Tuesday, August 18, 2015

Cenotes

CENOTES


cenote [seˈnote]; plural: cenotes; from Yucatec Maya dzonot or ts'onot, "well" is a natural pit, or sinkhole, resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula of Mexico, cenotes were sometimes used by the ancient Maya for sacrificial offerings.
The term derives from a word used by the low-land Yucatec Maya — ts'onot — to refer to any location with accessible groundwater. Cenotes are common geological forms in low latitude regions, particularly on islands, coastlines, and platforms with young post-Paleozoic limestones that have little soil development.

Formation

Cenotes are formed by dissolution of rock and the resulting subsurface void, which may or may not be linked to an active cave system, and the subsequent structural collapse. Rock that falls into the water below is slowly removed by further dissolution, creating space for more collapse blocks. The rate of collapse increases during periods when the water 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 circular ponds, measuring some tens of meters in diameter with sheer drops at the edges. Most cenotes, however, require some degree of stooping or crawling to access the water.

Penetration and extent

In the north and northwest of the Yucatán Peninsula, the cenotes generally overlie vertically extensive 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. In contrast, the cenotes along the Caribbean coast of the Yucatán Peninsula (within the state of Quintana Roo) often provide access to extensive underwater cave systems, such as Sistema Ox Bel HaSistema Sac Actun/Sistema Nohoch Nah Chich and Sistema Dos Ojos.

Freshwater/seawater interface

The Yucatán Peninsula contains a vast coastal aquifer system, which is typically density-stratified.[5] The infiltrating meteoric water (i.e., rainwater) floats on top of higher-density saline water intruding from the coastal margins. The whole aquifer is therefore an anchialine 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 a halocline, 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 by refraction 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 the water 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.[5]

Types

Radar topography reveals the 180 km (110 mi) ring of the crater; clustered around the crater's trough are numerous sinkholes, suggesting a prehistoric oceanic basin in the depression left by the impact (Image courtesy NASA/JPL-Caltech).
In 1936, a simple morphometry-based classification system for cenotes was presented.
  • Cenotes-cántaro (Jug or pit cenotes) are those with a surface connection narrower than the diameter of the water body;
  • Cenotes-cilíndricos (Cylinder cenotes) are those with strictly vertical walls;
  • Cenotes-aguadas (Basin cenotes) are those with shallow water basins; and
  • grutas (Cave cenotes) are those having a horizontal entrance with dry sections.
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 onwards with the initiation of cave diving exploration.

Monday, August 10, 2015

Prevent seasickness

Preventing Seasickness can save your day.


Seasickness or Motion Sickness – featured from DAN (Divers Alert Network)
Seasickness is a condition individuals may experience when on a moving platform. It involves a general feeling of illness, dizziness, nausea and vomiting. It is also called motion sickness. Passive motions disturb fluid movement within the labyrinth and affects one’s sense of balance and equilibrium. It is exaggerated when the brain receives conflicting messages delivered from the eyes, muscles and joint sensors (proprioceptors). In a closed room, the view indicates that the surroundings are still, while the signals from the labyrinth indicate that the body is moving. Motion sickness can occur when traveling on a ship, plane, train, bus or car. Some people are more sensitive than others, but if the motion stimuli are strong and the exposure lasts long enough, nearly all individuals will experience it.

Symptoms
The symptoms of motion sickness include dizziness, sweating, nausea, vomiting and a general feeling of discomfort or illness. Symptoms can strike suddenly and progress from simply not feeling well to cold sweats, dizziness and vomiting. Motion sickness is more common in women and in children 2-12 years old. Individuals who suffer from migraine headaches are also more prone to motion sickness. Motion sickness lasts as long as the motion lasts. Once the motion stops, symptoms quickly subside. Some people feel “sea legs” after a long sojourn at sea.

Prevention and Management
If you know you have motion sickness or might be prone to it, consider this advice:
  • On a boat: Stay on deck and focus on the horizon. Avoid inhaling exhaust fumes.
  • In a car: Sit in the front seat. If you are the passenger, look at the scenery in the distance.
  • Do not read in moving vehicles. Reading makes motion sickness worse.
  • Avoid heavy meals prior to diving.
  • Drink plenty of water.
  • Avoid alcohol the evening before you travel.
  • If possible, stand up. Sitting or lying down can make you feel worse.
  • Eat dry crackers to help settle a queasy stomach.
  • Avoid others who have become nauseous with motion sickness.

Treatment
Motion sickness can be treated with over-the-counter and prescription drug products.
  • Over-the-counter products: Antihistamines are commonly used both to prevent and treat motion sickness. A side effect of antihistamines is drowsiness, which is exaggerated when alcohol is consumed. Drowsiness may adversely affect diver safety.
  • Prescription products: The scopolamine skin patch (Transderm Scop) is a popular option. The patch is applied to the skin area behind the ear at least eight hours before exposure and can help prevent motion sickness for up to three days per patch. Scopolamine may cause dry mouth, blurry vision, drowsiness and dizziness. Patients with glaucoma, enlarged prostate and some other health problems should not use this drug. Be sure to tell your doctor of your existing health problems to help determine which drug is best suited for you.
  • Alternative remedies: Various alternative remedies have been promoted as being helpful in relieving or preventing motion sickness. In most cases, the evidence of efficacy is missing. However, if you have mild symptoms, you may try ginger or peppermint products to ease your symptoms without risking side effects.
To learn more tips, visit the DAN Health & Diving library at DAN.org/Health.