Chesapeake lanthanide fractionation

Fig. 41. Concentration of dissolved Ce, Nd, Eu, and Yb in the upper porewater (0-1 cm) as a function of time for the CHEER time-series study of Chesapeake Bay. Dashed line for Yb indicates that no heavy-lanthanide fraction was measured for samples on days 138 and 166. From Sholkovitz et al. (1992).

A time-series study of Chesapeake Bay demonstrated that the dissolved lanthanides have a large seasonal cycle in both the water column and pore water in response to the development of anoxia in the spring and reoxygenation in the fall (Sholkovitz et al. 1992). Short-term variations in fractionation accompany the release and removal of lanthanides from the water column under seasonally-varying redox conditions. 

Large scale fractionation accompanies the seasonal cycle of the dissolved lanthanide concentrations in Chesapeake Bay. This is illustrated in fig. 37 which presents the time series of the Nd/Yb ratio for bottom waters (and upper pore waters). This light to heavy lanthanide ratio increases in the spring and is followed by an equally large decrease in the fall. At its maximum in the summer, the Nd/Yb ratio is three times greater than its winter baseline ratio. Hence, the lighter lanthanides are preferentially released in the spring and summer and then preferentially removed in the fall. 

Shale-normalized patterns quantify the extent of fractionation during periods of anoxia as illustrated by the water column data collected from different depths of Chesapeake Bay on 26 July 1988 (fig. 38). The oxic surface water exhibits a heavy-enrichment and large negative Ce anomaly while anoxic bottom water has an almost flat pattern and a small negative Ce anomaly. Hence the lanthanide composition shifts away from the heavy-enriched pattern of oxic seawater toward one that is more crust-like. As redox conditions become more reducing, the relative order of trivalent lanthanide release to the... 

Fig. 37. Temporal variation of the Nd/Yb ratio in the bottom water and upper porewater for the CHEER time-series study of Chesapeake Bay. Four symbols refer to estimated values, as the heavy-lanthanide (Yb) fractions from these samples were lost (see sect. 3). The Yb concentrations were estimated by using the shale-normalized patterns generated from the light lanthanide and middle lanthanide fiactions and extrapolating to Yb. This leads to good ( 10%) estimates as the lanthanide patterns are smooth (see fig. 7). From Sholkovitz...

---