Volcanic impact on terrestrial and aquatic ecosystems in the Eastern Mediterranean

The study investigates how terrestrial vegetation and aquatic ecosystems in the Eastern Mediterranean (Lake Van, Turkey) respond to tephra deposition under differing climate states (glacial, interglacial, stadial, interstadial). The objectives are to (1) quantify the extent of volcanic disturbance, its effects, and the recovery of vegetation relative to climatic conditions, (2) evaluate the response of aquatic ecosystems following volcanic impact, and (3) investigate interactions among vegetation change, volcanic deposition, climate, and fire activity. Contextually, volcanic eruptions can directly damage vegetation via ash, gases, and pyroclastic flows, and indirectly alter soils, nutrients, and lacustrine chemistry and light regimes. Distal climatic effects include aerosol-driven cooling and altered weather. The Lake Van record, with numerous tephra over 600 ka, provides a long, varve-constrained archive to disentangle volcanic versus climatic drivers of ecosystem disturbance and recovery.

Prior work shows eruptions can cause abrupt, short-lived ecological shifts. Distal aerosol forcing yields regional-to-global cooling (e.g., −0.42 °C summer cooling within five years in Europe after large eruptions). Modern analogs (e.g., 1980 Mount St. Helens; 1907 Ksudach) document substantial initial impacts on herb/shrub communities and decadal to multi-decadal recovery, with ash thickness modulating survival. Tephropaleoecological studies in Europe show: after Laacher See tephra (LST, ~13 ka), birch–pine forests declined and grasses increased with recovery over ~10–20 varve-years; diatom populations increased due to nutrient/silica input. Following the Early Holocene Saksunarvatn Ash, pollen and geochemical tracers recorded short-term cooling and ecosystem responses persisting ~15–17 varve-years, with some pre-tephra disturbances. In Italy (Lago Grande di Monticchio), thick ash layers (>2.5 cm) reduced pollen accumulation and productivity, with 25–35 year regeneration. In SW Turkey (Lake Gölhisar), the ~4 cm Santorini (Minoan) tephra did not alter terrestrial vegetation composition but increased aquatic microfossils, implying fertilization and eutrophication. These findings suggest that the magnitude and nature of ecological responses depend on tephra thickness, distance from source, environmental sensitivity, and prevailing vegetation and climate.

Study site and stratigraphy: The Ahlat Ridge composite record (Lake Van, Turkey) spans ~600 ka (ICDP PALEOVAN). Chronology integrates Holocene varve counts, single-crystal 40Ar/39Ar-dated tephra, geomagnetic tie points, radiocarbon, cosmogenic nuclides, and synchronization of climate-sensitive proxies (TOC, reflectance b*, Ca/K, pollen) to NGRIP GICC05 and GL/T-syn. Selected events: Five volcaniclastic deposits embedded in laminated sediments and contrasting climate backgrounds were chosen: V-18a (~32.7 ka BP; thickness 275 cm; MIS 3, Nemrut Formation), V-60 (~81.5 ka BP; 203 cm; MIS 5a; İncekaya-Dibekli hyaloclastite cone), V-176 (220.4 ± 6.9 ka BP; 35 cm; MIS 7d; Nemrut?), V-233a (~321.1 ka BP; 4.2 cm; MIS 9d; Nemrut?), V-237 (~334.5 ka BP; 18 cm; MIS 9e; Nemrut?). Selection criteria: ash thickness >4 cm; reliable age control within laminated intervals; contrasting climate states; exclusion of Holocene layers to avoid anthropogenic confounding and exclusion of non-laminated/disturbed sections. Varve chronology: Annual-scale interpretations were derived from fine laminations (assumed varves beyond the Holocene) to construct floating varve-counted chronologies around each event, enabling recovery-time estimates. Sampling: Continuous sampling across pre- and post-tephra intervals focused on laminated sediments; 172 samples total, with an average temporal resolution of ~4.7 varve-years per sample. For each sample, thickness, number of varve-years, and volume were documented. Pollen and charcoal: Standard palynological preparation (HCl, KOH, HF, acetic acid, acetolysis; ultrasonic sieving 10 µm); Lycopodium tablets (batch 3862) for influx calculations. At least 500 terrestrial grains counted per sample. Aquatic NPP counted (e.g., Pseudopediastrum, Pediastrum, Monactinus, Botryococcus; dinoflagellates). Accumulation rates (grains/particles cm−2 yr−1) computed from concentrations and sedimentation rates. Microscopic charcoal (>20 µm) counted from the same slides as a wildfire proxy. Diagrams prepared with TILIA v1.7.16. Geochemistry: XRF core scanning (AVAATECH III, MARUM) every 2 cm over 1 cm2 with 12 mm slit, 10 s count time; Ultralen foil cover. Key ratios: Ca/K (carbonate precipitation vs detrital input), Mn/Fe (redox/sealing of sediment–water interface), and Si counts (nutrient input). TOC measured every 2.5 cm (elemental analyzer for TC; coulometric TIC; TOC = TC – TIC). Interpretation framework: Volcaniclastic deposits considered as mixed primary/reworked material per Sohn & Sohn; depositional processes interpreted without strict primary/secondary separation. Recovery assessed as return to pre-eruption levels in taxa composition, PAR, NPP, and charcoal at annual (varve) resolution.

