Katerina E. Petronotis

Vast amounts of carbon are stored beneath the seafloor in the form of methane hydrate. Hydrate is stable at moderate pressure and low temperature at a depth extending several hundred meters beneath the seafloor to the base of gas hydrate stability (BGHS) often marked by bottom simulating reflections (BSRs) in seismic profiles. However, data from logging‐while‐drilling and coring during Integrated Ocean Discovery Program Expeditions 372 and 375 offshore New Zealand identified hydrate ∼60 m beneath the BSR. This hydrate appears to be dissociating over thousands of years following a gradual temperature increase from sediment burial modulated by changes in bottom‐water temperature and sea‐level fluctuations. Slow hydrate dissociation significantly buffers the release of methane and therefore, carbon through glacial cycles. Dissociating hydrate beneath the BGHS may also increase estimated global budgets of methane stored in hydrate.

Data report: paleomagnetic and rock magnetic observations from sediments and basalts collected at Expedition 375 Site U1526 on the Tūranganui Knoll, Hikurangi Plateau

Increases in temperature and pressure caused by rapid burial of sediments seaward of the Sumatra subduction zone have been hypothesized to trigger dehydration reactions that diagenetically strengthen sediments and contribute to the formation of an over‐pressured pre‐décollement, which together facilitate the occurrence of large shallow earthquakes. We present paleomagnetic, rock magnetic, and electron microscopic analyses from drill cores collected offshore Sumatra at Site U1480 during IODP Expedition 362 that support this hypothesis. The older pre‐fan units (Late Cretaceous to early Paleocene) were deposited when Site U1480 was moving rapidly northward with the Indian plate from a paleolatitude of 50° to 30°S, which would equate to expected absolute paleomagnetic inclinations of 70°–43°. Most of the older pre‐fan sediments, however, have shallow observed inclinations (shallower than ±20°), indicating that the sediments were overprinted when Site U1480 was located near the paleoequator, as it has been since the early Oligocene. Electron microscopic observations reveal that the pre‐existing detrital magnetite grains have undergone pervasive dissolution and alteration by hydrothermal fluids. The diagenesis observed is consistent with mineral dehydration, possibly driven by rapid burial of pelagic sediments by the ∼1250 m thick Nicobar Fan sequence. In addition, the elevated burial temperature also facilitated the smectite to illite conversion reaction. We hypothesize that chemical reactions resulted in the formation of fine‐grained magnetite that records a chemical remanent magnetization overprint. This overprint is consistent with the alteration occurring after burial by the thick Nicobar Fan sequence sometime in the past few million years.

Authigenic greigite may form at any time within a sediment during diagenesis. Its formation pathway, timing of formation, and geological preservation potential are key to resolving the fidelity of (paleo‐)magnetic signals in greigite‐bearing sediments. In the cored sequence of the International Ocean Discovery Program Expedition 362 (Sumatra Subduction Margin), multiple organic‐rich mudstone horizons have high magnetic susceptibilities. The high‐susceptibility horizons occur immediately below the most bioturbated intervals at the top of muddy turbidite beds. Combined mineral magnetic, microscopic, and chemical analyses on both thin sections and magnetic mineral extracts of sediments from a typical interval (∼1,103.80–1,108.80 m below seafloor) reveal the presence of coarse‐grained greigite aggregates (particles up to 50–75 μm in size). The greigite formed under nonsteady state conditions caused by the successive turbidites. Organic matter, iron (oxy)(hydr)oxides, Fe²⁺, and sulfides and/or sulfate were enriched in these intensively bioturbated horizons. This facilitated greigite formation and preservation within a closed diagenetic system created by the ensuing turbidite pulse, where pyritization was arrested due to insufficient sulfate supply relative to Fe (oxy)(hydr)oxide. This may represent a novel greigite formation pathway under conditions modulated by turbidites and bioturbation. Paleomagnetic analyses indicate that the early diagenetic greigite preserves primary (quasi‐)syn‐sedimentary magnetic records. The extremely high greigite content (0.06–1.30 wt% with an average of 0.50 wt% estimated from their saturation magnetization) implies that the bioturbated turbiditic deposits are an important sink for iron and sulfur. Mineral magnetic methods, thus, may offer a window to better understand the marine Fe–S–C cycle.

Quantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage‐displacement scaling as continental faults.

We constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

Walvis Ridge Hotspot: drilling Walvis Ridge, Southeast Atlantic Ocean, to test models of ridge-hotspot interaction, isotopic zonation, and the hotspot reference frame

We constrain the orientation of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW-SE convergence leading to recurrent slow slip events. The direction of the horizontal maximum stress is E-W at an active thrust fault near the subduction margin trench. This trend changes to NNW-SSE in the Tuaheni Basin in the offshore accretionary wedge, and to NE-SW in the onshore forearc. Multiple, tectonic and geological processes, either individually or in concert, may explain this variability. A general offshore-onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike-slip and/or extensional stress regime closer to the Taupo Volcanic Zone. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

The Pāpaku Fault Zone, drilled at International Ocean Discovery Program (IODP) Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In logging‐while‐drilling data, the 33‐m‐thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P‐wave velocity, and resistivity. Methane hydrate is observed from ~30 to 585 m below seafloor (mbsf), including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy centimeter‐thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply sourced fluids and that the near‐seafloor Pāpaku Fault Zone has little to no active fluid flow.