Millennial-scale variations in sedimentary oxygenation in the western subtropical North Pacific and its links to North Atlantic climate

Abstract

The deep-ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ levels on millennial timescales is poorly constrained. An increase in the respired ${\mathrm{CO}}_{\mathrm{2}}$ concentration in the glacial deep-ocean due to biological pump generally corresponds to deoxygenation in the ocean interior. This link thus offers a chance to study oceanic ventilation and coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here, we investigate a suite of geochemical proxies in a sediment core from the Okinawa Trough to understand sedimentary oxygenation variations in the subtropical North Pacific over the last 50 000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals and after 8.5 ka, while oxygenation decreased during the Bölling-Alleröd (B/A) and Preboreal. The enhanced oxygenation during cold spells is linked to the North Pacific Intermediate Water (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast trade winds over the tropics. The enhanced formation of the NPIW during Heinrich Stadial 1 (HS1) was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in the high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A due to a decreased NPIW formation and enhanced export production, indicates an expansion of the oxygen minimum zone in the North Pacific and enhanced ${\mathrm{CO}}_{\mathrm{2}}$ sequestration at mid-depth waters, along with the termination of atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentration increase. We attribute the millennial-scale changes to an intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation.

1 Introduction

A more sluggish deep-ocean ventilation combined with a more efficient biological pump is widely thought to facilitate enhanced carbon sequestration in the ocean interior, leading to atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ drawdown during glacial cold periods (Sigman and Boyle, 2000). These changes are tightly coupled to bottom water oxygenation and sedimentary redox changes on both millennial and orbital timescales (Hoogakker et al., 2015; Jaccard and Galbraith, 2012; Sigman and Boyle, 2000). Reconstruction of past sedimentary oxygenation is therefore crucial for understanding changes in export productivity and renewal of deep-ocean circulation (Nameroff et al., 2004). Previous studies from the North Pacific margins as well as the open subarctic Pacific have identified drastic variations in export productivity and ocean oxygen levels at millennial and orbital timescales using diverse proxies such as trace elements (Cartapanis et al., 2011; Chang et al., 2014; Jaccard et al., 2009; Zou et al., 2012), benthic foraminiferal assemblages (Ohkushi et al., 2016, 2013; Shibahara et al., 2007) and nitrogen isotopic composition (${\mathit{\delta }}^{\mathrm{15}}\mathrm{N}$) of organic matter (Addison et al., 2012; Chang et al., 2014; Galbraith et al., 2004; Riethdorf et al., 2016) in marine sediment cores. These studies suggested that both the North Pacific Intermediate Water (NPIW) and export of organic matter regulate the sedimentary oxygenation variation during the last glaciation and Holocene in the subarctic Pacific. By contrast, little information exists on millennial-scale oxygenation changes to date in the western subtropical North Pacific (WSTNP).

The modern NPIW precursor waters are mainly sourced from the NW Pacific marginal seas (Shcherbina et al., 2003; Talley, 1993; You et al., 2000), spreading into the subtropical North Pacific at intermediate depths of 300 to 800 m (Talley, 1993). The pathway and circulation of the NPIW have been identified by You (2003), which suggested that cabbeling, a mixing process to form a new water mass with increased density than that of the parent water masses, is the principle mechanism responsible for transforming subpolar source waters into the subtropical NPIW along the subarctic–tropical frontal zone. More specifically, a small subpolar input of about 2 Sv ($\mathrm{1}\mathrm{Sv}={\mathrm{10}}^{\mathrm{6}}$ m${}^{\mathrm{3}}$ s${}^{-\mathrm{1}}$) is sufficient for subtropical ventilation (You et al., 2003). Benthic foraminiferal ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$, a quasi-conservative tracer for water mass, from the North Pacific indicates an enhanced ventilation (higher ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$) at water depths of $<\mathrm{2000}$ m during the last glacial period (Keigwin, 1998; Matsumoto et al., 2002). Furthermore, on the basis of both radiocarbon data and modeling results, Okazaki et al. (2010) suggested the formation of deep water in the North Pacific during the early deglaciation in Heinrich Stadial 1 (HS1). Enhanced NPIW penetration was further explored using numerical model simulations (Chikamoto et al., 2012; Gong et al., 2019; Okazaki et al., 2010). In contrast, substantial effects of an intensified NPIW formation during Marine Isotope Stage (MIS) 2 and 6 on the ventilation and nutrient characteristics of the lower-latitude mid-depth eastern equatorial Pacific have been suggested by recent studies (Max et al., 2017; Rippert et al., 2017). The downstream effects of an intensified NPIW are also reflected in the record of ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ of Cibicides wuellerstorfi in core PN-3 from the middle Okinawa Trough (OT), where lower deglacial ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ values were attributed to enhanced OC accumulation rates due to higher surface productivity by Wahyudi and Minagawa (1997).

The Okinawa Trough is separated from the Philippine Sea by the Ryukyu Islands and is an important channel of the northern extension of the Kuroshio in the WSTNP (Fig. 1). Initially the OT opened at the Middle Miocene (Sibuet et al., 1987) and since then, it has been a depositional center in the East China Sea (ECS), receiving large sediment supplies from nearby rivers (Chang et al., 2009). Surface oceanographic characteristics of the OT over glacial–interglacial cycles are largely influenced by the Kuroshio and ECS Coastal Water (Shi et al., 2014); the latter is related to the strength of the summer East Asian monsoon (EAM) whose source is the western tropical Pacific. Modern physical oceanographic investigations showed that intermediate waters in the OT are mainly derived from horizontal advection and mixing of the NPIW and South China Sea Intermediate Water (Nakamura et al., 2013). These waters intrude into the OT in two ways: (i) the deeper part of the Kuroshio enters the OT through the channel east of Taiwan (sill depth 775 m) and (ii) they enter through the Kerama Gap (sill depth 1100 m). In the northern OT, the subsurface water mainly flows through horizontal advection through the Kerama Gap from the Philippine Sea (Nakamura et al., 2013). Recently, Nishina et al. (2016) found that an overflow through the Kerama Gap controls the modern deepwater ventilation in the southern OT.

