Zunli Lu

Commemorating two centuries of iodine research: An interdisciplinary overview of current research

Iodine was discovered as a novel element in 1811 during the Napoleonic Wars. To celebrate the bicentennial anniversary of this event we reflect on the history and highlight the many facets of iodine research that have evolved since its discovery. Iodine has an impact on many aspects of life on Earth as well as on human civilization. It is accumulated in high concentrations by marine algae, which are the origin of strong iodine fluxes into the coastal atmosphere which influence climatic processes, and dissolved iodine is considered a biophilic element in marine sediments. Iodine is central to thyroid function in vertebrates, with paramount implications for human health. Iodine can exist in a wide range of oxidation states and it features a diverse supramolecular chemistry. Iodine is amenable to several analytical techniques, and iodine compounds have found widespread use in organic synthesis. Elemental iodine is produced on an industrial scale and has found a wide range of applications in innovative materials, including semiconductors--in particular, in solar cells.

Iodine to calcium ratios in marine carbonate as a paleo-redox proxy during oceanic anoxic events

Periods of globally distributed extreme oxygen depletion, so-called oceanic anoxic events (OAEs), have been recognized in the Mesozoic geological record and appear to be characteristic of episodes of extreme warmth. Here we explore the application of iodine/calcium ratios (I/Ca) in marine carbonate as a new geochemical proxy to constrain seawater redox change, and provide additional insights into the response of ocean chemistry to ancient climatic warming. Iodine has long been known as a redox-sensitive and biophilic element, mainly present as iodate and iodide in seawater, iodate converting to iodide under anoxic conditions. Laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure, likely substituting for the CO 3 2− ion. A fall in the I/Ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early Toarcian and Cenomanian-Turonian OAEs is interpreted both as a response to a decrease in the iodate/iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic-matter burial. The results suggest that I/Ca ratios in carbonates may be used to monitor seawater oxidation levels throughout Earth history.

An iodine record of Paleoproterozoic surface ocean oxygenation

Constraining oxygen levels in the early Precambrian surface ocean has been a longstanding goal, but efforts have been challenged by the availability of suitable proxies. Here we present a novel approach, iodine geochemistry, which broadens our perspective by providing constraints on shallow, carbonate-dominated marine settings. Iodate (IO 3 – ) persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals, allowing iodine-to-calcium ratios (I/Ca) in shallow carbonates to be used as a paleoredox indicator. Our data from a series of Mesoarchean through Paleoproterozoic carbonates deposited under shallow-marine conditions reveal a progressive surface ocean oxygenation in the early Paleoproterozoic. These data seem to indicate that a largely anoxic surface ocean extended throughout the Archean until the Great Oxidation Event (GOE) at ca. 2.4 Ga, implying that previous inferences of pre-GOE oxygen production may reflect oxygen oases, transient oxidation events, or oxygen levels below those required for IO 3 – accumulation. The data suggest formation and persistence of IO 3 – and, consequently, surface ocean oxygen concentrations of at least 1 μM during the GOE. Following the initial rise of oxygen, carbonate-associated iodine in globally extensive carbonate units deposited during the Lomagundi positive carbon isotope excursion at ca. 2.22–2.1 Ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event, synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions.

I/Ca evidence for upper ocean deoxygenation during the PETM

Anthropogenic global warming affects marine ecosystems in complex ways, and declining ocean oxygenation is a growing concern. Forecasting the geographical and bathymetric extent, rate, and intensity of future deoxygenation and its effects on oceanic biota, however, remains highly challenging because of the complex feedbacks in the Earth-ocean biota system. Information on past global warming events such as the Paleocene-Eocene Thermal Maximum (PETM, ~55.5 Ma), a potential analog for present and future global warming, may help in such forecasting. Documenting past ocean deoxygenation, however, is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column. As yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones (OMZs). We apply a novel proxy for paleoredox conditions, the iodine to calcium ratio (I/Ca) in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans, and compared our reconstruction with modeled oxygen levels. The reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the Atlantic, Indian Oceans, and possibly the Pacific Ocean, as well during the PETM, due to vertical and potentially lateral expansion of OMZs.

Upper ocean oxygenation dynamics from I/Ca ratios during the Cenomanian-Turonian OAE 2

Global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to catastrophic marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially for the suboxic window. To address this problem, we use bulk carbonate I/Ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the Cretaceous OAE 2. In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and excursions in I/Ca through the OAE interval, reflecting the importance of local controls and suggesting a high spatial variability in redox state. Both I/Ca and an Earth System Model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, indicating that anoxia was not global during OAE 2 and that important regional differences in redox conditions existed. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.

