Recent change of hydrographic features in the Cheju Strait (southeastern Yellow Sea), Korea: contrast between planktonic and benthic foraminifera isotope composition

Abstract

Abstract AMS 14 C dates on a 260 cm-long gravity core, NPC-2, collected from the Cheju Strait (southeastern Yellow Sea) revealed that the core sediments were deposited only during the last 1200 yr B.P. Under such relatively rapid sediment deposition (about 2 mm/yr of sedimentation rate), the δ 18 O and δ 13 C compositions of planktonic ( Globigerionoides bulloides ) and benthic ( Ammonia beccarii, s.l.) foraminifera provide a new information on the evolution of surface and bottom water masses in the study area. The discrepancies of δ 18 O and δ 13 C values between planktonic and benthic foraminifera were consistent between the bottom and 50 cm downcore, whereas both values diverged gradually in the upper 50 cm of core and the difference reaches at maximum at the top of core. In accordance with the linear covariances between δ 18 O and δ 13 C values of both planktonic and benthic foraminifera, a common principal factor to govern the isotopic composition of foraminiferal calcite may be judged. Considering not only temperature fractionation of calcite but gradients of salinity to δ 18 O w of seawater, the isotopic variation and excursion in the upper part of core NPC-2 suggest that the contribution from the gradual decreasing salinity be emphasized in contrast with increasing temperatures at the surface layer. The low-salinity waters have accumulated greatly during the last 250 yr, part of which is likely to result from the divergence of Huanghe River mouth. Thus, the modern hydrographic changes are recorded as the negative shift of δ 18 O and δ 13 C values in the shallow Yellow Sea.