Table 1

Locations of different sediment core records and their source references discussed in the text.

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Despite the effects of EAM and the Kuroshio, evidence of geochemical tracers (temperature, salinity, oxygen, nutrients and radiocarbon) collected during the World Ocean Circulation Experiment (WOCE) in the Pacific (transects P24 and P03) favors the presence of low-salinity, nutrient-enriched intermediate and deep waters (Talley, 2007). Dissolved oxygen content is $<\mathrm{100}$ $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ at water depths below 600 m in the OT, along WOCE transects PC03 and PC24 (Talley, 2007). Modern oceanographic observations at the Kerama Gap reveal that upwelling in the OT is associated with the inflow of the NPIW and studies using a box model predicted that overflow through the Kerama Gap is responsible for upwelling (3.8–$\mathrm{7.6}\times {\mathrm{10}}^{-\mathrm{6}}$ m s${}^{-\mathrm{1}}$) (Nakamura et al., 2013; Nishina et al., 2016).

4 Materials and methods

4.1 Chronostratigraphy of core CSH1

A 17.3 m long sediment core CSH1 (31${}^{\circ }$13.7${}^{\prime }$ N, 128${}^{\circ }$43.4${}^{\prime }$ E; water depth: 703 m) was collected from the northern OT, close to the main stream of Tsushima Warm Current (TWC) (Fig. 1b) and within the depth of the NPIW (Fig. 1c) using a piston corer during Xiangyanghong09 Cruise in 1998, carried out by the First Institute of Oceanography, Ministry of Natural Resources of China. This location is enabling us to reconstruct millennial-scale changes in the properties of the TWC and NPIW. Core CSH1 mainly consists of clayey silt and silt with occurrence of plant debris at some depth intervals (Ge et al., 2007) (Fig. 3a). In addition, three layers of volcanic ash were observed at depths of 74–106, 782–794 and 1570–1602 cm. These three intervals can be correlated with well-known ash layers, Kikai-Akahoya (K-Ah; 7.3 ka), Aira-Tanzawa (AT; 29.24 ka) and Aso-4 (roughly around MIS 5a) (Machida, 1999), respectively. The core was split and subsampled at 4 cm interval and then stored in the China Ocean Sample Repository at 4 ${}^{\circ }$C until analysis.

Table 2

Age control points adopted between planktic foraminifera species Globigerinoides ruber ${\mathit{\delta }}^{\mathrm{18}}\mathrm{O}$ of core CSH1 and Chinese stalagmite ${\mathit{\delta }}^{\mathrm{18}}\mathrm{O}$ (Cheng et al., 2016) for tuning the age model between 10 and 60 ka in this study. A linear interpolation was assumed between age control points.

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Both TOC and ${\mathrm{CaCO}}_{\mathrm{3}}$ have been used as proxies for the reconstruction of past export productivity (Cartapanis et al., 2011; Lembke-Jene et al., 2017; Rühlemann et al., 1999). Molar $\mathrm{C}/\mathrm{N}$ ratios of $>\mathrm{10}$ (Fig. 4c) suggest that terrigenous organic sources significantly contribute to the TOC concentration in core CSH1. The TOC content therefore may not be a reliable proxy for the reconstruction of surface water export productivity during times of the LGM and late deglaciation, when maxima in $\mathrm{C}/\mathrm{N}$ ratios co-occur with decoupled trends between ${\mathrm{CaCO}}_{\mathrm{3}}$ and TOC concentrations.

Several lines of evidence support ${\mathrm{CaCO}}_{\mathrm{3}}$ as a reliable productivity proxy, particularly during the last deglaciation. The strong negative correlation coefficient ($r=-\mathrm{0.85}$; $p<\mathrm{0.01}$) between $\mathrm{Al}$ and ${\mathrm{CaCO}}_{\mathrm{3}}$ in sediments throughout core CSH1 confirms the biogenic origin of ${\mathrm{CaCO}}_{\mathrm{3}}$ against terrigenous $\mathrm{Al}$ (Fig. 4f). Generally, terrigenous dilution decreases concentrations of ${\mathrm{CaCO}}_{\mathrm{3}}$. An inconsistent relationship between ${\mathrm{CaCO}}_{\mathrm{3}}$ contents and sedimentation rates indicates a minor effect of dilution on ${\mathrm{CaCO}}_{\mathrm{3}}$. Furthermore, the increasing trend in ${\mathrm{CaCO}}_{\mathrm{3}}$ associated with a high sedimentation rate during the last deglacial interval indicates a substantial increase in export productivity (Fig. 4a and d). The high coherence between the ${\mathrm{CaCO}}_{\mathrm{3}}$ content and alkenone-derived sea surface water (SST) (Shi et al., 2014) indicates a direct control on ${\mathrm{CaCO}}_{\mathrm{3}}$ by SST. Moreover, a detailed comparison between ${\mathrm{CaCO}}_{\mathrm{3}}$ concentrations and the previously published foraminiferal fragmentation ratio (Wu et al., 2004) shows, apart from a small portion within the LGM, no clear covariation between them. These pieces of evidence suggest that ${\mathrm{CaCO}}_{\mathrm{3}}$ changes are driven primarily by variations in carbonate primary production and are not overprinted by secondary processes, such as carbonate dissolution through changes in the lysocline depth and dilution by terrigenous materials. Likewise, a similar deglacial trend in ${\mathrm{CaCO}}_{\mathrm{3}}$ is also observed in core MD01-2404 (Chang et al., 2009), indicating a ubiquitous, not local picture in the OT. All these lines of evidence thus support ${\mathrm{CaCO}}_{\mathrm{3}}$ of core CSH1 as a reliable productivity proxy to a first-order approximation.

