[1] Centimeter-scale mineralogical and chemical analyses of a diffusely venting, $280°C sulfide structure (called Roane) from the Mothra Hydrothermal Field on the Juan de Fuca Ridge reveal a complex growth history. These analyses document four well-defined zones, which from the exterior to the interior of the structure include a barite-silica zone (Fossil Worm Tube Zone), a silica-sulfide zone (Silica Zone), a sulfide-silica zone (Outer Sulfide Zone), and a sulfide ± gordaite ± silica zone (Inner Sulfide Zone). These features are a product of a myriad of processes that include extensive mineral replacement, ingress of seawater, fracturing and breakout of hot hydrothermal fluids, significant conductive cooling, and the sustained, broad-scale outflow of warm fluids from the walls through a porous and permeable matrix.Roane lacks an open, throughgoing, chalcopyrite-lined, central conduit. Instead, it hosts an anastomosing, discontinuous network of tortuous channels within the interior of a sponge-like matrix of amorphous silica, sulfide, sulfate, and clay minerals. Megafaunal communities at the summit and sides of this mature, diffusely venting chimney provide a constructional framework for new growth. Isolation of hydrothermal fluid from seawater during progressive fossilization of megafauna and the early formation of barite-silica assemblages promote flow of higher-temperature fluids within the interior. Continued isolation of interior higher-temperature fluids, through increased mineral precipitation and expansion of the structure, leads to a Zn sulfide + pyrite-dominated permeable matrix and a network of tortuous channels that form a central porous conduit. This conduit is shielded by a weakly silicified, metal sulfide inner wall and a strongly silicified outer wall. The most outer portions of the walls are highly porous and sites of significant advection and mixing between hydrothermal fluids and seawater. The mineralogy and chemistry of Roane are strongly affected by ammonia-ammonium buffering of the pH during cooling of the hydrothermal fluids. The two major results of the buffering are (1) precipitation of Zn sulfide at higher temperatures than are typical for bare-rock systems, leading to the correlation of Cu, Zn, Cd, and Se and (2) increased deposition of amorphous silica and clay minerals.

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Early Cretaceous (145-100 Ma) rocks record a ~5‰ negative shift in the sulfur isotope composition of marine sulfate, the largest shift observed over the past 130 m.y. Two hypotheses have been proposed to explain this shift: (1) massive evaporite deposition associated with rifting during opening of the South Atlantic and (2) increased inputs of volcanically derived sulfur due to eruption of large igneous provinces. Each process produces a very different impact on marine sulfate concentrations, which in turn affects several biogeochemical phenomena that regulate the global carbon cycle and climate. Here we present sulfur isotope data from Resolution Guyot, Mid-Pacific Mountains (North Pacific Ocean), that track sympathetically with strontium isotope records through the ~5‰ negative sulfur isotope shift. We employ a linked sulfur-strontium isotope mass-balance model to identify the mechanisms driving the sulfur isotope evolution of the Cretaceous ocean. The model only reproduces the coupled negative sulfur and strontium isotope shifts when both hydrothermal and weathering fluxes increase. Our results indicate that marine sulfate concentrations increased significantly during the negative sulfur isotope shift and that enhanced hydrothermal and weathering input fluxes to the ocean played a dominant role in regulating the marine sulfur cycle and CO 2 exchange in the atmosphere-ocean system during this interval of rapid biogeochemical change.*

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While uniformitarianism underlies many aspects of the Earth sciences, modeling geochemical cycles throughout Earth history requires critically assessing the reliability and constancy of projecting modern fluxes and their isotope ratios into deep geologic time. In this study, we develop an iterative technique for modeling the radiogenic strontium isotope composition (R 87 = 87 Sr/ 86 Sr) of seawater (sw) during the Phanerozoic utilizing as a constraint the