Central Okhotsk Sea Research Articles

The Interannual Variability of Sea Ice Area, Thickness, and Volume in the Southern Sea of Okhotsk and Its Likely Factors

AbstractThe lowest latitude sea ice in the world (excluding coastal freezing) is in the southern Sea of Okhotsk (south of 46°N), where it has significant impacts on freshwater input and primary production. This region is subject to climate change, and accordingly the monitoring of sea ice conditions is important. However, the interannual variability of the region's sea ice is poorly understood due to its logistical challenges. Sea ice observations have been conducted in this region every winter for the period 1996–2020. The interannual variability of the ice conditions and the likely factors responsible for it were investigated using visual observations following the international ASPeCt protocol, combined with satellite SSM/I‐SSMIS ice concentration data (1988–2020). AMSR‐derived ice drift data sets and ERA5 meteorological reanalysis data sets were also analyzed to examine the effects of dynamic and thermodynamic processes. Our analysis revealed that (a) sea ice area in this region varies differently from that in the central and northern Sea of Okhotsk, where decreasing trends are reported, (b) sea ice volume has remarkable interannual variation and the peaks appeared much to more affected by dynamically deformed ice than freezing conditions, and (c) prominently deformed ice can be explained by taking shear components into account based on sea ice rheology. These results suggest the importance of including the proper sea ice rheology in numerical sea ice models to reproduce the realistic sea ice volume and deformation processes, for all seasonal ice zones.

Controls on Terrigenous Detritus Deposition and Oceanography Changes in the Central Okhotsk Sea Over the Past 1550 ka

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Spatial Distribution and Provenance of Detrital Minerals of Surface Sediment in the Okhotsk Sea

The Okhotsk Sea is a distinctive marginal sea in the northwestern Pacific Ocean, which is characterized by the prevalence of seasonal sea ice in winter. Sediment is sourced from the around region through sea ice transportation, rivers input, and volcanic eruptions. Surface sediments of the Okhotsk Sea shelf vary greatly in grain size, and the sand content is generally high, which is conducive to source-to-sink studies using ice-rafted detritus (IRD),detrital minerals and single mineral geochemistry methods. In this paper, the 63–125 μm grain size fraction was selected for the detrital minerals analysis of surface sediments (top 0–10 cm) from 58 sediment stations and 15 stations samples has been chosen for garnet chemistry. These stations are mainly located in the south central Okhotsk Sea. The distribution and composition of the heavy minerals are influenced by material derived from the Amur River, the north shelf (Okhotsk-Chukotka volcanic belt), Sakhalin Island, the Kamchatka Peninsula, and the Kuril Islands. The detrital mineral results show that hornblende, epidote and garnet are terrigenous material indicators. High contents of fresh hypersthene can be used as an indicator of volcanic eruption materials. And high content of abraded hypersthene can be used as an indicator of Okhotsk-Chukotka volcanic materials. In the northern Okhotsk Sea, the southward moving sea ice produced in Sakhalin Bay collides with the sea ice produced off the east coast of Sakhalin Island, which causes the sea ice to accumulate to the southeast. This results in the deposited ice-rafted debris having a southeastward facing fan shape, and the geochemical analysis of the garnet supports this conclusion. From west to east, the amount of material from Sakhalin Island gradually decreases, while the amount of material from the Amur River and Chukchi-Kamchatka increases.

Orbital and millennial-scale environmental and hydrological changes of the central Okhotsk Sea over the last 136 kyr inferred from micropaleontological (radiolarian and benthic foraminifera), geochemical and lithological proxies and the mechanisms responsible for them

The response of benthic foraminifera (BF) and radiolarian production in the central Okhotsk Sea (OS) to orbital and millennial changes of climate and environment over the last 136 kyr have been studied on the basis of the sediment core PC-7R, with available productivity, geochemical and lithological records and a modified age model. We used the calculated accumulation rate of microfossils (AR, fluxes) of the main ecological groups and species indicative of radiolarians and of BF as quantity responses in their production, which is related to environmental and hydrological changes of the water column and surface sediment in the central OS and to global and regional climate changes. The responses of the opportunistic BF Alabaminella weddellensis and the suboxic group during the last 136 kyr were mostly related to changes in organic fluxes from the surface, sea ice cover and surface sediment oxygenation at both orbital and millennial time scales. The AR of radiolarians from the surface, dichothermal and mesopelagic groups significantly differed from the AR of BF. The AR has a maximum during the middle and late Marine Isotope Stage 5e and lower values during the earlier Holocene, while the minimum happened during cold Marine Isotope Stages 6, 4 and 2 with maximal ventilation of OS intermediate water. Production of the mesopelagic radiolarian Cycladophora davisiana in the OS was mostly related to the lateral delivery of terrigenous material and OM into the water column from the northwestern shelf and/or from the western subarctic Pacific. Due to the sensitivity of marine organisms to environmental conditions, the production of radiolarians and BF sharply changed with the extinction of Amphimelissa setosa, since Heinrich Stadial 8 is related to the termination of global environment cooling and the hydrology changes of the early last glacial.

