Magnetic Properties Of Sediments Research Articles

Influence of Seasonal Post‐Depositional Processes on the Remanent Magnetization in Varved Sediments From Glacial Lake Ojibway (Canada)

AbstractThe natural remanent magnetization (NRM) of high sedimentation rate sediments provides significant information about paleomagnetic secular variation of the Earth's magnetic field and can also potentially be used for stratigraphy. However, NRM acquisition depends on conditions inherent to the depositional environment. In addition to recording a precise annual chronology, varved sediments reflect marked annual sedimentary changes. The Earth's magnetic field does not vary significantly over such a short period, so magnetic changes recorded by varves are expected to reflect the influence of depositional parameters on the recording process. We focus here on a sequence of 27 ± 1 varves from the former proglacial Lake Ojibway (∼8.5 ka cal BP) from which individual cm‐thick summer and winter beds were sampled. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. A mean inclination shallowing of 24.3° is observed in winter beds, along with an 11.3° shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20° occur between successive beds. Summer beds are thicker than winter beds and have stronger magnetic susceptibility, higher Ca/Fe ratios and coarser sedimentary and magnetic grains. This grain size pattern reflects the input of coarser detrital particles during summer, while the finer fraction remained in suspension until it was deposited in winter. A combination of differential compaction between the winter and summer beds, seasonally varying physical and magnetic properties of sediments, and delayed NRM acquisition explains the variable and coercivity‐dependent inclination shallowing.

Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey

The anaerobic oxidation of methane, a key geochemical process that is involved in the cycling of sulfate and iron (oxyhydr)oxides in marine sediments, results in the formation of iron sulfides. Although ferrimagnetic iron sulfides have been identified in seepage systems, the link between iron migration and sediment magnetic properties remains poorly understood. Here, we investigate two cores from the Sea of Marmara to evaluate biogeochemical iron cycling and iron sulfide mineralogy in gas hydrate-bearing sediments. Magnetic analyses indicate the presence of greigite and pyrrhotite in a core from a hydrate-rich site with a high hydrocarbon flux, which contrasts with a lack of these minerals in a core characterized by only mild seepage. This is supported by the results of rock magnetic and scanning electron microscope analyses of the sediments. The presence of authigenic greigite is critical for assessing local redox records and together with the occurrence of monoclinic pyrrhotite may suggest specific diagenetic processes in gas hydrate environments. Our analysis demonstrates the usefulness of these ferrimagnetic minerals, with a high saturation isothermal remanent magnetization to magnetic susceptibility ratio (SIRM/χ > 15 kAm−1) and a high index of hysteresis parameters (DJH > 0.2) indicative of magnetic mineralogy changes, for evaluating variability in the intensity of seepage fluxes and for estimating gas hydrate distributions.

Magnetic properties of surface soils in the upper and middle reaches of the Yarlung Zangbo River Basin, southern Tibetan Plateau, and their environmental significance

SUMMARY The relationship between climate and the magnetic properties of surface soils on the Chinese Loess Plateau (CLP) have been widely used to provide the basis for palaeoclimatic reconstruction, based on the underlying loess/palaeosol sequences. To date, however, there are few investigations of variations in the magnetic properties of surface soils on the Tibetan Plateau (TP), especially on the southern TP. Therefore, it remains unclear whether magnetic properties could be used as proxies for palaeoclimatic reconstruction in the region. In this study, environmental magnetic parameters and bulk sediment grain-size measurements were made on a set of surface samples from the Yarlung Zangbo River Basin (YZRB), on the southern TP, and their environmental significance was evaluated. The results reveal spatial differences in the magnetic properties of the surface soils, likely caused by regional climatic factors. The input of primary magnetic minerals is the driver of the magnetic properties of the samples from the western (Gar-Saga) part of the YZRB, where the magnetic variations are controlled by the influence of wind intensity on the local source material. However, the samples from the eastern region (Gongga-Nyingchi) are significantly affected by the topographic and sedimentary conditions, and there is no relationship between the magnetic properties and climate. The samples from the central part of the study area (Saga-Gongga) show obvious signs of a pedogenic influence on magnetic properties and ultrafine superparamagnetic (SP) and single domain (SD) ferrimagnetic minerals dominate the magnetic susceptibility. Combined with the variation of grain size, this implies the influence of wind intensity and pedogenesis contribute to the magnetic enhancement. However, the pedogenic intensity closely related to precipitation and temperature could offer the possibility of using sedimentary magnetic properties for palaeoclimatic reconstruction. Furthermore, the existence of a distinct pedogenic alteration boundary in the Saga area is consistent with the location of the Indian summer monsoon, which may be the dominant control on the pedogenic intensity.

