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Geomagnetic field models constrained by direct observations extend back some 400 years and the magnitude of decadal to centennial variations that they display is often assumed to be typical of the secular variation over longer timescales. Here we present archaeomagnetic directions from layered Neolithic hearths in Orkney, dating to around 3000 BCE, that challenge this assumption. Combining stratigraphic controls with radiocarbon dates allows for a precise chronology, which, in turn, implies directional change of more than 12 degrees/century lasting for 200 years, far in excess of anything seen at this latitude over the era of historical observations. These archaeomagnetic data are complemented by similarly rapid changes in inclination recorded in two high temporal resolution sediment cores from Ka<spacing diaeresis>lksjo<spacing diaeresis>n, Sweden. As well as raising the possibility of using archaeomagnetic dating within this important archaeological period, the new data pose questions regarding our understanding of the secular variation, the limitations of the historic field as a proxy for the past, and the underlying dynamical processes within the core. This research highlights the importance of archaeology to the wider scientific community and the potential of archaeological material to further our understanding of Earth processes.

Interpretation of palaeomagnetic data requires the detection of magnetofossils in sedimentary rocks and an understanding of their influence on magnetic properties. Subsamples collected from IODP site M0061 lost up to 90 per cent of their initial bulk magnetic susceptibility (MS) during cold-room storage of 5 months, which was attributed to the alteration of single-domain magnetosomal greigite (Fe3S4). To test if the magnetic susceptibility loss affected the anisotropy of MS (AMS) we resampled site M0061 with a Kullenberg piston corer (3 cores), took palaeomagnetic subsamples and undertook time-dependent AMS measurements over 1 yr in a controlled cool, humidified environment and exposed to air. Most subsamples possessed an initial normal oblate AMS fabric predicted for laminated sediments (horizontal with respect to the bedding plane) but we also detected a negative trend between the degree of anisotropy (Pj) and MS. In accordance with previous observations, MS decreased over 1 yr, which we accredit to oxidation of magnetosomal greigite and conversion into a less magnetic phase (probably FeO(OH)) that does not make a detectable contribution to AMS. These results allowed us to isolate, through application of an AMS tensor subtraction routine, the fabric of the magnetosomal greigite component that had decayed. In subsamples with the largest MS loss over one year, the decayed component had a prolate, inverse AMS fabric (defined as the principal susceptibility axis perpendicular to the bedding place) but relatively low Pj. We conclude that the initial (in-situ) AMS ellipsoid consisted of a mixture of a typical normal, oblate sedimentary fabric and the prolate, inverse magnetosomal fabric. The mostly inverse nature of the separated fabric indicates that the long axis of the magnetosomal greigite (as individual single-domain magnetofossils or chains) must be oriented parallel to the bedding plane, which implies that the magnetosomal greigite was deposited from the water column and contributes to a depositional remanent magnetization (DRM). Our results indicate that greigite magnetofossils can (i) explain the inverse AMS fabrics that have been reported in similar sedimentary environments and (ii) carry DRM with a median destructive field (MDF) of approximately 20 mT, although this remanence is transient under ambient laboratory conditions and is prone to oxidation.

A marked sedimentological change in subsurface sediments from the entire Baltic Proper, the Baltic Sea, has been previously noted. Our detailed work on a variety of multi-cores from basin-wide transects indicates that this sedimentological change was caused by a large shift in environmental conditions during the 1950s. Until the 1950s, the water column was rather weakly stratified and winter-time convection - although weakened during the post Little Ice Age warming - was still able to ventilate the bottom waters of the Baltic Proper. Therefore, complete sediment sequences only accumulated in calm waters deeper than 150-160 m. High-resolution benthic foraminiferal records of subsurface sediments obtained along the saline water inflow pathway in combination with historical data indicate that the depositional environment changed drastically owing to the giant saline water inflow in AD 1951. The accompanied sharpening of the halo(pycno)cline triggered a collapse in the ventilation of the basin, resulting in oxygen-deficient bottom waters. This deficiency, in turn, caused the onset of phosphate release from the sediments, which accelerated primary production. The ventilation collapse also enabled the onset of deposition of organic carbon-rich sediments also in shallower water areas as calm conditions prevailed up to the modern winter mixing depth (60-70 m). A slight return to Little Ice Age-type conditions was observed during the late 1980s when temperatures decreased and stratification weakened. These conditions gave rise to a reduction in hypoxic areas and to a bottom-water ventilation, most pronounced in the north of the so-called Baltic Sea Klint, a hydrographic and topographic barrier. However, the general environmental conditions essentially have not changed since the 1950s. Remarkably, external (temperature and stratification) in combination with internal factors (e.g. ventilation collapse and phosphate release) were able to change the redox conditions of the Baltic Proper from oxic to hypoxic within less than 10 years. A marked sedimentological change seen in sub-recent seabed sediment cores from the entire Baltic proper can be attributed to large hydrographic and environmental changes that started at the end of the Little Ice Age and were accelerated by a rapid change in stratification resulting from the massive inflow of saline water in AD 1951. The increase in stratification caused a collapse of the already weakened vertical winter-time deep water convection leading to hypoxia in the bottom waters, which in turn forced a sudden phosphate release from the sediments and increased primary production in the late 1950s.image

