Submerged bridge constructed at least 5600 years ago indicates early human arrival in Mallorca, Spain

The colonization timeline of the Balearic Islands, particularly Mallorca, remains debated due to limited archaeological evidence. Currently, human arrival is estimated around 4400 years ago. However, previous radiocarbon dating from sites like Cova de Moleta and Cova de Canet yielded controversial results due to sample preservation issues and unclear stratigraphic context, casting doubt on earlier proposed dates of 7000 and 9000 calibrated years before present (cal BP), respectively. More recent evaluations largely concur on a timeframe of 4600-4200 cal BP for the earliest settlement. This paper introduces a novel approach to dating the early presence of humans on Mallorca, based on the discovery of a submerged archaeological structure.

Existing literature on the early colonization of Mallorca presents conflicting narratives. Cherry and Leppard (2018), Dawson (2014), and Simmons (2014) discuss the complexities of island colonization, highlighting the debate surrounding Mallorca's relatively late settlement. While initial radiocarbon dates from Cova de Moleta and Cova de Canet suggested much earlier human presence (around 9000 cal BP), these were later challenged due to inconsistencies in sample integrity and association with human activity. Subsequent studies, incorporating new data, revised the earliest human settlement timeframe to 4600-4200 cal BP. Dawson (2014) further synthesizes different arrival models, suggesting early (~9000 cal BP), intermediate (~7600 cal BP), and late (~5000 cal BP) phases, with only the late model gaining general acceptance in the local archaeological community. The most recent research, based on radiocarbon dating of Myotragus balearicus remains and sheep bones, suggests human arrival approximately 4400 cal BP, coinciding with the extinction of Myotragus. However, the exact timing of these paleontological occurrences remains uncertain.

This study integrates uranium-series (U-series) age data from phreatic overgrowths on speleothems (POS) and stalactite tips in Genovesa and Drac caves with high-resolution Holocene sea-level data for Mallorca. The focus is on a submerged stone bridge in Genovesa Cave, exhibiting a distinctive light-colored band near its top. The depth of this coloration band, mirroring the depth of the phreatic overgrowth on speleothems (-1.1 meters), is crucial for the dating process. U-series dating of the POS in both caves revealed multiple periods of sea-level stability. Specifically, a period of stability at ~1.1 mbpsl between 5820 ± 140 and 5479 ± 120 years BP is significant, as it corresponds to the depth of the coloration band on the bridge. A high-resolution Late Holocene sea-level curve for Mallorca was used to model sea-level changes and relate them to the age and depth of the bridge and POS. The accuracy of sample location and depth was ensured through meticulous underwater mapping using a combination of instruments, including a Suunto SK-8 compass, a measuring tape, and a Suunto Gekko diving computer. The hydraulic gradient was measured using a Leica TC 405 total station to confirm that the water table in the cave was consistent with the sea level. U-series dating of the POS samples involved dissolving samples in acid, spiking with tracers, and using column chemistry to purify the uranium and thorium fractions before analysis with a mass spectrometer. Isochron ages were calculated using IsoplotR.

The U-series dating of the phreatic overgrowths on speleothems (POS) in Genovesa and Drac caves revealed a period of sea-level stability at approximately -1.1 meters below present sea level (mbpsl) between ~5964 and ~5359 years BP. A distinctive light-colored band, resembling a ‘bathtub ring’, is present on the submerged bridge at this same depth (-1.05 ± 0.1 m). This coloration mark suggests a period of stable water level at the bridge's upper surface, during which calcite precipitated. Integrating the bridge's depth, the coloration band's depth, and the high-resolution sea-level curve, the researchers conclude that the bridge's construction must have occurred between ~6000 and ~5600 years ago. The presence of pottery in the cave, tentatively attributed to the Naviform period (ca. 3550–3000 cal B.P.), is considered to represent later use of the cave after the bridge construction. The prior assumption of the bridge being constructed around 3600 years ago based on pottery findings in a nearby site, Closos de Can Gaià, is rejected due to inconsistencies in radiocarbon dating of that site. A reconstruction of sea level shows that the bridge would have been significantly submerged by 0.8m at 3500 years ago and 0.7m at 4400 years ago, making it highly improbable that the bridge was constructed that late.

The findings significantly push back the timeline of human presence on Mallorca, providing evidence for human activity at least 5600 years ago, and potentially as early as 6000 years ago. This challenges the established consensus of human arrival around 4400 cal BP. The submerged bridge, along with its associated stone-paved path, represents a substantial construction effort, suggesting a level of societal organization and planning that predates earlier estimates. The precise reasons behind the bridge's construction remain unclear but are potentially linked to accessing the deeper parts of the cave. The combined evidence from the bridge's depth, the coloration band, and the U-series dating of the POS strongly supports an early human presence on the island. This challenges the current model and adds new information to the ongoing debate of early human colonization of the Mediterranean Islands. Future research should focus on further exploring the cave and surrounding area for additional archaeological evidence to corroborate these findings and refine the timeframe of the early human presence on Mallorca.

This study presents strong evidence for human presence on Mallorca at least 5600 years ago, and potentially as early as 6000 years ago, based on the discovery and dating of a submerged bridge in Genovesa Cave. The integrated approach of U-series dating of speleothems and analysis of the bridge’s coloration band, coupled with a refined sea-level curve, provides a robust chronological framework. This significantly predates previous estimates and reshapes our understanding of early human colonization in the Balearic Islands. Future investigations should focus on locating and dating additional archaeological remains in the cave and nearby areas to further solidify this evidence and further illuminate the early human history of Mallorca.

While the methodology employed provides strong evidence for an earlier date of human presence in Mallorca, limitations exist. The study relies on a single archaeological feature, the submerged bridge. Further archaeological findings are needed to fully corroborate the results. The precision of the U-series dating is subject to inherent uncertainties associated with this method, though isochron dating techniques were used to improve accuracy. The interpretation of the coloration band's formation relies on the assumption of a relatively short-lived sea-level stillstand. Further investigation into the specific conditions leading to the band’s formation would enhance the interpretation.