Chemical weathering over geological timescales acts as a source or sink of atmospheric carbon dioxide (CO2), while influencing long-term redox cycling and atmospheric oxygen (O-2) at Earth's surface. There is a growing recognition that the oxidative weathering of rock organic carbon (OCpetro) can release more CO2 than is locally drawn down by silicate weathering, and may vary due to changes in erosion and climate. The element rhenium (Re) has emerged as a proxy to track the oxidative weathering of OCpetro, yet uncertainties in its application remain namely that we lack a systematic assessment of the comparative mobility of Re and OCpetro during sedimentary rock weathering. Here we measure Re and OCpetro loss across gradients in rock weathering at 9 global sites, spanning a range of initial OCpetro values from similar to 0.2 % to >10 %. We use titanium to account for volume changes during weathering and assess Re and OCpetro loss alongside major elements that reflect silicate (Na, Mg), carbonate (Ca, Mg) and sulfide (S) weathering. Across the dataset, Re loss is correlated with OCpetro loss but not with loss of any other major element. Across the weathering profiles, the average molar ratio of OCpetro to Re loss was 0.84 +/- 0.15, with 8 out of 9 sites having a ratio >0.74. At one site (Marcellus Shale), the average ratio was lower at 0.58 +/- 0.11. The excess loss of Re matches expectations that, typically, between similar to 0 and 20 % of the Re liberated by sedimentary rock weathering derives from silicate or sulfide phases, while some OCpetro may be physically or chemically protected from weathering. Overall, our measurements provide validation for the Re proxy of OCpetro oxidation and allow future work to further improve our knowledge of regional and global-scale rates of this important source of CO2 in the geochemical carbon cycle.

Post-glacial sea level rise led to a direct connection between the Arctic and Pacific Oceans via the Bering Strait. Consequently, the Bering Sea experienced changes in connectivity, size, and sediment sources that were among the most drastic of any ocean basin in the past 30,000 years. However, the sedimentary response to the interplay between climate change and sea level rise in high-latitude settings such as Beringia remains poorly resolved. To ascertain changes in sediment delivery, productivity, and regional oceanography from the Last Glacial Maximum (LGM) to the Holocene, we analyzed sedimentological, geochemical, and isotopic characteristics of three sediment cores from the Bering Sea. Interpretations of productivity, terrestrial input, nutrient utilization, and circulation are based on organic carbon isotopes (delta C-13(org)), total organic carbon (TOC), bulk nitrogen isotopes, total organic nitrogen, carbon/nitrogen ratios, elemental X-ray fluorescence data, grain size, and presence of laminated or dysoxic, green intervals. Principal component analysis of these data captures key climatic intervals. The LGM was characterized by low productivity across the region. In the Bering Sea, deglaciation began around 18-17 ka, with increasing terrestrial sediment and TOC input. Marine productivity increased during the BOlling-AllerOd when laminated sediments revealed dysoxic bottom waters where denitrification was extreme. The Younger Dryas manifested increased terrestrial input and decreased productivity, in contrast with the Pre-Boreal, when productivity markedly rebounded. The Pre-Boreal and Bolling-Allerod were similarly productive, but changes in the source of TOC and a delta C-13(org) depletion suggest the influence of a gradually flooding Bering Shelf during the Pre-Boreal and Holocene.

Weathering of organic carbon is the process in the geochemical carbon cycle in which organic carbon in the rock reservoir is restored to surficial carbon reservoirs. Approximately half of this weathering occurs as oxidation, in which organic carbon is oxidized to CO2. Organic carbon weathering is, thus, an important control on the O2 and CO2 content of Earth's atmosphere over geologic time. The remainder of carbon weathering occurs through chemical and physical weathering of ancient rocks, and delivers aged fossil organic carbon to soils, rivers, and ultimately the oceans where it may be reburied in modern sediments. During weathering, a suite of chemical changes in organic matter abundance and composition are observed, some of which may be the result of biodegradation. Geochemical models provide one means of estimating annual fluxes of organic carbon weathering and their changes over geologic time. Other approaches examine chemical, mineralogical, and isotopic signatures of aged fossil carbon in river particulates and sediments.

Molecular oxygen (O2) is generated and consumed by a wide span of biological and abiotic processes. Chief among these are photosynthesis and respiration, but on geologic timescales, burial and weathering of redox-active elements such as carbon and sulfur exert strong controls on atmospheric O2 concentrations. The global cycling of O2 is controlled by a coupled system of physical, chemical, and most importantly, biological processes acting on and in the Earth's surface. Large inroads have been made toward understanding the processes that control the concentration of atmospheric O2, especially regarding O2 as a component of coupled biogeochemical cycles of many elements, including carbon, sulfur, nitrogen, phosphorus, iron, and others. Numerical models of sedimentary rock abundance and of the isotope systematics of the surficial cycles of carbon and sulfur provide useful insights into the evolution of atmospheric O2 through time. At the same time, advances in isotope geochemistry and geobiology contribute to an ever more detailed understanding of the geologic history of atmospheric O2 from the early Earth through the Phanerozoic.