Differentiating detrital and pedogenic contributions to the magnetic properties of aeolian deposits in the southern Tibetan Plateau: Implications for paleoclimatic reconstruction

Aeolian deposits in the Yarlung Zangbo River (YZR) valley in the southern Tibetan Plateau (TP) have recorded the evolution of past climate and atmospheric circulation in the “Asian water tower” region. However, the paleoclimatic history of the region remains controversial, largely because of ambiguous proxy environmental indicators. Here, we conducted the first detailed investigation of the environmental magnetic properties of two Holocene aeolian sedimentary sequences in the valley. Combined with geochemical and sediment granulometry data, we clarify the magnetic enhancement mechanisms and the paleoclimatic implications of the magnetic parameters. The results reveal spatial differences in magnetic properties, which makes sense in relation to the climatic gradient in this region. Due to the stronger Indian summer monsoon (ISM), ferrimagnetic and imperfect antiferromagnetic minerals (both coarse and fine-grained) dominate the sedimentary magnetic properties in the middle reaches of the YZR, suggesting both detrital and pedogenic origins. Hence, although both wind activity and pedogenesis cause the observed magnetic enhancement, magnetic parameters sensitive to the presence of fine magnetic grains can be used as paleo-moisture and/or temperature proxies. With decreasing ISM precipitation, the sedimentary magnetic properties in the upper reaches of the YZR are dominated by coarse ferrimagnetic grains, indicating the dominant influence of detrital input. Hence, the variations in sedimentary magnetic properties are caused by changes in paleo-wind intensity, given the constant material source. Accordingly, we propose a conceptual model that explains the spatiotemporal variations in dust accumulation and magnetic properties in this region. Our results provide a sound basis for understanding the sedimentary magnetic properties and their application in paleoclimatic reconstruction in the southern TP.

Variation of sediment magnetic properties and their paleoenvironmental implications in Yangtze River Delta during 30–50 ka B.P.

Identifying the variability in and driving forces of relative sea-level (RSL) change in the Yangtze River Delta during 30–50 ka B.P. is vital for understanding global climatic and RSL changes. In this study, the environmental magnetism of sediments in the Yangtze River Delta was systematically studied for the period 30–50 ka B.P. based on the optically stimulated luminescence (OSL) dates. Detailed magnetic measurements, including high-temperature κ-T, magnetic hysteresis loop, isothermal remanent magnetization acquisition, and demagnetization curve measurements illustrate that the predominant magnetic contribution of core YZ07 is from magnetite, with a small contribution from hematite. Detrital input may be the primary source of magnetic minerals according to the χ/χfd% and χARM/χ correlations. In addition, the positive relationship between χ and the RSL curve indicates that χ is a valid indicator of RSL change. Additionally, comparing χ, RSL curves, and δ18O dates of the stalagmites demonstrated that climatic change influenced the RSL change, which mainly affected the concentration of magnetite. The results are conducive for understanding the response mechanism of magnetic indicators to relative sea-level change, and provide a scientific basis for encouraging for future work in understanding climate variability.

Temporal and Spatial Distribution of Surface Sediment and Its Implications in the Yangtze Subaqueous Delta

