The marine geochemistry of iron and iron isotopes

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2004This thesis addressed questions about the Fe cycle by measuring detailed profiles and transects of Fe species in the ocean and also by exploring the use of a new tracer of Fe, Fe isotopic fractionation. In the subtropical and tropical Atlantic Ocean, transects and profiles are presented for dissolved Fe (<0.4 μm), soluble Fe (<0.02 μm), and colloidal Fe (0.02 to 0.4 μm). Surface dissolved Fe distributions reflect atmospheric deposition trends with colloidal Fe following dust deposition more strongly than the soluble fraction of Fe. Observed surface maxima and shallow minima in dissolved Fe were always due to variations in the colloidal Fe fraction. Deep-water dissolved and colloidal Fe concentrations vary with water mass source, age, and transport path. Elevated dissolved Fe concentrations (>1 nmol/kg) were associated with an oxygen minimum zone in the tropical Atlantic at 10°N, 45°W. Fractionation of iron isotopes could be an effective tool to investigate the geochemistr of iron. Trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples were measured for their iron isotopic composition using a GV Instruments IsoProbe Multi-collector ICPMS. The Fe isotopic composition of plankton tow samples vared by over 4% (in 56Fe/54Fe). North Pacific plankton tow samples had isotopically lighter Fe isotopic compositions than samples from the Atlantic. The overall isotopic range observed in the Amazon River system was 1.5%, with variability observed for different types of tributaries. The main channel river dissolved Fe samples and suspended loads were isotopically similar (≈ -0.2 to -0.45% relative to igneous rocks). The isotopically heaviest sample collected was dissolved Fe from an organic rich tributary, the Negro River (+0.16%). In contrast, the suspended load from the Negro River was isotopically light (-1 %). The isotopically lightest sample from the Amazon region was shelf porewater (-1.4%). In river water-seawater mixing experiments, the Fe isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically heavy Fe. The observed range in the Fe isotopic composition of the natural samples including biological and aqueous samples demonstrates that significant and useful fractionation is associated with Fe biogeochemistry in the environment.This research was supported by NSF grants OCE-0002273 and OCE-99871442. The Amazon field trip was partially funded by the Houghton Fund at MIT. I was funded by the National Physical Science Foundation, Lawrence Livermore National Laboratory, and the Education Office of Woods Hole Oceanographic Institution

No evidence for a volcanic trigger for late Cambrian carbon-cycle perturbations

The early Paleozoic was marked by several carbon-cycle perturbations and associated carbon-isotope excursions (CIEs). Whether these CIEs are connected to significant (external) triggers, as is commonly considered to be the case for CIEs in the Mesozoic and Cenozoic, or result from small carbon-cycle imbalances that became amplified through lack of efficient silicate weathering or other feedbacks remains unclear. We present concentration and isotope data for sedimentary mercury (Hg) and osmium (Os) to assess the impact of subaerial and submarine volcanism and weathering during the late Cambrian and early Ordovician. Data from the Alum Shale Formation (Sweden) cover the Steptoean positive carbon-isotope excursion (SPICE; ca. 497–494 Ma), a period marked by marine anoxia and biotic overturning, and several smaller CIEs extending into the early Ordovician. Our Hg and Os data offer no strong evidence that the CIEs present in our record were driven by (globally) elevated volcanism or continental weathering. Organic-carbon and Hg concentrations covary cyclically, providing further evidence of an unperturbed Hg cycle. Mesozoic and Cenozoic CIEs are commonly linked to enhanced volcanic activity and weathering, but similar late Cambrian–early Ordovician events cannot easily be connected to such external triggers. Our results are more consistent with reduced early Paleozoic carbon-cycle resilience that allowed small imbalances to develop into large CIEs

Mercury isotope evidence for Arctic summertime re-emission of mercury from the cryosphere

During Arctic springtime, halogen radicals oxidize atmospheric elemental mercury (Hg-0), which deposits to the cryosphere. This is followed by a summertime atmospheric Hg-0 peak that is thought to result mostly from terrestrial Hg inputs to the Arctic Ocean, followed by photoreduction and emission to air. The large terrestrial Hg contribution to the Arctic Ocean and global atmosphere has raised concern over the potential release of permafrost Hg, via rivers and coastal erosion, with Arctic warming. Here we investigate Hg isotope variability of Arctic atmospheric, marine, and terrestrial Hg. We observe highly characteristic Hg isotope signatures during the summertime peak that reflect re-emission of Hg deposited to the cryosphere during spring. Air mass back trajectories support a cryospheric Hg emission source but no major terrestrial source. This implies that terrestrial Hg inputs to the Arctic Ocean remain in the marine ecosystem, without substantial loss to the global atmosphere, but with possible effects on food webs.Arctic warming thaws permafrost, leading to enhanced soil mercury transport to the Arctic Ocean. Mercury isotope signatures in arctic rivers, ocean and atmosphere suggest that permafrost mercury is buried in marine sediment and not emitted to the global atmospherePeer reviewe

