STABLE ISOTOPE PATTERNS IN A LAGOONAL LAKE SYSTEM: IMPLICATIONS FOR PALEO-SALINITY STUDIES

A geochemical survey of the Lake Shinji – Lake Nakaumi lagoonal system was undertaken to relate stable isotope patterns in water surface sediments to salinity. C and N isotope ratios of organics, C and O isotope rations in carbonates, and O and H ratios in water samples were measured. Samples were collected on two transects from the fresh-water Hii River, through the two brackish lakes to the Japan Sea. One transect was sampled repeatedly to document seasonal variation. Cores from four different locations in Lake Nakaumi show different scales of salinity variation in the lake basin.

Perhaps the most important feature of the lagoon geochemistry is the temporal and spatial complexity in the lagoon/estuary hydroenvironment. Although linear mixing relationships between salinity and water δ¹⁸O exist, the slope and intercept changes from season to season. Bulk shell oxygen and carbon isotope ratios are highly correlated with salinity, although uncertainties exist due to variability in the fresh-water end member of this mixing relationship. A strong permanent halocline exists in Lake Nakaumi and fresh-water surface layers can cover the whole system during the rainy season. This can lead to as much as 30 ppt difference between surface and bottom waters.

Based on C/N ratios, fine organic matter in the sediment is dominated by phytoplankton. Carbon isotope ratios increase as salinity increases from approximately -25 VPDB at points of fresh-water input to -21 in the Sea of Japan. Organic matter δ¹³C may indicate the salinity history of a lagoon system, although seasonal variability adds complexity to this relationship. Nitrogen isotope ratios do not correlate with salinity, rather they reflect the population density of the near-shore.

Shell δ¹⁸O shows that the salinity in Nakaumi was highest between 6000 and 8000 cal yrBP. Cores spanning the last 2000 years were sampled at high resolution. The δ¹³C of organics suggest that salinity remained in the 25 to 32ppt range throughout the last 2000 years, with highest salinities around 1800 and 650 cal yrBP. The lowest salinity is recorded around 2400 cal yrBP. Shell δ¹⁸O is in rough agreement with this salinity structure, but small differences exist, indicating that productivity changes may interfere with the interpretation of organic δ¹³C in terms of salinity.