Search

A zip file which contains three excel files and two slotting (*.sld) files, to be used with CPLSlot and with the worked examples S5.1, S5.2 and S5.3

The snowball Earth hypothesis - that a runaway ice-albedo feedback can cause global glaciation - seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re emergence of banded iron formation and ‘‘cap’’ carbonates. One of the most significant challenges to snowball Earth has been sedimentological cyclicity that has been taken to imply more climate dynamics than expected when the ocean is completely covered in ice. However, recent climate models suggest that as atmospheric CO2 accumulates, the snowball climate system becomes sensitive to orbital forcing. Here we show the presence of nearly all Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth. These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. Orbital glacial advance and retreat cycles provide a simple mechanism to reconcile both the sedimentary dynamics and the enigmatic survival of multicellular life during snowball Earth.

These guidelines provide advice on best practice for the effective use of scientific dating on Pleistocene sites. They are applicable to all archaeological projects, but are aimed primarily at those undertaken as part of the planning process. Pleistocene sites typically produce limited material that is suitable for dating. Some of the methods that can be employed are familiar to those working in later periods (eg Radiocarbon Dating), although special considerations for their effective use may apply. Other methods (eg the ’Vole Clock‘) are only used in the Pleistocene. The selection of appropriate techniques, given the available types of datable material, its taphonomic relationship to the archaeological objectives of the project, and the expected time-range of the site, is key. Different strands of evidence can be explicitly combined using Bayesian statistical modelling, and the resultant chronologies can be validated, not only by comparison to relative dating from stratigraphy, but also by employing multiple scientific dating techniques. Above all, seek expert advice. All laboratories will be happy to advise on applying their technique to Pleistocene deposits, and will welcome the opportunity to discuss sample selection and potential methods of cross-checking their results with you. It is by working together with a range of specialists that you will provide the best dating possible for your site.

Additional file 1: Figure S1. Major element compositions of volcanic glass shards from studied tephras in U1424A sediments compared with data from correlated known Japanese widespread tephras. FeO* is the total iron recalculated as FeO. The pale bars indicate the standard deviations.

Fine-grained magnetic particles in deep-sea sediments often statistically align with the ambient magnetic field during (and shortly after) deposition and can therefore record geomagnetic reversals. Correlation of these reversals to a geomagnetic polarity time scale is an important geochronological tool that facilitates precise stratigraphic correlation and dating of geological records globally. Sediments often carry a remanence strong enough for confident identification of polarity reversals, but in some cases a low signal-to-noise ratio prevents the construction of a reliable and robust magnetostratigraphy. Here we implement a data-filtering protocol, which can be integrated with the UPmag software package, to automatically reduce the maximum angular deviation and statistically mask noisy data and outliers deemed unsuitable for magnetostratigraphic interpretation. This protocol thus extracts a clearer signal from weakly magnetized sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406 (Newfoundland margin, northwest Atlantic Ocean). The resulting magnetostratigraphy, in combination with shipboard and shore-based biostratigraphy, provides an age model for the study interval from IODP Site U1406 between Chrons C6Ar and C9n (∼21-27 Ma). We identify rarely observed geomagnetic directional changes within Chrons C6Br, C7r, and C7Ar, and perhaps within Subchron C8n.1n. Our magnetostratigraphy dates three intervals of unusual stratigraphic behavior within the sediment drifts at IODP Site U1406 on the Newfoundland margin. These lithostratigraphic changes are broadly concurrent with the coldest climatic phases of the middle Oligocene to early Miocene and we hypothesize that they reflect changes in bottom-water circulation.

We produced a composite depth scale and chronology for Site U1385 on the SW Iberian Margin. Using log(Ca/Ti) measured by core scanning XRF at 1-cm resolution in all holes, a composite section was constructed to 166.5 meters composite depth (mcd) that corrects for stretching and squeezing in each core. Oxygen isotopes of benthic foraminifera were correlated to a stacked δ^18O reference signal (LR04) to produce an oxygen isotope stratigraphy and age model. Variations in sediment color contain very strong precession signals at Site U1385, and the amplitude modulation of these cycles provides a powerful tool for developing an orbitally-tuned age model. We tuned the U1385 record by correlating peaks in L* to the local summer insolation maxima at 37^oN. The benthic δ^18O record of Site U1385, when placed on the tuned age model, generally agrees with other time scales within their respective chronologic uncertainties. The age model is transferred to down-core data to produce a continuous time series of log(Ca/Ti) that reflect relative changes of biogenic carbonate and detrital sediment. Biogenic carbonate increases during interglacial and interstadial climate states and decreases during glacial and stadial periods. Much of the variance in the log(Ca/Ti) is explained by a linear combination of orbital frequencies (precession, tilt and eccentricity), whereas the residual signal reflects suborbital climate variability. The strong correlation between suborbital log(Ca/Ti) variability and Greenland temperature over the last glacial cycle at Site U1385 suggests that this signal can be used as a proxy for millennial-scale climate variability over the past 1.5 Ma. Millennial climate variability, as expressed by log(Ca/Ti) at Site U1385, was a persistent feature of glacial climates over the past 1.5 Ma, including glacial periods of the early Pleistocene (‘41-kyr world’) when boundary conditions differed significantly from those of the late Pleistocene (‘100-kyr world’). Suborbital variability was suppressed during interglacial stages and enhanced during glacial periods, especially when benthic δ^18O surpassed ~ 3.3-3.5‰. Each glacial inception was marked by appearance of strong millennial variability and each deglaciation was preceded by a terminal stadial event. Suborbital variability may be a symptomatic feature of glacial climate or, alternatively, may play a more active role in the inception and/or termination of glacial cycles.

