Dynamic sedimentary conditions during periods of enhanced sequestration of organic carbon in the central southern Tethys at the onset of the Cenozoic global cooling

Abstract

Stagnant bottom-water conditions (e.g., low and stable redox potential, long-water residence time) is an assumption commonly used to explain the preservation and burial of high amounts of organic carbon (Corg) in marine sediments. Rather than stagnant conditions, the evidence presented here from north-central Tunisia supports dynamic conditions during formation of variably Corg-rich, outermost shelf carbonates of the early–middle Eocene. The dynamic conditions are inferred by the deposition of four distinct lithofacies in this outermost shelf setting. Shedding of carbonate (i.e., mud and fragmented bioclasts) from the shallower source areas controlled the distribution of all lithofacies, with higher amounts of transported benthic debris occurring in the most proximal lithofacies and vice versa. This carbonate shedding also controlled the deposition of three orders of lithological cycles, from limestone/marly limestone couplets grading to cycles made up of groups of couplets. Bottom-water redox potential varied in intensity throughout this depositional setting, with moderate oxygen depletion (suboxic conditions) in the southern sector of north-central Tunisia and much higher oxygenation in the northern area. Evidence for suboxic bottom waters in the southern sector (higher Corg contents) is provided by higher trace metal (Cu, Ni, Zn, Cr, Mo, U and V) enrichments than in the northern area. Regionally heterogeneous primary productivity of surface waters is suggested to have caused a higher Corg burial flux in the southern sector compared to the north, a situation interpreted to have been related to varying upwelling patterns due to the effects of regional palaeogeography and the dominant wind patterns.The deposition of the studied Corg-rich carbonates spanned part of the calcareous nannofossil Zones NP13 to NP14 (~50–48m.y. ago) and coincided with the initiation of the Cenozoic global cooling subsequent to the early Eocene climatic optimum (EECO) (~52–50m.y. ago). An implication is that an increased Corg burial in north-central Tunisia could have been part of a major event sequestering atmospheric CO2 in marine sediments that caused climatic cooling immediately after the EECO. This implication, however, is difficult to reconcile with the data available elsewhere. Other than our Tunisian carbonates, major Corg sequestration in marine sediments of the early–middle Eocene transition are poorly documented globally. If this scenario is confirmed, other negative feedbacks, such as enhanced continental weathering, increased terrestrial carbon stock, decreased CO2 outgassing and/or changes in ocean circulation, would have been more influential to the onset of the Cenozoic global cooling.