21 research outputs found
On the Fractional Solubillity of Copper in Marine Aerosols: Toxicity of Aeolian Copper Revisited
Paytan et al. (2009) argue that the atmospheric deposition of aerosols lead to copper concentrations that are potentially toxic to marine phytoplankton in a large area of tropical and subtropical North Atlantic Ocean. A key assumption in their model is that all marine aerosols (mineral dust and anthropogenic particles) have a high (40%) fractional solubility of copper. Our data show that the fractional solubility of copper for Saharan dust over the Sargasso Sea and Bermuda is significantly lower (1-7%). In contrast, anthropogenic aerosols with non-Saharan sources have significantly higher values (10-100%). Hence, the potential Cu toxicity in the tropical and subtropical North Atlantic should be re-estimated, given the low fractional solubility of Cu in the Saharan dust that dominates aerosol deposition to this region
Development of an autonomous aerosol sampler for ocean buoys and land sites
The authors have successfully designed, built and tested an aerosol sampler
which is capable of collecting, in an unattended manner, a time-series set of aerosol
samples (aerosol-embedded filters) from moored ocean buoys and remote areas on
land. Research on aerosols, in particular, and atmospheric chemistry, in general, has
not been previously attempted from buoys. Aerosols entering and leaving the ocean
play an important role in climate change, ocean productivity, pollutant transport and
atmospheric optics.
This report discusses (1) the scientific applications of a buoy-mounted aerosol
sampler, (2) the advantages of using buoys as research platforms and (3) the authors'
new instrument. Also discussed are the results of a four month test of the aerosol
sampler on the AEROCE (Atmosphere/Ocean Chemistry Experiment) tower in
Bermuda and the results of a three month test on a buoy moored in Vineyard Sound
off Woods Hole, MA USA. The direct comparison between WHOI filters and
AEROCE filters from the Bermuda tower is very encouraging as the Fe
concentrations of aerosols compare to within 10-15% over a wide range of values.
Aerosol sampling from a buoy moored in coastal waters was successfully tested under
a variety of atmospheric and oceanic conditions.Funding was provided by the National Science Foundation through Grant No. OCE-943212
Trace element cycling in a subterranean estuary : part 2. Geochemistry of the pore water
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 811-826, doi:10.1016/j.gca.2005.10.019.Submarine groundwater discharge (SGD) is an important source of dissolved elements to
the ocean, yet little is known regarding the chemical reactions that control their flux from sandy
coastal aquifers. The net flux of elements from SGD to the coastal ocean is dependent on
biogeochemical reactions in the groundwater-seawater mixing zone, recently termed the
"subterranean estuary". This paper is the second in a two part series on the biogeochemistry of
the Waquoit Bay coastal aquifer/subterranean estuary. The first paper addressed the
biogeochemistry of Fe, Mn, P, Ba, U, and Th from the perspective of the sediment composition
of cores (Charette et al., 2005). This paper uses pore water data from the subterranean estuary,
along with Bay surface water data, to establish a more detailed view into the estuarine chemistry
and the chemical diagenesis of Fe, Mn, U, Ba and Sr in coastal aquifers.
Nine high-resolution pore water (groundwater) profiles were collected from the head of
the bay during July 2002. There were non-conservative additions of both Ba and Sr in the
salinity transition zone of the subterranean estuary. However, the extent of Sr release was
significantly less than that of its alkaline earth neighbor Ba. Pore water Ba concentrations
approached 3000 nM compared with 25-50 nM in the surface waters of the bay; the pore water
Sr-salinity distribution suggests a 26% elevation in the amount of Sr added to the subterranean
estuary. The release of dissolved Ba to the mixing zone of surface estuaries is frequently
attributed to an ion-exchange process whereby seawater cations react with Ba from river
suspended clay mineral particles at low to intermediate salinity. Results presented here suggest
that reductive dissolution of Mn oxides, in conjunction with changes in salinity, may also be an
important process in maintaining high concentrations of Ba in the pore water of subterranean
estuaries.
