Lithology and planktic foraminifera biostratigraphy of Aptian–Albian boundary as encountered in Jebel Sidi Salem (northeastern Tunisia)

Specimens of Recent foraminifera of Amphistegina radiata, Peneroplis planatus and Globigerinoides ruber, from fifty samples of surface sediments of the continental margin of the State of Ceará, Brazil, have been analyzed for carbon and oxygen isotopes to investigate oceanographic parameters and determine the values of delta18O of the oceanic water. From a comparison between values of delta18O obtained for ocean water using the linear equations by (Craig and Gordon 1965) and the one by Wolff et al. (1998), it became evident that the former yielded a more reliable value (0.2‰ SMOW) than the latter. Lower values of delta18O for the ocean water in this continental margin resulted from continental water influence. Values of 18O (-0.3‰ to -1.5‰ PDB for benthic foraminifera and -0.6‰ to -2.4‰ PDB for planktic foraminifera), attest to a variation of temperatures of oceanic water masses, in average, between 20 to 22ºC in deep water and 24 to 27ºC, in surface water. Values of delta13C from +3.2‰ to -0.2‰ PDB (benthic foraminifera) reflect a variation in the apparent oxygen utilization (AOU) in the continental margin and indicate that the environments of bacteriological decomposition of organic matter are not continuous along the investigated area.

Download Full-text

A record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera

Climate of the Past ◽

10.5194/cp-13-149-2017 ◽

2017 ◽

Vol 13 (2) ◽

pp. 149-170 ◽

Cited By ~ 17

Author(s):

Rosanna Greenop ◽

Mathis P. Hain ◽

Sindia M. Sosdian ◽

Kevin I. C. Oliver ◽

Philip Goodwin ◽

...

Keyword(s):

Carbon Cycle ◽

Middle Miocene ◽

Isotope Composition ◽

Boron Isotope ◽

Mineral Formation ◽

Secondary Mineral ◽

Cycle Model ◽

Planktic Foraminifera ◽

Carbon Cycle Model ◽

Secondary Mineral Formation

Abstract. The boron isotope composition (δ11B) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the δ11B of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater δ11B based on the δ11B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their δ13C values. Carbon cycle model simulations underscore that the ΔpH–Δδ13C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that δ11Bsw was ∼  37.5 ‰ during the early and middle Miocene (roughly 23–12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61 ‰. Strikingly, this pattern is similar to the evolution of the seawater isotope composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotope composition of riverine input to the ocean since 14 Ma.

Download Full-text