Remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity

Relative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well-documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis.

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title: Global changes in intensity of the Earth's magnetic field during the past 800 kyr
abstract: Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earth's past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earth's geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve, with the geomagnetic origin of the signal supported by an independent analysis of 10Be production made on different cores. The persistence of long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity into a composite curve spanning the past 800 kyr. We find that the intensity of the Earth's dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earth's orbital parameters have a direct and strong influence on the geodynamo.

title: North Atlantic palaeointensity stack since 75ka (NAPIS–75) and the duration of the Laschamp event
abstract: Six relative palaeointensity records from the north Atlantic Ocean were stacked together to produce a new record for the last 75 kyr (NAPIS–75). Five of these records have been previously correlated at millennial scale and placed on the GISP2 age scale, the sixth record was tied to the others using magnetic susceptibility. From 75 ka the field strength exhibits some oscillations, with a first minimum ca.65 ka, followed by a progressive increase to a broad maximum centred at ca.48 ka. There is then a well–marked low at 40 ka, corresponding to the directional anomaly of the Laschamp event. Another intensity low, observed at ca.34 ka, corresponds in age to the Mono Lake event. After a high at 33 ka and two lows at 30 and 24 ka with a broad maximum between, the field strength seems to slowly increase to the upper limit of the studied interval. In the 10–20 kyr interval some differences exist between individual records, and fine–scale details are not always resolved. In the 20–75 kyr interval, on the other hand, well–resolved millennial–scale features are superimposed to the broader trends. The duration of the Laschamp event, which is recorded directionally in five cores, appears to be about 1500 years, consistent with a recent suggestion on the origin of geomagnetic excursions.

title: South Atlantic and North Atlantic geomagnetic paleointensity stacks (0-80 ka): implications for inter-hemispheric correlation
abstract: Recent coring of high-deposition-rate Atlantic sediments has led to the development of North Atlantic (NAPIS) and South Atlantic (SAPIS) geomagnetic paleointensity stacks. The SAPIS stack comprises five records from the sub-Antarctic South Atlantic (41–471S, 6–101E). Four are from piston cores collected during the Ocean Drilling Program (ODP) Leg 177 site survey cruise (Earth Planet.  (2000) 145). The other is ODP Site 1089 (Leg 177) ( 2001, submitted for publication). All but one (4-PC03) of the SAPIS cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to Site 1089. The Site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core RC11-83 which has 14 calibrated radiocarbon ages in the 11–41 ka interval. Below this level, the chronology is derived by matching the Site 1089 benthic oxygen isotope data to the SPECMAP stack. The NAPIS stack comprises six records from a wide area of the North Atlantic (Philos.  (2000) 1009). Correlation between cores was based on marine isotopic stage boundaries, augmented by millennial-scale features (Heinrich events and fluctuations in concentration-dependent magnetic parameters). NAPIS was placed on the GISP2 chronology, using the marine to ice-core oxygen isotope correlation proposed by Voelker et al. (Radiocarbon 40 (1998) 517) for one of the NAPIS cores. In the resulting age model, the LaschampEvent, recorded in five of six cores, falls in a very narrow ( o1 kyr) age range and coincides with the 10 Be and 36 Cl peak measured in the ice cores. Comparison of the two stacks, placed on their own independent age models, indicates that common millennial scale paleointensity features are preserved. Although more work is needed to define the ‘‘true’’ global content of the paleointensity record as well as the precise age of many features, it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone. r 2002

title: Geomagnetic dipole strength and reversal rate over the past two million years
abstract: Independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment and thus provide information regarding the processes governing the geodynamo. So far, this procedure has been limited to the past 800,000 years (800 kyr; ref. 3), which does not include any geomagnetic reversals. Here we present a composite curve that shows the evolution of the dipole moment during the past two million years. This reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas, which were used for calibration. We show that, at least during this period, the time-averaged field was higher during periods without reversals but the amplitude of the short-term oscillations remained the same. As a consequence, few intervals of very low intensity, and thus fewer instabilities, are expected during periods with a strong average dipole moment, whereas more excursions and reversals are expected during periods of weak field intensity. We also observe that the axial dipole begins to decay 60–80 kyr before reversals, but rebuilds itself in the opposite direction in only a few thousand years.

title: A geomagnetic paleointensity stack between 0.8 and 3.0 Ma from equatorial Pacific sediment cores
abstract: We have conducted a paleomagnetic study of six sediment cores taken from the equatorial Pacific, three from the West Caroline Basin and three from the Manihiki Plateau, in order to make a relative paleointensity stack during the Matuyama and late Gauss Chrons. The age of the bottom of the cores ranges from 1.2 to 3.0 Ma. The sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity. The age of the cores is controlled by correlating variations in magnetic concentration (magnetic susceptibility and/or anhysteretic remanent magnetization (ARM)) to target oxygen-isotope (δ18O) curves. All cores from the Manihiki Plateau show an upward decrease of the natural remanent magnetization (NRM) intensity normalized by ARM and isothermal remanent magnetization (IRM) with a decrease in sedimentation rate. Such a trend was removed before being converted to relative paleointensity. This observation implies that a sedimentation-rate change can affect relative paleointensity estimation. Relative paleointensity records from the six cores coincide well with each other within uncertainty of age. We constructed a stacked curve between 0.8 and 3.0 Ma (the equatorial Pacific paleointensity stack EPAPIS-3Ma) after adjusting age; the number of cores stacked is three to four throughout the record. Quasiperiodic paleointensity lows occur in the EPAPIS-3Ma, and corresponding paleointensity lows can be found in previously published records with some shifts in age. At least, parts of the paleointensity minima seem to be accompanied by geomagnetic excursions. The “asymmetric sawtooth pattern” of paleointensity variations is not observed at the Matuyama-Gauss boundary and thereafter in our record. A spectral analysis shows that ∼100 kyr orbital eccentricity frequency may exist in paleointensity variations during the Matuyama and late Gauss Chrons.

title: Stacking paleointensity and oxygen isotope data for the last 1.5 Myr (PISO-1500)
abstract: Coupled relative geomagnetic paleointensity (RPI) and oxygen isotope records are used to construct RPI and oxygen isotope stacks for the last 1.5 Myr. The coupled correlations are accomplished using the Match algorithm (Lisiecki, L.E., and Lisiecki, P.A., 2002. Application of dynamic programming to the correlation of paleoclimate records. Paleoceanography, 17: 1049, doi:10.1029/2001PA000733) to simultaneously correlate isotope and RPI records. The simultaneous match reduces the degree of freedom associated with correlations using RPI or oxygen isotope records alone. The overall compatibility of RPI and oxygen isotopes indicates a dominant global (but independent) component in both signals. The local wavelet power spectrum (LWPS) for the RPI stack indicates little significant orbital power, although the accompanying oxygen isotope stack has all the LWPS characteristics expected of a high-resolution oxygen isotope record containing orbital frequencies. The PISO-1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production. Scaling the stack to values for virtual axial dipole moment (VADM) indicate maxima at ~ 15 × 1022 Am2 and minima that imply a threshold of ~ 2.5 × 1022 Am2 below which values are associated with either polarity reversals or magnetic excursions.

