Magnetic Properties Of Sediments Research Articles

Magnetic properties of sediments from Lake Chaohu and its response to sedimentary dynamics

对巢湖西部柱样以及杭埠河流域土壤的环境磁学研究表明,磁铁矿是决定磁性特征的主要磁性矿物,杭埠河流域是巢湖西部沉积物的主要来源．靠近巢湖西岸的柱样AC1砂含量较高,χ、SIRM值较大而χ_ARM/SIRM较低,说明磁铁矿含量较高、颗粒较粗．距岸稍远的柱样AC2砂含量较低,χ、SIRM值也较低,其变化与χ_ARM较相似,与粘土含量变化也有一定程度的相似,AC2沉积物χ、SIRM值较ACI小,但χ_ARM/SIRM较高,说明磁铁矿含量较低且颗粒较细．杭埠河流域土壤的磁性测量结果表明,砂、砾含量高的粗骨土磁性最强,富含粗颗粒磁铁矿．巢湖柱样沉积物砂的含量随踞岸远近而不同, 反映了沉积环境的水动力不同,由此产生明显的磁性特征差异,实质是对沉积动力的响应．研究表明磁性参数χ_ARM/SIRM 可以间接反映沉积物中粘土含量的高低变化．;Magnetic study on core sediments from western Lake Chaohu and soils from Hangbu River Catchment suggests that magnetite is the dominant magnetic mineral, and soils from Hangbu River Catchment are the main source of lake sediments. Core ACI near the bank shows higher proportion of sands and higher χ、SIRM values but lower χ_ARM values, indicating higher concentration of magnetite with coarse size. Core AC2 more far away from the bank shows lower proportion of sands and lower χ、SIRM values. The similar variations of χ、SIRM with χ_ARM, and to a lesser degree with clay fraction, in combination with higher χ_ARM/SIRM values, indicating lower concentration of magnetite with fine size. Magnetic properties of soils in the catchments indicate that skeletal soils with abundant sand and gravel fraction are magnetic strong which are contributed by coarse magnetite. Content of sand fraction in core is a reflection of water energy which is dependent on the distance to the bank. Therefore, the distinct magnetic properties of different cores are due to sedimentary dynamics. It is found that χ_ARM/SIRM can be used to indicate the variation of clay contents in sediments

Rock magnetic records of the sediments of the eastern Arabian Sea: Evidence for late Quaternary climatic change

Rock-magnetic measurements along with grain size, acid-insoluble residue (AIR), organic carbon (OC), CaCO 3 and δ 18O of the planktonic foraminifers of the sediments were determined for 15 gravity cores recovered from the western continental margin of India. Magnetic susceptibility (MS) values in the surficial sediments reflect the land-derived input and, in general, are the highest in terrigenous sediment-dominated sections of the cores off Saurashtra–Ratnagiri, followed by the sediments off Indus–Gulf of Kachchh and then Mangalore–Cape Comorin. The down-core variations in mineral magnetic parameters reveal that the glacial sediments off the Indus are characterized by low MS values/ S-ratios associated with high AIR-content, low OC/CaCO 3 contents and relatively high δ 18O values, while those off SW India are characterized by low MS values/high S-ratio% associated with low AIR content, and relatively high OC, CaCO 3 and δ 18O values. Conversely, the Early Holocene sediments of all cores are characterized by high MS values/ S-ratio% associated with high AIR content, low OC, CaCO 3 contents and gradually decreased δ 18O values. These results imply that during the Last Glacial Maximum (LGM), the cores off northwestern India received abundant continental supply leading to the predominance of eolian/fluvial sedimentation. In the SW region the influence of hinterland flux is less evident during this period, but convective mixing associated with the NE monsoon resulted in increased productivity. During the early Holocene intense SW monsoon conditions resulted in high precipitation on land, which in turn contributed increased AIR content/MS values in the continental margin sediments. A shallow water core off Kochi further suggests that the intense SW monsoon conditions prevailed until about 5 ka. The late Holocene organic-rich sediments of the SW margin of India were, however, subjected to early diagenesis at different intervals in the cores. Therefore, caution is needed when interpreting regional climatic change from down-core changes in sediment magnetic properties.

