The role of Southern Ocean processes in orbital and millennial CO2 variations – A synthesis

Abstract

a b s t r a c tRecent progress in the reconstruction of atmospheric CO 2 records from Antarctic ice cores has allowed for the documentation of natural CO 2 variations on orbital time scales over the last up to 800,000 years and for the resolution of millennial CO 2 variations during the last glacial cycle in unprecedented detail. This has shown that atmospheric CO 2 varied within natural bounds of approximately 170-300 ppmv but never reached recent CO 2 concentrations caused by anthropogenic CO 2 emissions. In addition, the natural atmospheric CO 2 concentrations show an extraordinary correlation with Southern Ocean climate changes, pointing to a significant (direct or indirect) influence of climatic and environmental changes in the Southern Ocean region on atmospheric CO 2 concentrations.Here, we compile recent ice core and marine sediment records of atmospheric CO 2 , temperature and environmental changes in the Southern Ocean region, as well as carbon cycle model experiments, in order to quantify the effect of potential Southern Ocean processes on atmospheric CO 2 related to these orbital and millennial changes. This shows that physical and biological changes in the SO are able to explain substantial parts of the glacial/interglacial CO 2 change, but that none of the single processes is able to explain this change by itself. In particular, changes in the Southern Ocean related to changes in the surface buoyancy flux, which in return is controlled by the waxing and waning of sea ice may favorably explain the high correlation of CO 2 and Antarctic temperature on orbital and millennial time scales. In contrast, the changes of the position and strength of the westerly wind field were most likely too small to explain the observed changes in atmospheric CO 2 or may even have increased atmospheric CO 2 in the glacial. Also iron fertilization of the marine biota in the Southern Ocean contributes to a glacial drawdown of CO 2 but turns out to be limited by other factors than the total dust input such as bioavailability of iron or macronutrient supply.

Mode change of millennial CO 2 variability during the last glacial cycle associated with a bipolar marine carbon seesaw

“…In addition, a rapid drop of about 10 ppmv within 350 y is found at the beginning of this transition comparable in duration and magnitude to a rapid increase found during the last deglaciation (21). Various processes are candidates to explain changes of CO 2 , such as the remarkable CO 2 decrease during the MIS 5-4 transition, or the millennial-scale variations associated with the CDM (6,22). In the following, we discuss mechanisms that could potentially explain the increase of the lag of CDM relative to the DO event from MIS 5 to 3 as identified in our CO 2 records and, hence, focus on the CO 2 increase after the DO events which explains-in case of the MIS 3 CDM-about one-third of the total CDM amplitude.…”

“…Changes in ocean circulation and the distribution of major water masses can influence atmospheric CO 2 concentrations (22,26). One proxy for ocean hydrographic changes is the Nd isotope ratio in ocean sediment cores (expressed as ε Nd ), which traces deep water origin at the drill site (Fig.…”

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity

“…It is generally believed that atmospheric δ 13 C provides useful constraint on the mechanisms of deglacial CO 2 rise (Schmitt et al, 2012;Joos et al, 2004;Fischer et al, 2010). Simulated atmospheric δ 13 C drops from the LGM level of about −6.4 ‰ to the minimum value of −6.7 ‰ between 16 and 14 ka (Fig.…”

A reconstruction of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the last glacial inception

“…Multiple processes operate simultaneously, and interact with each other non-linearly (Köhler et al, 2005;Brovkin et al, 2007;Sigman et al, 2010;Fischer et al, 2010;Tschumi et al, 2011), allowing for a wide range of possible scenarios to explain observed natural changes in atmospheric CO 2 . Furthermore, Goodwin et al (2011) point out that the way in which different climate proxies combine is crucial for accurate past carbon cycle reconstructions.…”