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A new hydrothermal vent site in the Southern Mariana Trough has been discovered using acoustic and magnetic surveys conducted by the Japan Agency for Marine-Earth Science and Technology's (JAMSTEC) autonomous underwater vehicle (AUV), Urashima . The high-resolution magnetic survey, part of a near-bottom geophysical mapping around a previously known hydrothermal vent site, the Pika site, during the YK09-08 cruise in June–July 2009, found that a clear magnetization low extends ∼500 m north from the Pika site. Acoustic signals, suggesting hydrothermal plumes, and 10 m-scale chimney-like topographic highs were detected within this low magnetization zone by a 120 kHz side-scan sonar and a 400 kHz multibeam echo sounder. In order to confirm the seafloor sources of the geophysical signals, seafloor observations were carried out using the deep-sea manned submersible Shinkai 6500 during the YK 10-10 cruise in August 2010. This discovered a new hydrothermal vent site (12°55.30′N, 143°38.89′E; at a depth of 2922 m), which we have named the Urashima site. This hydrothermal vent site covers an area of approximately 300 m×300 m and consists of black and clear smoker chimneys, brownish-colored shimmering chimneys, and inactive chimneys. All of the fluids sampled from the Urashima and Pika sites have chlorinity greater than local ambient seawater, suggesting subseafloor phase separation or leaching from rocks in the hydrothermal reaction zone. End-member compositions of the Urashima and Pika fluids suggest that fluids from two different sources feed the two sites, even though they are located on the same knoll and separated by only ∼500 m. We demonstrate that investigations on hydrothermal vent sites located in close proximity to one another can provide important insights into subseafloor hydrothermal fluid flow, and also that, while such hydrothermal sites are difficult to detect by conventional plume survey methods, high-resolution underwater geophysical surveys provide an effective means.

We analyzed bottom pressure recorder (BPR) data obtained by the Japanese Antarctic Research Expedition (JARE) from 4500 m depth off Lutzow-Holm Bay (66°50′S, 37°50′E) in the Antarctic Divergence Zone (ADZ). Data collected between December 16, 2004 and February 22, 2008 were processed after removing the tidal constituents with periods shorter than 1 month. The time series of monthly mean equivalent water height (OBP height) has a clear annual period. We correlated monthly GRACE Tellus OBP data with the JARE BPR heights. The GRACE data can account for about 38% of the variance of ocean signal observed by the JARE BPR. The variability in OBP height within the ADZ shows a maximum (2–4 cm) in the austral summer and a minimum (−2 to −4 cm) in the austral winter. In contrast, data from the coastal Syowa Station Tide Gauge (Syowa TG; 69.0°S, 39.6°E) have a maximum (7–8 cm) at the austral winter and a minimum (−5 to −6 cm) at the austral summer (opposite phase). We explained these OBP height variations in terms of the Ekman divergence mechanism using the wind velocity grids (2.5°×2.5°) of the NCEP/NCAR Reanalysis data. The monthly average of the vertical velocity (Ekman pumping) in the area of 65–67.5°S, 35–40°E is positive (upwelling) in all months, but varies seasonally, with its maximum in the austral winter and its minimum in the austral summer. This annual property is consistent with the variable differences observed between the JARE BPR and Syowa TG OBP heights. We subtracted this annual variation from the OBP monthly time series to obtain the non-seasonal OBP variability, and examined its correlation with the AAO index. The non-seasonal OBP variability of Syowa TG shows similar correlation strength (the coefficients of −0.50 to −0.52) for the short period (