A 4300km-Long Particulate Hydrothermal Plume West of the Southern East Pacific Rise (15°S): Particulate Minor and Trace Elements from the U.S. GEOTRACES Eastern Pacific Zonal Transect
Tuesday, 16 December 2014
Research over the last 35 years has demonstrated a substantial influence of deep-sea hydrothermal venting on seawater chemistry proximal to vent sites, but basin-scale hydrothermal effects are to date only hypothesized on the basis of limited seawater and sediment sampling downstream of vent sites. The U.S. GEOTRACES Eastern Pacific Zonal Transect cruise investigated these potential long-range hydrothermal influences by sampling along the 15°S southern East Pacific Rise (EPR) plume, which is known to extend >4000 km from the EPR based on He-3 distributions. We present measurements of 30 particulate elements (>0.45 µm, filtered from GO-Flo bottles) at 11 stations along this transect. Concentrations of pFe and pMn peaked at 90 nM and 9 nM at 2425m above the EPR ridge axis (Sta 18), with no measurable back-transport to the east. A clear plume signature was detected at all stations from the ridge axis to the last station 4300 km to the west, where attenuated but background-distinguishable pFe and pMn peaks of 0.31 and 0.11 nM persisted at 2500m (Sta 36). Notably, the pMn peak appeared slightly deeper than the pFe peak near-field, and distal pMn decreased faster than pFe, a prime example of variable particle composition and settling behavior along the plume. The oxyanions pV (377 pM max) and pP (20 nM max) and particle-reactive pNd (2.42 pM max) all showed strong correlations with pFe and similarly remained enriched at distal stations, suggesting scavenging by Fe oxyhydroxides. The chalcophiles pCu (0.27 nM), pZn (75 pM), pCd (0.35 pM), and pCo (5.7 pM) were all enhanced near-field but, except pCu, reached background concentrations by Sta 23, indicating that sulfide particles either preferentially settle or dissolve early in plume evolution. Elevated pCu, in contrast, persisted throughout the sampled plume length. No elevated pAl, pTi, or pNi was evident in the hydrothermal plume, yet a substantial nepheloid layer with high concentrations of crustally-dominated elements was detected from 4000m to near-bottom depths >5000m in the westernmost section (Sta 32 and 36). Further physical and chemical characterization and modeling of this 4300-km particulate hydrothermal plume will help reveal what combination of size, density, and chemical speciation permits such long-range hydrothermal particle transport in the South Pacific.