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Biogenic opal production in the surface water of the Bering Sea shows clear orbital-scale variations such as interglacial highs and glacial lows. In this study, we examined the variation patterns in δ 15 N of bulk sediment (δ 15 N bulk ) reflecting nitrate utilization, and total organic carbon (TOC) concentration in terms of organic carbon (OC) production, in response to the orbital-scale biogenic opal production during the last 600 kyrs at Site U1343 (IODP Exp. 323) in the Bering slope area. At Site U1343, δ 15 N bulk value and TOC concentration do not show the distinct glacial-interglacial change. Moreover, contrary to the weak correlation between TOC and biogenic opal concentrations, TOC concentration is positively correlated with δ 15 N bulk value, indicating the strong control of OC production on nitrate utilization. It should be noted that glacial terminations are characterized by high δ 15 N bulk value, high TOC and low biogenic opal concentration whereas the following full interglacials are characterized by low δ 15 N bulk value, low TOC and high biogenic opal concentration during the last 600 kyrs. In other words, glacial terminations are characterized by complete nitrate utilization associated with high OC production under low biogenic opal production, whereas the following full interglacial periods are characterized by decreased nitrate utilization associated with low OC production under high biogenic opal production. Such different glacial-interglacial variation patterns of OC production and nitrate utilization in response to biogenic opal production can best be explained by variable Si/C (or Si/N) uptake ratio by diatoms possibly owing to Fe availability. The glacial Fe input increase at Site U1343 is thought to be ascribed to more dust input or large discharge from the glacial Yukon River. These findings imply that Fe transport and contribution may play a significant role in governing the nutrient cycles in the Bering slope region of the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

Late Pliocene to early Pleistocene paleoproductivity changes in the Bering Sea were reconstructed using geochemical concentrations and mass accumulation rates (MARs) of CaCO 3 , biogenic opal, and total organic carbon (TOC), and sedimentary nitrogen isotope ratios (δ 15 N) at IODP Expedition 323 Hole U1343E, drilled in the northern slope area (1956 m deep) of the Bering Sea. CaCO 3 concentration is generally low, but prominent CaCO 3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation, regardless of glacial–interglacial variations. Biogenic opal concentrations reflect orbital-scale glacial–interglacial variations. However, TOC concentration did not show clear glacial–interglacial variation, probably due to poor preservation. The sedimentary δ 15 N values vary synchronously with biogenic opal concentration on orbital timescales. The co-varying pattern of opal productivity and δ 15 N values at Hole U1343E is a result of nutrient utilization controlled by diatom productivity in the Bering slope area where Fe is not a limiting factor. Biogenic opal and TOC MARs showed a temporal shift at around 1.9 Ma from a high productivity period under nutrient-enriched conditions to a low productivity period under relatively nutrient-depleted conditions. High diatom productivity with low δ 15 N values before 1.9 Ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system. This productivity decrease at about 1.9 Ma was also found in the southern Bering Sea (Site U1341) and may be related to global opal reorganization. [ABSTRACT FROM AUTHOR]

The clay mineral composition at IODP Exp. 323 Site U1343 in the Bering Sea was analyzed so as to unravel their provenance over glacial–interglacial cycles for the last 2.4 Ma. Smectite was negatively correlated with the sum of illite and chlorite; therefore, their ratio [S/(I + C)] was used as an indicator of clay mineral composition changes. In general, the S/(I + C) ratio was rather similar for glacial and interglacial periods during most of the last 2.4 Ma. In addition, these results overlap with those of surface sediments in the modern East Aleutian Basin, which suggests that smectite-rich clay particles are delivered from the Aleutians by the northward Bering Slope Current (BSC) rather than from rivers in western Alaska. However, some clay mineral compositions of the glacial periods after the Mid-Pleistocene Transition (MPT: 1.25–0.7 Ma) were characterized by low illite and relatively high smectite. During this period, extensive ice-rafting might have transported the smectite-rich clay particles to Site U1343 from the glacial shelf off Alaska and/or from glacial rivers from that area. [ABSTRACT FROM AUTHOR]