General patterns: Impact magnitude depends on tephra thickness and prevailing climate. The most common terrestrial response is a shift toward herbaceous taxa and abrupt fire activity; aquatic systems show rapid productivity increases driven by nutrient (notably silica) inputs. Herbaceous vegetation typically recovered to pre-eruption levels in ~20–40 varve-years; aquatic blooms were short-lived (often within 1–8 varve-years post-deposition). Fire intensity peaks coincide with ash deposition, scaling with available biomass (higher in interglacial/interstadial periods). Event-specific highlights: • V-18a (Nemrut, 275 cm; MIS 3 glacial): Pre-eruption desert-steppe with high Chenopodiaceae (mean 38.5%, 280 PAR) and Artemisia (24.5%, 177 PAR), low AP (mean 4.5%, 34 PAR). After thin V-18b, short declines in herb PAR and charcoal; between V-18b and V-18a, intensified algal bloom (Pediastrum mean 23 coenobia cm−2 yr−1). The thick V-18a fallout generated only short-lived terrestrial effects in an already open glacial landscape: NAP increased for ~4 varve-years, then decreased by ~12 varve-years with slight recovery by 16–20 varve-years; aquatic palynomorphs decreased abruptly (Pediastrum mean 6 coenobia cm−2 yr−1). Mean total PAR dropped post-events (mean 429) versus pre-event (mean 723). Low charcoal (mean 46 cm−2 yr−1) reflects low biomass. • V-60 (İncekaya-Dibekli basaltic, 203 cm; MIS 5a interstadial): Pre-eruption herb-dominated steppe-forest (NAP 88.0%, 336 PAR; AP 11.9%, 42 PAR; deciduous Quercus 7.4%, 27 PAR). First 3 varve-years show decreases in Chenopodiaceae and Poaceae (annual deposition 164 PAR) with increased detrital input (low Ca/K) and elevated charcoal (mean 192 cm−2 yr−1). Substantial pre- and post-eruption sedimentological disturbance and remobilization of ash increased allochthonous inputs and accumulation rates. Recovery of terrestrial and aquatic ecosystem to pre-eruption levels after ~35 varve-years. Notably, low-Si İncekaya ash did not trigger Pediastrum blooms. • V-176 (Nemrut?, 35 cm; MIS 7d stadial): Pre-eruption sparse steppe (NAP 96.8%, 1678 PAR; Chenopodiaceae 76.0%, 1301 PAR), low AP (3.3%, 70 PAR), high dinoflagellates (125 cysts cm−2 yr−1) and Pediastrum (91 coenobia cm−2 yr−1), high detrital input (low Ca/K). Post-deposition, abrupt increases in PAR across herb taxa (NAP mean 4440 PAR), aquatic NPP, fungal spores (mean 91 cm−2 yr−1), and charcoal (mean 1124 cm−2 yr−1). Up to 27 varve-years of intercalated ash/event layers. A 16-fold increase in aquatic NPP (e.g., Pediastrum/dinoflagellates/Botryococcus from ~32 to ~508 cm−2 yr−1) occurred immediately in ash-affected sediments. Charcoal PAR rose 16-fold (64 to 1052 cm−2 yr−1), indicating a large tephra-induced steppe fire. Recovery and stabilization after ~30 varve-years. • V-233a (Nemrut?, 4.2 cm; MIS 9d late interglacial to stadial transition): Pre-eruption oak steppe-forest (AP mean 27.9%, 1091 PAR; Quercus 18.9%, 760 PAR; Pinus 4.9%, 143 PAR; Poaceae 28%, 1363 PAR; Chenopodiaceae 16.2%, 773 PAR; Artemisia 9.0%, 708 PAR). Immediate effects include extreme peaks: total PAR 20,375; charcoal 9942 cm−2 yr−1; Botryococcus 331 coenobia cm−2 yr−1; strong detrital influx (low Ca/K). Within one varve-year, near-total vegetation destruction is reflected by low total PAR (mean 852) and a shift from oak steppe-forest to herbaceous steppe dominated by Chenopodiaceae (increase from 16.2% to 34.4%). Aquatic response (Pediastrum and dinoflagellates) elevated for ~6 varve-years. Due to unfavorable stadial transition, the vegetation did not recover to the prior state over decades; no recovery rate estimated. • V-237 (Nemrut?, 18 cm; MIS 9e transition from glacial to early interglacial): Pre-eruption desert-steppe (Chenopodiaceae 40.5%, 1020 PAR; Artemisia 32.1%, 833 PAR; Poaceae 11.3%, 268 PAR; AP 3.6%, 96 PAR), low fire (mean 289 cm−2 yr−1). Post-deposition, Chenopodiaceae dropped sharply and remained low (mean 4.6%, 96 PAR). Artemisia and Poaceae declined for ~24 varve-years, then recovered. Oak (mean 3.8%, 49 PAR), other steppe indicators, and charcoal increased markedly (mean 1060 cm−2 yr−1), consistent with enhanced fires at the onset of the interglacial with more biomass. Aquatic plants and non-siliceous microfossils increased; Pediastrum (mean 90 coenobia cm−2 yr−1) and dinoflagellates (mean 58 cysts cm−2 yr−1) elevated mainly in the first ~6 varve-years. Increased runoff and reduced alkalinity (higher Si) promoted algal blooms. Cross-cutting insights: • Climate-state control: Warm/humid periods (interglacial/interstadial) amplify fire peaks and can accelerate aquatic productivity boosts; cold/dry periods show lower biomass, weaker fire signals, and strong erosion signals (low Ca/K). • Tephra chemistry matters: High-silica Nemrut/Süphan deposits enhance aquatic NPP; low-Si İncekaya basalt did not induce Pediastrum blooms. • Pre-eruption disturbances: Some events show pre-tephra sedimentological changes (~20 varve-years before V-60). • Recovery times: Terrestrial herb/shrub communities typically recover within 20–40 varve-years except across major climate transitions (e.g., MIS 9e–9d), where recovery to prior composition does not occur.

