The Global Seamount Census Oceanography Bathymetric Seamount Studies

aBstr aCt. Seamounts are active or extinct undersea volcanoes with heights exceeding ~ 100 m. They represent a small but significant fraction of the volcanic extrusive budget for oceanic seafloor and their distribution gives information about spatial and temporal variations in intraplate volcanic activity. In addition, they sustain important ecological communities, determine habitats for fish, and act as obstacles to currents, thus enhancing tidal energy dissipation and ocean mixing. Mapping the complete global distribution will help constrain models of seamount formation as well as aid in understanding marine habitats and deep ocean circulation. Two approaches have been used to map the global seamount distribution. Depth soundings from single-and multibeam echosounders can provide the most detailed maps with up to 200-m horizontal resolution. However, soundings from the > 5000 publicly available cruises sample only a small fraction of the ocean floor. Satellite altimetry can detect seamounts taller than ~ 1.5 km, and studies using altimetry have produced seamount catalogues holding almost 13,000 seamounts. Based on the size-frequency relationship for larger seamounts, we predict over 100,000 seamounts > 1 km in height remain uncharted, and speculatively 25 million > 100 m in height. Future altimetry missions could improve on resolution and significantly decrease noise levels, allowing for an even larger number of intermediate (1–1.5-km height) seamounts to be detected. Recent retracking of the radar altimeter waveforms to improve the accuracy of the gravity field has resulted in a twofold increase in resolution. Thus, improved analyses of existing altimetry with better calibration from multibeam bathymetry could also increase census estimates. contrast between seamounts and water gives rise to gravity anomalies that perturb the geoid, an equipotential surface approximated by the sea surface. Altimeters can measure these perturbations , and processing can isolate their causes (i.e., seamounts). However, there are assumptions and uncertainties in this approach that need to be considered. Here, we briefly review results from both direct bathymetric and indirect altimetric mapping of seamounts, what the uncertainties are, and why smaller seamounts may be under-represented. For clarity, we follow Pitcher et al. (2007) and separate small from large seamounts at the height limit of 1500 m, which is the current threshold for characterizing seamounts with satellite altimetry. We then discuss why global estimates of seamount counts can vary considerably, depending on the methodology employed and the definition of a seamount. Finally, we discuss uncertainties in these estimates and arrive at our best …