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1.
Diatoms were studied in the KOMEX LV28-34-2 sediment core from the northwestern Sea of Okhotsk. The interval from the penultimate glacial to the Holocene was subdivided into stratigraphic units based on the distribution of biogeographic groups and individual index species. The analysis of the diatom assemblages indicated that, during the Last Glacial Maximum, of the Deryugin Basin was regularly covered with extensive seasonal ice of at least 70% concentration for a minimum of 6–7 months per year. Cold subarctic water from the adjacent open Pacific could have reached as far as the northwestern sea area.  相似文献   

2.
Lithological, geochemical, and micropaleontological data indicate that the Late Glacial of the northwestern Okhotsk Sea (OS) is characterised by severe climatic and environmental conditions with mainly perennial sea ice coverage and low productivity accompanied by weak deep-water ventilation and a temperate formation of the upper Sea of Okhotsk Intermediate Water (SOIW). The age model of the studied core sediments was constructed by AMS 14C dating. The most severe environmental conditions occurred during the period 15.8–14.8 kyr, synchronous with cold Heinrich event 1. Insignificant regional environmental amelioration accompanied by an increase of productivity and ice weakening during summer occurred almost simultaneously with the Bølling–Allerøed (BA) warming. The obtained results distinguished both the Bølling and Allerøed warmings as having different environmental conditions. Oxygen content in the surface sediment was low, as seen from the production of the benthic foraminifera (BF) species. During 12.6–11.1 kyr, synchronous with the Younger Dryas (YD) cold event, the regional environment conditions were cold, but not as severe as the glacial ones. Some climatic warming since the Preboreal has stimulated sea ice melting and surface amelioration during the summer season, which in turn led to a productivity rise and changes in the water column and bottom environment. Some increase in the surface water stratification and the intensified oceanic diatom and surface radiolarian production is parallel with the development of a mesopelagic regime of productivity. The surface sediment condition favours BF abundance and domination by BF species tolerant to oxygen deficiencies. During the Boreal period more stable surface conditions were accompanied by continuously high productivity and an intensifying of its mesopelagic regime.Significant regional climate warming since the Atlantic (9 kyr ago) strongly intensified the summer sea ice melting in the OS, and this created considerable surface environment amelioration with the preferential transport of bacteria and phytodetritus into the SOIW. Further considerable warming of the regional climate from 6 kyr ago contributed to slight sea ice changes, surface water warming, and the enhancement of its stratification; all typical for most of the OS. Along with a high nutrient supply from the Amur River, the NW OS experienced a strong diatom production increase with the maximum amount occurring during the last 3.6 kyr. This changed the productivity type and organic matter export into the water column while increasing the feeding of the “productive” Plagoniidae spp. group and decreasing the microbial biomass supply into the upper SOIW. Some sea surface water cooling or saltier conditions at the beginning of the Subatlantic (2.4–1.8 kyr) was followed by its warming or freshening 1.5–1.0 kyr ago, which likely correlated with the Medieval Warm Period. In turn, that probably led to strong surface water stratification, productivity deterioration and considerable changes in the overall NW OS environment. The established sequence of the northwestern OS environmental changes during the Late Glacial–Holocene is related to the Northern Hemispheric climate changes and was likely forced by atmospheric teleconnection in line with the polar circulation index variability.  相似文献   

3.
本文通过对南极斯科舍海东南部DC-11岩芯生物硅、有机氮、TFe2O3与有机氮同位素的年代学分析,重建了该海区3.4万年以来古生产力与环境演变历史。研究结果表明,生物硅、有机氮含量与南极温度变化基本一致,暖期高、冷期低;有机氮同位素值与南大洋海冰变化相吻合,暖期小、冷期大,冷期硝酸盐利用率大于暖期。从末次冰期、末次冰消期至全新世,研究区古生产力与环境变化显著,南极冷倒转等千年尺度的变化明显;海冰在气候、营养盐与古生产力之间起着重要的关联作用。冰期或冷期海冰的加强导致表层水层化加强,深层水及其营养盐的上涌减弱,表层海洋硝酸盐等相对匮乏,生产力降低。研究区现代与全新世铁供应充足,在风尘盛行的末次冰期和冰消期呈过剩状态,明显不同于亚南极。  相似文献   

4.
Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP25, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP25 data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bølling-Allerød and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes.  相似文献   

