Fluxes to sediments underlying the Rainbow hydrothermal plume at 36°14?N on the Mid-Atlantic Ridge

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of "hydrothermal" elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr?1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than "background" authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting

Vertical and lateral splitting of a hydrothermal plume at Steinaholl, Reykjanes Ridge, Iceland

The generation of multiple, neutrally-buoyant intrusions by a single, bubble-rich plume [Asaeda and Imberger, J. Fluid Mech. 249 (1993) 35–57] and plume bifurcation [Ernst et al., Bull. Volcanol. 56 (1994) 159–169; Lavelle, J. Geophys. Res. 102 (1997) 3405–3420], are predicted from theory and experiments but have yet to be documented for hydrothermal plumes. In contrast, bifurcation of volcanic plumes (which are dynamically analogous to hydrothermal plumes) is very common [Ernst et al., Bull. Volcanol. 56 (1994) 159–169] and thus bifurcation of hydrothermal plumes should be expected [Lavelle, J. Geophys. Res. 102 (1997) 3405–3420; Ernst et al., BRIDGE Newslett. 10 (1996) 76–77]. Recent 38 kHz echo-sounder and water-column chemical tracer studies near the Steinahóll vent site (Reykjanes Ridge, Iceland), identified a bubble-rich hydrothermal plume intruding at three levels (two main ones and a subsidiary one) before reaching the sea surface and spreading there [Ernst et al., BRIDGE Newslett. 10 (1996) 76–77; Olafsson et al., RIDGE Events 2 (1991) 35–38; German et al., Earth Planet. Sci. Lett. 121 (1994) 647–654]. The two main intrusions (ca. 100 and 200 m above the vent) show lateral development of two lobes away from the vent and are consistent with the 350 m rise of a bubble plume yielding seafloor and surface gas fluxes of ~2.5×10?3 and 0.75×10?1 m3/s, respectively. The bubble-rich core of the hydrothermal plume also penetrates the 150 m deep thermocline and generates an intrusion visible at the sea surface. Although bifurcation of the bent-over plume could have been initiated by crossflow alone [Ernst et al., Bull. Volcanol. 56 (1994) 159–169], several other processes may also have enhanced plume bifurcation; including interaction with the thermocline and entrainment of the plume lobes by horseshoe eddies in the lee of a 150 m high hill on the seafloor

Deposition of osmium and other platinum-group elements beneath the ultramafic-hosted rainbow hydrothermal plume

Osmium and other platinum-group elements (PGEs) have been measured in sediments accumulating directly under the Rainbow hydrothermal plume at 36[deg]N on the Mid-Atlantic Ridge. 187Os/188Os and PGE concentrations in the sediments of four cores taken 2-25 km from the active vent site reveal evidence of both radiogenic Os scavenged from seawater and, importantly, unradiogenic Os released by hydrothermal venting. The majority of analyses (n=16) yield 187Os/188Os ratios that fall in a narrow range (0.99-1.06) close to that of modern seawater. In these samples, Os concentrations correlate linearly with hydrothermal V enrichments, which are also derived from seawater scavenging. A smaller subset of samples, characterized by unusually high concentrations of Ni and Cr derived from ultramafic rock fragments, exhibit distinctly lower 187Os/188Os ratios. Their Ir content is also higher, and their PGE ratios are more similar to mantle material than the PGE patterns of the larger set of samples dominated by seawater-scavenged Os. Although the two most proximal cores lack evidence for any similar ultramafic 'detritus' in their surficial sediments, 187Os/188Os ratios in these samples are also lower than that of ambient seawater, suggesting they are likely to have been influenced by unradiogenic Os from vent fluid incorporated into the plume. Samples from the most distal core, by contrast, exhibit 187Os/188Os ratios indistinguishable from that of seawater. Compared with 'background' NE Atlantic sediments, Os is enriched four-fold in the seawater-dominated samples, while Pd, Pt and Ir are twice the background value. Burial fluxes of Os scavenged by hydrothermal Fe in these sediments are 200-400 pg cm-2 kyr-1, up to twice that found on the EPR, and several times the background (bg) of 50 pg cm-2 kyr-1. For Pt, burial fluxes are 1800-3000 pg cm-2 kyr-1 (bg ca. 1000 pg cm-2 kyr-1) and for Pd 2700-4600 pg.cm-2 kyr-1 (bg ca. 800 pg cm-2 kyr-1