5.2 Redox-sensitive elements

Figure 4 shows time series of selected redox-sensitive elements (RSEs) and proxies derived from them. Mn shows higher concentrations during the LGM and HS1 (16–22 ka) and middle–late Holocene but lower concentrations during the last deglacial and Preboreal periods (15.8–9.5 ka, Fig. 4g). Generally, concentrations of excess $\mathrm{Mo}$ and excess $\mathrm{U}$ (Fig. 4j and l) show coherent patterns with those of $\mathrm{Mo}$ and $\mathrm{U}$ (Fig. 4i and k), but both are out of phase with $\mathrm{Mn}$ over the last glacial period (Fig. 4h). Pronounced variations in $\mathrm{U}$ concentration after 8.5 ka are related to the occurrence of discrete volcanic materials. A significant positive $\mathrm{Eu}$ anomaly (Zhu et al., 2015) confirms the occurrence of discrete volcanic materials and its dilution effects on terrigenous components since 7 ka. Occurrence of discrete volcanic material is likely related to an intensified Kuroshio Current during the middle–late Holocene, as supported by higher hydrothermal $\mathrm{Hg}$ concentrations in sediments from the middle OT (Lim et al., 2017). A negative correlation between $\mathrm{Mn}$ and ${\mathrm{Mo}}_{\mathrm{excess}}$ during the last glaciation and the Holocene and the strong positive correlation between them during the LGM and HS1 (Fig. 5a and b) further corroborate the complex geochemical behaviors of $\mathrm{Mn}$ and $\mathrm{Mo}$. A strong positive correlation between ${\mathrm{Mo}}_{\mathrm{excess}}$ and $\mathrm{Mn}$ (Fig. 5b) may be attributed to co-precipitation of $\mathrm{Mo}$ by manganese oxyhydroxide under oxygenated conditions. Here, we thus use the $\mathrm{Mo}/\mathrm{Mn}$ ratio, instead of an excess $\mathrm{Mo}$ concentration to reconstruct variations in sedimentary redox conditions in our study area. Overall, the $\mathrm{Mo}/\mathrm{Mn}$ ratio shows a similar downcore pattern to that of ${\mathrm{Mo}}_{\mathrm{excess}}$ with higher ratios during the last deglaciation but lower ratios during the LGM and HS1. A strong correlation ($r=\mathrm{0.69}$) between $\mathrm{Mo}/\mathrm{Mn}$ ratio and excess $\mathrm{U}$ concentration (excluding Holocene data, due to the contamination with volcanic material; Fig. 5c) further corroborates the integrity of $\mathrm{Mo}/\mathrm{Mn}$ as an indicator of sedimentary oxygenation changes.

6 Discussion

6.1 Constraining paleo-redox conditions in the Okinawa Trough

In general, three different terms, hypoxia, suboxia and anoxia, are widely used to describe the degree of oxygen depletion in the marine environment (Hofmann et al., 2011). Here, we adopt the definition of oxygen thresholds by Bianchi et al. (2012) for oxic ($>\mathrm{120}$ $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ ${\mathrm{O}}_{\mathrm{2}}$), hypoxic ($<\mathrm{60}$–120 $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ ${\mathrm{O}}_{\mathrm{2}}$) and suboxic ($<\mathrm{2}$–10 $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ ${\mathrm{O}}_{\mathrm{2}}$) conditions, whereas anoxia is the absence of measurable oxygen.

Proxies associated with RSEs, such as sedimentary Mo concentration (Lyons et al., 2009; Scott et al., 2008) have been used to constrain the degree of oxygenation in seawater. Algeo and Tribovillard (2009) proposed that open-ocean systems with suboxic waters tend to yield ${\mathrm{U}}_{\mathrm{excess}}$ enrichment relative to ${\mathrm{Mo}}_{\mathrm{excess}}$, resulting in a sediment $(\mathrm{Mo}/\mathrm{U}{)}_{\mathrm{excess}}$ ratio less than that of seawater (7.5–7.9). Under increasingly reducing and occasionally sulfidic conditions, the accumulation of ${\mathrm{Mo}}_{\mathrm{excess}}$ increases relative to that of ${\mathrm{U}}_{\mathrm{excess}}$ leading the $(\mathrm{Mo}/\mathrm{U}{)}_{\mathrm{excess}}$ ratio either being equal to or exceeding that of seawater. Furthermore, Scott and Lyons (2012) suggested a non-euxinic condition with the presence of sulfide in pore waters when $\mathrm{Mo}$ concentrations range from $>\mathrm{2}$ $\mathrm{\mu }\mathrm{g}{\mathrm{g}}^{-\mathrm{1}}$ to $<\mathrm{25}$ $\mathrm{\mu }\mathrm{g}{\mathrm{g}}^{-\mathrm{1}}$. Given that the northern OT is located in an open-ocean setting, we use these two proxies to evaluate the degree of oxygenation in sediments.

Both the bulk Mo concentration (1.2–9.5 $\mathrm{\mu }\mathrm{g}{\mathrm{g}}^{-\mathrm{1}}$) and excess ($\mathrm{Mo}/\mathrm{U}$) ratio (0.2–5.7) in core CSH1 suggest that oxygen-depleted conditions have prevailed in the deep water of the northern OT over the last 50 ka (Fig. 4m). However, increased excess Mo concentrations with higher $\mathrm{Mo}/\mathrm{U}$ ratios during the last termination (18–9 ka) indicate more reducing conditions compared to the Holocene and the last glacial period, though $\mathrm{Mo}$ concentrations were less than 25 $\mathrm{\mu }\mathrm{g}{\mathrm{g}}^{-\mathrm{1}}$, a threshold for euxinic deposition proposed by Scott and Lyons (2012).

