Age determination and paleomagnetic investigations on a sediment profile from laguna Potrok Aike

Paleomagnetic inclination, declination and relative paleointensity were reconstructed from the sediments of Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). Here we present the u-channel-based full vector paleomagnetic field reconstruction since 51.2 ka cal BP. The relative paleointensity proxy (RPI) was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the ChRM interval (NRM/ARM10e40 mT). A grain size influence on the RPI was removed using a correction based on the linear relationship between the RPI and the median destructive field of the natural remanent magnetisation (MDFNRM). The new record is compared with other lacustrine and marine records and stacks from the mid- to high-latitudes of the Southern Hemisphere, revealing consistent millennial-scale variability, the identification of the Laschamp and possibly the Mono Lake geomagnetic excursions, and a direction swing possibly associated to the Hilina Pali excursion at 20 ka cal BP. Nonetheless, a global-scale comparison with other high-resolution records located on the opposite side of the Earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern South America at 46 ka cal BP.

Age-depth relation, radiocarbon ages and tephra geochemistry of core 5022-2CP from Laguna Potrok Aike, southern Patagonia

The 106 m long composite profile from site 2 of ICDP expedition 5022 (PASADO) at Laguna Potrok Aike documents a distinct change in sedimentation patterns from pelagic sediments at the top to dominating mass movement deposits at its base. The main lithological units correspond to the Holocene, to the Lateglacial and to the last glacial period and can be interpreted as the result of distinct environmental variations. Overflow conditions might have been achieved during the last glacial period, while signs of desiccation are absent in the studied sediment record. Altogether, 58 radiocarbon dates were used to establish a consistent age-depth model by applying the mixed-effect regression procedure which results in a basal age of 51.2 cal. ka BP. Radiocarbon dates show a considerable increase in scatter with depth which is related to the high amount of reworking. Validation of the obtained chronology was achieved with geomagnetic relative paleointensity data and tephra correlation.

Magnetic susceptibility of deep-sea cores from the Scotia Sea

We studied two deep-sea cores from the Scotia Sea to reconstruct past atmospheric circulation in the southern hemisphere and to resolve a long-standing debate on the interpretation of magnetic susceptibility (MS) records in Southern Ocean (SO) sediment. High-sedimentation sites MD07-3134 (0.2 - 1.2 m/kyr) and MD07-3133 (0.3 - 2 m/kyr) cover the last 92.5 kyr and 36 kyr, respectively. Both exhibit a one-to-one coupling of the MS and Ca2+ signal to the non-sea salt (nss) Ca2+ signal of the EDML ice core, clearly identifying atmospheric circulation as means of distribution. Comparison of additional proxies also excludes major influence by volcanic sources, sea-ice, icebergs, or oceanic current transport. The close resemblance of the dust proxies over the last glacial cycle, in turn, allows for the establishment of an age model of unprecedented resolution and precision for SO deep-sea sediment because atmospheric transport involves no major leads or lags. This is of particular importance because MS is routinely measured on deep-sea cores in the SO but the sediments usually lack biogenic carbonate and therefore had only limited stratigraphic control so far. Southern South America (SSA) is the likely source of eolian material because Site MD07-3133, located closer to the continent, has slightly higher MS values than Site MD07-3134, and also the MS record of Patagonian Site SALSA shows comparable variability. Patagonia was the dust source for both the Scotia Sea and East Antarctica. Dust fluxes were several times higher during glacial times, when atmospheric circulation was either stronger or shifted in latitude, sea level was lowered, shelf surfaces were exposed, and environmental conditions in SSA were dominated by glaciers and extended outwash plains. Hence, MS records of SO deep-sea sediment are reliable tracers of atmospheric circulation, allowing for chronologically-constrained reconstructions of the circum Antarctic paleoclimate history.