Arcellacean (thecamoebian) evidence of land-use change and eutrophication in Frenchman?s Bay, Pickering, Ontario

Frenchman’s Bay, on the northern shore of Lake Ontario, has been negatively impacted by eutrophication in the last 50 years through urbanization and the use of chemical fertilizers. Eutrophication began with wholesale land clearance and agricultural practises beginning in the mid-nineteenth century and reached a peak with urbanization after World War II. Eutrophication and the effects of land-use changes on the watershed were investigated by a combined analysis of the sediment magnetic properties and arcellacean (thecamoebian) microfauna. Micropaleontological analyses were conducted on two 2-m-long cores (FMB1 and FMB2) every 10 cm (42 samples) and magnetic susceptibility was measured at 2-cm intervals. Both cores showed a distinct correlative transition at 60 cm and 110 cm that was marked by a rapid increase in thecamoebian concentrations (from approx. 10×103 to 30×103 specimens per cc) and a large increase in Cucurbitella tricuspis (from approx. 10–20% to 40–70%). This transition correlated with a marked increase in magnetic susceptibility (150–200×10−8 m3 Kg−1) at the same depth, which was attributed to elevated levels of detrital magnetic minerals derived from land clearance and soil erosion in the watershed. It was indicated by 210 Pb dates indicate that there was a gradual onset of eutrophication in the mid-nineteenth century (AD 1850±56) and a more rapid rise in the mid-1940s to late 1950s. The initial increase in eutrophication was due to land clearing, agricultural development and increased nutrient loadings. The major eutrophication increase in the 1950s was from urbanization and storm sewer discharge loaded with high yield chemical fertilizers from lawns and gardens. This high concentration of nutrients has led to an unprecedented level of eutrophication within the wetland.

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway

Lake-based magnetic mapping of contaminated sediment distribution, Hamilton Harbour, Lake Ontario, Canada

The remediation of toxic sediment in harbours and urban waterways requires detailed mapping of contaminated sediment distribution and thickness. Conventional methods rely on interpolation of pollutant concentrations from widely spaced core samples but can lead to significant errors in estimating sediment distribution. An improved approach, as demonstrated by recent work in Hamilton Harbour in Lake Ontario, is to estimate pollutant levels from proxy measurements of sediment magnetic properties. Measurements from 40 core samples collected within the harbour show that the magnetic susceptibility of a contaminated upper layer of sediment is one to two orders of magnitude greater than in the underlying uncontaminated ‘pre-colonial’ sediments. The susceptibility contrast results from elevated levels of urban-source magnetic oxides and is sufficient to generate a total field anomaly (ca. 5–40 nT) that can be measured with a towed magnetometer. Systematic lake-based magnetic surveying (>500 line km) of the harbour using an Overhauser marine magnetometer identifies well-defined positive magnetic anomalies that coincide with mapped accumulations of contaminated sediments on the harbour bottom. Forward modelling of the anomalies shows that the magnetic response is consistent with a contaminated upper layer thickness of up to 5 m. Apparent susceptibility maps calculated from magnetic survey data show a close spatial correspondence with core-derived magnetic susceptibilities and provide a rapid means for classifying contaminated sediments. Detection of shallow magnetic anomalies is dependent upon a closely spaced survey grid (<75 m line spacing) and careful post-cruise processing to remove diurnal, regional and water-depth related variations in the magnetic field intensity.

Sediment-magnetic signature of land-use and drought as recorded in lake sediment from south-central Minnesota, USA

Sediment magnetic properties of a short core from Sharkey Lake, MN, record the effects of Euroamerican settlement and climate change over the last 150 yr. The onset of European-style farming led to increased erosion, reflected in high values of concentration-dependent parameters such as magnetic susceptibility (ĸ), Isothermal Remanent Magnetization (IRM), and Anhysteretic Remanent Magnetization (ARM). These high values are only partially due to increased supply of terrigenous material to the lake, and recent sediment contains an additional component of authigenic fine (single-domain) magnetite, most likely magnetosomes from magnetotactic bacteria. High organic productivity in the lake during the 1920s to 1940s drought increased this authigenic component resulting in highly magnetic fine-grained sediment. A comparison with older Holocene sediment from the same lake shows that, over time, most of the fine magnetic signal is lost after deposition, leading to decreases in magnetization and a bimodal grain size distribution of ultrafine, superparamagnetic grains and coarser multidomain particles, evident from measurements of ARM/IRM ratios, hysteresis measurements, and low-temperature analyses. The effects of dissolution and the superposition of climate and land-use signals complicate the use of recent sediments as modern analogs for sediment magnetic analyses.

