Mass spectrometers are versatile sensor systems, owing to their high sensitivity and ability to simultaneously measure multiple chemical species. Over the last two decades, traditional laboratory-based membrane inlet mass spectrometers have been adapted for underwater use. Underwater mass spectrometry (UMS) has drastically improved our capability to monitor a broad suite of gaseous compounds (e.g., dissolved atmospheric gases, light hydrocarbons, and volatile organic compounds) in the aquatic environment. Here we provide an overview of the progress made in the field of UMS since its inception in the 1990s to the present. In particular, we discuss the approaches undertaken by various research groups in developing in situ mass spectrometers. We also provide examples to illustrate how underwater mass spectrometers have been used in the field. Finally, we present future trends in the field of in situ mass spectrometry. Most of these efforts are aimed at improving the quality and spatial and temporal scales of chemical measurements in the ocean. By providing up-to-date information on UMS, this review offers guidance for researchers interested in adapting this technology as well as goals for future progress in the field.
Highly permeable sandy sediments dominate the productive continental shelves and likely play a key role in global biogeochemical cycles. Despite their prominence, we currently have a poor understanding of how sandy sediments function and how they will respond to climate change. This knowledge gap is largely due to the difficulty in accurately sampling sandy sediments, as no method yet exists for measuring a range of analytes while accounting for advective pore-water flow in these dynamic environments. To help address this, we developed a new pore-water sampler that, when coupled to a portable mass spectrometer, can measure a suite of dissolved gases (e.g., O 2 , N 2 , Ar, CO 2 , and CH 4 ) in sandy sediments. Here, we present a series of laboratory experiments to validate and calibrate the instrument, as well as proof-of-concept submersion and field tests. Our results show that with some design improvements, our system will be capable of sustained (hourly to diel) in situ measurements in sandy sediments. This new approach has the potential to provide a large volume of high-quality data in any aquatic system with sandy sediments and thus greatly advance our understanding of biogeochemical processes occurring in these environments.
Human activities release vast amounts of reactive nitrogen, profoundly influencing the functioning of marine ecosystems. On the inner shelf, where these effects are most pronounced, sediments have emerged as potential hotspots of reactive nitrogen removal-and they are therefore increasingly thought to play a key role in the ocean nitrogen cycle. Despite their potential importance, shelf sediments are still overlooked in global models of ocean nitrogen and carbon cycles, limiting model accuracy and predictive power. Through analysis of recent work, we identified unresolved questions and controversies, and propose a three-pronged approach to improve our mechanistic understanding and modeling of nitrogen removal on the continental shelf and in the global ocean.
Low-oxygen conditions plague coastlines worldwide. At present, little is known about how the transition from normoxic to low or even no oxygen conditions alters sediment biogeochemical cycling and ultimately ecosystem functioning. Conventional sediment core incubations cannot capture rapid (<hourly) changes in biogenic gas fluxes that may occur due to oxygen depletion. To better constrain the response of sediments to hypoxia, we employed a novel flow-injection system coupled to a membrane inlet mass spectrometer to quantify fluxes oxygen, dinitrogen, and methane across the sediment-water interface from a temperate estuary (Narragansett Bay, Rhode Island, United States). We evaluated how sediments from a site more impacted by nitrogen pollution compare to one less impacted by nitrogen in response to organic matter addition. Our system is able to sample every 10 minutes, allowing us to cycle through triplicate core measurements roughly every 30 minutes to track the response of sediments to increasing hypoxic severity. The high temporal-resolution data revealed dynamic changes in sediment-water gas fluxes, suggesting that reactive nitrogen removal is enhanced under mild hypoxia but dampened under prolonged hypoxia to anoxia. Further we found that organic matter loading enhances both net denitrification and methane emissions. Ultimately, our approach represents a powerful new tool for advancing our knowledge of short-term temporal dynamics in benthic biogeochemical cycling.
With Singapore serving as the subject of exploration, The Hard State, Soft City of Singapore explores the purview of imaginative representations of the city. Alongside the physical structures and associated practices that make up our lived environment, and conceptualized space engineered into material form by bureaucrats, experts and commercial interests, a perceptual layer of space is conjured out of people’s everyday life experiences. While such imaginative projections may not be as tangible as its functional designations, they are nonetheless equally vital and palpable. The richness of its inhabitants’ memories, aspirations and meaningful interpretations challenges the reduction of Singapore as a Generic City. Taking the imaginative field as the point of departure, the forms and modes of intellectual and creative articulations of Singapore’s urban condition probe the resilience of cities and the people who reside in them, through the images they convey or evoke as a means for collective expressions of human agency in placemaking.
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