Geol. 656 Isotope Geochemistry
Chapter 10
312 8/22/12
10,000 to 7500 years ago, CO2 decreased slightly while δ13C increased relatively rapidly. The principle
effect here was most likely expansion of the terrestrial biosphere to previously glaciated areas and stor-
age of isotopically light carbon in peat and soil at high latitudes. Changes in carbonate compensation
and growth of coral reefs (to keep up with rising sealevel) likely also had an influence. Coral reef
growth, perhaps counter-intuitively, has the effect of increasing atmospheric CO2. Reef organisms ef-
fectively precipitate calcite through the reaction:
Ca2+ + HCO3
– → CaCO3 + H+
Consequently, reef growth decreases ocean alkalinity and pH, allowing the oceans to dissolve less CO2.
Prior to 11,000 years ago, the variation in δ13C is more complex. Over this period, δ13C correlates with
the slope of the atmospheric CO2 curve rather than its value, such that δ13C is low when CO2 concentra-
tion is rising rapidly. An initial increase in atmospheric CO2 and decrease in δ13C marks the end of the
last glacial maximum around the time of Henrich Event 1 (Henrich events, first identified as layers of
ice-rafted debris in sediment cores from the North Atlantic, are now understood to be events where the
North American ice sheet destabilized and sent great flotillas of icebergs into the North Atlantic.) The is
followed by period when δ13C increased and atmospheric CO2 concentrations stabilized during the Bøl-
ling-Allerød period, a time recognized from pollen records of Europe as one of rapidly moderating cli-
mate and ice sheet retreat. Following this, δ13C again decreased and atmospheric CO2 increased in the
Younger Dryas period, a time recognized from European pollen records as a return to cold, almost gla-
cial, conditions. Laurantou et al. (2010) concluded from this record that changes in Southern Ocean
ventilation played the dominant role changing atmospheric CO2, but changes in marine productivity,
North Atlantic circulation, and the terrestrial biosphere were involved as well. The initial increase in
atmospheric CO2 is likely due to more rapid ventilation of the Southern Ocean in a poleward shift in
Westerlies as suggested by Tuggweiler et al. (2006), but reduced ocean productivity also played a role.
During the Bølling-Allerød, production of North Atlantic Deep Water intensified, enhancing warming
in the North Atlantic region. In response, expansion of the northern hemisphere terrestrial biosphere to
higher latitudes and consequent build up of carbon and peat eventually caused δ13C to decrease and the
increase in atmospheric CO2 to stall. Meltwater from retreating glaciers flooded the North Atlantic in
the Younger Dryas shut down NADW production, producing locally cooler conditions but resulting in
more vigorous Southern Ocean overturn, driving up atmospheric CO2 and increasing its δ13C.
The Long-Term Carbon Cycle
On geologic times scales, the carbon cycle model must be augmented by 3 reservoirs, sedimentary
carbonate, sedimentary organic carbon, and the mantle, as well as fluxes between these reservoirs and
the oceans and atmosphere. Such a long-term model is shown in Figure 10.37, where the anthropogenic
perturbations have been removed. The most important thing to notice is that there is much more car-
bon in the carbonate and sedimentary organic carbon reservoirs than in all the reservoirs in Figure
10.29 combined. However, the fluxes to and from the sedimentary reservoirs are small, so they play lit-
tle role in short-term (< 1 Ma) atmospheric CO2 variations (at least in natural ones: we could properly
consider fossil fuel burning as a flux from sedimentary organic carbon to the atmosphere). We should
also point out that only a small fraction of the sedimentary organic carbon is recoverable fuel; most is
present as minor amounts (typically 0.5% or less) of kerogen and other refractory organic compounds
in sediments. Even greater amounts of carbon are probably stored in the mantle, though the precise
amount is difficult to estimate. An order of magnitude figure might be 125-500 ppm CO2 in mantle,
which implies a total inventory of 1.3-5 × 10
8 Gt, or nearly 10
6 times the amount in the atmosphere.
Again, the flux from the mantle to the atmosphere, which results from volcanism, is small, so the man-
tle plays no role in short-term atmospheric CO2 variations. On long time scales (>106 yr), however, it is
the fluxes to and from sediments and the mantle that control the atmospheric CO2 concentration.
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