Seasonal distribution of Fe, Mn and Zn in sediment traps and suspended particles in response to atmospheric input to the Sargasso Sea

Date
2011
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University of Delaware
Abstract
Matching atmospheric samples (bulk and wet) and water column samples (sediment trap and suspended particles) were collected from July 1999 through April 2000, at the AEROCE tower and OFP Mooring Site in the Sargasso Sea. Using a ‘top-down’ approach, seasonal variability of the atmospheric input of Fe, Mn and Zn, and the subsequent processing in the water column in the surface, mid-water and deep-waters were investigated. Variability in mass flux at the 500 m, 1500 m, and 3200 m traps highlighted seasonal abiotic and biotic processes. The presence of salp fecal pellets in the sediment traps collected during the summer are thought to have rapidly processed atmospheric input in the surface ocean, supported by the coupling of increased input and output flux observed during that time. Increased winter mixing observed in temperature profiles in December indicates that aggregation and scavenging of surface particles. The increase in mass flux at all trap depths during April is supported by the sinking of aggregated phytoplankton from the spring bloom and subsequent grazing by zooplankton. Residence times for Fe, Mn and Zn in the ‘total’ reservoir (dissolved seawater and suspended particles) were calculated at different depth intervals corresponding to the mixed layer depth during the summer/fall, fall/winter and winter/spring. The input flux (soluble wet + soluble calculated dry) was compared with the output flux (500 m trap), in which variable residence times resulted. Fe ranged from 8-140 d when calculating residence time using the input flux, and 0.4-1.3 d range when using output flux, Mn ranged from 1-60 d using input flux and 30-184d using output flux and Zn ranged from 2-36 d using input flux and 7-16 d when using output flux. Residence times for Fe when using input fluxes agreed with literature values in surface waters. However, the residence times for Mn when using output flux were closer to literature values but were still much shorter. The lithogenic to biogenic component of the trace elements in trap particles and suspended particles were related to the loss of organic carbon with depth, with results in agreement to calculations with trap data from. Biogenic Zn decreased with depth in trap particles but increased in suspended particles. For Fe, the biogenic to organic carbon ratio both increase in addition to the lithogenic component. Results indicated the presence of Mn-oxides on particles with depth, in which there was an increase in the biogenic component of Mn and an increase in the lithogenic component of Fe through scavenging. Stoichiometric ratios of the surface most suspended particles collected in January (20 m) and May (50 m) revealed biotic and abiotic influence and a ratio comparable to ratios reported for phytoplankton composition (C1000Fe0.10Mn0.0035Zn0.005). Overall this investigation supports the relationship between trace element cycles and the influence of biotic and abiotic processing that impact the distribution in the water column.
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