Negative δ13C Excursions in Foraminiferal Records: The Holocene History of Methane Events in the Central Sea of Okhotsk

Abstract—Several negative δ13C excursions in benthic foraminifera from gas-bearing core LV50-05 sampled offshore on the eastern slope of Sakhalin Island, Sea of Okhotsk, in an area of active methane seepage record the local history of methane events (ME). The core chronostratigraphy has been contrained from AMS 14C ages and biostratigraphic data. Benthic foraminifera (Nonionellina labradorica and Uvigerina parvocostata) from some core intervals show normal marine δ13C values about –1‰ but some intervals are marked by extremely depleted compositions as low as –34.5‰ δ13C (relative to VPDB). The negative δ13C excursions are interpreted as a record of seabed methane emanation during primary and secondary biomineralization of carbonate foraminifera. The results reveal four Holocene methane events (ME) in the area: two brief (ME-1 at 700–900 yr BP and ME-2 at 1200–1400 yr BP) and two long (ME-3 at 2500–4700 yr BP and ME-4 at 7400–10000 yr BP) events.

A multiproxy study of past environmental changes in the Sea of Okhotsk during the last 1.5 Ma

Long-chain diols have been detected in a wide range of environments and have been used to reconstruct past environmental changes, however only a few long-term records exist to date. Here we reconstructed past environmental changes in the central Sea of Okhotsk over the last 1.5 million years, covering the Mid-Pleistocene Transition (MPT). Sea surface temperatures (SST) reconstructed using the Long-Chain Diol Index (LDI) reflects glacial/interglacial changes. However, when compared with other organic paleothermometers (Uk′37 and TEXL86) the LDI-SST is lower during interglacials and similar or higher during glacials, possibly suggesting a shift of diol production season during interglacials. The LDI-SST does not change in periodicity around the MPT as observed for the TEXL86, likely due to this seasonal shift. Diatom productivity, as recorded by 1,14-diols and biogenic opal content, increased during the main deglaciations with a succession from Proboscia diatoms to diatoms with a more heavily silicified shell, confirming that primary productivity in the central Sea of Okhotsk is driven by sea-ice progress and retreat. In contrast to the LDI-SST, the 1,14-diols record shows a change in periodicity around the MPT from 41- to 100-kyr cycle, suggesting an influence of orbital parameters on diatom productivity. In the central Sea of Okhotsk, the relative amount of C32 1,15-diol (FC32 1,15), a proxy for riverine input, correlates with sea-level change with more riverine-derived material reaching the core site when the Amur River mouth is closer at lower sea-levels. In agreement, FC32 1,15 shows a change in periodicity during the MPT, with the appearance of a 100-kyr cycle. Our results show that the long chain diols can provide important paleoceanographic information in subpolar environments over long time scales, but that temperature reconstructions can be severely impacted by changes in seasonality.

Large-scale habitat segregation of fish-eating and mammal-eating killer whales (Orcinus orca) in the western North Pacific

Top predators strongly impact the structure of ecosystems through the top-down cascading effects on prey species. Killer whales (Orcinus orca), the top predators in marine ecosystems, are increasing their presence in the Arctic following the ice cover loss. The impact of killer whales on marine ecosystems differs dramatically across ecotypes: ‘resident’ R-type killer whales feed mostly on large fish, while ‘transient’ T-type whales feed mostly on marine mammals. We analyze the differences in geographical distribution of killer whale ecotypes in the western North Pacific in order to predict the potential effects of climate change on their range and the consequential impact on Arctic ecosystems. We show that R-type whales prevail in the coastal waters of eastern Kamchatka, Commander and Kuril Islands and in the central Okhotsk Sea, while T-type whales dominate the coastal waters of Chukotka and the coastal Okhotsk Sea. The most prominent difference between these areas is depth: in the regions where R-type whales prevail, deep waters occur close to shore, while the regions dominated by T-type whales are represented by wide shallows covered with ice in winter. We propose several hypotheses to explain this large-scale segregation, including distribution of prey species and ice cover. Habitat preferences suggest that range expansion in the Arctic induced by climate change will likely involve mostly T-type killer whales and consequently increase the predation pressure on marine mammals to a greater extent than on fish stocks. This should be considered in further studies and prediction models highlighting Arctic marine ecosystems change.