title: Relative paleointensity (RPI) and oxygen isotope stratigraphy at IODP Site U1308: North Atlantic RPI stack for 1.2–2.2 Ma (NARPI-2200) and age of the Olduvai Subchron
abstract: Drilling Program (IODP) Site U1308 (49°53'N, 24°14'W; water depth 3871 m) provides a record of relative paleointensity (RPI) and benthic stable isotope stratigraphy back to 3.2 Ma. The record since 1.5 Ma was published previously, and here we present the interval from 1.5 Ma to 3.2 Ma (Early Pleistocene-Late Pliocene). The benthic oxygen isotope record in this interval is correlated to Marine Isotope Stage (MIS) 51 to KM2, with an apparent hiatus that removed part of the interval spanning MIS 104-G2 (2.6-2.65 Ma), at the Gauss-Matuyama boundary. The mean sedimentation rate for the 1.5-3.2 Ma period is 8.5 cm/kyr. The age model was built by correlation of the benthic oxygen isotope record to a global stack (LR04). Apart from the expected polarity reversals, three magnetic excursions are recorded: Punaruu in MIS 31/32 at 1092 ka, Gilsa in MIS 54/55 at 1584 ka, and a newly recognized excursion labeled Porcupine (after the nearby Porcupine Abyssal Plain) in MIS G6/G7 at 2737 ka. The ages of polarity reversals at Site U1308, on the LR04 time scale, are consistent with the current geomagnetic polarity timescale (GPTS) with the exception of the base of the Olduvai Subchron that occurred in MIS 73, corresponding to 1925 ka on the LR04 time scale, 25 kyr younger than in the current GPTS. The RPI record at Site U1308 is calibrated using the oxygen isotope age model, and combined with four other North Atlantic records to obtain a North Atlantic RPI stack for 1.2-2.2 Ma (NARPI-2200) that is compared with published RPI stacks: Epapis, Sint-2000 and PADM2M. For 2.2-3.2 Ma, the Site U1308 RPI record is compared with a RPI record from North Atlantic IODP Site U1314, and with the Pacific Epapis stack. The mean sedimentation rates of the North Atlantic sites in NARPI-2200 are greater (by about an order of magnitude) than most of the records incorporated in other stacks. The comparison of Epapis and NARPI-2200 yields an apparent lag for NARPI-2200 relative to Epapis, attributed the Epapis age model constructed by correlation of magnetic concentration parameters (a proxy for carbonate percentage) to a calibrated oxygen isotope record. The long RPI record from Site U1308 yields a very similar mean value for the Brunhes and Matuyama virtual axial dipole moments (7.05 × 1022 Am2), implying no polarity bias in the strength of the main geomagnetic dipole, in contrast to interpretations from Sint-2000 and PADM2M. The results strengthen the case that RPI can be used to improve global stratigraphic correlation for sites with mean sedimentation rates up to several decimeters/kyr.

title: PADM2M: a penalized maximum likelihood model of the 0–2 Ma palaeomagnetic axial dipole moment
abstract: We present a new time-varying model for palaeomagnetic axial dipole moment (PADM) for the past 2 Myr and compare it with earlier virtual axial dipole moment (VADM) reconstructions which have been based on stacking and averaging scaled relative palaeointensity records. The PADM is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood (PML) approach to recover a cubic B-spline representation of axial-dipole field variations on million year timescales. The PML method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average VADM reconstructions. We apply the PML method to a set of 96 032 published data (1800 palaeointensities from igneous rocks, 3300 archaeointensities and 86 relative palaeointensity time-series of variable lengths and resolutions). Two models are discussed: PADM2Mp is a trial model based on a subset of the nine longest available sedimentary records; PADM2M uses a comprehensive data set (76 records, 81 446 data; 10 records were eliminated) and is our preferred model. PADM2M has a lower mean than existing VADM reconstructions but shows similarities in long-period variability. Some differences in timing, amplitude and resolution of certain features can be attributed to variations in age assignments. Others result from our more comprehensive data set and a reduction in bias attributable to PML modelling. PADM2M has an average axial dipole moment over 0–2 Ma of 5.3 × 1022 Am2 with a standard deviation of 1.5 × 1022 Am2. The Brunhes chron average (6.2 × 1022 Am2) is higher than for earlier epochs of Matuyama (4.8 × 1022 Am2), as seen in some previous studies. The power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 Myr−1. We see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour.

title: An Atlantic lead over Pacific deep-water change across Termination I: implications for the application of the marine isotope stage stratigraphy
abstract: Globally synchronous changes in glacioeustatic sea-level provide a particularly useful stratigraphic reference in the study of glacial–interglacial climate change, and form the basis for the marine isotope stage (MIS) system, which posits a dominant glacioeustatic signal in benthic foraminferal δ18O. However, benthic δ18O strictly does not represent a pure sea-level record, and it is shown here that, due to local changes in deep-water hydrography, the MIS 2/1 boundary (i.e. benthic δ18O change) occurred ∼4000 years later in the deep eastern equatorial Pacific than in the deep Northeast Atlantic. This discrepancy is too large to be attributed entirely to the Pacific lag that may be expected due to the mixing time of the ocean basins. Instead, multiple proxies (including benthic foraminiferal Mg/Ca) suggest that the large offset in benthic δ18O most probably resulted from a late rise in deep-water temperature at the Pacific site and a series of coupled deep-water temperature and δ18O changes at the Northeast Atlantic site. These findings have important implications for the application of the MIS stratigraphy and the interpretation of benthic δ18O records in general, as they show that benthic δ18O fluctuations and MIS boundaries from different hydrological settings may be significantly diachronous. These results call into question the leads and lags that we might infer from a ‘naive’ interpretation of benthic δ18O as a sea-level proxy, with a direct bearing on the mechanisms that we may propose to explain glacial–interglacial climate transitions.

title: Sedimentary records of relative paleointensity of the geomagnetic field: Theory and practice
abstract: Sediments have proved irresistible targets for attempts at determining the relative variations in the Earth's magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution. The assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited. Early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short, no dependence on magnetic field was expected. Nonetheless, a number of experiments showed that sedimentary magnetizations varied in accordance with the field, albeit not always in a simple, linear fashion. Experiments in which the sediments were stirred in the presence of a field (to simulate bioturbation) showed a reasonably linear relationship with the applied field, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences. Examination of existing paleointensity data sets allows a few general conclusions to be drawn. It appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the Earth's magnetic field. The dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences. Moreover, when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences, there is considerable agreement among the various records. When viewed over timescales of hundreds to thousands of years, relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another, suggesting that they are dominated by nondipole field behavior. When viewed over timescales of a few tens of thousands to hundreds of thousands of years, however, the records show coherence over large distances (at least thousands of kilometers) and may reflect changes in the dipole field. On the basis of a sequence spanning the Brunhes and Matuyama chrons, the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years, but these oscillations are not clear in the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence.

title: Magnetic paleointensity stratigraphy and high-resolution Quaternary geochronology: successes and future challenges
abstract: Magnetic paleointensity stratigraphy is used to detect variations in the strength of Earth's ancient magnetic field. Paleointensity studies have demonstrated that a dominantly dipolar geomagnetic signal can be recorded in a globally coherent manner in different types of sediments and in non-sedimentary archives, including ice core records and marine magnetic anomaly profiles. The dominantly dipolar nature of geomagnetic paleointensity variations provides a global geophysical signal that has come to be widely used to date Quaternary sediments. Despite the many successful applications of paleointensity-assisted chronology, the mechanisms by which sediments become magnetized remain poorly understood and there is no satisfactory theoretical foundation for paleointensity estimation. In this paper, we outline past successes of sedimentary paleointensity analysis as well as remaining challenges that need to be addressed to place such work on a more secure theoretical and empirical foundation. We illustrate how common concepts for explaining sedimentary remanence acquisition can give rise to centennial to millennial offsets between paleomagnetic and other signals, which is a key limitation for using paleointensity signals for geochronology. Our approach is intended to help non-specialists to better understand the legitimate uses and limitations of paleointensity stratigraphy, while pointing to outstanding problems that require concerted specialist efforts to resolve.

