Devonian Upper Ocean Redox Trends Across Laurussia: Testing Potential Influences of Marine Carbonate Lithology on Bulk Rock I/Ca Signals

The Devonian is characterized by major changes in ocean-atmosphere O2 concentrations, colonialization of continents by plants and animals, and widespread marine anoxic events associated with rapid d13C excursions and biotic crises. However, the long-term upper ocean redox trend for the Devonian is still not well understood. This study presents new I/Ca data from well-dated Lower Devonian through Upper Devonian limestone sections from the Great Basin (western Laurussia) and the Illinois Basin (central Laurussia). In addition, to better address potential influences of lithology and stratigraphy on I/Ca redox signals, I/Ca data are reported here as carbonate lithology-specific. Results indicate that lithologic changes do not exert a dominant control on bulk carbonate I/Ca trends, but the effects of some diagenetic overprints cannot be ruled out. For the Illinois Basin, low I/Ca values (more reducing) are recorded during the Pragian to Emsian and increased but fluctuating values are recorded during the Eifelian to Givetian. The Great Basin I/Ca trends suggest local upper oceans were more reducing in the Lochkovian, more oxic in the Pragian- Emsian, return to more reducing in the Eifelian, then to increasingly more oxic, but fluctuating in the Givetian-Frasnian. The local I/Ca variations at Great Basin likely share more similarity with global upper ocean condition (compared to the Illinois Basin) based on its position adjacent to the Panthalassic Ocean and its temporal co-variation with global environmental volatility trends. The overall reducing and variable redox conditions of local upper ocean (if not a diagenetic signal) during the Middle and Late Devonian of Great Basin coincide with evidence of increased global environmental volatility suggesting seawater redox may have been an important part of environmental instability at this time

Pore fluid modeling approach to identify recent meltwater signals on the west Antarctic Peninsula

The sensitivity of sea level to melting from polar ice sheets and glaciers during recent natural and anthropogenic climate fluctuations is poorly constrained beyond the period of direct observation by satellite. We have investigated glacial meltwater events during the Anthropocene by adapting the pioneering approach of modeling trends in d18O in the pore waters of deep‐sea cores, previously used to constrain the size of ice sheets during the Last Glacial Maximum. We show that during recent warm periods, meltwater from glacier retreat drains into the coastal fjords, leaving a signature of depleted d18O values and low Cl concentrations in the pore water profiles of rapidly accumulating sediments. Here we model such pore water profiles in a piston core to constrain the timing and magnitude of an ice sheet retreat event at Caley Glacier on the west Antarctic Peninsula, and the result is compared with local ice front movement. This approach of pore water modeling was then applied in another kasten core and tested by a series of sensitivity analyses. The results suggest that our approach may be applied in fjords of different sedimentary settings to reconstruct the glacier history and allow insight into the sensitivity of polar glaciers to abrupt warming events

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 Earth’s 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

Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 463 (2017): 159-170, doi:10.1016/j.epsl.2017.01.032.The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today’s, although how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo- and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals.TL, ZL, and DH thank NSF EAR-1349252. ZL further thanks OCE-1232620. DH, ZL, and TL acknowledge further funding from a NASA Early Career Collaboration Award. TL, AB, NP, DH, and AK thank the NASA Astrobiology Institute. TL and NP received support from the Earth-Life Transitions Program of the NSF. AB acknowledges support from NSF grant EAR-05-45484 and an NSERC Discovery and Accelerator Grants. CW acknowledges support from NSFC grant 40972021

Halogen and ¹²⁹-I 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

Old iodine in fluids venting along the central American convergent margin

Focused fluid expulsion at cold vents is a common feature of subduction zones, serving as an important backflux of water and volatile elements to the oceanic reservoir. The strong enrichment of iodine in fluids collected from mounds along the Central American convergent Margin allowed the determination of 129I/I ratios for age calculations in order to determine potential source formations in this active, erosional margin. The majority of the determined iodine ages are between 40 and 20 Ma. Because these ages are older than the age of host sediments and underthrust sediments on the oceanic plate (<18 Ma), a major contribution of iodine must come from old, organic rich sources in the upper plate. Both the iodine concentrations and ages determined for the mounds in this study are similar to reported values for hydrate fields at accretionary margins, indicating that iodine and associated organic carbon cycling at both erosional and accretionary margins may occur on similar time scales

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

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 30 (2015): 510–526, doi:10.1002/2014PA002741.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.Z.L. thanks NSF OCE 1232620. J.D.O. is supported by an Agouron Postdoctoral Fellowship. T.W.L. acknowledges support from the NSF-EAR and NASA-NAI. A.R. thanks the support of NERC via NE/J01043X/1.2015-11-1