Site Description

IODP Expedition 366 (8 December 2016 to 7 February 2017) onboard the R/V JOIDES Resolution drilled into the Yinazao (previously known as Blue Moon), Fantangisña (previously known as Celestial), and Asùt Tesoru (previously known as Big Blue) serpentinite mud volcanoes on the Mariana forearc (Fryer et al., 2018b). These mud volcanoes are located to the west of the Mariana Trench and differ in distance to the trench and temperature of the underlying subducting slab (Figure 1). The three mud volcanoes also differ in geographic location, i.e., Fantagisña is located ~90 km north of Yinazao, and ~170 km south of Asùt Tesoru (Fryer et al., 2018b). Yet, distance to trench, which only differs by 17 km, and associated changes in the slab temperature, which varies from 80°C to around 250°C, are more important drivers of deep geochemical processes (Hulme et al., 2010). For this reason we schematically represent the three mud volcanoes as a transect in Figure 1.

Schematic dissection of the Yinazao, Fantangisña, and Asùt Tesoru mud volcanoes in relation to the Mariana Trench, which is formed by subduction of the Pacific Plate under the Philippine Plate. The Pacific Plate and its sediment cover are exposed to increasing temperatures as they are subducted. The Yinazao, Fantangisña, and Asùt Tesoru mud volcanoes are shown in blue due to the distinct blue color of the dominant lithology, upwelling serpentinite mud. Investigating how SCOA compositions, and origins (microbial, thermogenic, abiotic) change both horizonally and vertically between and within these mud volcanoes provides insights into the controls on carbon transformation reactions in the deep subseafloor.

All three mud volcanoes are formed as fluids, liberated during the subduction of both sediments and crustal rock from the Pacific plate, hydrate the overlying plate's mantle and drive serpentinization, whereby mud is produced by rock-alteration and breakdown, and the pH of fluids becomes highly alkaline (Mottl et al., 2003; Fryer, 2012). Due to volume expansion and corresponding density changes, this mud wells up via conduits, which are likely related to fault intersections, to the seafloor (Früh-Green et al., 2004), where it forms serpentinite mud volcanoes with heights and diameters of multiple kilometers (Fryer et al., 2018b). Maximum vertical mud flow velocities at Yinazao and Asùt Tesoru have been estimated to be 10.3 and 36.3 cm yr⁻¹, respectively (Hulme et al., 2010; no data for Fantangisña), which correspond to ascent times of ~130,000 (Yinazao) and ~50,000 years (Asùt Tesoru) from the décollement to the seafloor. Detailed site data from IODP Expedition 366 are publicly available on the IODP homepage (Fryer et al., 2018b).