ALBERT

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: Non-carbonaceous abyssal fine-grained sediments cover vast parts of the North Pacific’s deep oceanic basins and gain increasing interests as glacial carbon traps. They are, however, difficult to date at an orbital-scale temporal resolution and still rarely used for paleoceanographic reconstructions. Here, we show that sedimentary records of past geomagnetic field intensity have high potential to improve reversal-based magnetostratigraphic age models. Five sediment cores from Central North Pacific mid-latitudes (39–47°N) and abyssal water depths ranging from 3,900 to 6,100 m were cube-sampled at 23 mm resolution and analyzed by automated standard paleo- and rock magnetic methods, XRF scanning, and electron microscopy. Relative Paleointensity (RPI) records were determined by comparing natural vs. anhysteretic remanent magnetization losses during alternating field demagnetization using a slope method within optimized coercivity windows. The paleomagnetic record delivered well interpretable geomagnetic reversal sequences back to 3 Ma. This age span covers the climate-induced transition from a biogenic magnetite prevalence in the Late Pliocene and Early Pleistocene to a dust-dominated detrital magnetic mineral assemblage since the Mid-Pleistocene. Volcaniclastic materials from concurrent eruptions and gravitational or contouritic sediment re-deposition along extinct seamount flanks provide a further important source of fine- to coarse-grained magnetic carriers. Surprisingly, higher proportions of biogenic vs. detrital magnetite in the late Pliocene correlate with systematically lowered RPI values, which seems to be a consequence of magnetofossil oxidation rather than reductive depletion. Our abyssal RPI records match the astronomically tuned stack of the mostly bathyal Pacific RPI records. While a stratigraphic correlation of rock magnetic and element ratio logs with standard oxygen isotope records was sporadically possible, the RPI minima allowed to establish further stratigraphic tie points at ∼50 kyr intervals. Thus, this RPI-enhanced magnetostratigraphy appears to be a major step forward to reliably date unaltered abyssal North Pacific sediments close to orbital-scale resolution.

Description: Deglacial dissolved oxygen concentrations were semiquantitatively estimated for intermediate and deep waters in the western Bering Sea using the benthic foraminiferal-based transfer function developed by Tetard et al. (2017), Tetard et al. (2021a). Benthic foraminiferal assemblages were analyzed from two sediment cores, SO201-2-85KL (963 m below sea level (mbsl), the intermediate-water core) and SO201-2-77KL (2,163 mbsl, the deep-water core), collected from the Shirshov Ridge in the western Bering Sea. Intermediate waters were characterized by an oxygen content of ∼2.0 ml L−1 or more during the Last Glacial Maximum (LGM)–Heinrich 1 (H1), around 0.15 ml L−1 during the middle Bølling/Allerød (B/A)–Early Holocene (EH), and a slight increase in [O2] (∼0.20 ml L−1) at the beginning of the Younger Dryas (YD) mbsl. Deep-water oxygen concentrations ranged from 0.9 to 2.5 ml L−1 during the LGM–H1, hovered around 0.08 ml L−1 at the onset of B/A, and were within the 0.30–0.85 ml L−1 range from the middle B/A to the first half of YD and the 1.0–1.7 ml L−1 range from the middle to late Holocene. The [O2] variations remind the δ18O NGRIP record thereby providing evidence for a link between the Bering Sea oxygenation at intermediate depths and the deglacial North Atlantic climate. Changes in the deep-water oxygen concentrations mostly resemble the deglacial dynamics of the Southern Ocean upwelling intensity which is supposed to be closely coupled with the Antarctic climate variability. This coherence suggests that deglacial deep-water [O2] variations were primarily controlled by changes in the circulation of southern-sourced waters. Nevertheless, the signal from the south at the deeper site might be amplified by the Northern Hemisphere climate warming via an increase in sea-surface bioproductivity during the B/A and EH. A semi-enclosed position of the Bering Sea and sea-level oscillations might significantly contribute to the magnitude of oxygenation changes in the study area during the last deglaciation. Interregional correlation of different proxy data from a wide range of water depths indicates that deglacial oxygenation changes were more pronounced in the Bering and Okhotsk marginal seas than along the open-ocean continental margin and abyssal settings of the North Pacific.

