Impact of brine-induced stratification on the glacial carbon cycle
N. Bouttes, D. Paillard, D. M. Roche
2010
Climate of the Past Discussions
During the cold period of the Last Glacial Maximum (LGM, about 21 000 years ago) atmospheric CO 2 was around 190 ppm (Monnin et al., 2001) , much lower than the pre-industrial concentration of 280 ppm. The causes of this substantial drop remain partially unresolved, despite intense research. Understanding the origin of reduced 5 atmospheric CO 2 during glacial times is crucial to comprehend the evolution of the different carbon reservoirs within the Earth system (atmosphere, terrestrial
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... e and ocean). In this context, the ocean is believed to play a major role as it can store large amounts of carbon (Sigman and Boyle, 2000), especially in the abyss, which is a carbon reservoir that is thought to have expanded during glacial times. To create 10 this larger reservoir, one possible mechanism is to produce very dense glacial waters, thereby stratifying the deep ocean and reducing the carbon exchange between the deep and surface ocean (Paillard and Parrenin, 2004) . The existence of such very dense waters has been inferred in the LGM deep Atlantic from sediment pore water salinity (Adkins et al., 2002) . Based on these observations, we study the impact of a 15 brine mechanism on the glacial carbon cycle. This mechanism relies on the formation and rapid sinking of brines, very salty water released during sea ice formation, which brings salty dense water down to the bottom of the ocean. It provides two major features: a direct link from the surface to the deep ocean along with an efficient way of setting a strong stratification. We show with the CLIMBER−2 coupled carbon-climate 20 model (Petoukhov et al., 2000) that such a brine mechanism can account for a significant decrease in atmospheric CO 2 and contribute to the glacial-interglacial change. This mechanism can be amplified by low vertical diffusion resulting from the brineinduced stratification. The results obtained substantially improve the modeled glacial distribution of oceanic δ 13 C as well as the deep ocean salinity in line with reconstruc-25 tions from sediment cores (Curry and Oppo, 2005; Adkins et al., 2002) , suggesting that such a mechanism could have played an important role during glacial times. 682 CPD 6, 681-710, 2010 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion diate complexity climate model (Petoukhov et al., 2000) well suited for the long term 685 CPD Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Implementation of brines in the CLIMBER−2 model When sea ice is formed, a flux of ions, among which salt, is released into the ocean as sea ice is mostly composed of fresh water (Wakatsuchi and Ono, 1983; Rysgaard et al., 2007 Rysgaard et al., , 2009 . Because the underlying water is then enriched in salt it becomes denser and can thus sink and transport salt to deeper waters. The rejection of salt Brine-induced stratification Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion the abyss increases. The frac=0 simulation (no brine mechanism) corresponds to the standard LGM simulation (LGM-std), where CO 2 is 296 ppm. The maximum effect of the brines is obtained for frac=1, i.e. when all the salt released by sea ice formation sinks to the bottom of the ocean with the brine mechanism. The latter gives a maximum CO 2 drop of about 52 ppm (CO 2 is 244 ppm).
doi:10.5194/cpd-6-681-2010
fatcat:74jxlryvcjfu3jguiejkf236qm