Daily to decadal variability of size-fractionated iron and iron-binding ligands at the Hawaii Ocean Time-series Station ALOHA
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© 2015 Elsevier Ltd. Time-series studies of trace metals in the ocean are rare, but they are critical for evaluating both the residence times of the metals themselves and also the timescales over which the marine ecosystems that depend on micronutrient metals can change. In this paper we present two new time-series of the essential micronutrient iron (Fe) taken from the Hawaii Ocean Time-series (HOT) site, Station ALOHA (22.75°N, 158°W): a set of intermittent monthly surface samples taken from ~50 dates between 1999 and 2011 by the HOT program, and a daily-resolved sample set from summer 2012 and 2013 containing ~80 surface samples and 7 profiles to 1500 m depth. The long-term monthly climatology of surface total dissolvable Fe (TDFe) concentrations covaried with the seasonal cycle of continental Asian dust deposition at Hawaii, indicating dust as the major source of TDFe to ALOHA surface waters and a short residence time for TDFe (order ~ months). During the daily summer time-series, surface Fe was most variable in the larger size fractions (>0.4 μm particulate and 0.02-0.4 μm colloidal) and nearly constant in the smallest (<0.02 μm) soluble size fractions, confirming that the larger size fractions have shorter residence times with respect to scavenging/settling. The most significant Fe event lasted three days in early August and "cascaded" through the Fe size fractions from largest to smallest; lacking evidence that dust triggered this event, we correlated it with the arrival of the edge of an anticyclonic eddy and an increase in diatom abundance at ALOHA. The surface Fe-binding ligand daily time-series showed that excess ligand concentrations lagged dFe by 1-2. days, revealing a short residence time of ligands in the central North Pacific likely due to photochemical degradation. In the ferricline, the dissolved Fe (dFe) linear relationship with apparent oxygen utilization was used to establish a water column dFe:C ratio of 3.12 ± 0.11 μmol/mol and a pre-formed dFe concentration of 0.067 ± 0.009 nmol/kg that defines the central/mode waters of Station ALOHA. Finally, in deep waters near 1200 m, where minimal temporal variation in dFe might be expected, dFe instead ranged over a factor of two in concentration (0.72-1.44 nmol/kg), driven by the intermittent passing of the Loihi hydrothermal plume through Station ALOHA. This study not only provides the largest number of Fe measurements made at a single location anywhere in the global ocean to date but also reveals the strength of time-series measurements for exploring mechanisms and timescales of biogeochemical events.
author list (cited authors)
Fitzsimmons, J. N., Hayes, C. T., Al-Subiai, S. N., Zhang, R., Morton, P. L., Weisend, R. E., Ascani, F., & Boyle, E. A.