{"id":17426,"date":"2023-05-22T15:18:29","date_gmt":"2023-05-22T22:18:29","guid":{"rendered":"https:\/\/www.darkenergybiosphere.org\/?post_type=publication&#038;p=17426"},"modified":"2023-05-22T15:18:29","modified_gmt":"2023-05-22T22:18:29","slug":"chemistry-of-springs-across-the-mariana-forearc-carbon-flux-from-the-subducting-plate-triggered-by-the-lawsonite-to-epidote-transition","status":"publish","type":"publication","link":"https:\/\/www.darkenergybiosphere.org\/publication\/chemistry-of-springs-across-the-mariana-forearc-carbon-flux-from-the-subducting-plate-triggered-by-the-lawsonite-to-epidote-transition\/","title":{"rendered":"Chemistry of springs across the Mariana forearc: Carbon flux from the subducting plate triggered by the lawsonite-to-epidote transition?"},"content":{"rendered":"<div id=\"abstracts\" class=\"Abstracts u-font-gulliver text-s\">\n<div id=\"ab005\" class=\"abstract author\">\n<div id=\"as005\">\n<p id=\"sp0005\">Low-chlorinity springs sampled from ten sites on nine\u00a0serpentinite\u00a0mud volcanoes show systematic chemical gradients across the outer Mariana forearc that result from progressive devolatilization of the subducting\u00a0Pacific plate. Sites range from 50 to 90\u00a0km from the trench axis corresponding to depths to the top of the plate of \u223c15\u201329\u00a0km. Dissolved sulfate, Na\/Cl, K, Rb, Cs, and B in the springs all increase regularly with distance from the trench, leached from subducting sediment and altered basalt in response to increasing temperature at depth from \u223c80 to 350\u00a0\u00b0C. Sites nearer the trench have high Ca (up to 76\u00a0mmol\/kg) and Sr, low alkalinity, and pH 10.7, whereas sites farther from the trench have almost no Ca and Sr, alkalinity (some carbonate but mostly hydroxyl) as high as 69\u00a0meq\/kg, and pH 12.5. Springs with high alkalinity also have high methane (&gt;44\u00a0mmol\/kg) that feeds sulfate-reducing archaeal communities in the shallow subsurface and\u00a0macrofauna\u00a0at the seafloor. These distal springs form chimneys and crusts of CaCO<sub>3<\/sub>, whereas the proximal springs form chimneys of\u00a0brucite. High alkalinity at the distal sites apparently results from carbonate dissolution at the top of the subducting plate; because\u00a0serpentinization\u00a0during ascent generates both high pH and H<sub>2<\/sub>, the resulting dissolved carbonate is reduced to methane such that carbonate alkalinity is replaced by hydroxyl alkalinity: 4H<sub>2<\/sub>\u00a0+\u00a0HCO<sub>3<\/sub><sup>\u2013<\/sup>\u00a0\u2192\u00a0CH<sub>4<\/sub>\u00a0+\u00a02H<sub>2<\/sub>O\u00a0+\u00a0OH<sup>\u2013<\/sup>. This reaction can account for the much higher pH of the distal springs. Chlorinity of the springs varies from 234 to 546\u00a0mmol\/kg and is related to latitude N\u2013S rather than distance from the trench. Distal springs have otherwise similar compositions over this entire range of chlorinity, implying that chloride derives from depth rather than from mixing with seawater within the seamounts themselves. The range in chlorinity can readily be explained by serpentinization at reasonable water\/rock mass ratios of 0.2\u20131.0 if 30\u201340% of the spring water originates as residual\u00a0pore water\u00a0in subducted sediment and basalt rather than as H<sub>2<\/sub>O<sup>+<\/sup>\u00a0of dehydration.<\/p>\n<p id=\"sp0010\">The pH, alkalinity, and methane content of the springs increase abruptly, while Ca and Sr decrease, because carbonate dissolution joins dehydration as a major process at the top of the subducting plate at \u223c70\u00a0km from the trench, where\u00a0metabasites\u00a0recovered from the serpentinite mud indicate the transition from\u00a0lawsonite\u00a0blueschist facies to epidote blueschist facies also occurs. Replacement of lawsonite by epidote drastically depletes the solution in Ca and shifts the equilibrium toward massive dissolution of subducted carbonate. Fluxes of sulfate, C, Na, K, Rb, Cs, B, Ca, and Sr in the forearc springs represent only a few percent of the amounts subducted, consistent with continued supply at greater depth.<\/p>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"template":"","tags":[],"publication_type":[19],"_links":{"self":[{"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/publication\/17426"}],"collection":[{"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/publication"}],"about":[{"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/types\/publication"}],"wp:attachment":[{"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/media?parent=17426"}],"wp:term":[{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/tags?post=17426"},{"taxonomy":"publication_type","embeddable":true,"href":"https:\/\/www.darkenergybiosphere.org\/wp-json\/wp\/v2\/publication_type?post=17426"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}