The relative abundance of benthic foraminifera species that thrive in different oxygen concentrations has also been widely used to reconstruct variations in bottom water ventilation, such as the enhanced abundance of Bulimina aculeata, Uvigerina peregrina and Chilostomella oolina found under oxygen-depleted conditions in the central and southern OT from 18 to 9.2 ka (Jian et al., 1996; Li et al., 2005). An oxygenated bottom water condition is also indicated by abundant benthic foraminifera species Cibicidoides hyalina and Globocassidulina subglobosa after 9.2 ka (Jian et al., 1996; Li et al., 2005) in cores E017 (1826 m water depth) and 255 (1575 m water depth) and high benthic ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ values (Wahyudi and Minagawa, 1997) in core PN-3 (1058 m water depth) from the middle and southern OT during the postglacial period. The poorly ventilated deep water in the middle and southern OT inferred by benthic foraminiferal assemblages during the last deglaciation correlates with the one in the northern OT referring to our RSEs (Fig. 4). A link thus can be hypothesized between deepwater ventilation and sedimentary oxygenation in the OT. Overall, a combination of our proxy records of RSEs in core CSH1 with other records shows oxygen-rich conditions during the last glaciation and middle and late Holocene (since 8.5 ka) intervals but oxygen-poor conditions during the last deglaciation.

6.2 Causes for sedimentary oxygenation variations

Our observed pattern of RSEs in core CSH1 suggests that drastic changes in sedimentary oxygenation occurred on orbital and millennial timescales over the last glaciation in the OT. In general, four factors can regulate the redox condition in the deep water column and are as follows: (i) ${\mathrm{O}}_{\mathrm{2}}$ solubility, (ii) export productivity and subsequent degradation of organic matter, (iii) vertical mixing and (iv) lateral supply of oxygen through intermediate and deeper water masses (Ivanochko and Pedersen, 2004; Jaccard and Galbraith, 2012). These processes have been invoked in previous studies to explain the deglacial Pacific-wide variations in oxygenation by either one or a combination of these factors (Galbraith and Jaccard, 2015; Moffitt et al., 2015; Praetorius et al., 2015). Our data also suggest drastic variations in sedimentary oxygenation over the last 50 ka. However, the mechanisms responsible for sedimentary oxygenation variations in the basin-wide OT and its connection with ventilation of the open North Pacific remain unclear. In order to place our core results in a wider regional context, we compare our proxy records of sedimentary oxygenation (${\mathrm{U}}_{\mathrm{excess}}$ concentration and $\mathrm{Mo}/\mathrm{Mn}$ ratio) and export productivity (${\mathrm{CaCO}}_{\mathrm{3}}$) (Fig. 6a–c) with the abundance of Pulleniatina obliquiloculata (an indicator of Kuroshio strength) and sea surface temperature in core CSH1 (Shi et al., 2014), the bulk sedimentary nitrogen isotope (an indicator of denitrification) in core MD01-2404 (Kao et al., 2008), benthic foraminifera ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ (a proxy for ventilation) in cores PN-3 and PC23A (Rella et al., 2012; Wahyudi and Minagawa, 1997) and the abundance of benthic foraminifera (an indicator of hypoxia) in core E017 (Li et al., 2005) and ODP Site 1017 (Cannariato and Kennett, 1999) (Fig. 6d–k).

Figure 6

Proxy-related reconstructions of mid-depth sedimentary oxygenation at site CSH1 (this study) compared with oxygenation records from other locations in the North Pacific and published climatic and environmental records from the Okinawa Trough. From top to bottom: (a) ${\mathrm{CaCO}}_{\mathrm{3}}$ concentration, (b) ${\mathrm{U}}_{\mathrm{excess}}$ concentration, (c) $\mathrm{Mo}/\mathrm{Mn}$ ratio, (d) sea surface temperature (SST) (Shi et al., 2014), (e) abundance of P. obliquiloculata in core CSH1 (Shi et al., 2014), (f) bulk sedimentary organic matter ${\mathit{\delta }}^{\mathrm{15}}\mathrm{N}$ in core MD01-2404 (Kao et al., 2008), (g) ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ of epibenthic foraminiferal C. wuellerstorfi in core PN-3 (Wahyudi and Minagawa, 1997), (h) relative abundance of B. aculeata (hypoxia-indicating species) and (i) C. hyalinea (oxygen-rich indicating species) (Li et al., 2005), (j) dysoxic taxa (%) in core ODP 167-1017 in the northeastern Pacific (Cannariato and Kennett, 1999) and (k) ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ of benthic foraminiferal Uvigerina akitaensis in core PC23A in the Bering Sea (Rella et al., 2012). Light gray and dark gray vertical bars are the same as those in Fig. 4.

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6.2.1 Effects of regional ocean temperature on deglacial deoxygenation

Warming ocean temperatures lead to lower oxygen solubility. In the geological past, solubility effects connected to temperature changes in the water column were thought to enhance or even trigger hypoxia (Praetorius et al., 2015). Shi et al. (2014) reported an increase in SST of around 4 ${}^{\circ }$C (from $\sim \mathrm{21}$ to $\sim \mathrm{24.6}$ ${}^{\circ }$C) during the last deglaciation in core CSH1 (Fig. 6d). Based on thermal solubility effects, a hypothetical warming of 1 ${}^{\circ }$C would reduce oxygen concentrations by about 3.5 $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ at water temperatures around 22 ${}^{\circ }$C (Brewer and Peltzer, 2016); therefore a $\sim \mathrm{4}$ ${}^{\circ }$C warming at core CSH1 (Shi et al., 2014) could drive a conservatively estimated drop of $<\mathrm{15}$ $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$ in oxygen concentration, assuming no large salinity changes. However, given the semiquantitative nature of our data of oxygenation changes, which seemingly exceed an amplitude of $>\mathrm{15}$ $\mathrm{\mu }\mathrm{mol}{\mathrm{kg}}^{-\mathrm{1}}$, we suggest that other factors, e.g., local changes in export productivity, regional influences such as vertical mixing due to changes in the Kuroshio Current and far-field effects may have played decisive roles in shaping the oxygenation history of the OT.

