Planktonic foraminiferal δ¹⁸O-δ¹³C anomalies reveal earthquake-triggered transient fluid flow along the active Bokkoya strike-slip fault, Alboran Sea

In the Alboran Sea, oblique convergence between the African and Eurasian plates has driven the development of the active Al Idrissi-Bokkoya sinistral strike-slip fault system since ~1 Ma. Several moderate-magnitude earthquakes (Mw > 6) have been recorded along different segments of this fault system, highlighting its ongoing activity. This study investigates the dynamics of this nascent plate boundary by identifying seismic events recorded in sedimentary archives.

We focus on the Bokkoya transtensive fault system, which offsets the Small Al Idrissi Volcano and extends over ~20 km along strike. Sedimentation in this area is strongly influenced by the circulation of Deep Mediterranean Water masses, resulting in contourite deposition, and is likely punctuated by mass-movement processes triggered by seismic events.

A multidisciplinary dataset was acquired during the ALBACORE oceanographic campaign (R/V Pourquoi pas?, 2021), conducted within the framework of the ANR ALBANEO project, which aims to characterize the dynamics and seismic hazards of this emerging plate boundary. The dataset includes two 18 m-long Calypso sediment cores (ALB_CL54 and ALB_CL53) located directly above and within the subsiding basin of the main Bokkoya Fault. Analyzes include Multi-Sensor Core Logging (MSCL), X-Ray Fluorescence (XRF), Total Organic Carbon (TOC, Rock-Eval), and stable Isotope analyses (δ¹³C and δ¹⁸O), complemented by multibeam bathymetry and seismic reflection/sub-bottom profiler data.

Radiocarbon-calibrated δ¹⁸O records allow sedimentary sequences to be dated back to ~45 ka, encompassing major cold climatic intervals such as the Younger Dryas, Heinrich Stadial 1, and the Last Glacial Maximum (LGM). The mean sedimentation rate within the subsiding basin is approximately 35 cm.kyr^-1. Comparison of sedimentary successions across different fault compartments reveals pronounced contrasts during the LGM (at ~20-21 ka), when core ALB_CL54 -penetrating the fault plane- records an exceptionally high sedimentation rate (> 200 cm.kyr^-1), an absence of bioturbation within contouritic deposits, and a distinct coupled δ¹⁸O- δ¹³C (up to ~3 ‰) anomaly not observed in the adjacent core ALB_CL53, located in the fault zone.

The restriction of the isotopic anomaly to ALB_CL54 points to a localized, transient tectonic event involving the rapid expulsion of hot fluids along the fault zone., which temporarily served as a preferential fluid drainage pathway. The absence of a similar isotopic record in ALB_CL53 suggests limited lateral fluid dissipation, consistent with a brief, high-intensity fluid release occurring during a cold climatic period associated with low sea level. These results demonstrate that coupled δ¹⁸O and δ¹³C anomalies in planktonic foraminifera constitute a robust geochemical marker of tectonic events in marine sediments, providing a complementary tool to highlight episodes of fault activity beyond the resolution of sedimentological observations.