Abstract: Biogenic opal content and mass accumulation rate (MAR) at IODP Expedition 323 Site U1343 were found to fluctuate consistently, generally being high under warm conditions and low under cold conditions during the last 2.4Ma. Continuous wavelet transform analysis of the normalized biogenic opal content indicates that export production in the Bering Sea varied predominantly at 41-ka periodicity before 1.25Ma, and shifted to 100-ka periodicity at the onset of the Mid-Pleistocene Transition (MPT; 1.25–0.7Ma). The 100-kacycles dominated until the Holocene. Export production in the Bering Sea decreased markedly in the Bering Sea two times during the MPT: the first occurred at the beginning of the MPT (1.25Ma) and the second in the middle of the MPT (0.9Ma). These decreases coincided with both increases in the relative abundance of sea-ice diatoms and decreases in the warm-water diatom species Neodenticula seminae, indicating that reductions in export production in the Bering Sea during the MPT were associated with climate cooling. Decreases in export production in the Bering Sea during the MPT were most likely associated with the increased influence of polar/Arctic domains on the high-latitude North Pacific. [Copyright &y& Elsevier]

Detailed diatom records within surface and core sediments from the Southern Bering Continental Shelf (SBCS) reveal that the Holocene evolution of sea-ice distribution is associated with low pressure patterns. Holocene sea-ice distribution over the SBCS was mainly controlled by the location of the Aleutian Low. The corresponding paleoceanographic and paleoclimate conditions can be divided into three stages: (1) the early Holocene (before 7000 cal. yr BP) was characterized by extensive sea-ice distribution under two low-pressure cells, which covered the western Bering Sea and the Gulf of Alaska, respectively. (2) Between 3000 and 7000 cal. yr BP, the low-pressure system over the Gulf of Alaska became weak, causing total sea-ice mass over the SBCS to retreat. (3) In the past 3000 years, prevailing southwesterly winds over the SBCS due to the developing Aleutian Low have reduced further sea-ice cover on the SBCS. These paleoclimatic changes were probably a response to ENSO variation. The timings of water mass exchanges on the SBCS coincided with sea-level change along the Alaskan Peninsula. As a result, subsequent morphologic alterations have also influenced the paleoceanographic condition of the SBCS. The effect of the surface coastal water and bottom marine water on the SBCS intensified about 6000 cal. yr BP when sea level increased. [ABSTRACT FROM AUTHOR]

Isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) and grain size distribution were investigated in surface sediments from the western Arctic Ocean including the Chukchi Sea and the adjacent northern Bering Sea to understand their source and transportation in the Arctic region and test environmental proxies derived from the GDGT composition. Coarse sediments such as sand and silty sand are distributed in the Yukon and Mackenzie River estuaries, the northern Bering Sea near Bering Strait, and some areas of the outer shelf of the Bering Sea. In the Chukchi Sea, silt, grading from sandy to clayey silt, predominates and becomes finer northward towards the deep Arctic Ocean Isoprenoid GDGTs are abundant on the outer shelf and slope of the Chukchi Sea and the upper slope of the Bering Sea. The higher abundances are attributed to a combination of higher production of marine Archaea (Thaumarchaeota) at the shelf edge, redeposition of GDGT-carrying fine particles, and better preservation of GDGTs at sites with higher sedimentation rates. The TEX 86 - and TEX 86 L -derived temperatures are not consistent with sea surface temperatures in the study area, with unrealistically high TEX 86 - and TEX 86 L -derived temperatures in samples north of 73 °N probably biased by factors other than temperature. Branched GDGTs are abundant on the Chukchi shelf and in the Yukon and Mackenzie River estuaries. At the shelf edge of the Chukchi Sea, both branched and isoprenoid GDGTs are abundant, indicating common concentration processes such as sediment redeposition and efficient preservation at sites with high sedimentation rates. Sediments from the western Arctic Ocean north of 75 °N, the Yukon and Mackenzie River estuaries, and the Yukon River have higher cyclization ratio of branched tetraethers (CBT) than sediments from the Chukchi and Bering Seas, suggesting two different sources of branched GDGTs inferred as soil and marine bacteria. [ABSTRACT FROM AUTHOR]

