Astrochronology of the Late Turonian: implications for the behavior of the carbon cycle at the demise of peak greenhouse

作者：Laurin Jiri^*; Cech Stanislav; Ulicny David; Staffen Zdenek; Svobodova Marcela

来源：Earth and Planetary Science Letters, 2014, 394: 254-269.

DOI：10.1016/j.epsl.2014.03.023

• Turonian
• Coniacian
• Milankovitch
• astrochronology
• carbon isotopes
• carbon cycle

A series of prominent carbon-cycle perturbations, indicated by carbon-isotope excursions of up to 2 parts per thousand, marked climatic and oceanographic changes that followed the peak mid-Cretaceous greenhouse. The nature and forcing mechanisms of these changes remain poorly understood, partly due to the lack of a detailed temporal control in this interval. Here we present an orbitally calibrated time scale, which is based on hemipelagic rhythmites in the Bohemian Cretaceous Basin, and which can be linked to the established chronostratigraphic framework using inoceramid biostratigraphy and carbon-isotope signature. In the new age model, the Late Turonian, defined by the first occurrence of I. perplexus below and the first occurrence of C. deformis erectus above, spans 1.56(+/- 0.07) Myr. When anchored to the newly refined age of the Turonian-Coniacian boundary (Sageman et al., in press), the onset of the Late Turonian is 91.31(+/- 0.45) Ma. Inoceramid zones I. perplexus and M. scupini span 0.76(+/- 0.05) Myr and 0.80(+/- 0.05) Myr, respectively. A time-domain record of carbon-isotope variations was obtained by pinning published delta C-13(carb) data to the orbital time scale using 9 biostratigraphic and carbon-isotope markers. The timing of major carbon-isotope excursions suggests a link to 405-kyr eccentricity changes and, in the early Late Turonian, to similar to 100 kyr eccentricity variations. However, over 80% of the variance in the Late Turonian delta C-13 occurs at longer (million-year) scales. The key features of the delta C-13(crab) signature, including the amplified long-term variance, can be reproduced with simple models of orbitally driven isotopic mass balance. At a constant carbon-isotope fractionation, the best fit to the delta C-13(crab) signature is obtained with enhanced organic burial at high eccentricity (in accordance with monsoonal forcing of organic accumulation in the low-latitude Atlantic and/or buildup of low-latitude, terrestrial carbon reservoirs). Alternative solutions should consider changing carbon-isotope fractionation due to pCO(2) and/or nutrient-flux changes.

• 出版日期2014-5-15