Magnetic Mineral Grain Size Research Articles

Magnetic Mineral Dissolution in Heqing Core Lacustrine Sediments and Its Paleoenvironment Significance

The magnetic parameters within lacustrine sediments serve as invaluable proxies for deciphering the paleoenvironmental and paleoclimatic conditions. However, the dissolution of magnetic minerals can significantly alter detrital magnetic mineral assemblages, thereby complicating their interpretation in paleoenvironmental reconstructions. In an effort to clarify the impact of this dissolution on the grain size of magnetic minerals in lacustrine sediments, we undertook a thorough analysis of the rock magnetic properties on samples from the interval characterized by low ARM (anhysteretic remanent magnetization)/SIRM (saturation isothermal remanent magnetization) values between 140 and 320 ka in the Heqing (HQ) lacustrine drill core, located in Southwest China. Temperature-dependent magnetic susceptibility and FORC diagrams revealed a predominance of single-vortex and pseudo-single domain (PSD) magnetite and maghemite within the sample. When compared to samples from both the glacial and interglacial periods, the high SIRM, elevated magnetic susceptibility, and low ARM/SIRM ratio intervals from 140 to 320 ka suggested a high concentration of magnetic minerals coupled with a relatively low concentration of fine-grained particles in the sediments. The reductive dissolution of the fine-grained magnetic oxides is responsible for the reduction in the fine-grained magnetic particles in this interval. Our findings indicate that pedogenic fine-grained magnetite and maghemite are the first to dissolve, followed by the dissolution of coarser-grained iron oxides into finer particles. This process underscores the complex interplay between magnetic mineral dissolution and grain size distribution in lacustrine sediments, with significant implications for the reliability of paleoenvironmental interpretations derived from magnetic parameters.

Multi-Proxy Environmental Reconstruction of the Covacha de los Zarpazos in Galería Site (Atapuerca, Spain)

Magnetic properties of iron oxides provide invaluable data for environmental reconstruction during sediment deposition. Encouraged by the results of our previous studies in the near Gran Dolina site, we investigate the GI waterlain facies sediments that fill Galería Complex cave (Sierra de Atapuerca, Spain), in particular the Covacha de los Zarpazos cave considering two vertical profiles at different walls and comparing them with two profiles previously analysed. For the first time in this cave, we propose a correlation among these two walls based on the low field susceptibility values. An environmental reconstruction is proposed based on the pedogenesis and weathering processes, and water activity influencing the composition, concentration and grain size of magnetic minerals. In order to have a more comprehensive interpretation, elemental geochemistry was also analysed. The results allow to identify three different sections with the alternation of environmental characteristics.

Response of Magnetic Minerals to the Mid-Brunhes Climate Event Recorded in Deep-Sea Sediments of the West Philippine Sea

The Mid-Brunhes Event (MBE) was one of the most important global climate events since 800 ka. The deep-sea palaeoceanographic changes in the Western Pacific might have been more sensitive to the MBE and they have not been well documented yet. In this study, we investigated a deep-sea core collected from the Western Philippine Sea and then obtained the magnetism record since around 900 ka. The MBE signal in the deposition process was derived from the concentration-dependent (χ, χARM and SIRM) and the grain-size-dependent magnetic parameters (χARM/χ and χARM/SIRM). Across the MBE, the content of magnetic minerals and the fluctuation of the grain size decreased. Both the content and grain-size indices calculated from magnetic parameters are well correlated with the glacial–interglacial alternations and display a major shift at the MBE. The decreased grain size of magnetic minerals may be associated with the flourishing biota in the tropical Pacific during interglacial intervals. The accumulation of magnetic minerals in the Western Philippine Sea was mainly regulated by the sediment accumulation rate, which may be related to the shoaling of the carbonate compensation depth. Overall, this study expanded the environmental magnetism record for the MBE, suggested a new possible influence of this critical climatic event on the deep-sea deposition process in Western Philippine Sea and inferred the interactions between various environmental systems on glacial–interglacial timescales.

