Recognizing magnetostratigraphy in overprinted and altered marine sediments: Challenges and solutions from IODP Site U1437

Abstract

Core disturbance, drilling overprints, postdepositional acquisition of remanence, authigenic growth of magnetic iron sulfides, and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments. We address these issues in a sequence of tuffaceous muds and volcaniclastics at International Ocean Discovery Program Site U1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling. Remanence measurements were filtered to remove intervals affected by fluidization, plastic sediment disturbance, and core biscuiting. Drilling overprints are concentrated in the disturbed annulus surrounding intact core material. Bioturbation was limited to a vertical extent of at most 15 cm. Changes in sediment color, stiffness, and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment–water interface. We did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums. Authigenic growth of greigite, in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate, has led to magnetic overprinting. Anomalous polarity artefacts, extending <5 m and occurring within about 20 m below a real polarity transition, appear to be due to a chemical remanence acquired by greigite produced during early diagenesis. Diagenetic magnetic mineral alteration resulted in the progressive loss of fine‐grained magnetite, which enhanced susceptibility to drilling and postdrilling overprints and increased the resistance of these overprints to removal by conventional demagnetization. We recovered the magnetostratigraphic record from many samples with resistant overprints through low‐temperature demagnetization through the Verwey transition.

One Million Years tephra record at IODP Sites U1436 and U1437: Insights into explosive volcanism from the Japan and Izu arcs

“…The robust age models and multiple volcanic events at both sites, provide the opportunity for a high‐resolution, time‐precise record of volcanic activity in the Izu arc. Overall, the excellent biostratigraphic and magnetostratigraphic data (Musgrave & Kars, ; Tamura et al, ; Vautravers et al, ) in Units I–III at Site U1437 demonstrate that the tephra layers represent a time series of instantaneous volcanic events that were preserved within rapidly accumulating tuffaceous mud, and that shows no evidence for hiatuses (Tamura et al, ).…”

Magnetic Mineral Diagenesis in a High Temperature and Deep Methanic Zone in Izu Rear Arc Marine Sediments, Northwest Pacific Ocean

“…Iron oxides would have been preserved as inclusions within sheet silicates and would not have been affected by diagenesis (Chang, Bolton, et al, 2016;Shi et al, 2017). That may explain why Musgrave and Kars (2016) were able to establish a reliable magnetostratigraphy in the studied interval. Elemental composition variations are more likely associated with redox conditions rather than with detrital provenance.…”

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…We term this behavior “punctuated diagenesis.” In some cases (fluid anomalies 1, 3, 6, and 8) this results in decreased coercivity, presumably by selective destruction of SD or vortex state grains of either magnetite (by dissolution) or greigite (by conversion to pyrite), while in others (fluid anomalies 2, 4, 5, and 9) it enhances coercivity, which in most of the sequence appears to be due to repeated phases of greigite authigenesis, based on the SIRM/κ response and the lack of evidence for significant proportions of SD magnetite or pyrrhotite. Possible mechanisms for greigite authigenesis may involve neoformation on pyrite grains (Jiang et al, ) as seen in SEM images from Hole U1437B (Musgrave & Kars, ), on authigenic smectite and chlorite (Roberts & Weaver, ) and possibly on siderite (Sagnotti et al, ). The relationships of these diagenetic processes to the fluid anomalies at Site U1437 are complex.…”

“…Ghost polarity intervals found at intervals within the sequence resulted from episodes of early diagenetic greigite growth within the initial sulfate reduction zone, but the success in identifying the magnetic polarity record down to 1,320 mbsf suggests that neither the repeated episodes of greigite authigenesis, nor the progressive conversion of greigite to pyrite and continued dissolution of magnetite, completely degraded the remanence record within the mud‐rich sequence of Units I through V. Continuity of the polarity signal was only lost when the shift to the coarser, volcaniclastic‐dominated lithology of Units VI and VII compounded the effects of progressive magnetic mineral diagenesis. Even within Units VI and VII some samples still record a stable remanence, evidenced by the recognition of reversed polarity in a few isolated samples (Musgrave & Kars, ). Stable remanence in these reduced, coarse‐grained units may be carried by magnetite inclusions within silicates, as evidenced by the presence of a SD component in the sample from coarse‐grained Unit VII (Figure D).…”

“…Methane, which is present in markedly higher concentrations in the interval from 750 to 1,035 mbsf, may have been locally sourced, but the correlation between total organic carbon contents and gas concentrations is poor (Kars et al, ), favoring gas migration. Ethane appeared above detection limits for the first time in the first core below an interpreted fault at the break between the bottom of Hole U1437D and the beginning of coring at the same subbottom depth in Hole U1437E (Musgrave & Kars, ; Tamura et al, ). Contrasting ethane abundance in the hanging and footwalls of this fault are consistent with gas migration.…”