A Comparison of Late Quaternary Organic Proxy-Based Paleotemperature Records of the Central Sea of Okhotsk.

The long‐chain diol index (LDI) is a new organic sea surface temperature (SST) proxy based on the distribution of long‐chain diols. It has been applied in several environments but not yet in subpolar regions. Here we tested the LDI on surface sediments and a sediment core from the Sea of Okhotsk, which is the southernmost seasonal sea ice‐covered region in the Northern Hemisphere, and compared it with other organic temperature proxies, that is, U37k′ and TEXL 86. In the surface sediments, the LDI is correlated with autumn SST, similar to the U37k′ but different from the TEXL 86 that correlates best with summer sea subsurface temperature. Remarkably, the obtained local LDI calibration was significantly different from the global core‐top calibration. We used the local LDI calibration to reconstruct past SST changes in the central Sea of Okhotsk. The LDI‐SST record shows low glacial (Marine Isotope Stage, MIS 2, 4, and 6) and high interglacial (MIS 1 and MIS 5) temperatures and follows the same pattern as the U37k′‐SST and a previously published TEXL 86 temperature record. Similar to the modern situation, the reconstructed temperatures during the interglacials likely reflect different seasons, that is, summer for the TEXL 86 and autumn for U37k′ and LDI. During glacials, the reconstructed temperatures of all three proxies are similar to each other, likely reflecting summer temperatures as this was the only season free of sea ice. Our results suggest that the LDI is a suitable proxy to reconstruct subpolar seawater temperatures.

Precession and atmospheric CO2 modulated variability of sea ice in the central Okhotsk Sea since 130,000 years ago

Recent reduction in high-latitude sea ice extent demonstrates that sea ice is highly sensitive to external and internal radiative forcings. In order to better understand sea ice system responses to external orbital forcing and internal oscillations on orbital timescales, here we reconstruct changes in sea ice extent and summer sea surface temperature (SSST) over the past 130,000 yrs in the central Okhotsk Sea. We applied novel organic geochemical proxies of sea ice (IP25), SSST (TEXL86) and open water marine productivity (a tri-unsaturated highly branched isoprenoid and biogenic opal) to marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth 1123 m). To complement the proxy data, we also carried out transient Earth system model simulations and sensitivity tests to identify contributions of different climatic forcing factors. Our results show that the central Okhotsk Sea was ice-free during Marine Isotope Stage (MIS) 5e and the early-mid Holocene, but experienced variable sea ice cover during MIS 2–4, consistent with intervals of relatively high and low SSST, respectively. Our data also show that the sea ice extent was governed by precession-dominated insolation changes during intervals of atmospheric CO2 concentrations ranging from 190 to 260 ppm. However, the proxy record and the model simulation data show that the central Okhotsk Sea was near ice-free regardless of insolation forcing throughout the penultimate interglacial, and during the Holocene, when atmospheric CO2 was above ∼260 ppm. Past sea ice conditions in the central Okhotsk Sea were therefore strongly modulated by both orbital-driven insolation and CO2-induced radiative forcing during the past glacial/interglacial cycle.

Changes in detrital input, ventilation and productivity in the central Okhotsk Sea during the marine isotope stage 5e, penultimate interglacial period

By presenting benthic foraminifera isotope profiles and bulk geochemical composition of core sediments, we offer a mutiproxy reconstruction of the central Okhotsk Sea oceanography between ∼130 to and ∼115 kyr, related to the marine isotopic stage 5e (MIS 5e). Sediments from the site MR0604-PC7A have been compared with paleo- sea surface temperatures and other records. This multiparameter approach allowed to recognize three periods for MIS 5e evolution, characterized by variations in marine productivity and bottom oxygenation. These variations have been ultimately associated with the production of Okhotsk Sea Intermediate Water and the presence or absence of nutrient contribution from the Pacific Deep Water into central Okhotsk Sea. Aeolian input reconstructed by Th/Sc ratio indicates higher values during the MIS 6 and MIS 5d compared with the MIS 5e. Eu∗ values indicate an input of detrital material from Kurile Islands and East-Kamchatka during the late MIS 5e, absent during the early MIS 5e and glacial periods.

