Shallow Seafloor Gas emissions Near Heard and McDonald Islands on the Kerguelen Plateau, Southern Indian Ocean

Abstract

Bubble emission mechanisms from submerged large igneous provinces remains enigmatic. The Kerguelen Plateau, a large igneous province in the southern Indian Ocean, has a long sustained history of active volcanism and glacial/interglacial cycles of sedimentation, both of which may cause seafloor bubble production. We present the results of hydroacoustic flare observations around the underexplored volcanically active Heard Island and McDonald Islands on the Central Kerguelen Plateau. Flares were observed with a split‐beam echosounder and characterized using multifrequency decibel differencing. Deep‐tow camera footage, water properties, water column δ3He, subbottom profile, and sediment δ13C and δ34S data were analyzed to consider flare mechanisms. Excess δ3He near McDonald Islands seeps, indicating mantle‐derived input, suggests proximal hydrothermal activity; McDonald Islands flares may thus indicate CO2, methane, and other minor gas bubbles associated with shallow diffuse hydrothermal venting. The Heard Island seep environment, with subbottom acoustic blanking in thick sediment, muted 3He signal, and δ13C and δ34S fractionation factors, suggest that Heard Island seeps may either be methane gas (possibly both shallow biogenic methane and deeper‐sourced thermogenic methane related to geothermal heat from onshore volcanism) or a combination of methane and CO2, such as seen in sediment‐hosted geothermal systems. These data provide the first evidence of submarine gas escape on the Central Kerguelen Plateau and expand our understanding of seafloor processes and carbon cycling in the data‐poor southern Indian Ocean. Extensive sedimentation of the Kerguelen Plateau and additional zones of submarine volcanic activity mean additional seeps or vents may lie outside the small survey area proximal to the islands.

Glacial and Hydrothermal Sources of Dissolved Iron (II) in Southern Ocean Waters Surrounding Heard and McDonald Islands

“…Sampling station locations in the HIMI region were selected based on bathymetric features (Watson et al, 2016) and acoustic flare signals detected with shipboard echosounders (Spain et al, 2019) suggesting potential hydrothermal activity (see section 3.1). Acoustic flare signals had distinct characteristics when caused by seafloor gas seepage (Spain et al, 2019).…”

“…The other three stations of the crosshair (Lupton et al, 2017). Acoustic flares, detected by the shipboard echosounder, were observed at both Heard and McDonald Islands (Spain et al, 2019). This may suggest that while there were stronger glacial sources of DFe(II) at Heard Island, hydrothermalism may have contributed to elevated background concentrations observed around both islands, along with sedimentary and other sources (discussed below).…”

“…Moved up [1]: compared to 1.7% close to station 37, near 468 the northeast of Heard Island (Lupton et al, 2017). Acoustic 469 flares, detected by the shipboard echosounder, were observed 470 at both Heard and McDonald Islands(Spain et al, 2019).…”

High Lability Fe Particles Sourced From Glacial Erosion Can Meet Previously Unaccounted Biological Demand: Heard Island, Southern Ocean

“…Conversely, suspended particles are generally lower at depth near Heard Island (Figure 10A). Although seafloor sediment resuspension would be a likely candidate for this increase with depth at McDonald, sub-bottom profile images suggest that only a thin sediment veneer is present at McDonald Island (Spain et al, 2018). It is possible that diffuse venting and buoyant bubble plumes are aiding resuspension of seafloor sediments and direct supply of vent precipitates near the seafloor in the region.…”

“…Although over 200 bubble plumes were found around HIMI, they were all low energy, with no detectable heat signature. Spain et al (2018) suggested that bubble plumes emanating from thick seafloor sediments near Heard Island, as evidenced from the sub bottom profiler images, and gas pockets within the sediments, implied methanogenesis, as has been observed at South Georgia (Römer et al, 2014). Conversely, at McDonald Island, bubble plumes were observed emanating from areas with a thin (<5 m) seafloor sediment veneer, and sometimes almost solid bedrock with domed morphologies (Watson et al, 2016) similar to what has been observed in shallow gasohydrothermal fields in Naples Harbour, Italy (Passaro et al, 2016).…”