Characteristics of sediments vary significantly temporally and spatially in the Yangtze subaqueous delta and adjacent East China Sea shelf due to complicated dynamic conditions. Magnetic properties of sediments are sensitive to hydrodynamics and provenance, and therefore can indicate sediment transportation and deposition. High-resolution surface sediments collected in different seasons and years from the Yangtze subaqueous delta and neighboring East China Sea shelf were subjected to environmental magnetic analyses. In combination with granulometric analysis, this paper discusses temporal and spatial variations of magnetic properties and their implications for indicating sediment transportation, sediment source identification, and hydrodynamic response. The results show that magnetic parameters SIRM and XARM/SIRM suggest sediment transportation path from the Yangtze River into the East China Sea, with majority migrating to the south and southeast and then deposited to the west of 123°E, while a little fine sediment delivered to the northeast in summer. Cluster analysis demonstrates three sedimentation districts according to magnetic properties and particle size in the study area, which represent modern Yangtze sediment, late Pleistocene relict sand, and their mixture, respectively. Temporal-spatial changes in particle size of sediments suggests erosion outside the North Branch in the past few years, while opposite trend of changes on particle size of surface sediment in the adjacent area might reflect different degrees of erosion.

How strong was pedogenesis in Schirmacher Oasis during the Late Quaternary?

We compare the sediment magnetic properties of two lakes in Schirmacher Oasis, East Antarctica and discuss the intensity of pedogenesis during the Late Quaternary. The magnetic minerals are land-derived with no obliteration of the signal due to bacterial or anthropogenic sources. The magnetic grains of Lake L-49 are much smaller in size compared to Lake Sandy's (low ratio values of IRM20mT/ARM and S20 ). The iron oxide minerals in Lake Sandy's sediments are coarse stable single domain (SSD) and multi-domain (MD) grains with the SSD concentration increasing during the Holocene. Percentage frequency-dependent susceptibility, a marker for pedogenic iron oxide minerals shows a similar trend for Lake Sandy, with the values remaining low during the Last Glacial Stage (LGS) and increasing towards the Holocene. Such a pattern is not observed in the sediments of Lake L-49. Principal Component Analysis reveals a relatively stronger intensity of pedogenesis in the Lake Sandy catchment during the deglaciation. From the data, we observe that even though pedogenesis is active, the intensity is not strong enough to form superparamagnetic (SP) grains. The peaks in χfd for both the lakes show a direct relationship with Antarctic Isotope Maximum events recorded in an ice-core data suggesting that χfd can be used as a proxy for paleo-pedogenesis in Antarctica.

Magnetic Properties of Late Holocene Dead Sea Sediments as a Monitor of Regional Hydroclimate

AbstractDiagenetic processes in anoxic sedimentary environments influence sediment magnetic properties mainly through dissolution of detrital magnetite and precipitation of authigenic greigite. Recently exposed late Holocene Dead Sea sediments provide an opportunity to study the processes governing greigite formation and preservation, and their relation to different hydrological settings. Magnetic data and pore‐fluid compositions were obtained from three Holocene sections along a N‐S transect on the western Dead Sea shore: Og, Ein‐Feshkha (EF), and Ein‐Gedi. The northern sections are closer to the major freshwater source to the Dead Sea‐the Jordan River. Detrital titanomagnetite is present at all sections, but greigite is the dominant magnetic phase at Og and EF. Bulk rock magnetic data vary between and within the sections by over 3 orders of magnitude, where higher values indicate higher greigite concentrations. At the three sites, pore fluids have similar or lower salinity than the modern and Holocene Dead Sea brine, with variable and dissolved iron (Fe2+) and sulfate (SO42−). Magnetic property changes are reflected by iron and/or sulfate microbial reduction that controlled sedimentary greigite formation. We propose that the N‐S greigite decrease suggests that anoxic microbial activity was controlled by labile organic matter and/or reactive iron brought by, or formed as a result of, freshwater influx from the Jordan River. Hence, greigite concentration changes depended on past freshwater input to the hypersaline lake and proximity to the freshwater source. The apparent relationship between hydrological conditions and magnetic properties provides a new method to trace past hydrological changes in the Dead Sea.

The Birth of a Connected South Atlantic Ocean: A Magnetostratigraphic Perspective

The early Paleogene sedimentary record of the Sao Paulo Plateau holds the key to understanding the timing and mechanisms of the onset of Atlantic oceanic circulation, with implications for ocean - climate feedback systems. The timing and evolution of South Atlantic Ocean connectivity remain poorly understood, hindering a full understanding of modern circulation mechanisms. In this work, we develop an improved magnetostratigraphic age model for the late- Cretaceous to Eocene sedimentary record of Deep-Sea Drilling Project (DSDP) Site 356, located on the Sao Paulo Plateau at the edge of the Rio Grande Rise. The model constrains the timing of the beginning of Atlantic Ocean circulation, as the last obstacle to a connected Atlantic was removed. We use the age model to quantify sedimentary rates, and correlate with rock magnetic and published geochemical data to explore links between ocean current rearrangement and climatic anomalies. We find that periods characterized by variations in the magnetic properties of sediments and sedimentation anomalies in the Sao Paulo Plateau are reflected in carbon isotope events, supporting a possible link between ocean circulation and hyperthermal events.

