Stable isotopic evidence for relative and eustatic sea-level changes in Eocene to Oligocene carbonates, Baldwin County, Alabama

Eocene to Oligocene marine carbonate shelf deposits in a continuous core in Baldwin County, Alabama, were evaluated within a sequence stratigraphic framework to determine characteristic stable isotopic responses to inferred fluctuations in sea level. Due to a greater sampling density per unit of time, stable isotopes in this core reveal more defined trends than have previously been reported from DSDP/ODP (Deep Sea Drilling Project/Ocean Drilling Project) records and from more land-ward sections that have undergone intense sub-aerial alteration or erosion.

The δ¹⁸O signature records eustatic sea level. δ¹⁸O becomes heavier as eustatic sea level falls and lighter as eustatic sea level rises. δ¹⁸O records a secular shift to heavy from the early Eocene to Oligocene of 3.0‰.

The δ¹³C signature records relative sea-level change (paleobathymetry). δ¹³C generally becomes heavier in shallower water deposits and lighter in deeper water deposits. However, in planktonic oozes, δ¹³C may shift to heavy. δ¹³C also records a secular shift to heavy from the early Eocene to the Oligocene of 3.5‰

Although minor diagenetic perturbations in the stable isotopes are present, the depositional signal dominates the isotopic signal. Spar cements generally result in lighter δ¹⁸O and δ¹³C. If the dolomite is >20% dolomitization results in heavier δ¹⁸O and δ¹³C.