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The turbidity maximum zone (TMZ) is one of the most important regions in an estuary. However, the high concentration of suspended material makes it difficult to measure the partial pressure of CO ( pCO) in these regions. Therefore, very little data is available on the pCO levels in TMZs. To relatively accurately evaluate the CO flux in an example estuary, we studied the TMZ and surrounding area in the Changjiang (Yangtze) River estuary. From seasonal cruises during February, August, November 2010, and May 2012, the pCO in the TMZ and surrounding area was calculated from pH and total alkalinity (TA) measured in situ, from which the CO flux was calculated. Overall, the TMZ and surrounding area acted as a source of atmosphere CO in February and November, and as a sink in May and August. The average FCO was −9, −16, 5, and 5 mmol/(m·d) in May, August, November, and February, respectively. The TMZ's role as a source or sink of atmosphere CO was quite different to the outer estuary. In the TMZ and surrounding area, suspended matter, phytoplankton, and pH were the main factors controlling the FCO, but here the influence of temperature, salinity, and total alkalinity on the FCO was weak. Organic carbon decomposition in suspended matter was the main reason for the region acting as a CO source in winter, and phytoplankton production was the main reason the region was a CO sink in summer. [ABSTRACT FROM AUTHOR]