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The North Pacific’s sensitivity to forcing and feedbacks to background climate are an important, but largely open question in assessments of global climate, both in the modern and geological past. Enhanced knowledge of processes of past climate change is crucial to separate between natural and anthropogenic forcing, and to enhance the reliability of future climate projections. On a spatial scale, this region also comprises major oceanographic patterns including Boundary Current systems e.g., Kuroshio/Oyashio, or the Alaskan Stream and several frontal regions. In addition, complex exchange processes and interactions between the open North Pacific and its marginal seas from low to high latitudes create a spatially heterogenous region, with small-scale mixing and both temporal and spatial variations in the system at atmospheric, and oceanic surface, subsurface and deep levels.
We aim to provide a comprehensive collection of original contributions and syntheses that foster the dynamic and four-dimensional understanding of the evolution of climate and oceanic modes in the North Pacific, including links and teleconnections to low latitudes (e.g. West Pacific Warm Pool) and polar regions, as well as to global ocean circulation and climate patterns.
We welcome contributions across all time scales, from the geological past to present. Results may be based on instrumental or proxy data, as well as climate modelling. The session should advance our process-oriented understanding of the complex role of the North Pacific and its marginals seas in regulating biogeochemical cycles, ocean overturning circulation, and ocean-atmosphere carbon budgets. These past climate scenarios can be used to create a framework for the identification of potential thresholds in the current, warming Earth system.

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Co-organized by OS1
Convener: Xun GongECSECS | Co-conveners: Lester Lembke-JeneECSECS, Gerrit Lohmann, Xuefa Shi
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| Wed, 06 May, 16:15–18:00 (CEST)

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D3497 |
EGU2020-13597
| solicited
| Highlight
Samuel Jaccard

The North Pacific basin has undergone large changes in subsurface oxygenation in the past. In general, oxygen-depleted zones increased volumetrically as climate warmed, with the rate of warming playing a critical role in determining the spatial extent of subsurface deoxygenation. The most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation, an interval referred to as the Bolling/Allerod (B/A), associated with the reinvigoration of the Atlantic Meridional Overturning Circulation (AMOC). At this time, the upper Indo-Pacific ocean was probably less oxygenated than today. The B/A was characterized by substantial changes in intermediate water circulation, combined with efficient removal of oxygen associated with enhanced remineralization of labile organic matter, as export production increased throughout the subarctic North Pacific. The abrupt decrease in oxygenation affected large swaths of the North Pacific, including shelf environments with detrimental consequences for marine ecosystems.
This contribution will review the available paleoceanographic evidence spanning the last 3 million years and distill the salient constraints that can help better predicting the future evolution of North Pacific (de)oxygenation in the context of anthropogenic climate forcing.

D3498 |
EGU2020-7221
Jianjun Zou, Xuefa Shi, Aimei Zhu, Yuan-Pin Chang, Min-Te Chen, Xun Gong, and Lester Lembke-Jene

The deep ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. Here, we investigate a suite of geochemical proxies in a sediment core from the northern and middle Okinawa Trough to understand variations in intermediate-water ventilation of 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 during the last deglaciation and last glaciation, while oxygenation decreased during the Bölling-Alleröd (B/A) and warm interstadials. The enhanced oxygenation during cold spells is linked to the intensified 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 NPIW during Heinrich Stadials was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A and interstadials due to decreased NPIW formation and enhanced export production, indicates an expansion of oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters. We attribute the millennial-scale changes to intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation. Out study extends the millennial-scale links between ventilation in the subtropical North Pacific Ocean and the Atlantic Climate into the last glaciations, highlighting the key roles of Atlantic Meridional Overturning Circulation in regulating the North Pacific environment at millennial timescales. Note: Financial support was provided by the National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-04) and by the National Natural Science Foundation of China (Grant Nos.: 41876065, 41476056, and U1606401).

D3499 |
EGU2020-20325
| Highlight
James Rae, William Gray, Louisa Bradtmiller, Andrea Burke, Holger Gebhardt, Michael Sarnthein, and David Thornalley

The North Pacific has been thought of as a sleeping giant in Earth’s climate system.  Despite being a major reservoir of heat, nutrients, and carbon, the lack of deep water formation in this region today limits the exchange of these properties.  Here, using a variety of new and published sediment core data, alongside Earth system modeling, we provide evidence that the North Pacific giant is in fact a dynamic player in Earth’s climate system, with active PMOC during the LGM and deep water formation during HS1.  We also demonstrate a persistent Atlantic-Pacific seesaw in deep water formation during rapid climate change events, and discuss the impact of these changes on regional climate and global CO2.

