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

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Abstract

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.

Introduction

The existence of hydrothermal systems was postulated to provide the additional sources and sinks needed to balance the budgets of elements entering the ocean from rivers and being removed to sediments (Conway, 1943), and subsequently such systems have been shown to play an important role in the recycling of elements between seawater and crust (Kadko et al., 1995). Initial searches for modern submarine venting focused on restricted areas of sediments with high Fe and Mn contents discovered along mid-ocean ridge crests Arrhenius and Bonatti 1965, Boström and Peterson 1966. The discovery of active vents on the Galapagos Rift (Corliss et al., 1979) and of black smokers at 21°N on the East Pacific Rise (EPR; Spiess, 1980) then turned attention from broad-scale studies of ridge sediments to more localized studies of sediments within vent fields German et al 1993, German et al 1999, Lalou et al 1986, Metz et al 1988, Mills et al 1993 and of hydrothermal plumes themselves Feely et al 1991, Field and Sherrell 2000, German et al 1991, Metz and Trefry 1993, Trefry and Metz 1989, Trocine and Trefry 1988. Detailed studies have also been undertaken on individual ridge-flank cores German et al 1997, Olivarez and Owen 1989 and on the sedimentation from hydrothermal plumes using sediment traps Dymond and Roth 1988, Feely et al 1994, German et al in press, Khripounoff et al 2000. A gap nevertheless remains in our knowledge of how vent signals are imprinted on sediments accumulating beyond vent fields, because no systematic study has been undertaken on sediments at progressive distance under a specific plume.

The Rainbow vent field on the Mid-Atlantic Ridge (MAR) was discovered in 1994 (German et al., 1996b). Between 1994 and 1999, water column studies provided data on local hydrographic conditions, water column chemistry, plume path et al et al 1998b, Thurnherr and Richards 2001, Thurnherr et al in press and plume composition Edmonds et al 1998, Khripounoff et al 2001, Radford-Knoery et al 1998, Radford-Knoery et al 2001. Direct investigations of the vent field by the submersible Nautile in 1997 (Fouquet et al., 1997) revealed that the Rainbow field comprises at least 10 groups of active black smokers, with vent fluid exit temperatures up to 360°C. The outputs of these black smokers coalesce to form the most particle-rich hydrothermal plume so far observed on the MAR (German et al., 1996a). A suite of four cores was collected in 1998 from sites located downstream from the vent field and directly under the plume trajectory Cave and German 1998, Poseidon Cruise 240 shipboard scientific party et alPoseidon Cruise 240 shipboard scientific party et al 1998a. Here we present geochemical data from this series of cores to characterize the deposition of discharge from the Rainbow hydrothermal field and its history.

Section snippets

Sampling and methods

The Rainbow vent field is situated at 36°14′N in the N.E. corner of the S. AMAR segment of the MAR. It lies on a westward-facing fault scarp, near the intersection with the adjacent nontransform discontinuity (NTD) that exposes ultrabasic rock (Fouquet et al., 1997;Fig. 1). The vent field complex is at a depth of 2300 m, and its plume reaches neutral buoyancy at ∼2100 m water depth. Before core collection, it had already been ascertained that plume dispersion at Rainbow is predictable, but

Sediment accumulation rates and the surface mixed layer

210Pb is a relatively short-lived radioisotope (t1/2= 22.7 yr) so that it persists in surficial sediments for only ∼100 yr after deposition from the water column. In the absence of bioturbation, all 210Pbexcess present would therefore be expected to be present in the upper few millimeters of deep-sea sediments since they accumulate at rates of cm.kyr−1(Cochran, 1992). In fact, bioturbation has mixed 210Pbexcess down to ∼7 cm in Rainbow cores 343 and 316 closest to the vent site, and to >10 cm

Summary and conclusions

The geochemistry of four sediment cores taken at increasing distances downstream from the Rainbow hydrothermal field on the MAR has been investigated. The cores, which contain >80% CaCO3, show well-organized mean sediment accumulation rates of between 2.65 and 3.75 cm.kyr−1 throughout the Holocene, with a lower pre-Holocene sedimentation rate of 1.11 cm.kyr−1 observed in the longest core only. The Fe and Mn contents in the sediments show a significant hydrothermal input at all levels, with a

Acknowledgements

R.R.C. thanks the Natural Environment Research Council for support through studentship GT4/97/2661, and for the radiocarbon analyses provided by NERC Radiocarbon Laboratory allocation no. 791.0599. Ian Croudace kindly provided access to the XRF facility at SOC. Comments by Greg Ravizza and reviews by Rachel Haymon and an anonymous reviewer greatly improved the final version.

Associate editor: D. W. Lea

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    Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.

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