The Effect of Early Diagenesis in Methanic Sediments on Sedimentary Magnetic Properties: Case Study From the SE Mediterranean Continental Shelf

Microbial respiration in marine sediment can affect the magnetic properties of the sediment through a complicated interplay between reductive dissolution and authigenic precipitation of iron-bearing magnetic minerals. However, a direct link between the main diagenetic zones in the upper sedimentary column and sedimentary magnetic properties using high resolution multi parameter profiles has been demonstrated only in few studies. Here, we directly correlate early diagenetic processes and sedimentary magnetism using a composite high-resolution sedimentary record of pore water chemistry, solid phase chemical measurements and mineral-magnetic parameters. Measurements along the profiles include the entire redox cascade, from the water-sediment interface, down through the deep methanic zone, on a six-meter sediment core collected from the Southern Eastern Mediterranean continental shelf. The uppermost part of the sediment core, associated with oxic, nitrous, manganous, ferruginous, and sulfate reduction zones, is characterized by high ferrous iron and sulfate concentrations and high values of the measured magnetic parameters (susceptibility, Isothermal remanent magnetization (IRM) and Anhysteretic remanent magnetization (ARM)). This layer is underlain by a sulfate-methane transition zone (SMTZ) that shows a significant decrease in magnetic parameters due to the dissolution of magnetic minerals. Below the SMTZ, the methanic zone has been assumed to be magnetically inactive under steady-state conditions. However, we observe in the upper methanic zone an increase in microbial iron reduction, coupled to an abrupt increase in magnetic parameters. Our data indicate that the observed increase in the magnetic signal is related to the precipitation of authigenic magnetic minerals. These diagenetic changes should be considered when interpreting paleomagnetic data, and highlight the potential to use high-resolution magnetic data as a proxy for identifying diagenetic processes.

High Potency of Volcanic Contribution to the ∼400 kyr Sedimentary Magnetic Record in the Northwest Pacific

As the northwest Pacific has been subject to varying terrigenous input linked to paleoclimate change, the concentration of magnetic minerals in deep-sea sediments is often utilized as a proxy to reconstruct the past atmospheric circulation in the Northern hemisphere. Volcanic materials account for a significant portion of the terrigenous input, but their contribution to sedimentary magnetic properties has not been carefully investigated. This study reveals the magnetic contribution and characteristics of volcanic materials, based on particle-size specific magnetic measurements on sediments that span the last 400 kyr for five size-fractions, including ranges typically attributed to fine eolian ( 100 mT) magnetic minerals, showing an increased portion of 32–74%. From coercivity spectra analysis, the coarse volcanic fractions are characterized by an abundance of the ~100 mT coercivity minerals, which can result in an increased average coercivity of bulk sediments. Around the study area, magnetic susceptibility records show synchronized variations with volcanic proportions in terrigenous sediments, validating their close relationship. Consequently, our results indicate that volcanic materials have a high potency of magnetic concentration, which can control bulk sedimentary signals in the northwest Pacific.

Magnetic mineral dissolution recorded in a lacustrine sequence from the Heqing Basin, SW China, and its relationship with changes in the Indian monsoon

