Reconstructing seawater circulation on the Moroccan shelf of Gondwana during the Late Devonian: Evidence from Nd isotope composition of conodonts

Abstract

[1] The Nd isotope composition of conodonts is used for the first time as a paleoceanographic proxy to reconstruct seawater circulation. Conodonts from two stratigraphic levels in the eastern Anti-Atlas, the lower part of the Frasnian Zone 11 (base of the Kellwasser facies) and the boundary interval of the Uppermost crepida/rhomboidea zones (top of the Kellwasser facies), were selected for reconstruction of seawater circulation on the Moroccan shelf of Gondwana during Late Devonian times. The recognized lateral variations in e Nd values are in very good agreement with current-related sedimentary features. They provide evidence for communication of the Moroccan shelf with the Rheic Ocean to the southwest and with the Variscan Sea to the north. In the Frasnian, the general seawater flow was directed from the southwest to the east and was influenced by a local riverine input of low radiogenic Nd from the south (the West African Craton and/or the Pan-African Ougarta Belt) and from the northwest (the Precambrian basement of the Anti-Atlas). The combined set of Nd isotopic records and directional data from the middle Famennian reveals a circulation pattern different from that of the Frasnian. It was dominated by a general westward flow with a countercurrent in the central Tafilalt area. The establishment of a new circulation system during the Famennian suggests essential changes in paleogeography of the western part of the Variscan realm.

At the southern limits of the Devonian reef zone: Palaeoecology of the Aferdou el Mrakib reef (Givetian, eastern Anti‐Atlas, Morocco)

“…(a) Distribution of Devonian outcrops in the eastern Anti‐Atlas (adapted from Dopieralska, ). Localities of the Mader Basin, coral‐stromatoporoid assemblages (JOu = Jebel Oufatène; JR = Jebel Rheris; MM = Madène el Mrakib; Ou = Ouihlane) and mud mounds (JD = Jebel Ou Driss; JZ = Jebel Zireg) mentioned in the text are indicated.…”

Nd isotopic composition of Late Devonian seawater in western Thailand: Geotectonic implications for the origin of the Sibumasu terrane

“…In addition, Nd isotopes have the singular benefit of providing information about age domains of eroded source rocks (dePaolo, 1988). While numerous studies documenting the Nd isotopic composition of seawater and its circulation patterns were published for the Cenozoic and Mesozoic (e.g., Stille et al, 1996;Thomas et al, 2003;Scher and Martin, 2004;Puceat et al, 2005;Gourlan et al, 2008;Dera et al, 2009), reconstructions from the Paleozoic are still rare (Holmden et al, 1996(Holmden et al, , 1998Felitsyn et al, 1998;Fanton et al, 2002;Dopieralska, 2009). The largest dataset of Nd Paleozoic seawater signatures is available from the Upper Devonian, but it is limited to the Variscan realm in Europe and North Africa (Dopieralska, 2003;Dopieralska et al, 2006).…”

Protracted diagenetic alteration of REE contents in fossil bioapatites: Direct evidence from Lu–Hf isotope systematics

“…Diagenetically-introduced trace elements have also formed the basis for radiometric dating of bones and teeth in archaeological settings (Millard and Hedges, 1996;Pike et al, 2002). Recently, the Nd isotope compositions of fossil bioapatites have been pivotal in reconstructing the evolution of ocean circulation throughout the Cenozoic (e.g., Vennemann and Hegner, 1998;Martin and Scher, 2004;Thomas, 2004;Martin, 2006, 2008;Via and Thomas, 2006;Kocsis et al, 2009a) and in the Mesozoic (e.g., Stille et al, 1996;Pucéat et al, 2005;Dera et al, 2009) and for Palaeozoic (e.g., Dopieralska et al, 2006;Dopieralska, 2009). Bioapatite-based geochemical proxies depend on two assumptions: (1) REE are introduced into bioapatite (bone and tooth mineral) rapidly after death, and (2) after early diagenetic enrichment, bones and teeth remain closed with respect to REE throughout geological time.…”