Paleosalinity in a brackish lake during the Holocene based on stable oxygen and carbon isotopes of shell carbonate in Nakaumi Lagoon, southwest Japan

Based on the relationship between salinity and δ18O and δ13C of modern shells in the Lake Nakaumi–Shinji lagoon system (southwestern Japan), where the salinity changes regularly from ca. 1 PSU to 34 PSU, a paleosalinity record for Nakaumi Lagoon during the Holocene has been derived from bulk mollusk shell δ18O and δ13C data. The robust relationships between the salinity and modern shell δ18Oar and δ13Car (aragonite) were used to calibrate the paleosalinity reconstruction. The salinity relationships are expressed by the regressions:Salinity (PSU)=3.86 δ18Oar(‰ VPDB)+33.9 (n=18, r=0.978)Salinity(PSU)=4.05 δ13Car(‰VPDB)+31.8 (n=18, r=0.977). ng these equations to bulk shell δ18Oar and δ13Car in the 20 m sediment core, very similar salinity histories were reconstructed from either oxygen or carbon isotope ratio data. At the beginning of lacustrine sediment deposition (8000–7500 14C yr B.P.) paleosalinity of the bottom water in the central portion of Nakaumi Lagoon increased from 22 to 32 PSU. During the 7500–5500 yr B.P. interval, which approximately corresponds to the Holocene climatic optimum and high sea-level stand, salinity ranged from 27 to 37 PSU. After 5500 yr B.P. salinity lessened under the cooling climate, with a minimum value of 24–27 PSU at 3000–1000 yr B.P. The results show a consistent relationship between climate change, sea-level, and salinity in the lagoon during the Holocene.