A Geochemical Study of Hydrothermal Signals in Marine Sediments: The Rainbow Hydro thermal Area, 36 degrees on the Mid-Atlantic Ridge

Cave, Rachel R (2002) A Geochemical Study of Hydrothermal Signals in Marine Sediments: The Rainbow Hydro thermal Area, 36 degrees on the Mid-Atlantic Ridge. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The geochemistry of a suite of cores accumulating at increasing distance down-stream of a major hydrothermal vent site has been characterized for the first time, in order to examine the changing chemistry of particles being deposited from the plume with distance from the vent, to try to establish the different processes operating in the plume, particularly with regard to scavenging of elements from seawater, and to quantify the fluxes from seawater to the sediments as a result of venting, for a wide range of major and trace elements (Mg, Fe, V, Mn, Co, Cu, Zn, REE, PGE, Os isotopes).

Sediment cores were collected at 2-25km downplume of the Rainbow hydrothermal vent site at 36^o14'N on the Mid-Atlantic Ridge, a known site of vigorous high-temperature venting. Samples from long-term sediment traps located 0.5-2 km from this vent were also studied. All samples showed similar bulk chemistry, with an overprint of hydrothermally derived material. Sedimentation rates of 40-60 mg.m²d^-1 for the cores were established using bulk radiocarbon dating of the carbonate material, and were found to be of the same order as the sediment traps 11-25 mg.m².d^-1. The maximum age of the core samples ranged from 8-26 x 10³ radiocarbon years.

For many elements, particularly the PGEs, the element to iron ratios in plume derived material were found to be up to an order of magnitude higher than those found globally in metalliferous sediments of hydrogenous origin. Burial fluxes to the Rainbow sediments have been calculated for hydrothermally derived elements - Mn (16-37 μmol.cm^-2.kyr^-1), Fe (368-718μmol.cm^-2.kyr^-1), Co(0.3-0.5μmol.cm^-2.kyr^-1), Cu(3.5-12μmol.cm^-2.kyr^-1), Zn(0.2-0.6μmol.cm^-2.kyr^-1), - and elements scavenged from seawater by hydrothermal Fe-oxyhydroxides - P (36-64μmol.cm^-2.kyr^-1), V (3-5μmol.cm^-2.kyr^-1), As (0.6-1.4μmol.cm^-2.kyr^-1), Pd(20-53pmol.cm^-2.kyr^-1), Os (0.1-2.3pmol.cm^-2.kyr^-1), Ir(0.2-0.6pmol.cm^-2.kyr^-1), Pt(6.5 - 18.5pmol.cm^-2.kyr^-1). Magnesium from seawater appears to co-precipitate with Fe in the vent plume at this site, and Mg burial fluxes are calculated to be 73-146μmol.cm^-2.kyr^-1. Burial fluxes of the hydrothermally derived elements represent only a fraction of the estimated vent output, implying that vent products are dispersed over long distances. Element/Fe ratios for scavenged elements are high compared to other metalliferous sediments, indicating enhanced scavenging efficiency at this site.