Sulfur isotopic signatures of pyrite (δ³⁴S_py) sensitively react to local environmental change. Here we elucidate mechanism of δ³⁴S_py variation in the Holocene sedimentary strata of the Yangtze River Delta, through the analyses of multiple geochemical and physical indicators including total organic carbon, total nitrogen, stable isotopes of organic carbon, different iron solid phases, Sr/Ba ratios, and changes in sedimentation rates. Two heavy δ³⁴S_py (∼30‰) layers that formed under contrasting redox conditions are preserved in the early Holocene transgressive tidal flat and late Holocene regressive mouth bar facies, respectively. Two hypotheses are proposed to explain these heavy δ³⁴S_py layers: 1) in the transgression process, the heavy δ³⁴S_py layers formed under stronger reducing conditions due to methane leakage; 2) in the regression process, the heavy δ³⁴S_py layers formed under relatively weaker reducing conditions due to rapid deposition and low salinity. This study emphasizes the importance of physicochemical condition characterization in sediments, including redox conditions, in distinguishing the mechanisms that act to produce heavy δ³⁴S_py signals.

Deposits of contaminated wood fiber waste (fiberbanks), originating from sawmills and pulp and paper industries, have been found in the aquatic environment in boreal countries. In-situ isolation capping has been proposed as a remediation solution because it has the potential to prevent dispersal of persistent organic pollutants (POPs) from this type of sediment. However, knowledge about the performance of such caps when placed on very soft (un-consolidated), gaseous organic rich sediment is scarce. We investigated the effectiveness of conventional in-situ capping to limit POPs fluxes to the water column from contaminated fibrous sediments that produce gas. A controlled, large-scale laboratory column (40 cm diameter, 2 m height) experiment was performed over 8 months to study changes in sediment-to-water fluxes of POPs and particle resuspension before and after capping the sediment with crushed stones (& GE;4 mm grain size). Two different cap thicknesses were tested (20 and 45 cm) on two types of fiberbank sediment with different fiber type composition. Results showed that capping fiberbank sediment with a 45 cm gravel cap reduced the sediment-to-water flux by 91-95% for p,p'-DDD, o,p'-DDD, by 39-82% for CB-101, CB-118, CB-138, CB-153, CB-180 and by 12-18% for HCB, whereas for less hydrophobic PCBs, capping was largely ineffective (i.e. CB-28 and CB-52). Although cap application caused particle resus-pension, the long-term effect of the cap was reduced particle resuspension. On the other hand, substantial sediment consolidation released large volumes of contaminated pore water into the overlying water body. Importantly, both sediment types produced large amount of gas, observed as gas voids forming inside the sediment and gas ebullition events, which increased pore water advection and affected the structural integrity of the cap. This may limit the practical applicability of this method on fiberbank sediments.

The Yaojiang Valley (YJV) on the southern Hangzhou Bay Plain, eastern China, was home to Neolithic cultures that date back to 8 ka. Reconstructions of ancient human cultures on the coastal plain in response to sea-level and hydroclimate changes have been well documented. However, centennial-resolution geological records are still lacking. Here, we present a combined approach of environmental (mineral) magnetism, geochemistry and diffuse reflectance spectroscopy (DRS) on a sediment core (YJ1503) in YJV to decipher the centennial hydroclimate oscillations of the coastal plain since early Holocene. Our results show that ferrimagnetic greigite and paramagnetic pyrite bearing layers correspond to periods of reducing condition that were caused by a higher water table while the presence of antiferromagnetic hematite/goethite corresponds to drier periods favorable for soil formation. Centennial scale fluctuations of magnetic mineralogy around the 8.2 ka cooling event and 7.5–6.6 cal ka BP reflect frequent and unstable environments, with limited Neolithic occupation. A strong drought event occurred around 6.0 cal ka BP, which was preceded by an extensive marine inundation event in the study area. This inundation event interrupted rice farming. Our study indicates that magnetic mineralogy can be used to reconstruct high-resolution hydrological processes in coastal plain and floodplain environments. These results are important to understand hydroclimate variation and Neolithic civilization response in environmentally sensitive areas.

Late Pliocene and Early Pleistocene epochs 3.6 to 0.8 million years ago1 had climates resembling those forecasted under future warming2. Palaeoclimatic records show strong polar amplification with mean annual temperatures of 11–19 °C above contemporary values3,4. The biological communities inhabiting the Arctic during this time remain poorly known because fossils are rare5. Here we report an ancient environmental DNA6 (eDNA) record describing the rich plant and animal assemblages of the Kap København Formation in North Greenland, dated to around two million years ago. The record shows an open boreal forest ecosystem with mixed vegetation of poplar, birch and thuja trees, as well as a variety of Arctic and boreal shrubs and herbs, many of which had not previously been detected at the site from macrofossil and pollen records. The DNA record confirms the presence of hare and mitochondrial DNA from animals including mastodons, reindeer, rodents and geese, all ancestral to their present-day and late Pleistocene relatives. The presence of marine species including horseshoe crab and green algae support a warmer climate than today. The reconstructed ecosystem has no modern analogue. The survival of such ancient eDNA probably relates to its binding to mineral surfaces. Our findings open new areas of genetic research, demonstrating that it is possible to track the ecology and evolution of biological communities from two million years ago using ancient eDNA.

