Palaeoceanographic constraints on the Devonian evolution of the north-western Gondwana margin

The palaeogeographic configuration of the continental fragments and seaways that developed during the Devonian evolution of the Rheic Ocean remains insufficiently understood. One of the elusive elements is the palaeogeography of north-western Gondwana, and most notably the position of the Moroccan Meseta – the central part of the Moroccan Variscides, comprising a collage of blocks once located at the northern periphery of Gondwana. While some reconstructions place the Meseta as a distal, continuous segment of the Gondwana margin, others depict a very different scenario, envisaging that at some point the Meseta became separated from Gondwana by a wide oceanic basin. Here, we aim to better understand the Late Devonian position of the Meseta using a novel approach that combines two palaeoceanographic tracers: neodymium (Nd) and oxygen isotopes. These proxies, applied together on conodont apatite – an established archive of the composition (Nd and O isotopes) and temperature (O isotopes) of past seawater – provide new constraints on the pre-Variscan oceanography of the Gondwana margin. The analysed, uppermost Givetian-lower Famennian sections, which are representative of the Gondwana mainland (eastern Anti-Atlas) and the cratonward part of the Western Meseta (Middle Atlas) show similar, relatively unradiogenic ε_Nd values. These signatures point to dominance of continental weathering-derived Nd sources in the epicontinental seas of northwestern Gondwana. The temporal trends observed in the studied sections also show notable similarities, which are primarily interpreted as reflecting variations in the continental-runoff vs. open-oceanic contributions to the local marine Nd isotope budget. These variations were controlled by changes in sea level, local tectonic movements, and the evolution of vascular plants on land. The distal, outboard margin of the Western Meseta exhibits less variable and more radiogenic ε_Nd values, indicating a greater contribution from open-oceanic seawater. While the observed trends in  oxygen isotope signatures are generally consistent with global records, the δ¹⁸O values are significantly lower than those reported from other parts of the Rheic realm. The most likely explanation for the observed ¹⁸O depletion is the increased role of freshwater input in the relatively high-latitude, semi-restricted epicontinental basins. Overall, the observed ε_Nd–δ¹⁸O signatures are consistent with the location of Moroccan Meseta at the northern Gondwana margin. Some local variations in the isotope signals can be attributed to the semi-isolated nature of the studied basins, rather than to a presence of an extensive Late Devonian oceanic seaway between the Anti-Atlas and Meseta domains.

This work was supported by the National Science Centre, Poland, grant No. 2022/47/ST10/00205.