Reconstructing Depositional Rates and Their Effect on Paleoenvironmental Proxies: The Case of the Lau Carbon Isotope Excursion in Gotland, Sweden
Creators: name: Jarochowska, E.
name: Nohl, T.
name: Grohganz, M.
name: Hohmann, N.
name: Vandenbroucke, T. R. A.
name: Munnecke, A.
Publisher: name: FID GEO
Year: 2020

description: Variations in depositional rates affect the temporal depositional resolutions of proxies used for paleoenvironmental reconstructions; for example, condensation can make reconstructed environmental changes appear very abrupt. This is commonly addressed by transforming proxy data using age models, but this approach is limited to situations where numerical ages are available or can be reliably inferred by correlation. Here we propose a new solution, in which relative age models are constructed based on proxies for depositional rates. As a case study, we use the onset of the late Silurian Lau Carbon Isotope Excursion (LCIE) in Gotland, Sweden. The studied succession is a gradual record of shallowing upward in a tropical, neritic carbonate platform. As proxies for depositional rates we tested thorium concentration, carbonate content, and the concentration of pelagic palynomorphs. These three proxies were used to create relative age models using the previously published DAIME model. We applied these models to transform the δ13Ccarb values as well as concentrations of selected redox‐sensitive elements. The three relative age models yielded qualitatively similar results. In our case study, variations in depositional rates resulted in peaks of redox proxies appearing up to 76% higher when taken at face value, compared to when accounting for these rates. In the most extreme cases, our corrections resulted in a reversal in the stratigraphic trend of elemental concentrations. This approach can be applied and developed across depositional setting and types of paleoenvironmental proxies. It provides a flexible tool for developing quantitative models to improve our understanding of the stratigraphic record.
description: Plain Language Summary: The depositional rate reflects how quickly a given thickness of sedimentary rock forms. Abrupt changes in environmental signals recorded in an interval of strata can be the result of (1) fast changes in environmental conditions and average sedimentation rate or (2) average changes in environmental conditions and slow sedimentation rate. To correct for this effect, age models are used, but they are often not available or lack sufficient resolution to detect rapid changes in the environment. We propose a method to estimate relative changes in depositional rates and test it in a sedimentary section on the Swedish island of Gotland. The section preserves geochemical records of a carbon cycle perturbation, expressed as shifts in carbon isotopes. It has also been proposed to record periods of oxygen depletion in marine water, detectable as enrichment in elements sensitive to redox conditions. We measured these parameters and compared the original values as preserved in the section with values corrected for depositional rates. We show that (1) perturbations of the carbon cycle were most likely more rapid than they appear in the section and (2) high depositional rates during the carbon cycle perturbation partly disguised the intervals of oxygen depletion.
description: Key Points: Relative age models are reconstructed from concentrations of Th, carbonate, and pelagic palynomorphs. δ13Ccarb and elemental proxies for redox conditions are transformed using the age models. Accounting for depositional rates can decrease peaks in redox proxies by up to 45% and make the δ13Ccarb excursion appear more abrupt.
description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659