Volcanic impact on terrestrial and aquatic ecosystems in the Eastern Mediterranean

Volcanic eruptions are significant natural disturbances capable of causing abrupt climate variability and profoundly affecting terrestrial and aquatic ecosystems. The impacts are complex, with direct effects stemming from the eruption itself (explosions, lava flows, pyroclastic flows, tephra) leading to vegetation destruction through mechanical, chemical, and physical damage. Tephra, particularly fine ash, modifies soil chemistry, fertility, and nutrient cycles, potentially causing soil acidification and plant mortality due to toxic elements. The deposition of ash on lake surfaces similarly alters aquatic systems, impacting light penetration, turbidity, and the biological feedback within algal communities. Indirect effects are also crucial, with volcanic emissions (sulfuric aerosols, CO2) modifying weather patterns and influencing Earth's radiative balance, leading to regional and global cooling, unseasonable weather, and increased lightning strikes. While modern studies of eruptions like Mount St. Helens (1980) and Ksudach (1907) have provided insights into vegetation recovery times (20+ years and 90 years respectively), paleoecological studies offer a longer-term perspective to understand the full trajectory of ecosystem response and recovery. However, distinguishing between volcanic and climatic influences in paleoecological datasets presents a significant challenge. Previous studies analyzing the impacts of tephra falls, using biotic and abiotic proxy records in sediments, have shown varying effects depending on factors like tephra thickness, distance from the source, environmental sensitivity, and pre-existing vegetation. This research aims to address these complexities by investigating the effects of five volcanic tephra layers in the Lake Van sediments, spanning different glacial-interglacial cycles, using high-resolution biotic proxies (pollen, non-pollen palynomorphs, charcoal) and existing abiotic data (TOC, XRF elements). Lake Van, with its well-constrained age model and extensive tephra record, provides an ideal setting for this investigation.

Existing literature highlights the multifaceted impact of volcanic eruptions on ecosystems. Studies have demonstrated both direct effects, such as the destruction of vegetation and alteration of soil chemistry, and indirect effects, such as climate change and altered weather patterns. Modern studies, such as those following the 1980 Mount St. Helens eruption, have shown substantial initial impacts on vegetation, with recovery taking decades. Paleoecological studies utilizing tephra layers as stratigraphic markers have also contributed to our understanding of long-term effects. For instance, the analysis of late-glacial lake sediments containing the Laacher See tephra (LST) revealed impacts on vegetation, with a shift towards grasses, and increased diatom abundances due to nutrient input. Similarly, studies of northern German lakes after the Saksunarvatn ash showed short-term ecological changes lasting for several years. However, the literature also reveals variability in the response depending on factors like tephra thickness, distance from the source, and pre-existing vegetation. Some studies have shown significant effects from thick ash layers (>2.5 cm), while others have found minimal impact from thinner layers, indicating the sensitivity of the receiving environment is crucial. Distinguishing the impacts of tephra from climate change is a constant challenge in paleoecological data analysis. This study builds upon previous research by focusing on a location with a high frequency of eruptions and high-resolution data over extended timescales, allowing for a more nuanced understanding of the dynamic relationship between volcanism, climate, and ecosystem recovery.

This research utilizes a multi-proxy approach focusing on five selected volcanic tephra layers from the Ahlat Ridge composite record of Lake Van sediments, spanning Marine Isotope Stages (MIS) 3 to 9e (32-340 ka BP). The selection criteria for the tephra layers included a minimum thickness of 4 cm, reliable age control within laminated sediments, and occurrence under contrasting climatic conditions (glacial/interglacial, stadial/interstadial). The 219-meter Ahlat Ridge composite record, part of the ICDP PALEOVAN project, offers a robust chronology established through various dating techniques (varve-counting, 40Ar/39Ar dating, geomagnetic tie points, radiocarbon dating, cosmogenic nuclides) and climate-sensitive proxies (TOC, sediment color, Ca/K ratio, pollen data). High-resolution biotic proxies were analyzed from 172 samples: pollen, non-pollen palynomorphs (including aquatic indicators like *Pediastrum*, *Botryococcus*, dinoflagellates), and microscopic charcoal particles (>20 µm). Standard chemical preparation methods were used, with Lycopodium clavatum spores added for pollen accumulation rate (PAR) calculations. At least 500 terrestrial pollen grains were counted per sample. Geochemical data (XRF elemental profiling, TOC) from Kwiecien et al. (2014) and Stockhecke et al. (2014) provided complementary information on sediment composition, nutrient input (Si), and redox conditions (Mn/Fe). The Ca/K ratio served as an indicator of detrital input versus carbonate precipitation. The data were analyzed to quantify the extent and duration of volcanic disturbances, assess the response of both terrestrial and aquatic ecosystems, and investigate the interplay between vegetation changes, volcanic deposition, climate, and fire activity. The software TILIA (version 1.7.16) was used for pollen diagram construction.

