cGENIE Anthropocene d13C excursion

we use the Earth system model cGENIE and calculate the change in carbon isotopic composition of the deep ocean in response to the current anthropogenic perturbation of the global carbon cycle. The three-dimensional cGENIE model simulates ocean, atmosphere and carbon cycle dynamics at low resolution (36 x 36 cells) (Edwards & Marsh, 2005; Ridgwell et al., 2007), which makes it an appropriate tool for resolving first-order questions on the functioning of the Earth's system like the issue at hand here. We perturb cGENIE by introducing isotopically-light CO2 (-28‰  δ13C) into the atmosphere at rates that correspond to IPCC's emission scenario A1B (Nakicenovic et al., 2000). In our simulation, we consider human-induced fluxes of carbon into the atmosphere between 2000 and 2150 AD, with an initial emission of 8.0 Gt C/year in 2000 AD, peak emissions of 16.4 Gt C/year in 2050 AD and zero-emissions from 2150 AD onwards. Starting from a pre-industrial atmospheric CO2 concentration of 278 ppm, this scenario leads to 495 ppm pCO2 in 2050 AD and 810 ppm pCO2 in 2150 AD.
The cGENIE model experimental setup used for the present study is based on GENIE16 of Archer et al. (2009). This version of cGENIE (Ridgwell & Hargreaves, 2007; Ridgwell et al., 2007) consists of a 3-dimensional C-Goldstein ocean circulation model with reduced physics and 16 depth levels in the ocean. The ocean is coupled to a 2-dimensional energy-moisture balance model of the atmosphere (Edwards & Marsh, 2005; Singarayer et al., 2008) and a 2-dimensional dynamic-thermodynamic sea-ice model (Edwards & Marsh, 2005; Ridgwell et al., 2007). Ocean temperature, salinity and the concentration of biogeochemical tracers, including carbon, are circulated and coupled through the circulation model for the ocean (Ridgwell et al., 2007). This representation of the marine geochemical cycle also accounts for carbonate precipitation and their preservation as deep-sea sediments. We also make use of the sediment diagenesis function of the model, which allows sediments to settle at each ocean grid cell as well as being dissolved during subsequent processes and which was developed by Archer et al. (2009) based on Ridgwell et al. (2007) and Archer (1996). Further details on the cGENIE model setup can be found in Archer et al. (2009) and Singarayer et al. (2008). We applied a 2-part spin-up of modern marine CaCO3 cycling as described in Ridgwell and Hargreaves (2007). In the first stage, we spun up the experiment for 50 kyr in a closed carbon system that is designed to create equilibrium between climate and ocean circulation for a predefined atmospheric CO2 concentration (here: 278 ppm pCO2) and ocean alkalinity. Atmospheric restoring of CO2 and δ13C is included together with carbonate weathering that exactly balances CaCO3 burial in a forced "closed system". In a second spin-up for 200 kyr, the carbonate system is opened to develop freely and includes a temperature-dependent determination of weathering rates. The CO2 emission experiment presented here is based on IPCC's A1B scenario (Nakicenovic et al., 2000). 
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