D3500 |
EGU2020-236
Xiaopei lin and Cunjie Zhang

The occurrence of deep convection could redistribute ocean heat and materials, and induce robust climate and biogeochemical changes. The convection in the North Pacific is quite shallow now (typically 300-m), but paleo records and model simulations suggest that it might reach 2000-3000 m during stadials in the last deglaciation, such as the Heinrich event 1 (H1: ~17.5-15 ka) and Younger Dryas event (YD: ~12.8-11.5 ka). The deep convection during H1 has been explained by increased North Pacific surface salinity due to evaporation and precipitation changes, but this explanation conflicts with many paleo records for YD. Here we collected published paleo records in the northwest Pacific and carried out simulations for the YD period. We show that due to the weakened Atlantic Meridional Overturning Circulation (MOC) during YD, the oceanic Meridional Heat Transport (MHT) weakened. According to the Bjerknes compensation, the atmospheric MHT strengthened. Because atmospheric MHT mainly occurs through baroclinic eddies in extratropics, storm activities strengthened. The strengthened wintertime storm activities induced more oceanic turbulent heat loss and triggered deep convections in the North Pacific, and further contributed to a seesaw pattern of MOC strengths between the North Pacific and North Atlantic. Our result not only provides a new explanation for the North Pacific deep convection during YD but also suggest that synoptic-scale atmospheric variations are capable of influencing low-frequency paleoclimate changes.

D3501 |
EGU2020-13892
Jinfeng Ma, Pengfei Lin, Fei Chai, Peng Xiu, and Hailong Liu

The phytoplankton and zooplankton biomass as well as nutrients in the southern region of Kuroshio Extension (KE) presents obvious decadal variability. Both local and remote links between biomass and physical properties are investigated by comparing satellite observations and the outputs from a biological-physical coupled model. The Regional Ocean Model System (ROMS) and Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) cover the entire Pacific Ocean. The ROMS-CoSiNE model captures the spatial distribution and decadal variation of the key biological variables including phytoplankton and zooplankton biomass and nutrients in the upper ocean around the KE. The decadal variation in the region is mainly caused by the westward-propagating signals that originate from the central and eastern North Pacific. Specifically, these signals are induced by the decadal oscillation of vertical displacement related to large-scale decadal Pacific modes, such as the North Pacific Gyre Oscillation (NGPO).The evidence obtained here includes not only from surface variables (sea surface height and surface chlorophyll) but also from the variables in the deeper ocean (thermocline, subsurface nutrients, upper 100-m phytoplankton and zooplankton biomass) in the KE region. The signals of the variables in the southern KE region significantly lag that of the NPGO in the central and eastern North Pacific by about 2-4 years. The mixed layer nitrogen budget is conducted to evaluate the contribution of vertical and horizontal advection for the decadal variation of nutrients. 

D3502 |
EGU2020-4536
Juan Chen, Shaofeng Li, Anzhou Cao, and Jinbao Song

Based on the Vector Geometry method, the eddy identification is carried out by using the altimeter data in 2016-2018, and the ocean internal diapycnal mixing diffusivity near the eddy is calculated by using the finescale parameterization method and the Argo data. The influence of the eddy in the North Pacific on the internal mixing of the ocean is analyzed. The results show that the average diffusivity of the study region under the influence of eddies is 6% greater than that without the influence of the eddy. The cyclonic eddy enhances the mixing of 600-1200m depth, and the effect on the mixing at the depth of 600-900m is the maximal and can be up to 18%. The anticyclonic eddy significantly enhances the mixing at the depth of 300-900m, but the effect on the mixing at the depth of 900-1200m is not obvious. As the distance from the center of the eddy increases, the diffusivity outside the eddy slowly decreases and that inside the eddy does not change significantly, the result is consistent with the one of a single case analysis of the region (24°N-36°N and 132°E-152°E) from March to October 2014. Moreover, as the intensity of the eddy increases, the diapycnal mixing is significantly enhanced. The statistical result in the study region shows that the diffusivity value of 90 percent is within the range of 10-5.5-10-4 m2/s.

D3503 |
EGU2020-235
Baolan wu, Xiaopei lin, and Lisan yu

The North Pacific Subtropical Mode Water (mode water hereafter) is a vertically homogeneous thermocline water mass, occupying the entire subtropical Western Pacific Ocean. By transporting mass, heat and nutrients from the surface into the subsurface ocean, it provides memory of climate variability and is a potential source of predictability. Previous studies attributed decadal variability of the mode water mean temperature to the Pacific Decadal Oscillation (PDO). Using available observations and reanalysis data, here we show that decadal to multi-decadal variability of the mode water mean temperature is controlled by the Atlantic Multi-Decadal Variability (AMV) instead. During an AMV positive phase, warm sea surface temperatures (SSTs) in the north Atlantic Ocean weaken the subtropical North Pacific westerlies, and the anomalous easterlies in the subtropical west Pacific drive an anomalous northward Ekman transport of warm water into the mode water formation area. This increases the mode water temperature through subduction, driving variability of the upper-layer ocean heat content and fish catches in the Northwestern Pacific. This mechanism is supported by a long pre-industrial model simulation with multiple AMV cycles and by a Pacemaker model experiment, in which the AMV forcing alone is shown to drive the variability of the mode water. Our finding suggests that the AMV is an important driver for decadal climate and ecosystem variability and provides memory for prediction in the Pacific Ocean.