title: On the possibility of obtaining relative paleointensities from lake sediments
abstract: Relative paleointensities are obtained from a 6-m sediment core from Lake St. Croix, Minnesota, spanning the time range from 445 to 1740 years B.P. To normalize the natural remanent magnetization (NRM) for variations in the magnetic content, a laboratory-induced remanence is chosen, whose alternating field (AF) demagnetization curves most closely resemble the NRM demagnetization curves. By plotting the ratio of the NRM to the normalizing remanence versus AF demagnetizing field, H AF , for samples of the same sediment horizon, as well as for samples from different horizons, estimates are obtained for expected uncertainties in the relative paleointensities. For the Lake St. Croix sediments the anhysteretic remanence (ARM) demagnetization curves are very similar to those of the NRM's, and ARM is therefore used as the normalization parameter. Because the sediment exhibits homogeneous remanence properties throughout, and H AF = 100Oe is the optimum “cleaning” field for the entire core, NRM 100 /ARM 100 is evaluated to represent the fluctuations of the relative paleointensity. Our relative paleointensity data exhibit the same general features as obtained from archeomagnetic studies. The intensity increases as one goes back in time with a peak near 800 years B.P., representing an increase in the intensity of up to 60%. Apparent periodicities in the intensity of 300–400 years are observed.

title: A new rock‐magnetic approach to selecting sediments for geomagnetic paleointensity studies: Application to paleointensity for the last 4000 years
abstract: The hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization (DRM/ARM) for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches. One approach is a detailed rock-magnetic examination of the implicit assumptions of the DRM/ARM method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions. Sediments are ‘uniform’ with respect to DRM/ARM ratio if they contain magnetite in the 1–15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20–30 times the minimum concentration. The DRM/ARM ratios of these sediments should provide estimates of relative geomagnetic paleointensity. Relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility (XARM) versus low-field susceptibility (X) and the size range ≃ 1–15 μm is approximately identified by high-field hysteresis parameters. A rock-magnetic evaluation of LeBoeuf Lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study. The second approach to evaluating the DRM/ARM ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data. A comparison between the LeBoeuf Lake estimates and Thellier-Thellier results from the western United States supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations.

title: The fidelity of paleomagnetic records carried by magnetosome chains
abstract: May 2013 Received in revised form 19 September 2013  J. Lynch-Stieglitz Magnetotactic bacteria (MTB), which use aligned chains of magnetosomes (magnetic crystals) as a navi- gation tool, are found in a wide range of modern day marine, river and lacustrine environments and their fossilized remains are being increasing recognized in geological records. Despite an increasing realization that biogenically derived magnetic particles may play a key role in sedimentary magnetizations, little is known about the influence that they may have on the fidelity of paleomagnetic recordings. Using cultured Magnetospirillum magneticum strain AMB-1, we have conducted simple 2D (i.e., zero-inclination) deposi- tion experiments to assess the efficiency with which magnetosome chains align along magnetic field lines and the implications that this has for paleomagnetic records. Our results indicate that the natural rema- nent magnetization (NRM) acquired by deposited MTB is near linear with applied field (0-120 μT), but that NRM acquisition does not perfectly follow the assumed linear trend and over a six-fold increase in applied field only a factor ∼5.5 increase in NRM intensity is observed (i.e., ∼9% lower than the assumed trend). Both anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetiza- tion (SIRM) normalized relative paleointensities (RPIs) can successfully recover field strength variations under identical situations. When the MTB concentration of the initial solution is varied (up to a factor 8), NRM carried by MTB responds in the expected fashion (i.e., a doubling of bacteria produces a doubling of NRM intensity). Both ARM and SIRM, however, do not respond to as expected: A doubling of con- centration corresponds to an increase of ∼1.76 in SIRM intensity and an increase of only ∼1.63 in ARM intensity. Both are influenced by magnetostatic interactions that arise from the close packing of bacterial cells or subchain interactions within a single bacterial cell. First-order reversal curve (FORC) diagrams, however, are too insensitive to indicate the presence of paleomagnetically relevant magnetic interactions. Remanence based methods (i.e., the ARM ratio) are more appropriate to detect such interactions.

title: Variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites: Implications for relative paleointensity signal recording
abstract: Widespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals. We studied a sediment core from the South China Sea that passes the strict empirical criteria for magnetic “uniformity” used in relative paleointensity studies. Such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages. Using techniques that enable component-specific magnetic mineral identification, we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments. We analyzed normalized remanence signals associated with the two magnetite components to assess whether co-occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency. Paleomagnetic directions for the two components have no phase lag, which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times. However, we find that the biogenic magnetite is generally 2–4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization (NRM) despite their approximately equal magnetic contributions. Variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the NRM is controlled by nongeomagnetic factors that will affect relative paleointensity recording. We recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies.

title: The high-nutrient, low-chlorophyll regime of the ocean: limits on biomass and nitrate before and after iron enrichment
abstract: In high-nutrient, low-chlorophyll (HNLC) regions of the ocean, phytoplankton biomass remains low despite an abundance of major nutrients. Platt et al.  constructed a simple two-component (chlorophyll and nitrate) model of HNLC regions and used it to determine analytically the upper bound on chlorophyll and the lower bound on nitrate in terms of the bio-optical and physical properties of the system. Subsequently, Platt et al.  showed that the response of HNLC regions to iron addition could be captured through the effect of iron on the parameters of the growth term in the model. Here, we extend this approach to derive a procedure for less-conservative bounds on chlorophyll and nitrate. We also examine the consequences of replacing the linear loss term in the original model with a quadratic loss term. The application of the model is illustrated using parameters typical of the eastern Equatorial Pacific in its unperturbed state and also after enrichment with iron. The results are consistent with observations made during an experimental manipulation of the region by addition of iron (IronEx I). Our work emphasises the value of simple mathematical models as tools to address complex issues in biological oceanography. The method has generality as well as simplicity: it could be applied by non-modellers to investigate other problems in other regions, and to facilitate this we make our computer programs freely available on the Ecological Modelling Website at http://www.elsevier.com/locate/ecolmodel.

title: Ocean dynamics, not dust, have controlled equatorial Pacific productivity over the past 500,000 years
abstract: The equatorial Pacific is a key oceanographic region in Earth’s climate system. Biological production in this region is limited, in part, by the lack of the micronutrient iron. Atmospheric dust is a source of iron, as is upwelling of ocean waters from below. A longstanding question has been whether biological productivity has responded to variable dust supply over ice age cycles. We use geochemical proxies in three sediment cores spanning the breadth of the equatorial Pacific to show that biological productivity did not respond to dustier ice age conditions. Rather than atmospheric iron supply, we infer that ocean dynamics, linking the equatorial Pacific to nutrient supply from the Southern Ocean, played a crucial role in regulating equatorial Pacific productivity. Biological productivity in the equatorial Pacific is relatively high compared with other low-latitude regimes, especially east of the dateline, where divergence driven by the trade winds brings nutrient-rich waters of the Equatorial Undercurrent to the surface. The equatorial Pacific is one of the three principal high-nutrient low-chlorophyll ocean regimes where biological utilization of nitrate and phosphate is limited, in part, by the availability of iron. Throughout most of the equatorial Pacific, upwelling of water from the Equatorial Undercurrent supplies far more dissolved iron than is delivered by dust, by as much as two orders of magnitude. Nevertheless, recent studies have inferred that the greater supply of dust during ice ages stimulated greater utilization of nutrients within the region of upwelling on the equator, thereby contributing to the sequestration of carbon in the ocean interior. Here we present proxy records for dust and for biological productivity over the past 500 ky at three sites spanning the breadth of the equatorial Pacific Ocean to test the dust fertilization hypothesis. Dust supply peaked under glacial conditions, consistent with previous studies, whereas proxies of export production exhibit maxima during ice age terminations. Temporal decoupling between dust supply and biological productivity indicates that other factors, likely involving ocean dynamics, played a greater role than dust in regulating equatorial Pacific productivity.

title: Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation
abstract: Atmospheric CO2 fluctuations over glacial–interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial–interglacial atmospheric CO2 variations invoke changes in deep-ocean carbon storage, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO2 rise observed in ice-core records. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial–interglacial ocean–atmosphere exchange of carbon.

