Nathaniel J. Buck

Hydrothermal cooling along the Eastern Lau Spreading Center: No evidence for discharge beyond the neovolcanic zone

Heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries, as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades. Recent observations and models are now challenging this assumption of exclusively axis-centric high-temperature venting. One example is the proposal that intense cooling along the vertical boundaries of a broad low-velocity volume (LVV) of hot crust could generate high-temperature fluids several kilometers off axis. To test the hypothesis that substantial hydrothermal discharge might occur beyond the LVV, we conducted a dense survey grid of the ridge and surrounding seafloor (up to ±5 km) along 175 km of the Eastern Lau Spreading Center and Valu Fa Ridge (∼1800 km of track line). Our sampling array extended from ∼50 to 400 m above bottom and included light-scattering, oxidation-reduction potential, and hydrographic sensors attached to the tow line and beneath the IMI120 sonar mapping system. The surveys successfully mapped plumes from several vent fields in the neovolcanic zone (∼±1.5 km about the axis) but did not detect evidence of significant discharge anywhere farther off-axis. At a few locations on the Valu Fa Ridge, however, we did record oxidation-reduction potential anomalies with hydrographic density signatures that imply low-temperature hydrothermal sources on the axial flank. Although these sites are hundreds of meters deeper than the adjacent crest, they are above, not beyond, the previously mapped LVV. Our results thus do not support a simple picture of high-temperature fluids ascending undiluted through the crust to the seafloor several kilometers off-axis. However, we cannot exclude the possibilities that the largely unmapped LVV is narrower here than seen on other ridges, that hydrothermal fluids formed beyond the LVV are channeled to the axis, or that discharge beyond the neovolcanic zone occurs only as dispersed, very low-temperature fluids. Our observations do demonstrate that high-temperature discharge predominantly exits the seafloor within a narrow (∼±1.5 km) axial ribbon, regardless of the presence or absence of an axial magma chamber.

Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic

Recent analyses suggest that considerable CaCO3 dissolution may occur in the upper water column of the ocean (< 1500 m). This study uses the distribution of particulate calcium from high-resolution suspended matter sampling along the Climate Variability and Predictability/CO2 Repeat Hydrography A16N transect in 2003 to estimate CaCO3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO3 dissolution rate in this region is estimated to be 0.9 mmol CaCO3 m−2 d−1, indicating this region is a hotspot for upper water column CaCO3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO3 m−2 d−1) and are comparable to previous estimates of CaCO3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface ocean CaCO3 standing stocks suggests that biologically mediated CaCO3 dissolution may be occurring in the top 1000 m of the Atlantic.

Imaging of CO2 bubble plumes above an erupting submarine volcano, NW Rota‐1, Mariana Arc

NW Rota-1 is a submarine volcano in the Mariana volcanic arc located ∼100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ∼560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 midwater data set shows: (1) bubble plumes were present on every pass over the summit and they rose 200–400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ships acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that midwater multibeam sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.