D3504 |
EGU2020-2291
Jingrui Li and Xuefa Shi

A sedimentary multi-index comprehensive study on a gravity core collected from the central Bay of Bengal (BoB) was presented with an attempt to understand the sedimentary processes and their responses to climate changes since the last glaciation. The sea level is suggested to be responsible for significant distinction of the terrigenous input between the last glaciation and the Holocene period through the depositional center transition in the BoB at the glacial-interglacial scale. The monsoon controlled terrigenous input at precession-related scales since it showed similar patterns with solar radiation and precipitation before 18 ka. Terrigenous input responses to the climate changes in the north Atlantic Ocean during the last deglaciation and early Holocene suggested at millennial scales. The paleoproductivity in the central BoB was at a roughly equivalent level during the last glaciation and the Holocene period, as indicated by the authigenic element accumulation rates. Different terrestrial nutrient inputs and ocean surface stratifications related to the Indian Summer Monsoon (ISM) were suggested to be responsible for this pattern. This study provides a brief understanding of the sedimentary response to the climate and emphasizes the different roles of the sea level and ISM in the central BoB.

D3505 |
EGU2020-3192
Seongjung Kim, Young-Tae Son, and SungHyun Nam

Submesoscale dynamics and ocean-atmosphere exchange process in frontal regions play an important role in regulating ocean overturning circulation and cycles of materials (including carbon) and energy, yet our understanding on the dynamics is limited primarily due to lack of relevant observation. To investigate frontal processes such as symmetric instability (SI) and ageostrophic secondary circulation (ASC), multiple comprehensive hydrographic and current observations were made with marine meteorological measurements across a sharp front of the East Korea Warm Current (EKWC) over spring 2017, summer 2017 and fall 2018. Submesoscale features were identified from the observations, estimating diagnostic variables that are the Ertel’s potential vorticity (fq), balanced Richardson number angle ( ), and Ekman buoyancy flux (EBF). The results with fq < 0 along the front,  corresponding to SI regime, and enhanced EBF along the surface of front support that submesoscale overturning circulation induced by down-front wind is due to the SI and ASC. The ASCs with ageostrophic current estimated using the Omega equations further provide vertical motions in the vicinity of the front. Our results suggest that the western boundary currents like EKWC within the North Pacific marginal sea strongly interact with local wind to impact submesoscale overturning circulation and (re-)distribution of materials via SI and ASC.

D3506 |
EGU2020-6266
Zhi Dong, Xuefa Shi, Jianjun Zou, Yanguang Liu, Ruxi Dou, and Sergey Gorbarenko

The formation of intermediate and deep water plays a key role in regulating climate changes at a variety of time scales through the heat redistribution and carbon cycling. The Japan Sea has unique water-mass characteristics in the North Pacific with its own deep-water formation within the Sea itself called Japan Sea Proper Water (JSPW). Latitudinal ventilation changes in the Japan Sea were reconstructed using radiolarian assemblage from three sediment cores, extending from the southwestern, central to northwestern Japan Sea. Here, we present downcore faunal records spanning the last 25 ka as well as other existing ventilation records in the Japan Sea, and provide reliable evidence to evaluate the potential controlling mechanism that lead to onset and interruption of JSPW ventilation. Taking all together, we argue that radiolarian assemblage records have revealed a distinct basin-scale transition in deep-water conditions from anoxic to oxic during the deglaciation related to changing surface hydrography. However, it should be recognized that there is significant potential for bias in the timing of the ventilation changes among regions. Deep ventilation in the central Japan Sea has been in an interglacial mode during the Bølling/Allerød presumably related to northward volume transport of the Tsushima Warm Current. Moreover, the decrease of JSPW Assemblage at the B/A in southwestern Japan Sea was attributed to higher export productivity, facilitating suboxic deepwater condition through enhanced consumption of oxygen, which was probably caused by coastal upwelling. In contrast, the weakening ventilation of the northwestern Japan Sea during the B/A and YD periods was probably caused by the blocking effect of the sea ice. Note: This study was supported by the National Natural Science Foundation of China (Grant No. 41420104005, U1606401) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-04).

D3507 |
EGU2020-6533
Ruxi Dou, Jianjun Zou, Xuefa Shi, Aimei Zhu, Zhi Dong, Fengdeng Shi, Xinru Xue, and Sergey Gorbarenko

The Sea of Japan is a unique marginal sea in the northwest Pacific Ocean, which is known as "miniature ocean". Constrained by four shallow straits communicating with surroundings seas, it is very sensitive to glacio-eustatic sea level changes. Also, it is located beneath the East Asia Monsoon, which affects the hydrography of surface waters, deep circulations and accumulation of terrigenous materials. The presence of seasonal ice also plays a role in controlling the local distributions of terrigenous materials and deep ventilation in the Sea of Japan. An increasing body of studies revealed pronounced changes in past ocean environment in the Sea of Japan since the late Quaternary. However, it remains elusive for past environment changes in the western Sea of Japan. In this study, we investigate the lithology, rare earth elements and radiogenic isotopes of sediment core LV53-18 retrieved from the western Sea of Japan since the last glaciation.

