The significance of the location can hardly be overstated. Northwestern Africa and the Western Mediterranean are considered among the most climate-sensitive regions on Earth, yet the periodicities and forcing mechanisms behind their long-term environmental variability have remained poorly understood. Lake Sidi Ali sits at a transitional zone between Morocco’s wetter northwest and its arid southeast, and its hydrology is influenced by three major air masses: Atlantic, Mediterranean, and Saharan. With no significant surface inlet or outlet, a maximum water depth of 38 meters, and alkaline, evaporatively influenced waters, the lake functions as a sensitive natural rain gauge whose sediments preserve the chemical fingerprints of every shift in the precipitation-evaporation balance.

The core itself was recovered in 2012 from the deepest point of the lake using a floating platform and a piston corer, yielding 19.63 meters of finely layered gyttja. The team dated the sequence with a robust chronology built from 26 radiocarbon measurements on pollen concentrates, supplemented by lead-210 and cesium-137 dating for the most recent layers. Onto this temporal framework they overlaid an extraordinary dataset: more than 17,800 X-ray fluorescence measurements taken at one-millimeter intervals along the core, calibrated against quantitative benchtop XRF analyses to convert raw scanning intensities into true elemental concentrations. Elements such as calcium, strontium, iron, titanium, potassium, and silicon each tell a different story about what was happening in and around the lake.

The analytical heart of the study lies in two complementary statistical techniques. Redfit spectral analysis, which works with unevenly spaced data, breaks each proxy time series into its constituent periodicities and tests them against a conservative red-noise background to weed out spurious cycles. Wavelet analysis then adds the dimension of time, revealing when particular cycles were strong, when they weakened, and when they vanished altogether. By correlating the statistically significant spectra of eighteen XRF-based proxies with one another, the researchers identified three distinct groups of elements that behave in concert through the frequency domain, which they termed Redfit Proxy Groups.

The first group, dominated by calcium, strontium, and related elemental ratios, displays persistent cycles of roughly 2,000 and 1,000 years, along with a cluster of multi-centennial periodicities between about 340 and 700 years. These soluble elements are classic indicators of authigenic carbonate precipitation, which in an alkaline lake like Sidi Ali is controlled by the ratio of precipitation to evaporation. Crucially, the same 1,000-year rhythm appears in the oxygen isotope record of ostracod shells from the same core, where elevated values mark episodes of reduced effective moisture. The team links these millennial dry phases, clustered in the Early and Middle Holocene, to the well-known North Atlantic cooling events, suggesting a long-range teleconnection between ocean conditions in the far North Atlantic and winter rainfall over the Western Mediterranean.

Independent proxies reinforce this hydroclimatic interpretation. The abundance of Atlas cedar pollen, a tree highly sensitive to prolonged summer drought, follows the same 1,000-year beat during the Early Holocene, while total organic carbon shows a clear 2,000-year cycle that mirrors the carbonate signal in reverse. Even the potassium-to-titanium ratio, previously shown by provenance studies to trace dust arriving from the Sahara, falls within this group, with a notable peak around 4,200 years ago corresponding to a hyper-arid phase in the central Sahara. Together, these lines of evidence paint the first proxy group as a recorder of the lake’s water balance, paced by North Atlantic and solar forcing.

The second proxy group, comprising iron, titanium, potassium, silicon-to-titanium ratios, and sulfur, behaves entirely differently. Its dominant periodicities are approximately 3,500 and 1,500 years, and these elements are conventionally interpreted as tracers of detrital input from the catchment and of lake productivity. Magnetic susceptibility, an independent erosion proxy measured directly on the core, shows the same spectral signature, as does the coarse-grained sediment fraction delivered from the steep surrounding slopes. Intriguingly, the 1,500-year cycle only emerges clearly after the Middle Holocene, hinting that the sensitivity of catchment processes to external forcing shifted through time, possibly as vegetation cover and human land use began to modify the landscape’s response to rainfall.

A third, smaller group consisting of manganese and the iron-to-manganese ratio records redox-sensitive chemistry at the lake floor, governed by stratification and mixing of the water column. Its mixed periodicity pattern, borrowing elements from both of the other groups, suggests it integrates hydrological and detrital influences rather than reflecting a single forcing mechanism. The researchers are careful to note that spectral similarity alone does not prove identical environmental causes; the process attribution rests on the convergence of the frequency-based groupings with independent geological and biological evidence from the same archive.

