XBTs Provide First-Order Characterization of Seabed Physical Properties

Expendable Bathythermographs (XBTs) are oceanographic instruments that fall through the ocean's water column and measure ocean temperature with depth. In many instances, however, XBTs continue to record temperature after they impact the seabed. Here we show evidence that XBTs produce unique temperature responses when they impact the seabed that depend directly on seabed physical properties. Specifically, standard-use XBTs (e.g., T-4s and T-5s), when deployed above a mud-rich seabed, require significant time (tens of minutes) to equilibrate to steady-state seafloor temperatures after seabed impact. In contrast, XBTs deployed above sand-rich sediments equilibrate to seabed temperatures rapidly (<5 min) after seafloor impact. One explanation for this difference in temperature response is that XBTs deployed above mud-rich sediment penetrate into low permeability marine muds that jacket the XBT, where diffusive heat flow dominates. Both observations and numerical modeling results support the hypothesis that XBTs impacting muddy seafloors exhibit slow, diffusion-dominated heat flow, while XBTs impacting harder, sand-rich seabed sites exhibit rapid seafloor temperature equilibration, consistent with advection-driven heat flow and little if any XBT seabed penetration. Given that >644k XBT measurements exist publicly (via the National Oceanographic and Atmospheric Administration website), and >74,000 XBTs record temperatures post seabed impact, we suggest that XBT data represents a large, low-cost, and currently untapped data set for characterizing seabed physical properties globally.