Stable isotope data from lipid biomarkers and diatom silica recovered from lake sediment cores hold great promise for paleoclimate and paleohydrological reconstructions. However, these records rely on accurate calibration with modern precipitation and hydrologic processes and only limited data exist on the controls on the δD values for n-alkanoic acids from plant leaf waxes. Here we investigate the stable isotopic composition of modern precipitation, streams, lake water and ice cover, and use these data to constrain isotope systematics of the Lake El'gygytgyn Basin hydrology. Compound-specific hydrogen isotope ratios determined from n-alkanoic acids from modern vegetation are compared with modern precipitation and lake core top sediments. Multi-species net (apparent) fractionation values between source water (precipitation) and modern vegetation (e.g., ϵwax/precip mean value is −107 ± 12‰) agree with previous results and suggest a consistent offset between source waters and the δD values of alkanoic acids. We conclude that although there may be some bias towards a winter precipitation signal, overall δD values from leaf wax n-alkanoic acids record annual average precipitation within the El'gygytgyn Basin. A net fractionation calculated for 200-yr-integrated lake sediments yields ϵ30/precip = −96 ± 8‰ and can provide robust net "apparent" fractionation to be used in future paleohydrological reconstructions.

Marine isotope stages (MIS) 11 has been proposed as an analog for the present interglacial; however, terrestrial records of this time period are rare. Sediments from Lake El'gygytgyn (67°30' N, 172°5' E) in Far East Russia contain a 3.56 Ma record of climate variability from the Arctic. Here, we present the first terrestrial Arctic reconstruction of environmental and climatic changes from MIS 8 through 12 (289 to 464 ka) using organic geochemical proxies. Terrestrial vegetation changes, as revealed by plant leaf wax (n-alkane) indices and concentrations of arborinol (a biomarker for trees), show increased tree cover around the lake during interglacial periods, with higher concentrations observed during MIS 11 as compared to MIS 9. A similar pattern is also observed in records of aquatic productivity revealed by molecular indicators from dinoflagellates (dinosterol), eustigmatophyte algae (long-chain (C28–C32) 1, 15 n-alkyl diols) in addition to short-chain n-alkanes, where aquatic productivity is highest during MIS 11. Changes recorded in these molecular proxies show a similar structure to relative temperature variability as recorded by the MBT/CBT (Methylation of Branched Tetraether/Cyclization of Branched Tetraether) paleothermometer, based on branched glycerol dialkyl glycerol tetraethers (GDGTs). Additionally, relative MBT/CBT temperature changes generally track pollen and diatom δ18O temperature estimates, compiled by other studies, which suggest glacial-interglacial temperature changes of ~ 9 to 12 °C. These records of environmental and climatic change indicate Arctic sensitivity to external forcings such as orbital variability and atmospheric greenhouse gas concentrations. Overall, this study indicates that organic geochemical analyses of the Lake El'gygytgyn sediment archive can provide critical insight into the response of lake ecosystems and their sensitivity in high latitude regions.

Arctic paleoenvironmental archives serve as sensitive recorders of past climate change. Lake El'gygytgyn (Far East Russian Arctic) is a high-latitude crater impact lake that contains a continuous sediment record influenced by neither glaciation nor glacial erosion since the time of impact 3.58 Ma ago. Prior research on sediments collected from Lake El'gygytgyn suggest times of permanent ice cover and anoxia corresponding to global glacial intervals, during which the sediments are laminated and are characterized by the co-occurrence of high total organic carbon, microscopic magnetite grains that show etching and dissolution, and negative excursions in bulk sediment organic matter carbon isotope (δ13C) values. Here we investigate the abundance and carbon isotopic composition of lipid biomarkers recovered from Lake El'gygytgyn sediments spanning marine isotope stages 1–3 to identify key sources of organic matter (OM) to lake sediments, to establish which OM sources drive the negative δ13C excursion exhibited by bulk sediment OM, and to explore if there are molecular and isotopic signatures of anoxia in the lake during glaciation. We find that during marine isotope stages 1–3, direct evidence for water column anoxia is lacking. A ~4‰ negative excursion in bulk sediment δ13C values during the Local Last Glacial Maximum (LLGM) is accompanied by more protracted, higher magnitude negative excursions in n-alkanoic acid and n-alkanol δ13C values that begin 20 kyr in advance of the LLGM. In contrast, n-alkanes and the C30 n-alkanoic acid do not exhibit a negative δ13C excursion at this time. Our results indicate that the C24, C26 and C28 n-alkanoic acids do not derive entirely from terrestrial OM sources, while the C30 n-alkanoic acid at Lake El'gygytgyn is a robust indicator of terrestrial OM contributions. Overall, our results strongly support the presence of a nutrient-poor water column, which is mostly isolated from atmospheric carbon dioxide during glaciation at Lake El'gygytgyn.

