Instituto Geológico y Minero de España Madrid, Spain ; Institute of Earth Sciences Jaume Almera, CSIC Barcelona, Spain.
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences Beijing, China.
Front Microbiol. 2014 Mar 4;5:71. doi: 10.3389/fmicb.2014.00071. eCollection 2014.
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.
尽管最近的研究揭示了前第四纪沉积物中磁化石的分布比以前报道的更为广泛，但它们在古地磁和古环境研究中的意义尚未完全被理解。我们对从瓜达尔基维尔盆地（西班牙西南部）采集的晚中新世海洋沉积物进行了古地磁和岩石磁学研究。两个研究岩芯中明确的古地磁方向提供了一个稳健的磁地层年代学记录。岩石磁学结果表明，在整个研究沉积物中都存在完整的磁小体链。这些结果将最高质量的古地磁方向与更高的磁化石丰度联系起来。我们推断，细菌磁铁矿在表层沉积物混合层中形成，这些磁性颗粒以与碎屑颗粒相同的方式产生古地磁信号。因此，它们携带的磁化强度与后沉积剩余磁化强度基本相同，我们称之为生物沉积剩余磁化强度。一些研究的极性反转记录了古地磁方向，其记录延迟约为 60-70 千年前。在这些情况下，磁化石被解释为携带生物地球化学剩余磁化强度，这种磁化强度被锁定在沉积物柱的更深层。在研究钻孔中部，磁化石丰度急剧下降，与 Messinian 盐度危机（MSC）开始时沉积物堆积速率的大幅上升大致吻合，MSC 是由地中海与大西洋之间的连接中断引起的事件。这种相关性似乎是由于 MSC 开始时盆地中触发的沉积变化导致的陆源输入增加而使磁化石稀释所致。我们的研究结果强调了磁化石作为高质量古地磁和古环境信号载体的重要性，即使在以陆源沉积物为主的沉积物中也是如此。