Presence of ribbon seal vocalizations are related to sea ice extent in the Nemuro Strait, the Okhotsk Sea

Vocalizations of ice-breeding ribbon seals Histriophoca fasciata were recorded using underwater passive acoustic methods from November 2012 to March 2014 off the Nemuro Strait, Japan. Seal presence in the strait was examined in relation to the sea ice extent in the Okhotsk Sea. Ribbon seal downsweeps were only detected when sea ice was present in the strait (February and March), with more detections in March leading up to the spring breeding season. Since ribbon seals require ice for breeding, underwater communication for breeding could be needed during the sea ice presence in this strait. Northeasterly winds were another indirect driver of ribbon seal occurrence, since winds from this direction likely transport sea ice from the central Okhotsk Sea into the Nemuro Strait. Downsweep detections decreased in the middle of the day, which is consistent with observations that seals hauled out on the ice during this time, and thus were producing fewer underwater vocalizations. Our results suggest that a decrease in the sea ice extent in the Okhotsk Sea may change the distribution of ribbon seals and impact their breeding behaviour since the Nemuro Strait region of the Okhotsk Sea is likely the southern limit of their breeding range.

Geochemical multielement signatures of glacial and interglacial facies of the Okhotsk Sea deepwater sediments during the past 350 kyr: A response to global climate changes at the orbital and millennial scales

AbstractThe previously dated deepwater sediment core MR06‐04 PC‐7R (length 1723 cm; 350 kyr) recovered from the central Okhotsk Sea (OS) was analyzed for biogenic compounds and for 63 chemical elements (using the inductively coupled plasma–mass spectrometry method) with a high resolution (1 cm; ~200 years). A one box model with two main members” and mathematical methods (based on multielemental composition of sediments) for the calculation of weight fractions (at each time slice) of two main types of geochemical facies that dominate during considerably diverse climatic periods (glacial maxima and interglacial optima) were proposed and tested. This model can be applied to other analogous natural systems whose sedimentation is driven by two main types of geochemical facies. The application of the developed model to the studied core revealed that variations of weight fractions of the typical interglacial and glacial geochemical facies in the sediments along the core depth (named as warm and cold “covariators,” respectively) change synchronously with global and regional climate variability. Profiles of warm and inversed cold covariators coincide tightly, and their values increase during warm marine isotope stages and substages and decrease during cold ones over the last 350 kyr. Millennial scale changes in covariators had occurred simultaneously with abrupt variability in the OS productivity and sediment lithology and with millennial global climate variability. Some discrepancies in the warm and inversed cold covariators calculated using specific mathematical treatments revealed the episodic influence of volcanogenic matter presented in the core by visible tephra layers and cryptotephras.

Sea‐ice conditions in the Okhotsk Sea during the last 550 kyr deduced from environmental magnetism

Reconstructing past sea‐ice conditions in the Okhotsk Sea is important because sea‐ice conditions vary in response to global climate changes, which in turn may affect global ocean circulation through intermediate water mass formation. We conducted an environmental magnetic study of six cores from three stations in the central Okhotsk Sea to better understand temporal and spatial sea‐ice variations. Intercore correlations and age estimations are based mainly on geomagnetic paleointensity; an oxygen‐isotope stratigraphy is available for one station. Magnetic susceptibility (MS) minima are accompanied by maxima in color b*, the ratio of the anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), and the S‐ratio, which indicates a higher proportion of biogenic to terrigenous magnetic components. This reflects enhanced ocean productivity. First‐order reversal curve diagrams and IRM component analyses support the dominance of biogenic magnetite at MS minima. In contrast, color b*, kARM/SIRM, and S‐ratio values are low when MS is high, which indicates an increased proportion of the terrigenous component that was probably transported as ice‐rafted debris (IRD). For the southern two stations, IRD accumulation increased in glacial and deglacial periods, which implies mobile sea‐ice conditions even in full glacials. This was succeeded by extremely enhanced ocean productivity in early interglacials, which suggests nearly ice‐free conditions. For the northernmost station, on the other hand, IRD accumulation was low in glacials and increased in early interglacials, which indicates perennial sea‐ice coverage with little mobility in glacials. Succeeding ocean‐productivity enhancement was delayed compared to the southern stations.

