Matthew A. Charette

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

Differences in seawater particulate organic carbon concentration in samples collected using small- and large-volume methods : the importance of DOC adsorption to the filter blank

Particulate organic carbon (POC) data collected by small-volume (∼1–2 l) bottle filtration and large-volume (∼100–600 l) methods are compared for samples from the central Arctic, Equatorial Pacific, Equatorial and South Atlantic, Gulf of Maine, and Narragansett Bay. Small-volume samples were collected using Niskin® and Go-Flo® bottles and large-volume samples were collected using in situ pumps and large-volume bottle filtration. Results indicate that small-volume bottle POC data are often greater than large-volume results, by as much as 2–4 times, in regions with low POC concentration (<∼5 μM). The implication is that POC concentrations determined by small-volume bottle filtration in regions characterized by low POC concentrations, i.e., in the majority of surface open ocean and deep waters, may be erroneously high. We suggest the most likely explanation is adsorption of DOC to the filter, which is rarely quantified yet can significantly increase the filter blank. The magnitude of DOC adsorption was assessed using coastal seawater by determining the y-intercept of a plot of the organic carbon retained by a glass-fiber filter against the volume filtered. The intercept was approximately two-fold greater than the precombusted filter blank, which we attribute to DOC adsorbed to the filter. Thus, when seawater POC concentrations are similar to, or less than, the precombusted filter blank, not correcting for the true in situ blank can result in erroneously high POC concentrations. To avoid this artifact, we recommend using large-volume sampling methods, which result in a greater quantity of POC per unit area of the filter relative to the filter blank. When large-volume filtration is not possible, we recommend a simple method to evaluate the true in situ filter blank.

An intercomparison of small- and large-volume techniques for thorium-234 in seawater

Abstract In this paper, an intercomparison of methods for the determination of 234Th in seawater is discussed. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts in deep waters off Hawaii and the Southern Ocean. We compared large-volume techniques, which rely upon Mn cartridges for the collection of dissolved 234Th and its detection via gamma counting (>200-l samples), with small volume methods that employed either direct beta counting, or beta counting after radiochemical purification (2–20-l samples). Unique to this study is the presentation of small volume (2 and 5 l) 234Th methods. This new technique is an adaptation of 20-l methods that are based on the coprecipitation of thorium with Mn oxides followed by direct beta counting of the precipitate. The small volume Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection, processing and low overall analytical uncertainties.

Shelf-derived iron inputs drive biological productivity in the southern Drake Passage

water entrainment while Fe/ 228 Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 � 10 5 mol d � 1 and total acid leachable iron flux of 1.1 � 10 6 mol d � 1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m � 3 ) had an iron demand of 1.1 to 4 � 10 5 mol Fe. Net vertical export fluxes of particulate Fe derived from 234 Th/ 238 U disequilibrium and Fe/ 234 Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron-fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.

Global estimate of submarine groundwater discharge based on an observationally constrained radium isotope model

Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013 m3 yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.

Investigating the carbon cycle in the Gulf of Maine using the natural tracer thorium 234

The naturally occurring radionuclide 234Th (t1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August-September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three-dimensional steady state model of 234Th flux combined with measurements of the POC/234Th ratio on >53-μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m−2 d−1, between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid-Atlantic Bight) of dissolved organic carbon (2.4 mmol C m−2 d−1) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m−2 d−1. Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off-shelf export may not be as important as previously estimated in this shelf region.

Coupled radon, methane and nitrate sensors for large-scale assessment of groundwater discharge and non-point source pollution to coastal waters

We constructed a survey system of radon/methane/nitrate/salinity to find sites of submarine groundwater discharge (SGD) and groundwater nitrate input. We deployed the system in Waquoit Bay and Boston Harbor, MA where we derived SGD rates using a mass balance of radon with methane serving as a fine resolution qualitative indicator of groundwater. In Waquoit Bay we identified several locations of enhanced groundwater discharge, out of which two (Childs and Quashnet Rivers) were studied in more detail. The Childs River was characterized by high nitrate input via groundwater discharge, while the Quashnet River SGD was notable but not a significant source of nitrate. Our radon survey of Boston Harbor revealed several sites with significant SGD, out of these Inner Harbor and parts of Dorchester Bay and Quincy Bay had groundwater fluxes accompanied by significant water column nitrogen concentrations. The survey system has proven effective in revealing areas of SGD and non-point source pollution.

Particle transformations and export flux during an in situ iron-stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron-induced increases in phytoplankton production. During the 13-days that the SOIREE bloom was monitored, export fluxes within the iron-fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time-lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron-induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Chapter 5 Uranium- and Thorium-Series Nuclides as Tracers of Submarine Groundwater Discharge

Publisher Summary A number of U- and Th-series isotopes have become popular tools for quantifying submarine groundwater discharge (SGD). These isotopic techniques enable large-scale estimates of various components of SGD, allowing detailed studies on the processes involved. Radium isotopes have proven to be useful tracers of total SGD in many environments on both small- and large scales. The existence of four naturally occurring radium isotopes makes Ra particularly useful for quantifying multiple sources of SGD. The utility of 222 Rn as a tracer of SGD has been demonstrated in a wide range of environments from coastal embayments to the coastal ocean. The approach for quantifying SGD using 222 Rn is similar to radium ( 226 Ra), except for a few key differences. The 222 Rn signature of SGD is best observed near the source because of its short half-life. In contrast, 226 Ra has the ability to integrate the SGD signal over much wider spatial scales. Finally, of the intercomparison studies conducted till date, there appears to be no systematic difference in the two techniques based on the coastal hydrogeologic setting, though a wider range of estimates has been typically observed in areas with fractured crystalline rock aquifers and where springs have been a major conduit for SGD. More recently, the geochemical budget for uranium has been shown to be impacted by SGD. As uranium is enriched in seawater relative to most coastal groundwater, the observed depletion of uranium in estuarine and coastal waters may provide an estimate of the seawater component of SGD that recharges coastal aquifers, or submarine groundwater recharge (SGR). Additionally, new isotope techniques for in situ study are under development including underwater g-spectrometry and continuous radon monitoring. These techniques may be more useful for location of SGD discharges rather than estimation of their magnitudes.