Stalagmite evidence for Early Holocene multidecadal hydroclimate variability in Ethiopia

A multiproxy oxygen and carbon isotope (δ13 C and δ18 O), growth rate, and trace element stalagmite paleoenvironmental record is presented for the Early Holocene from Ethiopia. The annually laminated stalagmite grew from 10.6 to 10.4 ka and from 9.7 to 9.0 ka with a short hiatus at ∼9.25 ka. Statistically significant and coherent spectral frequencies in δ13 C and δ18 O are observed at 15–25 and 19–23 years, respectively. The observed ∼1‰ amplitude variability in stalagmite δ18 O is likely forced by nonequilibrium deposition, due to kinetic effects during the progressive degassing of CO 2 from the water film during stalagmite formation. These frequencies are similar to the periodicity reported for other Holocene stalagmite records from Ethiopia, suggesting that multidecadal variability in stalagmite δ18 O is typical. Several processes can lead to this multidecadal variability and operate in different directions. A hydroclimate forcing is likely the primary control on the extent of the partial evaporation of soil and shallow epikarst water and associated isotopic fractionation. The resulting oxygen isotope composition of percolation water is subsequently modulated by karst hydrology. Further isotopic fractionation is possible in-cave during nonequilibrium stalagmite deposition. Combined with possible recharge biases in drip-water δ18 O, these processes can generate multide- cadal δ18 O variability.