Late Holocene droughts and cave ice harvesting by Ancestral Puebloans

Water availability significantly impacted the Ancestral Puebloan culture in the Southwestern United States. The region's archaeological richness contrasts with a scarcity of consistent hydroclimate records, hindering our understanding of how water scarcity affected pre-contact societies. Limited paleoenvironmental data and contradictory regional hydroclimate records make it challenging to understand the impact of prolonged droughts on Native American populations. Archaeological evidence points to periodic shifts in Ancestral Puebloan settlements towards higher altitudes, potentially linked to population growth and climate change. Tree-ring data offers some insights into drought periods and their correlation with Puebloan site occupation, but more detailed information is needed to fully understand the nature and range of environmental variability impacting early Puebloans. This study utilizes well-dated charcoal fragments preserved within an ice core from a lava tube in El Malpais National Monument (ELMA), western New Mexico, to reconstruct past drought events and their influence on Ancestral Puebloan water resource management. This research aims to provide direct evidence of Ancestral Puebloan ice-melting practices for water procurement in the Southwest, adding a critical layer to understanding human adaptation in the face of climate variability. The study's unique approach combines paleoclimatic data with archaeological findings to illuminate human-environment interactions and the implications of climate change on water resources.

Existing research emphasizes the importance of water availability in shaping the cultural mosaic of the pre-contact Southwest. Water resource type, accessibility, and reliability heavily influenced settlement patterns, subsistence strategies, agricultural intensification, demographic trends, and migration. Archaeological evidence suggests periodic altitudinal shifts in settlements, possibly correlating with population growth and climate fluctuations, particularly between the Late Archaic and early Pueblo periods (AD 100–900). While tree-ring data provide valuable insights into drought frequency and severity, and their correlation with Puebloan site occupation, the available records are limited, particularly concerning the detailed nature and timing of shorter-term environmental variations influencing early Puebloan mobility. Previous studies have hinted at the use of fire by pre-contact populations to melt ice in ELMA's lava tubes, but direct and dated evidence remained lacking. This research fills this gap by using radiocarbon dating of charcoal found within an ice core to provide unambiguous evidence of such practices.

The study focuses on Cave 29, a lava tube in ELMA, containing a remnant of a large ice deposit. Of 453 documented lava tubes in ELMA, 94 have or had seasonal/perennial ice, often containing archaeological deposits. Cave 29 has a surveyed length of 171 m and a depth of ~14 m. The cave's unique ventilation pattern, with denser cold winter air sinking to the floor and warmer summer air restricted to the entrance, creates conditions favorable for perennial ice accumulation. The cave contains abundant charcoal and burnt wood deposits, particularly near the ice block. A 59-cm long core was extracted from the ice deposit, revealing several charcoal-rich layers. Radiocarbon dating of these charcoal fragments, along with a sample from within a Cibola Gray Ware sherd found on the ice, and additional samples from the cave floor, provided chronological information on the ice accumulation and human activity within the cave. These dates were calibrated using BetaCal3.21 software and the IntCal13 curve. The results were cross-checked between two AMS facilities (ROAMS and Beta Analytic) to ensure accuracy and reliability. Additional data sources such as July precipitation reconstructions using ELMA latewood width chronology, the Palmer Drought Severity Index (PDSI), and other paleoclimatic records from the Southwest, were used for contextualization and comparison. This allowed for correlation between charcoal horizons and historical drought events.

Radiocarbon dating of charcoal within the ice core from Cave 29 revealed five distinct drought events between approximately 167 AD and 933 AD. The charcoal's presence, along with abundant ash and soot, strongly suggests that Ancestral Puebloans used fire to melt cave ice for water. The presence of a Cibola Gray Ware sherd, dating to the 900s AD, on the ice block, along with charcoal residue inside the sherd, further supports the interpretation of ice harvesting activities. The timing of the charcoal horizons strongly correlates with periods of below-average summer rainfall indicated by independent paleoclimatic data, including ELMA latewood width chronology, PDSI, and other regional and continental-wide records. The study reveals a pattern of increased ice-melting activity during severe droughts, particularly during the Medieval Warm Period (approximately 800-1300 AD). The most intense ice-melting activity, evidenced by the largest charcoal deposits, aligns with the peak of the Medieval megadrought. While the lower charcoal levels (AD 167 and AD 368) show smaller-scale activities potentially related to smaller, temporary communities in the area, the later, larger charcoal deposits (AD 747, AD 829 and AD 933) indicate more extensive and sustained ice harvesting during the major drought periods. A final charcoal deposit around AD 1424 indicates a subsequent event during another period of drought. The presence of charcoal deposits and their radiocarbon ages in the Cave 29 ice core establish a clear link between periods of drought and human response, specifically the practice of obtaining water by melting ice in lava tube caves. The findings also imply an understanding by the Ancestral Puebloans of the water resources present in these caves, and their use as a reliable source of water during periods of severe drought. This understanding is supported by the discovery of modified cave floor features, suggesting water collection and handling strategies were developed and employed over time.

The findings address the research question by providing direct, dated evidence for Ancestral Puebloan adaptation to drought conditions. The correlation between charcoal horizons and independent paleoclimatic records substantiates the interpretation of fire-aided ice melting as a response to water scarcity. The study demonstrates the significance of utilizing diverse data sources (archaeological and paleoclimatological) to reconstruct past human-environment interactions. This research adds to our understanding of the resilience and adaptability of Ancestral Puebloans, highlighting their ingenuity in exploiting seemingly marginal water resources. The discovery of the Cibola Gray Ware sherd on the ice further suggests that the practice continued well into the Pueblo II period. The findings have broader relevance to studies focusing on human responses to climate change and the importance of considering various data sources in such analyses. This research shows how communities adapt to changing environments, using available resources and technologies, even if they are not immediately apparent.

This study provides compelling evidence of Ancestral Puebloan use of fire to melt cave ice as a crucial water procurement strategy during periods of prolonged drought, notably during the Medieval Warm Period. The well-dated charcoal layers within the ice core, combined with archaeological findings, offer a unique glimpse into human adaptation to climate variability. The results highlight the importance of integrating archaeological and paleoclimatic data to better understand past human-environment interactions. Future research should focus on further investigating other ice caves in ELMA to assess the regional scale of this practice and explore alternative hypotheses regarding the origin of the charcoal fragments. The study also emphasizes the need for urgent preservation efforts of these ice caves, as current climate change is accelerating ice melt, threatening the preservation of valuable paleoenvironmental and archaeological records.

The study focuses on a single cave, Cave 29, in ELMA, limiting the generalizability of the findings to the broader Southwest region. While the radiocarbon dating provides strong chronological information, the uncertainty associated with the dating method may influence the precise correlation with specific drought events. Furthermore, the study assumes that the charcoal and ash are predominantly from anthropogenic activities rather than natural wildfires. Although the specific ventilation characteristics of Cave 29 make this interpretation more likely, the possibility of wildfire contributions to the deposits cannot be completely excluded without further investigation.