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The Arctic shelf is an important carbon reservoir, especially the Chukchi Sea, which plays a vital role in the biogeochemical processes of polar carbon cycles. In this study, we measured the contents of black carbon (BC) and total organic carbon (TOC) in a sediment core of the Chukchi Sea shelf, to evaluate the controlling factors of distribution and burial of BC and TOC. Our results show that the distribution of TOC and BC was strongly influenced by the Pacific Water (PW) inflow, resulting in nearly synchronous change since the early Holocene. The early Holocene (∼9.9 to 8.0 cal kyr BP) was characterized by the minimum of TOC, total nitrogen (TN), and BC, and the most depleted δ13C TOC and δ13C BC values throughout the Holocene, which indicate low primary productivity and low BC supplies under the weak PW flux. The carbon (TOC and BC) burial on the Chukchi Sea shelf was significantly enhanced during the middle Holocene (8.0–4.0 cal kyr BP) due to the rapid increase in PW flux. Since 4.0 cal kyr BP, PW inflow is stable and active, which enables sea ice extent to modulate the stratigraphic variations of BC. In particular, an increase in summer Arctic sea ice during this period may enhance ice-rafted debris and terrestrial carbon supply by promoting erosion of coastal peat soils and permafrost. Our results suggest that PW inflow is the first-order factor controlling Arctic carbon burial during the Holocene, while insolation-induced sea ice increase started playing a role when the PW inflow is active and stable during the late Holocene. • Different source and controlling processes of BC and TOC occurred in the Chukchi Sea shelf. • Carbon burial in the Chukchi Sea shelf was highly enhanced during the middle Holocene. • PW inflow mainly impacts the carbon burial in the Chukchi Sea shelf since the Holocene. [ABSTRACT FROM AUTHOR]

Abstract: We present a paleoclimatic reconstruction of the mid-Holocene by geochemical analysis of a sediment core MZ01 retrieved from the mud area of the inner continental shelf of the East China Sea (ECS). Our results show that the downcore geochemical variations in the sediments of the core provide a link to historical climate changes. Relatively higher ratios of CIA, Ba/Sr and a lower ratio of CaO/MgO coincide with major regional warm and humid climate episodes, and vice versa. Therefore, these geochemical indicators are useful in reconstructing the paleoclimate in eastern China. Using the geochemical records of core MZ01, which are well constrained by AMS 14C dating, we found that from 8300yr BP to 4200yr BP the regional climate was moderately warm and humid, and from 4200yr BP to 2300yr BP the climate was cool and dry, with large fluctuations between cool–dry (3700yr BP, 2850yr BP and 2400yr BP) and warm-wet (3250yr BP and 2650yr BP). Since 2300yr BP, our records indicate that the climate has gradually become warmer, although a cold event centered at around 250yr BP (the Little Ice Age, LIA) interrupted this warming trend. We also found significant 238yr cycles in our CIA records that imply a possible solar influence on the regional climate changes since the mid-Holocene. [Copyright &y& Elsevier]

Paleontological records of six sediment cores in the Okhotsk Sea (OS) marked the regional environmental changes over the last 140 kyr on orbital time scales. The diatom assemblages and content of diatom frustules in the sediments during Marine Isotope Stage (MIS) 6–1 indicate the dramatic climatic and environmental changes in the OS. A small abundance and low diatom species diversity as well as the high percentage of near-ice species indicate the cold surface environmental condition during glacial time (MIS 6, 4, 2) with low temperatures, cold climate conditions and extended sea ice cover. The presence of extinct redeposited species in the glacial assemblages indicates a low sea level during this time. The proportion of ice species enlarged and diatom abundance reduced due to increase of the influence of the sea ice, reflecting the sharp climatic cooling of adjacent land and regional environmental deterioration. The subsequent increase in diatoms productivity at 129.8–117.0 kyr BP and 8.3–5.5 kyr BP indicates the strong climate warming accompanied by decrease of sea ice coverage and surface water stratification (mixing of surface and intermediate water) during the warmest MIS in the Okhotsk Sea. The diatom abundance and high content of the oceanic and warm-water species reflect the warm surface environmental condition during MIS 5e and 1 since 8.3 kyr due to decrease of the sea ice influence. [ABSTRACT FROM AUTHOR]

