A Late Holocene Paleomagnetic Record from Lago del Desierto, Southern Patagonia (Argentina)

Lago del Desierto (49°02′S, 72°51′W) is located in a climatically sensitive sector near the Southern Patagonian Ice Field (Argentina). Three sediment cores collected from two sites in the lake were analyzed using a multi-proxy approach to reconstruct past environmental variability (Kastner et al., 2010). Numerous turbidites were identified in the sedimentological record. After excluding these event layers, a new age–depth model was developed for the first 3 sections of the core DES05-3 (289 cm), and paleosecular variations (PSV) were reconstructed for the interval between ~1000 and 3500 cal. BP.

Standard paleomagnetic measurements (alternating-field demagnetization) were performed on 125 samples from the same core. In addition, rock-magnetic measurements, including Anhysteretic Remanent Magnetization (ARM, 100 mT peak AF, 0.05 mT DC field), Isothermal Remanent Magnetization (IRM, acquisition up to 1.5 T and backfield curves), hysteresis loops and thermomagnetic analyses, were applied to extract complementary paleoenvironmental information from the sediment cores.

Rock-magnetic measurements indicate that the magnetic mineralogy is dominated by a low-coercivity component (magnetite-type), accompanied by a secondary high-coercivity fraction (hematite/goethite-type). This downcore distribution mirrors the paleoenvironmental shift described by Kastner et al. (2010): the lower part of the sequence shows an enhanced contribution of high-coercivity Fe oxides, consistent with more stable and chemically weathered catchment conditions. In contrast, the upper part shows an increasing dominance of detrital magnetite, indicating strengthened minerogenic supply and enhanced erosion, matching the onset of warmer conditions and glacier retreat during the Medieval Climate Anomaly as inferred from geochemical and lithological proxies. This agreement between magnetic and non-magnetic sediment parameters suggests coherent changes in the provenience of the sediment and in catchment dynamics over the last millennia. As expected from the catchment instability and the numerous turbidites in the upper part of the sequence, this interval could not be used for PSV reconstruction due to its discontinuous directional record. In contrast, samples from the lower part (~1000–3500 cal. BP) provided a continuous sequence suitable for paleosecular variation analysis. Although samples from this unit were not completely demagnetized at 100 mT, due to the presence of a high-coercivity component, magnetization directions consistently decayed toward the origin with high precision. Characteristic remanent magnetization (ChRM) directions were determined using principal component analysis, with maximum angular deviation (MAD) values below 2.5° for all non-turbidite samples. The resulting PSV record compares well with geomagnetic field models and other Patagonian paleomagnetic reconstructions. Inclination values range from −40° to −70°, displaying coherent directional variability over the last ~3500 years.

References:
Kastner, S., Enters, D., Ohlendorf, C., Haberzettl, T., Kuhn, G., Lücke, A., Mayr, C., Reyss, J.-L., Wastegård, S., & Zolitschka, B. (2010). Reconstructing 2000   years of hydrological variation derived from laminated proglacial sediments of Lago del Desierto at the eastern margin of the South Patagonian Ice Field, Argentina. Global and Planetary Change, 72(3), 201-214. https://doi.org/10.1016/j.gloplacha.2010.04.007