Relevant Magnetic Parameters and Heavy Metals from Relatively Polluted Stream Sediments - Vertical and Longitudinal Distribution Along a Cross-City Stream in Buenos Aires Province, Argentina

Previous magnetic and geochemical studies of stream sediments in the Northeast of Buenos Aires Province, in the Greater La Plata area, have revealed a high level of trace elements, especially lead and zinc. Continuing these preliminary works, new sampling sites were considered in order to improve and increase the magnetic mapping of this area. This part of the study focuses especially on the magnetic properties of sediments from a cross-city stream. Such stream receives a variable pollutant load from urban, industrial and diffuse sources. Concentration-dependent magnetic parameters, magnetic carriers (para, anti and ferrimagnetic) and their features (e.g. softer or harder magnetic carrier, grain size distribution, etc.) have been calculated using magnetic measurements. According to magnetic parameters and heavy metal loads two main distinctive groups of sediment-cores are distinguished. The vertical distribution of sediments reveals a recent anthropogenic influence, possibly, belonging to the last 20-40 years. Four out of all the magnetic parameters (magnetic susceptibility, anhysteric remanent magnetisation, S-ratio and κ ARM/κ) were chosen in order to investigate their relationship with contents of heavy metals. In all the cases, good positive correlations were obtained using linear regression. However, more significant correlation factors were achieved for grain size and magnetic feature-dependent parameters (κ ARM/κ and S-ratio) than for magnetic concentration-dependent parameters. Therefore, the κ ARM/κ and the S-ratio seem to be the most relevant magnetic parameters to describe the vertical and longitudinal distribution of heavy metals present in these stream sediments. The non-significant relationship between heavy metals and concentration-dependent parameters could be explained taking into account the discrimination of the two distinctive groups. The group belonging to the Coastal Plain shows a clear linear trend between heavy metals and magnetic susceptibility and anhysteric remanent magnetisation.

Paleolimnology and Paleoclimate Studies in Upper Klamath Lake, Oregon

The subsiding Upper Klamath Lake Basin contains sediments that were continuously deposited in a shallow, freshwater lake for more than 40 000 years. Well dated by radiometric methods and containing volcanic ashes of known age, these sediments constitute a valuable paleoclimate record. Sediment constituents and properties that reflect past climatic conditions in the area include pollen, diatoms, sediment geochemistry, and sediment magnetic properties. Many of these proxy measurements are also useful for comparing natural conditions in the lake to conditions following human settlement. Because of its location, the paleoclimate record from Upper Klamath Lake is valuable for comparisons to offshore marine records and as part of latitudinal transects of paleoclimate records along the west coast of the Americas.

Magnetic properties of sediment in the Ría de Arousa (Spain): dissolution of iron oxides and formation of iron sulphides

A detailed magnetic study of mineral dissolution has been carried out in order to better characterise the sedimentary environment in the Ría de Arousa, an estuarine-like system in western Spain. There the large number (>3000) of mussel rafts has resulted in a high influx of organic matter causing a change of the original chemical environment. Magnetic measurements of sediment cores (reaching 4.1 m depth) combined with electron microscopy show that there is a distinctive depth trend for the concentration of magnetic minerals. The different iron oxides dissolve with increasing time and depth and at different rates, so that their relative proportions change. At the same time, pyrite is forming, replacing the iron oxides at shallow depths. The magnetic properties of the sediments indicate a high dissolution rate of magnetite (half-life ⩽5 year) and other iron oxides in a strongly reducing environment. Additionally, goethite and hematite in the studied system seem to be less reactive than magnetite.

Three million years of monsoon variability over the northern Sahara

We present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of 3400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the east- ern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (321� N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan wa- tershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from ''green Sahara'' climate models. Our results indicate that this northward monsoon penetration re- curred during insolation maxima throughout the last 3 million years. As would be expected, this orbital pre- cession-scale mechanism is modulated on both short (3100-kyr) and long (3400-kyr) eccentricity time scales. We also observe a strong expression of the 341- kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also ob- serve a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (30.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.

