Oxygen and carbon isotope study of the Holocene oyster reefs and paleoenvironmental reconstruction on the northwest coast of Bohai Bay, China

The northwest coast of Bohai Bay supports large areas of Holocene Crassostrea gigas (oyster) reefs. Their huge size and the fact that the shells are composed of consecutive microgrowth layers, consisting of primary calcite, make them an ideal object for a stable isotope study.The delta(18)O and delta(13)C compositions of the oysters reveal a number of interesting features. The most important are annual cycles related to seasonal variations, the regional delta(18)O (water) versus salinity ratio, and a stop in shell secretion below a water temperature of 11.5+/-1.5 degrees C. A positive correlation (covariance) between delta(18)O and delta(13)C values is present in most specimens. The delta(18)O and delta(13)C values of single specimens are believed to be mainly controlled by water temperature and the occurrence of phytoplankton blooms and decomposition. However, correlation among oysters of different ages indicate that the delta(18)O and delta(13)C values are mainly dependant of three variables: water temperature, salinity and phytoplankton blooms/decomposition. Between ca. 6500 yrs cal B.P. and 2300 yrs cal B.P. the salinity in Bohai Bay increased from less than 20 parts per thousand to 30 parts per thousand. Based on geological data combined with ecological data from C. gigas, we suggest that precipitation decreased during this period, while the input of fluvial sediments increased. The sediment load of rivers increased markedly after ca. 4000 yrs cal B.P., possibly due to a change in climate. In our study area this resulted in the development of the Oyster Plain