Description: A comparison of micropaleontological data on the distribution of the Cycladophora davisiana radiolarian species in the surface sediment layer and the Late Quaternary sediments from the Subarctic Pacific and Far East marginal seas allowed conclusions concerning the possible conditions and occurrence of intermediate waters during the last glacial maximum. We used the modern data on the C. davisiana species, which is a micropaleontological indicator of the cold oxygen-rich upper intermediate water mass, which is now forming only in the Sea of Okhotsk. The high amount of C. davisiana in sediments of the last glacial maximum may point to the possible formation and expansion of the ventilated intermediate water in the most part of the Subarctic paleo-Pacific: the Bering Sea, the Sea of Okhotsk, within the NW Gyre, and in the Gulf of Alaska.

Description: In this paper we show that the development of the sediment architecture at the leeward toe-of-slope of Great Bahama Bank (Ocean Drilling Project Leg 166, Bahama Transect) during the last 6 Ma is not only a response to sea-level fluctuations, but also to major paleo-oceanographic and climatic changes. A major sequence boundary close to the Miocene/Pliocene boundary (dated at 5.6–5.4 Ma) is interpreted to reflect a major sea-level drop that was followed by a sea-level rise, which led to the re-flooding of the Mediterranean Sea at the end of the Messinian and increasing sea-surface temperatures at Great Bahama Bank. Distinct erosional horizons occurred during the Pliocene (dated at 4.6 and 3.3–3.6 Ma) related to sea-level change and the intensification of the Gulf Stream when the emergence of the Isthmus of Panama reached a critical threshold. The Gulf Stream brings warm, saline and nutrient-poor waters to the Bahamas. Starting at the Early–Late Pliocene boundary at 3.6 Ma this paleo-oceanographic reorganization in combination with enhanced sea-level fluctuations associated with the Late Pliocene main intensification in Northern Hemisphere Glaciation (since 3.2 Ma) led to (1) a gradual change from a ramp-type to a flat-topped type morphology, and (2) a change from a skeletal to a non-skeletal-dominated sedimentary system (mainly peloidal). Increased sea-level fluctuations during the second half of the Pleistocene led to an intensified high stand-shedding depositional pattern within the surrounding basins.

Description: The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was the Yukon–Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and glacial cycles in Alaska.

Description: Five radiolarian datum events have been recorded in the Late Quaternary sediment section from the Sea of Okhotsk, using age assignments based on oxygen-isotope stratigraphy: the last occurrence datum (LOD) of Stylacontarium acquilonium (at about 329 ka), the LOD of Spongodiscus sp. (at about 287 ka), the LOD of Amphimelissa setosa (at about 72 ka), the LOD of Lychnocanoma nipponica sakaii (at about 28 ka), and the L. nipponica sakaii acme event (at about 72 ka). The LODs of S. acquilonium, Spongodiscus sp. and A. setosa appear to be synchronous in comparison with those in the Subarctic North Pacific, but the LOD and the acme event of L. nipponica sakaii show an apparent diachroneity with the corresponding North Pacific events.

Description: The benthic and planktonic foraminiferal assemblages and the distribution of coarse grain-size factions were studied in the upper 4.5 m of the Core SO201-2-85KL (57A degrees 30.30' N, 170A degrees 24.79' E, water depth 968 m) retrieved from the Shirshov Ridge. This part of the core covers 7.5 to 50 kyr BP. The glacial period is established to be characterized by low surface water productivity, the wide distribution of sea ice and/or icebergs in this area, and a high oxygen concentration in the bottom layer. Enhanced productivity is inferred from the maximum abundance of planktonic foraminifers at the very beginning of the deglaciation. The late Bolling-Allerod interstadial and the early Holocene were marked by the further two-phase increase in the surface productivity and the weakened ventilation of the bottom water.