Steven P. Lund

Global geomagnetic field models for the past 3000 years: transient or permanent flux lobes?

PSVMOD1.0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts, lava flows, and lake sediments at 24 sites evaluated at 100 year intervals from 1000 BC to AD 1800. We estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time–intervals of overlap, and use the 100–year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models. Global predictions of radial magnetic field at the coremantle boundary (CMB), as well as inclination and declination anomalies at the Earths surface, provide an unprecedented view of geomagnetic secular variations over the past 3000 years, and demonstrate a consistent evolution of the field with time. Resolution of the models is poorest in the Southern Hemisphere, where only six of the 24 sites are located, several with incomplete temporal coverage. Low–flux regions seen in the historical field near the North Pole are poorly resolved, but the Northern Hemisphere flux lobes are clearly visible in the models. These lobes are not fixed in position and intensity, but they only rarely venture into the Pacific hemisphere. The Pacific region is seen to have experienced significant secular variation: a strong negative inclination anomaly in the region, like that seen in 0–5 Ma models, persists from 1000 BC until AD 1000 and then gradually evolves into the smaller positive anomaly seen today. On average between 1000 BC and AD 1800, the non–axial–dipole contribution to the radial magnetic field at the core–mantle boundary is largest in the north–central Pacific, and beneath Central Asia, with clear non–zonal contributions. At the Earths surface, average inclination anomalies are large and negative in the central Pacific, and most positive slightly to the east of Central Africa. Inclination anomalies decrease with increasing latitude. Average declinations are smallest in equatorial regions, again with strong longitudinal variations, largest negative departures are centred over Australia and Eastern Asia. Secular variation at the Earths surface is quantified by standard deviation of inclination and declination about their average values, and at the CMB by standard deviation in radial magnetic field. All three show significant geographical variations, but appear incompatible with the idea that secular variation in the Pacific hemisphere is permanently attenuated by greatly enhanced conductivity in D00 beneath the region.

Non-annual laminations and expansion of anoxic basin-floor conditions in Santa Monica Basin, California Borderland, over the past four centuries

Sediments from box cores in the deep portion of Santa Monica Basin have been examined by X-radiography, and the upper 10–20 cm are characterized by laminations 1–5 mm in thickness. Age dating of 9 cores by 210Pb analysis has shown that accumulation rates on the central basin floor have been remarkably constant during the past century, averaging 16.0 ± 0.4 mg/cm2/yr. Densitometer analysis of the X-radiography films from 7 cores and spectral analysis of the resulting time series indicates that most lamination couplets have a periodicity in the 3–7 year range, a range matching that of the El Nino-Southern Oscillation periodicity. The median periodicity appears to have decreased slightly during the past 3 centuries, from 7.0 years during 200–300 yrs B.P. to 5.2 yrs during 0–100 yrs B.P. This trend could be due to a decreasing sedimentation rate, an increasing forcing frequency, or an increasingly faithful response of the Basin in more recent years. Multiple processes have contributed to creation of the laminations in Santa Monica Basin. The principal factors appear to be (1) variations in porosity due to the relative abundance of organic filaments probably formed by Beggiatoa mats, and (2) compositional variations that reflect variations in carbonate/lithogenous ratios and in grain size. Both of these are expected to respond to ENSO events. The presence of near anoxic bottom waters has prevented bioturbation, permitting laminations to be preserved. Basin bottom waters became nearly anoxic about 350 years ago, at the onset of warming as the Little Ice Age waned. Expansion of the near anoxic area during the past century may have been augmented by 20th Century anthropogenic effects. The style of these non-annual laminations in Santa Monica Basin matches that of the annual laminations in the adjacent Santa Barbara Basin, and demonstrates the need for caution in interpreting all laminations as representing annual varves.

Records of the Cobb Mountain Subchron from the Bermuda Rise (ODP LEG 172)

Abstract Duplicate records of the Cobb Mountain Subchron were obtained from two holes (Holes 1063B at 33°41.204′N, 57°36′W and 1063C at 33°41.181′N, 57°36′W) drilled in a sediment drift at Ocean Drilling Program (ODP) Site 1063 on the northeast Bermuda Rise. We sub-sampled the cores from both holes using U-channels (up to 1.5 m long samples with 2×2 cm cross-section) focusing on continuous sampling through the late Matuyama interval containing the Cobb Mountain Subchron. Results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low-coercivity drilling overprint. Additional discrete samples (∼7 cm 3 oriented cubes) used in progressive thermal demagnetization studies display similar behavior. By taking duplicate sets of U-channels, we were able to identify and remove several measurement artifacts, such as edge effects that affect measurements made near the ends of the U-channel samples. The upper normal-to-reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation. After removing measurement artifacts, the records from the two holes were stacked to obtain a composite record. Virtual geomagnetic poles (VGPs) computed from the composite record follow a path through the central Pacific during the lower polarity transition, very similar to VGP paths from Cobb Mountain Subchron records from other sites around the world. Furthermore, the VGPs cluster near southern Africa early in the reversal process and then group in a nearly antipodal patch in the north Pacific late in the reversal. Taken together, these records indicate that the transitional field remained nearly dipolar through the polarity reversal, or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20–40° to the spin axis. If a tilted dipole is an inherent part of the initiation and ending of a reversal, it may help explain why many sediment recorders, which are thought to smooth the field, record antipodal VGP paths through many reversals.

Nongeocentric axial dipole field behavior during the Mono Lake excursion

A new record of the Mono Lake excursion (MLE) is reported from the Summer Lake Basin of Oregon, USA. Sediment magnetic properties indicate magnetite as the magnetization carrier and imply suitability of the sediments as accurate recorders of the magnetic field including relative paleointensity (RPI) variations. The magnitudes and phases of the declination, inclination, and RPI components of the new record correlate well with other coeval but lower resolution records from western North America including records from the Wilson Creek Formation exposed around Mono Lake. The virtual geomagnetic pole (VGP) path of the new record is similar to that from another high-resolution record of the MLE from Ocean Drilling Program (ODP) Site 919 in the Irminger Basin between Iceland and Greenland but different from the VGP path for the Laschamp excursion (LE), including that found lower in the ODP-919 core. Thus, the prominent excursion recorded at Mono Lake, California, is not the LE but rather one that is several thousands of years younger. The MLE VGP path contains clusters, the locations of which coincide with nonaxial dipole features found in the Holocene geomagnetic field. The clusters are occupied in the same time progression by VGPs from Summer Lake and the Irminger Basin, but the phase of occupation is offset, a behavior that suggests time-transgressive decay and return of the principal field components at the beginning and end of the MLE, respectively, leaving the nonaxial dipole features associated with the clusters dominant during the excursion.