Introduction: The Quaternary period, particularly the Holocene epoch, has witnessed significant climatic oscillations that have profoundly influenced environmental conditions, geomorphic processes, and human civilizations across the globe. The concept of climate, defined as the long-term average of temperature and precipitation, has been continuously fluctuating throughout Earth's history, with inseparable connections to biological and cultural responses. The rise and fall of ancient civilizations have been directly linked to climatic transformations, as changing environmental conditions shaped settlement patterns, agricultural practices, and migration routes of early human societies. Closed basin systems, such as playas and seasonal lakes, serve as exceptional natural archives for reconstructing paleoenvironmental and paleoclimatic conditions due to their high sensitivity to the balance between evaporation and precipitation. These sedimentary environments record physical, chemical, and biological signatures that reflect past climatic conditions, making them invaluable for understanding long-term environmental changes. The southeastern region of Iran, influenced by both the Indian Ocean summer monsoon and mid-latitude westerly winds, has experienced a complex history of climatic and hydrological variations during the late Quaternary. Among these critical sites, the Kaji Namakzar playa, located in the northern part of Nehbandan in South Khorasan Province (eastern Iran), represents a significant yet understudied sedimentary archive. This playa, situated in an arid to semi-arid region with mean annual precipitation below 150 mm and characterized by high evaporation rates exceeding 3000 mm per year, provides an ideal setting for investigating Holocene climatic variability. Despite the importance of understanding past climate dynamics in this region, comprehensive geochemical studies on the sedimentary sequences of the Kaji Namakzar playa have been notably absent. This research aims to address this knowledge gap by conducting detailed geochemical analysis of sediment cores from the Kaji Namakzar playa to reconstruct Holocene climatic and environmental conditions, identify periods of aridity and humidity, determine sedimentary facies and depositional environments, and establish the chronostratigraphic framework of the basin.
Methodology: The study area, Kaji Namakzar playa, is located in South Khorasan Province, approximately 70 km north of Nehbandan and 120 km south of Birjand, between 31°45'34" to 32°29'56" North latitude and 59°32'37" to 60°12'52" East longitude. The playa exhibits typical geomorphic zonation from the margin to the center, including alluvial fans, clay flats, wet zones, salt crusts, and seasonal lakes. The geological setting comprises Flysch deposits (extensive in the northern region of the Sahlabad plain), an ophiolitic mélange complex including peridotite, gabbro, diabase dikes, pillow basalts, and sedimentary rocks such as pelagic limestone, radiolarite, and shale from the Late Cretaceous. The area is characterized by faulted, crushed, and folded structures dominated by thrust and strike-slip faults with a northwest-southeast trend, creating a pull-apart basin. Three undisturbed sediment cores (Kj-1, Kj-2, and Kj-3) with depths ranging from 691 to 880 cm were extracted from the playa bed using a manual hand auger, with precise locations recorded by GPS. The cores were described in detail based on sedimentary structures, bedding characteristics, color, texture, evaporite crystal types, relative hardness, and presence of plant and animal remains. A total of 168 sediment samples were collected from the three cores for geochemical analysis at the Geological Survey and Mineral Explorations of Iran laboratory. Elemental analysis was performed using ICP-OES (Varian 735-ES) for trace elements and XRF (MAGIC-PRO) for major elements expressed as oxides (wt%) and trace elements (ppm). Approximately 10% of samples were analyzed as duplicates for quality control. Geochemical data processing included the calculation of elemental ratios (Mn/Al, K/Al, V/Cr, Ti/K, Fe/Ca, Fe/Al, Ti/Al, Mn/Ca, Ca/Sr) that serve as paleoenvironmental proxies based on established relationships between element behavior and climatic conditions. Additionally, sedimentological analysis identified seven primary sedimentary facies through detailed core description.
