Reconciling independent chronologies of lake sediments in the Baza paleolake, Southern Spain

Abstract

Measuring time is an indispensable tool in the Earth sciences and lies at the heart of historical reconstructions. Some sedimentary rocks are difficult to date with absolute time methods due to the scarcity of suitable material. Paleomagnetism and rock magnetic cyclostratigraphy can be determined from most geologic materials. The Baza Basin in Southern Spain contained the largest paleolake in Europe (>600km2) and records Plio-Pleistocene paleoclimate and paleoenvironmental changes. We developed a new magnetic polarity stratigraphy and rock magnetic cyclostratigraphy on a 60m-thick section using an oriented sample interval of 2-3m for paleomagnetic analyses and an unoriented sample interval of 0.25m for rock magnetic studies. Sedimentary facies include lacustrine, palustrine, and fluvial deposits based on comparative sedimentology. Paleomagnetic samples yielded stable characteristic remanent magnetization directions after either thermal or alternating field (AF) demagnetization and reveal polarity changes from R-N-R-N going down-section. The upper N was previously interpreted as the Olduvai chron. Rock magnetic measurements included isothermal remanent magnetization and hysteresis loops suggesting magnetite as the main magnetization carrier. Rock magnetic cyclostratigraphy was based on the variation of magnetite concentration, using an anhysteretic remanent magnetization imparted to the unoriented samples (AF of 100mT, and a bias field of 0.05mT) and low field bulk susceptibility. We recovered significant stratigraphic cyclicity above the 99% confidence limit and interpreted sediment accumulation rates (SAR) using Milankovitch frequencies. The highest frequencies are modulated by a significant frequency 1/5 lower suggesting recovery of precessional and eccentricity controlled cyclicity. If this is correct, the SAR is 3.6cm/kyr, which conflicts with the magnetostratigraphic assessment. The polarity interpretation requires a higher SAR, which would result in subMilankovitch cyclicity. In addition, we determined electron spin resonance (ESR) ages on quartz samples from 2 horizons that suggest burial at ~1Ma, which is inconsistent with both the magnetic polarity stratigraphy and the SAR determinations. Various resolutions of these individually robust datasets are possible.