Abstract: Sediments from three Sites (U1341, U1343 and U1345) of IODP Expedition 323, located in the Bering Sea, were analyzed for variation of biogenic opal representing surface water paleoproductivity. Initial age models of three Sites (bottom age, U1341: 4.3 Ma, U1343: 2.4 Ma, U1345: 0.5 Ma) were established onboard by biostratigraphic and paleomagnetic investigations. The initial age models were tuned by δ18O stratigraphy, which was established on shore. Biogenic opal contents of sediments were analyzed for the interval of 1.3 Ma to the present. Generally, the %biogenic opal values at Site U1341 (Bowers Ridge) were significantly higher than those at the Beringian slope sites (U1343 and U1345). Higher natural gamma radiation (NGR) counts and magnetic susceptibility (MS) at the Beringian slope sites suggested a greater contribution of lithogenic matter than at Site U1341 on Bowers Ridge. The percentages of sea-ice diatoms at the Beringian slope sites are also higher than those at Bowers Ridge Sites. This is primarily attributed to the fact that the sediments from the Beringian slope area were significantly diluted by lithogenic matter derived from the Alaskan continent, and were largely affected by sea-ice cover. The cyclicity of biogenic opal accumulation rates at Site U1341 is determined to be about 160, 40, and 30 ky, whereas that at Site U1343 is determined to be about 330, 100, 40 and 20 ky. If these had been reflecting a global climate change, the cyclicity of biogenic opal contents at all sites would be consistently the same. However, the results do not show such a consistency. Obliquity is observed at both sites of the Bowers Ridge and Beringian slope. A strong eccentricity was observed only at U1343, but no signature for eccentricity was detected at U1341. Therefore, the Milankovitch cycle with 100 ky periodicity was only observed at the Beringian slope area. The results suggest that the %biogenic opal at Site U1341 indicated a local characteristic for biogenic productivity changes. Thus, the absence of the 100 ky cycle of biogenic opal at Site U1341 seems to be due to local phenomena such as water circulation, although the changes in biogenic opal productivity at Site U1341 are attributed to summer insolation. [Copyright &y& Elsevier]

Abstract: Biogeochemical processes involving acetate in sub-seafloor sediments from piston core PC23B from the Bering Sea shelf break were inferred by examining the stable carbon isotopic relationships between acetate and other relevant carbon compounds: total organic carbon (TOC) in the sediment solid phase, and dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in pore water. Throughout the core, the isotopic composition of acetate (δ13Cacetate), from −31‰ to −29‰, was 13C-depleted by ca. 7‰ vs. DOC (δ13CDOC) and its depth profile approximately paralleled that of δ13CDOC, suggesting that the principal process producing acetate was fermentation of dissolved organic compounds. However, the 13C depletion in δ13Cacetate indicates some contribution of acetogenesis to total acetate production, because acetogenesis results in 13C depletion of the acetate produced. The relative contribution of acetogenesis via the H2/CO2 reaction, calculated by using a two source isotope mixing model, increased with depth in the sulfate reduction zone from 10% to 15% and was constant at 19% in the methanogenic zone. The acetogenic contribution to acetate production in the methanogenic zone underlying the sulfate reduction zone is consistent with reported observations, whereas the occurrence of acetogenesis in the sulfate reduction zone may be related to the contribution of terrestrial organic matter (OM) to the sedimentary OM in that depth interval, because the terrestrial component likely includes precursors that favor organoautotrophic acetogenesis. The high acetate concentration (up to 81μM) and TOC content (up to 1.4%) at the same depth (<200 cmbsf) suggest that some relationship exists between acetate production rate and TOC content, or that a temperature increase during core storage at room temperature might stimulate acetate-producing microbial activity in the high TOC sediment. [Copyright &y& Elsevier]

Abstract: A high-resolution record of the radiolarian assemblage from 60 to 10ka was investigated using a piston core (PC-23A) obtained from the northern slope of the Bering Sea. Faunal changes based on the 29 major radiolarian taxa demonstrated that the surface and deep water conditions in the Bering Sea were related to the orbital and millennial-scale climatic variations known as glacial–interglacial and Dansgaard–Oeschger (D–O) cycles, respectively. During interstadial periods of the D–O cycles, the assemblage was characterized by increases in the high-latitude coastal species Rhizoplegma boreale and the upper-intermediate water species Cycladophora davisiana, while the sea-ice related species Actinomma boreale and A. leptodermum and many deep-water species such as Dictyophimus crisiae and D. hirundo tended to be reduced. This trend was more apparent in two laminated intervals at 15–13.5 and 11.5–11ka, which were correlated with well-known ice-sheet collapse events that occurred during the last deglaciation: melt-water pulse (MWP)-1A and MWP-1B, respectively. The radiolarian faunal composition in these periods suggests that oceanic conditions were different from today: (1) surface water was affected by increased melt-water discharge from continental ice-sheet, occurring at the same time as an abrupt increase in atmospheric temperature, (2) upper-intermediate water (ca. 200–500m) was well-ventilated and organic-rich, and (3) lower-intermediate water (ca. 500–1000m) was oxygen-poor. Conversely, the sea-ice season might have been longer during stadial periods of the D–O cycles and the last glacial maximum (LGM) compared to the interstadial periods and the earliest Holocene. In these colder periods, deep-water species were very abundant, and this corresponded to increases in the oxygen isotope value of benthic foraminifera. Our findings suggest that the oxygen-rich water was present in the lower-intermediate layer resulting from intensified ventilation. [Copyright &y& Elsevier]