Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain

Abstract

Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60–70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.

Post-depositional remanent magnetization lock-in depth in precisely dated varved sediments assessed by archaeomagnetic field models

“…A significant recording delay, similar to the effects of a lock-in delay, is usually associated with a BGRM acquired by magnetofossils that were produced in the oxic sediment column (e.g. Larrasoaña et al, 2014). Our results, however, show evidence for very deep lock-in depths in Kälksjön and Gyltigesjön even without the presence of oxic conditions in the sediment column.…”

“…The OATZ is located above the sediment-water interface in both lakes as observed from oxygen profiles (Zillén, 2003;Snowball et al, 2013) and evidenced by varves, which would not be formed if burrowing and crawling organisms exist on the sediment surface. An OATZ in the water column leads to the acquisition of BDRM, which Larrasoaña et al (2014) associate with low preservation of magnetofossils due to anoxic conditions in the sediment column, which would cause reductive dissolution of magnetite (e.g. Snowball, 1994).…”

“…The possibility that magnetofossils produced by magnetotactic bacteria (MTB) contribute to a bio-depositional remanent magnetization (BDRM) and/or a bio-geochemical remanent magnetization (BGRM) needs to be considered (Larrasoaña et al, 2014). The location at which such magnetic particles are produced and made available for remanence acquisition is important for the occurrence of a lock-in depth and its effects in sediments.…”

Magnetic mineral diagenesis

“…However, if they lived below the surface mixed layer as suggested by Tarduno and Wilkison (1996) and Tarduno et al (1998), magnetofossils will contribute to a biogeochemical remanent magnetization (Roberts et al, 2013a;Larrasoaña et al, 2014). Further work is needed to test these possibilities.…”

Intracellular biomineralization in bacteria

“…The authors note that ultraviolet-B radiation could affect both cell growth and magnetite synthesis, and suggest that magnetosomes may have evolutionary benefits for the survival of MTB under extreme environments. A paleo-and rock-magnetic study of marine sediments from the Guadalquivir Basin, Spain by Larrasoaña et al (2014) …”