Geofyzikální ústav Akademie věd ČR, v.v.i.


Laurin et al. - 2017 - Orbital signals in carbon isotopes: phase distortion as a signature of the carbon cycle

Laurin et al. - 2017 - Orbital signals in carbon isotopes: phase distortion as a signature of the carbon cycle

Jiří Laurin and Bohuslav Růžek, researchers from the Institute of Geophysics CAS, v.v.i., in cooperation with Martino Giorgioni from Instituto de Geociências da Universidade de Brasília investigated deformation of astronomical (Milankovitch) signals by climate processes involving changes in the carbon cycle. Using isotopic mass balance models and a new numerical tool EPNOSE they demonstrate that differences in the phase distortion of individual astronomical components (e.g., the 405-kyr cycle of long eccentricity vs. amplitude modulation of the 100-kyr cycle of short eccentricity) can assist in interpreting the mechanisms of orbital-scale climate change.

Isotopic mass-balance models are employed here to study the response of carbon-isotope composition (d13C) of the ocean-atmosphere system to amplitude-modulated perturbations on Milankovitch time scales. We identify a systematic phase distortion, which is inherent to a leakage of power from the carrier precessional signal to the modulating eccentricity terms in the global carbon cycle. The origin is partly analogous to the simple cumulative effect in sinusoidal signals, reflecting the residence time of carbon in the ocean-atmosphere reservoir. The details of origin and practical implications are, however, different. In amplitude-modulated signals, the deformation is manifested as a lag of the 405-kyr eccentricity cycle behind amplitude modulation (AM) of the short (~100-kyr) eccentricity cycle. Importantly, the phase of AM remains stable during the carbon-cycle transfer, thus providing a reference framework against which to evaluate distortion of the 405-kyr term. The phase relationships can help to (1) identify depositional and diagenetic signatures in d13C, and (2) interpret the pathways of astronomical signal through the climate system. The approach is illustrated by case studies of Albian and Oligocene records using a new computational tool EPNOSE. Analogous phase distortions occur in other components of the carbon cycle including atmospheric CO2 levels; hence, to fully understand the causal relationships on astronomical time scales, paleoclimate models may need to incorporate realistic, amplitude-modulated insolation instead of monochromatic sinusoidal approximations. Finally, detection of the lagged d13C response can help to reduce uncertainties in astrochronological age models that are tuned to the 405-kyr cycle.

Link to the publication.


Numerical model illustrating distortions of the astronomical signal in the carbon cycle. (a) Input: astronomically controlled changes in the rates of carbon burial and emission. (b) Output: carbon isotope composition of the ocean-atmosphere reservoir. (c) Amplitude modulation of the short-eccentricity cycle in the model input (black) and output (red). (d) The 405-kyr cycle of orbital eccentricity in the model input (black) and output (red).