Late‐glacial and Holocene climatic effects on fire and vegetation dynamics at the prairie–forest ecotone in south‐central Minnesota

Summary 1 Treeline ecotones, such as the prairie‐forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2 In this study, we examined the prairie‐forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3 Sediment magnetic and LOI data suggest four broad climatic periods occurring c . 11 350‐8250 BP (cool/humid), c . 8250‐4250 BP (warm/dry), c . 4250‐2450 BP (warm/ humid), and c . 2450‐0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4 The area surrounding both lakes was characterized by boreal forest c . 12 500‐ 10 000 BP, changing to an Ulmus-Ostrya forest c . 10 000‐9000 BP, changing to a community dominated by prairie ( Poaceae-Ambrosia-Artemisia ) and deciduous forest taxa c . 8000‐4250 BP, and finally shifting to a Quercus -dominated woodland/savanna beginning c . 4250‐3000 BP. 5 Charcoal influx increased from an average of 0.11‐0.62 mm 2 cm − 2 year − 1 during the early Holocene forest period ( c . 11 350‐8250 BP) to 1.71‐3.36 mm 2 cm − 2 year − 1 during the period of prairie expansion ( c . 8250‐4250 BP) and again increased to 4.18‐4.90 mm 2 cm − 2 year − 1 at the start of the woodland/savanna period ( c . 4250 BP). 6 As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation.

Magnetic properties of tidal flat sediments of the Yangtze Estuary and its relationship with particle size

This paper considers the magnetic properties of tidal flat sediments of the Yangtze Estuary and its relationship with particle size. The results indicate that magnetite of multi-domain (MD)/pseudo-single domain (PSD) in size dominates the magnetic properties of the samples. In addition to detrital magnetite, spherical magnetic particles are found in the magnetically strongest sediments near Shidongkou sewage outlet, which are ascribed to the product of industrial pollution. Particle size plays a strong role on the magnetic properties of the sediments. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) are positively correlated with the proportion of the fraction of 8-16 μm. However, magnetic parameters indicating fine-grained ferrimagnetic minerals, e.g. susceptibility of anhysteric remanent magnetization (X ARM), are highly correlated with the fine fraction of <4 μm and even the fraction of <32 μm. The results indicate that, whenX is used as a proxy for pollutant content, it is necessary to consider the possible influence of particle size on it. Due to the significant relationship betweenX ARM and the fine component proportion,X ARM can be applied to the normalization of pollutant content for particle size effect.

Magnetic characterisation of present-day deep-sea sediments and sources in the North Atlantic

The North Atlantic plays a key role in global climate through formation of North Atlantic Deep Water, which drives the thermohaline circulation. To understand past and future climate change, it is essential to understand the processes occurring within the present-day North Atlantic but ocean-wide studies of present-day sediment distribution and sources are relatively sparse. Here, we use magnetic measurements to characterise the surface sediments of the North Atlantic and identify the major climatic and oceanographic controls on their magnetic signatures. The magnetic data, and subsequent cluster analysis, identify distinct spatial patterns of sediment sources and transport pathways. Much of the sedimentary magnetic signal appears to be controlled by detrital inputs, especially of windblown dust, and ice-rafted debris (IRD), with a range of different sources. These sediment transport pathways can be validated by making direct comparison of the sediment magnetic properties with source rocks and soils, and with iceberg trajectory and observation data. The spatial distributions of the IRD-dominated sediments substantiate those mapped previously using lithological tracers, provide additional spatial information on sediment sources and pathways, and suggest that deep water currents are less significant than proposed in controlling present-day sediment mineralogy and distribution. The data form a present-day basis for comparison with glacial-stage IRD and dust distributions, and modes of ocean circulation.

A high-resolution Holocene sediment record from Húnaflóaáll, N Iceland margin: centuryto millennial-scale varability since the Vedde tephra

