Geoscientist Online

Ken McNamara provides a very interesting and stimulating review of modern Australian stromatolites (Geoscientist 19.12 December 2009). He mentions that some of the stromatolites are closely related to hot springs (Yellowstone). Here, he actually touches upon a cruicial aspect of the stromatolite enigma – their association with, not only hot springs, but all kinds of hydraulically focused fluid flow, i.e., ‘cold seeps’.

Because the substratum of stromatolites (including microbialites and other aquatic bioherms) is the poro-elastic soil, there is reason to suspect a seep-relationship. Soil, submerged in water (ocean and lake floors) consists of sand, silt, and clay, where the pores between the grains are saturated with water. In the intertidal zone, the soil is fully saturated only part of the time. In the context of bacterial and stromatolite growth, we suspect that seepage is a potent and stimulating process.

In this communication, we choose to define seepage as vertically focused fluid flow through the aquatic soil surface. Because the process of seepage is generally invisible it is often treated as a mere curiosity or as a rare and insignificant process. In fact, seepage is an ubiquitous process occurring inside large porous or semi-porous geological bodies containing fluids (liquids and gases). This is because the three phases of the body (solid, liquid, gas) behaves differently to external and internal forces.

The fluid movements inside a low permeable marine or lacustrine sediment, are governed by pressure-, temperature-, and chemical gradients. Movement of liquids, gases, and even solid particles are the result of these gradients, some of which are transient (e.g., tidal fluctuations). Seepage occurs under-ground and is rarely observed (i.e., micro-seepage). It is a physical and physico-chemical process, and has existed on Earth from long before the advent of stromatolites. In our view, the process is expected not only to be beneficial, but in many cases cruicial for the well-being of surface-dwelling micro-organisms, including some of the proto-stromatolites.

When discussing seepage, we therefore, face a severe dilemma. As a generally invisible process, it is notoriously difficult to document. Seepage belongs to the complex and diverse science of Hydrology. To document where and how such seepage occurs, careful monitoring and special instrumentation is needed. The effects of seepage (often inferred) are, generally, much easier to document. When detecting seep locations on the seafloor, it is often their acoustic effects in the water column and on the seafloor, together with their geochemical signatures and effects on topography and the local biology we can map. These features are the tell-tale manifestations of the seep processes. As seepage relies on delicate balances between various sources of fluids (e.g., aquifers), un-careful use of drilling techniques may destroy the sub-surface hydraulic balance of seepage.

Thus, a drill-hole in the wrong location can cause the puncturing and deflation of an over-pressured system. The character of aquatic seeps vary according to 1) fluid type, 2) chemistry, 3) sediment conditions, 4) presence or not of under-ground gas, and 5) the local and regional hydrological system. This means that nearly all seeps have their own specific character, which affects all biological utilization of the seep. The catalogue of seep manifestations in the ocean is, therefore, large and varied, ranging from craters (pockmarks), to mud volcanoes, and hydrothermal vents (Judd and Hovland, 2007).

As a curiosity, it is interesting to know that ice on lakes and seawater is a good recorder of seepage. Thus, during the current severe winter in Norway, it was easy to see the effects of seepage on the ice of lakes and seawater (see Figures 1 and 2).

Fig. 1 A seepage manifestation on ice (lake Homsa, in SW Norway). A concentric pattern has developed in the nearhore region of the lake. It has been caused by slow seepage of water and possibly gas from the sub-surface. It illustrates clearly how seeping water can be recorded on the surface, in this case by a freeze/thaw processes. For scale, a pair of skis (2 m length) and a 15 cm long knife in the middle of the structure.

Fig. 2. Another seep manifestation on ice. The illustration shows bubbles photographed in clear ice the Hafrsfjord in Norway. The gas bubbles (probably methane) accumulate in the ice, as the ice freezes and documents the gas that tricles out of discrete (focussed) locations on the seafloor below.

As bacteria often rely on chemical gradients for their electron exchange, i.e., their livelyhood, most chemical gradients and exotic mineral supplies will be ‘of interest’ to specialized types. Seep-locations can be compared to open ‘supermarkets’ for baceria, inviting them for a feast, continuously or periodically, often turned on and off by tidal forces. By us ‘seep-hunters’, bacterial mats found on the seafloor are, therefore, often used as evidence for seepage. A biofilm may also start forming in association with mats. Biofilms may also include the precipitation of salts or oxides, hydroxides, sulphides, etc., which result in a durable crust. Therefore, the biofilm combined with a thin precipitated layer of minerals will tend to affect the physical properties of the seepage itself, i.e., sealing it up (Hovland, 2002).

In addition to the hot-spring associated stromatolites of Yellowstone, mentioned by McNamara, we here, provide a short review of some other examples of live stromatolites that are suspected to rely on seepage (Hovland and Judd, 1988; Judd and Hovland, 2007).

Fig. 3 Stromatolite dome formation according to Warren (1982). The drawing clearly infers that the stromatolites originally form where groundwater seeps through discrete locations in the upper tepee-shaped surface layer.

According to our model and also that of Warren (1982) these features are seep-related organo-mineralizations, which are completely dependent upon the transport of elements to the location, such that a strong (sharp) compositional gradient is generated. This occurs typically in coastal salinas, which are partly cut off from the sea, where ground-water charges through the bottom, as in the Tepee-formation described by Warren (1982). In some of these locations stromatolites occur, and their relationship with tepee-structures have been described (Fig. 3).

