Establishing terrestrial paleoclimate interpretive framework for modern biogenic carbonate from Ethiopia and the East African Rift System
Abstract
Paleoclimatic studies based on stable isotopes from terrestrial biogenic carbonates rely on assumptions in the starting isotopic values of the water, the temperature during formation, and the biological fractionation of the organism. All of the controls on isotopic composition can be variable and poorly constrained in a continental setting. To better contextualize paleorecords, modern stable isotope data from coupled water and biogenic carbonate are necessary to constrain the controls on the isotopic composition of biogenic carbonate in lake systems. We present stable isotope ratios (δ18O and δD) of 46 water samples from 8 different Ethiopian Rift and highland lakes as well as rivers, springs and groundwater that feed these lakes. In addition, we analyzed 30 modern mollusks from 5 of the lakes for carbonate oxygen and carbon isotopes (δ18O and δ13C). Our results from the lake water δ18O yield a mean of 7.1 ± 2.0‰ (SMOW, 1σ SD). From the mollusk shells, the δ18O values have a mean of 4.3 ± 1.3‰ (PDB, 1σ SD). By applying an aragonite oxygen isotope temperature equilibrium equation, we estimate the shell formation temperature. The calculated and observed mean temperature for Lake Hayk are in good agreement. However, reconstructed temperatures from three other rift lakes (Ziway, Langano, Abiyata) underestimate the temperature and Lake Hawassa overestimate it. This disparity might be attributed to the strong seasonal lake temperature fluctuations and/or the balance between evaporation and 18O-depleted river input from the Ethiopian highlands. Additional future analyses of mollusk shell carbonate clumped isotopes will provide an additional opportunity to validate the growth temperatures from well-characterized modern lakes. This has significant implications for interpretations of paleotemperatures from fossil shell carbonate in the East African fossil record.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP51F1443L
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 4914 Continental climate records;
- PALEOCEANOGRAPHY;
- 4924 Geochemical tracers;
- PALEOCEANOGRAPHY;
- 4994 Instruments and techniques;
- PALEOCEANOGRAPHY