Searching for Life’s Essential Ingredient in the Benthic Boundary Layer

Last Sunday afternoon, the Revelle’s science party began their Benthic Boundary Layer transect. This set of nine, near-shore stations stretches south from Cambria and wraps around Point Conception to finish just at the northernmost point of the Southern California Bight. These shallow stations allow us to sample water that is close to the seafloor, where the continental shelf provides sedimentary nutrients to upwelled waters on their way to the surface. For a trace metal chemist, this area is particularly interesting, as we suspect the Benthic Boundary Layer (pronounced BBL by busy scientists) is a major source of iron during some of the most productive times of the year.

The path of our BBL transect.

            Iron is an essential and often limiting nutrient for marine primary producers, but its exceptionally low concentrations make it particularly difficult to measure (especially from a big rusty boat). Numerous precautions are taken to assure that our samples avoid contamination during their trip from the ocean back to our lab at Scripps. This starts right when we first collect the water. For BBL sampling, the trace metal team employs a large 30-liter Niskin “GoFlo” bottle that we attach directly to a weighted line. We send it down to as close to the seafloor as possible, then literally throw a heavy Teflon messenger down the line to hit a button that triggers the bottle to close. The design for this bottle was developed over a hundred years ago, and has barely changed since then. To keep our samples free from big-rusty-boat-contamination (BRBC), there is an additional switch on the GoFlo that keeps the bottle closed until it reaches a certain depth. This is important for later.

Sailor with Niskin:
An early, yet shockingly similar version of our Niskin bottles.

            The full GoFlo is likely close to 100 pounds. It may be even more, but as a strapping gentleman, I can’t tell. One of us lugs the GoFlo back into our trace metal clean van (usually one-handed and with little difficulty), where it’s tied into its box and pressurized with high purity nitrogen gas. This allows us to push the water through an in-line filter to get our dissolved metal samples. For the BBL, we took about 6 liters of water at each station, for a variety of iron, mercury, ligand, and nutrient measurements.

            The nine-station transect takes us about 16 hours, so efficiency is key. In the van, the trace metal team becomes one organism with four arms. Some are clean, some are dirty, but they have one mind. The hour is late, the music is loud, and after the fourth or fifth station, the filtering, pressurizing, rinsing, and filling take place with minimal discussion. But there is singing, for the trace metal organism has awoken.

            At the third station, tragedy struck. The GoFlo known as “The Beast” experienced catastrophe. The pressure switch that should have opened the bottle at around 15 meters failed to trigger. The bottle ended up under 100 meters of ocean, still full of air. When we sent down the messenger, the shock imploded the GoFlo, and The Beast was slain. 

The decapitated corpse of The Beast.

            But we are not funded to mourn, and the transect must go on. After switching out The Beast for The Boss (our second “GoFlo” bottle), we were back on track. Efficiency increased throughout the night, and our finish line was marked by a beautiful sunrise over Point Conception. But wait! We do not close our eyes yet, for it’s not a proper BBL transect without a special bonus station in the Santa Barbara Basin, where an underwater sill creates a unique anoxic region. Here, we use our trace metal rosette to get a full depth profile, to see what scientific mysteries lie hidden in this oxygen-deprived water-mass. 

Kiefer with crown:
Kiefer Forsch (Barbeau lab), a veteran trace metal chemist, honors a fallen God.

            We’re very excited to process this data, as we now have six years of BBL measurements. The BBL waters we sampled this transect seem to have noticeably fewer particulates than previous years, so it will be very interesting to start to investigate the phenomena behind this trend, and how it affects the overall micronutrient-dependent behavior of the CCE.

-Max Fenton (Barbeau lab)

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