Rock magnetic characterization of ferrimagnetic iron sulfides in gas hydrate-bearing marine sediments at Site C0008, Nankai Trough, Pacific Ocean, off-coast Japan

Abstract

A high-resolution rock magnetic study was carried out in Integrated Ocean Drilling Program (IODP) Expedition 316 Hole C0008A located in the Megasplay Fault Zone of the Nankai Trough, SW offshore Japan, in order to document changes in magnetic properties throughout gas hydrate-bearing horizons. A total of 169 Pleistocene discrete samples were collected from~110 to 153 m core depth below sea floor (CSF), and their magnetic minerals concentration, grain size, composition, and rock magnetic parameters were estimated. Results showed the presence of iron oxides ((titano)-magnetite), iron sulfides (greigite and pyrrhotite), and their mixture, among which single-domain greigite is the most major magnetic mineral present in the samples. Two horizons containing ferrimagnetic iron sulfides (114.5-127.5 and 129.5-150 m CSF) covering almost the entire studied interval were identified, both associated with slight local pore water anomalies, suggesting occurrence of gas hydrates and anoxic conditions. These results are different from the neighboring Hole C0008C (215 m away from Hole C0008A) where four pore water anomalies and six iron sulfide-rich intervals were identified for the same time slice. Comparison of the lithology, physical properties, and geochemical data of the two boreholes at Site C0008 suggests that a combination of processes (e.g., availability of reactive iron, microbial activity) is responsible for such laterally varying distribution of the ferrimagnetic iron sulfides.

Ferrimagnetic Iron Sulfide Formation and Methane Venting Across the Paleocene‐Eocene Thermal Maximum in Shallow Marine Sediments, Ancient West Siberian Sea

“…Sulfidic diagenetic activity lasted for a relatively short period of time through the PETM, after which pelitic marine sediments accumulated under stagnant bottom waters (Figure c) where pyrite formation occurred without preservation of ferrimagnetic iron sulfides. The association of diagenetic minerals (e.g., siderite, barite, and apatite) with the PETM interval, indications of increased paleoproductivity across the PETM, temperature homogenization of fluid inclusions in siderite, bulk organic δ 13 C results, and the presence of ferrimagnetic iron sulfides in the PETM section, are all consistent with observations from modern environments associated with gas hydrate dissociation (e.g., Kars & Kodama, ; Larrasoaña et al, ) and provide evidence for a biogenic methane source (Dickens, ; Dickens et al, ; Panchuk et al, ; Zeebe, ).…”

“…CO 2 and sulfide‐saturated fluids appear to have penetrated interstitial spaces and microcracks in detrital grains (Figure g), which gave rise to siderite and pyrrhotite formation around and inside detrital mineral grains. Pyrrhotite and greigite have been described in other marine sediments where concentration gradients of methane occur near disseminated gas hydrates (Jørgensen et al, ; Kars & Kodama, ; Neretin et al, ; Roberts et al, ). Formation of sphalerite in the studied Paleocene oolitic ironstones may also have been associated with localized bacterial sulfate reduction and HS − release.…”

“…The kinetics of monoclinic pyrrhotite formation preclude its formation during earliest burial; its presence as an authigenic mineral is indicative of formation over longer periods (e.g., Horng & Roberts, ), often in relation to anaerobic oxidation of methane and formation of gas hydrates (Horng & Chen, ; Kars & Kodama, ; Roberts et al, ; Shi et al, ; van Dongen et al, ; Weaver et al, ). The formation of these magnetic iron sulfide minerals (particularly pyrrhotite) has been associated with increasingly methane seepage in marine sediments (Jørgensen et al, ; Kars & Kodama, ; Larrasoaña et al, ; Neretin et al, ; van Dongen et al, ). This is important because methane emissions are a leading cause of past global climate change (Kvenvolden, ) and contribute to present‐day climate change (Crichton et al, ; Monnin, ; Shakhova et al, ; Tesi et al, ).…”

The Besnus Transition in Single‐Domain 4C Pyrrhotite

“…Magnetic information about this transition has so far been obtained from samples with relatively large crystallites in the multidomain (MD) magnetic state (Dekkers et al, ; Koulialias, Schäublin, et al, ; Rochette et al, ), from shock‐induced samples containing MD and single‐domain (SD) particles (Louzada et al, ), from crushed samples with significant contribution of SD particles (Dekkers et al, ; Wehland et al, ), or from nonideal SD pyrrhotite samples (Bezaeva et al, ). Complementary information from phase pure, SD monoclinic pyrrhotite has so far not been established although this mineral phase could be widespread in sedimentary environments such as gas hydrate‐bearing sedimentary deposits (e.g., Kars & Kodama, , ).…”

Special issue on “Recent advances in environmental magnetism and paleomagnetism”

“…The strongest climate signal is the 100-ka period, which indicates that sea-level change played a dominant role in the long-term environmental setting; additionally, the signals of the 40-and 20-ka periods possibly suggest the influence of regional precipitation. Kars and Kodama (2015) presented a high-resolution rock magnetic record from 169 samples collected at Hole C0008A in the Nankai Trough, offshore southwest Japan, during Integrated Ocean Drilling Program Expedition 316. The rock magnetic study, conducted at a core depth from 110 to 153 m below the sea floor, highlighted the widespread occurrence of magnetic iron sulfides, particularly greigite.…”