Atmospheric CO2 Reconstructions from Polar Ice: What Do High-Resolution CO2 Records and δ13CO2 Analyses Tell Us about Past Climate and Global Carbon Cycle Processes?

Abstract:

Today, a monitoring network measures atmospheric CO₂ at high temporal and spatial resolution. Atmospheric transport models then calculate regional CO₂ source and sink fluxes. Prior to this instrumental period, archived air, firn air, and air trapped in polar ice are the only direct atmospheric archives to reconstruct past CO₂ changes. Only ice from Antarctica allows reliable CO₂ measurements, either from classical ice cores or outcropping ice, while Greenland records are subject to in situ production. They provide high-resolution and high-precision CO₂ reconstructions up to 800,000 years back in time. Ice core records have revealed an intimate connection between CO₂ variations and major changes in Earth’s climate and have fundamentally shaped the community’s view of the global carbon cycle. Knowing the concentration of past atmospheric CO₂ and the other greenhouse gases is key to provide the radiative forcing for climate simulations. Ice core reconstructions broadly fulfilled this task. On the contrary, we are far from a coherent understanding of the mechanisms driving these changes. Analyzing phase relations between CO₂, other ice-core derived species, and proxies from marine sediment cores allow for the identification of factors likely responsible for the observed CO₂ changes. Specifically, the strength of the Atlantic overturning circulation and Southern Ocean upwelling are thought to be key players. However, the observed CO₂ changes cannot uniquely be related to a specific process. Here, stable carbon isotope analysis on CO₂ extracted from ice provides additional constraints as any process leads to isotope fractionation of the reservoir. Analytical progress during the last decade affords us with a growing data set on this long-awaited parameter. This presentation provides a state-of-the-art overview on ice-based CO₂ and its carbon isotopic signature focusing both on the long-term orbital changes as well as rapid changes documented during the last deglaciation.