Abstract. Marine Isotope Stages (MIS) 11 has been proposed as an analog for the present interglacial; however, terrestrial records of this time period are rare. Sediments from Lake El'gygytgyn (67°30´ N, 172°5´ E) in Far East Russia contain a 3.56 Ma record of climate variability from the Arctic. Here, we present an organic geochemical reconstruction of environmental and climatic changes from MIS 8 through 12 (289 to 460 ka). Terrestrial vegetation changes, as revealed by plant leaf wax (n-alkane) indices and concentrations of arborinol (a biomarker for trees), show increased tree cover around the lake during interglacial periods, with higher concentrations observed during MIS 11 as compared to MIS 9. A similar pattern is also observed in records of aquatic productivity revealed by molecular indicators from dinoflagellates (dinosterol), eustigmatophyte algae (long-chain (C28–C32) 1,15 n-alkyl diols) in addition to short-chain nalkanes, where aquatic productivity is highest during MIS 11. Changes recorded in these molecular proxies track relative temperature variability as recorded by the MBT/CBT paleothermometer, based on branched glycerol dialkyl glycerol tetraethers (GDGTs). Additionally, relative MBT/CBT temperature changes generally track pollen and diatom δ18O temperature estimates, compiled by other studies, which suggest glacial–interglacial temperature changes of ~ 9–12 °C. These records of environmental and climatic change indicate Arctic sensitivity to external forcings such as orbital variability and atmospheric greenhouse gas concentrations. Overall, this study indicates that organic geochemical analyses of the Lake El'gygytgyn sediment archive can provide critical insight into the response of lake ecosystems and their sensitivity in high latitude regions.

We consider the effect that commercial gas production has had on microbiology and water and gas geochemistry in the northern producing trend of the Antrim Shale, an unconventional gas reservoir in the Michigan Basin, USA. We analyzed gas, water, and microbial biomass samples collected from seven wells in 2009 and compared our findings to the result of analyses performed as early as 1991 on samples collected from the same wells. We also examined production records associated with six wells. Water production has decreased sharply over time and is currently at 0.2 to 14.6% of peak levels. While this has happened, the chemical and isotopic composition of gas and water produced from the wells has shifted. The proportion of CO2 has increased by as much as 15mole% while CH4 content has correspondingly decreased. Isotopically, the δ13C and δD values of CH4 decreased for most wells by averages of 1.3‰ and 9‰, respectively, while δ13C values of CO2 increased for most wells by an average of 1.7‰. Alkalinity in the water from each well decreased by 10mM on average and SO42− content increased from below 50μM to over 200μM on average in water from each well with initial values. Microorganisms most closely related to CO2-reducing methanogens were the most abundant group in archaeal clone libraries and SO42− reducers were the most abundant group in bacterial libraries. In contrast, no SO42− reducers were identified in a nucleic acid-based analysis of a sample collected in 2002 from one of the wells we sampled . Our results show that commercial gas production has not only caused chemical and isotopic changes in water and gas in the Antrim Shale but also an increase in the abundance of SO42−-reducing microorganisms, a change that can ultimately have a negative impact on biogenic CH4 formation. Processes that can explain these changes include ongoing biogeochemical reactions, groundwater flow, gas desorption, and open-system degassing.

Consortia of microbes degrade recalcitrant organic-matter in deep subsurface reservoirs, such as shales and coals, under anaerobic conditions into simple C molecules such as CO2 and acetate. These substrates are subsequently metabolized by methanogens into economic quantities of natural gas in sedimentary basins world-wide. This study explores organic matter in the Devonian New Albany Shale (Illinois Basin, USA) and associated fluids to investigate the extent of organic matter biodegradation, and evaluate the potential for stimulating in situ gas production. Identification of labile compound classes such as n-alkanes, fatty acids, and phenols in produced waters of the New Albany Shale, and low biodegradation indices in the shale core samples indicate limited biodegradation. Together with detectable acetate concentrations (up to 225.1 mu M), these observations suggest that both the supporting microbial consortia and methanogens are limited in extent and activity. By comparison, the New Albany Shale is much less biodegraded than the microbial CH4-producing Michigan Basin Antrim Shale, Powder River Basin coals, or San Juan Basin coals. In the New Albany Shale, the extent of biodegradation generally becomes more varied with higher salinities, suggesting diverse microbial adaptations to degrade OM at high salinities. Enhancement of in situ CH4 production may be most effective if targeted at stimulating production of the supporting microbial consortia as well as methanogens. (C) 2012 Elsevier Ltd. All rights reserved.

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Sediment cores from the continental shelf adjacent to the Mississippi River delta immediately after the passage of Hurricane Katrina were used to examine the magnitude, and implications for the carbon budget, of sediment and particulate organic carbon (POC) remobilized by the storm on the river‐dominated continental shelf. POC was sourced from incision of the innermost continental shelf (<25 m water depth) and from surge ebb advection from adjacent wetlands and shallow estuaries, and was re‐deposited in deeper water on the shelf. This pulse of young (<1,600 yBP) labile POC, mixed with relict (>5000 yBP) POC eroded from the seafloor, has major implications for the remineralization versus burial of POC in deltas. The scale of erosional deflation of the shelf in water depths beyond seasonal wave‐current conditions suggests that, over millennia, tropical cyclones may be responsible for partly removing prodeltaic strata from the geologic record in low‐to‐mid latitude deltas.