Marine Shallow-Water Hydrothermal Activity Along the West Side of Saba, Dutch Antilles

With help from fantastic volunteer divers and as part of the Sea & Learn Experience we carried out a sampling campaign of submarine hot springs along the coastline of Saba. The samples were collected from light orange colored sandy patches within the coral reefs along the west side of Saba. These patches were also warmer to the touch than the surrounding normal colored sand. We estimated the temperature to be somewhere between 35 and 40 °C. The hypothesis was that the coloration and the elevated temperature are a result of remnant volcanism and the associated heat flow, which causes water to circulate in the subsurface beneath the sediments. Cold seawater enters the seafloor where it is heated and then circulates back up. The sample Sulfur Springs (see left picture below) was taken from the intertidal zone along the north side of Saba with great help from the Saba Conservation Foundation .

The samples were analyzed for a suite of elements to evaluate their difference from seawater and to get clues about chemical processes at higher temperatures. The samples show many similarities to seawater, however bicarbonate (HCO3), calcium (Ca), magnesium (Mg), arsenic (As), strontium (Sr), manganese (Mn), lithium (Li), and uranium (U) are substantially different from seawater, which would indicate that the heated water reacted with volcanic rocks or sediments. Particularly the low value of magnesium in the sample “Sulfur Springs” indicates the hydrothermal history, because magnesium is removed from a fluid during higher temperature and contact with rock. This sample also shows high lithium and arsenic values, which are indicative of a mature hydrothermal fluid. In the same sense all the other samples, which were collected, also show lower magnesium and higher arsenic and lithium values when compared to seawater. This confirms their reaction with volcanic rocks and sediment at elevated temperatures. Because the samples have a relatively low pH of around 6 (seawater has 8) the fluids are capable to dissolve the minerals calcite and aragonite. These two minerals make up the skeletons of most reef organisms and thus the sediments surrounding the reefs. This explains the high calcium and strontium values seen in the samples. It may be surprising to some that we did not find elevated concentrations of iron (Fe). However this was exactly as expected since iron is immediately removed from the hydrothermal fluids. Iron reacts quickly with the oxygen in seawater and then forms a mineral similar to rust. When cars rust, the same reaction takes place: rain, oxygen and iron combine to form rust. Rust is orange, thus explaining the orange color of the patches.

Orange colored patches of sand. The color is caused by the precipitation of iron minerals, which are similar to rust.
Collection of water samples from the Sulfur Springs intertidal site (left) and from the orange colored patches of sand. The samples were collected into syringes through rhizom filters. The divers had to be patient, because it could take up to 20 minutes to collect just one sample.
In summary the results are exciting because they point towards a hydrothermal contribution to the coral reefs along the west coast of Saba. The different water chemistry and different temperatures relative to seawater will allow us to study how marine flora and fauna adapt to those conditions. In turn this will allow us to study how life adapts to extreme situations and how life flourishes under extreme conditions. A carefully executed temperature monitoring of the colored sandy patches may even give clues to the progress of volcanic activity on Saba.