The scavenging of U, 230Th and 231Pa during pulsed hydrothermal activity at 20°S, East Pacific Rise

Abstract

Uranium, Th and Pa isotopes were measured in eight box-cores from two transects to the west of the East Pacific Rise at 10°S and 20°S. Sediment geochemistry (Al, Ca, Fe, Mn) and water column paniculates (Mn) suggest that highly metalliferous plumes, rich in Fe and Mn, are emanating from the rise crest at 19°S. Measurements of 230Th xs and 231Pa xs indicate decreasing sediment accumulation rates from the rise flank (0.4 cm ky −1) to the Tiki Basin (0.23 cm ky −1). Over this geographical area Al is found to have a constant flux of 3.6 × 10 −4 g cm −2 ky −1. Application of an Al constant flux model to ridge crest metalliferous sediment reveals two 1,000 year pulses of hydrothermal activity during the Holoccne (since 11,000 y BP). These hydrothermal pulses are characterised by hydrothermal Fe/Mn ratios (3.5:1), enhanced 230Th and 231Pa scavenging, and low 230Th xs/ 231Pa xs ratios (3–4). The similarity of the 230Th xs and 231Pa xs ratio to the dissolved seawaterrauo suggests that in fracuonation of the isotopes occurs during scavenging. Supplementary supply of dissolved 230Th and 231Pa in a westwards-moving water mass is suggested.