Geochemistry of the Aptian bituminous limestones in Gümüşhane area, Eastern Black Sea region: new insight into paleogeography and paleoclimate conditions

Abstract

Aptian is characterized by widespread deposition of organic-rich sediment. The Aptian bitumen limestone horizon, which is thin decimetre-thick sequences, locally crops out in the Kırcaova area, Eastern Black Sea Region (Eastern Pontides). They are well correlated with Aptian bitumen limestone in the other Tethys Reams. They are proposed as episodes of increased organic matter. However, background factors controlling organic matter enrichment are poorly known. In this study, we present new inorganic geochemistry, including trace elements, rare earth elements (REE), redox-sensitive elements (RSE), stable-isotopes (δ¹⁸O and δ¹³C), and total organic carbon (TOC). We integrated new geochemical data with existing stratigraphy, paleontology, and organic chemistry data to provide new insight into the depositional environment and paleoclimate conditions during Aptian. The lacustrine bitumen limestone (LBL) samples have varied δ¹³C (ave. -1.45‰) and δ¹⁸O (ave.-4.50‰). They possess distinct REE patterns, with an average of REE (ave. 14.45 ppm) and Y/Ho (ave. 35) ratios. In addition, they have variable Nd/Yb_N (0.28–0.81; ave. 0.56) and Ce/Ce* (0.68–0.97; ave. 0.86), and relatively high Eu*/Eu (1.23–1.53; ave. 1.35). They display seawater signatures with reduced oxygen conditions. The enrichment in RSE (Mo, Cu, Ni, and Zn) and the low Mo/TOC (0.70–3.69; ave. 2.41) support a certain degree of water restriction. The high Sr/Ba, Sr/Cu, Ga/Rb, and K/Al records of the LBL facies suggest hot house climatic conditions. The sedimentary environment was probably an isolated basin that is transformed from the marine basin. In addition to depositional conditions, the regional parameters such as the climate, increased run-off period, nutrient levels, alkalinity level, and dominant carbonate producers favored the enrichment in organic matter of LBL facies. Thus, extreme greenhouse palaeoclimate conditions have an important role in organic matter enrichment in the isolated basin. Our results are conformable with the published data from marine, semi-restricted basin, and lacustrine settings in the different parts of the Tethys margin. Thus, this approach provides the first insight into the Aptian greenhouse paleo-climate conditions of the Eastern Black Sea Region, NE Turkey.