Juan C. Larrasoaña

What do the HIRM and S-ratio really measure in environmental magnetism?

The “hard” isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. We demonstrate that synthetic Al-substituted hematite and goethite exhibit a wide range of coercivities, which significantly influences the HIRM and S-ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT: IRM AF@300mT /IRM AF@100mT . These parameters are easily measured using modern vibrating sample or alternating gradient magnetometers. Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. The L-ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols. For example, the HIRM is equivalent to IRM AF@300mT . Likewise, 0.5*(SIRM + IRM -100mT ), where IRM -100mT represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back-field of -100 mT, is equivalent to IRM AF@100mT . The HIRM/[0.5*(SIRM + IRM -100mT )] ratio is therefore a suitable substitute for the L-ratio when measurements are made with a long-core magnetometer. The newly proposed L-ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism, the HIRM and S-ratio.

A new proxy for bottom-water ventilation in the eastern Mediterranean based on diagenetically controlled magnetic properties of sapropel-bearing sediments

Magnetic properties of eastern Mediterranean sediments recovered during Leg 160 of the Ocean Drilling Program (ODP) provide insight into non-steady-state diagenetic reactions associated with accumulation and degradation of organic matter in sapropels. According to their magnetic properties, sapropels can be classified as one of three types that correspond to increasingly anoxic conditions at the time of sapropel formation. A combination of magnetic and geochemical data suggests a causal relationship that enables determination of the relative role of bottom-water ventilation versus productivity in the resulting diagenetic stage reached for the three types of sapropels. It appears that increased productivity is a prerequisite for sapropel formation, but once organic matter is available in sufficient amounts, variable efficiencies in bottom-water ventilation are more important for modulating the diagenetic context in which different types of sapropels formed. Magnetic properties are more sensitive to variations in bottom-water ventilation than to productivity, and can be used to establish relative variations in bottom-water ventilation both at, and after, periods of sapropel formation. Magnetic results and the distribution and type of sapropels at ODP Site 966 (Eratosthenes Seamount) between 2.3 and 4.0 Ma suggest that bottom-water ventilation was modulated by the orbital eccentricity component, with ventilation being restricted during 400-kyr eccentricity maxima and enhanced during eccentricity minima. Enhanced ventilation during eccentricity minima, as indicated by magnetic data, is consistent with the occurrence of red intervals at Site 966 and at other eastern Mediterranean sites drilled during ODP Leg 160, and also with astronomically modulated variations in CaCO3 content found in Mediterranean land-sections. This suggests that variations in bottom-water ventilation modulated by 400-kyr eccentricity cycles operated at a basin-wide scale.

Dynamics of Green Sahara Periods and Their Role in Hominin Evolution

Astronomically forced insolation changes have driven monsoon dynamics and recurrent humid episodes in North Africa, resulting in green Sahara Periods (GSPs) with savannah expansion throughout most of the desert. Despite their potential for expanding the area of prime hominin habitats and favouring out-of-Africa dispersals, GSPs have not been incorporated into the narrative of hominin evolution due to poor knowledge of their timing, dynamics and landscape composition at evolutionary timescales. We present a compilation of continental and marine paleoenvironmental records from within and around North Africa, which enables identification of over 230 GSPs within the last 8 million years. By combining the main climatological determinants of woody cover in tropical Africa with paleoenvironmental and paleoclimatic data for representative (Holocene and Eemian) GSPs, we estimate precipitation regimes and habitat distributions during GSPs. Their chronology is consistent with the ages of Saharan archeological and fossil hominin sites. Each GSP took 2–3 kyr to develop, peaked over 4–8 kyr, biogeographically connected the African tropics to African and Eurasian mid latitudes, and ended within 2–3 kyr, which resulted in rapid habitat fragmentation. We argue that the well-dated succession of GSPs presented here may have played an important role in migration and evolution of hominins.