Results: The sediment cores revealed three distinct lithostratigraphic sequences reflecting the basin's depositional history. Core Kj-1 (860 cm) exhibited a complex stratigraphy with the upper section (0-200 cm) dominated by clay and silty mud with alternating sandy mud and sand layers, containing plant remains and organic matter indicating low-energy conditions with high biological productivity. The middle section (200-600 cm) comprised mixtures of sand, silty clay, and clayey sand with halite crystals indicating arid and saline conditions. The lower section (600-850 cm) consisted of thick evaporite layers and mud suggesting significant arid episodes. Core Kj-2 (880 cm) showed more uniform sequences dominated by mud and clayey sand with brown to gray alternations, suggesting a more stable depositional environment with less variation in energy conditions. Core Kj-3 (691 cm) displayed similar patterns to Kj-2 but with greater diversity in the upper 150 cm including sand, mud, clay, and clay nodules. Seven sedimentary facies were identified: Clay (Cl), Silty Clay (ClS), Mud (MU), Silty Clay (SiC), Sand (Sa), Muddy Sand (MS), and Evaporite facies. These facies indicate deposition under fluvial-lacustrine, playa, and alluvial fan environments. Geochemical analysis revealed significant elemental variations. In Core Kj-1, maximum concentrations were dominated by Ti (17 samples) and Sr (4 samples), while K consistently showed minimum values. Al concentrations ranged from 3.19% to 6.66%, Ca from 3.29% to 9.34%, and Sr exhibited dramatic spikes at 220-229 cm (6878 ppm) and 270-275 cm (3189 ppm) indicating intense evaporative events. Cr reached 388 ppm at 625-634 cm, while Ti peaked at 3042 ppm at 840-850 cm. In Core Kj-2, Al ranged from 4.84% to 8.05%, Ca from 2.80% to 9.51%, and an extreme anomaly at 750-760 cm showed Ca at 9.51% and Sr at 5227 ppm with simultaneous minimal values of other major elements. Core Kj-3 exhibited Al values between 4.99% and 8.13%, and Fe ranging from 3.11% to 5.99% with maximum at 240-250 cm. Elemental ratios showed significant fluctuations: Mn/Al ranged 73.43-236.89, V/Cr 0.18-1.07, Ti/K 1265-2458, and Ti/Al 417-592 across all cores. The Mn/Ca ratio varied from 26.67 to 212.44, clearly distinguishing wet and dry periods. The K/Al ratio remained relatively constant (0.22-0.34), suggesting mineralogically consistent clay composition throughout the depositional history. The V/Cr ratio indicated alternating oxic and anoxic conditions, with higher values suggesting increased water depth and enhanced inputs from the catchment. Sedimentation rates derived from stratigraphic correlations indicate higher rates during the early Holocene (up to 5900 years BP) with progressive decrease over time, closely linked to facies variations.
Conclusion: The integrated geochemical and sedimentological investigation of the Kaji Namakzar playa sediment cores reveals a complex history of climatic and environmental fluctuations during the Holocene in eastern Iran. The three sediment cores collectively demonstrate distinct climatic phases alternating between humid and arid conditions. Wet periods, characterized by elevated concentrations of Al, Fe, V, Ti, K, and Cr, correspond to enhanced chemical weathering, increased fluvial input, and higher erosion rates within the catchment basin due to intensified precipitation. These humid phases are clearly indicated by increased elemental ratios including Ti/K, Ti/Al, and K/Al, reflecting enhanced detrital input. Conversely, arid intervals are characterized by reduced terrigenous sediment influx, dominance of evaporitic processes, and formation of carbonate and evaporite minerals, as evidenced by elevated Ca and Sr concentrations. The dramatic Sr spikes at 220-229 cm (6878 ppm) and 750-760 cm (5227 ppm) in different cores represent two distinct and intense evaporative events, possibly indicating periods of extreme aridity when the lake completely desiccated. The relatively constant Ti/Al ratios suggest stable provenance of terrigenous materials throughout much of the depositional history, while variations in Fe/Ca and Ca/Sr ratios reflect diagenetic processes and changes in the chemical composition of the basin. The Mn/Al and Mn/Ca ratios effectively distinguish oxic versus anoxic conditions, with higher values indicating oxidizing environments suitable for manganese precipitation. The V/Cr ratio demonstrates the transition between seasonal (oxic) and permanent (anoxic) lake conditions. The sediment cores show that the depositional environment evolved from a relatively deep, stable lake (indicated by green and dark gray clay-rich layers in the lower sections) to a seasonal playa with high salinity (evidenced by halite crystals and evaporite layers in the upper sections). Core Kj-2 and Kj-3, located in deeper and more stable parts of the basin, show greater lithological uniformity with limited organic matter and the absence of halite crystals, suggesting they were less influenced by water level fluctuations and evaporative conditions. The identified sedimentary facies (clay, silty clay, mud, sand, muddy sand, and evaporite facies) confirm the transition from alluvial fan and fluvial environments at depth to playa and lacustrine conditions upward in the sequence. The presence of mud cracks, carbonate nodules, halite crystals, plant remains, and diverse color variations (black, gray, brown, red, green, olive) within the stratigraphic columns provides additional evidence for alternating wet and dry conditions, oxidation-reduction changes, and periodic exposure of sediments to atmospheric conditions. This research significantly enhances our understanding of how sensitive arid and semi-arid environments in eastern Iran have responded to past climatic changes, providing a high-resolution sediment archive that can serve as an analog for predicting future ecosystem responses to ongoing climate change. The findings underscore the vulnerability of playa systems to climatic perturbations and highlight the importance of paleoenvironmental studies for sustainable environmental management in water-scarce regions.