Do bottom mixed layers influence 234Th dynamics in the abyssal near-bottom water column?

Robert Turnewitsch, Barbara M Springer

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Abstract

Dynamics of the natural radioactive particle tracer Th-234 (half-life: 24.1 days) within the abyssal water column up to 1000m above bottom and within surface sediments of the northeast Atlantic (Porcupine Abyssal Plain; depth: approximate to 4845 m) were investigated, Distributions of transmissometer voltages and potential temperature indicated a subdivision of the near-bottom water column into a benthic mixed layer (BML; thickness: approximate to 10-65 m) and the layer above the BML up to the upper boundary of the bottom nepheloid layer (BNL; thickness: approximate to 1000m). Comparison of Th-234 fluxes (dpm m(-2) d(-1)) in sediment traps, vertical fluxes derived from Th-234/U-238-disequilibrium in the near-bottom water column and excess Th-234 inventories in the surface sediment provided evidence for lateral advection of Th-234-depleted water and a 'missing sink' for Th-234. A Simple one-dimensional steady-state box-model approach was applied to investigate Th-234 dynamics. Estimated residence times suggest the BML and the resuspension zone of the surface sediment to be highly dynamic systems with respect to particle cycling and sorptive reactions on a time scale of weeks. Model results indicate that, through the chemical forcing of changing particle concentration, a thickening BML results in (1) increasing residence times of particulate Th-234 in the BML with respect to the net fluxes across the upper boundary of the BML and into the surface sediment; (2) declining adsorption rate constants in the BML; (3) increasing desorptive fluxes in the BML resulting in (4) enhanced Th-234 decay in the BML; (5) decreasing net fluxes of particulate Th-234 from the BML to the upper BNL above the BML and to the sediment. Potential consequences for carbon cycling in the water column of the deep ocean are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.
Original languageEnglish
Pages (from-to)1279-1307
Number of pages29
JournalDEEP-SEA RES PT I
Issue number8
DOIs
Publication statusPublished - 2001

Keywords

  • SPATIAL VARIABILITY
  • NEPHELOID LAYER
  • OXYGEN-UPTAKE
  • BENTHIC BOUNDARY-LAYER
  • Oceanography
  • DEEP-SEA SEDIMENTS
  • ATLANTIC BLOOM EXPERIMENT
  • PARTICULATE ORGANIC-CARBON
  • CENTRAL EQUATORIAL PACIFIC
  • TEMPORAL VARIABILITY
  • EASTERN NORTH-ATLANTIC

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