title: Regulation of primary productivity rate in the equatorial Pacific
abstract: Analysis of the Chl-specific rate of primary productivity (P{sup B}) as a function of subsurface nutrient concentration at >300 equatorial stations provides an answer to the question: What processes regulate primary productivity rate in the high-nutrient, low-chlorophyll waters of the equatorial Pacific In the western Pacific where there is a gradient in 60-m (NO{sub 3}) from 0 to {approximately}12 {mu}M, the productivity rate is a linear function of nutrient concentration; in the eastern Pacific where the gradient is from 12 to 28 {mu}M, the productivity rate is independent of nutrient concentration and limited to {approximately}36 mg C(mg Chl){sup {minus}1} d{sup {minus}1}, or a mean euphotic zone C-specific growth rate ({mu}) of 0.47 d{sup {minus}1}. However, rates downstream of the Galapagos Islands are not limited; they are 46.4 mg C(mg Chl){sup {minus}1} d{sup {minus}1} and {mu} = 0.57 d{sup {minus}1}, very close to the predicted nutrient-regulated rates in the absence of other limitation. This pattern of rate regulation can be accounted for by a combination of eolian Fe, subsurface nutrients, and sedimentary Fe derived from the Galapagos platform. In the low-nutrient western Pacific the eolian supply of Fe is adequate to allow productivity rate to be set by subsurface nutrient concentration.more » In the nutrient-rich easter equatorial region eolian Fe is inadequate to support productivity rates proportional to the higher nutrient concentrations, so in this region eolian Fe is rate limiting. Around the Galapagos Islands productivity rates reach levels consistent with nutrient concentrations; sedimentary Fe from the Galapagos platform seems adequate to support increased nutrient-regulated productivity rates in this region.« less

title: Marine control of biological production in the eastern equatorial Pacific Ocean
abstract: The eastern equatorial Pacific Ocean is the site of approximately 20–50% of new biological production in the global oceans. This region is also responsible for the greatest efflux of CO2 from oceans to the atmosphere. New production, which fixes carbon in response to external inputs of nutrients as opposed to supply from local nutrient recycling, is thought to modulate the CO2 release. But what controls new production in this region is less clear. Here we present a quantitative reconstruction of biological production in the surface ocean for this region over the past 130,000 years, which shows that the equatorial Pacific Ocean exhibits higher-frequency variations than the South Equatorial Current. Comparison of these records with palaeotemperature reconstructions indicates that atmospherically driven mechanisms—such as aeolian flux of iron or wind-driven changes in upwelling rate of nutrient-rich waters—are unlikely to have influenced longer-term rates of production in this region. Instead, biological production appears to be governed by changes in ocean circulation and the chemical composition of upwelled water.

title: Lower Miocene to Present Stratigraphy of the Equatorial Pacific Sediment Bulge and Carbonate Dissolution Anomalies
abstract: The bulge is a 600-m-thick regional deposit of pelagic sediment accumulated around the equator. Its stratigraphy reflects a number of factors: how accumulation rates have varied over time, how accumulation has been spatially focused around the equator, how much carbonate dissolution and reworking or nondeposition by bottom currents have occurred, and how much the deposits have been translated northward by motion of the Pacific tectonic plate on which they have accumulated. In order to fully explore the effects of these processes, a spatially continuous stratigraphic database is desirable, as existing cores provide information at only discrete points, and they tend to be sited in locally thicker accumulations. We illustrate the utility of seismic data tied to drill cores by comparing the carbonate dissolution effect in two north-south lines crossing the sediment bulge. Bathymetry over our easterly seismic line deepens relatively little going north, whereas our westerly line deepens by 750 m from 4°N to 7°N. At a common latitude the primary pelagic input to both these lines will have been similar, so the difference of their sediment mass should reflect mostly their relative carbonate loss due to dissolution. We use this fact to calculate the slope of the carbonate dissolution curve and find a remarkably small slope (little relative carbonate dissolution) compared to that expected from previous studies. The techniques here may suggest a possible way forward for studying the equatorial carbonate stratigraphy more generally.

title: Improvements in long‐core measurement techniques: applications in palaeomagnetism and palaeoceanography
abstract: We report on new developments in the long-core measurement techniques using u channels in connection with new small-access cryogenic magnetometers. With these new instruments, u-channel samples allow us to measure a wide range of magnetic parameters along entire-core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples. An additional advantage is that the sediment disturbance when sampling is significantly less when using u channels. We present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques. Examples of continuous measurements of low-field susceptibility, NRM, ARM, IRM, Hrc from North Atlantic deep-sea cores show that it is possible to generate logs of combination of parameters related to rock-magnetic characteristics of the sediments useful in palaeoceanography. In particular, the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo-environmental conditions. The fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable (or not suitable) for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity.

title: The least-squares line and plane and the analysis of palaeomagnetic data
abstract: The classic, multivariate technique of principal component analysis can be used to find and estimate the directions of lines and planes of best least-squares fit along the demagnetization path of a palaeomagnetic specimen, thereby replacing vector subtraction, remagnetization circles and difference vector paths with one procedure. Eigenvalues from the analysis are the variance of the data along each principal axis, and provide a relative measure of collinearity or coplanarity which may be used to define a general palaeomagnetic precision index. Demagnetization planes found with principal component analysis may be used in place of difference vector paths for locating Hoffman—Day directions, avoiding unnecessary vector subtraction and intensity truncation steps. Two methods are discussed for jointly estimating an average remanence direction from demagnetization lines and planes.

title: Characterizing interactions in fine magnetic particle systems using first order reversal curves
abstract: We demonstrate a powerful and practical method of characterizing interactions in fine magnetic particle systems utilizing a class of hysteresis curves known as first order reversal curves. This method is tested on samples of highly dispersed magnetic particles, where it leads to a more detailed understanding of interactions than has previously been possible. In a quantitative comparison between this method and the δM method, which is based on the Wohlfarth relation, our method provides a more precise measure of the strength of the interactions. Our method also has the advantage that it can be used to decouple the effects of the mean interaction field from the effects of local interaction field variance.

title: First‐order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples
abstract: Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first-order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first-order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two-dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single-domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.

title: A protocol for variable‐resolution first‐order reversal curve measurements
abstract: High-resolution first-order reversal curve (FORC) diagrams are being increasingly used in rock and environmental magnetism, including for detection of biomagnetic signals in sediments. Resolution can be a major barrier to obtaining high-quality FORC diagrams, and time-consuming measurements that employ small field steps are necessary to resolve the finest features of a FORC distribution. We present a new experimental protocol with irregularly spaced field steps that allow different parts of a FORC diagram to be measured at different resolutions. Larger numbers of measurements can, therefore, be made in key regions of a FORC distribution to resolve diagnostic features at higher resolution. Specification of the field steps in the irregular measurement grid is based on major hysteresis properties; no a priori knowledge concerning the underlying FORC distribution is required. FORC diagrams obtained with conventional measurements and with our new measurement protocol give consistent results. Because of its variable resolution, the irregular protocol provides a clear representation of fine-scale features produced by quasireversible superparamagnetic and noninteracting single-domain particles. Although the proposed irregular measurement protocol is not as efficient at suppressing noise as recently developed postprocessing techniques (e.g., VARIFORC), it enables efficient high-resolution analysis for relatively strongly magnetized samples where measurement noise is not detrimental to FORC distribution estimation.

title: Estimation of significance levels and confidence intervals for first‐order reversal curve distributions
abstract: First-order reversal curve (FORC) distributions provide a means with which to describe a magnetic mineral assemblage in terms of coercivities and interaction fields. In recent years the use of experimentally derived FORC distributions has increased dramatically and they are being placed in an increasingly quantitative interpretational framework. An outstanding issue for calculation and interpretation of FORC data sets is the statistical significance that can be assigned to structures in experimentally determined distributions. Without this knowledge, the selection and characterization of structures that can be deemed interpretable within a FORC distribution is a subjective process. We demonstrate how FORC processing algorithms can be adapted to provide a measure of statistical significance and a confidence interval for each point in a FORC distribution. This information can guide measurement protocols and provides a more quantitative framework for interpretation of FORC distributions.

