Vent locations and temporal changes in hydrothermal venting

formed by surficial lava morphology, defines the northern edge of the Roman Ruins field.

A volcanic rise north of the trench shows thick sediment cover and a few small inactive chimneys and oxide mounds near its top. The Roger’s Ruins hydrothermal field lies at the foot of the northwestern slope of that hill (Fig. 2.9).

2.4.5.2 Roger’s Ruins

Roger’s Ruins is located about 35 m deeper than Roman Ruins on the northern flank of Pual Ridge in 1710 m water depth (Fig. 2.9). The total area covered by chimneys is

~319 m² with a small area of diffuse discharge ~70 m². Roger’s Ruins is separated from Roman Ruins by the volcanic hill described above and consists of a main hydrothermal active sulphide chimney complex, a small, inactive chimney group and an area with oxide deposits on a small topographic high. The main field (Tmax: 320°C in 2006) can be subdivided into one large and one small cluster of smokers. The small cluster is characterised by numerous active chimneys that are highly branched, in contrast to the larger cluster that is composed of mostly inactive, columnar chimneys (max. 9 m high) with diffuse fluid discharge on their base where oxides have accumulated. In addition, diffuse fluid discharge was observed east of the main complex in a small, ~8 m long zone populated with small oxide mounds. The immediate northern and western slope of Rogers Ruins is covered by sulphide and volcanic talus.

2.4.5.3 Solwara 7

Active hydrothermal vent site, Solwara 7, discovered by Nautilus Minerals at a depth of ~1800 m is located just outside the ABE micro-bathymetry map area (Fig. 2.4 and 9). The vent field lies downslope of Roger’s Ruins where the seafloor is dominated by blocky lava flows covered by a thick sediment cover with a few mega pillow features that emerge at the base of lava flow lobes. Vigorous fluid flow through a few ~12 m high chimneys were observed in 2011 by ROV MARUM Quest with a Tmax of 348°C. Several old discharge sites in the closer vicinity to Solwara 7 are marked by collapsed and sediment-covered inactive chimneys.

2.4.6.1 Tsukushi

Active venting of grey to black fluids was observed at Tsukushi during a Shinkai-2000 dive in 1996 with Tmax. 268°C (Hashimoto et al., 1999). We did not observe any active chimneys in 2006 or 2011. Diffuse venting through the bases of some inactive chimney structures was observed in 2006, and no venting at all in the chimney area in 2011. We conclude that focused hydrothermal venting has been waning since 1996 and ceased altogether between 2006 and 2011. In contrast, diffuse venting through oxide mounds west of the chimney field (Fig. 2.6a) has been continuous between 1996 and 2011, and constant maximum temperatures of venting of around 60°C were measured in 2006 and 2011.

2.4.6.2 Snowcap

Hydrothermal activity of the vent field at the foot of the west slope of Snowcap appears to have decreased in the recent past as patches of dead, unsedimented gastropods occur next to sediment free, small (< 1m), inactive chimneys.

To the southwest, the vent field is bordered by an area characterised by slabs of native sulphur. Sulphur accumulations were seen in other locations in the eastern Manus Basin, such as at Desmos (Gamo et al., 1997; Gena et al., 2001), and North Su (Tivey et al., 2006; Bach et al., 2012). Sulphur vents are also common in hydrothermal systems in submarine island arc volcanoes (Butterfield et al., 2011). These sulphur deposits form from disproportionation of SO2 delivered to the seafloor by degassing magmatic fluids that mix with seawater-derived fluids (e.g. Gamo et al., 1997). Sulphuric acid-rich vent fluids form in the process. The Snowcap vent fluids in 2006 were not sulphuric acid-type fluids (Reeves et al., 2011), indicating that the sulphur slabs must have formed during an earlier stage, when the magmatic SO2 flux was higher. Abundant native sulphur as breccia cement in samples collected from the Snowcap dome indicate that discharge of SO2 must have been pervasive during this earlier stage. This is consistent with the presence of pyrophyllite-rich alteration assemblages in the drill core recovered from Snowcap (Paulick and Bach, 2006; Binns et al., 2007).

2.4.6.3 Fenway

Hashimoto et al. (1999) mentioned the existence of diffuse venting at the location of Fenway but the main chimney sites of Fenway were not discovered until 2006. It is uncertain if the main focused venting through chimneys existed in 1996. In 2006, the hydrothermal activity at Fenway was very intense, with Big Papi being the most active

hydrothermal vent complex of the entire PACManus Hydrothermal District featuring the highest measured fluid temperature of 358°C corresponding to the boiling temperature of seawater at 172 bar (i.e. ~1710 m water depth) (Reeves et al., 2011). In 2011, focused venting of black smoker fluids had decreased dramatically and fewer anhydrite blocks and more sulphide debris was observed. The highest temperature measured in 2011 at Big Papi was 304°C. The bordering parapet did not show any fluid flow in 2006. In 2011, however, vigorous diffuse venting of ~90°C hot fluid was observed in the entire area of the parapet. The activity in the smaller chimney clusters around Big Papi and in the large field of diffuse venting in the north apparently have not changed much between 2006 and 2011.

2.4.6.4 Satanic Mills

The intensity of fluid venting has not changed much at Satanic Mills between 1999 and 2011. Maximum vent temperatures have increased from 280-290°C in 2006 to 335°C in 2011. CO2 bubbling observed in 2011, indicate that the fluids are at least as rich in CO2

as they were in 2006 (>200 mM CO2; Reeves et al., 2011).

2.4.6.5 Roman Ruins

Roman Ruins was discovered by Binns and Scott (1993) and was revisited by (Hashimoto et al., 1999) but detailed information about the fluid composition and temperature were not reported. In 2006 the highest fluid temperature of Roman Ruins (341°C) was measured in the chimney wall area, which was vigorously venting and creating expansive black plumes in the water column. Within the entire PACManus Hydrothermal District, the intensity of hydrothermal activity at Roman Ruins was only surpassed by Big Papi at Fenway in 2006. The orifices of the Roman Ruins sulphide wall were not inspected in 2011, but extensive black smoke constrained ROV work in the immediate environment, so it is likely that the vigour of hydrothermal venting had not subsided much over the period between 2006 and 2011. The activity of small chimneys scattered around the Roman Ruins area does seem to have decreased since 2006 however.

2.3.6.6 Roger’s Ruins

Roger’s Ruins vents are considered part of the Roman Ruins hydrothermal field despite the ~100 m wide area without hydrothermal activity between them. The fluid compositions of both fields are somewhat similar in that they are thought to have the same source with Roger’s Ruins fluids showing a higher amount of seawater mixing (Reeves et al., 2011). Part of the Roman Ruins fluids may be being diverted underground

to Roger’s Ruins as it is situated directly at the foot of a volcanic high that is prominent for the area between the two hydrothermal fields. In 2011, hydrothermal activity at Roger’s Ruins main cluster had declined and only diffuse venting at the base of the chimneys could be observed. A small cluster of chimneys characterised by highly branched tubes emitting large amounts of black fluids with temperatures up to 320°C was sampled in 2006 (Reeves et al., 2011) and had no discernible change in activity between 2006 und 2011.