Systematic study of the impact of fresh water fluxes on the glacial carbon cycle

Abstract

Abstract. During glacial periods, atmospheric CO 2 concentration increases and decreases by around 15 ppm. At the same time, the climate changes gradually in Antarctica. Such climate changes can be simulated in models when the AMOC (Atlantic Meridional Oceanic Circulation) is weakened by adding fresh water to the North Atlantic. The impact on the carbon cycle is less straightforward, and previous studies give opposite results. Because the models and the fresh water fluxes were different in these studies, it prevents any direct comparison and hinders finding whether the discrepancies arise from using different models or different fresh water fluxes.In this study we use the CLIMBER-2 coupled climate carbon model to explore the impact of different fresh water fluxes. In both preindustrial and glacial states, the addition of fresh water and the resulting slow-down of the AMOC lead to an uptake of carbon by the ocean and a release by the terrestrial biosphere. The duration, shape and amplitude of the fresh water flux all have an impact on the change of atmospheric CO 2 because they modulate the change of the AMOC. The maximum CO 2 change linearly depends on the time integral of the AMOC change. The different duration, amplitude, and shape of the fresh water flux cannot explain the opposite evolution of ocean and vegetation carbon inventory in different models. The different CO 2 evolution thus depends on the AMOC response to the addition of fresh water and the resulting climatic change, which are both model dependent. In CLIMBER-2, the rise of CO 2 recorded in ice cores during abrupt events can be simulated under glacial conditions, especially when the sinking of brines in the Southern Ocean is taken into account. The addition of fresh water in the Southern Hemisphere leads to a decline of CO 2 , contrary to the addition of fresh water in the Northern Hemisphere.

Tightened constraints on the time-lag between Antarctic temperature and CO<sub>2</sub> during the last deglaciation

“…Notably, the initial (within the first 250 yr) rapid response in CO 2 in Schmittner and Galbraith (2008) is attributed to CO 2 release from the terrestrial biosphere (triggered by cooling associated with the AMOC reduction) and not to physical or biogeochemical components of the Southern Ocean carbon cycle. However, the response of terrestrial vegetation models and the interplay between ocean and vegetation responses is known to be highly model-dependent (Bouttes et al, 2012). Recent studies also emphasise the sensitivity of the timescales of ocean ventilation to model resolution; at least under modern boundary conditions, meso-scale eddy-resolving ocean models suggest much faster ventilation times than the coarser resolution models that are currently used for palaeo-simulations (Maltrud et al, 2010).…”

Evolution of the stable carbon isotope composition of atmospheric CO2 over the last glacial cycle

“…The occurrence of Heinrich event 6 did not only have an influence on ocean circulation, but the effects of such events are recorded in chemical signals worldwide, including the Cariaco Basin [ Peterson et al , ], Hulu Cave in China [ Wang , ], Arabian Sea [ Schulz et al , ], Santa Barbara Basin [ Behl and Kennett , ], and the western Pacific Warm Pool [ Shiau et al , ], due to the global impact of the AMOC and related changes in the position of the intertropical convergence zone. Although some model results indicate that a weakening of the AMOC may have resulted in an increase in carbon storage in terrestrial vegetation [e.g., Schmittner and Galbraith , ; Bouttes et al , ], others indicate that a decrease in these stocks is possible [e.g., Menviel et al , ; Bozbiyik et al , ].…”

Past Carbonate Preservation Events in the Deep Southeast Atlantic Ocean (Cape Basin) and Their Implications for Atlantic Overturning Dynamics and Marine Carbon Cycling

“…Any changes in the ocean's DIC and alkalinity content that impact on the global average carbonate saturation state will tend to be restored (i.e., eliminated) by adjustments in the position of the lysocline (and possibly the CCD), which is referred to as “carbonate compensation”(Broecker, ; Broecker & Peng, ; Sigman et al, ). Over millennial time scales, the distinct chemical signatures of northern versus southern sourced water masses make sedimentary carbonate preservation specifically in the South Atlantic primarily dependent on prevailing ocean circulation patterns that may have shifted rapidly in the past (Gottschalk et al, ; Henry et al, ; Keigwin & Jones, ; McManus et al, ), with implications for atmospheric CO 2 levels (e.g., Bouttes et al, ; Schmittner & Galbraith, ). The sedimentary carbonate record of the deep Atlantic hence integrates the influence of various ocean processes that operate on different time scales (Hodell et al, ).…”