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

Abstract

Trace metal clean techniques were used to sample Hawaii Ocean Time-series (HOT) station ALOHA on seven occasions between November 1998 and October 2002. On three occasions, full water-column profile samples were obtained; on the other four occasions, surface and near-surface euphotic zone profiles were obtained. Together with three other published samplings, this site may have been monitored for “dissolved” (≤0.4 or ≤0.2 μm) Fe more frequently than any other open ocean site in the world. Low Fe concentrations (<0.1 nmol kg −1) are seen in the lower euphotic zone, and Fe concentrations increase to a maximum in intermediate waters. In the deepwaters (>2500 m), the concentrations we observe (0.4–0.5 nmol kg −1) are significantly lower than some other deep North Pacific stations but are similar to values that have been reported for a station 350 miles to the northeast. We attribute these low deepwater values to transport of low-Fe Antarctic Bottom Water into the basin and a balance between Fe regeneration and scavenging in the deep water. Near-surface waters have higher Fe levels than observed in the lower euphotic zone. Significant temporal variability is seen in near-surface Fe concentrations (ranging from 0.2–0.7 nmol kg −1); we attribute these surface Fe fluctuations to variable dust deposition, biological uptake, and changes in the mixed layer depth. This variability could occur only if the surface layer Fe residence time is less than a few years, and based on that constraint, it appears that a higher percentage of the total Fe must be released from North Pacific aerosols compared to North Atlantic aerosols. Surprisingly, significant temporal variability and high particulate Fe concentrations are observed for intermediate waters (1000–1500 m). These features are seen in the depth interval where high δ 3He from the nearby Loihi Seamount hydrothermal fields has been observed; the total Fe/ 3He ratio implies that the hydrothermal vents are the source of the high and variable Fe. The vertical profile of Mn at ALOHA qualitatively resembles other North Pacific Mn profiles with surface and intermediate water maxima, but there are some significant quantitative differences from other reported profiles. The ≤0.4 μm Mn concentration is highest near the surface, decreases sharply in the upper 500 m, then shows an intermediate water maximum at 800 m and then decreases in the deepest waters; these concentrations are higher than observed at a station 350 miles to the northeast that shows similar vertical variations. It appears that there is a significant Mn gradient (throughout the water column) from HOT towards the northeast. Compared to the first valid oceanic Pb data for samples collected in 1976, Pb at ALOHA in 1997–1999 shows decreases in surface waters and waters shallower than 200 m. Pb concentrations in central North Pacific surface waters have decreased by a factor of 2 during the past 25 yr (from ∼65 to ∼30 pmol kg −1); surface water Pb concentrations in the central North Atlantic and central North Pacific are now comparable. We attribute the surface water Pb decrease to the elimination of leaded gasoline in Japan and to some extent by the U.S. and Canada. We attribute most of the remaining Pb in Pacific surface waters to Asian emissions, more likely due to high-temperature industrial activities such as coal burning rather than to leaded gasoline consumption. A 3-year mixed-layer time series from the nearby HALE-ALOHA mooring site (1997–1999) shows that there is an annual cycle in Pb with concentrations ∼20% higher in winter months; this rise may be created by downward mixing of the winter mixed layer into the steep gradient of higher Pb in the upper thermocline (Pb concentrations double between the surface and 200 m). From 200 m to the bottom, Pb concentrations decrease to levels of 5–9 pmol kg −1 near the bottom; for most of the water column, thermocline and deepwater Pb concentrations do not appear to have changed significantly during the 23-yr interval.