Sea-level changes and geochemical anomalies across the Cenomanian–Turonian boundary: Pecı́nov quarry, Bohemia

Abstract

Relationships between geochemical anomalies, sea-level change and other events were studied in the Cenomanian–Turonian boundary interval in the Pecinov quarry in southwestern part of the Bohemian Cretaceous Basin (Czech Republic). A major 3rd-order sea-level rise at the base of the late Cenomanian M. geslinianum Zone was followed by deposition of organic-enriched mudstones in a succession of parasequences, deposited in response to high-frequency (4th-order) sea-level fluctuations and recording a stepwise decrease in bottom oxygenation towards intensely dysaerobic conditions. A complex δ13C excursion occurs in total organic matter of the late Cenomanian deposits. A sequence boundary of latest Cenomanian age in the Pecinov section is correlated to a global sea-level fall during the N. juddii Zone. A renewed sea-level rise occurred during the early Turonian W. coloradoense Zone and reached maximum flooding during the M. nodosoides Zone. During the early Turonian, bottom waters were generally aerobic, and the positive δ13C excursion waned early in the W. coloradoense Zone. The magnitude of the δ13C excursion, more than 4‰, is approximately the same as in North America and Northern Africa, confirming that it was controlled by a global paleoceanographic mechanism. The absence of anoxia and a δ13C anomaly during the peak flooding of early Turonian age suggests that widespread deposition of organic-enriched deposits, as well as the positive shift in δ13C generally did not depend on the absolute elevation of sea level but, rather, on the area of newly flooded land during transgression. Abrupt, small-scale shifts in δ13C towards higher values coincide with the flooding surfaces of parasequences and may reflect either regional changes in the proportion of marine and terrestrial organic matter or rapid global changes in isotopic composition of marine organic matter related to high-frequency sea-level changes. Abundances of Ir, Sc, Cr, V and other elements previously reported as forming anomalous concentrations in the boundary interval showed no enrichment in the Bohemian section. This is most probably due to the large distance of the depositional site from the presumed volcanic source of element-enriched deep waters in the proto-Caribbean region. Anomalous concentration of Mn at the base of the Turonian deposits was caused by diagenetic incorporation of Mn into siderite.