Triassic paleomagnetism from the Western Pyrenees revisited: implications for the Iberian–Eurasian Mesozoic plate boundary

Since the pioneering studies of Van der Voo [Tectonophysics 7 (1969) 5] and Van der Voo and Boessenkool [J. Geophys. Res. 78 (1973) 5118], paleomagnetism of Permo-Triassic redbeds and volcanics from the Western Pyrenees has furnished important contributions for delineating the Mesozoic boundary between the Iberian and Eurasian plates. In this paper, we present a new paleomagnetic study focussed on Triassic red beds (23 sites) of the Paleozoic Basque Massifs (PBM). The aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the Western Pyrenees. Two stable magnetic components have been isolated: (1) a dual polarity, pre-folding magnetisation carried by specular hematite; and (2) a secondary, normal polarity component also carried by hematite. Our data confirm both the origin and the rotation pattern of the primary remanence described in previous works. Nevertheless, field tests performed on the secondary component do not confirm the earlier interpretations by Schott and Peres [Tectonophysics 156 (1988) 75] as they indicate a synfolding nature of the remagnetisation instead of a post-folding origin. We consider that the secondary component is better explained if a Cretaceous age is considered. The presence of such remagnetisation in the western Pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern Iberia in connection with the extensional tectonic events that occurred during Cretaceous times. A comparison of the rotations recorded by the Triassic component and by the remagnetisation indicate that the Paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired, most likely in connection with the counterclockwise rotation of Iberia with respect to Eurasia. These results favour that the Mesozoic plate boundary between the Iberian and Eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary.

Rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism

Gas hydrate, a clathrate of methane and water widespread on continental margins, has been implicated as a trigger of climate change and submarine slides as a result of methane release when the base of its stability zone moves upward rapidly. Direct tests of these hypotheses are made difficult by the ephemeral record of gas hydrate in sediment. In places, a seismic reflector (double bottom simulating reflector, BSR) appears to mark the old base of the gas hydrate layer, but the occurrence of this feature is patchy and its interpretation is controversial. Microbial activity is stimulated in the presence of gas hydrate, and results in the production of magnetic iron sulfides; the base of the gas hydrate interval is marked by a sharp reduction in the magnetic hysteresis parameter D J H . At Hydrate Ridge on the Cascadia margin, sampled during Ocean Drilling Program Leg 204, this signature occurs between 20 and 65 m below the present-day base of the gas hydrate zone, at a depth consistent with predictions for the base of gas hydrate stability given water depths and bottom-water temperatures appropriate for the last glacial maximum. Seismic evidence for a double BSR over part of Hydrate Ridge corroborates the rock magnetic interpretation.

Thrust Ramp Geometry and Spurious Rotations of Paleomagnetic Vectors

Interaction of deformation axes during pure translation of a hanging-wall over a footwall composed by frontal and oblique ramps is carefully evaluated together with the evolution of associated paleomagnetic vectors. Four different cases are distinguished on the basis of the deflection on the paleomagnetic vectors when the bedding correction is applied during the restoration process. Two cases (frontal and oblique ramp without mutual interaction) do not produce any deflection. But two cases in the transition zone between both ramps will undergo non-coaxial axis of tilting during progressive deformation. One of them will produce spurious rotation if the bedding correction is applied. These errors will affect the oroclinal bending diagram as well as the fold test producing an apparent oroclinality and an apparent syn-folding magnetization respectively. A well-known geometry and kinematics of the thrust system is needed to properly restore the beds (and vectors) and to avoid the spurious rotations and its collateral effects in paleomagnetic investigations. A paleomagnetic study in the Pyrenean External Sierras is shown as an example. Primary Eocene vectors underwent a clockwise rotation (40° about) during the emplacement of the South Pyrenean sole thrust, however the Rasal-Gabardiella system of oblique ramps display spurious rotations ranging from −8° up to 13° if the inappropriate bedding correction is performed.

Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain.

Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60–70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.

