The Cenomanian/Turonian anoxic event at the Bonarelli Level in Italy and Spain: enhanced productivity and/or better preservation?

Abstract

The upper Cenomanian pelagic sediments of Furlo in the northern Apennines, Italy, are characterized by a 1.5-m-thick organic-rich stratigraphic horizon called the Bonarelli Level, which represents the second major oceanic anoxic event in the Cretaceous (OAE 2). The Bonarelli Level is depleted in carbonates and consists essentially of biogenic quartz, phyllosilicates, and organic matter, with values of TOC reaching 18%. The age of the Furlo section is constrained by correlating its d 13 C curve with that of the well-dated Pueblo (USA) and Eastbourne (UK) sections. The presence of all the planktonic foraminiferid zones and details of the OAE 2 d 13 C excursion indicates a relatively continuous but reduced sedimentation rate across the Cenomanian/Turonian (C/T) boundary. Sediment and TOC mass accumulation rates have been calculated and suggest a sedimentation break in the upper Bonarelli Level. This may be an artifact of the diachronous FAD of the planktonic foraminiferid Helvetoglobotruncana helvetica and suggests that in some sections the d 13 C curve may provide more reliable age control for dating the C/T boundary. In order quantitatively to explain the carbon isotope curve and the measured TOC mass accumulation rate, a simple dynamic model of the isotope effects of organic versus inorganic carbon burial was developed. In order to verify the consistency of the model we correlated the modeled output of the Furlo section with that of the Manilva section, in southeast Spain. The modeling shows that increasing productivity only partially explains the measured d 13 C excursion and is not the only factor relevant to black shales deposition. Preservation may play a central role, especially in the later stages of OAE 2. Phosphorus and TOC accumulation patterns in the Bonarelli Level in both Furlo and Manilva suggest a similar process although other factors may also be involved.

Geochemical characteristics in the Longmaxi Formation (Early Silurian) of South China: Implications for organic matter accumulation

“…Factors controlling organic matter accumulation include enhanced preservation of primarily marine organic matter under reducing conditions (Demaison and Moore, 1980;Mort et al, 2007), high marine surficial primary productivity (Pedersen and Calvert, 1990;Caplan and Bustin, 1998), or combinations of these models (Arthur and Sageman, 1994;Murphy et al, 2000). Other factors, such as influx of clastic material and supply of terrestrial organic matter, may also play an important role in organic matter accumulation (e.g., Murphy et al, 2000;Sageman et al, 2003).…”

“…Although much work has been conducted on these organic-rich sediments, their genesis is still a matter of debate. The debate concerning the origin of these sediments polarized many workers toward "preservation" (e.g., Demaison and Moore, 1980;Arthur and Sageman, 1994;Mort et al, 2007) versus "productivity" (e.g., Pedersen and Calvert, 1990;Sageman et al, 2003) models. The former argues that bottom water anoxia due to physical isolation of the benthic environment beneath a permanent pycnocline, as in the modern Black Sea, enhances organic matter preservation owing to the diminution or absence of aerobic decomposition.…”

The Albian to Turonian carbon isotope record from the Shilaif Basin (United Arab Emirates) and its regional and intercontinental correlation

“…Gale et al ., ; Jenkyns et al ., , ; Tsikos et al ., ; Scopelliti et al ., ; Voigt et al ., ; Gebhardt et al ., ; Jarvis et al ., ; Bomou et al ., ; Dickson et al ., ; Owens et al ., ). Other observations associated with OAE 2 include an increase of siliceous plankton in the Umbria‐Marche Basin during the event (Mort et al ., ) or pre‐dating it in the Natih Basin (Van Buchem et al ., ; Wohlwend et al ., ), and blooms of pelagic bivalve filaments from the earliest Turonian across Northern Africa and Oman to Tibet (Negra et al ., ).…”

Biogeochemical sulfur cycling during Cretaceous oceanic anoxic events: A comparison of OAE1a and OAE2

“…During OAEs, high rates of organic carbon production in the marine realm lead to the expansion of oxygen‐minimum zones and enhanced preservation of organic‐rich sediments [ Schlanger and Jenkyns , ]. There is general agreement that enhanced nutrient availability was required to sustain eutrophic conditions during OAEs [ Nederbraught et al , ; Mort et al, ; Kraal et al, ; Higgins et al , ; Westermann et al, ]. It is likely that large igneous province (LIP) emplacement played a role in enhancing rates of nutrient delivery to the ocean, either directly through volcanic input of trace metals or secondarily via higher rates of continental weathering due to volcanic outgassing of carbon dioxide [ Sinton and Duncan , ; Kerr , ; Snow et al, ; Jenkyns , ].…”