Chronicle of regional volcanic eruptions recorded in Okhotsk Sea sediments over the last 350 ka

Recovered from the central Okhotsk Sea, the dated marine sediment core MR PC-7R was used to reconstruct the history of volcanic eruptions taking place over the last 350 ka in the Kuril–Kamchatka area, one of the most active volcanic regions in the world. Three visible tephra layers previously identified in Okhotsk Sea sediments were identified in this core. A novel approach was applied to detect traces of tephra fallout from ancient eruptions, namely cryptotephras, based on the elemental content of bulk sediment and a new lithological index. The concentrations of 63 elements in 1723 samples of bulk sediment sampled every 1 cm throughout core length were measured via ICP-MS analysis, with an average resolution time of 200 years. Factor analyses of these 63 elements plus 2 further variables enabled the identification of Factor 1 elements, which had lowered content in tephra, and those in the Factor 2 group, with elevated content in tephra. A Volc/Ter ratio determined under a binocular microscope by means of visual assessment of volcanic and terrigenous grains in the >63 μm sediment fraction was used as an independently measured index of cryptotephra occurrence. The factor analysis and Volc/Ter ratio were then employed to facilitate the detection of cryptotephras in the sediments, with 31 detected in addition to the three visible tephra for the last 350 ka. The previously constructed age model for the core, based on marine isotope stages/sub-stages and millennial-scale cold events, allowed us to determine the age and stratigraphy of the newly-identified cryptotephras and therefore also of ancient regional eruptions.

Response of bottom environments in the central sea of Okhotsk to orbital- and millennial-scale climate changes during the last 130000 years according to benthic foraminifera

Response of bottom environments in the central sea of Okhotsk to orbital- and millennial-scale climate changes during the last 130000 years according to benthic foraminifera

Creation of a Heat and Salt Flux Dataset Associated with Sea Ice Production and Melting in the Sea of Okhotsk

Abstract Sea ice formation, its transport, and its melting cause the redistribution of heat and salt, which plays an important role in the climate and biogeochemical systems. In the Sea of Okhotsk, a heat and salt flux dataset is created in which such sea ice processes are included, with a spatial resolution of ~12.5 km. The dataset is based on a heat budget analysis using ice concentration, thickness, and drift speed from satellite observations and the ECMWF Interim Re-Analysis (ERA-Interim) data. The salt flux calculation considers both salt supplied to the ocean from sea ice production and freshwater supplied when the ice melts. This dataset will be useful for the validation and boundary conditions of modeling studies. The spatial distribution of the annual fluxes shows a distinct contrast between north and south: significant ocean cooling with salt supply is shown in the northern coastal polynya region, while ocean heating with freshwater supply is shown in the south. This contrast suggests a transport of freshwater and negative heat by ice advection. The annual fluxes also show ocean cooling with freshwater supply in the Kashevarov Bank (KB) region and the central and eastern Sea of Okhotsk, suggesting the effect of warm water advection. In the ice melt season, relatively prominent ice melting is shown in the coastal polynya region, probably due to large solar heating of the upper ocean. This indicates that the polynya works as a “meltwater factory” in spring, contrasting with its role as an “ice factory” in winter. In the coastal polynya region, the spatial distribution of phytoplankton bloom roughly corresponds with the ice melt region.

Geological structure of the Kashevarov Trough (central Sea of Okhotsk)

This paper reports the composition and age of rocks dredged from the Kashevarov Trough (central Sea of Okhotsk) during cruise 41 of the R/V Akademik M.A. Lavrentyev in 2006. It was found that the Late Cretaceous and Eocene volcanics from the Kashevarov Trough and Okhotsk-Chukotka volcanic belt, struc� tures of which are traceable in the Sea of Okhotsk, have similar petrographic and geochemical features. The Cenozoic sedimentary cover consists of three differentag e complexes: (1) the late Oligocene (~28.2-24.0 Ma); (2) the terminal late Oligocene-early Miocene (24.0-20.3 Ma); (3) the terminal late Pliocene-early Pleis� tocene (2.0-1.0 Ma). The upper Oligocene-lower Miocene sediments were deposited in relatively shallow� water settings, whereas the late Pliocene-early Pleistocene complex was formed in deeper environments, which was probably determined by tectonic processes. The geological data indicate that the Kashevarov Trough and the surrounding underwater rises represented in the Oligocene-early Miocene a single shelf zone of the Sea of Okhotsk, which is underlain by a structurally integral Mesozoic basement and is now subsided to depths of 800-1000 m.

