The deep-ocean carbon cycle, especially carbon sequestration and outgassing,
is one of the mechanisms to explain variations in atmospheric
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
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 (
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.
Locations of different sediment core records and their source references discussed in the text.
Both surface characteristics and deep ventilation in the OT varied
significantly since the last glaciation. During the last glacial period, the
mainstream of the Kuroshio likely migrated to the east side of the Ryukyu Islands
or also became weaker due to lower sea levels (Shi et al., 2014; Ujiié and Ujiié, 1999; Ujiié et al., 2003) and the hypothetical
emergence of a Ryukyu–Taiwan land bridge (Ujiié and Ujiié,
1999). In a recent study, based on the
Based on benthic foraminiferal assemblages, previous studies have implied a
reduced oxygenation in deep waters of the middle and southern OT during the
last deglacial period (Jian et al., 1996; Li et al., 2005) but a strong
ventilation during the Last Glacial Maximum (LGM) and the Holocene (Jian
et al., 1996; Kao et al., 2005). High sedimentary
The sedimentary redox conditions are governed by the rate of oxygen supply
from the overlying bottom water and the rate of oxygen removal from pore
water (Jaccard et al., 2016), processes that are related to the
supply of oxygen by ocean circulation and organic matter respiration,
respectively. Contrasting geochemical behaviors of redox-sensitive trace
metals (
In general, enrichment of
The elements
In general,
In this study, we investigate a suite of redox-sensitive elements and the
ratio of
Surface hydrographic characteristics of the OT are mainly controlled by the warmer, more saline, oligotrophic Kuroshio water and cooler, less saline, nutrient-rich Changjiang Diluted Water, and the modern flow path of the former is influenced by the bathymetry of the OT (Fig. 1a). The Kuroshio Current originates from the North Equatorial Current and flows into the ECS from the Philippine Sea through the Suao–Yonaguni Depression. In the northern OT, Tsushima Warm Current (TWC), a branch of the Kuroshio, flows into the Sea of Japan through the shallow Tsushima Strait. The volume transport of the Kuroshio varies seasonally due to the influence of the EAM with a maximum of 24 Sv in summer and a minimum of 20 Sv in autumn across the east of Taiwan (Qu and Lukas, 2003).
A lower sea surface salinity (SSS) zone in summer relative to the one in winter in the ECS migrates toward the east of OT, indicating an enhanced impact
of the Changjiang discharge associated with summer EAM (Fig. 2a and b).
An estimated
Spatial distribution of sea surface salinity in the East China Sea.
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
A 17.3 m long sediment core CSH1 (31
Previously, paleoceanographic studies have been conducted and a set of data
has been investigated for core CSH1, including the contents of planktic
foraminifers as well as their carbon (
Notably, the original age model, which used constant radiocarbon reservoir
ages throughout core CSH1 are suitable to reveal orbital-scale Kuroshio
variations (Shi et al., 2014) but insufficient to
investigate millennial-scale climatic events. The occurrence of a higher abundance of
Here, we recalibrated the radiocarbon dates using updated CALIB 7.04
software with Marine 13 calibration dataset (Reimer et al., 2013).
Moreover, on the basis of significant correlation between planktic foraminifera species
Age control points adopted between planktic foraminifera species
Sediment subsamples for geochemical analyses were freeze-dried and ground to a fine powder with an agate mortar and pestle. Based on the age model, 85 subsamples from core CSH1, representing a temporal resolution of about 600 years (every 4 cm interval) were selected for detailed geochemical analyses of major and minor elements, and total carbon (TC), organic carbon (TOC) and nitrogen (TN) contents. The pretreatment of sediment and other analytical methods have been reported elsewhere (Zou et al., 2012).
TC and TN were determined with an elemental analyzer (EA; Vario EL III,
Elementar Analysensysteme GmbH) in the Key Laboratory of Marine Sediment
and Environment Geology, First Institute of Oceanography, Ministry of
Natural Resources of China, Qingdao. Carbonate was removed from sediments by
adding 1 M
About 0.5 g of sediment powder was digested in double-distilled
Excess fraction
In addition, given the different geochemical behaviors of
The content of
Age vs.
Both TOC and
Several lines of evidence support
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
Rapidly decreasing
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
(
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
Both the bulk Mo concentration (1.2–9.5
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
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)
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:
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
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
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
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
On the other hand, the gradually increased alkenone-derived SST and abundance of
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
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
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)
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
During the YD, both the
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,
Our RSEs data in the Northern OT and endobenthic
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.
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
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.
One of the striking features of RSEs data is the presence of higher
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
All raw data are available to all interested researchers upon request (zoujianjun@fio.org.cn).
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.
The authors declare that they have no conflict of interest.
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.
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.
This paper was edited by Bjørg Risebrobakken and reviewed by two anonymous referees.