In contrast, pore water U was significantly depleted in the subterranean estuary, a result
of SGD-driven circulation of seawater through reducing permeable sediments. This finding is
supported by surface water concentrations of U in the bay, which were significantly depleted in
U compared with adjacent coastal waters. Using a global estimate of SGD, we calculate U
removal in subterranean estuaries at 20 x 106 mol U y-1, which is the same order of magnitude as
the other major U sinks for the ocean. Our results suggest a need to revisit and reevaluate the
oceanic budgets for elements that are likely influenced by SGD-associated processes.This research was supported by the National Science Foundation (OCE-0095384) to M.A.C. and
E.R.S., and a WHOI Coastal Ocean Institute Fellowship to M.A.C
Iron isotope systematics in estuaries : the case of North River, Massachusetts (USA)
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 4045-4059, doi:10.1016/j.gca.2009.04.026.Recent studies have suggested that rivers may present an isotopically light Fe
source to the oceans. Since the input of dissolved iron from river water is generally
controlled by flocculation processes that occur during estuarine mixing, it is important to
investigate potential fractionation of Fe-isotopes during this process. In this study, we
investigate the influence of the flocculation of Fe-rich colloids on the iron isotope
composition of pristine estuarine waters and suspended particles. The samples were
collected along a salinity gradient from the fresh water to the ocean in the North River
estuary (MA, USA). Estuarine samples were filtered at 0.22 μm and the iron isotope
composition of the two fractions (dissolved and particles) were analyzed using high
resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive
δ56Fe values (with an average of 0.43 ± 0.04 ‰) relative to the IRMM-14 standard and do
not display any relationships with salinity or with percentage of colloid flocculation. The
iron isotopic composition of the particles suspended in fresh water is characterized by
more negative δ56Fe values than for dissolved Fe and correlate with the percentage of Fe
flocculation. Particulate δ56Fe values vary from -0.09‰ at no flocculation to ~ 0.1‰ at
the flocculation maximum, which reflect mixing effects between river-borne particles,
lithogenic particles derived from coastal seawaters and newly precipitated colloids. Since
the process of flocculation produces minimal Fe-isotope fractionation in the dissolved Fe
pool, we suggest that the pristine iron isotope composition of fresh water is preserved
during estuarine mixing and that the value of the global riverine source into the ocean can
be identified from the fresh water values. However, this study also suggests that δ56Fe
composition of rivers can also be characterized by more positive δ56Fe values (up to 0.3
per mil) relative to the crust than previously reported. In order to improve our current
understanding of the oceanic iron isotope cycling, further work is now required to
determine the processes controlling the fractionation of Fe isotopes during continental
run-off.This study was supported by the National Science Foundation (OCE 0550066) to O. Rouxel and Edward Sholkovitz
Iron isotope fractionation in subterranean estuaries
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 72 (2008): 3413-3430, doi:10.1016/j.gca.2008.05.001.Dissolved Fe concentrations in subterranean estuaries, like their river-seawater
counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater
and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on
Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron
and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an
excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redoxstratified
environments and determine potential Fe-isotope signatures of groundwater sources to
coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters
beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct
sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in
the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration
consistent with anoxic conditions and yield δ56Fe values between 0.3 and –1.3‰. In contrast,
sediment porewaters occurring in the mixing zone of the subterranean estuary have very low
δ56Fe values down to –5‰. These low δ56Fe values reflect Fe-redox cycling and result from the
preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Feoxides
precipitated onto subsurface sands in two cores from the subterranean estuary revealed
strong δ56Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ56Fe
values between –2 and 1.5‰. The relationship between Fe concentration and δ56Fe of Fe-rich
sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the
subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to
5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by
very low δ56Fe values relative to river values.This study was supported by the National Science Foundation (OCE 0550066) to OR and
ES , (OCE 0095384) to MC and ES and NASA Astrobiology Institute - Cycle 3 CAN-02-OSS-02
to KJE
Impact of Anthropogenic Combustion Emissions on The Fractional Solubility of Aeroosol Iron: Evidence From The Sargasso Sea