Magnetic Properties of Cherts from the Basque-Cantabrian Basin and Surrounding Regions: Archeological Implications

We present the first rock magnetic study of archeologically-relevant chert samples from the Basque-Cantabrian basin (BCB) and surrounding regions, which was conducted in order to test the usefulness of non-destructive magnetic properties for assessing chert quality, distinguishing source areas, and identifying heated samples in the archeological record. Our results indicate that the studied BCB cherts are diamagnetic and have very low amounts of magnetic minerals. The only exception is the chert of Artxilondo, which has a mean positive magnetic susceptibility associated with larger concentrations of magnetic minerals. But even in this case, the magnetic susceptibility is within the lower range of other archeologically-relevant cherts elsewhere, which indicates that the studied BCB cherts can be considered as flint. The similar mean values for all magnetic properties, along with their associated large standard deviations, indicates that rock magnetic methods are of limited use for sourcing different types of flint except in some specific contexts involving the Artxilondo flint. With regards to the identification of chert heating in the archeological record, our results indicate only a minor magnetic enhancement of BCB natural flint samples upon heating, which we attribute to the low amount of non-silica impurities. In any case, the diamagnetic behavior of most BCB natural flints, along with the local use only of the Artxilondo type, suggests that any flint tool within the core of the BCB with positive magnetic susceptibility values is likely to have been subjected to heating for improving its knapping properties. Further studies are necessary to better identify the type, origin and grain size of magnetic minerals in BCB natural flints, and to apply non-destructive magnetic properties to flint tools in order to identify the use of heat treatment in the BCB archeological record.

Hydrological and ecosystem response to abrupt changes in the Indian monsoon during the last glacial, as recorded by sediments from Xingyun Lake, Yunnan, China

The Indian Summer Monsoon (ISM) is the major moisture source of precipitation in southwest China and plays an important role in regional environmental change and in economic and social development, since abrupt changes of the ISM can cause crop failures and flooding that impact the livelihood of a large population. However, with the exception of speleothem records, abrupt changes of the ISM during the last glacial are not well documented in terrestrial records. Here we present a sediment record from Xingyun Lake in Yunnan Province, southwest China, spanning the interval 36.4kaBP to 13.4kaBP (1ka=1000cala) which documents the occurrence of a series of millennial-scale episodes which may reflect the abrupt weakening of the ISM. Seven AMS 14C dates from terrestrial wood fragments provide a robust chronology. The results demonstrate that increases in the coarse sediment fraction (>30μm) correspond to increases in the concentration of Pediastrum, the grain size of magnetic minerals, carbonate content and C/N ratios, which together indicate episodes of low lake level. In addition, pollen analysis indicates that Quercus (oak) increased and that ferns decreased during intervals of abrupt weakening of the ISM, indicating a drier environment. Nine abrupt shifts to a low lake level are documented for the studied interval and three of them are correlative with Heinrich events 1 to 3. Our study supports the hypothesis that, in response to the cold events in the North Atlantic associated with massive iceberg influxes and the corresponding slowing down of Atlantic meridional overturning circulation, the mean latitudinal extent of the ITCZ shifted southwards. As a result of an atmospheric teleconnection there was a corresponding decrease in precipitation associated with ISM precipitation, which caused reduced fluvial runoff in Yunnan.

Changing intensity of human activity over the last 2,000 years recorded by the magnetic characteristics of sediments from Xingyun Lake, Yunnan, China

We report mineral magnetic measurements (magnetic susceptibility, anhysteretic and isothermal remanence, magnetic hysteresis and thermomagnetic properties) from a 2.46-m-long sediment core taken in Xingyun Lake, Yunnan, China. Results demonstrate that magnetic minerals in the lake sediment are dominated by stable single-domain (SSD) ferrimagnetic minerals derived mainly from in-washed catchment soil. Magnetic susceptibility increases sharply during the period AD ~500–1050, and the grain size of magnetic minerals decreases. These changes are accompanied by increases in the concentrations of Fe, Ti, Zr and Si, and by decreased tree pollen percentages. δ18O measures in speleothems from South China demonstrate that climate was relatively uniform during this interval. Documentary evidence, however, indicates a large increase in the human population of Yunnan Province at that time. We conclude that the late Holocene sediment record from Xingyun Lake reflects a dramatic increase in soil erosion that was caused by a growing human population, which in turn was responsible for increased deforestation and agricultural activity. Our results from Xingyun Lake are consistent with sediment magnetic records from Erhai Lake, located near our study site. We therefore conclude that the interval AD ~500–1050 corresponded to a regional-scale increase in the intensity of human activity on the Yunnan-Guizhou Plateau.