High-resolution paleomagnetic investigation in the Iceland Basin

Based on discrete samples, we report new high-resolution records of the ~185 kyr Iceland Basin (IB) geomagnetic excursion from Ocean Drilling Project (ODP) Site 1063 on the Bermuda Rise (sedimentation rate 32 cm/kyr) and from ODP Site 983 in the far North Atlantic (sedimentation rate 18 cm/kyr). Two records from Holes 1063A and 1063B are very consistent, and provide the highest resolution of the detailed field behaviour during the IB excursion obtained so far. Inclination records from Holes 983B and 983C in the far North Atlantic are also very consistent, whereas declination anomalies deviate more notably. The pseudo-Thellier (PT) technique was applied along with more conventional palaeointensity proxies (NRM/ARM and NRM/kappa) to recover relative palaeointensity (RPI) estimates from Hole 1063A and Hole 983B. As expected, these proxies indicate that the field intensity generally dropped at both sites during the IB excursion, but also that the history of RPI from the two sites is different. VGPs from Site 1063 indicate that the field at this location experienced some stop-and-go behaviour between patches of intense vertical flux over North America and the tip of South America, areas which coincide fairly well with patches of preferred transitional VGP clustering from reversals and zones of high seismic velocity in the lower mantle. Changes in RPI at this location were generally gradual, possibly due to the proximity of these flux patches, and the first period of VGP-clustering over North America was accompanied by a conspicuous increase in RPI. VGPs from Site 983 track along a different path, and the associated RPI changes are very abrupt and completely synchronous with the onset and termination of the excursion. The differing VGP paths from Sites 1063 and 983 indicate that the global field structure during the IB excursion was not dominated by a single dipole.

Magnetic observation of ODP Site 152-919 in the Irminger Basin, North Atlantic

Component natural remanent magnetizations derived from u-channel and 1-qcm discrete samples from ODP Site 919 (Irminger Basin) indicate the existence of four intervals of negative inclinations in the upper Brunhes Chronozone. According to the age model based on planktic oxygen isotope data, these "excursional" intervals occur in sediments deposited during the following time intervals: 32-34 ka, 39-41 ka, 180-188 ka and 205-225 ka. These time intervals correspond to polarity excursions detected elsewhere, known as Mono Lake, Laschamp, Iceland Basin and Pringle Falls. The isotope-based age model is supported by the normalized remanence (paleointensity) record that can be correlated to other calibrated paleointensity records for the 0-500 ka interval, such as that from ODP Site 983. For the intervals associated with the Mono Lake and Laschamp excursions, virtual geomagnetic poles (VGPs) reach equatorial latitudes and mid-southerly latitudes, respectively. For intervals associated with the Iceland Basin and Pringle Falls excursions, repeated excursions of VGPs to high southerly latitudes indicate rapid directional swings rather than a single short-lived polarity reversal. The directional instability associated with polarity excursions is not often recorded, probably due to smoothing of the sedimentary record by the process of detrital remanence (DRM) acquisition.

(Table 1) Viscosity coefficients at various temperatures of samples from ODP Hole 176-735B

