Too‐low magnetic inclinations in central Asia: an indication of a long‐term Tertiary non‐dipole field?

Observed Tertiary palaeolatitudes in central Asia are more southerly (by about 1600 km on average) than those predicted from the Eurasian reference palaeopoles. Subsequent northward displacements of the central Asian terranes are unlikely to have been this large. In this study we analyse to what extent non-dipole fields, especially octupole fields, can explain this phenomenon. A global (zonal) octupole field manifests itself in two ways. (1) Because the reference APWP is based mostly on results from the UK and North America, its palaeopoles will be far-sided as seen from the North Atlantic, but near-sided as seen from eastern Asia, giving predicted palaeolatitudes that are too high. (2) An octupole field contribution produces observed palaeolatitudes, as calculated with the dipole formula for central Asia, that are too low. Both effects therefore increase the palaeolatitude anomalies in Asia. We find that an octupole/dipole field ratio (G3) of 0.06 or greater will reduce the palaeolatitude discrepancies significantly and is of the same magnitude as the G3 estimate of a recent analysis of Early Tertiary European and North American data.     

Temperature variability over the past millennium inferred from Northwestern Alaska tree rings

We describe a new tree-ring width data set of 14 white spruce chronologies for the Seward Peninsula (SP), Alaska, based on living and subfossil wood dating from 1358 to 2001 AD. A composite chronology derived from these data correlates positively and significantly with summer temperatures at Nome from 1910 to 1970, after which there is some loss of positive temperature response. There is inferred cooling during periods within the Little Ice Age (LIA) from the early to middle 1600s and late 1700s to middle 1800s; and warming from the middle 1600s to early 1700s. We also present a larger composite data set covering 978–2001 AD, utilizing the SP ring-width data in combination with archaeological wood measurements and other recent collections from northwestern Alaska. The Regional Curve Standardization (RCS) method was employed to maximize potential low-frequency information in this data set. The RCS chronology shows intervals of persistent above-average growth around the time of the Medieval Warm Period (MWP) early in the millennium, which are comparable to growth levels in recent centuries. There is a more sustained cold interval during the LIA inferred from the RCS record as compared to the SP ring-width series. The chronologies correlate significantly with Bering and Chukchi Sea sea surface temperatures and with the Pacific Decadal Oscillation index. These atmosphere–ocean linkages probably account for the differences between these records and large-scale reconstructions of Arctic and Northern Hemisphere temperatures based largely on continental interior proxy data.     

4D seismic for oil-rim monitoring

In the central North Sea ‘Gannet‐A’ field, a 50 ft oil rim is overlain by a gas cap of variable thickness. Oil is produced from horizontal wells which initially produced dry oil, but as the field became more mature, a significant water cut was seen in several wells. A dedicated 4D seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves. By forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations (using 2D and 3D wedge models), and comparing the results with real seismic data, we are able to decipher the contact movements across the field. It is shown that, in one part of the field, the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap. This oil‐rim displacement produces a very different 4D seismic response from the case of a static gas–oil contact and rising oil–water contact (normal production). As a result of these observations, we are able to optimize field production by both re‐perforation of existing wells and by drilling sidetracks into the displaced rim: a brown‐field development opportunity that might otherwise be missed.     

Late Quaternary paleosols, stratigraphy and landscape evolution in the Northern Pampa, Argentina

The field properties, micromorphology, grain-size, geochemistry, and optically stimulated luminescence (OSL) ages of two late Quaternary sections have been used to reconstruct the sequence of pedosedimentary processes and to provide insights into landscape evolution in part of the Northern Pampa of Argentina. Paleosols developed in paludal sediments adjacent to the Paraná river at Baradero and in loess at Lozada can both be correlated and linked to other sites, thus enabling for the first time the tentative recognition and tracing of a diachronous soil stratigraphic unit that probably spans the equivalent of at least part of marine oxygen isotope stage (OIS) 5. The paleosol at Lozada was truncated and buried beneath fluvial sediments during the time span of OIS 4 and 3. Eolian gradually replaced paludal inputs at Baradero over this period, and there were also two clearly defined breaks in sedimentation and development of paleosols. The period corresponding to OIS 2 was marked by significant loess accumulation at both sites with accretion continuing into the mid-Holocene only at Lozada. The more developed nature of the surface soil at Baradero probably reflects a combination of a moister climate and a longer soil-forming interval.     

Syn-folding remagnetization of Cambro-Ordovician carbonates from the Pennsylvania Salient post-dates oroclinal rotation

Paleomagnetic directions of 35 sites of Cambro-Ordovician carbonates from 10 anticlines were analyzed to test models of curvature along the Pennsylvania Salient of the Appalachians and to constrain the relative timing of magnetization acquisition. The sites yield directions of magnetization that are almost all reversed with near-horizontal inclinations upon appropriate structural correction. The common, Late Paleozoic (Kiaman-aged) direction and incremental fold tests show that these directions represent remagnetizations carried by authigenic magnetite, acquired just before or during the earlier phases of folding. No convincing indications were found of primary magnetizations. Mean declinations from the northeastern and southwestern limbs of the salient differ by a few degrees, indicating negligible, if any, rotation between the limbs. The results are similar to prior studies of overlying Siluro-Devonian carbonates, showing coherent behavior of the entire Paleozoic cratonic cover. We conclude that the statistically negligible difference in declination indicates that (previously demonstrated) oroclinal bending occurred before carbonates of the Paleozoic stratigraphic cover were remagnetized during the Permian and before regional folding was completed.