Bitumen II from the Paleoproterozoic Here’s Your Chance Pb/Zn/Ag deposit: Implications for the analysis of depositional environment and thermal maturity of hydrothermally-altered sediments

The formation of sedimentary exhalative (SEDEX) Pb/Zn deposits is linked to ocean euxinia, but recent evidence suggests that ferruginous conditions may have dominated the deep ocean during the Middle Proterozoic, a maximum period for SEDEX distribution. Biomarkers of sulfate-reducing and sulfide-oxidising bacteria are valuable indicators of euxinic conditions in such settings. Organic matter (OM) from SEDEX deposits is often affected by alteration and/or migration, but OM entrapped within the kerogen/mineral matrix (Bitumen II) may be less affected than the freely-extractable OM (Bitumen I). We analysed Bitumen II from the Paleoproterozoic Here’s Your Chance (HYC) Pb/Zn/Ag deposit to find evidence of euxinic conditions in the depositional environment. n-Alkane distributions in Bitumen II are markedly distinct from previously reported Bitumen I. Bitumen II contains long-chain n-alkanes (up to C36 or C38) and a strong even-over-odd distribution in a number of samples, which are 4& to 7& depleted in 13C compared to n-alkanes in Bitumen I and verified as indigenous by comparison with d13C of isolated kerogen.These features are interpreted as evidence of sulfate-reducing and sulfide-oxidising bacteria, confirming that HYC was deposited under euxinic conditions. Bitumen II has the potential to reveal information from OM that is degraded and/or overprinted in Bitumen I. Commonly-used methylphenanthrene maturity ratios give conflicting information as to the relative maturity of Bitumens I and II. Bitumen I contains a far higher proportion of methylated phenanthrenes than Bitumen II. As Bitumen II is sequestered within the kerogen/mineral matrix it may have restricted access to the ‘methyl pool’ of organic compounds that can donate methyl groups to aromatic hydrocarbons. Parameters that include both phenanthrene and methylphenanthrenes do not appear suitable to compare the maturity of Bitumens I and II; hence there is no clear evidence that Bitumen II is of lower thermal maturity than Bitumen I

Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

Sedimentary δ15N (δ15Nsed) has been widely applied as a proxy for water column denitrification. When combined with additional productivity proxies, it provides insights into the driving forces behind long‐term changes in water column oxygenation. High‐resolution (~2 years) δ15Nsed and productivity proxy records (total organic carbon [TOC], Si/Ti, and Ca/Ti) from Santa Barbara Basin, California, were generated from a well‐dated Kasten core (SPR0901‐03KC). These records reveal the relationship between Southern California upwelling and oxygenation over the past 2,000 years. Inconsistencies between Si/Ti (coastal upwelling proxy) and TOC (total export productivity proxy) suggest wind curl upwelling influenced Southern California primary productivity, especially during intervals of weak coastal upwelling. Coherence between δ15Nsed, TOC, and drought indicators supports a local control of δ15Nsed by atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, δ15Nsed is used as a water mass tracer of denitrification signals transported north from the eastern tropical North Pacific (ETNP) via the California Undercurrent. A 1,200‐year δ15Nsed record from the Pescadero slope, Gulf of California, lies between denitrifying subsurface waters in the ETNP and Southern California. During the Medieval Climate Anomaly, coherence between Pescadero and Santa Barbara Basin δ15Nsed indicates connections between ETNP and Southern California on centennial timescales. Yet an out‐of‐phase relationship occurred when the Aleutian Low was anomalously strong during the Little Ice Age. We suggest intensified nutrient‐rich subarctic water advection might have transported high‐15N nitrate into Southern California when the California Undercurrent and ETNP denitrification weakened.Key PointsWind curl upwelling contributes to Southern California primary productivity, especially during weak coastal upwelling intervalsIntensified NPH leads to stronger denitrification through enhanced coastal upwelling and reduced rainfallCalifornia receives relatively more tropical water during the Medieval Climate Anomaly and more subarctic water during the Little Ice AgePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/1/palo20779_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/2/palo20779.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/3/palo20779-sup-0001-2019PA003577-SI.pd

Optimization of on-line hydrogen stable isotope ratio measurements of halogen- and sulfur-bearing organic compounds using elemental analyzer–chromium/high-temperature conversion isotope ratio mass spectrometry (EA-Cr/HTC-IRMS)

