Rock Magnetic Signature Research Articles

Rock Magnetic Signature of Red Beds From the Intermontane Tarom Basin (NW Iran): Insights Into Middle to Late Miocene Environmental Conditions and Possible Forcing Mechanisms

AbstractThe study of sedimentary magnetism in the intermontane Tarom Basin (northern Iran) offers insights into local paleoenvironmental conditions during global middle‐late Miocene climate changes and the topographic growth triggered by the Arabia‐Eurasia continental collision. Rock magnetic results reveal that the ∼16.2 to ∼10–9 Ma coarse‐grained deposits at the basin's southern margin present a homogenous magnetic mineral assemblage, reflecting sediment provenance. Conversely, the ∼13.2 to ∼7.6 Ma, fine‐grained deposits in the basin's depocenter include alternating playa‐lake and lacustrine deposits, recording dry, evaporative conditions, leading to hematite formation in a low‐temperature oxidizing environment, and wetter conditions that preserve the original detrital signal, respectively. Time series analyses show cyclicity in different period bands for magnetic susceptibility, but precession and obliquity cycles can hardly be resolved in the record. Comparison with deep‐sea oxygen isotope data suggests that from ∼13.2 to ∼10.8 Ma environmental conditions likely mirrored global climatic forcing, with lacustrine and playa‐lake deposits associated with increased and decreased global temperature, respectively. At ∼10.8 Ma, the basin likely recorded the Tortonian Thermal Maximum with the establishment of a lacustrine system. From ∼10.4 Ma, the magnetic susceptibility signal departed from the global climate record, possibly due to basin margin (western Alborz and Tarom mountains) and regional (Anatolian‐Iranian plateau) topographic growth, accompanied by increased precipitation seasonality, focused rainfall and augmented erosion rates. Finally, we suggest that before ∼10.8 Ma, the Hadley cells expanded northward, leading to a trade‐dominated system with moist air masses sourced from the Caspian, while from ∼10.8 Ma, westerlies dominance progressively prevailed.

Rock Magnetic Signatures of Hydrothermal Mineralization in the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field

AbstractThe Ocean Drilling Program Leg 158 drill holes from the Trans‐Atlantic Geotraverse hydrothermal field are investigated to understand the rock magnetic signatures of hydrothermal mineralization. A composite columnar section has been constructed through hole correlation to understand the stratigraphic variation of magnetomineralogy within the stockwork. Isothermal remanent magnetization components unmixing, first‐order reversal curve diagrams, low‐temperature magnetic signatures, and electron microscopic analyses disclose magnetic minerals of disparate occurrences related to predominating hydrothermal mineralization reactions in three broad zones: For basaltic basements, serpentinization of olivine phenocrysts during preliminary hydrothermal alteration produces magnetite, in addition to primary titanomagnetite; Chloritized and silicified zone samples contain relict titanomagnetite and exsolved magnetite that survived hydrothermal dissolution; Anhydrite and sulfide zone samples are dominated by magnetite and hematite, likely from oxidation of polymetallic sulfides due to exposure in oxidative seafloor environments during drilling. Our findings suggest that seafloor oxidation potentially modifies the magnetic properties of polymetallic sulfides in hydrothermal deposits, which applies to magnetic tomography of sophisticated subseafloor vent structures and prospecting seafloor massive sulfides (SMS) deposits therein. Meanwhile, we alert future deep‐sea mining that drilling may promote physicochemical alteration of SMS deposits, causing environmental risks. The established magnetic signatures ultimately contribute to understanding the in situ geological preservation of SMS deposits and optimizing exploitation procedures in the future.