In order to reconstruct the history of primary paleoproductivity changes during the last 160 kyr in the Okhotsk Sea (OS), the high-resolution down-core analyses of chlorin content were performed on 11 sediment cores obtained from various part of the studied area with wide ranged depths. Spatial and temporal distributions of the marine phytoplankton paleoproductivity depend on sea ice condition and nutrient supply. The observed discrepancies among the reconstructed records allow us to separate out three main regions with specific environmental conditions. During glacial stages the paleoproductivity was depressed and vice versa. Productivity in the central region is low comparing to other regions due to nutrient limitation. Increased paleoproductivity in the southeastern area was affected by influence of nutrient-rich Pacific waters. [ABSTRACT FROM AUTHOR]

Abstract: A diatom-based sea-ice concentration (SIC) transfer function was developed by using 72 surface samples from west of Greenland and around Iceland, and validated against associated modern SIC. Canonical correspondence analysis on surface sediment diatoms and monthly average of SIC indicated that April SIC is the most important environmental factor controlling the distribution of diatoms in the area, justifying the development of a diatom-based SIC transfer function. The agreement between reconstructed SIC based on diatoms from West Greenland and the satellite and modelled sea-ice data during the last ~75yr suggests that the diatom-based SIC reconstruction is reliable for studying the palaeoceanography off West Greenland. Relatively warm conditions with a strong influence of the Irminger Current (IC) were indicated for the early part of the record (~5000–3860cal. yr BP), corresponding in time to the latest part of the Holocene Thermal Maximum. Between 3860 and 1510cal. yr BP, April SIC oscillated around the mean value (55%) and during the time interval 1510–1120cal. yr BP and after 650cal. yr BP was above the mean, indicating more extensive sea-ice cover in Disko Bugt. Agreement between reconstructed April SIC and changes in the diatom species suggests that the sea-ice condition in Disko Bugt was strongly influenced by variations in the relative strength of two components of the West Greenland Current, i.e. the cold East Greenland Current and the relatively warm IC. Further analysis of the reconstructed SIC record suggests that solar radiation may be an important forcing mechanism behind the historic sea-ice changes. [Copyright &y& Elsevier]

The flux of low-salinity Pacific water (PW) through Bering Strait (BS) weights on the freshwater budget of the Arctic Ocean, which, in turn, impacts the Atlantic Meridional Overturning Circulation (AMOC). Through time, this PW flux has been tightly controlled by the bathymetry of the Strait (~50 m water depth at present), thus by sea-level (SL) changes, as documented in several papers based on a large array of sedimentological and geochemical tracers. The present study aimed at reassessing the robustness of these tracers of the PW-flux, based on five surface samples collected along a south–north transect from BS to the Chukchi Plateau, within the bathymetric range of the PW-mass dispersal. We also estimated their variation in relation to the post-glacial SL-rise in a sedimentary core (ARC4-R09), from the Chukchi shelf, spanning the last ~10 cal kyr. Potential indicators of PW-flux include sortable silt, clay minerals, and the particulate and exchangeable Nd-isotope composition of specific grain-size fractions. The results indicate that ɛNd-values in leachates, and to a lesser extent, the smectite abundance, both relating to volcanic rocks of the Bering Sea, are sensitive indicators of the PW-flux, whereas sortable silts and other clay minerals relative abundances are also influenced by other processes (e.g., west–east shelf currents, sea-ice rafting). The modern distributions of ɛNd-value and smectite abundance illustrate mixing between two endmembers: i) a northern Bering Sea endmember, characterized by ~14% of smectite (vs all clay minerals) and a ɛNd-value of the exchangeable fraction of ~ −2; ii) a Chukchi Sea endmember, characterized by ~8% of smectite and a ɛNd-value ranging from −7 to −8. The Holocene sedimentary record from the study core indicates a progressive increase in Bering Sea supplies through time, linked to an enhanced inflow of PW, which we associate with the deepening of the Strait due to the rising SL. Following a major reorganization of the Arctic Ocean circulation at ~8 cal kyr BP, a steady increase in PW flux prevailed until ~4 cal kyr BP. A tenuous but continuous positive trend towards modern εNd values followed during the late Holocene. The middle Holocene steep increase of warm, low salinity PW-flux through BS, led to important changes in the Arctic Ocean, in particular of its salinity budget. Assuming that the BS bathymetry is the main parameter governing the PW-flux towards the Arctic Ocean, a first-order estimate of PW-flux under a higher than modern SL can be made using the SL-elevation-ɛNd relationship of the major middle Holocene shift. • Assessment of the reliability of clay minerals and Nd-isotopes as tracers of PW-fluxes through Bering Strait. • Recording of Holocene PW-fluxes through Bering Strait by exchangeable ɛNd-values in a gravity core from the Chukchi Shelf. • Estimation of past changes in PW-fluxes towards the Arctic in relation with SL changes. [ABSTRACT FROM AUTHOR]

