Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO₂ and seawater composition over the last 130 000 years: a model study

We have developed and employed an Earth system model to explore the forcings of atmospheric <i>p</i>CO₂ change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus (DP), reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), ¹³C-DIC, and ¹⁴C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial <i>p</i>CO₂ drawdown may be attributed to marine regressions. The glacial sea-level low-stands implied steepened ocean margins, a reduced burial of particulate organic carbon, phosphorus, and neritic carbonate at the margin seafloor, a decline in benthic denitrification, and enhanced weathering of emerged shelf sediments. In turn, low-stands led to a distinct rise in the standing stocks of DIC, TA, and nutrients in the global ocean, promoted the glacial sequestration of atmospheric CO₂ in the ocean, and added ¹³C- and ¹⁴C-depleted DIC to the ocean as recorded in benthic foraminifera signals. The other half of the glacial drop in <i>p</i>CO₂ was linked to inferred shoaling of Atlantic meridional overturning circulation and more efficient utilization of nutrients in the Southern Ocean. The diminished ventilation of deep water in the glacial Atlantic and Southern Ocean led to significant ¹⁴C depletions with respect to the atmosphere. According to our model, the deglacial rapid and stepwise rise in atmospheric <i>p</i>CO₂ was induced by upwelling both in the Southern Ocean and subarctic North Pacific and promoted by a drop in nutrient utilization in the Southern Ocean. The deglacial sea-level rise led to a gradual decline in nutrient, DIC, and TA stocks, a slow change due to the large size and extended residence times of dissolved chemical species in the ocean. Thus, the rapid deglacial rise in <i>p</i>CO₂ can be explained by fast changes in ocean dynamics and nutrient utilization whereas the gradual <i>p</i>CO₂ rise over the Holocene may be linked to the slow drop in nutrient and TA stocks that continued to promote an ongoing CO₂ transfer from the ocean into the atmosphere.