Hydrothermal vents are naturally forming structures found in the ocean. They usually occur on divergent plate boundaries, where tectonic plates are moving apart. The vents expel a fluid that was heated to extreme temperatures when seeping through the Earth's crust from the ocean. Maggie explains, 'Hydrothermal vents are like hot springs on the seafloor.
As tectonic plates of oceanic crust move apart, the crust is stretched, and in places it breaks, forming cracks and fissures within it. As the heated seawater moves through the crust, it picks up dissolved gases and minerals.
The high temperature of the fluid makes it buoyant, and the superheated water eventually begins to move back up to the seabed where it is expelled through a vent. The minerals that are dissolved in the hot fluid then start to come out of solution.
As the minerals precipitate, they form a solid structure onto the seabed around the venting fluid known as a vent chimney. Maggie says, 'There is normally a temperature difference associated with the two types of vent chimney. Depending on how hot the fluid is, it can carry different minerals in solution, and different minerals will precipitate out at different temperatures. White smokers typically occur at lower temperatures.
The light appearance is due to the minerals carried, which can include silica and barite. When these precipitate, they appear white. Black smokers are hotter and spew out a fluid that carries mostly iron sulphides, which make them look darker. But despite the scalding heat, the environment around the vents is habitable for a range of animals. The giant vent mussel Bathymodiolus elongatus is found on hydrothermal vents at a depth of around m.
The food chain at these ocean oases relies on a core process called chemosynthesis, which is carried out by bacteria. This is similar to photosynthesis used by plants on land, but instead of using light energy from the Sun, the bacteria use chemicals drawn from the vent fluid.
There are lots of somewhat strange animals that have adapted to what may initially seem like fairly inhospitable conditions. Some species appear to have become fully reliant on the thermal sites. Animals such as scaly-foot gastropods Chrysomallon squamiferum and yeti crabs Kiwa species have only been recorded at hydrothermal vents. Large colonies of vent mussels and tube worms can also be found living there. In , the Pompeii worm Alvinella pompejana was identified living on the sides of vent chimneys.
Very close to the hot fluid, there are typically only microorganisms. Scientists believed that only small animals lived at the ocean bottom in seafloor sediments. These animals received their food from above. The food chain depended on sunlight and photosynthesis, just as the food chain on land does. To see how it works, click on the Photosynthesis vs.
Chemosynthesis icon at right. In the sunlit ocean surface, microscopic marine plants called phytoplankton flourish-like great fields of grasses on land. Marine animals eat the phytoplankton much the way insects or zebras eat plants on land. And predators eat other animals. When all these marine plants and animals die, they sink to the bottom. Occasionally, the carcass of a dead whale might sink down to provide a feast!
Scientists had thought this was the only way life could survive on the deep seafloor. The discovery of hydrothermal vents changed all that. Vast communities of animals grew big and fast in the depths! Instead of using light to create organic material to live and grow photosynthesis , microorganisms at the bottom of the food chain at vents used chemicals such as hydrogen sulfide chemosynthesis. At the seafloor, thriving ecosystems receive energy from a source that had never been thought of before-heat and chemicals from inside the planet itself.
The energy to sustain life was not coming down from the sun. It was coming up from the interior of the earth.
Since the discovery of hydrothermal vents in , scientists have found close to organisms that had never been seen before. To live at the vents, many of these organisms have unusual adaptations that were new to scientists. Scientists have found that hydrothermal vent sites around the globe are as unique as cities around the world.
Honolulu and Denver are both cities, but they have many differences. The same is true of vent sites. The early cruises first hinted that vents sites are not all alike. While national regulatory bodies and the ISA are currently drafting exploitation regulations for deep-sea mining and the measures for environmental protection Van Dover et al. For example, a major premise of deep-sea mining was recently demonstrated unreliable when researchers found that nearby vents 75 km apart did not support the same vent-associated communities Goffredi et al.
Because mining will be proceeding in back-arc settings e. To test this null hypothesis of little to no difference between these systems in this regard, we repeatedly surveyed five study sites on the side of active edifices in the Lau Basin over 10 years to resolve the natural rates of structural and functional changes experienced at back-arc basin hydrothermal vents.
