Climate change impacts and mitigation strategies will define our interaction with the oceans this century. Marine carbon sequestration could facilitate the enormous scaling necessary for gigaton-level carbon dioxide (CO2) removal: at least 10 GT/y by 2050 and 20 GT/y by 2100, required just to limit anthropogenic warming to below 2°C. Many proposed marine CO2 removal techniques involve the distributed capture of carbon, i.e., accelerating the biological carbon pump (e.g., iron ocean fertilization or artificial upwelling) or shifting the dissolution equilibrium of CO2 (e.g., ocean alkalinity enhancement). However, technology that enables rapid, inexpensive, persistent and accurate measurement and validation of drawn-down CO2 at the sequestration time- and regional ocean spatial-scales necessary to quantify carbon capture does not exist today. The accuracy and wholeness of these future techniques will be important for assigning financial value to marine CO2 removal processes in a carbon market, as well as enabling thorough evaluation of environmental impacts and a comprehensive understanding of ocean carbon dynamics. I will discuss ARPA-E’s interest in carbon sensing approaches, including passive and active acoustic techniques, which could rapidly quantify ocean carbon flux at scale and introduce powerful tools to address the challenges of mitigating climate impacts at sea.
Biologically complex coastal environments, such as coral reefs, demonstrate an equally rich ambient soundscape. Bioacoustic features of coastal soundscapes are closely tied with relative ecosystem health, functional groups present, and can be linked with specific behaviors. Biological contributions to ambient soundscapes have distinctive qualities as compared to sound associated with physical processes (i.e. wind and wave noise). While some biological components are readily identifiable, such as marine mammal or fish calls, the background noise associated with hundreds of thousands of biological clicks, snaps, and pops is not as well studied but contains a wealth of information about the ecosystem. A 64-element line array with 4.5 kHz design frequency was deployed for several field experiments off the coast of Kona, Hawaii in 2019 and 2020. Soundscape data from Hawaii were compared with comparable omnidirectional time series from Bermuda (2020) and coastal New England rocky reefs (2020–2021). Similarities in certain spectral features associated with biological sound sources were found between these unique ecosystems. The characteristic coral reef evening chorus, or significant increase in sound levels immediately prior to sunset, was consistent in Hawaii and Bermuda with comparable crepuscular changes in coastal New England.
Passive acoustic monitoring of biological soundscapes offers a long-term view into ecosystem state. This is particularly well studied for coral reefs and tropical littoral systems with evidence for similar capability in temperate and deep ocean biologically rich ecosystems. Monitoring ecosystem state under both climate change impacts and changing human usage is a critical piece of understanding how climate change and human use impact ecosystems. Passive acoustics allow for wide area coverage of an ecosystem heartbeat, and changes in key bioacoustic metrics in coral reefs indicate shifts from healthy coral dominated systems to more degraded systems with increased macroalgal cover. These shifts are typically associated with increased ocean temperatures and/or increased human use. The primary controls on coral reef soundscapes are time of day and year. Here, broad comparisons between warmer and cooler years at long term coral reef monitoring sites in Hawaii will be discussed, as well as summer versus winter biological soundscapes at temperate sites in New England. Reef soundscapes encompass contributions from a wide variety of marine flora and fauna, some of which can be identified to species level through characteristic calls and tracked with a high degree of fidelity through passive acoustics alone.
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