ALBERT

Description: Paleoceanographic evidence commonly indicates that Last Glacial Maximum surface temperatures in the Japan Sea were comparable to modern conditions, in striking difference to colder neighboring regions. Here, based on a core from the central Japan Sea, our results show similar UK′37- and TEXL86-derived temperatures between 24.7-16.3 ka BP, followed by an abrupt divergence at ~16.3 ka BP and a weakening of divergence after ~ 8.7 ka BP. We attribute this process to a highly stratified glacial upper ocean controlled by the East Asian Summer Monsoon, increasing thermal gradient between surface and subsurface layers during the deglaciation and the intrusion of Tsushima Warm Current since the mid Holocene, respectively. Therefore, we suggest threshold-like changes in upper-ocean temperatures linked to sea-level rise and monsoon dynamics, rather than just sea surface temperatures, play a critical role in shaping the thermal and ventilation history of this NW Pacific marginal sea.

Description: Collection of Alkenone temperatures derived from 42 Multicorer sediment surface samples of the subpolar North Pacific and its marginal seas (Okhotsk Sea and Bering Sea). Multicores were collected during several expeditions to the Okhotsk Sea, Bering Sea and North Pacific from 1997 to 2013 (R/V Akademik M.A. Lavrentyev cruise LV27 (Nürnberg et al., 1997); R/V Akademik M.A. Lavrentyev cruise LV28 (Biebow and Hütten, 1999); R/V Marshal Gelovany cruise GE99 (Biebow et al., 2000); R/V Akademik M.A. Lavrentyev cruise LV29 (Biebow et al., 2003); R/V Sonne cruise SO178, (Dullo and Biebow, 2004); R/V Sonne cruise SO201-2, Dullo et al., 2009); R/V Akademik M.A. Lavrentyev cruise LV55 (Gorbarenko, 2012); R/V Akademik M.A. Lavrentyev cruise LV63 (Gorbarenko, 2014).

Description: Arctic and subarctic regions are sensitive to climate change and, reversely, provide dramatic feedbacks to the global climate. With a focus on discovering paleoclimate and paleoceanographic evolution in the Arctic and Northwest Pacific Oceans during the last 20,000 years, we proposed this German–Sino cooperation program according to the announcement “Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany for a German–Sino cooperation program in the marine and polar research”. Our proposed program integrates the advantages of the Arctic and Subarctic marine sediment studies in AWI (Alfred Wegener Institute) and FIO (First Institute of Oceanography). For the first time, the collection of sediment cores can cover all climatological key regions in the Arctic and Northwest Pacific Oceans. Furthermore, the climate modeling work at AWI enables a “Data-Model Syntheses”, which are crucial for exploring the underlying mechanisms of observed changes in proxy records.

Description: Paleoceanographic evidence commonly indicates that Last Glacial Maximum surface temperatures in the Japan Sea were comparable to modern conditions, in striking difference to colder neighboring regions. Here, based on a core from the central Japan Sea, our results show similar UK′37‐ and TEXL86‐derived temperatures between 24.7 and 16.3 ka BP, followed by an abrupt divergence at ~16.3 ka BP and a weakening of divergence after ~8.7 ka BP. We attribute this process to a highly stratified glacial upper ocean controlled by the East Asian Summer Monsoon, increasing thermal gradient between surface and subsurface layers during the deglaciation and the intrusion of Tsushima Warm Current since the mid‐Holocene, respectively. Therefore, we suggest that threshold‐like changes in upper‐ocean temperatures linked to sea level rise and monsoon dynamics, rather than just sea surface temperatures, play a critical role in shaping the thermal and ventilation history of this NW Pacific marginal sea.

Description: Investigating the composition and distribution of pelagic marine sediments is fundamental in the field of marine sedimentology. The spatial distributions of surface sediment are unclear due to limited investigation along the Emperor Seamount Chain of the North Pacific. In this study, a suite of sedimentological and geochemical proxies were analyzed, including the sediment grain size, organic carbon, CaCO3, major and rare earth elements of 50 surface sediment samples from the Emperor Seamount Chain, spanning from ∼33°N to ∼52°N. On the basis of sedimentary components, we divide them into three Zones (I, II, and III) spatially with distinct features. Sediments in Zone I (∼33°N–44°N) and Zone III (49.8°N–53°N) are dominated by clayey silt, and mainly consist of sand and silty sand in Zone II. The mean grain size of the sortable silt shows that the hydrodynamic condition in the study area is significantly stronger than that of the abyssal plain, especially at the water depth of 1,000–2,500 m. The CaCO3 contents in sediments above 4,000 m range from 20 to 84% but decrease sharply to less than 1.5% below 4,000 m, confirming that the water depth of 4,000 m is the carbonate compensation depth of the study area. Strong positive correlations between Al2O3 and Fe2O3, TiO2, MgO, and K2O (R 〉 0.9) in the bulk sediments indicate pronounced contributions of terrigenous materials from surrounding continent mass to the study area. Furthermore, the eolian dust makes contributions to the composition of bulk sediments as confirmed by rare earth elements. There is no significant correlation between grain size and major and minor elements, which indicates that the sedimentary grain size does not exert important effects on terrigenous components. There is significant negative δCe and positive δEu anomalies at all stations. The negative Ce anomaly mainly exists in carbonate-rich sediments, inheriting the signal of seawater. The positive Eu anomaly indicates widespread volcanism contributions to the study area from active volcanic islands arcs around the North Pacific. The relative contributions of terrestrial, volcanic, and biogenic materials vary with latitude and water depth in the study area.

Description: The Pacific hosts the largest oxygen minimum zones (OMZ) in the world ocean, likely to intensify and expand under future climate warming, with consequences for ecosystems, biogeochemical cycles and living resources. Today, better-oxygenated subsurface North Pacific Intermediate Water mitigates OMZ development, but on instrumental time scales, data indicate decreasing NPIW ventilation, induced by surface freshening and increased stratification of seasonal thermocline water. However, longer variations in oceanographic boundary conditions were potentially large and hinder assessment of anthropogenic influences against natural background shifts. We previously provided evidence that modern well-ventilated waters underwent significant millennial-scale variations over the last ca. 12 ka, with a tipping point ca. 4.5 ka before present. Crossing this mid-Holocene threshold led to the Okhotsk Sea losing its modern ventilation source characteristics, although underlying forcing and physical boundary conditions remain largely enigmatic. A combination of sea ice loss, water temperatures, and remineralization rates may have conceivably induced a nonlinear switch into a different mean state in this region. To constrain these factors, we present surface ocean proxy records from Okhotsk Sea key study sites with multi-decadal resolution to assess changes in upper ocean stratification, nutrient characteristics and resulting mid-depth water ventilation. Our results imply that under assumed past warmer- than-present conditions, regional surface temperatures and upper ocean stratification were increased and changed in a nonlinear mode during the last ca. 6,000 years, associated with changing primary productivity patterns and biogeochemical feedback mechanisms. Complementary results from model simulations corroborate our results and provide evidence for close coupling the Okhotsk Sea and the North Pacific Subarctic Gyre, thus exporting marginal sea signals into large oceanic regions.