Carbonate chimneys at the highly productive point Dume methane seep: Fine-scale mineralogical, geochemical, and microbiological heterogeneity reflects dynamic and long-lived methane-metabolizing habitats.

Methane is a potent greenhouse gas that enters the marine system in large quantities at seafloor methane seeps. At a newly discovered seep site off the coast of Point Dume, CA, ~ meter-scale carbonate chimneys host microbial communities that exhibit the highest methane-oxidizing potential recorded to date. Here, we provide a detailed assessment of chimney geobiology through correlative mineralogical, geochemical, and microbiological studies of seven chimney samples in order to clarify the longevity and heterogeneity of these highly productive systems. U-Th dating indicated that a methane-driven carbonate precipitating system at Point Dume has existed for ~20 Kyr, while millimeter-scale variations in carbon and calcium isotopic values, elemental abundances, and carbonate polymorphs revealed changes in carbon source, precipitation rates, and diagenetic processes throughout the chimneys' lifespan. Microbial community analyses revealed diverse modern communities with prominent anaerobic methanotrophs, sulfate-reducing bacteria, and Anaerolineaceae; communities were more similar within a given chimney wall transect than in similar horizons of distinct structures. The chimneys represent long-lived repositories of methane-oxidizing communities and provide a window into how carbon can be transformed, sequestered, and altered over millennia at the Point Dume methane seep.

Balance and imbalance in biogeochemical cycles reflect the operation of closed, exchange, and open sets.

Biogeochemical reactions modulate the chemical composition of the oceans and atmosphere, providing feedbacks that sustain planetary habitability over geological time. Here, we mathematically evaluate a suite of biogeochemical processes to identify combinations of reactions that stabilize atmospheric carbon dioxide by balancing fluxes of chemical species among the ocean, atmosphere, and geosphere. Unlike prior modeling efforts, this approach does not prescribe functional relationships between the rates of biogeochemical processes and environmental conditions. Our agnostic framework generates three types of stable reaction combinations: closed sets, where sources and sinks mutually cancel for all chemical reservoirs; exchange sets, where constant ocean-atmosphere conditions are maintained through the growth or destruction of crustal reservoirs; and open sets, where balance in alkalinity and carbon fluxes is accommodated by changes in other chemical components of seawater or the atmosphere. These three modes of operation have different characteristic timescales and may leave distinct evidence in the rock record. To provide a practical example of this theoretical framework, we applied the model to recast existing hypotheses for Cenozoic climate change based on feedbacks or shared forcing mechanisms. Overall, this work provides a systematic and simplified conceptual framework for understanding the function and evolution of global biogeochemical cycles.

Adapting automated instrumentation for high-throughput calcium isotope measurements by multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: Large, efficiently generated datasets of calcium isotope ratios offer tremendous utility in an increasing number of scientific disciplines. Modern analytical capabilities in mass spectrometry present unique challenges to previously established sample preparation techniques, the extent of which must be thoroughly examined before these data are interpreted and reported. This study addresses key methodological challenges in the determination of calcium isotopes using state-of-the-art, commercially available instruments. METHODS: Automated ion chromatography was used to isolate calcium from carbonate- and seawater-like samples prior to analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). A new, membrane-bearing desolvator (Apex-Ω) is implemented and achieves enhanced sensitivity, yet necessitates an update to established sample preparation techniques due to previously unobserved matrix effects. Performance of this method was assessed through analyses of multiple standard reference materials (SRM 915b, USGS EN-1, and seawater) and several in-house standards using a Neptune MC-ICP-MS instrument. RESULTS: The enhanced sensitivity afforded by the Apex-Ω also yields pronounced matrix effects during mass spectrometry, resulting specifically from heightened sample Na and Sr content and deviations in sample [Ca] from their bracketing standards. While the latter can be addressed by a concentration correction, the first two matrix effects are mitigated using a modified chromatography technique that implements unique rinsing protocols. CONCLUSIONS: This precise (0.14‰ 2σ), high-throughput method is very reproducible for small sample amounts and, at optimal efficiency, can generate approximately 140 sample δ44/40 Ca values per week with 5 h of in-person effort per day. Documented matrix effects are successfully mitigated through corrections and modified chromatographic techniques. Additionally, this method may be permuted to accommodate most major cations.

Dataset of characteristic remanent magnetization and magnetic properties of early Pliocene sediments from IODP Site U1467 (Maldives platform).

This data article describes data of magnetic stratigraphy and anisotropy of isothermal remanent magnetization (AIRM) from "Magnetic properties of early Pliocene sediments from IODP Site U1467 (Maldives platform) reveal changes in the monsoon system" [1]. Acquisition of isothermal magnetization on pilot samples and anisotropy of isothermal remanent magnetization are reported as raw data; magnetostratigraphic data are reported as characteristic magnetization (ChRM).

The abrupt onset of the modern South Asian Monsoon winds.

The South Asian Monson (SAM) is one of the most intense climatic elements yet its initiation and variations are not well established. Dating the deposits of SAM wind-driven currents in IODP cores from the Maldives yields an age of 12. 9 Ma indicating an abrupt SAM onset, over a short period of 300 kyrs. This coincided with the Indian Ocean Oxygen Minimum Zone expansion as revealed by geochemical tracers and the onset of upwelling reflected by the sediment's content of particulate organic matter. A weaker 'proto-monsoon' existed between 12.9 and 25 Ma, as mirrored by the sedimentary signature of dust influx. Abrupt SAM initiation favors a strong influence of climate in addition to the tectonic control, and we propose that the post Miocene Climate Optimum cooling, together with increased continentalization and establishment of the bipolar ocean circulation, i.e. the beginning of the modern world, shifted the monsoon over a threshold towards the modern system.