Acenote (English:/sɪˈnoʊti/or/sɛˈnoʊteɪ/;Latin American Spanish:[seˈnote]) is a naturalpit, orsinkhole, resulting when a collapse oflimestonebedrock exposesgroundwater. The term originated on theYucatán Peninsula of Mexico, where the ancientMaya commonly used cenotes for water supplies, and occasionally for sacrificial offerings. The name derives from a word used by the lowlandYucatec Maya—tsʼonoʼot—to refer to any location with accessible groundwater.[2][3]
In Mexico the Yucatán Peninsula alone has an estimated 10,000 cenotes,[4] water-filled sinkholes naturally formed by the collapse of limestone, and located across the peninsula. Some of these cenotes are at risk from the construction of the new tourist Maya Train.[4]
Cenotes are common geological forms in low-altitude regions, particularly on islands (such as Cefalonia, Greece), coastlines, and platforms with young post-Paleozoic limestone with little soil development. The termcenote, originally applying only to the features in Yucatán, has since been applied by researchers to similarkarst features in other places such as inCuba,Australia,Europe, and theUnited States.
Cenotes are surface connections to subterranean water bodies.[5] While the best-known cenotes are large open-water pools measuring tens of meters in diameter, such as those atChichen Itza inMexico, the greatest number of cenotes are smaller sheltered sites and do not necessarily have any surface exposed water. Some cenotes are only found through small <1 m (3 ft) diameter holes created by tree roots, with human access through enlarged holes, such as the cenotesChoo-Ha, Tankach-Ha, and Multum-Ha nearTulum. There are at least 6,000 cenotes in the Yucatán Peninsula of Mexico.[6][better source needed] Cenote water is often very clear, as the water comes from rain water filtering slowly through the ground, and therefore contains very little suspended particulate matter. The groundwater flow rate within a cenote may be very slow. In many cases, cenotes are areas where sections of the cave roof have collapsed revealing an underlying cave system, and the water flow rates may be much faster: up to 10 kilometers (6 mi) per day.[7]
The Yucatan cenotes attract cavern andcave divers who have documented extensive flooded cave systems, some of which have been explored for lengths of 376 km (234 mi) or more.[8]
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Cenotes are formed by the dissolution of rock and the resulting subsurface void, which may or may not be linked to an activecave system,[clarification needed] and the subsequent structural collapse. Rock that falls into the water below is slowly removed by further dissolution, creating space for more collapse blocks. Likely, the rate of collapse increases during periods when thewater table is below the ceiling of the void since the rock ceiling is no longer buoyantly supported by the water in the void.
Cenotes may be fully collapsed, creating an open water pool, or partially collapsed with some portion of a rock overhanging above the water. The stereotypical cenotes often resemble small circularponds, measuring some tens of meters in diameter with sheer rock walls. Most cenotes, however, require some degree of stooping or crawling to access the water.
In the north and northwest of theYucatán Peninsula in Mexico, the cenotes generally overlie vertical voids penetrating 50 to 100 m (160 to 330 ft) below the modern water table. However, very few of these cenotes appear to be connected with horizontally extensive underground river systems, with water flow through them being more likely dominated by aquifer matrix and fracture flows.[7]
In contrast, the cenotes along the Caribbean coast of the Yucatán Peninsula (within the state ofQuintana Roo) often provide access to extensive underwater cave systems, such asSistema Ox Bel Ha,Sistema Sac Actun/Sistema Nohoch Nah Chich andSistema Dos Ojos.
The Yucatán Peninsula contains a vast coastalaquifer system, which is typically density-stratified.[7] The infiltratingmeteoric water (i.e., rainwater) floats on top of higher-densitysaline water intruding from the coastal margins. The whole aquifer is therefore ananchialine system (one that is land-locked but connected to an ocean). Where a cenote, or the flooded cave to which it is an opening, provides deep enough access into the aquifer, the interface between the fresh and saline water may be reached. The density interface between the fresh and saline waters is ahalocline, which means a sharp change in salt concentration over a small change in depth. Mixing of the fresh and saline water results in a blurry swirling effect caused byrefraction between the different densities of fresh and saline waters.
