STABLE ISOTOPE STRATIGRAPHY OF THE PLIO-PLEISTOCENE ZÚJAR SECTION, GUADIX-BAZA BASIN (SE SPAIN)

Continental sediments exposed along a ~200-m outcrop near the village of Zújar in the Guadix-Baza Basin, SE Spain, serve as a Pliocene mammal biostratigraphic reference section for Western Europe. This section also records a detailed history of facies changes during a time of important climatic transitions, including the expansion of temperate grasslands and onset of Northern Hemisphere glaciation. In this study, we report new stratigraphic and geochemical analyses of lacustrine, palustrine, and pedogenic carbonates from the Zújar section, which we use to interpret the climatic history of this area. We complement analyses of the stable carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O, respectively) of carbonate and bulk organic matter with thin-section petrography, cathodoluminescence microscopy, scanning electron microscopy, and semi-quantitative X-ray diffraction.

Periods of wet climate and lake-level highstand in the Zújar section are recorded by shallow lacustrine carbonate beds dominated by microcrystalline calcite. The δ¹⁸O values of the lacustrine carbonates range from ‑8.1‰ to ‑5.7‰ VPDB, and are comparable to those of modern and Holocene carbonates from Spanish lakes at elevations similar to that of the modern Guadix-Baza Basin (~1,000 m). Periods of aridification and lake-level lowstand are recorded by palustrine carbonates bearing evidence of subaerial exposure (root traces, microkarst). These carbonates are commonly rich in microcrystalline dolomite (up to 99% relative to calcite) and have δ¹⁸O values ranging from ‑3.6‰ to +1.9‰ VPDB, consistent with carbonate precipitation in an evaporative environment. The calcite- and dolomite-rich samples are indistinct in terms of their δ¹³C values, which range from ‑8.8‰ to ‑4.2‰ VPDB. When adjusted to account for past variation in the δ¹³C of atmospheric CO₂, the δ¹³C values of bulk organic matter occluded within carbonate fall within the upper end of the range of δ¹³C values for modern vegetation using the C3 photosynthetic pathway, and are consistent with independent isotopic and palynological evidence for the dominance of C3 vegetation in southern Spain throughout the Pliocene.