Abstract :
[en] The evolutions of atmospheric CO2 partial pressure (pCO2) and of the carbonate compensation depth and the
calcite and aragonite saturation horizons (CSH and ASH, respectively) have been studied with the coupled oceansediment model MBM-MEDUSA [1], over the next 50,000 years. MBM-MEDUSA includes a full description of
sedimentary exchange processes, taking into account chemical carbonate erosion in a consistent way. The adopted emission scenarios were based upon logistic functions [2], considering total emissions of 500, 1000, 2000 and 4240 GtC); the adopted stabilisation scenarios were the S350, S450, S550, S650 and S750 from the IPCC [3]. While the evolutions of atmospheric pCO2 and pH have got a great deal of attention so far (e.g., [4, 5]), only a few studies have considered the saturation horizons [5, 6], and, to our best knowledge, this is the first study also focusing on compensation depth variations.
Simulation experiments were started with a 50,000 year spin-up to 1750 A.D. (at steady state). This state was
characterised by an atmospheric pCO2 of 277 ppm, a CSH depth of 3350 m and a CCD of 4300 m (in the Indo-Pacific, which can be considered the most representative reservoir for the global ocean).
In all experiments, we found that CCD variations were considerably greater than CSH variations. The 500 GtC
emission scenario yielded CSH and CCD maximum shoalings of 450 and 800 m, respectively, in the year 3400 A.D. about; with the 4240 GtC emission scenario, both CSH and CCD became shallower than 500 m in 2650 A.D. With the highly optimistic S350 stabilisation scenario, CSH and CCD become even shallower than with the 500 GtC emission scenario (650 m and 1000 m shoaling, respectively), although in the year 5000 A.D. only. For the close-to-CO2-doubling scenario S550, CSH and CCD shoaled by about 1950 and 2450 m (to depths of 1400 and 1900 m, respectively). As a result, most of the sea-floor environment bathed in water that was highly
corrosive to carbonate material. In the S650 and S750 scenarios experiments, the CCD becomes shallower than
500 m, leaving little space for benthic carbonate producers to survive.
[1] Munhoven (2007) Deep-Sea Res. II 54, 722-746. [2] Bacastow and Dewey (1996) Energy Convers. Mgmt. 37,
1079-1086. [3] IPCC (1994) Climate Change 1994, Cambridge University Press. [4] Caldeira and Wickett (2003)
Nature 425, 325-325. [5] Orr et al. (2005) Nature 437, 681-686. [6] Cao and Caldeira (2008) Geophys. Res. Lett. 35, L19609.
