Pamela M. Barrett

Estimating the benthic efflux of dissolved iron on the Ross Sea continental shelf

Continental margin sediments provide a potentially large but poorly constrained source of dissolved iron (dFe) to the upper ocean. The Ross Sea continental shelf is one region where this benthic supply is thought to play a key role in regulating the magnitude of seasonal primary production. Here we present data collected during austral summer 2012 that reveal contrasting low surface (0.08 ± 0.07 nM) and elevated near-seafloor (0.74 ± 0.47 nM) dFe concentrations. Combining these observations with results from a high-resolution physical circulation model, we estimate dFe efflux of 5.8 × 107 mol yr−1 from the deeper portions (>400 m) of the Ross Sea continental shelf; more than sufficient to account for the inferred “winter reserve” dFe inventory at the onset of the growing season. In addition, elevated dFe concentrations observed over shallower bathymetry suggest that such features provide additional inputs of dFe to the euphotic zone throughout the year.

Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic

Recent analyses suggest that considerable CaCO3 dissolution may occur in the upper water column of the ocean (< 1500 m). This study uses the distribution of particulate calcium from high-resolution suspended matter sampling along the Climate Variability and Predictability/CO2 Repeat Hydrography A16N transect in 2003 to estimate CaCO3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO3 dissolution rate in this region is estimated to be 0.9 mmol CaCO3 m−2 d−1, indicating this region is a hotspot for upper water column CaCO3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO3 m−2 d−1) and are comparable to previous estimates of CaCO3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface ocean CaCO3 standing stocks suggests that biologically mediated CaCO3 dissolution may be occurring in the top 1000 m of the Atlantic.

Dissolved Fe and Al in the upper 1000 m of the eastern Indian Ocean: A high‐resolution transect along 95°E from the Antarctic margin to the Bay of Bengal

A high-resolution section of dissolved iron (dFe) and aluminum (dAl) was obtained along ~95°E in the upper 1000?m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the U.S. Climate Variability and Predictability (CLIVAR) CO2 Repeat Hydrography I08S and I09N sections (February–April 2007). In the Southern Ocean, low concentrations of dAl (

Ocean chemistry: Fingerprints of a trace nutrient

Lack of dissolved iron in the sea limits biological productivity and the uptake of carbon dioxide. The sources of dissolved iron in the North Atlantic Ocean have been identified from isotopic variations of this trace nutrient. See Letter p.212 Iron availability limits phytoplankton growth throughout the oceans, acting as a key influence on the global carbon cycle and the oceanic response to changing climate. But large uncertainties remain as to the relative importance of the various sources of iron, including windblown dust and hydrothermal vents. This paper presents a high-resolution transect of seawater dissolved stable iron isotope ratios and iron concentrations in the North Atlantic Ocean. Saharan dust aerosol emerges as the dominant source of dissolved iron along the section, with sediments and hydrothermal vents also significant. Changes in these sources through time may have wide-ranging implications for the global carbon cycle.