Rock Magnetic Signature of Heterogeneities Across an Intraplate Basal Contact: An Example From the Northern Apennines

AbstractHeterogeneities in the magnetic signature along intraplate shear zones complicate their correlation with the physical processes that are involved in the geodynamic evolution of megathrusts. Isolating the preferred orientation of different magnetic minerals may provide insights into faulting processes, tectonics, and strain partitioning. Studies of exhumed analogs are fundamental to constrain variations in the magnetic properties with respect to a geodynamic context of intraplate shear zones. This study uses a combined statistical and magnetic approach to separate the contribution of coexisting petrofabrics to better interpret the heterogeneities in the magnetic signature. The main results indicate that there is a strong dependency among the variation in the magnetic properties and their anisotropy with the distance from the thrust faults and strain localization within the shear zone. Close to the main thrust, we observed a shear‐related fabric, indicating a high degree of non‐coaxial strain. Away from the thrust faults, the degree of anisotropy and the ellipsoids' oblateness gradually diminishes and subfabrics related to previous tectonic events or less intense deformation become dominant. Our observations also indicate strain decoupling across the basal thrust with dominant vertical uniaxial strain within the footwall. In contrast characterization of the anisotropy of magnetic remanence provides significant information on the variable response of different subpopulations of ferromagnetic minerals to shearing, indicating different degree of reworking or independent deformation processes.

Rock Magnetic Signatures of the Dalma Formation in the Singhbhum Mobile Belt, Eastern India

Abstract Petrography and rock magnetic properties of the Dalma volcanics (DV) and associated meta-sediments of Singhbhum mobile belt (SMB) are presented to describe magnetic grain size and Fe-Ti oxide mineralogy of the volcano-sedimentary sequence. Petrography revealed the presence of Ti-magnetite, hematite, ilmenite and pyrite in quartz-biotite-sericite-muscovite schists, quartzites, and volcanic rocks. The average value of magnetic susceptibility (χ) is 5.6 × 10−7 m3/kg, anhysteretic remanent magnetization (ARM) is 2.25 × 10−5 Am2/kg, saturating isothermal remanent magnetization (SIRM) is 403.33 × 10−5 Am2/kg, soft IRM is 240.2 × 10−5 Am2/kg, hard IRM is 77.56 × 10−5 Am2/kg, and natural remanent magnetization (NRM) is 54.1 mA/m. The central tendency of the inter-parametric ratio ARM/χ is 0.06 kAm−1, SIRM/χ is 111.04 kAm−1 and S-ratio is 0.8. Based on results, magnetic mineralogy indicated that the bulk magnetic carrier is stable single domain (SSD) or pseudo single domain (PSD) grains of Fe-Ti oxides with minor iron sulphides such as pyrite, greigite and pyrrhotite. Petrography and rock magnetic studies suggest that the magnetic carriers are chiefly secondary in origin and the NRM is modified over time.

Rock magnetic evidence of tectonic control on the sedimentation and diagenesis in the Andaman Sea over ~1 million years

We evaluate the sedimentary (magnetic, mineralogical, geochemical, grain size) records of a long sediment core (NGHP-01-17 A) to elucidate the tectonic control on the sedimentation and diagenesis in the Andaman Sea (AS) over the past ~ 1 Ma. Two distinct sedimentary magnetic zones (Z-I and Z-II) are identified based on the rock magnetic signatures. A significant shift in the depositional environment is observed around ~90 ka which is marked by a rapid change in magnetite susceptibility (χlf), Fe, Ti and organic carbon content in Z-I. A trend of upward increase in χlf from ~90 ka to 3 ka in core NGHP-01-17 A suggest increase in detrital flux to the AS and represents the onset of a new sedimentary regime following most recent event of tectonic uplift in Indo-Burman Ranges (IBR) which occurred at ~ 150 ka. Methane influenced progressive diagenetic dissolution of detrital iron oxides and subsequent precipitation of iron sulfides in Z-II is clearly evident in the rock magnetic and mineralogical data. Multiple bands of diagenetically formed iron sulfide nodules at different periods in Z-II indicate episodic intensification of anaerobic oxidation of methane (AOM) due to enhanced methane-flux. We propose that fluid migration forced by tectonic processes facilitated episodic release of methane from deep reservoir that led to the formation of iron-sulfide nodules in the AS.

Controls of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record from the Bay of Bengal

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/SIRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record.

