Abstract:
We have studied the magnetic properties of wet and dry late Pleistocene Lake Lisan sediments and the Holocene Dead Sea sediments. Our initial prediction was that the properties of both would be quite similar, because they have similar source and lake conditions, unless diagenetic change had occurred. Rock magnetic and paleomagnetic experiments revealed three stages of magnetization acquisition. Our fi ndings suggest two magnetic carriers in the Holocene Dead Sea and wet Lisan sediments: titanomagnetite and greigite. The titanomagnetite grains are detrital and carry a detrital remanent magnetization (DRM), whereas the greigite is diagenetic in origin and carries a chemical remanent magnetization (CRM) that dominates the total natural remanent magnetization (NRM) of Holocene Dead Sea and wet Lisan sediments. The magnetization of dry Lisan sediments is a DRM and resides in multidomain (MD) grains. We propose that magnetic properties of the Lisan Formation and Holocene Dead Sea sediments can be explained by a model that incorporates dissolution, precipitation, and alteration of magnetic carriers. At the time of deposition, titanomagnetite grains of varying size were deposited in Lake Lisan and the Holocene Dead Sea, recording the geomagnetic field via a primary DRM. Sedimentation was followed by partial or complete dissolution of titanomagnetite in anoxic lake bottom conditions. As the kinetics of dissolution depends upon surface area, the single-domain (SD) grains dissolved faster, leaving only the larger pseudo-single domain (PSD) and MD grains. Titanomagnetite dissolution occurred simultaneously with precipitation of greigite in anoxic, sulfatereducing conditions probably related to bacterial degradation of organic matter. This process added a secondary CRM that overwhelmed the DRM and the primary geomagnetic record. Later, when the level of Lake Lisan dropped, these sediments were exposed to air. At this time, the greigite was oxidized, removing the CRM from the system and leaving only the original detrital PSD and MD titanomagnetite grains as the dominant DRM carriers. Presently, wet Lisan sediments have not been completely altered and therefore contain secondary greigite preserved by the original formation water that carries a secondary CRM. Thus, the magnetization in the Holocene Dead Sea and the wet Lisan magnetic record cannot be considered as an accurate, reliable geomagnetic record, while magnetization of dry Lisan sediments is a primary DRM. © 2006 Geological Society of America.
