Wallmann, Klaus , Schneider, Birgit and Sarnthein, M. (2016) Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric pCO2 and seawater composition over the last 130 000 years: a model study. Climate of the Past, 12 (2). pp. 339-375. DOI 10.5194/cp-12-339-2016.

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Abstract

We have developed and employed an Earth system
model to explore the forcings of atmospheric pCO2 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 14CDIC
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
pCO2 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 pCO2 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
pCO2 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 pCO2 can be explained by fast changes
in ocean dynamics and nutrient utilization whereas the gradual
pCO2 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.

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