Oceanic carbon and water masses during the Mystery Interval: A model‐data comparison study

Abstract

The ‘Mystery Interval’ (17.5–14.5 ka BP) is characterized by a large decline in atmospheric Δ14C synchronous with an increase in atmospheric CO2. The most widely accepted hypothesis to explain these observed shifts involves the existence of an isolated ‘old’ ocean carbon reservoir that was subsequently ventilated. Here we use the UVic Earth System Climate Model to locate a potential carbon rich and Δ14C depleted water mass under 17.5 ka BP boundary conditions. We then investigate two mechanisms for the potential ventilation of such a reservoir, namely the weakening of the North Atlantic Meridional Overturning due to iceberg calving and latitudinal shifts in Southern Hemisphere Westerlies (SHW) due to southern hemispheric warming. We find that simulations derived from an equilibrium state forced with present‐day SHW and moderate North Atlantic Deep Water (NADW) formation are in better agreement with atmospheric and ocean Δ14C reconstructions than simulations derived from an equilibrium state forced with a northward shifted SHW belt resulting in a shut‐down of the Atlantic Meridional Overturning and formation of North Pacific Deep Water. For simulations with present‐day SHW, the oldest water masses are found in the North Pacific, although the Southern Ocean cannot be ruled out as a potential ‘Mystery Reservoir’. According to our simulations, the strength of Atlantic overturning is the dominant mechanism in increasing the ocean‐atmosphere carbon flux, while shifting SHW results in a rearrangement of deep ocean carbon largely between the Atlantic and Pacific basins. In our ‘best case’ scenario, the model can account for 58% of the atmospheric CO2 increase and 48% of the atmospheric Δ14C decline. While the rate of ventilation and the age of ventilated water masses are comparable with observations, the ventilation in the model could not be sustained long enough to account for the full excursion seen in paleodata.

An Atlantic–Pacific ventilation seesaw across the last deglaciation

“…Menviel et al, 2014;Huiskamp and Meissner, 2012; see supplementary info) also show a Pacific-Atlantic seesaw, although this is most strongly expressed in the Northern Hemisphere. In these idealized model experiments, the North Atlantic is perturbed with freshwater, resulting in a cessation of NADW formation and decreased 14 C over most of the North Atlantic (including our study site) and below 2000 m in the South Atlantic (Fig.…”

“…Mikolajewicz et al, 1997;Okazaki et al, 2010;Chikamoto et al, 2012;Huiskamp and Meissner, 2012) (Fig. 6).…”

Atlantic-Pacific seesaw and its role in outgassing CO2during Heinrich events

“…A similar mechanism occurring during HS1 is also consistent with paleoproxy records as it lowers atmospheric δ 13 CO 2 [ Schmitt et al , ; Tschumi et al , ], decreases the ventilation age of deep Southern Ocean waters [ Skinner et al , ], and enhances mixing of Upper and Lower Circumpolar Deep Waters [ Burke and Robinson , ]. It is also consistent with a decrease in atmospheric Δ 14 C [ Huiskamp and Meissner , ; Matsumoto and Yokoyama , ]. Antarctic ice cores record an atmospheric CO 2 increase of 33 ppmv at the beginning of the last deglaciation and across HS1 [ Monnin et al , ].…”

“…Previous studies examined either the terrestrial carbon response [ Scholze et al , ; Köhler et al , ], the marine carbon cycle response [ Marchal et al , ; Chikamoto et al , ], or the combined marine and terrestrial carbon response [ Obata , ; Schmittner et al , ; Menviel et al , ; Schmittner and Galbraith , ; Bozbiyik et al , ; Bouttes et al , ; Huiskamp and Meissner , ; Matsumoto and Yokoyama , ] to an AMOC shutdown. While some modeling studies concluded that the oceanic carbon content increased as a result of an AMOC shutdown [ Obata , ; Menviel et al , ; Bozbiyik et al , ; Chikamoto et al , ], others suggested the opposite [ Marchal et al , ; Schmittner et al , ; Schmittner and Galbraith , ; Bouttes et al , ; Matsumoto and Yokoyama , ].…”

An improved land biosphere module for use in the DCESS Earth system model (version 1.1) with application to the last glacial termination

“…For example, enhanced ocean remineralization length scales during the glacial, due to less active bacteria at low temperatures, could trap more DIC in the deep ocean, which then could account for additional CO 2 outgassing but would also reduce deep-ocean dissolved oxygen concentrations. Also the volume of isolated deep waters in the SO is uncertain; moreover, water masses in other oceans may also have contributed to the overall atmospheric pCO 2 change (Rose et al, 2010;Okazaki et al, 2010;Kwon et al, 2012;Huiskamp and Meissner, 2012). The T glob and pCO 2 changes after the MI across the BA, the Younger Dryas and the Holocene are not expected to be simulated in detail by the DCESS model.…”