Grain Size Analysis Reveals Late Holocene Climate Variability from the Halk El Menjel Lagoon (Central Mediterranean)

Examining Late Holocene climate variability is crucial for understanding past environmental changes and predicting future patterns, especially in the Mediterranean, a region sensitive to climate shifts. Positioned between temperate and arid zones, the Mediterranean is vulnerable to even minor climatic fluctuations, impacting ecosystems and water systems. The Halk El Menjel lagoon in the South-Central Mediterranean is a vital paleoenvironmental archive, recording over 4000 years of climate variability through hydrological and sedimentary cycles.

Granulometric analysis is an essential tool for interpreting sediments as environmental indicators. This study focuses on grain size distribution, revealing polymodal distributions linked to specific depositional environments and transport processes, whether by water or wind. Detailed granulometric analysis of 750 sediment samples from a 150-cm core at Halk El Menjel Lagoon was conducted using Principal Component Analysis (PCA), dendrogram clustering (DC), and temporal distribution analysis. These methods help reconstruct sediment transport dynamics and investigate long-term climatic variability in this semi-arid coastal environment.

Twelve distinct grain size sub-populations (ranging from 1.2–2 µm to 1000–1350 µm) were identified, each linked to different depositional environments. PCA revealed three main sedimentary groups: aeolian (dominated by 600–1000 µm), fluvial (dominated by 250–580 µm), and hydraulic runoff (dominated by 1.2–2 µm). These groups reflect different transport processes and depositional environments, with varying levels of homogeneity. DC further refined the categorization, highlighting significant differences between aeolian and fluvial processes.

Temporal analysis, based on an age-depth model using LANDO and Bayesian Bchron scripts, revealed key climatic shifts over the 4000-year period. The analysis, visualized through various charts, showed significant changes in sediment composition linked to historical climate variability. From 3500 to 3000 BP, all twelve sub-populations coexisted, reflecting highly variable climatic conditions. During the Iron Age (IA) to Roman Classical Period (RCP), the 600–1000 µm sub-population increased, peaking during the 2.8 ka transition in the IA. This shift suggests arid conditions and growing aeolian influence.

The Roman Humid Period (RHP) exhibited alternating arid and humid phases. During arid periods, the 600–1000 µm fraction dominated, while the 250–580 µm fraction, often associated with the 2–5.2 µm fraction, prevailed in humid periods. By the RCP, the 600–1000 µm sub-population reached near-total dominance, indicating a shift toward predominant aeolian sedimentation.

In contrast, the Dark Age (DA) and Medieval Climate Anomaly (MCA) periods saw a significant increase in the 250–580 µm sub-population, exceeding 75%, indicating wetter conditions favoring fluvial sedimentation. A new component, represented by the 1.2–2 µm fraction, emerged, accounting for approximately 25% of the sediment, signaling evolving depositional environments.

The Little Ice Age (LIA) and Modern Period (MP) were dominated by the 1.2–2 µm sub-population, comprising over seven-eighths of the sediment in the MP. This reflects lagoon infilling and deposition of suspended material transported by hydrological processes.

These findings align with Mediterranean climatic events, highlighting shifts in precipitation and aeolian activity. The Halk El Menjel lagoon provides valuable insights into long-term environmental dynamics, illustrating the interplay between climate variability and sedimentary processes.