So, what is the big deal about salt? The phrases “worth your weight in salt” and “salt of the Earth” derive from the fact that salt that has been a highly valued commodity for millennia (since about 4700 B.C.). The use of salt to preserve food enabled the establishment of civilization. The earliest known European settlement was located around a salt production facility. And, the Romans, among others, valued salt as if it were gold. Salt was used as currency and widely transported and traded. So, humans have long been interested in and vested in salt.
But, there’s another, more nerdy, side to the salt story. Super-salty fluids flow along the seabed at many sites in the deep sea and the habitats they create are fascinating and extreme. In this post, I explain the major reservoirs where salts, like sodium, magnesium, calcium, potassium, and chloride, are found on Earth and how the super-salty seafloor ecosystems we study form.
There are two major reservoirs of salt on Earth – in seawater and in salt deposits on land and below the seabed, both of which formed through evaporative concentration tens of millions of yeas ago. The composition of evaporate salt deposits reflects that when seawater reaches salt saturation, the major salts precipitate out in sequence, first gypsum (calcium sulfate), then anyhydrite (hydrated calcium sulfate), and then halite (sodium chloride). However, the concentration of sodium and chloride are so much greater, that halite dominated the fraction of salts precipitated. As salts precipitate, seawater becomes more and more concentrated (i.e. hyper-saline).
On Earth, the majority of water is in the ocean (95.6% or 1.35 billion cubic kilometers of fluid). The salt content (3.5%) and the proportion of major salt ions in seawater is constant, known to oceanographers as the “Principle of Constant Proportions”. This principle applies to all fluids in the ocean, except the super-salty brines that result from the dissolution of deeply buried salt deposits. Such brines form under specific conditions across many marine habitats, including the Gulf of Mexico, Mediterranean Sea and the Red Sea.
The Gulf of Mexico is somewhat unique in that there are two modes of brine formation.
Sub-seafloor salt deposits – known as salt diapirs – that lie beneath surface sediments (they can be exposed, fairly shallow or deeply buried hundreds of m below the sediment surface) are fascinating and abundant in the Gulf (see image 2). Dissolution of these Jurassic-age salt deposits drive production of super-salty brine solutions. These solutions flow subsequently from the seabed, creating extreme and unique microbial habitats, such as brine flows and brine-influenced sediments (see image 1), brine pools, and brine basins (more about the latter two habitats in upcoming blog posts).
Deeply-sourced brines derive from salt diaper dissolution. These brines have unique chemical composition, depending on the salt body from which they were derived (halite = sodium chloride, gypsum = calcium sulfate, or some mixture of salts) and the characteristics of fluid into which the salt dissolves. In the Gulf of Mexico, most brines derive from halite though evidence of gypsum-derived brines exists at a few locations.
Brines can also form near the seafloor and at shallower depths as a by-product of gas hydrate formation (images 3 & 4). Gas hydrates, also knows as methane clathrates, are ice-like deposits that form at the seafloor where the temperature is low enough, the pressure is high enough, and the flux of methane and other low molecular weight gases, such as propane, is substantial. When this “methane ice” forms, water is removed from seawater to form the hydrate and the residual salts become concentrated, generating brines. These brines are likely chemically distinct from brines derived from salt diapir dissolution.
The next phase of our work during this cruise focuses on understanding the coupled geological, physical, microbiological and geochemical processes that occur at different brine-influenced sites, some where the brine is derived from gas hydrate formation and some where the brine is formed by salt-dissolution.
The first brine site we’re working is GC600. This site is unique in that brine, oil, and gas flows are comingled creating a harsh, yet luxurious habitat for micro- and macro- organisms alike. I told the story of this site’s mysterious oil chimneys and oil-saturated gas hydrate several days ago.
Today, I’ll tell the story of hydrate-derived brines. Our recent dive there (last Sunday, 4/6/2014) and the next one (on Friday, 4/11/14) will focus on collecting brine-impacted sediments and fluids as well as finishing up some oil chimney collections.
The first image on this blog is a mosaic I made showing a brine flow down the wall of a small (100m across) crater. The brine flow was approximately 18m long, and the arrowhead shaped pool at the terminus was approximately 3 long and 1.5m across.
The sediments around these brines are often black and reducing (image 5). All brines are loaded with dissolved organic matter, but these brines are also loaded with oil and gas. Oil can be seen seeping from the brine-charged sediments in image 4. Gas bubbles gush from these sediments as the Alvin lands gently on the bottom. This oil and gas and sulfide fuel a dynamic and diverse microbial community as well as larger animals.
Such brine flows are home to microbial mats (images 6-8) and chemosynthetic mussels and clams (images & 10) that are able to endure the harsh conditions arising from exposure to a fluid 3 times more salty than seawater.
The microorganisms in the white mats, such as those shown in images 6-8, thrive in brine flows and serve as primary producers in brine-influenced habitats. Such bacteria convert inorganic carbon dioxide into biomass, just as grass and trees do on land. On land, primary production is fueled by photosynthesis; at the seafloor, primary production is fueled by chemosynthesis. The difference between photosynthesis and chemosynthesis is simple: photosynthesis uses energy derived from sunlight to produce biomass where as chemosynthesis uses energy derived from chemical oxidation (such as the oxidation of sulfide to sulfate) to harvest energy for biomass formation. Chemosynthetic microorganisms, along with chemosymbiotic macrofauna, form the base of the food web at brine seeps, at hydrocarbon seeps, and at hydrothermal vents.
While the mussels and clams (images above may look like organisms you have seen before. These are not the mussels and clams you’d see in a coastal salt marsh or mudflat. These deep sea mussels and clams different: they live in harsh, stinky muds, where high hydrocarbon fluxes fuel high rates of microbial activity and it’s primary by-product is hydrogen sulfide (ever smell a rotten egg?). Inside these invertebrates live unique symbiotic microorganisms that oxidize hydrogen sulfide, methane (and at higher temperature hydrothermal vents, hydrogen, H2). The symbiotic microorganisms essentially “feed” the macro-organisms, creating an interdependent relationship. The mussels and clams position themselves at sites where their symbionts have access to their primary substrates (such as oxygen and sulfide or oxygen and methane, etc.). The symbionts then provide nutrition to the host.
Other animals, such as eels, fish, and crabs, take advantage of the bounty of the chemosynthetic production, living in or visiting the seeps to feed (images 11 & 12). Others, such as the anemone seen here (image 13), are sessile yet they still take advantage of the tremendous productivity of these amazing deep sea oases of life.
Upon completion of the all the tasks on the dive plan and being low on battery, we drop weights and the Alvin rises slowly from the seafloor (1225m) to the sea surface. The transit takes about 45 minutes. When we reach the surface, we see the small boat and R/V Atlantis waiting for us (image 14). On this dive we collected a variety of samples– 24 sediment cores, 2 brine samples, bottom water samples, and some animal specimens - a treasure trove that will keep us busy all night and into the following day. I can’t wait to get back to this site the day after tomorrow to search for additional brine flows and pools along the seabed.