title: Rock magnetic criteria for the detection of biogenic magnetite
abstract: We report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS1, MV1 and MV2 and dissimilatory iron-reducing bacterium strain GS-15, grown in pure cultures, were used in this study. Our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetization (IRM) using static, pulse and alternating fields; (2) acquisition of anhysteretic remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (FC) from 300 K. However, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero-field and field cooled SIRMs on warming through the Verwey transition (T ≈ 100 K). Intact chains of unoxidized magnetite magnetosomes have ratios of δFC/δZFC greater than 2, where the parameter δ is a measure of the amount of remanence lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the δFC/δZFC ratio to around 1, similar to values observed for some inorganic magnetite, maghemite, greigite and GS-15 particles. Numerical simulations of δFC/δZFC ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the δFC/δZFC parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made, compared to techniques such as electron microscopy.

title: Giant magnetofossils and hyperthermal events
abstract: Magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size, morphology, and stoichiometry. These cosmopolitan bacteria are widely observed in aquatic environments. If preserved after burial, the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations. They also have potential to provide paleoenvironmental information. In contrast to conventional magnetofossils, giant magnetofossils (most likely produced by eukaryotic organisms) have only been reported once before from Paleocene-Eocene Thermal Maximum (PETM; 55.8 Ma) sediments on the New Jersey coastal plain. Here, using transmission electron microscopic observations, we present evidence for abundant giant magnetofossils, including previously reported elongated prisms and spindles, and new giant bullet-shaped magnetite crystals, in the Southern Ocean near Antarctica, not only during the PETM, but also shortly before and after the PETM. Moreover, we have discovered giant bullet-shaped magnetite crystals from the equatorial Indian Ocean during the Mid-Eocene Climatic Optimum (∼40 Ma). Our results indicate a more widespread geographic, environmental, and temporal distribution of giant magnetofossils in the geological record with a link to “hyperthermal” events. Enhanced global weathering during hyperthermals, and expanded suboxic diagenetic environments, probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils. Our micromagnetic modelling indicates the presence of magnetic multi-domain (i.e., not ideal for navigation) and single domain (i.e., ideal for navigation) structures in the giant magnetite particles depending on their size, morphology and spatial arrangement. Different giant magnetite crystal morphologies appear to have had different biological functions, including magnetotaxis and other non-navigational purposes. Our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils, and that these organisms were globally distributed. Much more work is needed to understand the interplay between magnetofossil morphology, climate, nutrient availability, and environmental variability.

title: Late Pleistocene Paleoclimatology of the Central Equatorial Pacific: Flux Patterns of Biogenic Sediments
abstract: High-resolution records of the concentration and mass accumulation rate (MAR, or flux) of biogenic sediments from core RC11-210 in the central equatorial Pacific are compared for the entire late Pleistocene. The fluxes of calcium carbonate and organic carbon correlate well. The opal flux pattern shows no direct correlation with the other two; rather, opal flux maxima correlate with slopes of (i.e. both precede and follow) the organic carbon and carbonate MAR peaks. Organic carbon and calcite therefore may both be better indicators of past productivity than opal, which is an indicator of transition at this location. We interpret the sequence low biogenic fluxes, high opal fluxes, high carbonate, carbon, and opal fluxes as representing the transition from low to high sea surface biological productivity. The phase relationships for eccentricity of the biogenic sedimentation records were compared to those of other climatic indications including the oxygen isotope record of ice volume, the ice core CO2 record, the Pacific Δδ13C record, the deep nutrient (Cd/Ca) record, and the eolian grain-size record of wind intensity. We find that starting from maximum ice volume (phase angle of 0°) the phase relationships of the 100,000-year periodicity show, in the first quadrant, maximum wind intensity (30°) and maximum calcite and carbon flux (45°) occurring first, followed by maximum opal flux (75°) and then maximum deep nutrients (95°). Minimum Pacific Δδ13C occurs at a phase angle of 155°, the maximum CO2 in the Vostok record and minimum ice volume is at 180°. Thus, in the eccentricity climate cycle, glacial conditions lead to stronger wind-driven circulation which enhances productivity which in turn results in enhanced oceanic and atmospheric carbon dioxide and minimum ice volume.

title: Barium in Deep‐Sea Sediment: A Geochemical Proxy for Paleoproductivity
abstract: We used sediment traps to define the particulate fluxes of barium and organic carbon and investigate the use of barium as a proxy for ocean fertility. Strong correlations between Corg and Ba fluxes indicate a link between upper ocean biological processes and barium flux to the seafloor. The ratio of organic carbon to barium decreases systematically with water depth. Data from 10 sites indicate that organic debris settling from the 200-m depth has a Corg /Ba ratio of approximately 200. The systematic decrease in this ratio with increasing water depth results from the simultaneous decay of organic matter and uptake of Ba in settling particles. This behavior provides additional evidence that the formation of barite in oceanic particles is a consequence of decomposition/uptake in microenvironments rather than the secretion of barite by specific organisms. The decrease of the Corg/Ba ratio with depth is greatest in the North Pacific followed by the equatorial Pacific and is lowest in the western Atlantic. Since this spatial pattern is consistent with the variations in the deep-ocean barium contents which increase along the path of bottom water flow from the Atlantic to the North Pacific, it suggests that the particulate barium uptake and flux is enhanced by higher barium contents in the intermediate and deep waters of the ocean. Consequently, we have combined our particle flux data with existing water column Ba data to define an algorithm relating new productivity, dissolved barium contents, water depth, and particulate barium flux. This relationship provides a basis of applying barium flux measurements in sediments to estimating new production. In order to use barium as an indicator of productivity, it will be necessary to evaluate inputs from hydrothermal and aluminosilicate sources and xenophyophors. The application of a sequential leach procedure to the trap material indicates that 50-70% of the Ba in settling particles is in the form of barite and the remaining is adsorbed or bound to carbonates. Normative analysis demonstrates that in nearshore areas the contribution of barium from aluminosilicate sources can dominate that from biogenic inputs. It appears that normative estimates of biogenic barium contents can be made with accuracy if less than 50% of the Ba is associated with aluminosilicates; i.e., is of terrigenous origin. Since diagenetic mobilization of Ba can occur in reduced and suboxic sediments, highly productive nearshore areas also are likely to be inappropriate sites to use Ba measurements as productivity indicators. Comparisons between the rain rates of particulate Ba to the seafloor and the burial rate indicate that approximately 30% of the Ba rain is preserved. Although the preservation factor does not appear to be constant, it may be possible to predict the extent of preservation from an empirical relationship with the mass accumulation rate. These observations indicate that measurement of Ba burial fluxes in sediments can provide quantitative information on the paleoproductivity of the oceans. Joining the relationship between barium rain and burial with the barium and organic carbon algorithm, we make estimates of the new production in the northern California Current during the last 18,000 years. This calculation suggests that new production was at least a factor of 2 lower at this site during the last glacial maximum.

title: Geomagnetic excursions: Knowns and unknowns
abstract: Geomagnetic excursions are short-lived episodes when Earth’s magnetic field deviates into an intermediate polarity state. Understanding the origin, frequency, amplitude, duration, and field behavior associated with excursions is a forefront research area within solid earth geophysics. Recent advances in excursion research are summarized here, and key further research is suggested to resolve major unanswered questions. Improving the global distribution of excursion records, particularly from the southern hemisphere, obtaining high-resolution sedimentary excursion records with good age control from sites with sedimentation rates >10 cm/kyr, obtaining volcanic excursion records coupled with high-precision geochronology, and estimating excursion duration with high chronological precision will all facilitate hypothesis testing concerning the deep earth dynamics that generate geomagnetic excursions.