RATIONALE: Accurate hydrogen isotopic analysis of halogen- and sulfur-bearing organics has not been possible with traditional high-temperature conversion (HTC) because the formation of hydrogen-bearing reaction products other than molecular hydrogen (H 2 ) is responsible for non-quantitative H 2 -yields and possible hydrogen isotopic fractionation. Our previously introduced, new chromium-based EA-Cr/HTC-IRMS (Elemental Analyzer – Chromium/High Temperature Conversion – Isotope-Ratio Mass Spectrometry) technique focused primarily on nitrogen-bearing compounds. Several technical and analytical issues concerning halogen- and sulfur-bearing samples, however, remained unresolved and required further refinement of the reactor systems. METHODS: The EA-Cr/HTC reactor was substantially modified for the conversion of halogen- and sulfur-bearing samples. The performance of the novel conversion setup for solid and liquid samples was monitored and optimized using a simultaneously operating dual- detection system of IRMS and ion trap MS. The method with several variants in the reactor, including the addition of manganese metal chips, was evaluated in three laboratories using EA-Cr/HTC-IRMS (on-line method) and compared with traditional uranium-reduction-based conversion combined with manual dual-inlet IRMS analysis (off-line method) in one laboratory. RESULTS: The modified EA-Cr/HTC reactor setup showed an overall H 2 -recovery of more than 96 % for all halogen- and sulfur-bearing organic compounds. All results were successfully normalized via 2-point calibration with VSMOW-SLAP reference waters. Precise and accurate hydrogen isotopic analysis was achieved for a variety of organics containing F-, Cl-, Br-, I-, and S-bearing heteroelements. The robust nature of the on-line EA-Cr/HTC technique was demonstrated by a series of 196 consecutive measurements with a single reactor filling. CONCLUSIONS: The optimized EA-Cr/HTC reactor design can be implemented in existing analytical equipment using commercially available material and is universally applicable for both heteroelement-bearing and heteroelement-free organic-compound classes. The sensitivity and simplicity of the on-line EA-Cr/HTC-IRMS technique provide a much needed tool for routine hydrogen-isotope source tracing of organic contaminants in the environment

Changes of palaeoenvironmental conditions recorded in Late Devonian reef systems from the Canning Basin, Western Australia: A biomarker and stable isotope approach

Although the Late Devonian extinctions were amongst the largest mass extinction events in the Phanerozoic, the causes, nature and timing of these events remain poorly restrained. In addition to the most pronounced biodiversity loss at the Frasnian–Famennian (F–F) boundary and the end Famennian, there were also less extensively studied extinction pulses in the Middle to Late Givetian and the Frasnian. Here we used a combination of palynological, elemental, molecular and stable isotope analyses to investigate a sedimentary record of reef-systems from this time period in the Canning Basin, Western Australia. The acquired data generally showed distinct variations between sediments from (i) the time around the Givetian–Frasnian (G–F) boundary and (ii) later in the Frasnian and indicated a distinct interval of biotic stress, particularly for reef-builders, in the older sediments. Alterations of pristane/phytane ratios, gammacerane indices, Chlorobi biomarkers, δDkerogen and chroman ratios describe the change from a restricted marine palaeoenvironment with an anoxic/euxinic hypolimnion towards a presumably open marine setting with a vertically mixed oxic to suboxic water column. Simultaneous excursions in δ13C profiles of carbonates, organic matter (OM) and hydrocarbons in the older sediments reflect the stratification-induced enhancement of OM-recycling by sulfate reducing bacteria. Alterations in sterane distributions and elevated abundances of methyltrimethyltridecylchromans (MTTCs) and perylene indicate an increased terrigenous nutrient input via riverine influx, which would have promoted stratification, phytoplankton blooms and the development of lower water column anoxia.The detected palaeoenvironmental conditions around the G–F boundary may reflect a local or global extinction event. Our data furthermore suggest a contribution of the higher plant-expansion and photic zone euxinia to the Late Devonian extinctions, consistent with previous hypotheses. Furthermore, this work might contribute to the understanding of variations in Devonian reef margin and platform-top architecture, relevant for petroleum exploration and development in the global Devonian hydrocarbon resources