Deep-sea limestone block as a source of14C-depleted dissolved inorganic carbon at the Palau Trench

The submersible Shinkai 6500 located a large limestone block on the landward slope of the Palau Trench at approximately 6500 m depth, significantly deeper than the lysocline. This study aims to document the deep-sea carbon flux from this limestone block to seawater, investigating its implications for carbon cycling in deep marine environments. The limestone block's Sr isotope age suggests it formed in the middle and late Miocene, approximately 13–7 million years ago. Accelerator mass spectrometry measurements of the 14C of dissolved inorganic carbon (DIC) extracted from seawater surrounding the limestone block yielded values as low as -295‰. Isotope mass-balance calculations were employed to estimate the flux of limestone-derived carbonate ions into deep seawater, revealing that these ions contribute up to 6.7% of the bottom water DIC concentration. Additionally, the annual DIC flux from the limestone block was estimated to be 2.1 (±1.4) × 108 mol/y or 2.5 (±1.7) × 109 gC/y. These results highlight a substantial carbon flux entering the deep-sea environment and represent the first documented instance of carbonate dissolution occurring below the lysocline.