<p>The Toba volcano super-eruption on the island of Sumatra occurred about 74,000 years ago<sup>[1]</sup>, close to the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, called Youngest Toba Tuff (YTT), is currently described as the largest cataclysmic eruption of the Quaternary. However, the impact of this super-eruption on climate is widely debated and its effects on the ocean remains poorly understood.</p><p>The aim of this work is to estimate its impact on oceanic pH at a site near the eruption center. To do so, we measured &#948;<sup>11</sup>B values (pH proxy) on monospecific samples of planktonic foraminifera <em>Globigerinoides ruber</em> and <em>Pulleniatina obliquiloculata</em> from sediment core BAR94-25 (Andaman Sea) using a recently developed method at the Institut de Physique du Globe de Paris (IPGP)<sup>[2]</sup>. <em>G. ruber</em> is a species that thrives preferentially in surface waters, while <em>P. obliquiloculata</em> lives<em> </em>at the thermocline. Therefore, &#948;<sup>11</sup>B measurements on their shells can reconstruct pH variations in surface and thermocline waters, respectively.</p><p>We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. This interval contains two clearly visible tephra layers corresponding to the YTT, at the transition from MIS 5 to MIS 4, and to a post-YTT explosive activity during MIS 4. These layers are correlated with a significant decrease in carbonate content (CaCO<sub>3</sub>). Our results indicate a complex pH response during the two concerned volcanic episodes. Thermocline seawater doesn&#8217;t show significant pH decrease during the volcanic episodes compared to the overall signal recorded throughout the studied interval. Conversely, surface seawater shows a much more important pH decrease during part of the volcanic episodes than during the all studied interval. Such decrease in pH during the transition to a glacial state is particularly surprising because an increase in pH, due to the global reduction in atmospheric CO<sub>2</sub>, is rather expected, as shown by previous foraminifera &#948;<sup>11</sup>B&#160;records<sup>[3]</sup>.</p><p>The coupling of CaCO<sub>3</sub> and pH decrease during tephra levels suggests acidification in the Andaman Sea as a consequence of the Toba volcanic eruptive activity. The seawater surface seems much more sensitive to pH changes than the thermocline zone. However, the reduction of carbonate in the two tephra layers may also be due to dilution from ash falling into the sediment. Other analyses, such as measuring the variation of calcification intensity in planktonic foraminifera, are therefore necessary to better interpret these paleo-pH data.</p><p>[1] Storey et al., 2012, <em>PNAS</em>, 109 (46), 18684-18688</p><p>[2] Buisson et al., 2021, <em>JAAS</em>, 36, 2116-2131</p><p>[3] Foster et al., 2008, <em>EPSL</em>, 271, 254-266</p>
Abstract The Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.
Dataset for "Seamount subduction and accretion dynamics of the Franciscan complex"by Bonnet et al. - D1: Sample location, description and mineralogy - D2 : Bulk rock analyses - D3 : EPMA data - D4 : Synthesis of temperatures from Raman Spectroscopy of Carbonaceous Material - D5 : Titanite U-Pb-trace element data - D6 : Zircon U-Pb-trace element data
Dataset for "Seamount subduction and accretion dynamics of the Franciscan complex"by Bonnet et al. - D1: Sample location, description and mineralogy - D2 : Bulk rock analyses - D3 : EPMA data - D4 : Synthesis of temperatures from Raman Spectroscopy of Carbonaceous Material - D5 : Titanite U-Pb-trace element data - D6 : Zircon U-Pb-trace element data
Abstract IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island‐arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low‐gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat‐lying block of intact strata. The most likely mechanism for generating these large‐scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block‐rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high‐resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.