The dissolution of magnetic oxides is an important process in many lake environments. Understanding this phenomenon is crucial for the paleoenvironmental interpretation of the magnetic properties of lake sediments. In order to reveal its effects on sedimentary magnetic properties, and to assess the possible associated paleoenvironmental implications, we carried out detailed rock magnetic analyses of the selected samples from a 920-kyr lacustrine sequence from the Heqing Basin, SW China. The results indicate that the sedimentary magnetic properties are controlled by the concentration and grain size of magnetite and maghemite. High magnetic susceptibility (χ) intervals contain more fine-grained magnetite and maghemite, while low χ intervals contain only minor amounts of residual magnetite. The decreased content of fine-grained maghemite from high χ to low χ intervals reflects the dissolution of magnetic oxides during deposition. Intervals affected by strong magnetic dissolution have a high TOC content and correspond to times of high Antarctic temperatures, suggesting that magnetic mineral dissolution intensity was associated with variations in the strength of the Indian Summer Monsoon (ISM). Notably, the ISM is sensitive to Southern Hemisphere warming. Weak magnetic dissolution indicates a dry climate occurred since ~320 kyr in the Heqing Basin. This dry/cool event was widespread across the Eastern Bay of Bengal, Equatorial Indian Ocean and Northern Australia, and was linked to a strengthened Indian Ocean Dipole (IOD). Since the moisture source of the Heqing Basin was mainly from the above regions, we infer that the influence of the IOD extended northwards to SW China.

Magnetic properties and correlation with heavy metals in mangrove sediments, the case study on the coast of Fujian, China

To characterize the magnetic signature of sediment heavy metal contamination and identify sources of heavy metals in mangroves, 83 sediment specimens were collected from three mangroves in Fujian, China; various magnetic parameters and heavy metal concentrations were then determined. Variation in magnetic magnetization among specimens was linked to changes in pseudo-single-domain magnetite. Average values of Co, Cu, Ni, and Zn (but not Cr or Pb) were slightly lower than background levels. Geochemical evidence suggested that Co, Cr, Ni, Ti, and V were associated with lithogenic minerals in the sediment, while Cu, Pb, and Zn were associated with terrigenous minerals. A strong positive correlation was seen between magnetic concentration-dependent parameters and metal concentrations (Cu, Pb, and Zn), suggesting enrichment of metal-containing magnetic minerals with heavy metal pollution. The combined assessment of both sediment magnetic properties and heavy metal concentrations thus provides insight into the pollution status of mangrove sediments under complex conditions.

Magnetic mineral diagenesis in sediments of saline lake Lop Nur

Saline lakes are useful repositories for paleo-climatic records. In recent years, magnetic properties of saline lake sediments have been widely applied to establish paleo-climatic change. However, the influence factors of magnetic properties in saline lakes have not been fully understood, which complicates the paleoenvironmental interpretation. Here, we present a rock magnetic analyses result of LOP1 profile (40°26′09″′N, 90°21′23″E) from Lop Nur, a well-known saline lake, located in the eastern Tarim Basin in northwestern China. We combined the particle size, total organic content, and mineral characteristics analysis to assess the influence factors of magnetic properties in Lop Nur and its environmental significance. The results indicate that early diagenesis is the major influence factor on magnetic properties of Lop Nur saline sediments. Authigenic greigite and pyrite are identified within organic-rich sediments, which produce zones with high and low magnetic susceptibilities, respectively. The different authigenic iron sulfide contents in different layers are related to sedimentary environment changes. Sufficient supplies of organic matter and sulphate and low sedimentation rates favour the pyritization process. Moreover, if pyritization was constrained, intermediate greigite formed and was preserved. In oxidizing environments, sediment magnetic properties are consistent with those of source materials from Tarim Basin and are mainly controlled by particle size and hydrodynamic sorting of mainly detrital magnetite is largely unaffected by early diagenesis. Our study demonstrates that magnetic properties can provide a robust approach for studying depositional environment change in saline lake. In addition, the information obtained in this study would also provide insights into the geochemical processes of iron element in saline lakes.

Formation of greigite (Fe3S4) in the sediments of saline lake Lop Nur, northwest China, and its implications for paleo-environmental change during the last 8400 years