Rock magnetic signature of a Miocene playa cycle in Central Asia and environmental implications

The increased aridification of Central Asia during the Miocene coincides in time with lake formations and the evolution of playa environments in the region. However, Miocene continental climate dynamics and the forcing of aridification are still not well constrained. Neogene lacustrine mudflat deposits in the Ili Basin in southeast Kazakhstan provide a well-exposed paleoclimate archive. Here, we present a detailed rock magnetic study of a middle Miocene playa cycle deposited in a closed basin. We use high-resolution rock magnetic parameters, lithological studies and geochemistry to reconstruct the playa environment and the depositional conditions. The rock magnetic mineralogy of the playa cycle is controlled by hematite and two fine-grained magnetite phases. Increased magnetic concentrations occur during dry mudflat conditions, with a lower groundwater table and increased aridity. The underlying processes controlling the observed variation in magnetic concentrations are a complex interplay of diagenetic processes during and after deposition. The data support an authigenic origin of both magnetite phases, one formed before and the other after sediment consolidation. Early diagenetic formation of fine-grained magnetite by microbial activity is followed by post-depositional formation of a secondary fine-grained magnetite phase. The rock magnetic results such as magnetic concentration-dependent parameters, ARM/SIRM and s-ratio indicate a sensitive record of (ground) water availability and aridity changes in the Ili Basin. We suggest that they can serve as an effective proxy for detailed paleo-environment reconstruction of playa evolution, not only in the middle Miocene Ili Basin but also in comparable floodplain/playa lake settings.

Sedimentary and rock magnetic signatures and event scenarios of deglacial outburst floods from the Laurentian Channel Ice Stream

Eastern Canadian margin sediments bear testimony to several catastrophic deglacial meltwater discharges from the retreating Laurentide Ice Sheet. The reddish-brown plumite layers deposited on the levees of the Laurentian Fan valleys have been recognized as indications of multiple outburst floods between Heinrich events 2 and 1. Five event layers have been consistently recorded in three new gravity cores retrieved on the SW Grand Banks slope and comply with the previously published Laurentian Fan core MD95-2029. The apparently huge extent of these outburst plumes around the Laurentian Fan as well as their causes and consequences are investigated in this study using physical properties, rock magnetic and grain-size analyses, together with seismoacoustic profiling. We provide the first detailed 14C ages of the outburst event sequence and discuss their recurrence intervals in the context of regional ice retreat. Compared to the hemipelagic interlayers, event layers have overall uniform and systematic changes of rock-magnetic properties. Hematite contents increase over time and proximally while magnetite grain sizes fine upwards and spatially away from the fan. Based on the sediment composition and load, we argue that these plumites were formed by recurrent erosion of glacial mud deposits in the Laurentian Channel by meltwater outbursts. Three alternative glaciological scenarios are evaluated: in each case, the provenance of the transported sediment is not an indicator of the precise source of the meltwater.

Detrital and early chemical remanent magnetization in redbeds and their rock magnetic signature: Zicapa Formation, southern Mexico

Detrital and early chemical remanent magnetization in redbeds and their rock magnetic signature: Zicapa Formation, southern Mexico

Hydrothermally-induced changes in mineralogy and magnetic properties of oxidized A-type granites

Abstract The changes in magnetic mineralogy due to the hydrothermal alteration of A-type granitic rocks have been thoroughly investigated in samples from the granite of Tana (Corsica, France), and compared with other A-type granites: Meruoca (NE Brazil), Bushveld (South Africa), Mount Scott (Wichita Mountains, Oklahoma, USA) and the stratoid hypersolvus granites of Madagascar. The altered red-colored samples and their non-altered equivalents were magnetically characterized by means of magnetic susceptibility measurements, hysteresis loops, remanent coercivity spectra, and Lowrie test. It is shown that hydrothermalization in magnetite-bearing granites is related to the formation of fine-grained magnetite and hematite, and to coeval depletion in the content of primary low-coercive coarse-grained magnetite. These mineralogical changes give typical rock magnetic signatures, namely lower susceptibility magnitudes and anisotropy degrees, prolate AMS (anisotropy of magnetic susceptibility) fabrics and increased coercivities. Optical microscopy and SEM (scanning electronic microscopy) images suggest that the orientation of the secondary magnetic minerals is related to fluid-pathways and micro-fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics. Changes in magnetic mineralogy and grain-size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites.

