DO TEMPORAL VARIATIONS IN SM/ND RATIOS RECORD SEA-LEVEL CHANGE IN CARBONATE PLATFORMS?

Temporal variations in Sm/Nd ratios have been suggested as a tool for tracking sea-level changes in carbonate successions. High Sm/Nd ratios may record low weathering Nd flux into carbonate depositional environments and thus reflect increase of water depth (Fanton et al., 2002). To test if Sm/Nd ratios increase with sea-level rise (transgression) and decrease with sea-level fall (regression), we analyzed Sm/Nd ratios, along with other major, trace, and rare earth elements, across three third-order (million-year scale) transgressive-regressive cycles deposited on an Early Mississippian carbonate ramp in the southern Great Basin. The results demonstrate that Sm/Nd ratios have higher values up to 0.206, 0.256, and 0.30 during the three sea-level highstands and lower values down to 0.175, 0.152, and 0.164 below the cycle boundaries. The data suggest that Sm/Nd ratios tracked at least the third-order sea-level changes in both the shallow-water Mountain Home and deep-water Pahranagat sections. There are also Sm/Nd changes of smaller magnitudes accompanying the meter-scale cycles, but such variations are less apparent in the deep-water section. The low Sm/Nd ratios are accompanied with negative shifts in Ce anomaly and increase of Mn contents, confirming that the negative Ce anomalies down to 0.3–0.5 (lower than the average modern ocean seawater value of 0.55) in the Early Mississippian carbonates may have resulted from enhanced Ce removal by Fe-Mn oxides in the Antler foreland basin during sea-level regression. [Ref.: Fanton, K.C., Holmden, C., Nowlan, G.S. and Haidl, F.M., 2002, Geochimica et Cosmochimica Acta, v. 66, p. 241-255]