The mineral magnetic record of magnetofossils in recent lake sediments of Lake Ely, PA

摘要

Mineral magnetic and ferromagnetic resonance (FMR) measurements made on the sediments from Lake Ely, Pennsylvania reveal the presence of magnetotactic bacteria magnetofossils. Saturation isothermal remanent magnetization (SIRM) data from a 1.25 m long piston core taken from the deepest part of the lake show a large intensity decrease at depths between 30 and 75 cm in the sediment column most likely the result of reductive diagenesis. Modeling of isothermal remanent magnetization (1RM) and anhysteretic remanent magnetization (ARM) acquisition data indicates the presence of biogenic soft (BS) and biogenic hard (BH) magnetosome coercivity components, even at depths in the core greatly affected by reductive diagenesis. First-order reversal curve distribution diagrams for two samples support this interpretation. Modeling of the FMR data also indicates the presence of the BS and BH magnetosome coercivity components, but the relative importance of the two components is opposite to that seen for the IRM and ARM acquisition modeling. A correlation between SIRM variations and local rainfall recorded over the past 70 years suggests that magnetofossil concentrations recorded paleo-rainfall variations in the most recent lake sediments. Multi-taper method spectral analysis of SIRM variations in the piston core, tied to time by varve counting from 1600 common era (CE) to 1128 before CE, show centennial scale periodicities similar to those observed in spectral analysis of a 230 year long historical rainfall record for the northeastern US reconstructed to Philadelphia, PA. This result indicates that enough magnetofossils can survive reductive diagenesis to retain a record of paleorainfall variations, suggesting that they can be a powerful paleoclimate proxy.