).…”

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

“…The cores are closely clustered (within 50 km) and from similar depth range (2,655–2,794 m), such that they can be combined as if multiple iterations of the same paleorecord (e.g., Costa & McManus, ). Age models for the JdFR piston cores are well constrained based on radiocarbon dates, benthic δ 18 O, and stratigraphically tuned density cycles (Costa et al, ). Additionally, two multicores (06MC and 10MC) corresponding to piston cores (09PC and 12TC/PC respectively) are added to cover the Holocene.…”

“…Additionally, two multicores (06MC and 10MC) corresponding to piston cores (09PC and 12TC/PC respectively) are added to cover the Holocene. Age models for these multicores are primarily based on benthic δ 18 O due to bioturbative effects on core top radiocarbon dates (Costa, McManus, & Anderson, ). Sedimentation rates over the past 500 kyr generally range from 1 to 3 cm/kyr, with background pelagic sedimentation rates close to 1 cm/kyr, and they are spatially heterogeneous, likely reflecting sediment remobilization caused by the high relief and stronger bottom currents of the near‐ridge environment (Costa et al, ).…”

230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

“…Over 50 years' (1966–2019) worth of data have been compiled to create the global thorium database ( n = 1,167) presented here (Adkins et al, ; Anderson et al, , Anderson et al, , Anderson et al, , Anderson et al, ; Bausch, ; Böhm et al, ; Bohrmann, ; Borole, ; Bradtmiller et al, , , ; Broecker, ; Broecker et al, ; Brunelle et al, , ; Causse & Hillaire‐Marcel, ; Chase et al, , ; Chong et al, ; Costa, McManus, & Anderson, ; Costa & McManus, ; Crusius et al, ; Dekov, ; Denis et al, ; Dezileau et al, , ; Durand et al, ; Fagel et al, ; Francois et al, , Francois et al, ; Frank, Eisenhauer, Bonn, et al, , Frank, Eisenhauer, Kubik, et al, , ; Fukuda et al, ; Galbraith et al, ; Geibert et al, ; Gherardi et al, , ; Gottschalk et al, ; Hickey, ; Hillaire‐Marcel et al, ; Hoffmann et al, , ; Jaccard et al, , ; Jacobel et al, ; Jonkers et al, ; Kienast et al, ; Ku & Broecker, ; Kumar et al, ; Lam et al, ; Lamy et al, ; Lao et al, ; Lippold et al, , Lippold et al, , Lippold et al, , Lippold et al, ; Loubere et al, ; Loveley et al, ; Lund et al, ; Mangini & Dominik, ; Marcantonio et al, , Marcantonio et al, ...…”

“…Over 50 years ' (1966-2019) worth of data have been compiled to create the global thorium database (n = 1,167) presented here (Adkins et al, 2006;Anderson et al, 2006Bausch, 2018;Böhm et al, 2015;Bohrmann, 2013;Borole, 1993;Bradtmiller et al, 2006Bradtmiller et al, , 2007Bradtmiller et al, , 2009Broecker, 2008;Broecker et al, 1993;Brunelle et al, 2007Brunelle et al, , 2010Causse & Hillaire-Marcel, 1989;Chase et al, 2003Chase et al, , 2014Chong et al, 2016;Costa, McManus, & Anderson, 2017;Crusius et al, 2004;Dekov, 1994;Denis et al, 2009;Dezileau et al, 2000Dezileau et al, , 2004Durand et al, 2017;Fagel et al, 2002;Francois et al, 1990, Francois et al, 1993Frank, Eisenhauer, Bonn, et al, 1995Fukuda et al, 2013;Galbraith et al, 2007;Geibert et al, 2005;Gherardi et al, 2005Gherardi et al, , 2009Gottschalk et al, 2016;Hickey, 2010;Hillaire-Marcel et al, 2017;Hoffmann et al, 2013Hoffmann et al, , 2018Jaccard et al, 2009Jaccard et al, , 2013…”