Climate change in southern Illinois, USA, based on the age and δ¹³C of organic matter in cave sediments

Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P. floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four 14 C ages on humified organic matter provide chronologic control. The δ 13 C values of the organic matter reflect the proportion of C 4 -type to C 3 -type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C 4 -type vegetation was more prevalent relative to C 3 -type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ 13 C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.