MD99-2269 is a 25 m long core from Hunaflóadll, a deep trough which runs northward towards the shelf break from the narrow neck of land which joins the Northwest Peninsula of Iceland with the ‘mainland’. The core was recovered from the thickest part of an elongate shelf sediment body, underlain by glacial diamictons with ages 26 ka BP. 3.5 kHz profiles indicate the presence of a reflector, RI, which can be traced over long distances. Twelve AMS radiocarbon dates and the presence of the Saksunarvatn tephra (which is the origin for the RI reflector at 21 m core depth) define a linear depth/age relation with an accumulation rate of 200 cm/ky. The date on the core top is ‘modem’. X-radiographs indicate that the sediments are principally massive. bioturbated muds. The data were measured at resolutions of between 5 and 40 yr/sample. Our primary data represent aspects of (a) multisensor track measurements, (b) colour, (c) grain size, (d) sediment properties and (e) sediment magnetic properties. Principal component analysis (PCA) was used to simplify the analysis of these five data sets, and resulted in nine significant axes compared to over 70 original variables. The explained variance on the first axes varied between 47 and 96%. These time series were reduced to a common 50-yr sample interval which were then also subject to PCA. Using the resulting PC scores we present three primary axes of environmental variability in Hunaflóaall that have century-scale quasi-periodicities, and demonstrate that these are highly correlated with specific ‘raw’ variables. We focus on variables linked to changes in grain size and productivity and show regional coherence (cores B997-321 and 330) in the coercivity/grain size measure ARMJ(20)/ARMJ(0), which have a close similarity to the detrended A14C series. This provides a potential link to the vigour of the thermohaline circulation to the north of the Iceland shelf in the Iceland and Greenland Seas. Our data indicate increased variability in our proxies during the Neoglacial period.

Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yr: long-term North Atlantic oceanographic variability and solar forcing

Giant piston core MD99-2269 recovered 25 m of sediment in Hunáfloáall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (∼12 cal ka). The sediment was sampled at between 5 and 50 yr/sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM 20 mT/ARM) and magnetic concentrations (kARM). In turn ARM 20 mT/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (>99%) periodicities at millennial to multidecadal scales, including those at ∼200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the ∂ 18O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low ∂ 18O values define a Holocene thermal maximum between ∼8 and 6 cal ka.

Sediment magnetic properties reveal Holocene climate change along the Minnesota prairie-forest ecotone

We propose a model that explains variations in magnetic parameters of lake sediments as a record of Holocene climate change. Our model is based on records from four lakes and incorporates the effects of erosion, dust deposition, and the authigenesis and diagenesis of the magnetic component of the sediment. Once checked against high resolution multi proxy climate records, which are currently being established for some of our study sites, it will allow us to use magnetic proxies to establish high-resolution climate reconstructions on a regional scale. Our model utilizes a combination of concentration-dependent parameters (magnetic susceptibility, IRM) and grainsize-dependent parameters (ARM/IRM, hysteresis parameters). Magnetic mineralogy is characterized by a combination of low-temperature measurements and S-ratios, and our magnetic measurements are complemented by XRD, LOI and smearslide analyses. During periods of forest growth within the watershed, deposition of terrigenous material is low and the sediment magnetic properties are characterized by low concentrations of mainly authigenic minerals (low values of IRM, high ratios of ARM/IRM). During the early to mid-Holocene dry period, deposition of terrigenous material increased due to intensified dust deposition and the erosion of lake margins caused by lowered water levels. Concentration of magnetic minerals increases (high IRM, χ) and so does the grain-size of the magnetic fraction (low ARM/IRM). During the late Holocene, sediment magnetic properties depend on the varied position of the site with respect to the prairie–forest ecotone.

Late Quaternary ice extent and glacial history from the Dj�p�ll trough, off Vestfirdir peninsula, north-west Iceland: a stacked 36 cal. Ky environmental record

Djupall is a O90 km long by 15 km wide trough which extends from Isafjardardjup to the shelf break above Blosseville Basin, north of the Denmark Strait. We present 3.5 kHz seismic profiles from this trough and data from cores collected in 1996 (JM96-1232 and ?1234) and five cores collected on cruise B997. We pay particular attention to B997-338 as this core recovered sediments ranging in age between 12 and 36 cal. Ky BP. This is the first such record from the Iceland continental shelf. Dating control is provided by AMS 14C dates and the occurrence of the Saksunarvatn tephra. X-radiographs of the cores enable us to quantify the input of iceberg-rafted detritus (IRD) and to describe the lithofacies. The sediment matrix is fine-grained and might represent either rain-out of suspended sediment plumes or distal turbidites. IRD is present from ca. 12 cal. Ky BP throughout the next 24 cal. Ky with some IRD-free intervals. Using sediment magnetic properties, sampled at 1 cm (o100 yrs/sample) resolution, we provide a stacked environmental record which includes marine isotope stages 1, 2 and part of 3. The sediment magnetic properties kARM and IRM(60), and carbonate and TOC, show multi-millennia quasi-periodic cycles, but there are no obvious events coeval with the North Atlantic Heinrich events. Our data indicate that at the Last Glacial Maximum on the Vestfirdir peninsula (VP), north-west Iceland, ice did not reach the shelf break, but was probably grounded near the mouth of Isafjardardjup. A rapid increase in the rate of sediment accumulation suggests that deglaciation of the VP occurred mainly between 11 and 15 cal. Ky BP.