The contents of coarse fraction of sediment grain size suggest an advance in sea ice cover during the last deglaciation and the early Holocene (15-8 ka) and potential perennial sea ice cover during Heinrich Stadial (HS) 1 and HS2. The variation in sea ice cover is explained by changing strength of East Asian Winter Monsoon (EAWM). On millennial timescales (HS2, HS1 and Younger Dryas), our grainsize data shows a reverse correlation between the EAWM and the East Asian Summer Monsoon (EASM), indicating by Chinese stalagmite δ18O record, and it is ascribed to the slowdown of Atlantic Meridional Overturning Circulation (AMOC). The brine rejection related to sea-ice generation enhances local deep ventilation.

Both the concentration of ∑REEs and positive Eu anomaly (1.2~1.4) reveal a sustained contribution of calcium-rich volcanic materials after 8 ka, which coincides with the onset and intensity of Liman Cold Current during the sea-level highstand. Furthermore, the 87Sr/86Sr values (0.706347 to 0.711713) decrease after 8 ka while εNd (-5.09 to -2.45) are more radiogenic, which further corroborate the presence of volcanic materials. On the basis of a binary mixture of volcanic material and upper crust, we estimated qualitatively the relative contributions of these two end-members. In summary, our study underlines the importance of EAWM in controlling the environment in the western Sea of Japan and reveals increasing volcanic contribution since 8 ka, which is related to the intensity of Liman Cold Current.

Note: This study was supported by the National Natural Science Foundation of China (Grant No. 41420104005, U1606401) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-04).

D3508 |
EGU2020-6677
Jie Chen, Jianjun Zou, Xuefa Shi, Lester Lembke-Jene, Dirk Nürnberg, and Ralf Tiedemann

The Emperor Seamount chain is located in the North Pacific Ocean and beneath the Northern Westerly wind belt. It extends from the subtropical to subarctic North Pacific oceans between 30°N-50°N. Modern observations have shown this region has complex physical oceanic processes, including the Kuroshio Extension, the Oyashio Current, the polar front and the subarctic front. A large amount of dust from the central Asian continent is delivered to this area, which affects the regional marine ecosystem and the global carbon cycle. Due to the lack of sediments from the Emperor Seamount chain, few studies have examined the composition of surface sediments in this ocean realm. On the basis of 50 samples collected during the SO264 Expedition in 2018 using multicorers, we investigate the spatial distributions of sediment grainsize, total organic carbon, CaCO3 and major and minor elements in surface sediments of this ocean realm. Our data show that the detritus sediments mainly consist of siltly sand and clayey silt with more coarse fractions between ~45°N and 48°N, which has strong negative correlations with water depth. The content of CaCO3 varies between 0.04% and 83.67% with higher values at the south of 48°N. The TOC content ranges between 0.07% and 1.36% with lower values (<0.3%) occurring at the north of ~45°N. The concentration of ∑REEs ranges from 31 ppm to 136 ppm with lower values between ~45° N and 48°N. There is significant positive Eu anomaly at all stations, indicating widespread occurrence of volcanic detritus. A significant negative correlation between sediment grainsize and ∑REEs and some lithophile elements, such as Al2O3, Fe2O3, K2O, Th, REEs, etc., indicates a strong effect of sediment grainsize on sediment geochemical composition. A strong negative correlation between Al and CaCO3 suggests contrasting sources, such as terrigenous vs biogenic sources, respectively. Our data confirms the contributions of terrigenous, volcanic and biogenic materials to the bulk sediment with contrasting spatial distribution along the Emperor Seamount Chain.

Note: This study was supported by the National Natural Science Foundation of China (Grant No.41876065, U1606401) and National Program on Global Change and Air-Sea Interaction(GASI-GEOGE-04). 

D3509 |
EGU2020-7703
Sabine Kasten, Jessica Volz, Walter Geibert, Ingrid Stimac, Denise Bethke, Ingrid Dohrmann, Annika Schnakenberg, and Weng-Si Chaoa

The deep water of the North Pacific Ocean is enriched in CO2 and nutrients as a result of organic matter degradation in the water column and surface sediments. Due to its large volume, the deep North Pacific may have played a fundamental role for the postulated glacial carbon sequestration leading to the observed drawdown of atmospheric CO2. As a consequence of increased CO2 levels in the deep glacial ocean, bottom-water oxygen concentrations must have been correspondingly low compared to current oxygenated conditions (e.g., Anderson et al., 2019). Previous studies on sediments from the NW Pacific Ocean have provided evidence that glacial bottom‑water O2 concentrations were significantly lower than today, which have induced suboxic surface sediment redox conditions (Jaccard et al., 2009) and have altered the primary sediment composition and properties of glacial deposits (e.g., magnetic susceptibility) due to diagenetic processes (Korff et al., 2016).