We present the results of rock magnetic and SEM/EDX analyses of the sediments of Lop Nur, a former saline lake in the Tarim Basin, China. The aim of the study was to assess the impacts of authigenic iron sulfide minerals (e.g., greigite) on the sedimentary magnetic properties and their possible relationship with environmental and climate change. Five layers with relatively high values of χ, χARM, SIRM, S-ratio, χARM/χ and SIRM/χ are identified within organic-rich sediments; the five layers are dated to ca. 8150–7000, 5300–4900, 3600–3100, 2750–2550 and 1820–1680 cal. yr BP. Authigenic greigite is the dominant magnetic mineral within the five layers and it is present in the stable single domain state. The magnetic properties of the intervening layers are dominated by detrital multidomain magnetite. Combined with results of a previous study, we infer that the greigite-bearing sediments were deposited during cold and wet periods which were probably related to the North Atlantic cold events during the past 8400 yr. During these periods, greigite formation was likely driven by an enhanced organic matter supply, but pyritization was constrained by the decreased sulfate content and high sedimentation rate. The greigite and its diagnostic magnetic parameters (e.g., SIRM/χ) can be regarded as an indicator of the hydrological and climatic evolution of the Tarim Basin.

The Holocene Environmental Evolution of the Inner Hangzhou Bay and Its Significance

The Holocene environmental evolution in coastal area, including deltas and estuaries, is vital to the Neolithic colonization in the eastern China. A 29-m long sediment core XL was obtained from the region between the Taihu Lake and Qiantang Estuary, and multiple analyses on the core sediments have been done to investigate the evolution history of the inner coastal zone of Hangzhou Bay during the Holocene. Six sedimentary facies are identified since the late Pleistocene, including the paleosol at the bottom, overlain by tidal-flat-channel complex (TFC), tidal-distributary-channel (TDC), inner estuarine basin (IEB), subtidal-to-intertidal-flat, and fluvial-plain facies. The TFC and TDC facies are dated to be formed before 8500 cal yr BP, featured by relatively coarse sediments, high magnetic properties and less abundant foraminifera, denoting the tide-influenced setting with large terrestrial inputs. The IEB facies characterized by the most abundant foraminifera fossils and the lowest sediment magnetic properties, is inferred to be deposited during the maximum flooding period between 8500 and 7900 cal yr BP. The branch estuary was then infilled by local sediments with a shallowing-upward subtidal-to-intertidal sequence during 7900–4300 cal yr BP. At last the fluvial plain system developed since no later than 4300 cal yr BP, favouring the rapid development of Neolithic cultures in the region.

Climate-induced discharge variations of the Nile during the Holocene: Evidence from the sediment provenance of Faiyum Basin, north Egypt

Abstract The Faiyum Basin of northern Egypt occupies a key location on the source-to-sink pathway of the Nile. High-resolution grain size, organic carbon and carbonates, as well as geochemical and magnetic analyses, were conducted on a sediment core retrieved from the basin to infer changes in sediment provenance in association with climate-induced variations in Nile River discharge during the Holocene. A high Si/Al ratio in the coarse sediment with weak magnetic properties prior to ca. 9.7 cal ka BP indicated prevalent aeolian activities in the lake basin, revealing no Nile discharge inputs and thus a dry Nile. Initial high values of the mobile elements (CaO, MgO and Sr) after ca. 9.7 cal ka BP with well-sorted finer sediment marked the establishment of a flow connection between the basin and the Nile since then. This establishment was followed by a durative hydrological connection to the Nile until ca. 5.0 cal ka BP inferred from high organic matter and stable magnetic and geochemical properties of the finer sediment, implying a humid climate phase of the Nile between ca. 9.7–5.0 cal ka BP. Afterwards, an increasing Ti/Al ratio and magnetism of the sediment indicated a brief change in sediment provenance, as material from the volcanic Ethiopian Uplands carried by the Blue Nile and the Atbara became dominant. A drying climate prevailed in the Nile Basin after ca. 5.0 cal ka BP. Although the Ti/Al ratio and sediment magnetic properties still showed a prominent sediment contribution from the volcanic Ethiopian Uplands, the natural discharge from the river Nile to the Faiyum Basin was weakened, especially when increasing water extraction by humans occurred after ca. 4.0 cal ka BP. The sediment provenance pattern of the Faiyum Basin was substantially influenced by climate-induced variations of the Nile discharge in response to the advance/retreat of the African Summer Monsoon (ASM) driven by the northward/southward migration of the Inter-Tropical Convergence Zone (ITCZ).