The Odds and Evens of Mercury Isotopes: Applications of Mass-Dependent and Mass-Independent Isotope Fractionation

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155682/1/Bergquist_et_al_2009_Odds_and_evens.pd

Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155870/1/Bergquist_et_al_2007_Mass-dependent_and_independent.pd

Reporting of variations in the natural isotopic composition of mercury

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155865/1/Blum_et_al_2007_Reporting_of_variations.pd

Effects of ultraviolet radiation on mercury isotope fractionation during photo-reduction for inorganic and organic mercury species

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155743/1/Rose_et_al_2015_Effects_of_ultraviolet.pd

Mercury Stable Isotopes in Ornithogenic Deposits As Tracers of Historical Cycling of Mercury in Ross Sea, Antarctica

Production of methylmercury (MeHg) in ocean waters and its bioaccumulation in marine organisms are critical processes controlling the fate and toxicity of mercury (Hg). However, these processes are not well understood in the Antarctic, where high levels of MeHg are observed in the subsurface ocean (100–1000 m). We explored the use of Hg stable isotope compositions in historical and modern biological deposits as a new approach for discerning Hg sources and tracing MeHg cycling in the ocean and bioaccumulation in marine biota. We found similar mass independent isotope fractionation (MIF) of Hg between a sediment profile containing historical penguin and seal feces deposits from coastal Antarctica and modern penguin and seal feces, suggesting that penguin and seal feces were the dominant sources of Hg to the sediments at different time periods. Furthermore, sediments dominated by seal feces displayed a significantly lower MIF slope (Δ¹⁹⁹Hg/Δ²⁰¹Hg) than those dominated by penguin feces despite similar extents of MIF. Since seals forage at greater depths (>400 m) than penguins (<100 m), the high MIF values and lower Δ¹⁹⁹Hg/Δ²⁰¹Hg in seal feces suggest that a significant fraction of MeHg accumulated by seals was produced in situ in the subsurface ocean from residual inorganic Hg­(II) that sank from the euphotic zone after partial photoreduction. Our results suggest that in situ Hg methylation can be an important source of MeHg for marine biota, and Hg isotope compositions in biological archives can be valuable tracers of MeHg cycling

Determination of the decay-constant of Rb-87 by laboratory accumulation of Sr-87

The decay-constant of the geochronometer87Rb (λ87) has been measured numerous times over the course of nearly a century, yet consensus over an accurate determination has remained elusive. This has resulted in persistent uncertainty, with different labor

Mercury Isotope Fractionation during Aqueous Photoreduction of Monomethylmercury in the Presence of Dissolved Organic Matter

Monomethylmercury (MMHg) is a toxic pollutant that bioaccumulates in aquatic food webs. A major mechanism that limits MMHg uptake by biota is photodemethylation in surface waters. Recently, the extent of mass-independent fractionation (MIF) of Hg isotopes preserved in fish is being used to quantify this MMHg sink. Here, the effects of different types and amounts of DOM on Hg MIF during MMHg photodemethylation were investigated to assess how variable MIF enrichment factors may be with respect to changing DOM binding sites. From experiments conducted with varying amounts of reduced organic sulfur (S_red–DOM), the extent and signature of MIF is likely dependent on whether MMHg is dominantly bound to S_red–DOM. Similar enrichment factors were observed for low MMHg:S_red–DOM experiments, where S_red–DOM was in far excess of MMHg. In contrast, significantly lower and variable enrichment factors were observed for experiments with higher MMHg:S_red–DOM ratios. Additionally the relationship between the two odd Hg isotopes that display MIF (Δ¹⁹⁹Hg/Δ²⁰¹Hg) was consistent for the low MMHg:S_red–DOM experiments, while lower Δ¹⁹⁹Hg/Δ²⁰¹Hg relationships were observed for the higher MMHg:S_red–DOM experiments. These results suggest that both the extent and signature of MMHg MIF are sensitive to different ligands that bind MMHg in nature