Determinants on foraminiferal stable carbon isotopes offshore Cape Blanc, NW Africa

The region offshore Cape Blanc, NW Africa, is one of the most productive zones in the world’s oceans due to the occurrence of an eastern boundary upwelling system and dust input from the Sahara. It is also a well-known site for benthic foraminiferal research, where vertical zonation of the living foraminifera was first described in detail. However, earlier studies are based only on >150μm or >250μm specimens collected along one transect. Therefore, the relative proportion of the smaller, phytodetritus-related species (e.g., Alabaminella, Epistominella) is likely underestimated. This region was also the focus of foraminiferal stable carbon isotope (δ¹³C) studies.

However, previous area works concentrated either on the sea surface or on the deep-water signals and their oceanographic meaning. Thus, there is a lack of data linking these two realms.

Here we investigated foraminiferal assemblages, δ¹³C determinants, patterns, and shifts from the continental shelf to the abyssal plain. We analyzed ca. 360 samples from 12 multicore split in two parallel depth transects from ca. 100 to 3400 m water depth. The topmost 10 cm of each multicore was sampled in 1 cm intervals. We analyzed the δ¹³C composition of benthic species Cibicidoides wuellerstorfi (epifauna) and Uvigerina peregrina (infauna), as well as planktonic species Globigerinoides ruber white (>150 μm).  Benthic and planktonic foraminiferal assemblages (>63 μm) were also determined.

The δ¹³C difference between the sea-surface and the ocean-floor values (∆δ¹³C_{G. ruber white - C. wuellerstorfi}) do not show notable changes along the two studied transects. However, the δ¹³C difference between the epifaunal and the infaunal species (∆δ¹³C_{C. wuellerstorfi - U. peregrina}) shows a major increase from the continental shelf to the abyssal plain. Moreover, the benthic and planktonic foraminiferal assemblages present vertical and horizontal zonation related to the environmental parameters (e.g., oxygen, temperature, nutrients) and this is in accordance with the changes observed in geochemical signals showcasing a faunal and δ¹³C shift from the shore to the abyssal plain.

Our findings argue for different determinants of δ¹³C off Cape Blanc. In the nearshore (ca.100-1500 m water depth) the main control is mineral-rich dust input from the Sahara, while below this depth and further away from the continental shelf lateral advection plays the dominant role.