6.2.2 Links between deglacial primary productivity and sedimentary deoxygenation

Previous studies have suggested the occurrence of high primary productivity in the entire OT during the last deglacial period (Chang et al., 2009; Jian et al., 1996; Kao et al., 2008; Li et al., 2017; Shao et al., 2016; Wahyudi and Minagawa, 1997). Such an increase in export production was due to favorable conditions for phytoplankton blooms, which were likely induced by warm temperatures and maxima in nutrient availability, the latter being mainly sourced from an increased discharge of the Changjiang River, erosion of material from the ongoing flooding of the shallow continental shelf in the ECS and upwelling of Kuroshio Intermediate Water (Chang et al., 2009; Li et al., 2017; Shao et al., 2016; Wahyudi and Minagawa, 1997). On the basis of sedimentary reactive phosphorus concentration, Li et al. (2017) concluded that export productivity increased during warm episodes but decreased during cold spells on millennial timescales over the last 91 ka in the OT. Gradually increasing concentrations of ${\mathrm{CaCO}}_{\mathrm{3}}$ in core CSH1 during the deglaciation (Fig. 6a) and little changes in foraminiferal fragmentation ratios (Wu et al., 2004), are indicative of high export productivity in the northern OT. Accordingly, our data indicate an increase in export productivity during the last deglaciation, which was previously evidenced by concentrations of reactive phosphorus (Li et al., 2017) and ${\mathrm{CaCO}}_{\mathrm{3}}$ (Chang et al., 2009) from the middle OT and thus was a pervasive synchronous phenomenon in the entire study region at the outermost extension of the ECS.

Similar events of high export productivity have been reported in the entire North Pacific due to the increased nutrient supply, high SST, reduced sea ice cover, etc. (Crusius et al., 2004; Dean et al., 1997; Galbraith et al., 2007; Jaccard and Galbraith, 2012; Kohfeld and Chase, 2011). In most of these cases, increased export productivity was thought to be responsible for oxygen depletion in mid-depth waters, due to exceptionally high oxygen consumption. However, the productivity changes during the deglacial interval, very specifically ${\mathrm{CaCO}}_{\mathrm{3}}$, are not fully consistent with the trends of the excess $\mathrm{U}$ and $\mathrm{Mo}/\mathrm{Mn}$ ratio (Fig. 6b and c). The sedimentary oxygenation thus cannot be determined by export productivity alone.

6.2.3 Effects of the Kuroshio dynamics on sedimentary oxygenation

The Kuroshio Current, one of the main drivers of vertical mixing, has been identified as the key factor in controlling modern deep ventilation in the OT (Kao et al., 2006). However, the flow path of the Kuroshio in the OT during the glacial interval remains a matter of debate. Planktic foraminiferal assemblages in sediment cores from inside and outside the OT indicated that the Kuroshio migrated to the east side of the Ryukyu Islands during the LGM (Ujiié and Ujiié, 1999). Subsequently, Kao et al. (2006) based on modeling results suggested that the Kuroshio still enters the OT, but the volume transport was reduced by 43 % compared to the present-day transport, and the outlet of Kuroshio switches from the Tokara Strait to the Kerama Gap at $-\mathrm{80}$ and $-\mathrm{135}$ m lowered sea level. Combined with sea surface temperature (SST) records and ocean model results, Lee et al. (2013) argued that there was little effect of deglacial sea-level change on the path of the Kuroshio, which still exited the OT from the Tokara Strait during the glacial period. Because the main stream of the Kuroshio Current is at a water depth of $\sim \mathrm{150}$ m, the SST records are insufficient to decipher past changes in the Kuroshio (Ujiié et al., 2016). On the other hand, low abundances of P. obliquiloculata in core CSH1 in the northern OT (Fig. 6e) indicate that the main flow path of the Kuroshio migrated to the east side of the Ryukyu Island (Shi et al., 2014). Such a flow change would have been caused by the proposed block of the Ryukyu–Taiwan land bridge by low sea level (Ujiié and Ujiié, 1999) and an overall reduced Kuroshio intensity (Kao et al., 2006), effectively suppressing the effect of the Kuroshio on deep ventilation in the OT. Our RSEs data show that oxygenated sedimentary conditions were dominant in the northern OT throughout the last glacial period (Fig. 6b and c). The Kuroshio thus likely had a weak or even no effect on the renewal of oxygen to the sedimentary environment during the last glacial period. More recently, a lower hydrothermal total $\mathrm{Hg}$ concentration during 20–9.6 ka, associated with reduced intensity and/or variation in flow path of KC, relative to that of Holocene recorded in core KX12-3 (1423 water depth) (Lim et al., 2017), further validates our inference.

On the other hand, the gradually increased alkenone-derived SST and abundance of P.obliquiloculata (Fig. 6d and e) from 15 ka onwards indicate an intensified Kuroshio Current. At present, mooring and float observations revealed that the KC penetrates to the 1200 m isobath in the East China Sea (Andres et al., 2015). However, the effect of Kuroshio on sedimentary oxygenation was likely very limited during the glacial period and only gradually increasing throughout the last glacial termination. Therefore, while its effect on our observed deglacial variation in oxygenation may provide a slowly changing background condition in vertical mixing effects on the sedimentary oxygenation in the OT, it cannot account for the first-order, rapid-oxygenation changes, including indications of millennial-scale variations, that we observe between 18 and 9 ka.

Better-oxygenated sedimentary conditions since 8.5 ka coincided with an intensified Kuroshio (Li et al., 2005; Shi et al., 2014), as indicated by rapidly increased SST and P. obliquiloculata abundance in core CSH1 (Fig. 6d and e) and C. hyalinea abundance in core E017 (Fig. 6i). Re-entrance of the Kuroshio into the OT (Shi et al., 2014) with rising eustatic sea level likely enhanced the vertical mixing and exchange between bottom and surface waters, ventilating the deep water in the OT. Previous comparative studies based on epibenthic ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ values indicated well-ventilated deep water feeding both the inside of the OT and the outside of the Ryukyu Islands during the Holocene (Kubota et al., 2015; Wahyudi and Minagawa, 1997). In summary, the enhanced sedimentary oxygenation regime observed in the OT during the Holocene is mainly related to the intensified Kuroshio, while the effect of the Kuroshio on OT oxygenation was limited before 15 ka.

