13:45 〜 15:15
[SCG52-P11] New observations of precisely dated circum-Antarctic mid-ocean ridges
キーワード：中央海嶺、海底地形、海底磁気異常
Mid-ocean ridges (MORs) play an important role in the Earth system by shaping ocean basins and transporting of heat and material from the sub-seafloor to the ocean. Abyssal hills are common bathymetric features in MORs, and it has been considered that they are formed and shaped by the combination of normal faulting in the spreading environment and magmatic constructional processes (e.g., Buck et al., 2005). Recent studies presented the “Milankovitch cycle hypothesis” for abyssal hill construction (Crowley et al., 2015; Tolstoy, 2015) and implied its expectable value for reconstructing a record of global sea level fluctuations and climate history over >10Myr. While the hypothesis has gained independent support from sedimentological evidence (e.g., Lund et al., 2016), it has been the focus of substantial criticism from a systematic statistical approach (e.g., Goff, 2018). In addition, the discussion about the upper mantle dynamics and the presence of chemical heterogeneities is essentially limited (e.g., Roth et al., 2019).
To understand the temporal and/or spatial variation of MORs in detail, further constraints from integrated observational records with precise age determination in several regions are needed. Here, we present a new geophysical dataset across circum-Antarctic MORs. We newly conducted underway geophysical mapping and deep-tow magnetics during the R/V Hakuho-maru and R/V Mirai cruises of KH-19-1 (Jan. to Feb. 2019), KH-19-6 Leg 3 (Nov. to Dec. 2019), KH-19-6 Leg 4 (Dec. 2019 to Jan. 2020), KH-20-1 (Jan. to Feb. 2020), and MR19-4 (Dec. 2019 to Feb. 2020). Systematic data of multibeam bathymetry, total and vector magnetic fields, sub-bottom profiler, and gravity were acquired across the Chile Ridge, the southern Mid-Atlantic Ridge, and two segments of Southeast Indian Ridge.
Analytical results of bathymetry and sea-surface magnetic anomalies show clear magnetic stripes of C2An.3n –C2Ar boundary (3.58Ma) on both sides of the axial valley of the Chile Ridge and Southeast Indian Ridges. No large changes in spreading rates were observed. Interpretation of magnetic stripes of the Mid-Atlantic Ridge is more complicated, probably due to the complex melt supply in slow-spreading ridges. Well-ordered abyssal hills (a few hundred meters variation) were developed from ridge-axis to C2n (1.77–1.95 Ma) in the Chile Ridge, while the morphological features were more smooth on the seafloor older than C2n. Its eastern and western slopes show different bathymetric signature that includes topographic highs probably caused by excess volcanism. These variations are likely caused by a complex combination of ~106-years scale variation in the mantle partial melting process (Bonatti et al., 2003) and ~105-years scale variation in the melt supply process (Shinevar et al., 2019; Parnell-Turner et al., 2020). Our results show that bathymetric change of shorter times scale (<105) exists at least, but their amplitudes are not remarkable. As for the further understanding of mid-ocean ridge processes in the Milankovitch timescale, the following are essentially needed in addition to geophysical mapping observations; 1) sequences of off-axis sediments with volcanic glasses as direct evidence to evaluate the timing of volcanic activities, and 2) a numerical model approach to evaluate melt transport response to stress field change in glacier and interglacial periods.
To understand the temporal and/or spatial variation of MORs in detail, further constraints from integrated observational records with precise age determination in several regions are needed. Here, we present a new geophysical dataset across circum-Antarctic MORs. We newly conducted underway geophysical mapping and deep-tow magnetics during the R/V Hakuho-maru and R/V Mirai cruises of KH-19-1 (Jan. to Feb. 2019), KH-19-6 Leg 3 (Nov. to Dec. 2019), KH-19-6 Leg 4 (Dec. 2019 to Jan. 2020), KH-20-1 (Jan. to Feb. 2020), and MR19-4 (Dec. 2019 to Feb. 2020). Systematic data of multibeam bathymetry, total and vector magnetic fields, sub-bottom profiler, and gravity were acquired across the Chile Ridge, the southern Mid-Atlantic Ridge, and two segments of Southeast Indian Ridge.
Analytical results of bathymetry and sea-surface magnetic anomalies show clear magnetic stripes of C2An.3n –C2Ar boundary (3.58Ma) on both sides of the axial valley of the Chile Ridge and Southeast Indian Ridges. No large changes in spreading rates were observed. Interpretation of magnetic stripes of the Mid-Atlantic Ridge is more complicated, probably due to the complex melt supply in slow-spreading ridges. Well-ordered abyssal hills (a few hundred meters variation) were developed from ridge-axis to C2n (1.77–1.95 Ma) in the Chile Ridge, while the morphological features were more smooth on the seafloor older than C2n. Its eastern and western slopes show different bathymetric signature that includes topographic highs probably caused by excess volcanism. These variations are likely caused by a complex combination of ~106-years scale variation in the mantle partial melting process (Bonatti et al., 2003) and ~105-years scale variation in the melt supply process (Shinevar et al., 2019; Parnell-Turner et al., 2020). Our results show that bathymetric change of shorter times scale (<105) exists at least, but their amplitudes are not remarkable. As for the further understanding of mid-ocean ridge processes in the Milankovitch timescale, the following are essentially needed in addition to geophysical mapping observations; 1) sequences of off-axis sediments with volcanic glasses as direct evidence to evaluate the timing of volcanic activities, and 2) a numerical model approach to evaluate melt transport response to stress field change in glacier and interglacial periods.