IDENTIFIKASI MINERAL MAGNETIK PADA GUANO DI GUA BAU-BAU KALIMANTAN TIMUR MENGGUNAKAN SCANNING ELECTRON MICROSCOPE (SEM)

Change of environment of a cave can be seen from the presence of magnetic minerals in guano. In this research the magnetic minerals identified a Guano in Bau-Bau caves, East Kalimantan. This research aim to look the morphology, elemental composition, the kind of magnetic minerals, and how magnetic minerals transport into the cave. Magnetic minerals in guano identified by using Scanning Electron Microscope (SEM) that completed with Energy Dispersive Spectroscope (EDS). The result showed the morphology of magnetic minerals of the guano is oval and have some cracks, in addition to the grain size of magnetic minerals is multidomain, guano elements are Fe, O, Ti , Si, C, N , and Br. To determine the kind of magnetic minerals using Proust's Law. The results of calculation the kind of magnetic minerals that can form Hematite (Fe 2 O 3 ), Magnetite (Fe 3 O 4 ), Ilmenite (FeTiO 3 ) and Rutile (TiO 2 ) . This is indicates the magnetic grains estimated come from the external environment and during the rainy, transportation into the cave. Keywords : Change of environment, magnetic minerals, guano, SEM

Environmental magnetic studies on surface sediments: a proxy for metal and hydrocarbon contamination

Visakhapatnam is one of the major port cities, and it is developed into a hub of many large- and medium-scale industries. Due to growing industrialization and urbanization, coast is vulnerable to both organic and inorganic micro-pollutants. Twenty-five surface sediments were collected along the Visakhapatnam coast for the measurement of texture size, petroleum hydrocarbons, trace metals and environmental magnetic parameters. The percentage of coarser particles was more in the northern region, whereas the percentage of fine particles was increased toward south. Elevated levels of petroleum hydrocarbons and trace metals were attributed due to marine and land-based sources, in particular, those were due to shipping activities, treated and partially treated sewage and industrial wastes. The concentrations of trace metals, petroleum hydrocarbons and magnetic minerals were decreased from nearshore to seaward. Our results revealed that the magnetic mineralogy is dominated by magnetite with a small proportion of hematite, and the grain size of magnetic minerals was in the range of pseudo-single domain to multidomain nature with detrital origin. From the principal component analysis, the magnetic concentration and mineralogy-dependent parameters co-vary with the heavy metal and PHC concentrations, suggesting that the inputs of magnetic minerals, petroleum hydrocarbons and heavy metals in the Visakhapatnam shelf sediments were derived from the same anthropogenic sources. Thus, the large magnetic dataset can be used to reduce the number of chemical analysis; hence, environmental magnetic parameters were used as a proxy for both organic and inorganic micro-pollutants.

Rock magnetic properties and paleoenvironmental implications of an 8-Ma Late Cenozoic terrigenous succession from the northern Tian Shan foreland basin, northwestern China

