Description: Despite the important role that volcanogenic aluminosilicate (VA) alteration has on elemental cycles in marine sediments, there is no mechanism to arrive at a global assessment of this process. To quantify the VA alteration rates from Japan, New Zealand (NZ), and Costa Rica, we developed a mass balance approach that is constrained by the strontium concentration and 87Sr/86Sr ratios in pore fluid, authigenic carbonates, and VA. We derived VA alteration rates ranging from 101 to 103 nmole Sr/m3 bulk sediment/yr with the highest rate obtained for Tuaheni, NZ (Site GeoB80202), which has the youngest sediment. We showed that 87Sr/86Sr ratios of VA derived from this mass balance approach are significantly higher than the reported ratios from volcanic glass samples, indicating a concomitant role of volcanogenic feldspar dissolution and/or authigenic clay formation. Most of the strontium released during VA alteration is precipitated as authigenic carbonate, with important implications for carbon inventories. The VA alteration rates derived from this approach can also be used to quantify the release of other critical elements, such as release of iron that can stimulate formation of Fe‑carbonates and/or fuel microbial activity at depth.

Description: Highlights • A saline formation fluid originated from great depths was documented. • Gas hydrates are currently at a dynamic equilibrium due to the low methane flux. • Fluids were diverted by the buried seep carbonates in Lunde pockmark. Abstract Seafloor seepage sites along the Vestnesa Ridge off west-Svalbard have been, for decades, a natural laboratory for the studies of fluid flow and gas hydrate dynamics at passive continental margins. The lack of ground truth evidence for fluid composition and gas hydrate abundance deep in the sediment sequence however prohibits us from further assessing the current model of pockmark evolution from the region. A MARUM-MeBo 70 drilling cruise in 2016 aims to advance our understanding of the system by recovering sediments tens of meters below seafloor from two active pockmarks along Vestnesa Ridge. We report pore fluid composition data focusing on dissolved chloride, stable isotopes of water (δ18O and δD), and the isotopic composition of dissolved boron (δ11B). We detect a saline formation water around two layers where gas hydrates were recovered from one of the seepage sites. This saline formation pore fluid is characterized by elevated chloride concentrations (up to 616 mM), high B/Cl ratios (9×10-4 mol/mol), high δ18O and δD isotopic signatures (+0.6 ‰ and +3.8 ‰, respectively) and low δ11B signatures (+35.0 ‰), which collectively hint to a high temperature modification at great depths. Based on the dissolved chloride concentration profiles, we estimated up to 47 % of pore space occupied by gas hydrate in the sediments shallower than 11.5 mbsf. The observation of bubble fabric in the recovered gas hydrates suggests formation during past periods of intensive gaseous methane seepage. The presence of these gas hydrates without associated positive anomalies in dissolved chloride concentrations however suggests that the decomposition of gas hydrate is as fast as its formation. Such a state of gas hydrates can be attributed to a relatively low methane supply transported by the saline formation water at present. Our findings based on pore fluid composition corroborate previous inferences along Vestnesa Ridge that fluids sustaining seepage have migrated from great depths and that the variable gaseous and aqueous phases through the gas hydrate stability zone controls the distributions of authigenic carbonates and gas hydrates.

Description: Alteration of volcanogenic aluminosilicates (VAs) in marine sediments is recognized as critical in regulating geochemical cycles and sustaining the oceanic deep biosphere, but rates of VA alteration and its associated authigenic mineral formation are not commonly reported. Here we present results on analyses of sediments and pore water recovered from the upper 150 mbsf of four sites drilled on the northern Hikurangi margin during IODP Expeditions 372 and 375. Petrographic analyses show that volcanogenic materials (glass shards, feldspar, volcanic lithoclasts) constitute important components (15–45 wt%) of the hemipelagic mud, and reveal ongoing glass alteration with accompanying authigenic phase formation. A reaction-transport model constrained by pore water Sr, 87Sr/86Sr, Ca, Mg, and Si was applied to simulate VA diagenetic reactions. Our model results yield VA alteration rates of 0.047–0.64 mmol Sr m−2 yr−1, with substantially higher values at Sites U1517 and U1520 that experienced rapid sediment emplacement. In addition, our simulations show that 〉99% of the dissolved Si generated by VA alteration is fixed in silica cement and authigenic clay, and that ∼50% of Ca incorporated in the authigenic carbonate is supplied by VA alteration. First-order estimates suggest that, in addition to authigenic carbonate precipitation, authigenic clay formation may represent an important sink for dissolved Mg. This study quantitatively examines the linkage between VA alteration and formation of authigenic phases, highlights its role in subsurface geochemical cycles, and indicates that slope instability may play an important role in promoting VA diagenesis.