5.
The molluscan shells from three drill and two piston cores obtained in the southern Russian sector of the Chukchi Sea are dated by the AMS 14C measurement method back to 0.8–3.5 and 9.2–10.5 ka. The period of 9–10 ka was marked by increased sedimentation rates related to the transgression onset. The fossils in the lower Holocene section exhibit the successive upward replacement of brackish-water organisms by their marine counterparts. After the opening of the Bering Strait in the middle Holocene, the sedimentation was under influence of the increased bioproductivity of the waters. The climatic optimum in the Chukotka region corresponds to the early Holocene, while the late Holocene was characterized by the wider development of the ice cover on the shelf.  相似文献   

6.
本文基于白令海北部陆坡ARC6-B11岩心的沉积物粒度、颜色反射率、有机碳/氮含量和高分辨率化学元素XRF扫描数据,结合底栖和浮游有孔虫AMS14C测年资料,重建了研究区23 ka以来至中全新世的沉积记录,重点研究了海冰的扩张变化对生产力和沉积物输入的影响。研究结果显示,末次盛冰期白令海北部陆坡受到海冰南向扩张的影响而生产力较低,沉积物中砂粒级和冰筏碎屑（IRD）含量较高。HS 1时期由于受到常年冰的覆盖,沉积物中砂粒级和IRD含量存在低值,沉积物中较高的总有机碳（TOC）含量受陆源有机质的影响较大,异常的TOC含量峰值主要受到低海平面和海冰形成过程产生的富氧水团控制下的高浓度细粒物质携带的陆源有机质的快速沉积作用的影响,难以作为反映白令海古生产力的替代指标。B/A暖期和全新世生产力较高,在B/A暖期存在Ca/Ti峰值现象,Ca/Ti峰值现象的产生与碳酸盐补偿深度增加有关,水体通风条件的改变可能起到一定的作用,也不能忽视颗石藻对其产生的潜在影响。  相似文献   

7.
This paper presents reconstructions of ice sheet boundaries, lacustrine and marine paleobasins, as well as the connections of the Barents and Baltic seas with the North Atlantic from the Last Glacial Maximum to the Holocene. The reconstructions are based on original and published data obtained from the northern and western parts of the Barents Sea and Baltic depressions with account for the available regional schematic maps of deglaciation. The early deglaciation of the Scandinavian–Barents ice sheet culminated with the Bølling-Allerød interstadial (14.5–12.9 cal ka BP), which was characterized by a more vigorous Atlantic meridional overturning circulation (AMOC) and a corresponding increase in surface Atlantic water inflow into the Barents Sea through deep troughs. The Baltic Ice Lake (BIL) remained a dammed-up isolated basin during deglaciation from 16.0 to 11.7 cal ka BP. In the Younger Dryas (YD), the lake drained into the North Sea and was replaced by a brackish Yoldia Sea (YS) at the beginning of the Holocene (Preboreal, 11.7–10.7 cal ka BP), due to a limited connection between two basins through the Närke Strait. In the Barents Sea, the next increase in the Atlantic water influx into the deep basins corresponded to terminal YD and Preboreal events with a culmination in the Early Holocene. The Yoldia Sea became a lake again during the next stage, the Ancylus (~10.7–8.8 cal ka BP). Atlantic water inflow both into the Barents and Baltic seas varied during the Holocene, with a maximum contribution in the Early Holocene, when the Littorina Sea (LS, 8–4 cal ka BP) connection with the North Sea via the Danish Straits was formed to replace the Ancylus Lake. The recent, post-Littorina stage (PS, the last 4 cal ka) of the Baltic Sea evolution began in the Late Holocene.  相似文献   

8.
本文研究采自位于南极罗斯海西部的ANT32-RB16C柱状沉积物,根据粒度、有机碳和生物标志化合物数据探讨末次冰盛期(24.8 ～20 ka BP)以来该地区有机质来源及沉积环境特征。该柱状样记录了冰架下沉积、冰架前沉积、开阔海域沉积的沉积环境。指示有机质来源的生物标志化合物指标表明该柱状样中所含有机质主要为浮游植物、细菌等海源输入,同时伴有少量陆源物质混合输入。末次冰盛期,由于冰架的影响有机质含量较低,环流影响使得有机质受低等浮游藻类生物等海源影响较大。末次冰消期(20～11.7 ka BP),罗斯冰架消退,冰川溶解释放的有机质在此沉积,使得陆源有机质输入增多,有机质含量升高。进入全新世,有机质含量较末次冰盛期和末次冰消期明显升高,海源输入比例增大,同时细菌等原核生物增多,导致短链正构烷烃降解程度较大。研究区的氧化还原环境主要受冰架与海冰限制作用的影响,与有机质含量和高氧的南极底层水关系不大。总体来说,从末次冰盛期到末次冰消期,研究区沉积环境受罗斯冰架进退影响,全新世以来受气候变化影响。  相似文献   