In the northern Tian Shan foreland basin, northwestern China, the thick Cenozoic terrigenous succession is crucial for paleoclimate–environmental reconstruction of the Asian interior. Here we present a detailed rock magnetic investigation on 245 samples from the ~1200-m-thick Neogene Taxi He section with a magnetostratigraphic age span of ca. 8.0 to 2.0Ma in the northern Tian Shan foreland basin. Our rock magnetic results indicate that the significant variations in composition, concentration and grain size of magnetic minerals occurred at ca. 6.0, 3.7 and 2.7Ma. The comparable compositions of rare earth elements (REEs) throughout the Neogene Taxi He section suggest no significant modification of the source materials during the interval between ca. 8.0 and 2.0Ma, and thus sediment provenance is not regarded as responsible for these observed variations in rock magnetic properties. Our further analyses show that the variations in magnetic properties of the Taxi He section are casually linked mainly with lithofacies transition due to range encroachment into foreland basin as well as climate aridification. Identified enhancement of aridification was chronologically constrained at ca. 6.0 and 2.7Ma. Such climate events are important archives for reconstructing the Late Cenozoic paleoclimatic history of the Asian interior. Further comparison between different paleoclimate records clearly indicates that magnetic parameters such as S−100mT are potentially effective proxy indices for paleoclimate–environmental reconstruction in the Tian Shan foreland basins and the nearby areas.

MAGNETIC PROPERTY AND DOMINATING FACTORS OF THE LOESS AT NORTHERN PIEDMONT OF TIANSHAN,XINJIANG

We carried out systemic rock-magnetic investigations on the loess/paleosol sequence at the northern piedmont of the Tianshan Mountain,Xinjiang.Results show that the main magnetic minerals of this sequence are magnetite,maghaemite,haematite,and the main magnetic grain size of magnetic minerals are PSD and MD.The magnetic susceptibility of the loess layer in the Tianshan Mountains is higher than that of paleosoil,which is different from the loess in Loess Plateau.Magnetic susceptibility of the sediments is controlled by native ferrimagnetic minerals from source.Fine grain magnetic materials produced by pedogenic progress make limited contribution to magnetic susceptibility of the loess.

Diagenetic alteration of magnetic minerals in Labrador Sea sediments (IODP Sites U1305, U1306, and U1307)

In order to reveal the potential effects of early diagenesis on magnetic minerals in deep‐sea sediments, we studied early diagenetic zones and magnetic mineral characteristics of Lower Pliocene hemipelagic sediment samples from IODP Sites U1305, U1306, and U1307 on Eirik Drift, Labrador Sea. All samples analyzed were unlithified silty clay sediments recovered by a piston corer from depths down to 200 meters composite depth (mcd). Based on shipboard interstitial‐water geochemistry, we divided the sediment column from each site into six early diagenetic zones. Magnetite (Fe3O4) was present at all analyzed depths, whereas maghemite (γFe2O3) was found only above the iron reduction zone. We attribute this to associated changes in interstitial redox conditions, which induced preferential dissolution of maghemitized surfaces on magnetite grains. Mineral magnetic results indicate a general down‐hole change in mean grain size of magnetic minerals. At Site U1307, which has relatively low organic carbon contents, the diagenetic zones occur at greater depths than at the other studies sites. This suggests that interstitial oxygen levels at this site remained high enough to degrade organic matter through oxic bacterial activity, and that detrital magnetic minerals have been preserved even at depth.

Comparison of Topsoil Magnetic Properties Between the Loess Region in Tianshan Mountains and Loess Plateau, China, and Its Environmental Significance

AbstractThe topsoils of the loess region of Tianshan Mountains and Loess Plateau in northwestern China are systematically collected and analysed on their rock magnetism and grain size. Their magnetic characteristics indicate that high magnetic minerals are magnetite and maghemite, and the weakly magnetic minerals are hematite, goethite or lepidocrocite. There are at least parts of maghemite deposited in the Loess Plateau that was originally from aeolian source area. The grain size of magnetic minerals (and particle size) in Tianshan topsoil samples is generally much coarser; mostly within range of multi‐domain (MD). Hard magnetic minerals in Tianshan topsoils are in higher percentage than that in Loess Plateau. The magnetic susceptibility of topsoils from Loess Plateau shows a good positive relation with frequency dependent susceptibility. Climatic condition, such as temperature and precipitation, especially the latter one, is explained as a major factor effecting susceptibility enhancement in the Loess Plateau. Tianshan topsoil samples contain a very small amount of ultrafine magnetic grains produced during pedogenic process, such ultrafine magnetic grains have very limited contribution to the susceptibility enhancement. Tianshan topsoils generally show a characteristic of less weathering or weak pedogenesis, aeolian coarser grains therefore play an important role in mechanism of magnetic susceptibility. Frequency dependent susceptibility shows a close relation with mean annual precipitation (MAP) in all studied areas, However, the relations between magnetic susceptibility and MAP are different in various regions: showing positive correlation in the central Loess Plateau (study area), but negative in Tianshan regions. The precondition of ‘wind vigor' model is discussed, ‘wind vigor' theory is debated as not suitable for interpretation of Alaskan and Siberian loess, but suitable for Xinjiang loess.

