Exponential growth of hadal science: perspectives and future directions identified using topic modelling

Abstract

Abstract The hadal zone is a cluster of deep-ocean habitats that plunge to depths of 6000–11000 m below sea level. Research of the deepest marine zone has occurred on a disjunct timeline and slower than shallower zones. Over the past 20 years, research efforts have surged with greater sampling capabilities and an expansion of expeditions. We aimed to assess the state of hadal science by quantitively assessing the publishing landscape. We applied a topic modelling approach and fit a Latent Dirichlet Allocation model for 12 topics to 520 abstracts from peer-reviewed papers, reviews, and conference proceedings available on the Web of Science's Core Collection between 1991 and 2021. The model outputs were analysed with ecological modelling approaches to identify the main lines of research, track trends over time, and identify strengths and gaps. We found that hadal science is occurring across all five broad disciplines of oceanography and engineering. Hadal research has exponentially grown in the past 30 years, a trend that shows no signs of slowing. The expansion is most rapidly occurring to understand the biogeochemistry of trenches, the functions of microbial communities, and the unique biodiversity inhabiting these ecosystems, and then the application of ‘omics techniques to understand hadal life. The topic trends over time are largely driven by available technology to access and sample the deepest depths and not necessarily the pursuit of specific scientific questions, i.e. the hadal research topics are bounded by the capabilities of available exploratory vehicles. We propose three recommendations for future hadal research: (1) conduct multifeature studies that include all hadal geomorphologies across their depth range, (2) establish a programme for seasonal or long-term sampling, and (3) strengthen cross-disciplinary research. This continued acceleration in hadal research is pertinent for this last marine frontier given its vulnerability to multiple anthropogenic pressures and cascading threats from global change.