Characteristics and ages of tephra layers in the central Okhotsk Sea over the last 350 kyr

A sequence of 9 ash layers (tephra) found in the Middle-Late Pleistocene and Holocene sediments of the central Okhotsk Sea was established on the basis of glass chemical composition and refractive indices, grain-size distributions and heavy mineral assemblages. Five tephras were also characterized by rare earth element distributions. Volcanic sources and ages of the identified tephra were estimated based on pre-existing marine sediment core chronostratigraphy and correlation between these marine tephra deposits with geochemically similar terrestrial deposits on the Kamchatka Peninsula and adjacent Kurile Islands. Areal distributions and approximate eruptive volumes of pyroclastic material were also estimated for some of these tephras.

Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350 kyr

We measured productivity proxies (chlorin, carbonate and organic carbon, opal, and biogenic Ba content) and lithophysical proxies (magnetic susceptibility, water content, density, and coarse sediment fraction) in sediment of central Okhotsk Sea core PC-7R. The age model covering the last 350kyr of this core was constructed by correlating the dated series of relative paleointensity lows recognized in geomagnetic intensity records, marine isotope stage (MIS) boundaries determined in broad variations of the lithophysical and productivity proxies, and tephrochronology. The orbital changes of the lithophysical and productivity proxy stacks lag behind Northern Hemisphere summer radiation by approximately 6.3 and 5.9kyr, respectively. This lag is consistent with a Milankovich model of climate control by solar radiation through the northern ice sheet volume and sea surface and surrounding land responses, which are fast compared with sedimentological evidence. Productivity proxies of the Okhotsk Sea also demonstrate 52 abrupt, pronounced productivity minima associated with regional climate coolings during the last 350kyr, which present useful indicators of millennial-scale climate changes in this marginal sea. Based on the postulated synchronicity of Dansgaard–Oeschger cycles in the Northern Hemisphere and the established simultaneity of 11 Okhotsk Sea coolings in the last 77kyr with abrupt severe cold events in the Greenland ice core and North Atlantic Heinrich events, all of these may be regarded as Heinrich-equivalent event anomalies. The Okhotsk Sea events have their counterparts in the records of North Atlantic sediments, the Greenland ice sheet, East Asia summer monsoon, and the Antarctic ice sheet. Probably the Arctic Oscillation was the main factor determining orbital and millennial climate oscillations in the high-latitude Northern Hemisphere, including the Okhotsk Sea region.

Magnetostratigraphy of marine sediment core MD01-2414 from Okhotsk Sea and its paleoenvironmental implications

This study used a paleomagnetic method to analyze the marine sediment core, called MD01-2414, taken from the Okhotsk Sea during the IMAGES VII cruise in 2001. The results indicate that the Brunhes/Matuyama boundary appeared at the depth of 27.76 m, with the age of 0.78 Ma. The lower and upper bounds of the Jaramillo event are located at 36.96 and 34.74 m depth, respectively. Cobb mountain event is found at a depth between 40.67 m and 41.06 m. Close to the bottom of the core, normal polarity is observed; it is proposed to be the termination of the Olduvai normal event. This event implies that this core signifies an age interval of about 1.8 Ma. Excursions could be identified from the paleomagnetic directional record, such as Kamikatsula, Santa Rosa; the relative paleo-intensity secular variation pattern also helped to delimit magnetic excursions in the Brunhes normal epoch, e.g. Iceland Basin event, Big Lost event, etc. Thus, the age model of this core can be well established. Based on the age model, the variations of magnetic susceptibility and ARM/χ of this core indicated that highly abundant but relatively coarse grain of magnetic minerals appeared in the even stages of oxygen isotope chronology, while low abundance with finer grain appeared in the interglacial periods. This suggests that there are different sources and transportation pathways of the sediments deposited in the area studied during glacial and interglacial periods. During the glacial time, terrigenous sediments containing abundant magnetic minerals eroded by the ice river from the Kamchatka peninsula were brought into the central Okhotsk Sea by the West Kamchatkan current (WKC), while the central and western parts of the Okhotsk Sea were completely covered by sea ice. During the interglacial time, the WKC and the East Sakhalin current (ESC) brought lots of terrigenous sediments from the north and west (the Siberia continent), which contain less abundant magnetic minerals and dilute the signal of magnetic susceptibility.