title: 5.10 – Geomagnetic Excursions
abstract: Although evidence for geomagnetic polarity reversal can be traced to the work of Brunhes (1906), it was during the 1960s that efforts got fully underway to resolve the time sequence of polarity reversals (the so-called geomagnetic polarity timescale or GPTS) from both volcanic rocks and sediments. The importance of the GPTS to the geosciences became apparent when it was realized, in the early 1960s, that polarity records in oceanic magnetic anomalies and in sedimentary sequences could be used as a powerful means of correlation and, hence, dating. Early in the process of GPTS construction (and its correlation to geologic and absolute ages), brief polarity excursions began to emerge from volcanic and sedimentary paleomagnetic records. These were often characterized by magnetization directions transitional between the two polarity states, that did not correlate to polarity intervals elsewhere, and could be attributed to high amplitude secular variation or, alternatively, to artifacts of sampling, remagnetization, and sedimentological processes. The brief (few kiloyears) duration of polarity excursions means that their recording is often fortuitous and, until recently, no single excursion was unequivocally recorded at more than one location. The uncertainty in correlation of excursions has been a result of poorly constrained excursion ages in both volcanics and sediments. As aberrant magnetization directions can be attributed to a myriad of nongeomagnetic causes, records of excursions were generally treated with skepticism. 
 
The change in attitude toward excursions began about 15 years ago and can be attributed to two important developments: (1) Improved age control in young volcanic rocks and sediments allowed coeval excursions recorded in the two media to be unequivocally correlated. (2) The development of the hydraulic piston corer by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP), along with enhanced penetration from other more conventional piston coring devices, such as the CALYPSO corer on board of the Research Vessel Marion Dufresne of the French Polar Institute (IPEV), led to the recovery of long sequences of high-sedimentation-rate sediments from the deep sea, that enhanced the likelihood of recording brief polarity excursions. 
 
Although the subject of polarity excursions can now be considered relatively mature compared to the situation just 15 years ago, records of polarity excursions are largely restricted to the last 2 My. We can be confident in the existence of at least seven polarity excursions in the Brunhes Chron, and perhaps as many as 11 in the Matuyama Chron. Prior to the Matuyama Chron, records of polarity excursions are rare, apart from an interval in the Middle–Late Miocene where nine excursions (or brief microchrons) are recorded at more than one location, and another four are recorded at a single location. The Brunhes and Matuyama Chrons (or the Middle–Late Miocene interval) are unlikely to be intrinsically different from other polarity chrons; therefore, we may conclude that the documentation of polarity excursions is in its infancy. Progress will require access to volcanic or sedimentary sequences that have sufficiently high eruption/sedimentation rates to facilitate recording of brief polarity excursions. 
 
The (few) polarity excursions that appear to be more or less adequately recorded are manifest as paired polarity reversals with intervening directions that define a distinct, but brief, interval of opposite polarity relative to the polarity either side of the excursion. For the best-documented excursions, such as the Laschamp and Iceland Basin excursions, the directional changes appear to be manifest globally, and there is no clear evidence that excursions can be locally manifest. Although excursional records are often characterized by transitional magnetization directions without establishment of the opposite polarity, these excursional records may represent inadequate or incomplete recording of the polarity excursion, rather than pristine records of high amplitude secular variation. The smearing of the paleomagnetic record, by the detrital remanence (DRM) acquisition process, is likely to influence all sedimentary excursional records, and to be more severe in sediments with low accumulation rates. 
 
It is now well established that polarity excursions correlate with minima in sedimentary relative paleointensity records. In the future, we expect the catalog of excursions to expand considerably, as relative paleointensity minima are increasingly associated with excursional magnetization directions. Polarity excursions combined with sedimentary paleointensity records have great potential for high-resolution stratigraphic correlation. One of the major challenges in the study of abrupt climate change is the quest for a means of correlation of high-resolution climate records at an appropriate resolution that is not easily achieved using standard marine isotopic methods. The study of polarity excursions is, therefore, not only relevant to understanding geomagnetic field behavior and for constraining numerical simulation of the geodynamo, but is also important for paleoceanographic and paleoclimatic studies that require precise, millennial-scale, stratigraphic correlation.

title: Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences
abstract: We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ18O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19. 
 
Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed. 
 
The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of 40Ar/39Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28). 
 
The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface. 
 
Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.

title: Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron
abstract: We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ18O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ18O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Réunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Réunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

title: Post-depositional remanent magnetization lock-in and the location of the Matuyama-Brunhes geomagnetic reversal boundary in marine and Chinese loess sequences
abstract: article i nfo (PDRM) lock-in loess paleosol Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama-Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences.

title: Age of Matuyama-Brunhes boundary constrained by U-Pb zircon dating of a widespread tephra
abstract: ABSTRACTThe youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary (MBB), provides an important datum plane for sediments, ice cores, and lavas. Its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records, and is supported by 40 Ar/ 39 Ar dating of Hawaiian lavas using recent age calibrations. Challenging this age, how-ever, are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high-sedimentation-rate marine records, and cosmogenic nuclides in marine sediments and an Antarctic ice core. Here, we present a U-Pb zircon age of 772.7 ± 7.2 ka from a marine-deposited tephra just below the MBB in a forearc basin in Japan. U-Pb dating has a distinct advantage over 40 Ar/ 39 Ar dating in that it is relatively free from assumptions regarding stan-dardization and decay constants. This U-Pb zircon age, coupled with a newly obtained oxygen isotope chronology, yields an MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbitally tuned marine sediment records and on an Antarctic ice core. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratig-raphy for the MBB, fulfilling a key requirement in calibrating the geological time scale.

title: A method for unmixing magnetic hysteresis loops
abstract: Hysteresis loops provide essential information concerning both induced and remanent magnetizations and are an important tool for characterizing magnetic mineral assemblages. Although the hysteresis behavior of mixed natural magnetic assemblages has been a focal point of much recent work, little progress has been made in unmixing of hysteresis loops into characteristic components. Unmixing strategies can act as cornerstones for interpretation of rock magnetic data and have become popular for characterizing isothermal remanent magnetization acquisition curves. Unmixing of hysteresis loops is, however, a challenging task because the individual component loops in the mixture must meet stringent shape constraints. We present a new technique for decomposing an ensemble of hysteresis loops into a small number of end-members based on linear mixing theory. The end-members are not based on type curves but instead are derived directly from the hysteresis data. Particular attention is paid to the form of the end-members, ensuring they meet the shape constraints expected for hysteresis loops of natural magnetic mineral assemblages. Marine sediments from the Southern Ocean and lake sediments from Butte Valley, northern California, provide case studies on which the proposed unmixing method is tested.

title: Magnetic properties of pelagic marine carbonates
abstract: Pelagic carbonates are deposited far from continents, usually at water depths of 3000–6000 m, at rates below 10 cm/kyr, and are a globally important sediment type. Recent advances, with recognition of widespread preservation of biogenic magnetite (the inorganic remains of magnetotactic bacteria), have fundamentally changed our understanding of the magnetic properties of pelagic carbonates. We review evidence for the magnetic minerals typically preserved in pelagic carbonates, the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates, and what magnetic properties can tell us about the open-ocean environments in which pelagic carbonates are deposited. We also discuss briefly late diagenetic remagnetisations recorded by some carbonates. Despite recent advances in our knowledge of these phenomena, much remains undiscovered. We are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation. Recently developed techniques have potential for testing how different magnetotactic bacterial species, which produce different magnetite morphologies, respond to changing nutrient and oxygenation conditions. Future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well-calibrated modern magnetotactic bacterial occurrences. A tantalizing link between giant magnetofossils and Paleogene hyperthermal events needs to be tested; much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies. Rather than being a well-worn subject that has been studied for over 60 years, the magnetic properties of pelagic carbonates hold many secrets that await discovery.