We have investigated seven 10- to 15-m-long sediment cores along a S-N transect at the Emperor Seamount Chain taken during RV SONNE cruise SO264 in order to (1) geochemically characterize the sediments and, (2) reconstruct past sediment redox conditions. The cores were retrieved from water depths between 3.5 and 5.7 km from organic-poor siliciclastic‑carbonaceous sediments in the South to more organic-rich siliciclastic‑siliceous sediments in the North with tephra layers found throughout all cores (Nürnberg et al., 2018).

Mn2+ is released into the pore water at all study sites with increasing Mn2+ concentrations below 20‑30 cm sediment depth. Pore-water Mn2+ reraches up to 190 µM in siliciclastic‑siliceous sediments most likely associated with high rates of dissimilatory Mn(IV) reduction. The solid‑phase composition of a core taken from the Minnetonka Seamount (47°44’N, 168°40’E) at 4 km water depth shows Mn/Al ratios below 0.0003. These ratios are much lower than the average MORB Mn/Al value of 0.013 (Klein, 2004), which further indicates that Mn has been diagenetically lost from these sediments. As pore-water Fe2+ concentrations are below detection limit at the Minnetonka Seamount and the depth distribution of solid-phase Fe/Al is mostly constant with ratios close to the average MORB Mn/Al value of 0.59 (Klein, 2004), Fe has probably not been diagenetically redistributed at this site. Pore‑water Fe2+ concentrations of up to 20 µM are only found at two sites most likely as a result of dissimilatory Fe(III) reduction due to higher fluxes of organic material to the seafloor compared to the other sites.

References

Anderson, R.F., et al., 2019. Deep-sea oxygen depletion and ocean carbon sequestration during the last ice age. Global Biogeochem. Cycles 33, 301-317.

Jaccard, S.L., et al., 2009. Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool. Earth Planet. Sci. Lett. 277, 156‑165.

Klein, E.M., 2004. Geochemistry of the Igneous Oceanic Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, Vol.3. Elsevier, Amsterdam, pp. 433‑463.

Korff, L., et al., 2016. Cyclic magnetite dissolution in Pleistocene sediments of the abyssal northwest Pacific Ocean: evidence for glacial oxygen depletion and carbon trapping. Paleoceanography 31, 600‑624.

Nürnberg, D., 2018. RV SONNE Fahrtbericht /Cruise Report SO264, SONNE-EMPEROR, 30.6. – 24.8.2018.

D3510 |
EGU2020-8181
Qiuling Li, Shuqing Qiao, Xuefa Shi, Limin Hu, Yazhi Bai, Aimei Zhu, and Jingjing Cui

    Grain size, clay minerals and major and trace elements of surface sediment samples collected from the East Siberian Arctic shelf are analyzed. Based on factor analysis and cluster analysis the study area is classified into four provinces, the main sediment sources of each provinces is discussed. The results show: province I covers the coastal estuary of the Kolyma River and the Indigirka River. The sediments are mainly composed of silt and sandy silt, and characterized by highest content of SiO2, TiO2, Zr, Sr and low content of other elements. Illite is dominant which accounting for 70% of the whole clay minerals. This area is strongly influenced by terrestrial sources from the Kolyma River and the Indigirka River. ProvinceⅡis located in the middle of the East Siberian Sea, where the sediments are generally silt and mud. The content of Al2O3, K2O, MnO and Ni are relatively high. Clay minerals composition is similar to Province I, but MnO/TiO2 ratio is higher. The sediments in this area are mainly fine-grained imported by rivers, which are also influenced by sea ice process. As the distance increasing offshore, the content of marine authigenic components begin to increase. Province Ⅲ is located in the northern East Siberian Sea, sediments there are mainly mud. Elements such as Al2O3, K2O, V, Li reach the maximum value in this area. The content of illite is the lowest, semctite and kaolinite reach the maximum (>10%). Fine sediments in this area are probably influenced by Atlantic waters and the Beaufort Gyre. Province Ⅳ is located in Chukchi Sea where the sediments consist of silt and sandy silt. Elements are characterized by higher contents of CaO, P2O5, and the content of Chlorite reach peak (>20%). Sediments in this area are significantly influenced by the Pacific inflow water.

