Atmospheric carbon dioxide variations across the middle Miocene climate transition

The middle Miocene climate transition similar to 14 Ma marks a fundamental step towards the current ice-house climate, with a similar to 1 parts per thousand delta O-18 increase and a similar to 1 parts per thousand transient delta(3C) rise in the deep ocean, indicating rapid expansion of the East Antarctic Ice Sheet associated with a change in the operation of the global carbon cycle. The variation of atmospheric CO2 across the carbon-cycle perturbation has been intensely debated as proxy records of pCO(2) for this time interval are sparse and partly contradictory. Using boron isotopes (delta B-11) in planktonic foraminifers from Ocean Drilling Program (ODP) Site 1092 in the South Atlantic, we show that long-term pCO(2) varied at 402 kyr periodicity between similar to 14.3 and 13.2 Ma and follows the global delta C-13 variation remarkably well. This suggests a close link to precessional insolation forcing modulated by eccentricity, which governs the monsoon and hence weathering intensity, with enhanced weathering and decreasing pCO(2) at high eccentricity and vice versa. The similar to 50 kyr lag of delta C-13 and pCO(2) behind eccentricity in our records may be related to the slow response of weathering to orbital forcing. A pCO(2) drop of similar to 200 mu atm before 13.9Ma may have facilitated the inception of ice-sheet expansion on Antarctica, which accentuated monsoon-driven carbon cycle changes through a major sea-level fall, invigorated deep-water ventilation, and shelf-to-basin shift of carbonate burial. The temporary rise in pCO(2) following Antarctic glaciation would have acted as a negative feedback on the progressing glaciation and helped to stabilize the climate system on its way to the late Cenozoic ice-house world.

Automated summary

The middle Miocene climate transition around 14 million years ago marked a fundamental step towards the current ice-house climate, with an increase in delta O-18 and transient delta(3C) in the deep ocean, indicating rapid expansion of the East Antarctic Ice Sheet. The variation of atmospheric CO2 during this time interval has been debated, with sparse and contradictory proxy records. Using boron isotopes in planktonic foraminifers, researchers found that long-term pCO2 varied at 402 kyr periodicity between 14.3 and 13.2 Ma, closely linked to global delta C-13 variation influenced by precessional insolation forcing modulated by eccentricity. The drop in pCO2 before 13.9Ma may have facilitated Antarctic ice-sheet expansion, leading to carbon cycle changes through sea-level fall, deep-water ventilation, and shelf-to-basin shift of carbonate burial.