INVESTIGATING LINKS BETWEEN CLIMATE AND NUTRIENT CYCLING IN THE EARLY PERMIAN: AN INTEGRATED STUDY OF NEW INORGANIC AND STABLE ISOTOPE DATA FROM THE MIDLAND BASIN OF WEST TEXAS

New stable isotope data (δ¹³C, δ¹⁸O, δ¹⁵N) and inorganic geochemistry (major, trace elements) permit a high resolution zonation and paleoenvironmental reconstruction for the lower Permian Wolfcamp, Spraberry, and Dean formations in the Midland Basin of west Texas. Organic carbon isotope values peak in the Wolfcamp D and C, coincident with exceptionally low inorganic δ¹³C_carb values. However, above this δ¹³C_carb and δ¹³C_org values that move in lock-step through the Wolfcamp B, Wolfcamp A, Dean, and Spraberry Sections. Highest δ¹³C_carb occurs within the Wolfcamp A, producing a distinct positive shift in values that is interpreted as an Asselian/Sakmarian boundary excursion recognized throughout the central United States; a useful chronostratigraphic anchor for these units. Values of δ¹⁵N rise sharply through the Wolfcamp A and remain high as deposition of the Dean and Spraberry progressed. This coincides with relatively low and flat-lying carbon isotope values.

These observations are coupled with significant shifts of inorganic geochemistry, which reflects the varied influence of sediment provenance and paleoenvironmental conditions. A nested hierarchy of patterns in the data are interpreted as the cyclic variation induced by sea-level and climate. Superimposed on this is an overall upward-decline in the abundance of redox sensitive trace elements (e. g. Cu, Ni, Zn) commonly associated with organic matter and seafloor anoxia.

We interpret these trends as the signal of transition from cool, dynamic climates of the late Pennsylvanian to warmer greenhouse-type conditions that prevailed in the middle to late Permian. Positive δ¹³C excursions in the lower section are interpreted as organic carbon burial events, with a probable influence of meteoric diagenesis, particularly during times of lowstand. Low and relatively stable δ¹³C values through the Dean and Spraberry, coupled with high δ¹⁵N suggest nitrate limitation, a possible signal of poorly circulated seas and stable climates.