A continuous record of organic carbon δ
13C from a buried soil sequence in south-central Texas demonstrates: 1) strong coupling between marine and adjacent continental ecosystems in the late Pleistocene as a result of glacial meltwater entering the Gulf of Mexico and 2) ecosystem decoupling in the Holocene associated with a reduction of meltwater and a shift in global circulation patterns. In the late Pleistocene, reduction in C
4 plant productivity correlates with two well-documented glacial meltwater pulses (
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15,000 and 12,000
14C yr B.P.), indicating a cooler-than-present adjacent continental environment. Increased C
4 production between 11,000 and 10,000
14C yr B.P. suggests that the Younger Dryas was a warm interval responding to the diversion of glacial meltwater away from the Mississippi River. With waning meltwater flow, C
4 productivity generally increased throughout the Holocene, culminating in peak warm intervals at
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5000 and 2000
14C yr B.P. Shifts in the abundances of C
3–C
4 plants through the late Quaternary show no correlation to ecophysiological responses to atmospheric CO
2 concentration.