6.2.4 Effects of GNPIW on sedimentary oxygenation

Relatively stronger oxygenated Glacial North Pacific Intermediate Water (GNPIW), coined by Matsumoto et al. (2002), has been widely documented in the Bering Sea (Itaki et al., 2012; Kim et al., 2011; Rella et al., 2012), the Okhotsk Sea (Itaki et al., 2008; Okazaki et al., 2014, 2006; Wu et al., 2014), the waters off of east Japan (Shibahara et al., 2007), the eastern North Pacific (Cartapanis et al., 2011; Ohkushi et al., 2013) and the western subarctic Pacific (Keigwin, 1998; Matsumoto et al., 2002). The intensified formation of GNPIW due to additional source region in the Bering Sea was proposed by Ohkushi et al. (2003) and Horikawa et al. (2010). Under such conditions, the invasion of well-ventilated GNPIW into the OT through the Kerama Gap would have replenished the water column oxygen in the OT, although the penetration depth of GNPIW remains under debate (Jaccard and Galbraith, 2013; Max et al., 2014; Okazaki et al., 2010; Rae et al., 2014). Both a gradual decrease in the excess $\mathrm{U}$ concentration and an increase in the $\mathrm{Mo}/\mathrm{Mn}$ ratio during the last glacial period (25–50 ka) validate such an inference, suggesting pronounced effects of an intensified NPIW formation in the OT.

During HS1, a stronger formation of GNPIW was supported by proxy studies and numerical simulations. For example, on the basis of paired benthic–planktic (B–P) ${}^{\mathrm{14}}\mathrm{C}$ data, enhanced penetration of the NPIW into a much deeper water depth during HS1 relative to the Holocene has been revealed in several studies (Max et al., 2014; Okazaki et al., 2010; Sagawa and Ikehara, 2008) and was also simulated by several models (Chikamoto et al., 2012; Gong et al., 2019; Okazaki et al., 2010). On the other hand, increased intermediate-water temperature in the subtropical Pacific recorded in core GH08-2004 (1166 m water depth) (Kubota et al., 2015) and young deep water observed in the northern South China Sea during HS1 (Wan and Jian, 2014) along the downstream region of the NPIW are also related to an intensified NPIW formation. Furthermore, the pathway of GNPIW from numerical model simulations (Zheng et al., 2016) was similar to modern observations (You, 2003). Thus, all of this evidence implies a persistent cause-and-effect relation between GNPIW ventilation, the intermediate and deep water oxygen concentration in the OT, and sediment redox state during HS1. In addition, our RSEs data also suggested a similarly enhanced ventilation in HS2 (Fig. 6b and c) that is also attributed to intensified GNPIW formation.

Hypoxic conditions during the B/A have been also widely observed in the mid- and high-latitude North Pacific (Jaccard and Galbraith, 2012; Praetorius et al., 2015). Our data of the excess $\mathrm{U}$ concentration and $\mathrm{Mo}/\mathrm{Mn}$ ratio recorded in core CSH1 (Fig. 6b and c), together with enhanced denitrification and B. aculeata abundance (Fig. 6f and h), further reveal the expansion of oxygen-depletion at mid-depth waters down to the subtropical NW Pacific during the late deglacial period. Based on high relative abundances of radiolarian species, indicators of upper intermediate-water ventilation in core PC-23A, Itaki et al. (2012) suggested that a presence of well-ventilated waters was limited to the upper intermediate layer (200–500 m) in the Bering Sea during warm periods, such as the B/A and Preboreal. Higher B–P foraminiferal ${}^{\mathrm{14}}\mathrm{C}$ ages, together with increased temperature and salinity at intermediate waters recorded in core GH02-1030 (off east Japan) supported a weakened formation of the NPIW during the B/A (Sagawa and Ikehara, 2008). These lines of evidence indicate that the boundary between GNPIW and North Pacific Deep Water shoaled during the B/A, in comparison to HS1. Based on a comparison of two benthic foraminiferal oxygen and carbon isotope records from off northern Japan and the southern Ryukyu Island, Kubota et al. (2015) found a stronger influence of Pacific Deep Water on intermediate-water temperature and ventilation at their southern compared their northern locations, though both sites are located at similar water depths (1166 and 1212 m for cores GH08-2004 and GH02-1030, respectively). A higher excess $\mathrm{U}$ concentration and low $\mathrm{Mo}/\mathrm{Mn}$ ratio in our core CSH1 during the B/A and Preboreal suggest reduced sedimentary oxygenation, consistent with reduced ventilation of GNPIW, contributing to the subsurface water deoxygenation in the OT.

During the YD, both the $\mathrm{Mo}/\mathrm{Mn}$ ratio and excess $\mathrm{U}$ show a slightly decreased oxygen condition in the northern OT. By contrast, benthic foraminiferal ${\mathit{\delta }}^{\mathrm{18}}\mathrm{O}$ and ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ values in a sediment core collected from the Oyashio region suggested a strengthened formation and ventilation of GNPIW during the YD (Ohkushi et al., 2016). This pattern possibly indicates a time-dependent, varying contribution of distal GNPIW to the deglacial OT oxygenation history, and we presume a more pronounced contribution of organic matter degradation due to high export productivity during this period, as suggested by increasing the ${\mathrm{CaCO}}_{\mathrm{3}}$ content.