Stable Eocene Magnetization Carried by Magnetite and Iron Sulphides in Marine Marls (Pamplona-Arguis Formation, Southern Pyrenees, Northern Spain)

In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of the southern Pyrenees (N Spain). The unblocking temperatures suggest that the characteristic remanent magnetization (ChRM) is carried by magnetite and iron sulphides. The ChRM has both normal and reversed polarities regardless of whether it resides in magnetite or iron sulphides, and represents a primary Eocene magnetization acquired before folding. Rock magnetic results confirm the presence of magnetite and smaller amounts of magnetic iron sulphides, most likely pyrrhotite, in all the studied samples. Framboidal pyrite is ubiquitous in the marls and suggests that iron sulphides formed during early diagenesis under sulphate-reducing conditions. ChRM directions carried by magnetic iron sulphides are consistent with those recorded by magnetite. These observations suggest that magnetic iron sulphides carry a chemical remanent magnetization that coexists with a remanence residing in detrital magnetite. We suggest that the south Pyrenean Eocene marls are suitable for magnetostratigraphic and tectonic purposes but not for studies of polarity transitions, secular variations and geomagnetic excursions, because it is difficult to test for short time differences in remanence lock-in time for the two minerals. The presence of iron sulphide minerals contributing to the primary magnetization in Eocene marine marls reinforces the idea that these minerals can persist over long periods of time in the geological record.

Quantifying the post-tectonic topographic evolution of closed basins: The Ebro basin (northeast Iberia)

Closed (endorheic) sedimentary basins are key recorders of the climatic, erosional, and tectonic history of their surrounding topography, playing an active role in its evolution by changing the local geomorphological base level. When these basins become exorheic, the accelerated incision along the new fluvial network can excavate excellent stratigraphic outcrops, but this often removes the uppermost infill, and essential information about the late basin history is lost. Here we propose estimating the opening age and past elevation of captured closed basins by combining the flexural isostatic compensation of the eroded volume with available constraints on sediment age. We use this method to constrain the post-tectonic evolution of the Cenozoic Ebro basin in northeast Iberia. The similar results obtained for 4 dated stratigraphic columns show the robustness of the model and date the basin opening as 12.0–7.5 Ma, with a maximum paleoelevation of the basin of 535–750 m. The isostatic rebound associated with basin erosion, as much as 630 m in the center of the basin, may explain the absence of a canyon excavated by the Ebro River during the Mediterranean sea-level fall associated with the Messinian salinity crisis.

Source-to-sink magnetic properties of NE Saharan dust in Eastern Mediterranean marine sediments: review and paleoenvironmental implications

We present a review of the magnetic properties of NE Saharan dust that was conducted, following a source-to-sink approach, to unravel the paleoclimatic significance of environmental magnetic records from Eastern Mediterranean marine sediments. Our synthesis indicates that pedogenic hematite, which formed during past wetter Green Sahara periods (GSPs), is the most common magnetic mineral in Eastern Mediterranean marine sediments as a result of its eolian transportation, along with smaller amounts of lithogenic hematite, from the NE Sahara. Coupled with the limited impact of reductive sedimentary diagenesis on hematite abundances in Eastern Mediterranean Sea sediments, this indicates that hematite concentrations provide reliable quantitative estimates of NE Saharan dust supply. Our results indicate that variations in NE Saharan dust supply record an on-off mechanism in which a key control on eolian input is provided by the monsoon-driven spread and retreat of savannah vegetation through the Sahara. Magnetite/maghemite is also a common magnetic mineral in NE Saharan dust, and also formed mainly pedogenically during GSPs but in much smaller amounts compared to hematite. Limited production of magnetite/maghemite in the source area during GSPs, along with the later imprint of diagenetic processes, indicates that magnetite/maghemite abundances cannot be used to estimate dust inputs from the NE Sahara. Goethite cannot be used either to estimate variations in NE Saharan dust supply, because its occurrence in Eastern Mediterranean marine sediments also appears to be linked to fluvial inputs. Our results reinforce the view that a source-to-sink approach should be routinely conducted in environmental magnetic studies to understand the complex combination of processes involved in the production, transportation, sedimentation and diagenetic evolution of magnetic minerals in sedimentary environments.