The marine geochemistry of iron and iron isotopes

Abstract

Fractionation of iron isotopes could be an effective tool to investigate the geochemistr of iron. Trace metal clean plankton tows, river samples, aerosolleachates, and porewater samples were measured for their iron isotopic composition using a GV Instruments IsoProbe Multi-collector ICPMS. The Fe isotopic composition of plankton tow samples vared by over 4%0 (in 56Fe/54Fe). North Pacific plankton tow samples had isotopically lighter Fe isotopic compositions than samples from the Atlantic. The overall isotopic range observed in the Amazon River system was 1.5%0, with variability observed for different types of tributaries. The main channel river dissolved Fe samples and suspended loads were isotopically similar (= -0.2 to -0.45%0 relative to igneous rocks). The isotopically heaviest sample collected was dissolved Fe from an organic rich trbutar, the Negro River (+0.16%0). In contrast, the suspended load from the Negro River was isotopically light (-1 %0). The isotopically lightest sample from the Amazon region was shelf pore water (-1.4%0). In river water-seawater mixing experiments, the Fe isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically heavy Fe. The observed range in the Fe isotopic composition of the natural samples including biological and aqueous samples demonstrates that significant and useful fractionation is associated with Fe biogeochemistry in the environment. 3 THESIS SUMMARYThis study addressed questions about the Fe cycle by measuring detailed profiles and transects of Fe in the ocean and also by exploring the use of a new tracer of Fe, stable Fe isotopic fractionation. Iron distribution, speciation, and dissolution were investigated on three cruises in the sub-tropical and tropical Atlantic Ocean in regions where dust deposition varies by three orders of magnitude. Detailed profiles and transects were collected and analyzed for "dissolved" Fe (DFe, 0.4 i.m filtered) and "soluble" Fe (SFe, 0.02 i.m filtered). The difference between DFe and SFe is inferred to be the "colloidal" fraction of Fe (CFe). Iron concentrations were measured by a new isotope dilution multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) method, which allows manganese and chromium concentrations to be measured simultaneously. Iron and manganese comparsons are useful because the source for manganese is aeolian deposition and it is removed by scavenging like iron, but the DMn profile (0.4 i.m filtered) is not indicative of a nutrient-type element. In the subtropical and tropical Atlantic Ocean, surface DFe and DMn concentrations reflect dust deposition trends. CFe followed dust deposition trends more strongly than the SFe, and observed maxima in DFe profiles were always due to maxima in the CFe fraction. Where dust deposition was low (e.g., the South Atlantic), CFe concentrations were also low and sometimes negligible in surface waters.SFe and CFe profiles had distinct profiles both in the upper water column and in deeper waters. SFe profiles were always depleted in sur...

Iron, manganese, and lead at Hawaii Ocean Time-series station ALOHA: Temporal variability and an intermediate water hydrothermal plume

“…At the same latitude in the western North Atlantic, we find that Ͻ0.4 m Fe is ϳ0.9 nmol kg Ϫ1 (only a factor of 2-3 higher than typical levels at HOT-ALOHA; Bergquist, 2004), but dust flux estimates for the North Atlantic are much higher than at HOT-ALOHA. For example, estimated dust fluxes are ϳ40-fold higher near Barbados than near Hawaii (Gao et al, 2001;Mahowald et al, 2003).…”

“…Hand et al (in press) found that 1.7 Ϯ 0.8% Fe(II) was released into pH 4.5 formic acid solution for Ͻ2.5 m North Pacific aerosol samples collected in April 2001. Bergquist (2004) treated two fine-fraction North Pacific aerosol samples (April 2001) with surface seawater (refreshed daily) for 3 days and found 7% and 37% release of Ͻ0.4 m Fe. Zhuang et al (1992b) reported that ϳ15% of total Fe was present as Fe(II) in Barbados aerosols (after correction for a typographic error noted by Zhu et al, 1997).…”

The marine biogeochemistry of zinc isotopes

“…GO-FLO samplers were attached to a stainless steel CTD cable to collect waters below 1000 m. Seawater samples from the North Atlantic were collected using individual MITESS samplers modified to hold 1L bottles and attached to a hydrowire with a VANE to keep the sampler upstream of the wire (Bell et al, 2002;Bergquist, 2004). Seawater samples from the Peru Upwelling Region were collected using either a Plankton tows were collected using a 50 jxm mesh nylon net with no metal fittings attached to a Kevlar hydrowire with plastic rope (Sea Gear Corp.).…”

“…They were reacted at low heat overnight with 2mL 16N HNO 3 to burn off some of the organic material. When the sample was dry quartz beakers were placed in a shallow beaker covered by a watch glass and combusted overnight at 450'C in a muffle furnace to eliminate residual organic material (Bergquist, 2004).…”

Variability in oceanic dissolved iron is dominated by the colloidal fraction

“…1). Iron concentrations were measured by a new isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) method (Bergquist, 2004;Bergquist and Boyle, 2006). (30°N, 45°W and 10°N, 45°W), which allowed for sampling at both times of year in the upper 200 m. The northern site at 30°N is in the subtropical gyre and is characterized by oligotrophic conditions, a deep pycnocline, and lower atmospheric dust inputs than the 10°N site.…”