Isotopic Compositions of Modern Seawater at the Mariana and Yap Trenches

Abstract

The isotopic composition of ocean water is crucial in studying water masses and mixing in deep oceans, isotope mass balance in ocean water regulated by high-temperature and low-temperature hydrothermal alterations, and the exchange of water among crust-ocean-mantle reservoirs. We collected 40 water samples from Challenger Deep and the water column above at the Mariana Trench (down to 10,923 m) and 12 from the Yap Trench (down to 6,300 m) in the western Pacific Ocean in three hadal cruises from 2016 to 2018. The δ²H values at the Mariana and Yap Trenches average 0.1 ± 0.2 ‰ (1σ error). The δ²H records from this study, together with existing databases, manifest that deep waters have δ²H values varying between −2 and +2 ‰ (except for the Weddell Sea, the Greenland, Iceland, and Norwegian Seas, and the Mediterranean Sea), with increasing values from the Southern Ocean to the Pacific and Indian Oceans, and to the Atlantic Ocean. The average δ¹⁸O value of water samples from both trenches is –0.04 ± 0.03 ‰ (1σ error). The correlation between δ¹⁸O and salinity distinguishes abyssal water masses at the study region, UCDW (Upper Circumpolar Deep Water) and LCDW (Lower Circumpolar Deep Water). These water samples from the Mariana and Yap Trenches gave an average ¹⁷O_excess value of −6 ± 1 ppm (1σ error). Our 52 data records of ¹⁷O_excess expand the 38 existing records for the deep ocean. Both δ²H and ¹⁷O_excess of modern ocean have rolled as anchor points to reconstruct compositions of Earth’s early ocean.