title: Detection of noninteracting single domain particles using first-order reversal curve diagrams
abstract: We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

title: Searching for single domain magnetite in the “pseudo‐single‐domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations
abstract: Magnetic hysteresis measurements of sediments have resulted in widespread reporting of “pseudo-single-domain”-like magnetic properties. In contrast, the ideal single domain (SD) properties that would be expected to be responsible for high quality paleomagnetic records are rare. Determining whether SD particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment. We apply a range of such techniques and find that SD particles are much more common than has been reported in the literature and that magnetite magnetofossils (the inorganic remains of magnetotactic bacteria) are widely preserved at depth in a range of sediment types, including biogenic pelagic carbonates, lacustrine and marine clays, and possibly even in glaci-marine sediments. Thus, instead of being rarely preserved in the geological record, we find that magnetofossils are widespread. This observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments. Magnetofossils also have grain sizes that are substantially smaller than the 1–15μm size range for which there is reasonable empirical support for relative paleointensity studies. The different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies. This issue needs much closer attention. Despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years, much remains to be understood about how they become magnetized.

title: First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature
abstract: The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Neel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.

title: Analysis of the field dependence of remanent magnetization curves
abstract: A new method to calculate and analyze coercivity distributions of measured acquisition/demagnetization curves of remanent magnetization is presented. The acquisition/demagnetization curves are linearized by rescaling both the field and the magnetization axes. An appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions. The filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components. An error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data. Three sediment samples from Baldeggersee (Switzerland) and three samples of urban atmospheric particulate matter (PM) have been analyzed using this method. It is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic Gaussian function. Large deviations are found also in the coercivity distributions of theoretical AF demagnetization curves of single-domain and multidomain particles. Constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible. The generalized probability density function presented in this paper is suitable for appropriate modeling of Gaussian and a large number of non-Gaussian coercivity distributions.

title: Characterization of Individual Rock Magnetic Components by Analysis of Remanence Curves, 1. Unmixing Natural Sediments
abstract: Natural sediments are a complex mixture of magnetic minerals with different origins and different geochemical history, each of which is called a magnetic component. Magnetic components practically never occur in isolated form, and their characterization using bulk magnetic measurements relies on the individuation of the systematic variation of some parameters within a large group of samples. These variations can be interpreted either as a mixing trend or as the result of natural processes, which affect the physical and chemical properties of the magnetic particles. An alternative approach is offered by the analysis of magnetization curves using model functions, which are supposed to represent the magnetic properties of individual components. The success of this approach relies on (1) the choice of model functions that can reproduce the natural properties of a component with sufficient accuracy by varying a minimum number of parameters and (2) on very precise and accurate measurements, which are necessary to overcome the extreme sensitivity of the method to noise. In this paper, the analysis of remanent magnetization curves proposed by Egli (2003) is applied to a large set of representative sediments from the most variable environments and to a set of artificial magnetite samples. Despite the variety of materials and natural processes involved in the formation of these sediments, seven groups of magnetic components with well-defined and consistent properties could be identified. It has been found that both lacustrine and marine sediments contain two magnetically distinct groups of magnetosomes, which react differently to changes of the redox potential. The effects of some natural processes, such as weathering, reductive dissolution and transport could be observed on the individual components.

title: The identification and biogeochemical interpretation of fossil magnetotactic bacteria
abstract: Magnetotactic bacteria, which most commonly live within the oxic-anoxic transition zone (OATZ) of aquatic environments, produce intracellular crystals of magnetic minerals, specifically magnetite or greigite. The crystals cause the bacteria to orient themselves passively with respect 
to the geomagnetic field and thereby facilitate the bacteria’s search for optimal conditions within the sharp chemical gradients of the OATZ. The bacteria may also gain energy from the redox cycling of their crystals. 
 
Because magnetotactic bacteria benefit from their magnetic moments, natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells. These traits also allow crystals produced by magnetotactic bacteria (called magnetofossils when preserved in sediments) to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism. Magnetofossils are recognizable based on their narrow size and shape distributions, distinctive morphologies with blunt crystal edges, chain arrangement, chemical purity, and crystallographic perfection. This article presents a scheme for rating magnetofossil robustness based on these traits. 
 
The magnetofossil record extends robustly to the Cretaceous and with lesser certainty to the late Archean. Because magnetotactic bacteria predominantly live in the OATZ, the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column. The magnetofossil record therefore provides an underutilized archive of paleoenvironmental information. Several studies have demonstrated a relationship between magnetofossil abundance and glacial/interglacial cycles, likely mediated by changes in pore water oxygen levels. More speculatively, a better-developed magnetofossil record might provide constraints on the long-term evolution of marine redox stratification. More work in modern and ancient settings is necessary to explicate the mechanisms linking the 
abundance and character of magnetofossils to ancient biogeochemistry.

title: Magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron
abstract: article i nfo Article history: Magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording palaeomagnetic signals. However, bacterial magnetite has only been reported in a few pre-Quaternary records because progressive burial into anoxic diagenetic environments causes its dissolution. Deep-sea carbonate sequences provide optimal environments for preserving bacterial magnetite due to low rates of organic carbon burial and expanded pore-water redox zonations. Such sequences often do not become anoxic for tens to hundreds of metres below the seafloor. Nevertheless, the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known. We document the preservation of bacterial magnetite, which dominates the palaeomagnetic signal throughout Eocene pelagic carbonates from the southern Kerguelen Plateau, Southern Ocean. We provide evidence that iron fertilization, associated with increased aeolian dust flux, resulted in surface water eutrophication in the late Eocene that controlled bacterial magnetite abundance via export of organic carbon to the seafloor. Increased flux of aeolian iron- bearing phases also delivered iron to the seafloor, some of which became bioavailable through iron reduction. Our results suggest that magnetotactic bacterial populations in pelagic settings depend crucially on particulate iron and organic carbon delivery to the seafloor.

title: Quantifying magnetite magnetofossil contributions to sedimentary magnetizations
abstract: Under suitable conditions, magnetofossils (the inorganic remains of magnetotactic bacteria) can contribute to the natural remanent magnetization (NRM) of sediments. In recent years, magnetofossils have been shown to be preserved commonly in marine sediments, which makes it essential to quantify their importance in palaeomagnetic recording. In this study, we examine a deep-sea sediment core from offshore of northwestern Western Australia. The magnetic mineral assemblage is dominated by continental detritus and magnetite magnetofossils. By separating magnetofossil and detrital components based on their different demagnetization characteristics, it is possible to quantify their respective contributions to the sedimentary NRM throughout the Brunhes chron. In the studied core, the contribution of magnetofossils to the NRM is controlled by large-scale climate changes, with their relative importance increasing during glacial periods when detrital inputs were low. Our results demonstrate that magnetite magnetofossils can dominate sedimentary NRMs in settings where they are preserved in significant abundances.

title: Magnetic detection and characterization of biogenic magnetic minerals: A comparison of ferromagnetic resonance and first‐order reversal curve diagrams
abstract: Australian Research Council. Grant Number: DP120103952
Australia-New Zealand IODP Consortium (ANZIC). Grant Number: LE0882854

title: Critical superparamagnetic/single-domain grain sizes in interacting magnetite particles: implications for magnetosome crystals
abstract: Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosome crystals), which they use for navigation (magnetotaxis). To improve magnetotaxis efficiency, the magnetosome crystals (usually magnetite or greigite in composition) should be magnetically stable single-domain (SSD) particles. Smaller single-domain particles become magnetically unstable owing to thermal fluctuations and are termed superparamagnetic (SP). Previous calculations for the SSD/SP threshold size or blocking volume did not include the contribution of magnetic interactions. In this study, the blocking volume has been calculated as a function of grain elongation and separation for chains of identical magnetite grains. The inclusion of magnetic interactions was found to decrease the blocking volume, thereby increasing the range of SSD behaviour. Combining the results with previously published calculations for the SSD to multidomain threshold size in chains of magnetite reveals that interactions significantly increase the SSD range. We argue that chains of interacting magnetosome crystals found in magnetotactic bacteria have used this effect to improve magnetotaxis.