Figure 1. The environmental settings of the study. These vent fields are all located within deep-sea mining license blocks see text for details. The edifice sulfide deposits host three faunal groups of chemoautotrophic bacteria-containing invertebrates, C the provannid snails Alviniconcha spp. Table 1. All five study sites are within deep-sea exploration tenements currently licensed to Nautilus Minerals Inc which require renewal in ; pers comm Alison Swaddling. Within these tenements, the ABE and Tu'i Malila vent fields are specifically identified as mining prospect sites Jankowski, While remote sensing of the geomorphology of vent fields and the geochemistry of hydrothermal plumes can provide some evidence of stochastic geological events, investigating the natural variability in structure and function of vent habitats requires in situ time-series of the vents and associated fauna.
During each expedition, we used high-resolution sub-centimeter photo mosaics Figure 1B and spatially explicit in situ measurements to document changes in the structural deposits, hydrothermal fluid, and vent communities. We imaged the edifices with a forward-mounted camera and made spatially explicit in situ thermal measurements using a temperature probe and the ROV manipulators.
Details on the hardware used before are available in Podowski et al. Although equipment varied between expeditions, we regard the differences as negligible since image resolution was consistently sub-centimeter and there was little to no variability in ambient seawater measurements between years Table 2.
Table 2. Spatio-temporal characteristics and analyses of the hydrothermal fluids and chemoautotrophic bacteria-containing invertebrates foundation species at the long-term study sites: Tow Cam TC , ABE, and Tu'i Malila TM vent fields. The collection, processing, and digitization of images and in situ thermal measurements followed procedures initially outlined in Podowski et al.
For hydrothermal vent research, it is often necessary to consolidate data from a series of discrete cruises that were not originally designed for temporal studies Glover et al. In contrast, our long-term, high-resolution time-series was designed for repeated surveys. To replicate the surveys, repositioning with the same coordinates, depth, and heading relative to deployed physical markers, was crucial to locate the study sites, minimalize the effect of parallax, and enable paired statistical comparisons of replicated measurements Sen et al.
Using a house-painting photo acquisition technique and a customized large-area mosaicing Matlab program Pizarro and Singh, , we generated a high-resolution photo mosaic of one side of each edifice, for each expedition. Mosaics were georeferenced and digitized in ArcMap 9. To map the physical structure of the edifices, we digitized a vector layer area polygon for the two main edifice substrates, sulfide and anhydrite deposits, which are visually distinct with different physical characteristics Goldfarb et al.
We also digitized vector layers for the three local faunal groups of chemoautotrophic bacteria-containing invertebrates, the provannid snails Alviniconcha spp. These primary producers and foundation species are community-structuring species and hence are the focus of our investigation concerning the natural stability of biological processes and ecosystem function on the edifices.
For the substrate vectors, we inferred that the obscured edifice beneath clusters of foundation species were sulfide deposits since these molluscs avoid anhydrite surfaces Sen et al. We performed statistical analyses in R For the thermal data, we used paired t -tests parametric and Wilcoxon signed-rank tests non-parametric , for a total of 13 tests temperature measurements were not collected for TC1C in To assess the temporal variability of the faunal groups, we ran a Friedman test for each faunal group using the edifices as replicates, followed by three paired tests to investigate each temporal interval separately i.
To assess the spatio-temporal variability on each edifice, we tested for spatial overlap over time. We consolidated the foundation species for this analysis because the three faunal groups have substantial overlaps in realized niches Podowski et al. We also calculated the expected spatial overlap assuming each year's microdistribution was by chance, independent and random, based on the percent of edifice area and the addition rule of probability. The inverse of the CI test indicates if there was a significant change in the microdistribution H 1.
We interpreted results as one of two spatio-temporal patterns, either redistribution occurred, randomly or to new areas H 1 : observed overlap was not significantly greater than expected , or the microdistribution was spatially stable H 0 : observed overlap was significantly greater than expected. Despite the spatial and temporal extent of our surveys Figure 1A , our research was not affected by large-scale stochastic geological events over the year study period.
While the processing of bathymetry data is ongoing, preliminary analyses of bathymetric surfaces and visual observations of the seafloor indicate that no significant changes in surface geology occurred over the decade pers comm Vicki Ferrini. Here we document that three medium spreading rate back-arc basin vent fields have provided stable hydrothermal environments for over a decade, with no evidence of regional disruption from sub-surface hydrothermal activity or catastrophic eruptions or lava flows.
These findings support the hypothesis that, unlike the well-studied dynamic mid-ocean ridge systems, the associated communities of these vent fields need not be as naturally resilient to the same fast-paced large-scale disturbance regime.
In comparison, mid-ocean ridge time-series have shown catastrophic events occurring at interannual to decadal-scales, with system dynamics positively correlated with the ridge spreading rate Haymon et al.
The sides of the edifices surveyed ranged from 1. All five edifices were predominantly sulfide deposits. While the anhydrite spires were prone to cycles of growth and collapse or dissolution cf. Tunnicliffe and Juniper, , in comparison, the sulfide deposits were physically stable between surveys.
New sulfide deposition was slow, and the general shapes and sizes of the edifices did not change over the decade. As previously mentioned, time-series data is limited, but the general model is that hydrothermal vent system dynamics are positively correlated with the ridge spreading rate.
However, the sulfide areas of our Lau Basin edifices grew by 2. Although deposition rates can slow from clogging or reaching a growth-weathering equilibrium as edifices mature Goldfarb et al.
These growth rates are much slower than the rapid formation of edifices documented on mid-ocean ridges with similar spreading rates e. Following a mining event, these slow sulfide deposition rates could further hinder recovery through slow reversibility of the substrate removal and habitat loss Gollner et al.
Figure 2. The natural changes in A the size of the edifices and the two substrate types, B the fluid temperatures, and C—E the three chemoautotrophic bacteria-containing invertebrates for the five edifices over the study decade , , Lines show the pairings between repeated temperature measures and asterisks indicate a significant difference over time.
For summarized statistics, see Tables 2 , 3. On four of the five edifices, the number and locations of orifices venting hydrothermal fluid did not change between years visual observation of shimmering or discolored fluid Table 2.
On the exception edifice, TM1C, three of the six orifices were stable. The distribution and intensity of hydrothermal flow, detected with spatially explicit in situ temperature measurements, was also remarkably stable on all five edifices at the sub-decadal and decadal scales Figure 2B ; Table 2.
Subsurface mixing dilutes vent fluids with cooler water, and our point measurements ranged from ambient at 2. These multiple lines of evidence indicate a stability in hydrothermal venting on the outer portions of Lau Basin edifices that is different from vent fields on mid-ocean ridges of similar spreading rates e.
Figure 3. The colored polygons represent the microdistributions observed during each survey: pink in , orange in , and blue in Thick black lines denote the study sites' boundaries in , while thin black lines denote previous years' boundaries i. The three values provided underneath each edifice are the percent of spatial overlap in the microdistribution between years i. For detailed statistical outputs, including the differences between observed and expected overlap, see Table 2.
Without focused high-temperature flow measurements, we cannot directly speak to the stability of the end-member fluid i. However, as the source of the diffuse hydrothermal venting, we would expect the end-member fluid temperature to be equally or more stable over the monitoring period than our time-series cf. Tivey et al.
In the present study, at least some stability in the end-member fluid chemical composition is indirectly evident from observations of consistent edifice deposition hydrothermal fluid precipitate and the realized distributions of vent-associated fauna. The realized distribution of the three study faunal groups reflects their requirement for reduced substrates, their temperature and chemistry tolerances, responses to biological interactions, and the physical stability of the substrate Henry et al.
It then follows that the unusual habitat stability of the Lau Basin edifices would be expected to reflect similarly high stability in the composition, coverage, and microdistribution of the foundation species at the same temporal scales.
That said, hydrothermal vents are not static systems and any habitat stability experienced would be the result of dynamic systems in equilibrium. All five study sites were inhabited by I. The coverage of each faunal group between years was remarkably constant, with no significant changes at the sub-decadal and decadal temporal scales Figures 2C—E ; Table 3.
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