Carlo Laj

Correlation of marine 14C ages from the nordic seas with the GISP2 isotope record : Implications for 14C calibration beyond 25 ka BP

We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous (super 14) C ages up to 54 (super 14) C ka BP. Based on various lines of evidence, the local (super 14) C reservoir effect was restricted to 400-1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the delta (super 18) O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1-6. This correlation indicates large fluctuations in the calibration of (super 14) C ages between 20 and 54 (super 14) C ka BP. Generally the results confirm the (super 14) C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our (super 14) C shifts after 45 (super 14) C ka BP clearly exceed the geomagnetic prediction near 40-43 and 32-34 calendar (cal) ka BP. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic (super 14) C and (super 10) Be production, giving rise to excess (super 14) C in the atmosphere of up to 1200 per mil.

The Tertiary geodynamical evolution of the Aegean arc: a paleomagnetic reconstruction

Abstract The paleomagnetic results obtained in the last 5–6 years from Tertiary formations in the Aegean domain indicate that the Lower Miocene arc was almost rectilinear with an E-W trend and that its curvature has been acquired tectonically in two major phases. During the Middle Miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc, clockwise in the west (Epirus) anticlockwise in the east (southeastern Anatolia). A second phase of rotation occurring in the last 5 Ma about a pole situated in the southern Adriatic Sea has affected only the northwestern part. The data also indicate that the Antalya basin has not been involved in the geodynamical evolution of the arc so that the Bey Daglari represent its easternmost unit. A more complex rotational pattern is observed in the internal zones: the large rotations measured in Evia (48°) and Skyros (26°) probably occurred in the last 5 Ma and result from rotation of fault bounded blocks in a shear zone connecting the North Aegean trough to the external arc. In the Izmir region, in the Karaburun peninsula of western Anatolia and in the island of Lesbos significant counterclockwise, clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime. Furthermore, the values of the inclination obtained from both sedimentary and volcanic Oligo-Miocene formations are systematically shallower than expected on the basis of a geocentered dipole field. This suggests that the entire studied area has undergone a northward drift of over 1000 km since the Middle Miocene.

RAPID CLIMATIC VARIATIONS DURING MARINE ISOTOPIC STAGE 3 : MAGNETIC ANALYSIS OF SEDIMENTS FROM NORDIC SEAS AND NORTH ATLANTIC

Abstract The bulk magnetic parameters of seven deep-sea cores distributed from the Nordic Seas (67°N) to the North Atlantic as far south as the Bermuda Rise (33°N) exhibit short-term variations which correlate with rapid climatic changes during marine isotopic stage 3 (MIS3). The magnetic mineralogy is uniformly dominated by well sorted low Ti-content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals. Because the magnetic minerals predominantly originate from one common source area (the Nordic basaltic province), these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition. These results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep-sea circulation. The latter was active/reduced during the interstadials/stadials and Heinrich events transporting the magnetic particles from the Norwegian Sea into the North Atlantic ocean along a path similar to the present path of the NADW. It is tentatively suggested that the Faeroe-Shetland Channel and the Denmark Strait were the only two active paths for the overflow water during MIS3. The presence of magnetic oscillations in the Bermuda Rise core in phase with those from the North Atlantic indicates that the activity of the southern Newfoundland Basin gyre was linked to that of the NADW during MIS3.

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

Abstract A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.

Presence of the Solar de Vries Cycle (∼205 years) during the Last Ice Age

Certain characteristic periodicities in the Δ14C record from tree rings, such as the well-known 11-yr Schwabe cycle, are known to be of solar origin. The origin of longer-period cycles, such as the 205-yr de Vries cycle, in the Δ14C record was less certain, and it was possible to attribute it either to solar or climatic variability. Here, we demonstrate that the de Vries cycle is present in 10Be data from the GRIP ice core during the last ice age (25 to 50 kyr BP). Analysis of the amplitude of variation of this cycle shows it to be modulated by the geomagnetic field, indicating that the de Vries cycle is indeed of solar, rather than climatic, origin.

Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10Be and 14C records

The variations in atmospheric radiocarbon (C-14) concentration during the last 50 000 years can be attributed to changes in the C-14 production rate (due to changes in solar activity, the geomagnetic field and/or interstellar galactic cosmic ray flux) and to changes in the global carbon cycle. The relative contributions of these processes is the subject of current debate. Although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age, the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed C-14 variations for this period. Using Be-10 measurements from Greenland Summit ice cores, we show that, in addition to solar and geomagnetically induced production rate changes, significant changes in the carbon cycle have to be considered to explain the measured C-14 concentrations. There is evidence that these changes are connected to: (1) global deglaciation and (2) climate changes in the North Atlantic region on centennial to millennial time scales related to changes in the ocean circulation. Differences between Be-10 and geomagnetic field records, however, suggest that uncertainties of about 20% still exist in determinations of past changes in the C-14 production rate