When the Sidi Ali cycles are compared with other regional archives, a coherent picture emerges. A Scottish peat record of North Atlantic storminess shows both the 2,000-year Hallstatt-type cycle and a 1,000-year cycle, while speleothems from Middle Atlas caves display comparable millennial and multi-centennial periodicities in their rainfall proxies. The multi-centennial modes at Sidi Ali, clustered around 700 and between 600 and 340 years, match solar cycles identified in cosmogenic isotope records and periodicities in a reconstructed North Atlantic Oscillation index, strengthening the case that westerly wind dynamics delivered these rhythms to Moroccan winter rain. A tentative 170-year cycle in redox-sensitive ratios may even echo the famous de Vries solar cycle, though the authors caution that chronological uncertainty makes such short periodicities statistically fragile.

The overarching conclusion is that Holocene environmental change at Lake Sidi Ali was structured by two partly decoupled regimes: a hydroclimatic regime of roughly 2,000- and 1,000-year cycles tied to precipitation and evaporation, and a catchment-related regime of roughly 3,500- and 1,500-year cycles tied to erosion, productivity, and terrestrial sediment supply. For a region projected to face severe drying in the coming decades, the message is sobering and useful in equal measure. The lake sediments demonstrate that rainfall at the North African desert margin has never varied randomly; it has oscillated to identifiable beats driven by ocean, atmosphere, and sun. Understanding those natural rhythms provides an essential baseline against which the magnitude and pace of future, human-driven hydroclimatic change in the Western Mediterranean can be judged.

Subject of Research: Holocene hydroclimatic variability and periodicity analysis of lake sediments at Lake Sidi Ali, Morocco

Article Title: Holocene hydroclimatic variability and multi-frequency analyses at Lake Sidi Ali (Morocco)

Article References: Schmidt, J., Reichert, M., Kertscher, C., Schneider, B., Dietze, E., Bergmann, L., Benkaddour, A., Mikdad, A., Pichat, S., Fletcher, W., Mischke, S., & Zielhofer, C. (2026). Holocene hydroclimatic variability and multi-frequency analyses at Lake Sidi Ali (Morocco). Climate of the Past, 22(8), 1537-1557. https://doi.org/10.5194/cp-22-1537-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1537-2026

Keywords: Lake Sidi Ali, Morocco, Holocene, paleoclimate, hydroclimate, XRF core scanning, wavelet analysis, Redfit spectral analysis, North Atlantic forcing, solar cycles, Middle Atlas, lake sediments

The study, led by Dina Hanifah of MARUM, the Centre for Marine Environmental Sciences at the University of Bremen, together with Steffen Kutterolf of GEOMAR and Katrin Huhn of MARUM, focused on the eastern offshore sector of Montserrat, directly downslope of the Tar River Valley. This valley served as the main conduit for pyroclastic density currents during the Soufrière Hills eruption of 1995 to 2010, a prolonged crisis of andesitic lava-dome growth and collapse that delivered more than 90 percent of the erupted material into the sea. Because the onshore chronology of that eruption is exceptionally well documented by the Montserrat Volcano Observatory, the surrounding seabed offers a rare opportunity to test how faithfully marine sediments record known eruptive events.

Two gravity cores, each roughly 88 centimetres long, were recovered in 2019 during RV METEOR cruise M154-2 from water depths of around 850 to 950 metres. Their locations tell much of the story. Core GeoB23710-1 was taken from a topographically confined depression near the margin of a large mass-transport deposit, in the direct submarine extension of the dome talus fan. Core GeoB23711-1, recovered only a short distance away but roughly 90 metres higher on the adjacent slope, sits outside the principal transport pathway. Despite their proximity, the two cores could hardly be more different. GeoB23710-1 contains about 30 centimetres of volcaniclastic layers underlain by a muddy-sandy debrite, whereas GeoB23711-1 preserves just a one to two centimetre ash horizon near the top of an otherwise homogeneous mud sequence.

To interrogate these contrasting successions, the team applied a multi-proxy toolkit that goes far beyond visual description. Multi-sensor core logging provided continuous profiles of gamma-ray density, magnetic susceptibility and P-wave velocity at one-centimetre resolution. X-ray fluorescence scanning at ten-millimetre resolution yielded elemental ratios such as Zr/Sr, which distinguishes volcaniclastic input from background hemipelagic sediment, and Zr/Rb, which tracks grain-size changes. Most strikingly, the archive half of GeoB23710-1 was scanned with a medical-grade computed tomography scanner at 0.3-millimetre voxel resolution, allowing the researchers to reconstruct internal structures invisible to the naked eye: erosional basal contacts, imbricated clasts, cross-lamination, loaded beds and subtle density laminae.