The depth of the halocline is a function of several factors: climate and specifically how much meteoric water recharges the aquifer,hydraulic conductivity of the host rock, distribution and connectivity of existing cave systems, and how effective these are at draining water to the coast, and the distance from the coast. In general, the halocline is deeper further from the coast, and in the Yucatán Peninsula this depth is 10 to 20 m (33 to 66 ft) below thewater table at the coast, and 50 to 100 m (160 to 330 ft) below the water table in the middle of the peninsula, with saline water underlying the whole of the peninsula.[7]
In 1936, a simple morphometry-based classification system for cenotes was presented.[9]
The classification scheme was based on morphometric observations above the water table, and therefore incompletely reflects the processes by which the cenotes formed and the inherent hydrogeochemical relationship with the underlying flooded cave networks, which were only discovered in the 1980s and later with the initiation of cave diving exploration.[citation needed]
Flora and fauna are generally scarcer than in the open ocean; however, marine animals do thrive in caves. In caverns, one can spotmojarras,mollies,guppies,catfish, smalleels andfrogs. In the most secluded and darker cenotes, the fauna has evolved to resemble those of many cave-dwelling species. For example, many animals don't havepigmentation and are often blind, so they are equipped with long feelers to find food and make their way around in the dark.[10]
Although cenotes are found widely throughout much of the Yucatán Peninsula, a higher-density circular alignment of cenotes overlies the measured rim of theChicxulub crater. This crater structure, identified from the alignment of cenotes,[11] but also subsequently mapped using geophysical methods (includinggravity mapping) and also drilled into with core recovery, has been dated to the boundary between theCretaceous andPaleogene geologic periods, 66 million years ago. This meteorite impact at theCretaceous–Paleogene boundary is therefore associated with the mass extinction of the non-avian dinosaurs and is also known as theCretaceous–Paleogene extinction event.[12]
In 2001–2002 expeditions led by Arturo H. González and Carmen Rojas Sandoval in the Yucatán discovered three human skeletons; one of them,Eve of Naharon, wascarbon-dated to be 13,600 years old.[13][14][15] In March 2008, three members of theProyecto Espeleológico de Tulum andGlobal Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava, explored a section ofSistema Aktun Hu (part ofSistema Sac Actun) known as the pitHoyo Negro.[14][16] At a depth of 57 m (187 ft) the divers located the remains of amastodon and a humanskull (at 43 m [141 ft]) that might be the oldest evidence ofhuman habitation in the region.[14]
The Yucatán Peninsula has almost no rivers and only a few lakes, and those are often marshy.[17] The widely distributed cenotes are the only perennial source of potable water and have long been the principal source of water in much of the region. MajorMaya settlements required access to adequate water supplies, and therefore cities, including the famousChichen Itza, were built around these natural wells. Many cenotes like theSacred Cenote in Chichen Itza played an important role in Maya rites. The Maya believed that cenotes were portals toXibalba or the afterlife, and home to the rain god,Chaac.[18] The Maya often deposited human remains as well as ceremonial artifacts in these cenotes.
The discovery of golden sacrificialartifacts in some cenotes led to the archaeological exploration of most cenotes in the first part of the 20th century.Edward Herbert Thompson (1857–1935), an American diplomat who had bought the Chichen Itza site, began dredging the Sacred Cenote there in 1904. He discovered human skeletons and sacrificial objects confirming a local legend, theCult of the Cenote, involvinghuman sacrifice to the rain godChaac by the ritual casting of victims and objects into the cenote.[19] However, not all cenotes were sites of human sacrifice. The cenote atPunta Laguna has been extensively studied and none of the approximately 120 individuals show signs of sacrifice.[20] The remains of this cultural heritage are protected by theUNESCO Convention on the Protection of the Underwater Cultural Heritage.[21]
Cenotes have attracted cavern and cave divers, and there are organized efforts to explore and map these underwater systems. They are public or private and sometimes considered "National Natural Parks". Great care should be taken to avoid spoiling this fragile ecosystem when diving. In Mexico, theQuintana Roo Speleological Survey maintains a list of the longest and deepest water-filled and dry caves within the state boundaries. When cavern diving, one must be able to see natural light the entire time that one is exploring the cavern (e.g., Kukulkan cenote nearTulum, Mexico). During a cave dive, one passes the point where daylight can penetrate, and one follows a safety guideline to exit the cave. Things change quite dramatically once moving from a cavern dive into a cave dive.[22]
Contrary to cenote cavern diving, cenote cave diving requires special equipment and training (certification for cave diving). However, both cavern and cave diving require detailed briefings, diving experience, and weight adjustment to freshwater buoyancy. The cenotes are usually filled with rather cool fresh water. Cenote divers must be wary of possiblehalocline; this produces blurred vision until they reach a more homogeneous area.
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