Colorful salt diapirs and conversations from the deep
Written by Mandy Joye, 18 April 2014; posted by Mandy Joye, 19 April 2014.
To have a brine-filled basin like Orca, the super-salty fluid has to flow in from somewhere. But where? The published literature on Orca geology suggests that the salt diapirs that donate salt to the deep basin are located on the eastern flank of the central basin. We wanted to find it and better yet, SEE it, and, even better, SAMPLE it!
Salt diapirs are best imagined as pipes or square blocks that move upwards or are pushed up from underlying sediments. The overlying sediments are heavier than the salt and salt is ductile, so it deforms under tensil stress and moves upward, driving salt tectonics and creating diapirs than sometimes breach the sediment surface.
Alvin dive #4699 was tasked with two primary objectives, sampling the sponge gardens and finding a salt diaper where salty-brine was entering the basin. We also had an exceptionally splendid and special opportunity on this dive: to do a radio call with my daughter’s class at the Montessori School in Athens, GA, while sitting in the Alvin 2169m below the sea surface amongst a garden of sponges. Well, that conversation made my day!
But first, a little about the Orca brine and glass sponges. When seawater evaporates, a series of salt crystals precipitate out as a function of their relative solubility at that salinity. The most insoluble salts precipitate first. Anhydrite (image 2), gypsum and halite precipitate from seawater in sequence as it becomes more and more salty. Because there is much more sodium and chloride in seawater than calcium and sulfate, there is more halite (rock salt or sodium chloride) than calcium sulfate (anhydrite or gypsum, both are comprised of calcium and sulfate, but gypsum is hydrated while anhydrite is not).
The Louann salt deposit, which lies deep beneath the sediments of the Northern Gulf and drives much of the regional geology, formed during the mid-Jurassic, about 165 million years ago. This evaporative deposit is primarily halite but most evaporites usually contain more than one salt. The chemical composition of the deep Orca brine suggests that calcium sulfate contributes to this brine. We suspect this because the brine’s sulfate concentration is elevated significantly above that expected for seawater and an obvious source of the excess sulfate is calcium sulfate dissolution.
Anhydrite can have rose-colored hue (image 2) and if anhydrite is mixed with rhodochrosite (manganese carbonate), it is bright pink (image 3). And, oddly enough, the sediments of central Orca basin are bright pink. Halite can also have a pinkish coloration, but it is most often opaque. So, we believe the “Orca Pink” is derived from calcium sulfate. We set out to test this hypothesis on this dive .
As the Alvin descended into the Orca Deep, I was nervous and excited. I had spent literally hours the evening before poring over geological maps of the central basin attempting to pick a spot where a diaper would arise from the basin bottom, allowing us access to sample it. This is not straightforward because the basin floor is around 2200m deep but the Alvin, even loaded with extra weights, can only reach a depth of 2198m in the brine. This is because the brine is so dense – and the Alvin so buoyant – that the Alvin literally comes to a stop and floats along the surface when it reaches the neutral density point. So, my job was to find a spot where a diapir breached the depth where we could sample, around 2190m. Talk about finding a needle in a haystack. After five hours, I finally picked my point, 26º 56.22N and 91º 16.97W. I crossed my fingers and hoped this spot would be a lucky one.
When we reached our cruising depth, 2188m, we punched in a heading to the E, heading towards what I hoped would be the land of salt fingers reaching up from the seafloor. Around 45 minutes later – and after much hand wringing on my part – I spotted the first one. Wow. There it was, a diapir. Well, it looked like what I thought a diapir would look like, though it was deeper than I expected.
Nonetheless, there it was, poking up through the brine haze and we gently nudged into it creating one big mess of brine fog. Since we had no idea the size of the feature – it literally rose up in front us giving us little reaction time – we had to get a little lighter to get out of the fog (image-4). Luckily the top of the feature was at about 2185m so we went up and sat on top of it to have a look. There, we saw a spectacular garden of glass sponges (image-5).
Glass sponges are uncommon in shallow water but they are known to reside in deep water. This particular group of sponges, the Heactinellidae, are thought to be the most ancient lineage of sponges, having evolved in the pre-Cambrian period, well over 500 million years ago. They were first discovered in the late 1800’s and were described in detail by Ernst Haeckel, a famous German zoologist, in 1904 (image-6). These sponges are thought to live for thousands of years and their anchoring roots look oddly like fiber optic cable (image-7). In the Orca Basin, these filter-feeding sponges thrive where other macro-organisms cannot. They live at salinities well above those tolerated by fish and other invertebrates and are likely significant predators of free-living bacteria in the water column.