The study revealed distinct responses of terrestrial and aquatic ecosystems to volcanic ash deposition, significantly influenced by the thickness of the tephra layer and prevailing climatic conditions. **Glacial vs. Interglacial Periods:** The thickest tephra layer (V-18a, 275 cm, MIS 3) deposited during a glacial period showed only short-lived effects on vegetation due to pre-existing disturbance. A thinner layer (V-18b) caused a short-term decline in herb and charcoal accumulation rates, but the vegetation remained dominated by *Chenopodiaceae* and *Artemisia*. Intensified algae blooms were observed due to increased *Pediastrum*. A high Mn/Fe ratio indicated rapid sealing of the sediment-water interface and potentially higher wind intensities. **Interglacial Period:** The V-233a tephra (4.2 cm, MIS 9d) deposited at the transition from an interglacial to a stadial period resulted in a sudden peak in accumulation rates, a large-scale fire event, and a shift in vegetation from an open oak steppe-forest to a herbaceous-dominated steppe. The aquatic system showed increased *Pediastrum* and dinoflagellates. Recovery was slow due to unfavorable climate conditions. **Transition Glacial to Interglacial:** The V-237 tephra (18 cm, MIS 9e) deposited during the transition from a glacial to an interglacial period triggered a drop in *Chenopodiaceae*, a short-term reduction in *Artemisia* and *Poaceae*, and an increase in oak values and charcoal fragments. Aquatic plants and microfossils showed increased values, indicating enhanced lake productivity. **Interstadial Period:** The V-60 tephra (203 cm, MIS 5a) deposited during an interstadial period caused a decrease in *Chenopodiaceae* and *Poaceae*, but the ecosystem recovered after 35 years. Increased charcoal values indicated frequent steppe-forest fires. **Stadial Period:** The V-176 tephra (35 cm, MIS 7d) deposited during a stadial period caused increased accumulation rates of both terrestrial and aquatic materials, reflecting remobilization of allochthonous material. The high charcoal suggests a large-scale tephra-induced steppe fire. The ecosystem recovered after 30 years. Across all periods, the most common response to ash deposition was a shift towards herbaceous taxa, increased fire activity, and recovery of herbaceous vegetation within 20-40 years. Lake productivity increased due to nutrient input.

The findings demonstrate the significant impact of volcanic eruptions on both terrestrial and aquatic ecosystems in the Eastern Mediterranean, but also highlight the crucial role of pre-existing environmental conditions, particularly climate. The resilience of herbaceous vegetation, particularly grasses (*Poaceae*), reflects their adaptation to frequent disturbances. The rapid response of aquatic organisms, with short-term algae blooms, underscores the sensitivity of these systems to nutrient input from tephra. The study indicates that the recovery time of terrestrial vegetation is relatively short (20-40 years), consistent with findings from other volcanic eruption studies. However, climate transitions, particularly during glacial-interglacial shifts, significantly influence recovery, sometimes preventing a return to the pre-eruption state. The correlation between volcanic ash deposition and increased fire frequency highlights the potential for synergistic effects. The varying responses observed across different climatic phases emphasize the importance of considering regional climatic context when assessing the impact of volcanic eruptions on ecosystem dynamics.

This study provides valuable insights into the complex interplay between volcanic eruptions, climate, and ecosystem responses in the Eastern Mediterranean. The findings emphasize the importance of considering both direct and indirect effects of tephra deposition, along with pre-existing environmental conditions, to fully understand the impact and recovery of affected ecosystems. Future research could focus on refining the understanding of specific plant responses to tephra composition, investigating the long-term effects of volcanic aerosols on climate and vegetation beyond the 40-year recovery period observed here, and expanding the analysis to other volcanic regions to assess the generality of the observed patterns.

The study's reliance on paleoecological proxies has inherent limitations. Distinguishing between the effects of volcanic eruptions and other environmental factors, particularly climate change, remains challenging. The temporal resolution of some data may not fully capture the nuances of short-term ecological responses. Furthermore, the precise timing of eruptions relative to the growing season could influence the observed vegetation responses, although this was not determined in this study. While the study covers several climate periods, the relatively limited number of selected tephra layers could not provide comprehensive coverage for all past volcanic eruptions, potentially missing important nuance in the broader picture of volcanic impact. Finally, the study is limited to Lake Van, and the extent to which these findings are generalizable to other regions requires further investigation.