Rock-magnetic signature of precipitation and extreme runoff events in south-eastern Patagonia since 51,200 cal BP from the sediments of Laguna Potrok Aike

A 106-m long sediment sequence from the maar lake Laguna Potrok Aike in southern Patagonia was recovered in the framework of the International Continental Scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). About half of the sedimentary sequence is composed of mass movement deposits (MMDs) and the event-corrected record reaches back to 51,200 cal BP. Here we present a high-resolution rock-magnetic study revealing two sedimentary facies associated with MMDs and characterized by two different types of spurious gyroremanent magnetization (GRM) acquired during static alternating field demagnetization. The first rock-magnetic signature is detected in MMDs composed of reworked sand and tephra material. The signature consists of GRM acquired during demagnetization of the natural remanent magnetization (NRM) and other rock-magnetic properties typical of iron sulfides such as greigite. We interpret these intervals as authigenic formation of iron sulfides in suboxic conditions within the MMD. The second rock-magnetic signature consists of a series of 10 short intervals located on the top of MMDs characterized by GRM acquisition during demagnetization of the isothermal remanent magnetization (IRM). Based on geological, limnological, stratigraphic and climatic evidence these layers are interpreted as reflecting pedogenic hematite and/or goethite brought to the lake by runoff events related to precipitation and permafrost melt. The pedogenic iron minerals mobilized from the catchment most likely settled out of suspension on top of MMDs after a rapid remobilization event. The series of runoff events corresponds to periods of increased lacustrine productivity in Laguna Potrok Aike and are coeval within the limit of the chronology to warm periods of the Last Glacial as recorded in Antarctica, the deglaciation in the mid-latitudes of the Southern Hemisphere and enhanced precipitation during the Early Holocene in southeastern Patagonia.

Signatures and significance of aeolian, fluvial, bacterial and diagenetic magnetic mineral fractions in Late Quaternary marine sediments off Gambia, NW Africa

Two gravity cores retrieved off NW Africa at the border of arid and subtropical environments (GeoB 13602–1 and GeoB 13601–4) were analyzed to extract records of Late Quaternary climate change and sediment export. We apply end‐member (EM) unmixing to 350 acquisition curves of isothermal remanent magnetization (IRM). Our approach enables to discriminate rock magnetic signatures of aeolian and fluvial material, to determine biomineralization and reductive diagenesis. Based on the occurrence of pedogenically formed magnetic minerals in the fluvial and aeolian EMs, we can infer that goethite formed in favor to hematite in more humid climate zones. The diagenetic EM dominates in the lower parts of the cores and within a thin near‐surface layer probably representing the modern Fe2+/Fe3+ redox boundary. Up to 60% of the IRM signal is allocated to a biogenic EM underlining the importance of bacterial magnetite even in siliciclastic sediments. Magnetosomes are found well preserved over most of the record, indicating suboxic conditions. Temporal variations of the aeolian and fluvial EMs appear to faithfully reproduce and support trends of dry and humid conditions on the continent. The proportion of aeolian to fluvial material was dramatically higher during Heinrich Stadials, especially during Heinrich Stadial 1. Dust export from the Arabian‐Asian corridor appears to vary contemporaneous to increased dust fluxes at the continental margin of NW Africa emphasizing that meltwater discharge in the North Atlantic had an enormous impact on atmospheric dynamics.

Rock magnetism of remagnetized carbonate rocks: another look

Abstract Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks. Authigenic magnetite grains, dominantly in the superparamagnetic and stable single-domain size range, also give rise to distinctive rock-magnetic properties, now commonly used as a ‘fingerprint’ of remagnetization. We re-examine the basis of this association in terms of magnetic mineralogy and particle-size distribution in remagnetized carbonates having these characteristic rock-magnetic properties, including ‘wasp-waisted’ hysteresis loops, high ratios of anhysteretic remanence to saturation remanence and frequency-dependent susceptibility. New measurements on samples from the Helderberg Group allow us to quantify the proportions of superparamagnetic, stable single-domain and larger grains, and to evaluate the mineralogical composition of the remanence carriers. The dominant magnetic phase is magnetite-like, with sufficient impurity to completely suppress the Verwey transition. Particle sizes are extremely fine: approximately 75% of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single-domain. Although it has been proposed that the single-domain magnetite in these remagnetized carbonates lacks shape anisotropy (and is therefore controlled by cubic magnetocrystalline anisotropy), we have found strong experimental evidence that cubic anisotropy is not an important underlying factor in the rock-magnetic signature of chemical remagnetization.