Magnetic analyses of soils from the Wind River Range, Wyoming, constrain rates and pathways of magnetic enhancement for soils from semiarid climates

In order to constrain the rate of magnetic enhancement in soils, we investigated modern soils from five fluvial terraces in the eastern Wind River Range, Wyoming. Profiles up to 1.2 m deep were sampled in 5-cm intervals from hand-dug pits or natural riverbank exposures. Soils formed in fluvial terraces correlated to the Sacajawea Ridge (730–610 ka BP), Bull Lake (130–100 ka BP) and Pinedale-age (∼20 ka BP) glacial advances. One soil profile formed in Holocene-age sediment. Abundance, mineralogy, and grain size of magnetic minerals were estimated through magnetic measurements. Magnetic enhancement of the A-horizon as well as an increase in fine-grained magnetic minerals occurred mostly in Bull Lake profiles but was absent from the older profile. Such low rates of magnetic enhancement may limit the temporal resolution of paleosol-based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols. A loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age. Our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age, or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis. Absolute and relative hematite abundance increase with age, making both useful proxies for soil age and the dating of regional glacial deposits. All coercivity proxies are consistent with each other, which suggests that observed changes in HIRM and S-ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions, even though the modified L-ratio varies widely.

Rock magnetic properties of the Gardar Drift sedimentary sequence, Site IODP U1314, North Atlantic: Implications for bottom current change through the mid-Pleistocene

The sediment obtained at Integrated Ocean Drilling Program (IODP) Site U1314 on the Gardar Drift in the eastern flank of the Reykjanes Ridge was analyzed using rock magnetic techniques to study the depositional environment through the mid-Pleistocene climate transition. The study area is under the influence of Iceland–Scotland Overflow Water (ISOW). Consequently, the sedimentary record represents directly ISOW behavior over time. A change in the depositional environment was documented as magnetic property changes that are the result of changes in concentration and grain size of magnetic minerals, both of which decrease upward from around 0.9 Ma. Because these shifts are accompanied by a change in magnetic fabrics, which is interpreted as a change in bottom current directions from E–W to NNE–SSW, it is considered that the sedimentation pattern inferred from magnetic records at the site was modified by a change in the bottom current through the middle Pleistocene.

Magnetic properties of the urban soils in Shanghai and their environmental implications

To study the feasibility of using magnetic techniques for monitoring soil pollution in Shanghai, magnetic properties and heavy metals in the topsoils in an urban site (Songnan Town) and a less-urbanized agricultural site (Luojing Town) in Baoshan District, Shanghai, were studied. Compared with the background, magnetic signals of the urban topsoils are extremely enhanced with magnetic susceptibility ( χ lf) from 127.3–1959 × 10 − 8 m 3 kg − 1 ; while those of the agricultural topsoils are only slightly increased. However, both the urban and agricultural topsoils contain few pedogenic SP grains, as indicated by their low χ fd% (< 3.6%). Ratios of χ arm/SIRM, χ arm/ χ lf and SIRM/ χ lf indicate that the grain size of magnetic minerals in the urban topsoils is significantly coarser than that in the background and the agricultural topsoils. Furthermore, the urban topsoils show low coercivity and magnetic soft behaviors, as indicated by higher SOFT%, lower HARD%, higher IRM 300 mT/SIRM (close to 1) and lower IRM − 200 mT /SIRM (close to − 1). It suggests that the urban topsoils have received some coarse ferrimagnetic particles. Heavy metals are highly enriched in the magnetic fractions of the topsoils. Geochemical properties of the magnetic fraction of the urban topsoils are significantly different from those of the agricultural topsoils, further indicating that the extra magnetic minerals accumulated in the urban topsoils are neither inherited from soil parent materials nor from pedogenic processes, but originate from anthropogenic activities. The significant correlations between heavy metals and χ lf, χ arm, SIRM, SOFT and HIRM of the topsoils in the district indicate that the magnetic techniques can be used for monitoring soil pollution in Shanghai. The soils with χ lf from 39–50 × 10 − 8 m 3 kg − 1 in the district are tentatively defined as “slightly polluted soils”; those with χ lf > 50 × 10 − 8 m 3 kg − 1 are defined as “polluted soils”.

