We have conducted a paleomagnetic study of Holocene sediments from Lake Victoria in order to develop a high-resolution record of paleomagnetic secular variation (PSV). This study has recovered PSV records from two cores (V95-1P and V95-7P) in northern Lake Victoria (0.5°S). The PSV is recorded in fine-grained detrital magnetite/titanomagnetite grains, but the rock magnetic data suggest that significant magnetic mineral dissolution has occurred, which limits our paleomagnetic studies to the uppermost ~5 m of both cores. Detailed alternating field (af) demagnetization of the natural remanence (NRM) shows that a distinctive characteristic remanence (ChRM) is demagnetized from ~10 to 40 mT, which decreases simply toward the origin. The resulting directional PSV records for 1P and 7P are correlatable with 22 distinct inclination features and 19 declination features. Radiocarbon dating of the cores is based on eight radiocarbon dates from core 1P, which can be correlated into core 7P using both the PSV and rock magnetic/environmental measurements. The final PSV time series cover the last 11,000 years with an average sediment accumulation rate of ~40 cm/kyr. The Lake Victoria PSV records can be correlated with new PSV records from Lake Malawi. Comparison of the correlatable PSV feature ages between the two lakes indicates that the PSV records are not significantly different in age, although Lake Victoria PSV ages might average ~100 years younger. We think that the Lake Victoria and Lake Malawi PSV records, together, provide the most accurate, well-dated, and consistent record of Holocene PSV for Africa yet developed.
International Ocean Drilling Program (IODP) Expedition 341 recovered sediments from the south Alaska continental slope that preserves a well resolved and dated inclination record over most of the past ∼43 000 yr. The Site U1419 chronology is among the highest resolution in the world, constrained by 173 radiocarbon dates, providing the ability to study Palaeomagnetic Secular Variation (PSV) on centennial to millennial timescales. This record has an exceptionally expanded late Pleistocene sedimentary record with sedimentation rates commonly exceeding 100 cm kyr–1, while also preserving a lower resolution Holocene PSV record at the top. Natural and laboratory-induced magnetic remanences of U1419 u-channels from the 112-m-long spliced record were studied using stepwise AF demagnetization. Hysteresis loops were obtained on 95 and IRM acquisition curves on 9 discrete samples to facilitate magnetic domain state, coercivity and magnetic mineralogical determinations. Due to complexities related to lithology, magnetic mineralogy, and depositional and post-depositional processes, Site U1419 sediments are not suitable for palaeointensity studies and declination could not be robustly reconstructed. Progressive (titano-)magnetite dissolution with depth results in decreasing NRM intensity and signal-to-noise that is exacerbated at higher demagnetization steps. As a result, inclination measured after the 20 mT AF demagnetization step provides the most reliable directional record. Inclination appears to be well resolved with removal of just a few intervals influenced by depositional and/or sampling and coring deformation. The shipboard inclination stack from nearby IODP Site U1418, on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC, verifies centennial to millennial scale variations in inclination observed in U1419. Comparisons with other independently dated records from the NE Pacific and western North America suggest that these sites likely capture regional geomagnetic variability. As such, this new high-resolution and well-dated inclination record, especially robust between 15 and 30 cal kyr BP, offers new geomagnetic insights and a regional correlation tool to explore this generally understudied part of the world.
more » « less- Award ID(s):
- 1929486
- NSF-PAR ID:
- 10381955
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 229
- Issue:
- 1
- ISSN:
- 0956-540X
- Format(s):
- Medium: X Size: p. 345-358
- Size(s):
- ["p. 345-358"]
- Sponsoring Org:
- National Science Foundation
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