Rock magnetic signatures in diagenetically altered sediments from the Niger deep‐sea fan

Diagenesis has extensively affected the magnetic mineral inventory of organic‐rich late Quaternary sediments in the Niger deep‐sea fan. Changes in concentration, grain size, and coercivity document modifications of the primary magnetic mineral assemblages at two horizons. The first front, the modern iron redox boundary, is characterized by a drastic decline in magnetic mineral content, coarsening of the grain size spectrum, and reduction in coercivity. Beneath a second front, the transition from the suboxic to the sulfidic anoxic domain, a further but less pronounced decrease in concentration and bulk grain size occurs. Finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment. Low‐ and high‐temperature experiments were performed on bulk sediments and on extracts which have also been examined by X‐ray diffraction. Thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components. Their broad range of titanium contents reflects the volcanogenic traits of the Niger River drainage areas. Diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition. Low‐titanium compounds are less resistant to diagenetic dissolution. Intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments. At the Fe redox boundary, precipitation of authigenic, possibly bacterial, magnetite is documented. The presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain. Grain size–specific data argue for a gradual development of a superparamagnetic and single‐domain iron sulfide phase in this milieu, most likely greigite.

Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy

Detailed cyclostratigraphic analyses of the Valanginian to Hauterivian part of the Biancone Formation, a pelagic nannofossil limestone in the Southern Alps of Italy, were carried out. The Cismon section in the Belluno Trough near Feltre and the Pra da Stua section on the Trento Plateau near Avio were studied. Carbonate content, magnetic susceptibility and natural remanent magnetization were measured on densely spaced samples from Cismon. The first two properties vary in a cyclic fashion in this pelagic limestone section and are almost perfectly negatively correlated, while cyclicity in natural remanent magnetization is only vaguely indicated. Quantitative time-series analysis is critical in cyclic stratigraphy. The geostatistical method of cova functions (a generalization of the cross-variogram) which has proven to be the most versatile and robust time-series-analysis method is applied. Cova functions can be calculated from unevenly and non-correspondingly spaced time series without any preprocessing. This method also retains relatively more of the signal when noise and extreme outliers obscure the picture. The periodicities detected in the Cismon time series fall in the range of Milankovitch cycles. Cycle periods of 45cm, 80cm and 180cm likely correspond to dominant precession, obliquity and eccentricity cycles. Owing to the inaccuracy of the Cretaceous time scale, periods cannot be matched exactly, but cycle ratios are extremely close to expected ratios so that Milankovitch climate cycles could be positively identified in this Early Cretaceous section. In the Pra da Stua section bedding thickness was measured and analyzed quantitatively. A cycle period of 55cm is dominant in this data set, while periods of 115cm and 170cm are only vaguely indicated, although bedding in the sampled interval visually appears cyclic and even hierarchically structured. It can be expected that densely spaced measurements of sedimentary properties such as susceptibility and carbonate content will reveal the cyclicity much better. This identification of Milankovitch cyclicity in the pelagic Biancone Formation has important consequences for our understanding of the climate system in the past. These results demonstrate that orbital forcing was effective enough to create palaeoclimatic cycles even in the Cretaceous warm, equable, ice-free climate state. Magnetic susceptibility proved to be a reliable proxy for carbonate content reflecting palaeoproductivity cycles in this pelagic setting.

Ferrimagnetic Cr And Mn spinels in sediments: Residual magnetic minerals after diagenetic dissolution

Ferrimagnetic Cr‐Fe and Mn‐Fe spinels are the dominant remanence carrying phases in the late Triassic Lunde Formation. These have largely been identified by magnetic mineral extraction from the sediments, combined with microscopy. The extensive removal of most of the originally deposited magnetic Fe‐Ti oxides during diagenesis has left the Cr‐Fe and Mn‐Fe spinels as a mineral residue which is resistant to dissolution. The rock magnetic signature of these minerals is not sufficiently known, to be able to identify these minerals on the basis of their magnetic properties alone.