Halogen and 129I systematics in gas hydrate fields at the northern Cascadia margin (IODP Expedition 311): Insights from numerical modeling

We measured halogen concentrations and I-129/I ratios in five drilling sites of Integrated Ocean Drilling Program Expedition 311 (offshore Vancouver Island, Canada) in order to identify potential sources of fluids and methane in gas hydrate fields. Iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope I-129 (T-1/2 = 15.7 Ma) allows the age determination of organic sources for iodine. Here we report halogen concentrations in 135 pore water samples, I concentrations in 48 sediment samples, and I-129/I ratios measured in a subset of 20 pore water samples. Most I-129/I ratios fall into a range around 500 x 10(-15), corresponding to a minimum age of 25 Ma and the lowest ratio of 188 x 10(-15) (T-min = 47 Ma) was observed at 208 m below sea floor (mbsf) in Site 1326. These ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the Juan de Fuca plate, indicating that old, accreted sediments in the accretionary wedge contribute a significant amount of iodide and, by association, of methane to the gas hydrate occurrences. A geochemical transport-reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide, bromide, and ammonia in the host sediments due to organic matter degradation. The model was first tested with data from two well studied areas, Ocean Drilling Program Site 1230 (Peru margin) and Site 1245 (Hydrate Ridge). The model results for the Expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids, carrying large amounts of old iodine from deep sources. The comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry, especially at the Cascadia margin. The iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields.

Age variation of pore water iodine in the eastern Nankai Trough, Japan: Evidence for different methane sources in a large gas hydrate field

The 129 I geochronology of marine pore water is useful for the understanding of the origin of methane in gas hydrates because of the close association between I and marine organic materials responsible for methane generation. We report 129 I/I ratios in pore waters from three deep cores in the eastern Nankai Trough gas hydrate field, two located on the outer ridge and one in the forearc basin. As in previous studies of gas hydrate fields, I ages of pore water are consistently older than those of the host sediments. For the first time, however, the results demonstrate that the potential I source formations vary considerably across the forearc setting: While I at the basin site reaches ages close to 50 Ma, all I ages at the two ridge sites are

Oxygen depletion recorded in upper waters of the glacial Southern Ocean

Oxygen depletion in the upper ocean is commonly associated with poor ventilation and storage of respired carbon, potentially linked to atmospheric CO2 levels. Iodine to calcium ratios (I/Ca) in recent planktonic foraminifera suggest that values less than ∼2.5 μmol mol−1 indicate the presence of O2-depleted water. Here we apply this proxy to estimate past dissolved oxygen concentrations in the near surface waters of the currently well-oxygenated Southern Ocean, which played a critical role in carbon sequestration during glacial times. A down-core planktonic I/Ca record from south of the Antarctic Polar Front (APF) suggests that minimum O2 concentrations in the upper ocean fell below 70 μmol kg−1 during the last two glacial periods, indicating persistent glacial O2 depletion at the heart of the carbon engine of the Earths climate system. These new estimates of past ocean oxygenation variability may assist in resolving mechanisms responsible for the much-debated ice-age atmospheric CO2 decline.

Using discriminant analysis to determine sources of salinity in shallow groundwater prior to hydraulic fracturing.

High-volume hydraulic fracturing (HVHF) gas-drilling operations in the Marcellus Play have raised environmental concerns, including the risk of groundwater contamination. Fingerprinting water impacted by gas-drilling operations is not trivial given other potential sources of contamination. We present a multivariate statistical modeling framework for developing a quantitative, geochemical fingerprinting tool to distinguish sources of high salinity in shallow groundwater. The model was developed using new geochemical data for 204 wells in New York State (NYS), which has a HVHF moratorium and published data for additional wells in NYS and several salinity sources (Appalachian Basin brines, road salt, septic effluent, and animal waste). The model incorporates a stochastic simulation to predict the geochemistry of high salinity (>20 mg/L Cl) groundwater impacted by different salinity sources and then employs linear discriminant analysis to classify samples from different populations. Model results indicate Appalachian Basin brines are the primary source of salinity in 35% of sampled NYS groundwater wells with >20 mg/L Cl. The model provides an effective means for differentiating groundwater impacted by basin brines versus other contaminants. Using this framework, similar discriminatory tools can be derived for other regions from background water quality data.

Methane occurrence is associated with sodium‐rich valley waters in domestic wells overlying the Marcellus shale in New York State

Prior work suggests spatial parameters (e.g., landscape position, distance to nearest gas well) can be used to estimate the amount of dissolved methane in domestic drinking water wells overlying the deep Marcellus Shale. New York (NY) provides an opportunity to investigate methane occurrence prior to expansion of high-volume hydraulic fracturing because unconventional gas production is currently banned in the state. We sampled domestic groundwater wells for methane in 2013 (n = 137) across five counties of NY bordering Pennsylvania, and then resampled a subset of those wells in 2014 for methane concentrations and δ13C-CH4 and δD-CH4. The majority of waters from wells sampled (77%) had low concentrations of methane ( 10 mg/L). Dissolved methane concentrations did not change as a function of proximity to existing vertical gas wells, nor other parameters indicating subsurface planes of weakness (i.e., faults or lineaments). Methane levels were significantly higher in wells closer to hydrography flow lines, and most strongly correlated to Na-HCO3 water type. The distribution of methane between Ca-HCO3 (n = 76) and Na-HCO3 (n = 23) water types significantly differed (p < 0.01), with median methane concentrations of 0.002 and 0.78 mg/L, respectively. Combined classification of sampled waters based on the dominant water cation, well topographic position, and geologic unit of well completion effectively identified wells with a greater than 50% probability of having methane concentrations exceeding 1 mg/L. Such classification schemes may be useful as a screening tool to assess natural versus gas production-related sources of methane in domestic wells.