The findings address how volcanic ash deposition perturbs ecosystems and how recovery is modulated by climate state and tephra properties. Thick tephras superimposed on already open glacial/stadial landscapes chiefly reduce pollen accumulation and increase detrital input via erosion; vegetation composition often remains herb-dominated but shows short-lived suppression. In interglacial/interstadial contexts, ash deposition can catalyze marked shifts (e.g., from oak steppe-forest to herbaceous steppe) and strong but brief fire peaks, with recovery contingent on broader climatic trajectories; across climate transitions, prior-state recovery may be precluded for decades. Aquatic systems respond rapidly and sensitively to ash, with short-lag surges in green algae and dinoflagellates driven by nutrient inputs (notably silica), transient pH/alkalinity reductions, and sealing of sediment–water interfaces that alter redox and nutrient recycling. Tephra composition governs fertilization strength: silica-rich Nemrut/Süphan ashes stimulate blooms; Si-depleted İncekaya basalt does not. Fire responses co-occur with ash fall but scale with biomass availability; during cold periods, large charcoal peaks may be diagnostic of tephra-induced fires rather than climate-driven burning. Palynological and XRF records show synchronized steps across ash boundaries, supporting depositional-process coupling (erosion/runoff, allochthonous input, ash remobilization). Plant functional traits typical of Eastern Mediterranean steppe (small leaves, thick cuticles, deep roots) likely confer some resistance to ash and acid stress, helping explain rapid decadal-scale recovery except when overridden by climate regime shifts. Overall, the ecosystem response is a compound product of ash thickness, chemistry, depositional dynamics, catchment sensitivity, and prevailing climate.

A multiproxy, varve-resolved analysis of five substantial tephra deposits in Lake Van demonstrates that volcanic disturbances consistently alter terrestrial and aquatic ecosystems, with impact magnitude controlled by ash thickness and climate state. The most frequent terrestrial signal is a transient suppression of herbaceous taxa and a shift toward resilient herb steppe, accompanied by abrupt, biomass-dependent fire peaks. Aquatic systems display pronounced but short-lived productivity increases due to nutrient (silica) inputs and transient physicochemical changes. Terrestrial herb/shrub communities typically return to pre-eruption levels within ~20–40 varve-years, except across major climate transitions where vegetation composition shifts and does not revert. These results underscore the need to distinguish the effects of tephra deposition, volcanic volatiles, and volcanically induced climate change when tracking short-term ecosystem dynamics over long timescales. Future work should refine separation of these drivers using higher-resolution, multi-proxy approaches across additional sites and events, and further resolve how tephra chemistry and climate state jointly modulate recovery trajectories.

• Primary vs secondary deposits: Volcaniclastic layers contain mixed primary, reprocessed, and reworked particles, making it impossible to strictly classify deposit origin; interpretations focus on depositional processes. • Chronological precision: Beyond the Holocene, varve identification is assumed; some intervals lack well-preserved lamination, preventing precise varve-year estimates. • Proxy resolution: Abiotic datasets (e.g., XRF, TOC) are at lower resolution than biotic proxies, constraining fine-scale interpretations. • Event exclusion: Thin tephras (<4 cm) and non-laminated/disturbed intervals were excluded; Holocene layers omitted to avoid anthropogenic effects, limiting generality. • Seasonality unknown: Eruption season is unknown, which can influence plant impact and recovery. • Site specificity and wind/distance effects: Regional oak stands may be unevenly affected due to prevailing wind directions and distances (>20 km) from vents. • Potential remobilization: Post-eruption erosion can remobilize older pollen/charcoal, inflating accumulation rates and complicating attribution.