The Concentration and Distribution of Particulate Iron in Waters of the Northwestern Atlantic Ocean and Caribbean Sea

• Main Author: Betzer, Peter Robin
• Format: Text
• Language: unknown
• Published: DigitalCommons@URI 1971
• Subjects: Pacific; Northwest Atlantic
• Online Access: https://digitalcommons.uri.edu/oa_diss/1322; https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=2334&context=oa_diss

Measurements of particulate iron concentrations have been carried out on samples from the northwestern Atlantic Ocean, Caribbean Sea and Gulf of Mexico. Water samples were collected in 30-1 Niskin bottles; iron analyses were done by atomic absorption spectroscopy. The average of 319 iron determinations on shallow open-ocean water (0-300 m) was 0.142 ± .146 μg/1; the average of 165 determinations on samples deeper than 500 m and shallower than 1000 m from the bottom was 0.137 ± .091 μg/1. These averages are lower than previous estimates by an order of magnitude or more. Spectrophotometric and atomic-absorption analyses carried out on a set of duplicate samples from the surface water of the Puerto Rican Trench as well as comparison of neutron activation and atomic-absorption analyses on the samples collected in the Pacific Ocean indicate that the method of analysis is not the reason for the large discrepancy between the concentrations of particulate iron observed in this study and those reported in previous investigations. Studies of sampling methods showed that rusty hydrographic wire and the Nansen bottle, the sample bottle most frequently used by other investigators, introduced significant iron contamination. Concentrations of particulate iron in 10 seawater samples collected in Teflon®-coated Nansen bottles were 15-86 times higher than those collected in 30-1 Niskin bottles at the same time, depth, and station. Water collected with Niskin bottles on rusty hydrographic wire contained 5 times as much particulate iron as water collected with the samplers on new, aluminized wire. It is concluded that the collection of water samples in metal bottles attached to corroded wire seems the most likely source of the high concentrations of particulate iron which have been reported in the literature. Through the use of all-plastic water samplers and coated hydro-wire, oceanographically meaningful patterns of particulate iron concentrations were found in the deep ocean, near river mouths and in the anoxic waters of the Cariaco Trench. Greatly increased concentrations of particulate iron were found within 1000 m of the bottom in the northwest Atlantic and eastern Gulf of Mexico (6 times and 3 to 4 times the average of shallower water, respectively) while only slightly increased concentrations were found within 1000 m of the bottom in the Caribbean and western Gulf of Mexico (2 times the average concentration in shallower water). These distributions agree with published light-scattering studies of deep ocean water, and are chemical evidence of a near-bottom nepheloid layer. It is concluded that the increase in the near-bottom concentrations of particulate iron is not a water mass effect, but arises from interaction of water with the bottom. Greatly increased concentrations of particulate iron were found in ocean waters adjacent to the mouths of the Magdalena, Mississippi, and Orinoco Rivers. The estimated annual particulate iron input of the three rivers is significant when compared to the total amount present in the ocean areas into which they discharge. Particulate iron settled more rapidly than the bulk of suspended materials near the Magdalena and Mississippi Rivers; this could be due to precipitation and flocculation of iron hydroxides or the differential settling of clays. Concentrations of particulate iron in the anoxic zone of the Cariaco Trench showed an increase with depth and a linear relationship with the reciprocal of the sulfide ion concentration at sulfide ion concentrations greater than 15 x 10-6 μM. It is concluded that the solubility of ferrous sulfide is controlling particulate iron concentrations and that particulate iron may be in equilibrium with soluble iron and sulfide below 600 min these waters.