We report empirical estimates of the fractional solubility of aerosol iron over the Sargasso Sea during periods characterized by high concentrations of Saharan dust (summer 2003) and by low concentrations of aerosols in North American/maritime North Atlantic air masses (spring 2004 and early summer 2004). We observed a strong inverse relationship between the operational solubility of aerosol iron (defined using a flow-through deionized-water leaching protocol) and the total concentration of aerosol iron, whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low. This relationship is consistent with source-dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model, wherein bulk aerosols represent a conservative mixture of two air mass end-members that carry different aerosol types: Saharan air,\u27\u27 which contains a relatively high loading of aerosol iron (27.8 nmol Fe m-3) that has a low fractional solubility (0.44%), and North American air,\u27\u27 which contains a relatively low concentration of aerosol iron (0.5 nmol Fe m-3) that has a high fractional solubility (19%). Historical data for aerosols collected on Bermuda indicate that the low iron loadings associated with North American air masses are typically accompanied by elevated V/Al, Fe/Al, and V/Mn mass ratios in the bulk aerosol, relative to Saharan dust, which are indicative of anthropogenic fuel-combustion products. The identification of similar compositional trends in our Sargasso Sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with North American air masses reflects the presence of anthropogenic combustion products, which contain iron that is readily soluble relative to iron in Saharan soil dust. We thus propose that the source-dependent composition of aerosol particles (specifically, the relative proportion of anthropogenic combustion products) is a primary determinant for the fractional solubility of aerosol iron over the Sargasso Sea. This hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean
Fractional solubility of aerosol iron : synthesis of a global-scale data set
Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to bioavailability is the proportion of aerosol iron that enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and %FeS values for ~1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including new data from the Atlantic Ocean. The global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data falling along an inverse hyperbolic trend. The large dynamic range in %FeS (0-95%) varies with FeT in a manner similar to that identified for aerosols collected in the Sargasso Sea by Sedwick et al. (2007), who posit that the trend reflects near-conservative mixing between air masses that carry lithogenic mineral dust (with high FeT and low %FeS) and non-soil-dust aerosols such as anthropogenic combustion emissions (with low FeT and high %FeS), respectively. An increasing body of empirical evidence points to the importance of aerosol source and composition in determining the fractional solubility of aerosol iron, such that anthropogenic combustion emissions appear to play a critical role in determining this parameter in the bulk marine aerosol. The robust global-scale relationship between %FeS and FeT may provide a simple heuristic method for estimating aerosol iron solubility at the regional to global scale
A compilation of the rare earth element composition of rivers, estuaries and the oceans
This technical report serves as an appendix to a recent article by Byrne and Sholkovitz (1996) in the Handbook on the Physics
and Chemistry of Rare Earths (vol. 23, chapter 158, pg. 497-592) edited by K. A. Gschneidner Jr. and L. Eyring and published by
Elsevier Science. This article, Marine Chemistry and Geochemistry of the Lanthanides, discusses the physical chemistry of the
lanthanides in natural waters, describes the major features of the lanthanides in rivers, estuaries and oceans and discusses the
chemical and biogeochemical processes controlling the speciation and distribution of the lanthanides in the ocean.
The article by Byre and Sholkovitz (1996) refers to a large set of published and unpublished data on the rare earth (RE)
composition of rivers, estuaries, seawater, marine pore waters and marine hydrothermal waters. In order to conserve space in the
Handbook arctile, a compilation of concentration data for natural waters will be presented in this report. Publications through 1995
are cited
Fractional solubility of aerosol iron : synthesis of a global-scale data set [revised]
Aerosol deposition provides a major input of the essential micronutrient iron to the open ocean. A critical parameter with respect to biological availability is the proportion of aerosol iron that enters the oceanic dissolved iron pool – the so-called fractional solubility of aerosol iron (%FeS). Here we present a global-scale compilation of total aerosol iron loading (FeT) and estimated %FeS values for ~1100 samples collected over the open ocean, the coastal ocean, and some continental sites, including a new data set from the Atlantic Ocean. Despite the wide variety of methods that have been used to define 'soluble' aerosol iron, our global-scale compilation reveals a remarkably consistent trend in the fractional solubility of aerosol iron as a function of total aerosol iron loading, with the great bulk of the data defining an hyperbolic trend. The hyperbolic trends that we observe for both global- and regional-scale data are adequately described by a simple two-component mixing model, whereby the fractional solubility of iron in the bulk aerosol reflects the conservative mixing of 'lithogenic' mineral dust (high FeT and low %FeS) and non-lithogenic 'combustion' aerosols (low FeT and high %FeS). An increasing body of empirical and model-based evidence points to anthropogenic fuel combustion as the major source of these non-lithogenic 'combustion' aerosols, implying that human emissions are a major determinant of the fractional solubility of iron in marine aerosols. The robust global-scale relationship between %FeS and FeT provides a simple heuristic method for estimating aerosol iron solubility at the regional to global scale