6.3 Subtropical North Pacific ventilation links to North Atlantic climate

One of the characteristic climate features in the Northern Hemisphere, in particular the North Atlantic is millennial-scale oscillation during glacial and deglacial periods. These abrupt climatic events have been widely thought to be closely related to the varying strength of the Atlantic Meridional Overturning Circulation (AMOC) (Lynch-Stieglitz, 2017). One of dynamic proxies of ocean circulation, ${}^{\mathrm{231}}\mathrm{Pa}{/}^{\mathrm{230}}\mathrm{Th}$ reveals that severe weakening of the AMOC only existed during Heinrich stadials due to increased freshwater discharges into the North Atlantic (Böhm et al., 2015; McManus et al., 2004). On the other hand, several mechanisms, such as a sudden termination of freshwater input (Liu et al., 2009), an increase in atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentration (Zhang et al., 2017), an enhanced advection of salt (Barker et al., 2010) and changes in background climate (Knorr and Lohmann, 2007), were proposed to explain the reinvigoration of the AMOC during the B/A.

Our RSEs data in the Northern OT and endobenthic ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ in the Bering Sea (Fig. 7a–c) both show a substantial millennial variability in intermediate-water ventilation in the subtropical North Pacific. Notably, enhanced ventilation during HS1 and HS2 and oxygen-poor conditions during the B/A, respectively, correspond to the collapse and resumption of the AMOC (Fig. 7d). Such an out-of-phase millennial-scale pattern is consistent with the results of various modeling simulations (Chikamoto et al., 2012; Menviel et al., 2014; Okazaki et al., 2010; Saenko et al., 2004), although these models had different boundary conditions and causes for the observed effects in GNPIW formation and ventilation ages derived from B–P ${}^{\mathrm{14}}\mathrm{C}$ (Freeman et al., 2015; Max et al., 2014; Okazaki et al., 2012). These lines of evidence confirm a persistent link between the ventilation of the North Pacific and North Atlantic climate (Lohmann et al., 2019). Such links have also been corroborated by proxy data and modeling experiment between the AMOC and East Asian monsoon during the 8.2 ka event (Liu et al., 2013), the Holocene (Wang et al., 2005) and 34–60 ka (Sun et al., 2012). The mechanism linking East Asia with the North Atlantic has been attributed to an atmospheric teleconnection, such as the position and strength of the westerly jet and Mongolia-Siberian High (Porter and Zhisheng, 1995). However, the mechanism behind such an out-of-phase pattern between the ventilation in the subtropical North Pacific and the North Atlantic deep water formation remains unclear.

Figure 7

Proxy records favoring the existence of out-of-phase connections between the subtropical North Pacific and North Atlantic during the last deglaciation and enhanced carbon storage at mid-depth waters. (a) ${\mathrm{U}}_{\mathrm{excess}}$ concentration in core CSH1; (b) $\mathrm{Mo}/\mathrm{Mn}$ ratio in core CSH1; (c) benthic ${\mathit{\delta }}^{\mathrm{13}}\mathrm{C}$ record in core PC-23A in the Bering Sea (Rella et al., 2012); (d) indicator of the strength of Atlantic Meridional Ocean Circulation (${}^{\mathrm{231}}\mathrm{Pa}{/}^{\mathrm{230}}\mathrm{Th}$) (Böhm et al., 2015; McManus et al., 2004); (e) atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentration (Marcott et al., 2014). Light gray and dark gray vertical bars are the same as those in Fig. 4.

[Figure omitted. See PDF]

Increased NPIW formation during HS1 may have been caused by enhanced salinity-driven vertical mixing through higher meridional water mass transport from the subtropical Pacific. Previous studies have proposed that intermediate-water formation in the North Pacific hinged on a basin-wide increase in sea surface salinity driven by changes in strength of the summer EAM and the moisture transport from the Atlantic to the Pacific (Emile-Geay et al., 2003). Several modeling studies found that freshwater forcing in the North Atlantic could cause a widespread surface salinification in the subtropical Pacific Ocean (Menviel et al., 2014; Okazaki et al., 2010; Saenko et al., 2004). This idea has been tested by proxy data (Rodríguez-Sanz et al., 2013; Sagawa and Ikehara, 2008), which indicated a weakened summer EAM and reduced transport of moisture from Atlantic to Pacific through the Isthmus of Panama owing to the southward displacement of the Intertropical Convergence Zone caused by a weakening of the AMOC. Along with this process, as predicted through a general circulation modeling, a strengthened Pacific Meridional Overturning Circulation would have transported more warm and salty subtropical water into the high-latitude North Pacific (Okazaki et al., 2010). In accordance with comprehensive $\mathrm{Mg}/\mathrm{Ca}$ ratio-based salinity reconstructions, however, Riethdorf et al. (2013) found no clear evidence for such higher-salinity patterns in the subarctic northwest Pacific during HS1.

On the other hand, a weakened AMOC would deepen the wintertime Aleutian Low based on modern observation (Okumura et al., 2009), which is closely related to the sea ice formation in the marginal seas of the subarctic Pacific (Cavalieri and Parkinson, 1987). Once stronger Aleutian Low, intense brine rejection due to sea ice expansion, would have enhanced the NPIW formation. Recently modeling-derived evidence confirmed that enhanced sea ice coverage occurred in the southern Okhotsk Sea and off the east Kamchatka Peninsula during HS1 (Gong et al., 2019). In addition, stronger advection of low-salinity water via the Alaskan Stream to the subarctic NW Pacific was probably enhanced during HS1, related to a shift in the Aleutian Low pressure system over the North Pacific, which could also increase sea ice formation, brine rejection and thereafter intermediate-water ventilation (Riethdorf et al., 2013).