Magnetic mineral diagenesis in the post-glacial muddy sediments from the southeastern South Yellow Sea: Response to marine environmental changes

Core YSDPlO3 was retrieved in the muddy deposit under the cold eddy of the southeastern South Yellow Sea, and the uppermost 29.79 m core represents the muddy sediments formed in the shelf since about 13 ka BP. The lower part from 29.79 to 13.35 in, called Unit A2, was deposited during the period from the post-glacial transgression to the middle Holocene (at about 614c ka BP) when the rising sea level reached its maximum, while the upper part above 13.35 m (called Unit Al) was deposited in a cold eddy associated with the formation of the Yellow Sea Warm Current just after the peak of post-glacial sea level rise. Rock-magnetic properties of the uppermost 29.79 m core were investigated in detail. The experimental results indicate that the magnetic mineralogy of the core is dominated by magnetite, maghemite and hematite and that, except for the uppermast 2.35 m, the magnetic minerals were subject to reductive diagenesis leading to significant decline of magnetic mineral content and the proportion of low-coercivity component. More importantly, ferrimagnetic iron sulphide (greigite) is found in Unit A2 but absent in Unit Al, suggesting the control of marine environmental conditions on the magnetic mineral diagenesis. Magnetic parameters show abrupt changes across the boundary between Units A1 and A2, which reflects a co-effect of environmental conditions and primary magnetic components of the sediments on the diagenesis. Alternating zones of high and low magnetic parameters are observed in Unit A2, which is presumably due to periodic changes of the concentration and/or grain size of magnetic minerals carried into the study area.

Magnetic properties of the profiles of polluted and non-polluted soils. A case study from Ukraine

SUMMARY This report deals with the preliminary study of the magnetic properties of the chernozem depth profiles from polluted and non-polluted areas situated in the Volnovaha–Mariupol agro-soil region in southeast Ukraine. The profiles of soil for study were taken from the chernozem virgin land of the Homutovski steppe and from the nearby polluted area of Mariupol belonging to the Dniprovsko–Donetskij industrial region. The magnetic mineralogy and magnetic properties were examined along the profiles to determine the vertical structure of the non-polluted soil and to find differences caused by pollution. The soil from the non-polluted area is characterized by maghemite and haematite content. Magnetite is present in the topsoil of profiles from the polluted area of Mariupol. Blocking temperatures increase with depth in all profiles, becoming close to the temperature characteristic for haematite. Hysteresis parameters show that the deep soil is characterized by higher coercivity and coercivity of remanence values and lower saturation isothermal remanent magnetization and saturation magnetization values than the respective parameters in the topsoil. The grain size of magnetic minerals also changes with depth, becoming larger for deeper horizons. The surface values of susceptibility, saturation magnetization, saturation isothermal remanent magnetization and anhysteretic remanent magnetization for the polluted profiles from Mariupol show enhancement in relation to the values for the Homutowski steppe as a result of contamination by magnetite and haematite. However, starting from a depth of approximately 80–100 cm, the magnetic parameters approach the same values. This suggests that in deep horizons a lithogenic contribution prevails, similar for both areas.