Rock-magnetic signature of gas hydrates in accretionary prism sediments

Sediments from two Ocean Drilling Program Leg 146 sites from the Cascadia margin of western North America have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates. Two indices combining coercivity, remanence, and susceptibility parameters, D JH ( = {J rs/J s}{H cr/H c”) and D S ( = {J rs/k}H cr), when combined with thermo-magnetic data, can be diagnostic of these changes. At Site 892, D S values are distinctly higher and more scattered above the bottom simulating seismic reflector (BSR), which marks the base of the hydrate stability zone. Within the hydrate stability zone at Site 892, D JH shows two trends: an increase from about 50 mbsf to the BSR at 73 mbsf, corresponding to an expected increase in hydrate concentration near the BSR; and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf, associated with hydrate recovered in cores above 19 mbsf. At Site 889/890 D JH increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. High D JH sediments within a low Cl − zone at this site have magnetic mineralogies which are dominated by fine-grained magnetic sulfides, whereas sediments from above and below this zone are characterised by magnetite-magnetic sulfide mixes. This trend at Site 889/890 is consistent with an interpretation based on pore-water geochemistry (low Cl −) and bottom-water temperature that a ‘fossil gas hydrate zone’ extended downwards to about 295 mbsf during the last glacial. The observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through SD greigite to pyrite (or to overgrowth of SD greigite to MD size). Diagenetic growth of magnetic iron sulfides (greigite and/or pyrrhotite) has been reported in other accretionary wedge sediments. Thermal demagnetization of multi-component isothermal remanent magnetization (mIRM) indicates the presence of a low-coercivity magnetic mineral with an unblocking temperature (T ub) between 310° and 350°C. High J rs/k ratios suggest that the low-coercivity, low unblocking temperature mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with T ub ca. 580°C — magnetite — is also present in varying amounts. Hydrate apparently controls the presence of greigite by incorporating H 2S, shown to be present as a hydrate phase together with methane in hydrate recovered at Site 892. Release of H 2S below the base of the hydrate layer allows overgrowth of greigite grains to MD size or the conversion of some of the greigite to pyrite.

Rock magnetic signature of the Cretaceous-Tertiary boundary : Worm, H.-U. and S.K. Banerjee, 1987. Geophys. Res. Letts, 14(11):1083–1086

Rock magnetic signature of the Cretaceous-Tertiary boundary : Worm, H.-U. and S.K. Banerjee, 1987. Geophys. Res. Letts, 14(11):1083–1086

Rock magnetic signature of the Cretaceous‐Tertiary boundary

Black magnetic microspherules, commonly found in Cretaceous‐Tertiary (K‐T) boundary sediments and believed to be of an impact melt origin, were found to be responsible for a strong magnetic anomaly at the K‐T boundary in Petriccio, Italy, and in almost all Deep Sea Drilling Project (DSDP) sediment cores investigated in which the K‐T boundary was present. The enriched concentrations of magnetic minerals have been detected by measurements of the magnetic susceptibility which is in a good approximation proportional to the magnetic microspherule content. A rock magnetic characterization of extracted microspherules by hysteresis, Isothermal Remanent Magnetization acquisition, demagnetization and thermomagnetic measurements describes the magnetic minerals in the microspherules as ferrites with a near magnetite composition and ‘soft’, i.e. multidomain, magnetic behavior, in agreement with earlier nonmagnetic studies. On the contrary, volcanic ash from Hawaii has distinctively ‘harder’ magnetic properties, an important finding with regard to the proposed alternative, a volcanic origin of the well‐known Ir‐anomaly at the K‐T boundary. It is suggested here that from magnetic susceptibility scanning of numerous cross‐sections of the K‐T boundary a distinct worldwide pattern might evolve that can potentially reveal the impact/volcano site because of a presumed higher microspherule concentration near the source. The advantage of mapping the microspherule distribution over the previous unsuccessful attempt using Ir‐concentrations lies in the large size (hundreds of microns) of the microspherules with relatively short airborn life times while the iridium might have adhered to small dust particles that circled the globe several times before settling.