Mapping the distribution of global carbonate cover from 0 to 100 Ma by modelling the carbonate compensation depth

Abstract:

The oceans play an important part in regulating the carbon cycle and climate system, acting as a buffer between the carbon in the atmosphere and the deep earth. Of all dissolved inorganic carbon (DIC) in the ocean, only carbonate can exist in a solid state (mostly as calcite). As such, the carbonate compensation depth (CCD) acts as control on this buffer, governing the distribution of sedimentary carbonate

The CCD today is around 4.5 km depth, though previous work that looked at the composition of sediments on the ocean floor has suggested that CCD was different in the past (e.g. Pälike et al., 2012; Sclater et al., 1977). These studies mostly show the CCD decreasing to shallower depths through the Cenozoic and the Mesozoic. The deepening of the CCD through time is consistent with the decrease in atmospheric CO₂ over time shown in the GEOCARB models (Berner, 1987; Berner and Kothavala, 2001; Berner, 2006); more carbon is being stored in the ocean as sediment.

We look at the evolution of the CCD since 100 Ma and how this has affected the distribution of sedimentary carbonate on the ocean floor. We combine recent advancements in determining palaeobathymetry into the Mesozoic from reconstructed ages of the ocean floor (Müller et al., 2008) in conjunction with a geochemical model by Boudreau et al. (2010) for the average CCD today, applying it from 0 to 100 Ma. Assuming values for ocean ion concentrations, productivity rates, and solubility constants we make a first order model. The model is sensitive to changes in the dissolved concentration of carbonate.

In the reconstruction where the surface saturation state of calcite was decreased going back to 100 Ma, the CCD gradually deepens with time, consistent with other independent studies. The CCD reconstructions were then used to map the theoretical extent of global sedimentary carbonate and determine proximity to subduction zones. The maps suggest that the amount of sedimentary carbon being subducted has increased with time, despite an overall decrease in volcanic activity since the Mesozoic.