    Note: This study was supported by the Marine S&T Fund of Shandong Province for Qingdao National Laboratory for Marine Science and Technology (Grant No. 2018SDKJ0104-3) , National Natural Science Foundation of China (Grant No. U1606401,41722603) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-03)

D3511 |
EGU2020-8765
Lara Jacobi, Dirk Nürnberg, Weng-si Chao, Ralf Tiedemann, Lester Lembke- Jene, Mariem Saavedra Pellitero, Thomas Frederichs, and Tilo von Dobeneck

The North Pacific plays a key role in shaping the Earth’s climate, yet there still is a lack in understanding the complex interplay of atmosphere and ocean, and their respective circulation patterns reacting to a varying Pleistocene climate. Proxy records established on marine sediment core SO264-28-2, recovered from the Emperor Seamount Chain (Suiko Seamount; ~45°N, close to the Subarctic Front) during R/V SONNE Cruise SO264 in 2018, allow to reconstruct changes of surface and subsurface water masses in order to provide unique insight in spatial and temporal shifts of North Pacific Subarctic vs. Subtropical gyres. According to the preliminary age model based on radiocarbon dating, benthic oxygen isotopes, combined magneto-, tephra- and biostratigraphical approaches, the only 7 m long core covers the last ~1.35 Myr. This core was chosen due to its highly characteristic pattern in magnetic susceptibility and a prominent lithological change from carbonate oozes to more siliciclastic sediment sequences at ~1.2 Ma. Thus, numerous other cores from the study area can be correlated with it suggesting this core as a reference record for the North Pacific.

A continuous and synchronous cooling of both surface and subsurface ocean temperatures since ~1.35 Ma changed rapidly at 1.2 Ma to a continuous warming surface from <4 °C to ~ 8 °C while subsurface temperature remained constant below 4 °C. The long-term diverging temperatures and increasing salinities at both surface and subsurface point to the continuous northward displacement of the Subarctic Front and an increased influence of the North Pacific Tropical Water at Suiko Seamount, with most prominent, millennial-scale, changes of the gyre system and the related Kuroshio Current during interglacials. Around ~430 ka, the influence of warm and saline subtropical surface water masses declines, reflected by a rapid decrease of sea surface temperatures of 4-5 °C and a salinity inversion, whereby the subsurface water mass becomes more saline than the surface water. After ~430 ka, interglacials are very pronounced and with the prominent presence of low saline and cooler surface waters, conditions are similar to present.

D3512 |
EGU2020-9416
Zhengquan Yao, Xuefa Shi, Yanguang Liu, and Shuqing Qiao

Sediment accumulation in the continental margin is largely influenced by both sea-level fluctuations and climate changes during the Quaternary Period. However, the response of sediment accumulation to these changes at orbital timescale, remains poorly understood, mainly due to (i) the scarce of sedimentary records with high-resolution chronology and (ii) the difficulty of distinguishing the role of sea-level from climate signals. Here we present sediment color reflectance (c*), grain size and total organic carbon (TOC) data of core BH08 (212.4 m; ~1 Myr) recovered from the Bohai Sea, China. The chronology of core BH08 was constrained at orbital timescale by using magnetostratigraphy and astronomical tuning methods. Sedimentary facies analysis suggests that the core sequence is dominated by alternations of deltaic system and floodplain deposits. Principal components analysis on grain size data reveals two principal components (PCs), including PC1 (31–500 µm, coarse fraction) and PC2 (18–66 µm, fine fraction). Comparison of PC1, PC2, c* and TOC with sedimentary environments, we found that PC1 and c* corresponds well with cycles of deltaic and floodplain deposits at ~100/40-kyr cycles, while PC2 and TOC display ~20-kyr cycle, in addition to the ~100/40-kyr cycles. We interpret that PC1 and c* are mainly sea-level dependent, whereas PC2 and TOC are controlled by a combination of monsoonal climate and sea level. We suggest that Milankovitch-scale monsoon climate controlled the sediments supply to the Bohai Sea during the last 1 Myr, while the redistribution of sediments by marine process (e.g. tidal currents) seem to have obscured the monsoonal signal in the grain size proxy (e.g. PC1) which is sensitive to sea-level change. Our results provide an example of climate and sea-level influenced sediment accumulation in the shallow continental margin influenced by monsoonal climate in an icehouse world.

D3513 |
EGU2020-10522
| Highlight
Weng-Si Chao, Lara Jacobi, Lester Lembke-Jene, Ralf Tiedemann, and Dirk Nürnberg

At present, the North Pacific constitutes one of the main marine natural carbon sinks and thus helps regulate atmospheric COconcentrations. Understanding past changes in North Pacific deep water circulation and biological productivity are of particular importance, since the region likely changed these characteristics on both orbital and millennial time scales, and may have even undergone switches between being a carbon source and sink. We present a suite of new sediment records retrieved from the subarctic Northwest Pacific along the Emperor Seamount Chain in order to contribute to the Pleistocene stratigraphy and reconstruct changes in the physical and biological carbon pump on millennial to orbital timescales. We used high-resolution AMS 14C-derived benthic-planktic (B-P) foraminiferal ventilation ages, and stable carbon and oxygen isotopes of epibenthic foraminifera along both meridional and water depth transects in order to establish deep water ventilation patterns and reconstruct nutrient concentrations over the last 200 ka. We used X-ray fluorescence (XRF)-scanning records combined with radiocarbon dating to correlate prominent patterns between sediment cores, and to develop a stratigraphic framework for the study area. We used changes in Ba/Ti, Ca/Ti, Si/Ti ratios to assess variationsin biological productivity. Biogenic Barium (Ba/Ti) and Calcium (Ca/Ti) ratios generally show high values during interglacials and low values during glacials. This pattern resembles subpolar Northwest Pacific ODP Site 882, which shows a good correlation to the global COrecord. These results provide evidence for the close link between global climate, the ocean carbon cycle and marine biogeochemistry in North Pacific.