First paleomagnetic results from Mio-Pliocene series of the hellenic sedimentary ARC

Abstract Paleomagnetic measurements have been conducted on the Mio-Pliocene series along the external sedimentary Hellenic arc, in order to detect the presence or absence of rotational deformations in different sections of the arc. In the central-eastern part of the arc (Crete and Rhodes) paleomagnetic directions are very closely aligned along the North-South direction, showing that this portion of the arc has not undergone significant rotation since at least 7 m.y. (for Crete) and 4 m.y. (for Rhodes). In the western and northwestern parts, results from South Peloponnesos, Zakinthos, Kephallinia and Corfu indicate that this section of the arc has undergone a clockwise rotation. Detailed analysis of the data from the three Ionian islands shows that their rotation began about 5 m.y. B.P. and has proceeded since then at roughly 5°/m.y., giving a total amplitude of about 26°. It is suggested that the beginning of this rotation is related to the lower Pliocene compressive phase, which most probably initiated the tectonic activity of the present day Ionian margin.

Geomagnetic intensity and 14C abundance in the atmosphere and ocean during the past 50 kyr

We have used a new sedimentary record of geomagnetic field intensity, combined with published volcanic data, to examine the extent to which geomagnetic changes have affected 14C production in the past. The production record was then converted into an atmospheric Δ14C signal using a 4-box model of past oceanic circulation. The results indicate that changes in geomagnetic field intensity account for at least 80% of the Δ14C shift documented by published U-Th data, which are of the order of 500 per mil with respect to a constant production hypothesis. Model simulations show that changes in past oceanic circulation account for only ±100 per mil of Δ14C variation, which is within the uncertainty of the geomagnetic correction. In terms of dating, these results show that the radiocarbon ages have to be shifted by 2 to 3.5 kyr towards older ages during the 20–40 ka interval.

Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core

Abstract A distinct peak has been discovered in the 36 Cl data from the GRIP ice core between the Dansgaard Oeschger (D–O) events 6 and 7 at approximately 32 kyr BP. This peak can be attributed to a minimum of the geomagnetic dipole field associated with the Mono Lake event. Since the 36 Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14 C dating of the last ice age. Furthermore, it provides an additional time marker similar to a peak found earlier corresponding to the Laschamp event at approximately 39 kyr BP.

Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale

[1] A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama-Brunhes (M-B) reversal is recorded in five transitionally magnetized lava flows from the north (TN) section. The isochrons obtained from three of the lower four M-B lavas are defined by 14 incremental heating experiments that, together with a previous age determination, yielded a weighted mean of 798.4 ± 6.2 ka (all uncertainties ±2s). In addition, a 780.3 ± 10.3 ka isochron was determined for the overlying transitionally magnetized flow, indicating that it was erupted during a distinctly younger portion of the transition. Near the base of the south (TS) section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole (VGP) positions in the southwest Indian Ocean between latitudes 56� S and 65� S, suggesting instability of the geomagnetic field beyond that of typical secular variation. 40 Ar/ 39 Ar isochrons from three of these flows, defined by 11 separate incremental heating experiments, gave a weighted mean of 822.2 ± 8.7 ka. This anomalous field behavior recorded 24 ± 11 kyr prior to the M-B reversal may coincide with an event featured in several marine sediment records. Directly above two normal polarity flows ( 40 Ar/ 39 Ar isochrons of 751.9 ± 18.1 ka and 675.0 ± 15.7 ka) are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude VGPs spanning 23� –60� N. These flows are then capped by a single flow possessing normal polarity. Based on 12 incremental heating experiments, 40 Ar/ 39 Ar isochrons of five of these nine lavas, along with the uppermost flow, gave a weighted mean age of 580.2 ± 7.8 ka for this period of transitional to normal field behavior. From these same transitional lavas, Quidelleur et al. [1999] reported three unspiked K-Ar ages with a weighted mean of 602 ± 24 ka and proposed a new event called the ‘‘La Palma’’ excursion. However, the 40 Ar/ 39 Ar age presented here is three times more precise than the K-Ar age and is indistinguishable at the 95% confidence level from the 40 Ar/ 39 Ar age of a lava from the Snake River Plain, Idaho, that originally defined the Big Lost event. Transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the Big Lost event recorded at La Palma was indeed global in extent. Rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process. INDEX TERMS: 1560 Geomagnetism and Paleomagnetism: Time variations—secular and long term; 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1035 Geochemistry: Geochronology; 1513 Geomagnetism and Paleomagnetism: Geomagnetic excursions; KEYWORDS: argonargon, dating, lavas, excursion, paleomagnetism, reversal