Description: A geochemical investigation was carried out on two sediment cores collected at 2 and 5 km from the Rainbow hydrothermal vent site. Bulk sediment compositions indicate that these cores record clear enrichments in Fe, Cu, Mn, V, P and As from hydrothermal plume fallout (Cave et al., 2002, doi:10.1016/S0016-7037(02)00823-2). Sequential dissolution of the bulk sediments has been used to discriminate between a leach (biogenic and oxy-hydroxide) component and a residual phase (detrital and sulphide/sulphate fractions). Major element data (Al, Fe, Ti, Mn, Mg, Ca, Si and index%) reveal that the hydrothermal input, as recorded in the leach phase, is much stronger than apparent from bulk sediment analyses alone. REE patterns for the leach phase record contributions from both biogenic carbonate (mimicking seawater REE patterns) and hydrothermal oxy-hydroxides, with the latter exhibiting positive Eu anomalies (hydrothermal derived) and negative Ce anomalies (seawater derived). Based on major element and REE data, the residue contains contributions from aeolian dust input, local MORB material and a hydrothermal component. Ternary REE mixing calculations indicate that most of the REE within the residual fraction (~80%) is derived from hydrothermal material, while detrital contributions to the REE budget, as deep-sea clay and volcanic debris, are 〈20%. By combining bulk and REE data for the various end-member components of the residue, we calculate that the chemical composition of the residue hydrothermal end-member is high in Ca (6-15%) and with a Nd/Sr ratio of 0.004. These characteristics indicate the presence of low-solubility hydrothermal sulphate (rather than sulphide) material within the residue component of Rainbow hydrothermal sediments.

Description: Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) 〉99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.

Description: New major and trace element analyses of Ocean Drilling Program Leg 170 and 205 mafic igneous samples are presented along with Sr and Nd isotopic results. Samples were cored from a sill unit (Subunit 4A) and a lower igneous unit (Subunit 4B) whose lower boundary was not drilled. The samples are dominantly microcrystalline to medium-grained glomerocrystic plagioclase-clinopyroxene gabbro in a nearly holocrystalline groundmass. Samples are dominantly low-K subalkaline tholeiites with trace element systematics that correlate with unit stratigraphy. Using isotopic data, trace element ratios little affected by fractionation, and abundance data from the least fractionated samples, it is possible to model their mantle sources and subsequent igneous processing. Melting models suggest that small differences in degrees of partial melting can generate the two geochemical groups identified within the igneous complex. Crystal fractionation and accumulation have further modified the melt compositions, particularly within the lower stratigraphic unit. Mixing models indicate that these units are derived from a mantle source that is 50%-70% enriched compared to depleted mid-ocean-ridge source mantle. In the context of the complex regional tectonic and volcanic history of the Cocos plate, this igneous complex likely represents melts of depleted upper mantle that were previously enriched during transit near the Galápagos hotspot. A postenrichment change in the local-regional tectonic stress regime may have triggered decompression melting and emplacement of the igneous complex far from any active spreading center or plume. Preexisting plate fractures, ridge jumps, and abandoned spreading centers may have facilitated distal reach of plume-overprinted material.

Description: Bulk geochemical analyses were carried out on 27 sediment samples located just above the igneous section of the incoming plate along the Costa Rican convergent margin. This pilot study was intended to determine whether hydrothermal activity within the sediment section could be detected within the bulk sediment. The metaliferous index (100 x Al/[Al + Fe + Mn]) values on bulk sediment (24.3 〈 x 〈 43.3) are all much lower than the value for deep pelagic sediment (50.8) but higher than the value for the hydrothermal sulfide from the East Pacific Rise (1). The high Zn, Cu, and Co concentrations associated with high Ba content for a constant detrital input (molar Ti/Al = 0.121 ± 0.026) indicate that the transition metal enrichment is not related to volcanic ash but rather to Fe-Mn oxides, sulfides, and barite. These observations testify to a record of hydrothermal activity within the 70 m of sediment overlying the basement.

Description: Oceanic dissolved organic carbon (DOC) is an important carbon pool, similar in magnitude to atmospheric CO2, but the fate of its oldest forms is not well understood (Dittmar and Stubbins, 2014; Hansell, 2013, doi:10.1146/annurev-marine-120710-100757). Hot hydrothermal circulation may facilitate the degradation of otherwise un-reactive dissolved organic matter, playing an important role in the long-term global carbon cycle. The oldest, most recalcitrant forms of DOC, which make up most of oceanic DOC, can be recovered by solid-phase extraction. Here we present measurements of solid-phase extractable DOC from samples collected between 2009 and 2013 at seven vent sites in the Atlantic, Pacific and Southern oceans, along with magnesium concentrations, a conservative tracer of water circulation through hydrothermal systems. We find that magnesium and solid-phase extractable DOC concentrations are correlated, suggesting that solid-phase extractable DOC is almost entirely lost from solution through mineralization or deposition during circulation through hydrothermal vents with fluid temperatures of 212-401 °C. In laboratory experiments, where we heated samples to 380 °C for four days, we found a similar removal efficiency. We conclude that thermal degradation alone can account for the loss of solid-phase extractable DOC in natural hydrothermal systems, and that its maximum lifetime is constrained by the timescale of hydrothermal cycling, at about 40 million years (Elderfield and Schultz, 1996, doi:10.1146/annurev.earth.24.1.191).