Ocean Drilling Program (ODP) Hole 735B was drilled to a depth of 1.5 km in a tectonic window of gabbroic lower oceanic crust created at the Southwest Indian Ridge. The gabbros have a very stable natural remanent magnetization (NRM) of reversed polarity with most unblocking temperatures slightly below the Curie temperature of magnetite. The NRM includes a drilling-induced overprint but its intensity decays strongly towards the interior of the drill core. The demagnetization data yield no or only a very small secondary magnetization component acquired during the present Brunhes chron or an earlier normal chron, suggesting cooling through most of the blocking temperature range during chron C5r and a strong resistance against the acquisition of thermoviscous magnetization. A novel furnace has been designed to measure magnetizations and their time dependences at high temperatures (up to 580 deg C) inside a commercial SQUID magnetometer. Magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550 deg C to determine the time and temperature stability of remanent magnetization. Viscosities are generally small and increase little with temperature below the main blocking temperature, where the increase becomes almost an order of magnitude. Extrapolations to geological times infer viscous acquisitions that would be 5-25% of a thermoremanence in 100 kyr and at temperatures of 200-500 deg C. At ocean bottom temperature the predicted magnetization of one sample acquired in the present Brunhes chron should be 10% of the NRM. However, this is not recognized during NRM demagnetization and partial thermoremanent magnetization (pTRM) acquisitions at 250 deg C are also much smaller than predicted. It thus appears that the NRMs are generally magnetically harder than magnetizations acquired after heating to 570 deg C in the laboratory. Susceptibility changes during heating are small (<5%) indicating a seemingly stable magneto-mineralogy, but conspicuous minima occur after heating to 520 deg C. Also, quasi paleointensity experiments reveal characteristic patterns in the NRM/pTRM ratios and also large increases in pTRM capacity after heating to 570 deg C. Moreover, anhysteretic remanent magnetization acquisition in the low field range (<=10 mT) is strongly enhanced after heating by factors up to three. The alteration of the magneto-mineralogy is interpreted to result from the annealing of defects in magnetite that originate from tectonically induced strain. The oceanic gabbros of Hole 735B are thus ideal source layer material for marine magnetic anomalies, and secondary thermoviscous acquisition, as a possible cause for anomalous skewness, is essentially absent.

Age model of ODP Site 162-983

At ODP Site 983, relative geomagnetic paleointensity and planktic and benthic delta18O records have been acquired for the last 350 kyr. The mean sedimentation rate in this interval is 11.3 cm/kyr. Magnetic properties and hysteresis ratios indicate that pseudo-single domain magnetite is the remanence carrier. Volume susceptibility (kappa), anhysteretic (ARM) and isothermal (IRM) remanence values vary by a factor of 3-4, well within the criteria usually cited for paleointensity studies. Natural remanent magnetization (NRM) is normalized by ARM and IRM to acquire the paleointensity proxy. Arithmetic means of NRM/ARM and NRM/IRM, calculated for five demagnetization steps in the 25-45 mT range, constitute the relative paleointensity estimates. Some paleointensity lows (particularly those at ~40, ~120 and ~188 ka) are associated with directional excursions of the field, especially the event at ~188 ka (referred to here as the Iceland Basin Event) that constitutes a short-lived polarity reversal. For the last 200 kyr, the records can be correlated with other high-resolution paleointensity records such as those from the Labrador Sea, Mediterranean/Somali Basin and Sulu Sea, implying that the millennial scale features are globally synchronous. A labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages.

Magnetic properties of two cores from the Iberian Margin

Magnetic properties of late Quaternary sediments on the SW Iberian Margin are dominated by bacterial magnetite, observed by transmission electron microscopy (TEM), with contributions from detrital titanomagnetite and hematite. Reactive hematite from eolian dust, together with low organic matter concentrations and the lack of sulfate reduction, lead to dissimilatory iron reduction and availability of Fe(II) for abundant magnetotactic bacteria. Magnetite grain-size proxies (kARM/k and ARM/IRM) and S-ratios (sensitive to hematite) vary on stadial/interstadial timescales, contain orbital power, and mimic planktic d18O. The detrital/biogenic magnetite ratio and hematite concentration are greater during stadials and glacial isotopic stages, reflecting increased detrital (magnetite) input during times of lowered sea level, coinciding with atmospheric conditions favoring hematitic dust supply. Magnetic susceptibility, on the other hand, has a very different response being sensitive to coarse detrital multidomain (MD) magnetite associated with ice-rafted debris (IRD). High susceptibility and/or magnetic grain size coarsening, mark Heinrich stadials (HS), particularly HS2, HS3, HS4, HS5, HS6 and HS7, as well as older Heinrich-like detrital layers, indicating the sensitivity of this region to fluctuations in the position of the polar front. Relative paleointensity (RPI) records have well-constrained age models based on planktic d18O correlation to ice-core chronologies, however, they differ from reference records (e.g. PISO) particularly in the vicinity of glacial maxima, mainly due to inefficient normalization of RPI records in intervals of enhanced detrital/eolian hematite input.