PALEOCLAK · Paleomagnetic applications for dating and identifying Holocene climate variability in southwestern Europe and Azores Islands as recorded by lake sediments

Paleomagnetic applications for dating and identifying Holocene climate variability in southwestern Europe and Azores Islands as recorded by lake sediments

The main objective of the PALEOCLACK project, developed at Géosciences-Rennes UMR-6118 (Rennes, France) was to give a forward step towards the use of the paleomagnetic and environmental magnetic techniques for dating and identifying Holocene climate variability in southwestern Europe and Azores Islands as recorded by lacustrine sediments. This project takes its real potential as part of a multidisciplinary integration in collaboration with specialists of different disciplines. The new magnetic data obtained have been analyzed together with other environmental parameters obtained with other sedimentological, geochemical and biological proxies available for the different records. To achieve this main goal several objectives have been addressed: 1. Objective 1. We have increased our knowledge on past changes of the geomagnetic field during the Holocene by acquiring new archeo- and paleomagnetic data from the study of well-dated archeological baked material and Holocene lacustrine sediments form the Iberian Peninsula and the Azores Islands. The improved characterization of geomagnetic field changes in the past provides a new tool for the dating -or at least to provide a regional synchronization- of Holocene sedimentary sequences. The paleomagnetic dating technique is based on the recognition of changes in the direction and intensity of the Earth´s magnetic field in the sedimentary record. The main result obtained in the framework of the project is the establishment of a new regional European geomagnetic field intensity model for the last millennia (Pavón-Carrasco et al., 2014) which provides an improved reconstruction of past geomagnetic field intensity fluctuations (see Figure 1), one of the main challenges of geomagnetic research nowadays. High-resolution geomagnetic field reconstructions are essential to better probe the Earth´s dynamo. In addition, this study demonstrates the necessity of establishing a pre-selection of high-quality intensity data in order to obtain a robust description of the geomagnetic field in the past; and highlights that new reliable data are still crucial to obtain a detailed reconstruction of the Earth´s magnetic field, specially for the Southern Hemisphere. 2. Objective 2. New environmental magnetic datasets, based on bulk magnetic parameters and detailed rock-magnetic experiments, for different lacustrine sediments form the Iberian Peninsula and the Azores Islands have been obtained. The magnetic data have been used to identify paleoenvironmental variations as recorded by changes in the type, concentration and grain size of magnetic minerals. By comparing magnetic parameters with other available sedimentological, geochemical and biological data, the reliability of magnetic parameters as proxies for Holocene climate variability and human activity has been evaluated. A main output of the project comes from Lake Sanabria (NW Spain), which is the largest lake of glacial origin in the Iberian Peninsula (Borruel et al., 2015). In this work, we present a source-to-sink environmental magnetic study of a Late Pleistocene–Holocene sediment core from Lake Sanabria and from different rocks of its catchment that complements previous sedimentological and geochemical studies. The new approach gives new insight into the climatic evolution of the north- west Iberian Peninsula during the last deglaciation. The results indicate that the magnetic assemblage of sediments from the lower half of the studied sequence, accumulated between 26 and 13 ka BP in a proglacial environment, and of the Palaeozoic rocks that make up most of the catchment of the lake, is characterized by magnetite and, probably also, pyrrhotite. The occurrence of these minerals both in the catchment rocks and in the lake sediments indicates that sedimentation was then driven by the erosion of glacial flour, which suffered minimal chemical transformation in response to rapid and short routing to the lake. Sediments from the upper half of the studied sequence, accumulated after 12.6 ka BP in a lacustrine environment with strong fluvial influence, contain magnetite and smaller amounts of maghemite and greigite. These findings point to a significant role of post-depositional reducing conditions once the sediments were accumulated in the lake, but also to a significant imprint of pedogenic activity in the lake catchment before detrital material was transported into the lake. The sharp change in magnetic properties observed in the lake sediments between 13 and 12.6 ka BP, coupled with more subtle changes in rock magnetic data at ca. 14 cal ka BP, supports the rapid deglaciation of the catchment of Lake Sanabria inferred in previous studies on the basis of sedimentological, geochemical and geomorphological data.

Data: CORDIS, © European Union