9.
Geochemical and rock-magnetic investigations were carried out on a sediment core collected from the SE Arabian Sea at 1420 m depth in oxygenated waters below the present-day oxygen minimum zone. The top 250 cm of the core sediments represent the last 35 kaBP. The · 18O values of Globigerinoides ruber are heaviest during the Last Glacial Maximum (LGM) and appear unaffected by low-saline waters transported from the Bay of Bengal by the strong northeast monsoon and West Indian coastal current. The signatures of Bølling-Allerød and Younger Dryas events are distinct in the records of magnetic susceptibility, organic carbon (OC) and · 18O. Glacial sediments show higher OC, CaCO3, Ba, Mo, U and Cd, while the early-to-late Holocene sediments show increasing concentrations of OC, CaCO3, Ba, Cu, Ni and Zn and decreasing concentrations of Mo, U and Cd. Productivity induced low-oxygenated bottom waters and reducing sedimentary conditions during glaciation, and productivity and oxygenated bottom waters in the Holocene are responsible for their variation. The core exhibits different stages of diagenesis at different sediment intervals. The occurrence of fine-grained, low-coercivity, ferrimagnetic mineral during glacial periods is indicative of its formation in organic-rich, anoxic sediments, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea and Japan Sea. The glacial sediments exhibiting reductive diagenesis with anoxic sedimentary environment in this core correspond to reductive diagenesis and intermittent bioturbation (oxygenation) reported in another core in the vicinity. This suggests that the poorly oxygenated bottom water conditions during glacial times should not be generalized, but are influenced locally by productivity, sedimentation rates and sediment reworking.  相似文献   

10.
Vertical profiles of tritium in seawater were determined for samples collected during the period from 1988 to 1990 at fourteen stations in the northwestern North Pacific (the Oyashio region) including the Okhotsk Sea and the Bering Sea. The profiles usually had a maximum in the surface layer and decreased gradually with depth down to 1,000 m. The water column inventory of tritium averaged 63% of the total atmospheric input in this region.The horizontal distribution of tritium showed a maximum in the region facing the Okhotsk Sea near 45°N for every isopycnal surface of 0 ranging from 26.60 to 27.40. The ages of the intermediate water were calculated for the respective isopycnal surfaces in the maximum region. This calculation assumed that the intermediate water was formed by the isopycnal mixing of two water masses—the Okhotsk Sea and the Bering Sea Component Waters, which had been produced in wintertime by the diapycnal mixing of the surface and the deep waters in the respective marginal seas. The results show that the intermediate water in this region was formed in the late 1980's for the water which has 0 of 26.60 to 26.80 and about 1970 for the water which has 0 of 27.00 to 27.40. Although we have estimated the mean ages of the intermediate water, the horizontal profile of dissolved oxygen suggests that the Okhotsk Sea Component Water is younger than the mean age.  相似文献   

11.
In January to April, floating ice drifts from the Sea of Okhotsk via the Kuril straits to the northwestern part of the Pacific Ocean. In its turn, ice from the Bering Sea moves southwestward along the eastern coast of the Kamchatka Peninsula. The amount of the ice supplied to the ocean features a significant interannual variability. In this paper, we use the data for the period from 1958 to 2005. During the years with the heaviest ice conditions, floating ice from the Bering Sea moved to the south along Kamchatka Peninsula down the latitude of the Fourth Kuril Strait forming a tongue 30–50 miles wide. The ice drifting from the Sea of Okhotsk to the northwestern part of the Pacific Ocean may move via the passages from the Kunashir to the Kruzenshtern straits and form a band with a width of 30–40 miles depending of the severity of the winter.  相似文献   

