11:00 AM - 1:00 PM
[MIS18-P12] Sedimentary rhythms of ferromanganese crust induced by glacial-interglacial cycles
Keywords:Ferromanganese, Glacial-interglacial cycles, Global environmental changes
Ferromanganese crusts and nodules are widely distributed in the deep-sea floor and seamounts around the world (e.g., Usui et al., 2017). The growth rate of ferromanganese crusts is very slow, about several millimeters per million years. It is also known that some areas of ferromanganese crusts preserve the rhythmical laminations, which may reflect glacial-interglacial cycles, according to the corresponding age of the laminations (Eisenhauer et al., 1992; Han et al., 2003; Takahashi, 2015MS). However, the detailed formational mechanisms of such sedimentary rhythms remain uncertain due to their micro-scale structures. In this study, we performed micro-scale analysis of the elemental composition and 3D internal structure to unravel its formation mechanism.
We used 4 ferromanganese crust samples from 1940~2700 m depth around the Shotoku Seamounts. Ultrafine-scale magnetostratigraphy measured by scanning SQUID microscopy has been reported for one sample (Oda et al., 2011). We focused on the past 1-million-year interval where the sedimentary rhythms were particularly developed. We observed the internal structure using a reflected light fluorescence microscope and micro-focus X-ray CT, and analyzed the elemental compositions using EPMA.
The X-ray CT observation of 3D internal structure in ferromanganese crusts reveals that the sedimentary rhythms were formed by the alternating layers of the columnar structure, which resembles to stromatolite and the fenestral structure (Hofmann, 2000). The columnar structure has a higher density (high CT value), whereas the fenestral structure has a much lower density. The fenestral structure also consists of a double-layered structure with slightly different densities inside and outside. The outside layer of the fenestral structure shows weak fluorescence features.
EPMA analysis reveals that the columnar structure was mainly composed of Mn and Fe, while the fenestral structure was composed of Si (outside layer) and Al and K (inside layer). Comparison with magnetostratigraphic time-scale based on SQUID microscopy and benthic foraminiferal oxygen isotope record (Lisiecki & Raymo, 2005) suggests that the columnar structure with Mn enrichment likely formed during the interglacial period.
Given that Mn accumulation is thought to be higher during the interglacial period (Eisenhauer et al., 1992) and eolian dust deposition is enhanced during the glacial period (Maher et al., 2010; Lamy et al., 2014), we interpreted that the columnar structure of the Mn-enriched area was formed during interglacial while the fenestral structure consisting of clastic particles of eolian dust origin was formed during glacial.
We used 4 ferromanganese crust samples from 1940~2700 m depth around the Shotoku Seamounts. Ultrafine-scale magnetostratigraphy measured by scanning SQUID microscopy has been reported for one sample (Oda et al., 2011). We focused on the past 1-million-year interval where the sedimentary rhythms were particularly developed. We observed the internal structure using a reflected light fluorescence microscope and micro-focus X-ray CT, and analyzed the elemental compositions using EPMA.
The X-ray CT observation of 3D internal structure in ferromanganese crusts reveals that the sedimentary rhythms were formed by the alternating layers of the columnar structure, which resembles to stromatolite and the fenestral structure (Hofmann, 2000). The columnar structure has a higher density (high CT value), whereas the fenestral structure has a much lower density. The fenestral structure also consists of a double-layered structure with slightly different densities inside and outside. The outside layer of the fenestral structure shows weak fluorescence features.
EPMA analysis reveals that the columnar structure was mainly composed of Mn and Fe, while the fenestral structure was composed of Si (outside layer) and Al and K (inside layer). Comparison with magnetostratigraphic time-scale based on SQUID microscopy and benthic foraminiferal oxygen isotope record (Lisiecki & Raymo, 2005) suggests that the columnar structure with Mn enrichment likely formed during the interglacial period.
Given that Mn accumulation is thought to be higher during the interglacial period (Eisenhauer et al., 1992) and eolian dust deposition is enhanced during the glacial period (Maher et al., 2010; Lamy et al., 2014), we interpreted that the columnar structure of the Mn-enriched area was formed during interglacial while the fenestral structure consisting of clastic particles of eolian dust origin was formed during glacial.