D3514 |
EGU2020-12806
Xuefa Shi, Jianjun Zou, and Sergey Gorbarenko

The Japan Sea, one of the marginal seas of the North Pacific, communicates with adjacent seas through four shallow straits (<130 m) and the present environment in the Japan Sea is mainly forced by the Tsushima Warm Current (TWC), East Asia Monsoon (EAM) and seasonal sea ice. During the Quaternary, the pronounced effects of glacial eustatic sea level on the hydrography, ocean biogeochemistry and sediment depositions in the Japan Sea over glacial-interglacial cycles. However, the spatial heterogeneity of these forcings exerting on environment of the Japan Sea may results in contrasting response. On the basis of a suite of sediment cores collected during the China-Russia joint expedition in 2010, we investigate the sedimentary processes and paleoenvironment changes in the Japan Sea. We found enhanced extent of seasonal sea-ice coverage in the western Japan Sea, which is synchronous with the intensification of East Asian Winter Monsoon (EAWM) from 15ka to 8ka. During the early last deglaciation (17ka-15ka), perennial sea ice cover at investigated site occurs and thus inhibits the deepwater formation in the Japan Sea. Since 8 ka, increased deep ventilation and dampened sea ice coverage are closely related to enhanced EAWM and invasion of high-salinity TWC into the Japan Sea. In the southern Japan Sea, the sediment provenance is mainly derived from the Yangtze and old yellow rivers, while the terrigenous matter was mainly sourced from the Yangtze River after 7 ka, on the basis of elemental and radiogenic isotopic data (Sr and Nd) of fine-sized (<63 μm) sediments. Abrupt shifts in sediment provenance occurred at ~18 ka and ~7 ka and these time periods are synchronous with changes in surface hydrography and deep ventilation in the Ulleung Basin. In the central Japan Sea, eolian dust sourced from central Asia and Chinese Loess Plateau by westerly was delivered to the central Japan Sea. In addition, deep ventilation in the southern and central Japan Sea evidenced by redox-sensitive elements and ventilation-like radiolarian species suggest intensified ventilation since 8ka and during cold spells of the last deglaciation, which is closely related to the invasion of the Tsushima Warm Current into the Japan Sea. Our data suggest that sea level is a first-order factor in controlling the environment and sediment deposition in the Japan Sea at orbital timescales, while the East Asian Monsoon and Kuroshio Current play a secondary role. Note: This study was supported by the National Natural Science Foundation of China (Grants No. 41420104005, U1606401) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-03 &-04). 

D3515 |
EGU2020-12967
Yonghua Wu, Xuefa Shi, Jianjun Zou, Xun Gong, Yanguang Liu, Gerrit Lohmann, Lester Lembke-Jene, and Sergey Gorbarenko

We measure and analyse the alkenone and tetraether lipid records over the past 25 ka, from a sediment core recovered from the central Japan/East Sea. In our results, UK′37- and TEX86- derived temperatures commonly represent warm signals during the period of 25-16.2 ka BP, indicating fresher and thus a stratified surface ocean. In comparison, the UK′37- and TEX86- derived temperatures become diverged abruptly after 16.2 ka BP, suggesting a thermal gradient between surface and subsurface water. In addition, the isoprenoidal glycerol dialkyl glycerol tetraethers (GDGT) community structure index, GDGT-[2]/[3] ratio is high during the period of 25-16.2 ka BP and drops sharply along with the TEX86- derived temperatures at 16.2 ka BP, which is likely attributed to changes in the depth of GDGT export and/or in archaeal community structure. Specifically, the high GDGT-[2]/[3] ratio (larger than 8) can be related to strongly stratified surface Japan/East Sea. Moreover, the UK′37- and TEX86- derived temperatures start converging at 5.8 ka BP, representing the impact of the Tsushima Warm Current until nowadays. Note: This study was supported by the National Natural Science Foundation of China (Grants No. 41420104005, U1606401) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-03 &-04).