Abstract: Two gravity cores collected off the modern Huanghe (Yellow River) delta in the southern Bohai Sea were analyzed for grain size, the total organic carbon (TOC)/total nitrogen (TN) ratio, color diffraction, magnetic susceptibility, 14C dating and 137Cs and 210Pb isotope contents to clarify changes in the sedimentary environment during the Holocene. In particular, the effect of natural and artificial river-course shifts of the Huanghe on the Bohai Sea sediment was investigated. A peat layer, scouring surface and sharp changes in the grain size, TOC/TN ratio, sediment color (L ⁎, a ⁎) and magnetic susceptibility were identified and are likely to be due to the early-Holocene sea-level rise resulting in environmental changes from coastal to shelf environments in the Bohai Sea. After the sea level reached its maximum at 6–7kaBP, the lateral shifts in the river course of the Huanghe formed 10 superlobes, and superlobe 7 (11–1048 AD) and superlobe 10 (1855–present) of the Huanghe delta affected the core sites. The northern site of BH-239 has been more affected by the Huanghe since the middle Holocene. Notably, in the superlobe 10 period, the reshaping of the northern Huanghe delta due to an artificial river-course shift from northward to eastward in 1976 (e.g., a∼10km shoreline retreat due to coastal erosion) was recorded in the core sediments, particularly in terms of the TOC/TN ratio, sediment color (L ⁎ and a ⁎) and magnetic susceptibility, owing to the huge sediment supply from coastal erosion of the former river mouth area. [Copyright &y& Elsevier]

This study investigates Holocene sedimentary evolution and hypoxia development using borehole cores CJK06 and CJK09, in combination with other published core data. Based on lithology and microfossil (benthic foraminifera) characteristics, seven types of sedimentary facies were identified from the base upward: river, tidal flat, tide-influenced river, transgressive lag, estuary, inner-shelf, and prodelta. Isochronous correlation among the cores was established relying on accelerator mass spectrometry 14C dates. Facies succession indicates that a weak progradation occurred in coastal environments (12–10 ka BP) due to the Younger Dryas-induced deceleration of sea-level rise; rapid deposition locally occurred in the southern marginal area of tide-dominated estuary environments (10–8 ka BP), likely due to the junction of the Yangtze and Qiantangjiang River currents; and marine current-induced fluid mudflows prevailed in the shallow marine environments (8–1 ka BP), with the cooling climates of 5–1 ka BP. Furthermore, prodelta and shallow marine environments co-occurred with an uneven progradation of the delta within the last 1 ka, while deposition occurred just inside the more southern, delta-influenced area. The occurrence of euryhaline benthic foraminifera suggests that an enhanced freshwater discharge of the mid-Holocene (8–5 ka BP) was followed by a sharp decline in the late Holocene (5–1 ka BP) with climate change. The occurrence of cold-water benthic foraminifera indicates a severe cold-water condition during the mid-Holocene due to the intrusion of upwelling currents triggered by the propelling force of warm currents and summer monsoon winds. In addition, the occurrence of low-oxygen foraminiferal assemblages reveals that hypoxia has developed since 10 ka BP in stages consistent with the sedimentary evolution: localized hypoxia formation coincident with the southern depocenter during the early Holocene (10–8 ka BP); severe hypoxia with enhanced freshwater discharge and upwelling current intrusion during the mid-Holocene; and weakened hypoxia of the late Holocene, mainly due to the sharp decline in freshwater discharge. Within 1 ka BP, freshwater discharge from the approaching river mouth and increased nutrient inputs from enhanced human activities on land have contributed to the prevalence of hypoxia, especially in the southern deltaic area. Overall, it was revealed that the freshwater discharge modulated with climate variations and delta progradation plays a primary role in controlling the sedimentary evolution and hypoxia development during the mid-late Holocene. • Gravity flows (fluid mudflow) prevailed during the late Holocene (5–1 ka BP). • Holocene freshwater discharge was enhanced (8–5 ka BP) and then sharply declined. • Severe upwelling current intrusion occurred during the mid-Holocene (8–5 ka BP). • Hypoxia boomed during the mid-Holocene (8–5 ka BP). [ABSTRACT FROM AUTHOR]