Paleomagnetic analysis and radiocarbon dating of an expanded Holocene deep-sea sediment sequence recovered by Integrated Ocean Drilling Program (IODP) Expedition 303 from Labrador Sea Site U1305 (Lat.: 57 degrees 28.5 N, Long.: 48 degrees 31.8 W, water depth 3459 m) provides insights into mechanisms that drive both paleomagnetic secular variation (PSV) and magnetization acquisition in deep-sea sediments. Seventeen radiocarbon dates on planktonic foraminifera define postglacial (ca. 8 ka) sedimentation rates as ranging from 35 to > 90 cm/kyr. Alternating field (AF) demagnetization of u-channel samples show that these homogeneous sediments preserve a strong, stable, and consistently well-defined component magnetization. Normalized remanence records pass reliability criteria for relative paleointensity (RPI) estimates. Assuming that the age of magnetization is most accurately defined by well dated PSV records with the highest sedimentation rates, allows us to estimate and correct for temporal offsets at Site U1305 interpreted to result from postdepositional remanence acquisition at a depth of similar to 20 cm. Comparisons indicate that the northern North Atlantic PSV and RPI records are more consistent with European than North American records, and the evolution of virtual geomagnetic poles (VGP) are temporally and longitudinally similar to global reconstructions, though with much larger latitudinal variation. The largest deviations from a geocentric axial dipole (GAD) are observed during times of the highest intensities, in contrast to the usual assumption. These observations are consistent with the idea that PSV in the North Atlantic and elsewhere during the Holocene results from temporal oscillations of high-latitude flux concentrations at a few recurrent locations.

International audience; Paleomagnetic analysis and radiocarbon dating of an expanded Holocene deep‐sea sediment sequence recovered by Integrated Ocean Drilling Program (IODP) Expedition 303 from Labrador Sea Site U1305 (Lat.: 57°28.5 N, Long.: 48°31.8 W, water depth 3459 m) provides insights into mechanisms that drive both paleomagnetic secular variation (PSV) and magnetization acquisition in deep‐sea sediments. Seventeen radiocarbon dates on planktonic foraminifera define postglacial (ca. 8 ka) sedimentation rates as ranging from 35 to > 90 cm/kyr. Alternating field (AF) demagnetization of u‐channel samples show that these homogeneous sediments preserve a strong, stable, and consistently well‐defined component magnetization. Normalized remanence records pass reliability criteria for relative paleointensity (RPI) estimates. Assuming that the age of magnetization is most accurately defined by well dated PSV records with the highest sedimentation rates, allows us to estimate and correct for temporal offsets at Site U1305 interpreted to result from postdepositional remanence acquisition at a depth of ∼20 cm. Comparisons indicate that the northern North Atlantic PSV and RPI records are more consistent with European than North American records, and the evolution of virtual geomagnetic poles (VGP) are temporally and longitudinally similar to global reconstructions, though with much larger latitudinal variation. The largest deviations from a geocentric axial dipole (GAD) are observed during times of the highest intensities, in contrast to the usual assumption. These observations are consistent with the idea that PSV in the North Atlantic and elsewhere during the Holocene results from temporal oscillations of high‐latitude flux concentrations at a few recurrent locations.

Submitted by Paula Serrano (paula.serrano@ineti.pt) on 2013-05-14T17:12:08Z No. of bitstreams: 1 35611.pdf: 4642239 bytes, checksum: 4321a8aea0910c342b8051bc8b38f416 (MD5) Approved for entry into archive by Paula Serrano (paula.serrano@ineti.pt) on 2013-05-14T17:12:18Z (GMT) No. of bitstreams: 1 35611.pdf: 4642239 bytes, checksum: 4321a8aea0910c342b8051bc8b38f416 (MD5) Made available in DSpace on 2013-05-14T17:12:18Z (GMT). No. of bitstreams: 1 35611.pdf: 4642239 bytes, checksum: 4321a8aea0910c342b8051bc8b38f416 (MD5) Previous issue date: 2012