12.
The study of foraminifers from sediments accumulated during the last 30 ky revealed their similar distribution in the central and marginal parts of the Deryugin Basin, which implies similar environmental changes throughout this morphological structure. The lack or extreme taxonomical impoverishment of the benthic foraminiferal assemblages during the Last Glacial Maximum indicates a combination of several factors: the significant deterioration of the parameters controlling their bioproductivity in response to the general cooling; the development of anoxic bottom conditions; and, probably, the unfavorable influence of cold barite-methane seeps on the geochemical parameters of the bottom waters. The weaker activity of these seeps in the central part of the basin during the Holocene is evident from some deviations in the structure of the benthic foraminiferal assemblages against the background of their general taxonomic similarity.  相似文献   

13.
《Marine Geology》2005,214(4):309-322
Gravity cores from the continental slope in the northwestern Black Sea were studied using high-resolution stable isotope, grain size and XRF-scanning data. The measurements provide a 30 000 years AMS 14C-dated record of variations in the hydrological regime of the Black Sea and give insight into changing paleoenvironments in the surrounding areas. Stable climatic conditions during the Last Glacial Maximum were followed by a series of meltwater pulses most likely originating from the Scandinavian ice sheet between 18 000 and 15 500 yr BP.1 This meltwater input rose the level of the Caspian Sea to a point that Caspian water could spill into the Black Sea via the Manych-depression north of the Caucasian mountains. High-frequency oscillations in the XRF-data during this period suggest a probable link to the arctic climate regime. Later, during the Bølling/Allerød and the early Holocene, prevailing high temperatures led to authigenic calcite precipitation through increased phytoplankton activity, interrupted by the Younger Dryas and the “8200 yr BP cold event” with dominant clastic sedimentation.  相似文献   

14.
On the recent warming of the southeastern Bering Sea shelf   总被引：1，自引：0，他引：1  
During the last decade, the southeastern Bering Sea shelf has undergone a warming of 3 °C that is closely associated with a marked decrease of sea ice over the area. This shift in the physical environment of the shelf can be attributed to a combination of mechanisms, including the presence over the eastern Bering Sea shelf of a relatively mild air mass during the winter, especially from 2000 to 2005; a shorter ice season caused by a later fall transition and/or an earlier spring transition; increased flow through Unimak Pass during winter, which introduces warm Gulf of Alaska water onto the southeastern shelf; and the feedback mechanism whereby warmer ocean temperatures during the summer delay the southward advection of sea ice during winter. While the relative importance of these four mechanisms is difficult to quantify, it is evident that for sea ice to form, cold arctic winds must cool the water column. Sea ice is then formed in the polynyas during periods of cold north winds, and this ice is advected southward over the eastern shelf. The other three mechanisms can modify ice formation and melt, and hence its extent. In combination, these four mechanisms have served to temporally and spatially limit ice during the 5-year period (2001–2005). Warming of the eastern Bering Sea shelf could have profound influences on the ecosystem of the Bering Sea—from modification of the timing of the spring phytoplankton bloom to the northward advance of subarctic species and the northward retreat of arctic species.  相似文献   

15.
A comparative analysis was conducted on climate variability in four sub-arctic seas: the Sea of Okhotsk, the Bering Sea shelf, the Labrador Sea, and the Barents Sea. Based on data from the NCEP/NCAR reanalysis, the focus was on air–sea interactions, which influence ice cover, ocean currents, mixing, and stratification on sub-seasonal to decadal time scales. The seasonal cycles of the area-weighted averages of sea-level pressure (SLP), surface air temperature (SAT) and heat fluxes show remarkable similarity among the four sub-arctic seas. With respect to variation in climate, all four seas experience changes of comparable magnitude on interannual to interdecadal time scales, but with different timing. Since 2000 warm SAT anomalies were found during most of the year in three of the four sub-arctic seas, with the exception of the Sea of Okhotsk. A seesaw (out of phase) pattern in winter SAT anomalies between the Labrador and the Barents Sea in the Atlantic sector is observed during the past 50 years before 2000; a similar type of co-variability between the Sea of Okhotsk and the Bering Sea shelf in the Pacific is only evident since 1970s. Recent positive anomalies of net heat flux are more prominent in winter and spring in the Pacific sectors, and in summer in the Atlantic sectors. There is a reduced magnitude in wind mixing in the Sea of Okhotsk since 1980, in the Barents Sea since 2000, and in early spring/late winter in the Bering Sea shelf since 1995. Reduced sea-ice areas are seen over three out of four (except the Sea of Okhotsk) sub-arctic seas in recent decades, particularly after 2000 based on combined in situ and satellite observations (HadISST). This analysis provides context for the pan-regional synthesis of the linkages between climate and marine ecosystems.  相似文献   