Along the shores of Lake Tanganyika, several seeps and warm submarine springs are known to occur. They document ongoing hydrothermal activity in the crust below the lake. Groups of stromatolites grow along a cliff down to a depth of 30 m at the Luhanga hydrothermal field. Cohen et al. (1997) studied these modern stromatolites, but only in the context of palaeoclimate. Even so, they noticed that some of the lake water consisted of “ground-water input from small hydrothermal springs”. (Cohen et al., 1997; Judd and Hovland, 2007). We consider this seepage activity to be essential and by no means a coincident with regards to the formation of the rare stromatolites.

In the very narrow (800 m) and long (5.8 km) “Pavillion Lake” in Marble Canyon, British Colombia, Canada, there are microbialites (Laval et al., 2000). Because surface streams do not enter this very clear lake, karst (seepage) hydrology dominates. This means that whereas most other lakes rely on riverine (meteoric) water flow, this lake acquires water through cracks and crevices in its sides and lake-bottom, i.e., groundwater flow, and hydrothermal seepage. The microbialites generally occur at three depths: shallow (~10 m), intermediate (~20 m), and deep (>30 m). The shallow ones range in height from several cm to a few dm and comprise interconnected clusters of discrete round aggregates of calcite grains covered by photosynthetic microbial communities and their calcified remains. At intermediate depth, large microbialite domes (< 3 m high) consist of closely spaced aggregate clusters with a preferred orientation forming vertically ribbed structural components reminiscent of cones and leaves (Laval et al., 2000). In deeper waters the structures are similar, but the individual ‘cones’ and ‘leaves’ are larger (20-35 cm in height). The researchers noted that the cones “often have one or more internal conduit up to 5 mm in diameter” and concluded: “Based on their appearance and the presence of internal conduits, it is probable that the distribution of the intermediate to deep, cone-topped microbialites correspond to regions of groundwater seepage into the lake.” Therefore, it seems likely that calcification is a consequence of microbial activity below surface bacterial mats. (Judd and Hovland, 2007).

The largest microbialites ever found, occur in the alkaline Lake Van in eastern Turkey. Here, Kempe et al. (1991) described “enormous (~40 m high) tower-like microbialites”. This is a remarkable lake, with a high pH (9.7-9.8) and a salinity of 21.7‰. Mantle-derived gas enters the lake together with other minerals and fluids by hydrothermal, mainly diffusive, seepage through the lake bottom. This fluid flow actually accounts for 0.04-0.06 % of the total global helium flux (Kipfer et al., 1994). Because the chemistry of the lake is similar to that of the Precambrian ocean, Kempe et al. speculated that the Lake Van microbialite structures are analogues to Pre-Cambrian stromatolites, and therefore, by analogy, Judd and Hovland (2007) noted that many of these structures are seepage related.

Finally, we offer a couple of comments on the Hamelin Pool stromatolites, Shark Bay, in Western Australia, currently the best known live stromatolites on Earth. They are localized in an intertidal zone, where there can be special hydraulic conditions. Parts of this belongs to the vadose zone (on land), where freshwater seeps and artesian springs occur. In the Hamelin Pool, it is evident that the stromatolites form clusters and sometimes lineated stromatolites. In our view these aspects may reflect seepage conditions in the pool. If one looks at the satellite images presented on Earth Google for the Hamelin Pool area, including the immediate shore-line, it is evident that there is very shallow groundwater in the coastal area, due to the pattern of shrubs seen surrounding the pool (Actually, that pattern mimics that of the stromatolites, or vice versa….).

Perhaps some carefully conducted hydrological investigations may unravel som very interesting sub-surface hydraulic conditions. And, furthermore, as McNamara (2009) points out, some of these still living stromatolites are ‘under threat’ (by human land development). Our comment is that – yes – they really need to be protected, actively, against modern development, especially against drainage and piping systems, and not least against under-ground sewage development.

• Martin Hovland, Centre for Geobiology (CGB), University of Bergen, Bergen, Norway and Statoil ASA, Stavanger, Norway
• Haakon Rueslåtten, Numerical Rocks, Trondheim, Norway
• Rolf Birger Pedersen, CGB, Bergen, Norway

References

• Cohen, A.S., Talbot, M.R., Awramik, S.M., Dettman, D.L., Abell, P., 1997. Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites. Geological Society of America (Bulletin) 109, 444-460.
• Hovland, M., 2002. On the self-sealing nature of marine seeps. Continental Shelf Research, 22, 2387-2394.
• Hovland, M. and Judd, A.G., 1988. Seabed Pockmarks and Seepages. Impact on Geology, Biology and the Marine Environment. Graham & Trotman Ltd., London, 293 pp
• Kempe, S., Kazmierczak, J., Landmann, G., Konuk, T., Reimer A. and Lipp, A., 1991. Largest known microbialites discovered in Lake Van, Turkey. Nature, 349, 605-608.
• Kipfer, R., Aeschebach-Hertig, W., Baur, H., Hofer, M., Imboden, D.M., Signer, P., 1994. Injection of mantle type helium into Lake Van (Turkey): the clue for quantifying deep water renewal. Earth & Planetary Science Letters, 125, 357-370.
• Laval, B., Cady, S.L., Pollack, J.C., McKay, C.P., Bird, J.S., Grotzinger, J.P., Ford, D.C., Bohm, H.R., 2000. Modern freshwater microbialite analogues for ancient dendritic reef structures. Nature, 407, 626-629.
• McNamara, K., 2009. Stromatolites – great survivors under threat. Geoscientist 19 (12), 16-22.
• Warren, J.K., 1982. The hydrological significance of Holocene tepees, stromatolites, and boxwork limestones in coastal salinas in South Australia. Journal of Sedimenatary Petrology 52, 1171- 1201.