Isotope and elemental geochemistry of black shale‐hosted fossiliferous concretions from the Cretaceous Santana Formation fossil Lagerstätte (Brazil)

Abstract

Carbonate concretions hosted within organic carbon-rich shale sequences represent unique archives of often exceptionally preserved fossil biota. Besides providing high-fidelity preservation, their geochemical signatures can provide insight into the physical and chemical processes during early and later-stage concretion growth. Here, two fossiliferous carbonate concretions of the late Early Cretaceous Santana Formation (Araripe Basin, northeast Brazil) are analysed with an integrative geochemical approach including l-XRF scanning, d 13 C, d 18 O, 87 Sr/ 86 Sr and D 47 (clumped isotope thermometry). Individual concretions show a concentric internal zonation with the outermost layer being composed of millimetre thick cone-in-cone calcite. A strong covariance of d 13 C and d 18 O values of the fine-crystalline concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages. Microbially-mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass, forming localized environments within a zone of sulphate reduction. Microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the concretion body and by high pyrite abundance in the surrounding shale. Information on the later-stage diagenetic processes affecting the Santana concretions can be derived from the outermost fringing cone-in-cone calcite. The carbonate precipitating fluid was characterized by a more or less marine d 18 O composition (calculated d 18 O porewater = À1Á0 to À1Á8&) and by radiogenic Sr-isotope ratios (up to 0Á713331 AE 7Á0*10 À6 ), the latter probably reflecting modification due to interaction with the surrounding shale or, alternatively, with underlying evaporitic sulphate deposits 150 influenced by strong continental inflow or with crystalline basement rocks. The D 47 -derived temperature estimates range between 37°C and 42°C AE 5, indicating precipitation of the cone-in-cone calcite at a depth of 650 to 850 m, which is only half as much as the maximum burial depth derived from existing fission-track data. Overall, the study of fossiliferous carbonate concretions in organic carbon-rich sedimentary sequences can reveal a complex growth history spanning incipient microbially-influenced precipitates as well as later-stage burial diagenetic phases.

Carbonate clumped isotope analyses with the long‐integration dual‐inlet (LIDI) workflow: scratching at the lower sample weight boundaries

“…Clumped isotope measurements were background-corrected using the background scans performed before each run and the built-in background subtraction algorithm of the Easotope software. In detail, we performed peak shape scans in 5 V steps from 30 V to 10 V intensity on m/z 44, which is the most abundant CO 2 isotopologue, 12 C 16 O 2 . These scans are essential to determine and correct for the m/z 44 dependent backgrounds that are responsible for non-linearity effects in the clumped isotope signal.…”

“…During the last 10 years clumped isotope geochemistry of carbonate minerals has become an important tool in the fields of paleoclimatology [1][2][3][4][5] and paleoaltimetry, [6,7] and in characterizing diagenetic processes. [8][9][10][11][12] Although much progress has been achieved over the last decade, a major obstacle to the applicability of clumped isotopes to a wider range of applications is the relatively large samples that are needed for analyses.…”

The transgressive-regressive cycle of the Romualdo Formation (Araripe Basin): Sedimentary archive of the Early Cretaceous marine ingression in the interior of Northeast Brazil

“…Although freshening events by runoff (Beurlen, 1971a) and associated anoxia have been suggested as the main causative factors for the high fish mortality and the genesis of the fossil-rich concretions (Martill, 1988(Martill, , 1997, the intrusion of sulphide-containing waters and photic-zone euxinia are more likely (Heimhofer et al, 2008). The genesis of the concretions was also considered to result from microbially mediated processes, and controlled by fermentation and methanogenesis, generating a zone of sulphate reduction around the decaying carcasses (Heimhofer et al, 2017).…”

Mesozoic climates and oceans – a tribute to Hugh Jenkyns and Helmut Weissert

“…Furthermore, high organic‐carbon burial rates and associated low‐oxygen conditions have a significant effect on preservation and diagenetic processes, which can result in exceptional palaeontological archives (e.g. Heimhofer et al ., ).…”