16.
《Marine Geology》2001,172(3-4):309-330
The link between smectite composition in sediments from the northern North Atlantic and Labrador Sea, and deep circulation is being further investigated through detailed studies of the X-ray pattern of smectites and cation saturations. This allows clear distinction of dominant terrigenous sources associated to the main components of the modern Western Boundary Undercurrent. Time variations of smectite characteristics in two piston cores from the inlet and outlet of the Western Boundary Undercurrent gyre in the Labrador Sea indicate: (1) a more southern circulation of North East Atlantic Deep Water during the Late Glacial; (2) a step by step transition to the modern pattern of deep circulation during the Late Glacial/Holocene transition, with intensification of North East Atlantic Deep Water and Davis Strait Overflow; (3) an expansion of Davis Strait Overflow and Labrador Sea Water circulation in relation to ice surges and deposition of detrital layers; (4) an intensified circulation of North East Atlantic Deep Water during the Younger Dryas; and (5) a very recent increased influence of Denmark Strait Overflow Water beginning between 4.4 and <1 kyr.  相似文献   

17.
The paper presents the records of several paleoproductivity proxies (PP) (biogenic opal and Ba (Si_bio, Ba_bio), organic carbon (C_org) and carbonate Ca_carb, chlorin and benthic foraminifera abundance (BFA)) in sediments of the Okhotsk Sea for the last 80 kyr with a resolution of ∼100–300 years. The sediment age model was based on the AMS 14C data, records of benthic foraminifera δ18O, paleointensity of the Earth’s geomagnetic field and magnetic susceptibility. PP values demonstrate series of severe prolonged productivity drops in the Okhotsk Sea followed by a sharp increase during the last glaciation. On the basis of quantitative estimations of the paleoproductivity in the Okhotsk Sea during the cold MIS 2 and warm Holocene (Gorbarenko and Goldberg, 2005), it is suggested that the millennium scale relationship in productivity-climate changes of this basin was similar: an increase in the sea’s productivity was related with regional climate warming and vice-versa. The PP records of productivity/climate oscillations in the Okhotsk Sea during MIS 2–4 occurred contemporaneously with the North Atlantic cold Heinrich events (HE) and Greenland Dansgaard-Oyeshger interstadial (DOI). Observed successions of prolonged climate cooling events followed by rapid, abrupt warming are similar to climate and environmental oscillations in the N. Atlantic and Greenland, that confirms the millennium-scale climate changes synchronicities in the Northern Hemisphere including the far NW Pacific, the hydrology and climate conditions of which are close to those of the Okhotsk Sea. Synchronism of the N. Hemisphere severe cooling (including the Okhotsk Sea) with the Antarctic warming suggests that mechanisms of the “seesaw” effect (Blunier and Brook, 2001) in the low latitude heat redistribution between high latitudes of both hemispheres were more complicated than direct NADW formation forcing and encompasses the global atmospheric reorganization. Within the PP used a closer connection in the Okhotsk Sea millennium oscillations was observed for the C_org, BFA and chlorin; Ba_bio increases more closely to DOI. Si_bio variability does not show any evident correlation with productivity changes.  相似文献   