The CT data resolved the volcaniclastic succession in GeoB23710-1 into three internally distinct packages, labelled V1, V2 and V3. V1, about five centimetres thick, begins with a sharp erosional base and coarsens dramatically, with bulk densities reaching 1.9 grams per cubic centimetre and CT intensities exceeding 1450 Hounsfield Units in its clast-rich base. V2, the thickest interval at roughly 33 centimetres, displays alternating high- and low-density laminae, normal grading, small-scale loading and cross-lamination, all signatures of repeated fluctuations between tractional transport and suspension fallout. V3, the uppermost eleven centimetres, fines upward into increasingly structureless sediment. Radiocarbon dating of planktonic foraminifera from hemipelagic layers bracketing V1 returned calibrated ages of AD 181 plus or minus 57 years above the unit and around 2131 calibrated BC below it, confirming that V1 predates the modern eruption and is chronologically compatible with a regional andesitic turbidite deposited roughly 1.5 to 2 thousand years ago.

V2 and V3, which overlie the younger dated horizon, are interpreted as the most likely offshore expression of the 1995 to 2010 eruptive phase. Yet the researchers are careful not to over-interpret. The two intervals are separated only by a thin carbonate-rich layer, V2b, which likely records a brief lull in volcaniclastic supply, possibly during the relative quiescence around 2000 to early 2001, though the chronological resolution cannot confirm this. Critically, the composite architecture of V2 and V3, with its erosional contacts, renewed coarse basal influxes and tractional reworking, demonstrates that bed number cannot be equated with event number. A single dome collapse entering the sea can partition into multiple submarine flows, each depositing sediment at different times and places, while successive pulses amalgamate into what appears to be a single bed.

Microscopic point counts of smear slides added a compositional dimension to this story. V1 contains a comparatively high proportion of juvenile pyroclasts, glassy and vesicular particles freshly fragmented by explosive eruptions, alongside volcanic lithics, dome fragments, dense minerals and bioclasts. In contrast, V2 and V3 are dominated by dense minerals, at 35 to 55 percent, and lava-dome fragments, with juvenile pyroclasts generally falling below 15 percent. This shift suggests that the younger package incorporated substantial reworked volcaniclastic detritus and dome-derived material during repeated submarine transport, rather than receiving pristine eruptive fallout. The bioclasts and sediment clasts scattered through the volcaniclastic layers provide direct evidence that the currents eroded and entrained the seafloor as they travelled.

The comparison between the two cores carries the study’s most consequential message. Under a simple model, the off-axis site should preserve a thinned but ordered equivalent of the axis-proximal succession. Instead, GeoB23711-1 holds only a thin ash veneer that cannot be confidently correlated with any specific interval in GeoB23710-1. The two sites evidently retained different fractions of the passing currents: the confined depression accumulated coarse, concentrated flow components repeatedly, while the elevated slope received only dilute, ash-bearing portions, or was bypassed entirely. Analogous relief-controlled routing has been documented in the Valparaíso Basin off Chile, the Shikoku Basin and the Izu-Bonin region, but the Montserrat pair provides an unusually crisp core-scale demonstration that depositional position, not eruption magnitude alone, determines what the archive retains.

The implications extend beyond stratigraphy into hazard assessment. The sharp density and fabric contrasts between coarse volcaniclastic beds and fine-grained interlayers define a heterogeneous shallow mechanical stratigraphy that, in principle, could host weak layers prone to failure during future loading or earthquakes. The authors are explicit, however, that their data do not establish mechanical weakness or tsunami-generating landslide potential; CT intensity and P-wave velocity are not measurements of shear strength, and direct geotechnical testing would be required. What the study does establish is a practical framework: repeat bathymetric surveys to map confined pathways and depocentres, dated core transects spanning axes, margins and elevated slopes, and integrated multi-proxy analysis to distinguish fresh eruptive input from reworked material. For volcano monitoring and seabed infrastructure planning around dome-dominated island volcanoes, the lesson is clear: a thin or absent deposit at one site does not mean the currents were weak or absent, and a thick succession does not simply count eruptions. The seafloor archive off Montserrat is a dynamically filtered record, and reading it demands that the filter itself be understood.

Subject of Research: Marine volcaniclastic stratigraphy and preservation of Soufrière Hills eruption records offshore Montserrat

Article Title: Volcanic stratigraphy offshore East Montserrat: Selective preservation and subaqueous remobilisation in marine records

Article References: Hanifah, D., Kutterolf, S., & Huhn, K. (2026). Volcanic stratigraphy offshore East Montserrat: Selective preservation and subaqueous remobilisation in marine records. Environmental Earth Sciences, 85(16), Article 414. https://doi.org/10.1007/s12665-026-13151-5

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13151-5

Keywords: Montserrat, Soufrière Hills Volcano, volcaniclastic sediments, marine stratigraphy, pyroclastic density currents, turbidity currents, computed tomography, XRF core scanning, mass-transport deposits, submarine slope stability, Lesser Antilles arc, radiocarbon dating