Influence of Sea Level Change and Centennial East Asian Monsoon Variations on Northern South China Sea Sediments Over the Past 36 kyr

AbstractSouth China Sea (SCS) sediments can provide important paleoclimate records because of their generally high deposition rates and strong regional influence of East Asian summer monsoon (EASM) variations. However, interpretation of sedimentary records from the northwestern SCS slope is complicated because sedimentation is sensitive to sea level and provenance variations. We seek to develop a paleoclimatic record of SCS sediments by analyzing sediment magnetic properties over the past ∼36 kyr from core PC338. The dominant magnetic minerals are magnetite and hematite in most parts of the core. The hematite concentration increased profoundly over the ∼15–32 kyr B.P. period when sea level was lower than −90 m below the present level. This suggests that Red River inputs were enhanced significantly in this period. Moreover, increased coarse magnetite contents were recorded in association with an enhanced East Asian winter monsoon (EAWM) during the Last Glacial Maximum and Heinrich event 1. Low‐field and frequency‐dependent magnetic susceptibility variations were coeval with EASM‐controlled climate change recorded in South China and the SCS over the past 15 kyr, probably because regional terrestrial weathering and erosion of iron‐bearing minerals, and their transportation are dominated by climate. Rhythmic coarsening of Holocene magnetic mineral concentrations coincided with global cooling events, which indicates that core PC338 also contains signals of enhanced EAWM variations. The study demonstrates that magnetic proxies are indicative of sediment supply and deposition variations on the SCS continental slope that are useful for understanding the effects of sea level and climate change on SCS sediments.

Chronostratigraphy and spatial distribution of magnetic sediments in the Chukchi and Beaufort seas since the last deglaciation

Palaeomagnetic investigation of three sediment cores from the Chukchi and Beaufort Sea margins was performed to better constrain the regional chronostratigraphy and to gain insights into sediment magnetic properties at the North American Arctic margin during the Holocene and the preceding deglaciation. Palaeomagnetic analyses reveal that the sediments under study are characterized by low‐coercivity ferrimagnetic minerals (magnetite), mostly in the pseudo‐single domain grain‐size range, and by a strong, stable, well‐defined remanent magnetization (MAD <5°). Age models for these sediment cores were constrained by comparing their palaeomagnetic secular variations (inclination, declination and relative palaeointensity) with previously published and independently dated sedimentary marine records from the study area. The magnetostratigraphical age models were verified by AMS radiocarbon dating tie points, tephrochronology and 210Pb‐based sedimentation rate estimate. The analysed cores 01JPC, 03PC and 02PC span c. 6000, 10 500 and 13 500 cal. a BP, respectively. The estimated sedimentation rates were stable and relatively high since the deglaciation in cores 01JPC (60 cm ka−1) and 03PC (40–70 cm ka−1). Core 02PC shows much lower Holocene sedimentation rates with a strong decrease after the deglaciation from ~60 to 10–20 cm ka−1. Overall, this study illustrates the usefulness of palaeomagnetism to improve the dating of late Quaternary sedimentary records in the Arctic Ocean.

Connection of the proto-Yangtze River to the East China Sea traced by sediment magnetic properties

The evolution of the Yangtze River, and specifically how and when it connected to the East China Sea, has been hotly debated with regard to possible linkages with the so-called ‘Cenozoic Topographic Reversal’ (tectonic tilting of continental east China in the Cenozoic) and particularly the relationship to the uplift history of the Tibetan Plateau. Resolving this key question would shed light on the development of large Asian rivers and related changes in landforms and monsoon climate during this interval. Here, we use the magnetic properties of both Plio-Quaternary sediments in the Yangtze delta and of surficial river sediments to identify a key mid-late Quaternary switch in sediment source-sink relationships. Our results reveal a fundamental shift in sediment magnetic properties at this time; the upper 145m of sediment has magnetic mineral concentrations 5 to 10 times higher than those of the underlying late Pliocene/early Quaternary sediments. We show that the distinctive magnetic properties of the upper core sediments closely match those of surficial river sediments of the upper Yangtze basin, where the large-scale E'mei Basalt block (2.5×105km2) is the dominant magnetic mineral source. This switch in sediment magnetic properties occurred at around the Jaramillo event (~1.2–1.0Ma), which indicates that both the westward extension of the proto-Yangtze River into the upper basin and completion of the connection to the East China Sea occurred no later than at that age.