δ¹³C and δ¹⁸O enrichment near cycle tops and sequence boundaries in the Capitan backreef, McKittrick and Slaughter Canyons

Bishop, James W.¹, Dawn Y. Sumner¹, David A. Osleger¹, Isabel Montanez¹, Scott W. Tinker² (1) University of California, Davis, Davis, CA (2) Jackson School of Geosciences,

The ~260 My Seven Rivers and Yates formations, West Texas and New Mexico, show substantial δ¹³C and δ¹⁸O variation over <<50 kyrs. Isotopic values from well-preserved backreef whole rock, cement, and packstone matrix are enriched up to 3‰ in δ¹³C and δ¹⁸O approaching cycle tops and sequence boundaries. This enrichment is unlikely to be due to diagenetic alteration because: (1) sampled calcites are chemically and petrographically well preserved; (2) the degree of recrystallization does not correlate with isotopic depletion; (3) meteoric diagenesis should deplete, not enrich, δ¹³C and δ¹⁸O at cycle tops; and (4) isotopic values do not correlate with porosity or petrographic texture, as expected from burial diagenesis. Rather, isotopic fluctuations likely reflect global- or basin-scale processes that changed the isotopic composition of basin water. Isotopic enrichment during lowstands could be global, with increased productivity causing enriched δ¹³C, coupled to cooler temperatures and ice sheet expansion, which caused enriched δ¹⁸O. Alternatively, enrichment may have resulted from basin-scale processes. The silled Delaware Basin was restricted, with evaporated waters replaced by more open marine waters via the Hovey Channel. Varying sealevel could change the rate and depth of the open marine water column that exchanged with basin water. During highstands, replenishing flow would have included waters depleted in δ¹³C from the oxygen minimum zone. However, during lowstands, the shallower channel may only have admitted surface waters, enriched in δ¹³C. Shallowing might also have restricted the outflow of saline bottom waters, causing elevated salinity. Thus, δ¹³C and δ¹⁸O could covary with sealevel.