D3516 |
EGU2020-20927
Yu Yonggui, Shi Xuefa, Wu Bin, and Qiao Shuqing

Chinese Huanghe (Yellow River) provides an extreme case of human controlled large river system. Since 2002, a unique Water-Sediment Regulation (WSR) regime was implemented annually through Xiaolangdi Dam to buffer pool infilling and scour the hanging riverbed. This involves transfers of large-volume of water and sediment between reservoirs, becoming a human-made flooding event. 37 surface sediments 8 box cores sampled during the 2018 WSR were analyzed for grain-size, C/N, 13C, radionuclides, etc. Satellite images together with high-resolution bathymetric data were incorporated to depict the flooding sedimentation at the river mouth. The results show that 7Be and 210Pb activity is pretty low, implying its incapability of tracing flood sediments at the Huanghe River mouth. The results also uncover that a majority of the sediment was deposited in the vicinity of the river mouth where the water depth is less than 15 m. Two depocenters with a maximum thickness of 9 m were formed expanding within a very restricted area, which was largely controlled by tidal currents.

D3517 |
EGU2020-21680
William Dow, Amanda Maycock, and Marcus Lofverstrom

There is an incomplete understanding of the mechanisms that govern the Pacific Decadal Oscillation (PDO), a major mode of climate variability that plays a key role in the evolution of global climate on decadal time-scales. Recent research has suggested that regional anthropogenic aerosol (AA) emissions could modulate the behaviour of the PDO, including the transition to a negative PDO phase starting in the late 1990s (Smith et al., 2016). However, other studies have questioned whether this connection is robust (Oudar et al., 2018). East Asia is a region of particular focus, where AA emissions having increased in recent decades (Bartlett et al., 2017). Here we combine analysis of an ensemble of coupled climate models running idealised AA perturbation experiments and a steady-state primitive equation model (LUMA) forced by diabatic heating anomalies to examine whether AA emissions influence the behaviour of the Aleutian low - a climate feature closely associated with the PDO  - and if so, test the posited teleconnection mechanisms proposed by Smith et al. (2016). We further compare the response of the Aleutian low to well mixed greenhouse gases to examine if AAs and GHGs influence the Aleutian low in a similar manner.

D3518 |
EGU2020-22599
Xun Gong, Lars Ackermann, and Gerrit Lohmann

North Pacific Intermediate water (NPIW) is a dominant water mass controlling ~400-1200m depth North Pacific Ocean, characterized by its low salinities and relatively lower temperatures. In the modern climate, the interplay between NPIW-related physical and biogeochemical processes among seasons determines annual-mean budget and efficiency of carbon sink into the North Pacific Ocean. Thus, to understand the NPIW physics is key to project roles of the North Pacific Ocean in changing Earth climate and carbon systems in the future. In this study, we provide a modelling view of the NPIW history since Yr 1850 (historical experiment) and its projection to near future (IPCC-defined RCP 4.2 and 8.5 experiments until Yr 2100), using new-generation Alfred Wegener Institute Earth System Model (AWI-ESM). Our results suggest an important role of regional hydroclimate feedback over the NW Pacific and Sea of Okhotsk in determining the NPIW from recent past to near future.

D3519 |
EGU2020-22629
Lester Lembke-Jene, Ralf Tiedemann, Dirk Nürnberg, Xun Gong, Jianjun Zou, Weng-si (JC) Chao, Xuefa Shi, and Gerrit Lohmann

The North Pacific hosts the both one of the largest oceanic reservoirs of sequestered carbon and extensive oxygen minimum zones in the world ocean, which will likely intensify and expand under future climate warming scenarios, yielding significant consequences for ecosystems, biogeochemical cycles, and living resources. At present, relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development, but on instrumental time scales, data the past decades indicate decreasing NPIW ventilation, induced by a freshening and increased stratification of surface and thermocline waters. Longer variations in these oceanographic boundary conditions were, however, large and are thus able to hinder assessment of anthropogenic influences against natural background shifts. We previously provided evidence modern well-ventilated waters underwent significant millennial-scale variations over the last ca. 12,000 years (Lembke-Jene et al., 2018), with a prominent “tipping point” around 4,500 years before present.Crossing such mid-Holocene threshold led to the Okhotsk Sea becoming the modern ventilation source it is today, although the underlying forcing and physical boundary conditions characteristics remain largely enigmatic. A combination of sea ice loss, higher water temperatures, and remineralization rates may be able to induce a nonlinear change into a different mean state in this region. To constrain these factors we present combined surface, mesopelagic and bathyal ocean proxy records from key study sites in the Western Subarctic Pacific, the Okhotsk Sea and Bering Sea, and the Gulf of Alalska, with submillennial-scale resolution to assess changes in upper ocean stratification, nutrient characteristics and resulting changes on mid-depth water ventilation. Our results imply that under assumed past hemispheric warmer- than-present conditions, regional surface temperatures and upper ocean stratification were increased and changed in a nonlinear mode during the last 4-5,000 years, associated with changing primary productivity patterns and biogeochemical feedback mechanisms. Results from complementary Earth System Model simulations provide evidence for the interaction between the high-latitude North Pacific marginal seas and thePacific Western Subarctic Gyre circulation, with effects on mesopelagic ventilation dynamics and its consequences for large oceanic regions.