18.
《Marine Geology》1999,153(1-4):57-75
Reconstructions with comprehensive estimates of confidence intervals are presented of changes in the W–E stable oxygen isotope gradient in Mediterranean surface waters between the Holocene Climate Optimum and the Present, and between the Last Glacial Maximum and the Present. Rigorous statistical assessment is made of the significances of the mean geographic trends observed in these reconstructions. Firstly, it is concluded that any reconstruction should strictly be based on values obtained by analyses of one single foraminiferal species throughout the basin, as different species are found to respond with isotopic variations of different amplitudes to climatic/hydrographic change. This difference is tentatively related to differences between the habitats and seasons of growth of the various species. Secondly, a significant increase of roughly a factor 3 is found in the Mediterranean W–E oxygen isotope gradient during the Last Glacial Maximum, relative to the Present. This difference is almost entirely due to increased glacial values in the Levantine Sea, which are considered to be a result of a combination of increased evaporation rates and/or somewhat cooler than anticipated surface water conditions. Thirdly, an eastward increase of roughly half the present-day magnitude is found for the W–E oxygen isotope gradient during the Holocene Climate Optimum. Values in the Levantine Sea appear to have undergone up to 0.3‰ more depletion than those elsewhere in the eastern Mediterranean. However, no significant trends are found between the eastern and western parts of the Levantine Sea, nor between values near the Nile delta and those from elsewhere in the Levantine Sea. The Holocene Climate Optimum's eastward increase in the Mediterranean oxygen isotope gradient, although weaker than the Present, suggests that the Mediterranean continued to function as a concentration basin, albeit in a less vigorous way than today. Finally, simple mixing arguments are used to argue that inferred oxygen isotope ratios of surface waters may not be used as an indication of conservative property (e.g. salinity) distribution on geological timescales, but instead show amplitudes of response to climatic/hydrographic changes that likely are >2 times larger than the corresponding amplitudes of response for truly conservative properties.  相似文献   

19.
Five assemblages of benthic foraminifers reflecting the changes in the properties of the intermediate water masses and the biological productivity during different periods of the last glacial-interglacial cycle are defined based on the abundances and relative contents of indicative species in the upper 9 m of giant Core MD02-2529 from the Cocos Ridge. High bioproductivity and low oxygen content in the bottom water layer and sediments are established for the interstadial period based on the high abundance of the species Uvigerina peregrina, U. hispida, and C. pachyderma (Assemblage I) and especially for the Last Glacial Maximum (Assemblage II) with the dominant role of the same species and E. smithi. The transition from the glaciation to the current interglacial (Termination I, Assemblage III) is characterized by a high share of the epifaunal species C. wuellerstorfi and H. subhaidingeri, which indicates enhanced hydrodynamic activity and ventilation of the intermediate water washing the bottom. The end of Termination I was marked by strengthened selective dissolution of carbonate microfossils due to the increased influx of fresh organic matter to the bottom and/or advection of more aggressive intermediate waters. The diverse Early Holocene assemblage (Assemblage IV) points to variable ecological niches, while the abundance of the infaunal U. hispida in Assemblage V indicates low productivity of the surface waters and a low oxygen concentration in the bottom sediments of the Late Holocene.  相似文献   

20.
High-resolution geochemical, isotope and elemental data from core PC23A in the northern margin of the Aleutian Basin (Bering Sea) were used to reconstruct distinct paleoceanographic features of the last deglaciation (pre-Boreal[PB], Bølling-Allerød[BA], Younger Dryas[YD]). The PB and BA intervals are characterized by increased siliceous (diatom) and calcareous (coccolithophores and foraminifers) productivity represented by high biogenic opal and CaCO3 contents, respectively. The enhanced productivity can plausibly be attributed to an elevated sea-surface nutrient supply from increased melt-water input and enhanced Alaskan Stream injection under warm, restricted sea-ice conditions. High Corg/N ratios and low δ13C values of sediment organic matter during the PB and BA intervals reflect the contribution of terrestrial organic matters. The PB and BA intervals were also identified by laminated sediment layers of core PC23A, characterized by high Mo/Al and Cd/Al ratios, indicating that the bottom water condition remained anoxic. High δ15N values during the same period were attributed mainly to the increased nutrient utilization and subsequent denitrification of seawater nitrate. Part of high δ15N values may also be due to incorporation of inorganic nitrogen in the clay minerals. It is worthy of note that high total organic carbon (TOC) deposition occurred approximately 3,000 years before onset of the last deglaciation. Simultaneous high Corg/N ratios and low δ13C values clearly suggest that the high TOC content should be related to terrestrial organic carbon input. Low δ15N values during the high TOC interval also confirm the contribution of terrigenous organic matter. Although abundant calcareous phytoplankton production under cold, nutrient-poor conditions represented by Baex data was reported for high TOC deposition preceding the last deglaciation in an earlier study of the Okhotsk Sea, the main reason for the enhanced TOC deposition in the Bering Sea is an increased terrigenous input from the submerged continental shelves (Beringia) with a sea-level rise; this is further supported by Al enrichment of bulk sediments during the high TOC deposition.  相似文献   

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