The geomagnetic field behavior in polar regions remains poorly understood and documented. Although a number of Late Holocene paleomagnetic secular variation (PSV) records exist from marginal settings of the Amerasian Basin in the Arctic Ocean, their age control often relies on a handful of radiocarbon dates to constrain ages over the past 4,200 years. Here we present well-dated Late Holocene PSV records from two sediment cores recovered from the Chukchi Sea, Arctic Ocean. The records are dated using 26 C-14 measurements, with local marine reservoir corrections calibrated using tephra layers from the 3.6 cal ka BP Aniakchak eruption in Northern Alaska. These C-14-based chronologies are extended into the post-bomb era using caesium-137 dating, and mercury isochrons. Paleomagnetic measurements and rock magnetic analyses reveal stable characteristic remanent magnetization directions, and a magnetic mineralogy dominated by low-coercivity minerals. The PSV records conform well to global spherical harmonic field model outputs. Centennial to millennial scale directional features are synchronous between the cores and other Western Arctic records from the area. Due to the robust chronology, these new high-resolution PSV records provide a valuable contribution to the characterization of geomagnetic field behavior in the Arctic over the past few thousand years, and can aid in developing age models for suitable sediments found in this region.

Unregulated discharges of wastewater from pulp and paper factories resulted in the formation of relatively thick organic (cellulose) rich sediments in shallow waters along the Swedish coast. These deposits are known as fiberbanks and are contaminated by persistent organic pollutants (POPs), metals and methylmercury, which can be dispersed by diffusion and advective processes coupled to propeller wash, high river discharges, strong wind waves and submarine landslides. Based on a case study of polychlorinated biphenyls (PCBs), one group of prevalent POPs in the fiberbanks, we present a probabilistic approach to estimate the potential risk of dispersion of fiberbank contaminants. The approach allows for estimation of the dispersal pathways that dominates the risk within a given time and provides more insight about the significance of various dispersion processes. We show that it is highly likely that chemical diffusion and advection triggered by ship-induced resuspension will disperse PCBs (sum of seven congeners; sigma 7PCB) above a threshold level for environmental impact, while the likelihood of river and wind-wave generated resuspension dispersion pathways are lower (similar to 20%, respectively). We further show that there is approximately 5% likelihood that a submarine landslide will disperse sigma 7PCB above the threshold level. The study implies that the governing parameters for risk assessment specifically should include reliable data on contaminant concentration, water depth above the fiberbank, estimation of concerned fiberbank areas, time duration of erosive fluid flows and measured diffusion. The approach provides insight into the importance of various dispersion processes. We suggest that it can be applied to support risk assessment, especially when there are limited available data and/or knowledge about the system under study.

In several locations along Sweden's northern Baltic coast there are deposits of contaminated fibrous sediments that are known as fiberbanks. Historically, these anthropogenic fiberbanks and associated contaminants were formed outside pulp and paper factories because of unregulated waste discharges. Some of the fiberbanks are located on seafloor slopes that are unstable and may fail, which could result in large amounts of fiber and contaminant dispersion. Hydroacoustic surveys have revealed that some parts of the fiberbanks have been dispersed to a wider area by submarine landslides. However, the unusual nature of these cellulose-rich sediments makes it difficult to apply conventional techniques for assessing submarine slope stability. Therefore, a combination of investigation techniques and interpretation methods was tested to assess the stability and triggering mechanisms for submarine slope failure identified in fiberbanks from the Angermanalven river estuary on the Baltic sea coast. The integration of bathymetric data, sediment lithology and geotechnical characteristics from in-situ and laboratory measurements, enabled the characterisation of underlying natural sediments and the fiberbanks at two sites. Despite low densities, which indicate that the fiberbank material is near the buoyancy level, the fiberbank deposits seem to be relatively stable. Our results indicate the underlying natural sediments are also stable and would require an external triggering mechanism to generate the observed slope failures. We suggest that pore water pressure, potentially related to groundwater table fluctuations, may be an important trigger for submarine landslides in the study area.

Management of fiberbanks and associated fiber-rich sediments, which are numerous along Sweden's north east coast, requires that sites are risk assessed and prioritized for remediation. To do so, under water slope stability should be considered as a possible dispersion pathway and therefore needs to be investigated. The study illustrates the value of using a combination of geophysical and geotechnical field and laboratory methods, together with empirical relationships, to assess relevant input parameters for slope stability calculations for this type of sediment.