While parked here, we collected sediment cores and to no one’s surprise, the sediments were pink (image-8). Then we continued our journey to the SE to find the target diapir. Around an hour later, we found it (image-1). And I was shocked to realize that it was located at almost the exact spot I picked. Amazing. This diapir was spectacular.
It had bright pink and white flow features all along its sides. And fissures several inches thick that looked like flow channels, perhaps channels where brine has flowed previously. We collected a lot of high definition video of the area and came against the diapir to core within the white and pink flow features (image-9). We collected 7 cores from the white areas, 7 from the pink area and 4 from generic brown areas. When we removed cores, the holes quickly filled with brine. The pore water and solid phase chemistry of these cores should help us nail down whether these features are in fact the source of brine feeding the deep basin.
Next, we moved to the top of the diapir, at a depth of 2169m below the sea surface. Here, again, we found ourselves in a sponge garden (image-10). The diverse array of sponges atop this diapir was not surprising – it’s what we always see. The variety of shapes, sizes and morphologies is striking and mysterious but they all share one thing in common, the deep anchoring roots (image-11). These roots make them extremely tricky to collect.
Looking outside my view port was this beautiful, rather large, sponge – about 15 cm across – sitting right at the edge of the wall. It was a lovely site that I could have spent hours simply starting at. If you look past the bright yellow sponge, you’ll see an interesting sight – a cluster of sponges that look like the smiley face of an octopus wearing a Christmas hat.
Finally, it was time for the finale of the dive: a radio call to my daughter’s class back in Athens, GA. The children had a lesson on talking over a radio and they did such a great job: they spoke loud and clearly and once finished with their question, said “OVER”. I was very impressed. It took me a while to learn to say “OVER” when talking on the radio but not one of the kids missed a beat.
And they asked great questions!
1. My daughter asked me, “Where are you right now Momma, over?” I answered that I was in the submersible Alvin sitting atop a salty mud mound at a depth 6300 feet below the sea surface. That’s over a mile, we’re way beneath the surface of the ocean.
2. Then, another child asked, “What do you see when you look outside the window, over?” I answered that if the lights were off it would be completely dark and I’d see nothing. But with the Alvin’s light illuminating the area, we can see colorful sediments and beautiful animals.
3. The next child asked, “Who is with you in the submarine, over?” I was with the pilot, Bob Waters, and a Univ. of GA PhD student, Ryan Sibert. While I was talking to the children, Bob was sampling and I noted that Bob was an excellent pilot who could catch or sample anything you asked him to get (which resulted on a bit grin on Bob’s face). I then asked the children “how many of you want to be an Alvin pilot when you grow up?” We could hear them all cheering and saying yes. That made all of us grin.
4. And then, “How long have you been on the bottom, over?” I answered that we had been lowered into the water around 8:15AM and at that point it was around 1:15PM so we’d been in the water about 5 hours and on the bottom about 3.5 hours. Typically an Alvin dive lasts for about 8 or 9 hours total and we get from 4 to 6.5 hours of bottom time, depending on the water depth (which determines how long it takes to get down and then back up). Usually we are in the water by 8:15AM and we are back on deck by around 5PM, give or take an hour here or there.
5. Finally, a child asked “Is it cold in the submarine, over?” This was a very good question. We’re inside a submarine but there is no heater, so it quickly becomes cold, very cold. The water outside is about 39º F and inside the sub it was probably around 60º F. Maybe a bit more than that, but I am always cold and I was sitting beside the titanium hull, which was in direct contact with the 39º F seawater. So, it’s cold in there. Very cold, brrrr!
After that, I spent a few minutes telling the children about what we were doing the next few days, how much fun it is to dive, explore, and discover new things at the seafloor, and that I had a big surprise for them when I return home next week. Each child in the class colored a Styrofoam cup and I took the cups down in a bag outside the submarine where the pressure of the deep sea shrunk them. Next Thursday, I’ll take those cups to the classroom for the children to take home. Thinking about it, I grin ear-to-ear knowing the squeals of joy that seeing those cups will generate.
Today was a great day.