Mineral magnetic properties of loess/paleosol couplets of the central loess plateau of China over the last 1.2 Myr

We have conducted a multiparameter investigation of 15 loess‐paleosol couplets (S0/L1 to S14/L15) from the Jiaodao section in the central loess plateau of China using environmental magnetic approaches coupled with soil science techniques. The magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis. High‐temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the Lishi Formation to the Holocene black loam, possibly indicating a decrease in weathering intensity. This may reflect a long‐term increase in aridity and/or a general long‐term cooling trend of the interior of the Asian continent from 1.2 Ma to the present. Several samples display wasp‐waisted hysteresis loops. These are most pronounced in moderately enhanced paleosols, less pronounced in the practically unaltered loess, and subdued in the well‐developed paleosols, but wasp waistedness reappears in the most developed paleosols. This wasp‐waistedness sequence suggests that the composition, concentration, and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess‐paleosol sequence, but each factor has a different effect at different stages of pedogenesis.

Magnetic susceptibility variations in Upper Pleistocene deep‐sea sediments of the NE Atlantic: Implications for ice rafting and paleocirculation at the Last Glacial Maximum

Magnetic susceptibility (MS) variations are used to intercorrelate 17 Upper Pleistocene sediment cores taken from the NE Atlantic, between 40° and 60°N. The MS‐based correlation depends on regionally consistent patterns of variation in the deposition of ice‐rafted detritus (IRD) in response to Pleistocene glaciations, and especially to high‐frequency ice‐rafting episodes referred to in recent studies as “Heinrich events.” The sedimentological and rock‐magnetic basis for the apparent relationship between the MS signal and IRD content of NE Atlantic sediments is examined by (1) comparing the MS profiles of selected cores with their records of coarse fraction (&gt; 150 µm) lithic fragment abundance and Neogloboquadrina pachyderma (sin) percentages, and (2) normalizing MS by expressing it both on a carbonate‐free basis, and as a quotient with anhysteretic remanent magnetization (a parameter sensitive to magnetic mineral grain size variations). These comparisons show that variations in bulk‐sediment MS are only partly driven by simple carbonate dilution (± productivity and dissolution) effects. Changes in both the concentration and grain size of magnetic minerals within the lithogenic noncarbonate fraction also impose a significant influence on bulk MS values. In particular, horizons rich in IRD are associated with significant increases in the relative proportion of coarse grained (multidomain) ferrimagnetic particles in the sediment. This is because ice‐rafting, in contrast to most other mechanisms capable of transporting detrital magnetic minerals to pelagic environments, has a high potential for delivering large ferrimagnetic grains as components of sand‐sized, polycrystalline lithic fragments. This fundamental linkage between the IRD content and MS signal of NE Atlantic sediments is used to reconstruct the patterns of variation in IRD deposition and, by inference, surface currents of the last glacial maximum (LGM, ∼18–19 ka) relative to the present‐day NE Atlantic, using the time‐slice mapping approach developed by the CLIMAP project group. Our LGM/Holocene MS ratio map, based on sample pairs from over 80 deep‐sea cores, confirms that there was a weak, cyclonic gyre north of the polar front in the LGM North Atlantic. The gyre comprised a sluggish warm current in the NE Atlantic flowing north between latitudes 47° and 62°N, partly fed by subtropical waters from south of the polar front, and carrying large numbers of icebergs derived from several sources, most of which melted between Latitudes 45° and 52°N. The warm current probably continued its flow into the Iceland Basin, where it fed into a south‐flowing current which transported melting icebergs from Iceland and Scandinavia along the western flank of the Reykjanes Ridge.

Contributions of fine-particle magnetism to reading the global paleoclimate record (invited)

Paleoclimate changes are recorded by proxy as variations in concentration, composition, and grain size of magnetic minerals, principally magnetite (Fe3O4), in the sediments deposited in lakes, oceans, and continental eolian deposits. Cross-validated multiple-parameter magnetic measurements of such sediment cores provide global change data of high temporal resolution, useful for constructing a base-line record against which anthropogenic modifications may be discerned. Theories of superparamagnetism and magnetic domains are used to explain the physical basis of magnetic proxy recording. Examples of applications to validation of Milankovitch theory of climate change and delineation of the glacial and interglacial stages of the last 1 000 000 years are provided.