High frequency measurements of soil carbon dioxide flux at Harvard Forest

Soil carbon dioxide flux was measured by automatic chambers at Harvard Forest over a four-year period, 2003-2006. The autochambers were installed along a moisture gradient from upland to wetland soils. In 2003, fluxes from the upland and mid-slope chambers exceeded the fluxes from the wetland margin. In 2004-2006, the mid-slope fluxes were significantly larger than both the upland and wetland margin chambers. The differences in flux between chamber location were most pronounced in the late summer and early fall. Residuals from a non-linear temperature regression exhibit a distinct seasonal pattern in 2003, 2004, and 2006, but not in 2005. On short time scales, the residuals are correlated with soil moisture, responding to precipitation events. The seasonal pattern of soil flux reaches a maximum later in the year than ecosystem respiration measured at the eddy covariance flux tower

Unraveling depositional and diagenetic signals in magnetic susceptibility in methane-bearing sediments along the Indian, Cascadia, and Japanese margins

Magnetic susceptibility is a bulk measure that reveals variation in ferromagnetic mineral content. High-resolution measurement of magnetic susceptibility in ocean drilling records reveals variability that can be attributed to primary depositional processes and/or secondary diagenetic processes that occur after deposition. Each chapter of my dissertation investigates magnetic susceptibility records along with geochemical, mineralogical, and rock magnetic techniques in methane-bearing marine sediments along the Indian, Cascadia, and Japanese margins. The overall goal of this work is to improve the understanding of the effects of detrital and biogeochemical processes on magnetic mineralogy, and thus magnetic susceptibility, in these continental margin marine environments. In the first study (Chapter 1; Phillips et al., 2014), using a multi-proxy approach, I investigated variation in productivity and weathering over the last 110,000 years in the northern Indian Ocean within the core Indian monsoon rainfall zone. These results reveal an increase in productivity due to reduced stratification and a decrease in weathering during the last glacial period due to a weakened summer monsoon. This work reveals a relationship between Zr/Rb and magnetic susceptibility that can be utilized to predict primary detrital magnetic susceptibility. In the second study, I used an elemental analysis and rock magnetic approach to decouple detrital and diagenetic patterns in magnetic susceptibility at three sites along the Cascadia accretionary wedge. Each site reveals intervals of diagenetic loss in magnetic susceptibility that is balanced by an increase in sulfur due to dissolution of magnetite and precipitation of pyrite. The diagenetic loss of magnetic susceptibility is influenced by organic matter availability as well as upward methane flux. In the third study, I used a rock magnetic approach to investigate the magnetic mineralogy in a deep sediment record down to ~2.5 km below the seafloor offshore northern Honshu, Japan. The magnetic susceptibility record reveals cm-scale increases that are likely the result of density sorting causing concentration of heavy minerals. The magnetic mineral assemblage is dominated by titanomagnetite with an increase in Ti-rich titanomagnetite associated with deeply buried (~2 km) coal beds. This change along the titanomagnetite solid solution series, may represent selective dissolution of Ti-poor, iron(III)-rich magnetite during microbial iron reduction since burial

Interannual, seasonal, and diel variation in soil respiration relative to ecosystem respiration at a wetland to upland slope at Harvard Forest

Soil carbon dioxide efflux (soil respiration, SR) was measured with eight autochambers at two locations along a wetland to upland slope at Harvard Forest over a 4 year period, 2003–2007. SR was consistently higher in the upland plots than at the wetland margin during the late summer/early fall. Seasonal and diel hystereses with respect to soil temperatures were of sufficient magnitude to prevent quantification of the influence of soil moisture, although apparent short‐term responses of SR to precipitation occurred. Calculations of annual cumulative SR illustrated a decreasing trend in SR over the 5 year period, which were correlated with decreasing springtime mean soil temperatures. Spring soil temperatures decreased despite rising air temperatures over the same period, possibly as an effect of earlier leaf expansion and shading. The synchronous decrease in spring soil temperatures and SR during regional warming of air temperatures may represent a negative feedback on a warming climate by reducing CO2 production from soils. SR reached a maximum later in the year than total ecosystem respiration (ER) measured at a nearby eddy covariance flux tower, and the seasonality of their temperature response patterns were roughly opposite. SR, particularly in the upland, exceeded ER in the late summer/early fall in each year, suggesting that areas of lower efflux such as the wetland may be significant in the flux tower footprint or that long‐term bias in either estimate may create a mismatch. Annual estimates of ER decreased over the same period and were highly correlated with SR

Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)

Sediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a fore‐arc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3–7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coal‐bearing unit (∼2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very low‐coercivity minerals with unblocking temperatures between 350 and 580°C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300–400°C), higher ARM, higher‐frequency dependence, and isotopically heavy δ56Fe across a range of lithologies in the coal‐bearing unit (between 1925 and 1995 mbsf) indicate the presence of fine‐grained authigenic magnetite. We suggest that iron‐reducing bacteria facilitated the production of fine‐grained magnetite within the coal‐bearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply iron‐reducing bacteria in the surrounding siliciclastic sediments. These results indicate that coal‐bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere

Primary deposition and early diagenetic effects on the high saturation accumulation of gas hydrate in a silt dominated reservoir in the Gulf of Mexico

On continental margins, high saturation gas hydrate systems (\u3e60% pore volume) are common in canyon and channel environments within the gas hydrate stability zone, where reservoirs are dominated by coarse-grained, high porosity sand deposits. Recent studies, including the results presented here, suggest that rapidly deposited, silt-dominated channel-levee environments can also host high saturation gas hydrate accumulations. Here we present several sedimentological data sets, including sediment composition, biostratigraphic age from calcareous nannofossils, grain size, total organic carbon (TOC), C/N elemental ratio, δ13C-TOC, CaCO3, total sulfur (TS), and δ34S-TS from sediments collected with pressure cores from a gas hydrate rich, turbidite channel-levee system in the Gulf of Mexico during the 2017 UT-GOM2-1 Hydrate Pressure Coring Expedition. Our results indicate the reservoir is composed of three main lithofacies, which have distinct sediment grain size distributions (type A-silty clay to clayey silt, type B-clayey silt, and type C-sandy silt to silty sand) that are characteristic of variable turbidity current energy regimes within a Pleistocene (\u3c 0.91 Ma) channel-levee environment. We document that the TOC in the sediments of the reservoir is terrestrial in origin and contained within the fine fraction of each lithofacies, while the CaCO3 fraction is composed of primarily reworked grains, including Cretaceous calcareous nannofossils, and part of the detrital load. The lack of biogenic grains within the finest grained sediment intervals throughout the reservoir suggests interevent hemipelagic sediments are not preserved, resulting in a reservoir sequence of silt dominated, stacked turbidites. We observe two zones of enhanced TS at the top and bottom of the reservoir that correspond with enriched bulk sediment δ34S, indicating stalled or slowly advancing paleo-sulfate-methane transition zone (SMTZ) positions likely driven by relative decreases in sedimentation rate. Despite these two diagenetic zones, the low abundance of diagenetic precipitates throughout the reservoir allowed the primary porosity to remain largely intact, thus better preserving primary porosity for subsequent pore-filling gas hydrate. In canyon, channel, and levee environments, early diagenesis may be regulated via sedimentation rates, where high rates result in rapid progression through the SMTZ and minimal diagenetic mineralization and low rates result in the stalling of the SMTZ, enhancing diagenetic mineralization. Here, we observed some enhanced pyritization to implicate potential sedimentation rate changes, but not enough to consume primary porosity, resulting in a high saturation gas hydrate reservoir. These results emphasize the important implications of sedimentary processes, sedimentation rates, and early diagenesis on the distribution of gas hydrate in marine sediments along continental margins

Isolating detrital and diagenetic signals in magnetic susceptibility records from methane-bearing marine sediments

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Johnson, J. E., Phillips, S. C., Clyde, W. C., Giosan, L., & Torres, M. E. Isolating detrital and diagenetic signals in magnetic susceptibility records from methane-bearing marine sediments. Geochemistry Geophysics Geosystems, 22(9), (2021): e2021GC009867, https://doi.org/10.1029/2021GC009867.Volume-dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post-depositional alteration by H2S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane-bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X-ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high-resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ13C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo-positions of the sulfate-methane transition zone, past enhancements of OSR or paleo-methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps.This research was supported by the American Chemical Society-Petroleum Research Fund Award #53006-ND8 and U.S. Department of Energy Grant #DE-FE0010120

Isolating Detrital and Diagenetic Signals in Magnetic Susceptibility Records From Methane-Bearing Marine Sediments

Volume-dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post-depositional alteration by H2S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane-bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X-ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high-resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ13C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo-positions of the sulfate-methane transition zone, past enhancements of OSR or paleo-methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps