9St questions and answers: 18th April 2008
9St questions and answers: 18th April 2008
How many seamounts are there around NZ and how often does a new one form?
At NIWA we keep a database of information about all known seamounts in ‘the New Zealand region’ – this is an area around NZ for which bathymetric and other oceanographic charts were produced by NIWA’s forerunner the NZ Oceanographic Institute. The area includes not only the NZ Exclusive Economic Zone (EEZ) but also areas of the Australian EEZ around Lord Howe, Norfolk and Macquarie Islands, as well as international waters. Currently we have listed over 1000 "seamounts" in the database. However, it is worth noting that our definition of the word seamount includes features with elevations of 100s of metres. The geological definition of a seamount specifies that such features should be above 1000 m in height. About 30% of the seamounts listed in the database are above 1000m.
I don’t know how often new seamounts form – I’ll leave the answering of that part of the question to a geologist.
What is the tallest seamount you have found? Are NZ seamounts taller than in other places?
The tallest seamounts listed in the NIWA seamount database are between 4000-5000 m in height. For comparison that’s over twice the height of Mount Taranaki. These very tall seamounts, whilst in the NZregion, rise from the seabed outside of the NZ EEZ. The tallest seamount in the EEZ is Bollons seamount which is 3828 m in height. Bollons is also the largest seamount, in terms of area covered, in the region. It covers a staggering 34,700 square kilometres. In a comparative terms that’s...well its more rugby fields than I can imagine.
I’m not sure off the top of my head how the height of NZ seamounts compares to seamounts elsewhere in the world, but it would be possible to find out by comparing those data in our database with information for seamounts, or the height of predicted seamounts, in global databases.
What range of seamounts are you in? Is seamount #375 in a named area of seamounts?
The seamounts we are studying on this voyage are either on or are associated with the Macquarie Ridge – so it is the name of this feature to which they are primarily referred. Seamount #375 occurs near the ridge in an area known as "Puysegur", named after the trench on the western side of the ridge in this region.
[for further information on the physical characteristics of NZ seamounts see: Rowden et al (2005) Physical characteristics and a biologically focused classification of "seamounts" in the New Zealand region. New Zealand Journal of Marine and Fershwater Research, Vol .39, pages 1039-1059.]
Why is there more life on the seamounts than the surrounding sea floor?
Short answer: Because there is more food and a larger number of habitats which means a lot more species compared with the soft sediment of the surrounding sea floor.
For a much longer answer ............. read on!
Well perhaps this question should be phrased "Is there more life on seamounts than the surrounding sea floor?" – and then the answer would begin with "It depends..." (aaahh, conservative, cautious scientists strike again...).
For first we’d need to agree on what "life" the question refers to, and what is meant by "more life" would have to be understood as well. In addition, the spatial extent and type of "surrounding" seafloor would have to be defined.
For example let’s consider the large invertebrates (those you can see with the naked eye) as the seabed life of interest, and ‘more’ as being both the number of species and the number/biomass of individuals present. In this case the surrounding seafloor can be limited to that immediately adjacent to the base of the seamount, and let’s assume that the surrounding seafloor is comprised of mud or sand (so-called soft sediment or substratum). A reasonable prediction, backed up by a limited number of observations, is that seamounts possess a greater number and mass of macro-and mega- invertebrates species than a surrounding seafloor of soft sediment.
The reasons for such an elevated diversity and abundance of life include the higher levels of habitat heterogeneity and food supply on seamounts as opposed to the surrounding soft sediment. In general there is a greater variety of habitats for seabed animals on seamounts – there are often both hard and soft sediment types (mud, sand, gravels and cobbles, small to large boulders) with the hard rock having cracks and crevices, overhangs etc and the soft sediment occurring in small patches or sometimes as relatively large areas of rippled sand; some of the organisms that can live on seamounts such as corals and sponges provide additional physical habitat; and because of the vertical extent of seamounts there are further habitats which occur with water depth-related changes in such variables as temperature, light and oxygen etc.
For each particular combination of environmental conditions there will be a suite of species that have adapted to survive best in such a habitat. Thus, overall the more habitats that a location possesses the more species that can be supported in that area. With respect to abundance of all or some of the species found on our example seamount – the physical morphology of some seamounts and their interaction with ocean currents can increase the amount of nutrients that arrive at a seamount which in turn can promote elevated levels of phytoplankton. This enhanced primary production at the base of the food web (see also next question) can support, either directly or indirectly, large numbers of filter-feeding organisms as well as detritivores and ultimately predators.
For further information on this topic see relevant chapters (4, 5, 7 & 8) in the book "Seamounts: Ecology, Fisheries & Conservation" (eds Pitcher et al, 2007, Blackwell Publishing). Click here for more information
So whilst the previous example matches with the original questioner’s assumption, if alternatively we considered a different group of organisms and different type of surrounding seafloor and physical characteristics of the seamount then the answer might be quite different. The fact that other possibilities exist is why scientists like to qualify statements about observations of life on seamounts so as not to promote the misleading impression that in all cases seamounts are oases of life. It’s worth noting that the same forces that promote diversity and abundance of large invertebrates exist elsewhere in the deep-sea, for example on the slopes of continental shelves. Indeed recent studies are beginning to demonstrate the invertebrate assemblages on seamounts are not as ‘unique’ as was previously thought
[Tim O’Hara recently published an article in Global Ecology and Biogeography testing the hypotheses that seamounts exhibit high rates of endemism and/or species richness compared to surrounding areas of the continental slope and oceanic ridges. Early online PDFs can be downloaded from http://www.blackwellsynergy.com/toc/geb/0/0 or contact Tim for further information– tohara@museum.vic.gov.au)
What are the producers in the seamount food chain?
The primary producers in a seamount food chain are phytoplankton. Bacteria and detritus (for example from dead and dying organisms) are also at the base of the food chain. Food chains or rather food webs can be quite complicated and varied for seamounts. Relatively little research has been conducted for seamounts at the ‘ecosystem’ level. However, recent advances have been made by science programs such as OASIS and food webs are now beginning to be modeled.
For further information on this topic see two chapters (14 and 15) in the book "Seamounts: Ecology, Fisheries & Conservation" (eds Pitcher et al, 2007, Blackwell Publishing).
Did you feel the earthquake recently? What do they feel like in a boat?
No we didn’t feel the earthquake. Last time I was out at sea – just off the west coast of the South Island in October 2007 there was a big earthquake in Fiordland and we didn’t feel anything then either. You would only feel an earthquake on a boat if it caused a tsunami wave. Earthquake waves (P, S and surface waves) do not travel through liquid, that is how we know the earth’s outer core is liquid.
How long would it take for the biggest sea mounts to form?
The seamounts along the Macquarie Ridge started forming during the Eocene, 52 million years ago when the Pacific and Australian continental plates started spreading apart. As the plates moved apart hot volcanic lava, or magma, comes up to the surface and cools rapidly to form mid ocean ridge basalt or MORB – as seen today on the Mid Atlantic Ridge. The MORB oozes out at the sea floor and forms a very distinctive shape and texture as it rapidly cools, they are round with lots of air bubbles and are called "pillow lavas".
Volcanoes can form very quickly during a single eruption of days to weeks, but they can also take many thousands of years to build a big volcano, which is made up of many layers of lava from many different eruptions over time. The Macquarie Ridge continued to spread until the mid Miocene, 12.5 million years ago, and there are volcanic rocks on Macquarie Island that have been dated at 9.7 million years – so some of the volcanoes/seamounts on the Macquarie Ridge could have taken 42.3 million years to form!
The strange thing about the Macquarie Ridge is that it is no longer spreading. Recent earthquake events along Macquarie Ridge indicate a dextral strike slip motion (movement to the right as you look across the fault), similar to the San Andreas Fault in California. In fact the largest strike slip event ever recorded was on the 23rd May 1989 just north of Macquarie Island. There is also evidence of some subduction in the northern and southern end of Macquarie Ridge. Both of these areas have deep trenches to the west of the ridge, down to depths of 6000 metres below sea level, where the Australian plate is being subducted below the Pacific plate.
To the east of the southern end of ridge there is also another group of seamounts that have a more typical volcanic cone shape, and these were probably formed by subduction related volcanism. This is similar to the volcanoes found in the Andes, where the subducting plate material is heated up as it is forced into the mantle and the molten magma rises up through the crust and erupts as a volcano. I don’t think any of these volcanoes east of the ridge have been studied yet by geologists.
If the sea mounts are formed by magma coming up, are they a lot warmer than the surrounding sea floor?
Initially when the volcano is still active there will be a lot of heat coming out of several vents on the volcano. These are called hydrothermal vents and can reach temperatures of up to 1000°C. Hydrothermal vents were initially discovered along the Mid Atlantic Ridge using the first scientific submarine called "Alvin". These vents are not only very hot, but the fluid coming out of them has some very strange chemistry. A range of chemosynthetic organisms (similar to photosynthetic – but use chemicals rather than sunlight) have evolved to use these hydrothermal fluids, with large communities of strange looking organisms found around the vents. There is no volcanic activity currently along the Macquarie Ridge and so far we have not found any hydrothermal vents during the seamount surveys.