During the late deglaciation, ameliorating global climate conditions, such as a warming Northern Hemisphere and a strengthened East Asian summer monsoon, are a result of changes in insolation forcing, greenhouse gases concentrations and variable strengths of the AMOC (Clark et al., 2012; Liu et al., 2009). During the B/A, a decrease in sea ice extent and duration was indicated by combined reconstructions of SST and mixed layer temperatures from the subarctic Pacific (Riethdorf et al., 2013). At that time, the rising eustatic sea level (Spratt and Lisiecki, 2016) would have supported the intrusion of the Alaska Stream into the Bering Sea by deepening and opening glacially closed straits of the Aleutian Islands chain, while reducing the advection of the Alaska Stream to the subarctic Pacific Gyre (Riethdorf et al., 2013). In this scenario, saltier and more stratified surface water conditions would have inhibited brine rejection and subsequent formation and ventilation of the NPIW (Lam et al., 2013), leading to a reorganization of the Pacific water mass, closely coupled to the collapse and resumption modes of the AMOC during these two intervals.

6.4

Increased storage of ${\mathrm{CO}}_{\mathrm{2}}$ at mid-depth water in the North Pacific at the B/A

One of the striking features of RSEs data is the presence of higher $\mathrm{Mo}/\mathrm{Mn}$ ratios and excess $\mathrm{U}$ concentrations across the B/A, supporting an expansion of the oxygen minimum zone in the North Pacific (Galbraith and Jaccard, 2015; Jaccard and Galbraith, 2012; Moffitt et al., 2015) and coinciding with the termination of atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentration rise (Marcott et al., 2014) (Fig. 7e). As described above, it can be related to the upwelling of nutrient- and ${\mathrm{CO}}_{\mathrm{2}}$-rich Pacific Deep Water due to resumption of the AMOC and enhanced export production. Notably, boron isotope data measured on surface-dwelling foraminifera in core MD01-2416 situated in the western subarctic North Pacific did reveal a decrease in near-surface pH and an increase in ${\mathrm{pCO}}_{\mathrm{2}}$ at the onset of B/A (Gray et al., 2018), indicating that the subarctic North Pacific was a source of the relatively high atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ concentration at that time. Here we cannot conclude that the same processes could have occurred in the subtropical North Pacific due to the lack of well-known drivers to draw out of the old carbon from the deep sea into the atmosphere. In combination with published records from the North Pacific (Addison et al., 2012; Cartapanis et al., 2011; Crusius et al., 2004; Galbraith et al., 2007; Lembke-Jene et al., 2017; Shibahara et al., 2007), an expansion of the oxygen-depleted zone during the B/A suggests an increase in respired carbon storage at mid-depth waters of the North Pacific, which likely stalls the rise of atmospheric ${\mathrm{CO}}_{\mathrm{2}}$. Our results support the findings by Galbraith et al. (2007). Given the sizable volume of the North Pacific, potentially, in the past the respired carbon could have been emitted to the atmosphere in stages, which would have brought the planet out of the last ice age (Jaccard and Galbraith, 2018).

7 Conclusions

Our geochemical results of sediment core CSH1 revealed substantial changes in intermediate water redox conditions in the northern Okinawa Trough over the last 50 ka on orbital and millennial timescales. Enhanced sedimentary oxygenation mainly occurred during cold intervals, such as the last glacial period, Heinrich stadials 1 and 2, and during the middle and late Holocene, while diminished sedimentary oxygenation prevailed during the Bölling-Alleröd and Preboreal. The sedimentary oxygenation variability presented here provides key evidence for the substantial impact of ventilation of the NPIW on the sedimentary oxygenation in the subtropical North Pacific and shows an out-of-phase pattern with North Atlantic climate during the last deglaciation. The linkage is attributable to the disruption of the NPIW formation caused by climate changes in the North Atlantic, which are transferred to the North Pacific via atmospheric and oceanic teleconnections. We also suggest an expansion of the oxygen-depleted zone and accumulation of respired carbon at the mid-depth waters from previously reported subarctic locations into the western subtropical North Pacific during the B/A, coinciding with the termination of the atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ rise. A step-wise injection of such respired carbon into the atmosphere would be helpful to maintain high atmospheric ${\mathrm{CO}}_{\mathrm{2}}$ levels during the deglaciation and bring the planet out of the last ice age.

Data availability

All raw data are available to all interested researchers upon request ([email protected]).

Author contributions

JZ and XS conceived the study. AZ performed geochemical analyses of bulk sediments. JZ, XS, SK and XG led the write-up of the paper. All other authors provided comments on the manuscript and contributed to the final version of the paper.

Competing interests

The authors declare that they have no conflict of interest.

Acknowledgements

Jianjun Zou and Xuefa Shi acknowledge financial support from the National Program on Global Change and Air Sea Interaction sponsored by the Ministry of Natural Resources of China, the National Natural Science Foundation of China, the Basic Scientific Fund for National Public Research Institutes of China, International Cooperative Projects in Polar Study of Chinese Arctic and Antarctic Administration and Taishan Scholars Program of Shandong. This study is a contribution to the bilateral Chinese–German cooperation project “Sino-German Pacific-Arctic Ocean Experiment (SIGEPAX)”. Xun Gong, Lester Lembke-Jene, Gerrit Lohmann, and Ralf Tiedemann thank the bilateral Chinese-German Cooperation Project “The North Pacific in Warming Climates (NOPAWAC)”. Lester Lembke-Jene and Ralf Tiedemann acknowledge financial support through the national Helmholtz Association REKLIM Initiative. We would like to thank the anonymous reviewers, who helped to improve the quality of this paper.

Financial support

This research has been supported by the National Program on Global Change and Air-Sea Interaction (grant no. GASI-GEOGE-04), the National Natural Science Foundation of China (grant nos. 41876065, 41476056, 41420104005, 41206059 and U1606401), the Basic Scientific Fund for National Public Research Institutes of China (grant no. 2016Q09), the International Cooperative Projects in Polar Study of Chinese Arctic and Antarctic Administration (grant no. 201613), the Taishan Scholars Program of Shandong (Xuefa Shi), and the Chinese–German cooperation projects (funding through BMBF) SIGEPAX (grant no. 03F0704A) and NOPAWAC (grant no. 03F0785A) and the national Helmholtz Association REKLIM Initiative.

Review statement

This paper was edited by Bjørg Risebrobakken and reviewed by two anonymous referees.