Natural Chlorine Enhances Antineutrino Detection in the Deep Ocean
Large-volume neutrino telescopes such as IceCube and KM3NeT exploit natural ice or water as detection media, but their sparse instrumentation makes MeV neutrino interactions difficult to identify individually. We show that chlorine naturally abundant in seawater provides an intrinsic neutron-capture target that enables efficient tagging of electron antineutrinos through inverse beta decay. In contrast to the 2.2 MeV $γ$-ray from neutron capture on hydrogen, capture on chlorine produces an $\sim$8.6 MeV $γ$-ray cascade on a shorter time scale, yielding a distinctive delayed-coincidence signature. Using measured seawater properties at the TRIDENT site and a detector-level simulation, we find that chlorine-assisted tagging can extend the core-collapse supernova 5$σ$ discovery reach from $\sim$50 to over 100 kpc. We validate the underlying detection principle with a salted-water experiment at the China Jinping Underground Laboratory, observing correlated neutron-capture signatures under suppressed cosmic muon background conditions. Natural seawater thus transforms the deep ocean into an intrinsically enhanced antineutrino detection medium, opening new opportunities for MeV neutrino astronomy and distributed measurements of reactor and geophysical antineutrinos.