title: Widespread occurrence of silicate‐hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording
abstract: Magnetic mineral inclusions occur commonly within other larger mineral phases in igneous rocks and have been demonstrated to preserve important paleomagnetic signals. While the usefulness of magnetic inclusions in igneous rocks have been explored extensively, their presence in sediments has only been speculated upon. The contribution of magnetic inclusions to the magnetization of sediments, therefore, has been elusive. In this study, we use transmission electron microscope (TEM) and magnetic methods to demonstrate the widespread preservation of silicate-hosted magnetic inclusions in marine sedimentary settings. TEM analysis reveals detailed information about the microstructure, chemical composition, grain size, and spatial arrangement of nanoscale magnetic mineral inclusions within larger silicate particles. Our results confirm the expectation that silicate minerals can protect magnetic mineral inclusions from sulfate-reducing diagenesis, and increase significantly the preservation potential of iron oxides in inclusions. Magnetic inclusions should, therefore, be considered as a potentially important source of fine-grained magnetic mineral assemblages, and represent a missing link in a wide range of sedimentary paleomagnetic and environmental magnetic studies. In addition, we present depositional remanent magnetization (DRM) modeling results to assess the paleomagnetic recording capability of magnetic inclusions. Our simulation demonstrates that deposition of larger silicate particles with magnetic inclusions will be controlled by gravitational and hydrodynamic forces rather than by geomagnetic torques. Thus, even though these large silicates may contain ideal single domain particles, they can not contribute meaningfully to paleomagnetic recording. However, smaller silicate grains (e.g., silt- and clay-sized) silicates with unidirectionally magnetized magnetic inclusions can potentially record a reliable DRM.

title: Low‐temperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and magnetic mineral reduction
abstract: Magnetic properties of pelagic sediments from the western equatorial Pacific (Ocean Drilling Program (ODP) Site 805C) have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth. A saturation isothermal remanent magnetization (Mrs) was given to the samples by applying a strong magnetic field (2.5 T) at 20 K after cooling both in the absence (zero field cooling, ZFC) and presence of a 2.5 T magnetic field (field cooling, FC). The thermal demagnetization of both MrsZFC and MrsFC was measured from 20 to 300 K. No pronounced Verwey transition was observed in the data from samples shallower than the iron redox boundary. Below the boundary the Verwey transition is visible for all samples studied. We interpret this behavior as reflecting low-temperature oxidation (maghemitization) of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary. The low-temperature oxidation may occur both at the water-sediment interface and deeper in the suboxic zone of sediments as a result of bacteria-mediated processes, including the reduction of nitrate compounds. MrsZFC and MrsFC thermal demagnetization curves of all samples diverge for the entire temperature range measured. The divergence may be caused by the low-temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system. Together, maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low-temperature properties of the sediment studied. As a result, low-temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments. The changes in effective oxidation state below the iron redox boundary detected by low-temperature methods may be useful in the study of some paleoenvironmental processes.

title: Low-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains
abstract: Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C) and on samples containing whole magnetotactic bacteria cells. We document a range of low-temperature magnetic properties, including reversible humped low-temperature cycling (LTC) curves. Different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of LTC curves. A dipole spring mechanism in magnetosome chains is introduced to explain reversible LTC curves. This dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite, similar to published results for uniaxial stable single domain (SD) particles. The dipole spring mechanism reversibly restores the remanence during warming in LTC measurements. This supports a previous idea that remanence of magnetosome chains is completely reversible during LTC experiments. We suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains, although the presence of isolated uniaxial stable SD particles and magnetically interacting particles can complicate this test. Magnetic measurements through the Eocene section of ODP Hole 738B reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic “hard” component. Co-occurrence of these two magnetic fingerprints during the late Eocene in the Southern Ocean indicates less oxic conditions, probably due to increased oceanic primary productivity and organic carbon burial.

title: Low-temperature magnetic behavior of titanomagnetites
abstract: Titanium substitution has a significant effect on the temperature dependence (5–300 K) of low-temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite (Fe3−xTixO4, 0≤x 0.4, and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range. These features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples. Moreover, the apparent similarities between the thermal decay of low-temperature remanence of titanomagnetite with pyrrhotite, pure magnetite, and superparamagnetic phases can lead to complications in interpreting low-temperature remanence data when titanomagnetite is present.

title: Depositional remanent magnetization: Toward an improved theoretical and experimental foundation
abstract: The first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field, yet redeposition experiments showed a strong (and quasi-linear) dependence on the external field. This empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades. We present redeposition experiments which suggest instead that the relationship between depositional remanence (DRM) and applied field may frequently be curved for magnetic fields in the range of the Earth's. Numerical simulations using a flocculation model can explain the redeposition data and suggest that DRM will be significantly non-linear when the flocs are small (several microns). There is a strong dependence of floc size on salinity particularly in low salinity environments. Floc size has a profound influence on the efficiency of DRM, hence low salinity environment may give results with poor reproducibility. The size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization, yet is the most important parameter. On the bright side, however, it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained.

title: Are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization
abstract: Recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column. Magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached, making remanence acquisition an inefficient process. Alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel, reducing the overall magnetic torque. Existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other. In the case of non-spherical flocs, the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column. Calculations will be presented that show the torque exerted on a non-spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque. Non-spherical flocs will, therefore, align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence.

title: Least-squares fitting of a straight line.
abstract: A detailed discussion of the calculation of the "best straight line" by the method of least squares is given. The most general solution is found and the conditions under which certain previously derived special solutions are valid are clearly stated. The "best" slope is shown to be given by the solution of the "Least-Squares Cubic". An example is given to illustrate the method. It is shown that the best slope is not necessarily bounded by values found from the regressions of x on y and y on x.

title: Magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment: First experimental results and theory
abstract: The widespread occurrence of magnetotactic bacteria (MTB) in several types of marine and freshwater sediment, and the role of fossil magnetosomes (magnetofossils) as main remanent magnetization carriers therein, has important paleomagnetic and paleoenvironmental implications. Despite numerous studies on MTB biology and on magnetofossil preservation in geological records, no detailed information is yet available on how magnetotaxis (i.e., the ability to navigate along magnetic field lines) is performed in sedimentary environments, and on how magnetofossils possibly record the Earth magnetic field. We provide for the first time experimental evidence for these processes. MTB living in sediment are poorly aligned with the geomagnetic field, contrary to what is observed in water. This can explain the seemingly excessive magnetic moment of most MTB. The observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients. Experiments with magnetofossil-rich sediment suggest that a natural remanent magnetization (NRM) is acquired by magnetofossils in the so-called benthic mixed layer, where natural MTB populations usually occur. The acquired NRM is proportional to the applied field at least up to ∼160 µT, and its intensity is compatible with values observed in nature for same sediment types. Therefore, if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes, they can provide useful relative paleointensities. We propose a preliminary model to explain early stages of magnetofossil NRM acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing.

title: Hysteresis properties of titanomagnetites: Grain-size and compositional dependence
abstract: Sized fractions of x = 0.6, 0.4, 0.2 and 0.0 titanomagnetites were studied with a vibration magnetometer. In the course particles (d > 150 μm), no compositional dependence of hysteresis parameters was found. HC was less than 50 Oe, HR/HC > 4 and JR/JS < 10−2, reflecting multi-domain behaviour. In contrast, fine particles (d ⋍ 0.1 μm) revealed systematic grain-size dependence of parameters with coercive force as high as 2,000 Oe in x = 0.6 titanomagnetite. Grain-size dependence studies revealed broad transition sizes for the onset of true multi-domain behaviour depending upon which factor is chosen. In magnetite it varies from 10 to 20 μm. The experimental critical size for single-domain behaviour for magnetite is about 0.1 μm and for x = 0.6 titanomagnetite 1–2 μm.

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