Fig 1.
The Carajás National Forest, in southeastern Pará State, Brazilian Amazonia.
Darker areas are terra firme forest, while lighter areas represent deforested areas. The bat caves sampled are grouped in two major iron ore extraction areas, Serra Norte (top) and Serra Sul (bottom). Satellite image: NASA Landsat Program, 2021, Landsat 7 ETM+, scene LE72240642021357ASN00, LE07_L1GT_224064_20211223_20220118_01_T2, SLC-Off, USGS, 12/23/2021.
Fig 2.
Examples of speleothems and corrosion processes in bat caves (A, cave N3-0023; C, cave N4WS-0067; and E, cave S11B-0094) and non-bat caves (B, cave N4E-0008; D, cave N4WS-0015; and F, cave N5SM2-0021). In A and C, deep corrosion processes can be observed in the cave floor. Phosphate stalactites, like in C and E, were very rare and, so far, identified only in active or inactive bat caves. In B, D and E, the roughness of the walls and ceiling is a product of erosive action, not corrosive action. In D and F, the large rebates stand out, which is also an erosive process. (Photo credits: Ataliba Coelho, for A, C and E; Allan Calux, for B, D and F).
Table 1.
Comparison of length (in meters), area (in square meters) and volume (in cubic meters) between bat caves and non-bat caves in the Carajás National Forest, Brazilian Amazonia.
Fig 3.
A) Profile of guano deposit 65 cm deep at cave N3-0023-P2, in the Carajás National Forest, Brazilian Amazonia. B) Detail of small guano aggregates with remains of insect exoskeletons. Sample from bat cave S11A-0036.
Table 2.
Chemical profile of guano samples from bat caves in the Carajás National Forest, Brazilian Amazonia.
Depth of sample in centimeters.
Fig 4.
pH and phosphorus pentoxide variation in samples from different depths taken from guano deposits in three bat caves in Carajás National Forest, Brazilian Amazonia.
Table 3.
Chemical parameters (ammonium, iron, phosphate, and nitrate) of circulating waters samples from bat caves in the Carajás National Forest, Brazilian Amazonia.
Table 4.
Estimated ages based on radiocarbon analysis for guano samples and one stalagmite from bat caves in the Carajás National Forest, Brazilian Amazonia.
Guano samples were obtained from trenches or from a sample hole, all excavated on the cave floor.
Fig 5.
Examples of speleothems identified in iron ore caves in the Carajás National Forest, Brazilian Amazonia.
A) Coralloids in cave S11D-0078; B) Flowstones on the wall of cave S11D-0001; C) Pingente at the ceiling of cave S11D-0047; D) Crusts on the floor of cave N4WS-0015. None of those caves are bats caves.
Fig 6.
Examples of corrosion on the floor and walls in bat caves in the Carajás National Forest, Brazilian Amazonia.
A) Dripping holes on the floor covered by bat guano, in the active bat cave N5SM2-0099; B) Corrosion features on fresh guano, in the active bat cave S11A-0036; C) Floor without guano, already washed by water, evidencing pinnacles, in the inactive bat cave N4WS-0067; D) Pinnacles, dripping holes and flutes on the walls, in the inactive bat cave N3-0023.
Fig 7.
A variety of complex reactions takes place in the guano, especially bacterial decomposition, liberating phosphoric and sulphuric acids which react with rock or the sediments to form speleothems (stalactites, stalagmites, crusts and coralloids) of phosphates, with a predominance of leucophosphite, phosphosiderite, strengite and spheniscidite, and sulfates (gypsum).
In bat caves, conditions are very favorable for the microbial reduction of iron(III), including an environment that is often extremely acidic and with an abundance of organic matter represented by guano. This microbiological mechanism would be the main responsible for the dissolution of the iron rock. Important values of total iron, including soluble iron, were recorded in pools containing guano. The dripping and temporary drainage processes would be responsible for the elaboration of the forms of corrosion on the walls and floors of bat caves, including pinnacles, drainage channel, vertical fluting and guano holes. Fresh guano deposits also release CO2 and water vapor, which rise through the convection process of hot air produced by the exothermic decomposition of guano. These gases can produce carbonic acid which eventually corrodes the altered host rock and speleothems of phosphates and sulfates. Figure based on Onac and Forti [41].