Distribution, sediment magnetism and geochemistry of the Saksunarvatn (10 180 ± 60 cal. yr BP) tephra in marine, lake, and terrestrial sediments, northwest Iceland

AbstractIn 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub‐bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99‐2269 in Húnaflóaáll, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic‐wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine‐ to medium‐grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and ‘hard’ magnetisation in a −0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice‐core that dates to 10 180 ± 60 cal. yr BP, and which was correlated with the 9000 14C yr BP Saksunarvatn tephra. We present accelerator mass spectrometry 14C dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 14C yr BP off northwest Iceland. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Magnetic properties of sediments from Ocean Drilling Program sites 1109, 1115, and 1118 (Leg 180), Woodlark Basin (Papua New Guinea)

Latest Miocene-Pleistocene synrift sediments at Ocean Drilling Program sites 1109, 1115, and 1118 (Leg 180), located on the hanging wall margin north of the Moresby fault in the Woodlark Basin, showed variations in magnetic parameters carried by magnetite and maghemite related to sedimentation process in the basin. At sites 1109 and 1115, an increase in the sedimentation rate at 3.8 Ma was accompanied by the deposition of sediments with low ferrimagnetic mineral concentrations. An increase in the ferrimagnetic mineral concentrations occurred between 3.4 Ma and 3.2 Ma at the three sites. The onset age of the change became younger with distance from the subsidence center of the basin near the Moresby fault: 3.4 Ma at Site 1118, 3.3 Ma at Site 1109, and 3.2 Ma at Site 1115, which implies a northward onlapping of sediments with high ferrimagnetic mineral concentration. Sediments with finer-grained ferrimagnetic minerals were deposited between 2.3 and 2.0 Ma at sites 1118 and 1109 and later, 2.8 Ma at Site 1115 during a period of a low sedimentation rate. The upper parts of sites 1109 and 1115 had a diamagnetic contribution, which is attributed to relatively high concentrations of diamagnetic pelagic materials at a low sedimentation rate associated with the low frequency of turbidites.

Late Quaternary ice extent and glacial history from the Djúpáll trough, off Vestfirdir peninsula, north-west Iceland: a stacked 36 cal. Ky environmental record

Djúpáll is a Ø90 km long by 15 km wide trough which extends from Ísafjardardjúp to the shelf break above Blosseville Basin, north of the Denmark Strait. We present 3.5 kHz seismic profiles from this trough and data from cores collected in 1996 (JM96-1232 and ?1234) and five cores collected on cruise B997. We pay particular attention to B997-338 as this core recovered sediments ranging in age between 12 and 36 cal. Ky BP. This is the first such record from the Iceland continental shelf. Dating control is provided by AMS 14C dates and the occurrence of the Saksunarvatn tephra. X-radiographs of the cores enable us to quantify the input of iceberg-rafted detritus (IRD) and to describe the lithofacies. The sediment matrix is fine-grained and might represent either rain-out of suspended sediment plumes or distal turbidites. IRD is present from ca. 12 cal. Ky BP throughout the next 24 cal. Ky with some IRD-free intervals. Using sediment magnetic properties, sampled at 1 cm (ø100 yrs/sample) resolution, we provide a stacked environmental record which includes marine isotope stages 1, 2 and part of 3. The sediment magnetic properties kARM and IRM(60), and carbonate and TOC, show multi-millennia quasi-periodic cycles, but there are no obvious events coeval with the North Atlantic Heinrich events. Our data indicate that at the Last Glacial Maximum on the Vestfirdir peninsula (VP), north-west Iceland, ice did not reach the shelf break, but was probably grounded near the mouth of Ísafjardardjúp. A rapid increase in the rate of sediment accumulation suggests that deglaciation of the VP occurred mainly between 11 and 15 cal. Ky BP.