Sites 889 and 890

Westbrook, G.K.; Carson, B.; Musgrave, R. J.

doi:10.2973/odp.proc.ir.146-1.008.1994

Cited by 5 publications

References 39 publications

(67 reference statements)

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

Musgrave

Kars

2016

Geochem Geophys Geosyst

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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.

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

Musgrave

Kars

2016

Geochem Geophys Geosyst

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Implications for focused fluid transport at the northern Cascadia accretionary prism from a correlation between BSR occurrence a d near-sea-floor reflectivity anomalies imaged in a multi-frequency seismic data set

Zühlsdorff

Spieß

Hübscher

et al. 2000

International Journal of Earth Sciences

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Subseafloor Temperature Variations Influenced by Variations in Bottom Water Temperature and Pressure: New High Resolution Observations and Implications

Davis

Villinger

2019

JGR Solid Earth

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A new instrument developed for monitoring acceleration, tilt, and pressure at the ocean floor also measures sediment temperature 1 m below the seafloor. Four deployments have been completed and connected to the Ocean Networks Canada cabled observatory, one on the inner Cascadia accretionary prism, two on the outer prism, and one on the sedimented eastern flank of the Juan de Fuca Ridge. Relative amplitudes and phases of temperature variations measured at the seafloor and in the sediment at periods greater than roughly 1 week constrain the thermal diffusivity of the upper meter of subseafloor sediment to be 4 × 10 À7 m 2 /s. Clear ±0.1-mK amplitude tidal sediment temperature variations are also resolved. These are too large and regular to be the consequence of downward thermal diffusion from the seafloor and too large to be the consequence of fluid migration driven along the sediment geotherm by poroelastic response to tidal loading. The variations are closely correlated with tidal pressure variations, however, and we infer that these temperature signals reflect adiabatic heating and cooling. The lapse rates inferred from the observations at two of the sites are close to the values for seawater but significantly higher than predicted for a mixture that includes sediment grains. The values observed by both instruments at the outer prism site, located near methane-bearing-fluid springs, are particularly high, 20% higher than predicted for a sediment-seawater mixture. This discrepancy could be reconciled if free gas or methane hydrate were present within the pore volume.Plain Language Summary A new tool developed for deep ocean seismic and geodynamic monitoring also measures sediment temperature 1 m below the seafloor to very high precision. Temperature variations that have diffused from week-to month-long temperature variations at the ocean bottom provide a direct measurement of the thermal diffusivity of the sediments. Small (0.0001°C) sediment temperature variations are also observed at tidal periods, and these are synchronized with ocean tidal pressure variation. These observations provide a new method for the in situ determination of the adiabatic lapse rate for the sedimentary material.

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Sites 889 and 890

Cited by 5 publications

References 39 publications

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

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

Implications for focused fluid transport at the northern Cascadia accretionary prism from a correlation between BSR occurrence a d near-sea-floor reflectivity anomalies imaged in a multi-frequency seismic data set

Subseafloor Temperature Variations Influenced by Variations in Bottom Water Temperature and Pressure: New High Resolution Observations and Implications

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